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Non-Energetic Formation of Ethanol via CCH Reaction with Interstellar H2O Ices. A Computational Chemistry Study

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  • University of Grenoble and Universitat Autònoma de Barcelona
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Ethanol (CH$_3$CH$_2$OH) is a relatively common molecule, often found in star forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). Yet, the formation route of this species remains debated. In the present work, we study the formation of ethanol through the reaction of CCH with one H$_2$O molecule belonging to the ice, as a test case to investigate the viability of chemical reactions based on a "radical + ice component" scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical-radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH$_3$CH$_2$OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H$_2$O on the water ice clusters can be barrierless (thanks to the help of boundary icy water molecules acting as proton transfer assistants) leading to the formation of vinyl alcohol precursors (H$_2$CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings.
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Non-Energetic Formation of Ethanol via CCH
Reaction with Interstellar H2O Ices. A
Computational Chemistry Study
Jessica Perrero ,,Joan Enrique-Romero ,,,Berta Martínez-Bachs ,
Cecilia Ceccarelli ,Nadia Balucani ,§,,kPiero Ugliengo ,and Albert
Rimola ,
Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193,
Catalonia, Spain
Dipartimento di Chimica and Nanostructured Interfaces and Surfaces (NIS) Centre,
Università degli Studi di Torino, via P. Giuria 7, 10125, Torino, Italy.
Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble
(IPAG), 38000 Grenoble, France
§Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di
Sotto 8, 06123 Perugia, Italy
kOsservatorio Astrosico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
E-mail: juan.enrique-romero@univ-grenoble-alpes.fr; albert.rimola@uab.cat
1
arXiv:2202.11406v1 [astro-ph.GA] 23 Feb 2022
Abstract
Ethanol (CH3CH2OH) is a relatively common molecule, often found in star forming re-
gions. Recent studies suggest that it could be a parent molecule of several so-called interstellar
complex organic molecules (iCOMs). Yet, the formation route of this species remains debated.
In the present work, we study the formation of ethanol through the reaction of CCH with
one H2O molecule belonging to the ice, as a test case to investigate the viability of chemical
reactions based on a "radical + ice component" scheme as an alternative mechanism for the
synthesis of iCOMs, beyond the usual radical-radical coupling. This has been done by means
of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results
indicate that CH3CH2OH can potentially be formed by this proposed reaction mechanism. The
reaction of CCH with H2O on the water ice clusters can be barrierless (thanks to the help of
boundary icy water molecules acting as proton transfer assistants) leading to the formation of
vinyl alcohol precursors (H2CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol
yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss
the astrophysical implications of these findings.
Keywords
interstellar medium, astrochemistry, DFT, iCOMs, grains
2
Introduction
Interstellar complex organic molecules (iCOMs)
are compounds between 6-12/13 atoms, in
which at least one is carbon, conferring the or-
ganic character.1–3 These molecules are impor-
tant because i) they can be considered as the
simplest organic compounds that are synthe-
sised in space (hence representing the dawn of
organic chemistry), and ii) they are the precur-
sors of more complex organic molecules, which
can be of biological relevance, such as amino
acids, nucleobases and sugars. Indeed, there
is robust evidence that the iCOMs formed in
the interstellar medium (ISM) were inherited by
small objects of the Solar system4–8 (e.g., car-
bonaceous chondrites), which upon alteration
mechanisms (e.g., hydrothermal) can be con-
verted into more evolved organic species,9–12
thereby having a potential contribution to the
emergence of life on Earth.
The first detection of iCOMs took place
in 1971 in massive star formation regions,13
but we had to wait for the beginning of this
century for detections in regions that could
eventually evolve in Solar-like planetary sys-
tems.6,14 Currently, complex organic molecules
have been detected in different astrophysical en-
vironments such as prestellar cores, protostellar
outflows, protoplanetary disks, and even in ex-
ternal galaxies.3,8,15–18
Two different prevailing paradigms have been
postulated for the formation of iCOMs: one
based on gas-phase reactions,19–24 the other
based on radical-radical couplings occurring on
grain surfaces,25,26 although other paradigms
have been postulated later, like those based
on the condensation of atomic C,27,28 ex-
cited O-atom insertion,29,30 or the formation
of HCO on surfaces as a parent precursor
of other iCOMs.31,32 Both prevailing reaction
models have the same initial step: formation
of hydrogenated species (e.g., CH3OH, NH3,
H2CO, CH4) by H-addition on icy grain sur-
faces in the cold pre-stellar phase. In the
gas-phase paradigm, the process follows with
the desorption of these species, either ther-
mally when the grain surface reaches ca. 100
K like in hot cores/corinos, non-thermally by
photodesorption due to UV incidence on the
grains33–35 or by chemical desorption once they
are formed36–41, or induced by cosmic rays
(CR)42–45. In the gas phase they react with
other gaseous species forming iCOMs. In the
on-grain paradigm, the hydrogenated species
act as parent species of molecular radicals
(e.g., CH3, HCO, NH2), formed by the irra-
diation of UV photons and/or energetic ions,
partial hydrogenation, and H-abstraction reac-
tions2,25,26,46–50 during the cold pre-protostellar
stage. Later on, these radicals, due to the warm
up of the protostar surroundings (ca. 30K), can
diffuse on the icy surfaces and encounter one
each other to couple and form the iCOMs.
These two reaction paradigms have been
largely used in combined observational and
astrochemical modelling studies to rationalize
the presence of iCOMs in the given sources,
e.g.,.3,51–53 And, additionally, they have also
been assumed to study the formation of iCOMs
by means of quantum chemical simulations.54–57
By compiling all the available works, it seems
that both paradigms are necessary to explain
the presence, distribution and abundance of the
wide and rich organic chemistry in the ISM.
Thus, knowing whether the formation of an
iCOM is dominated by surface or gas-phase
chemistry is a case-by-case problem. It depends
on the nature of the iCOMs and each one has
to be addressed as a particular case.
With the different quantum chemical studies
addressing on-grain radical-radical couplings,
some drawbacks of this paradigm have been
identified. One is that the chance for the cou-
pling is a delicate trade-off between the diffu-
sion of the radicals and their desorption. That
is, the temperature increase that enables the
radical diffusion must be lower than their des-
orption temperatures. This can give rise to a
small temperature window in which the cou-
pling can take place, because this coupling tem-
perature window is defined by the lowest dif-
fusion temperature and the lower desorption
temperature among the two radicals. For in-
stance, in acetaldehyde formation by the cou-
pling of CH3and HCO, the coupling tempera-
ture window was found to be between the tem-
peratures at which CH3becomes mobile on the
3
surface (between 9 and 15 K, depending on the
adopted diffusion barrier) and the temperature
at which the methyl radical would desorb (30
K).55 Another drawback is that radical-radical
couplings are often assumed to be barrierless
because they are driven by the coupling of the
opposite electronic spins of the radicals. How-
ever, the reactions can exhibit energy barriers
because the radicals, to proceed with the cou-
pling, need to break the interactions with the
icy surfaces. Obviously, as a rule of thumb, the
stronger the interaction, the higher the energy
barrier. A third limitation of this paradigm is
that radical-radical couplings can have compet-
itive channels inhibiting the efficiency of the
iCOMs formation. The competitive reactions
are based on H-abstractions from one radical
to the other. For instance, the CH3+ HCO
coupling can give CH3CHO (acetaldehyde as
iCOM) but also CH4+ CO (the H-abstraction
product).
In a previous work by some of us,58 an alter-
native on-grain mechanism different from the
radical-radical coupling was proposed. It is
based on the reaction of a radical (coming from
the gas phase or generated by UV irradiation)
with neutral, entire components of the ice, i.e.,
H2O and CO as the most abundant compo-
nents. In that work, this mechanism was tested
to form formamide (NH2CHO) through the re-
action of the radical CN with a water molecule
belonging to the ice, i.e., CN + H2O(ice)
NH2CO, in which the resulting species can be
easily hydrogenated to form formamide. This
alternative mechanism overcomes the problem
of i) the coupling temperature window (the dif-
fusion of the radical is not needed because it
reacts with an abundant ice component, this
way increasing the chance of the encounter-
ing among the two reactants) and ii) the com-
petitive channel (they a priori do not present
any other possible reaction). They, however,
present energy barriers since the reaction is
between a radical and a neutral closed-shell
species.
The present work aims to investigate this
alternative on-grain mechanism by simulating
with quantum chemical computations the reac-
tivity of the CCH radical with a water molecule
of the ice. The goal is to study the possibil-
ity to form ethanol (EtOH), with the formation
of vinyl alcohol (VA) as intermediate species,
through the following reactions:
CCH + H2OH2CCOH/HCCHOH (1)
H2CCOH/HCCHOH + H H2CCHOH (VA)
(2)
H2CCHOH + 2H CH3CH2OH (EtOH)
(3)
We consider this path towards ethanol be-
cause the electronic structure of CCH is isoelec-
tronic with CN, hence exhibiting a similar re-
activity with water as shown in Rimola et al.58
CCH is one of the first detected interstellar
polyatomic molecules59. It is a fundamental
and common carbon chain (in fact, the sim-
plest) species in the ISM. It is found in re-
gions near UV sources, the so-called Photon
Dominated Regions (PDRs) e.g.60,61, in diffuse
and translucent clouds e.g.62,63, in protostellar
objects,64,65 in the cavities of the protostellar
outflows66,67 . In addition, CCH has been de-
tected in protoplanetary disks30,68–71 and exter-
nal galaxies e.g.72, probably in their PDR re-
gions/skins. CCH is also abundant towards the
lukewarm (40 K) envelopes of the so-called
Warm Carbon Chain Chemistry(WCCC) ob-
jects, which are young Class 0/I sources charac-
terized by higher abundances of carbon chains
and lower abundances of iCOMs than those ob-
served in hot corinos (see for example ref.65 ).
CCH appears at scales of few 1000s AU around
the protostellar centre at densities of some 106
cm3(e.g., ref.73). Particularly relevant to
this work, CCH is also relatively abundant in
cold molecular clouds e.g.74,75 and prestellar
cores.76 Typical CCH column densities range
from 1012–1015 cm261,77, with the highest ones
in PDRs.61 In molecular clouds, the CCH col-
umn density is around 1014 cm2, equivalent to
abundances of 109108.74,75 Similar abun-
dances are found in prestellar cores, as probably
CCH resides in the least depleted zone, simi-
lar to the molecular clouds.76,78,79 In summary,
CCH is abundant (109108) in cold (20
K) regions where the interstellar dust grains are
4
enveloped by icy mantles. Thus, it is worth to
investigate the possibility that it interacts with
the water molecules of the ice to form ethanol,
following the path described above (reactions 1
to 3).
The reaction of CCH + H2O has been stud-
ied as a gas-phase process at high temperatures,
both experimentally and theoretically. Sem-
inal experiments by Van Look and Peeters80
suggested that the outcome of the reaction
was not the result of a direct H-abstraction
forming HCCH + OH, and hence that au-
thors proposed a mechanism based on, first,
an association between the two reactive part-
ners, forming H2C=CHOH or HC=CHOH, fol-
lowed by an elimination giving rise H2CCO
+ H and/or HCCH + OH. Subsequent theo-
retical calculations,81 however, indicated that
among the different reactive channels, the di-
rect H-abstraction was the most kinetically
favourable one, in detriment of the association-
elimination mechanism. Investigations on this
reaction ended with a combined experimen-
tal/theoretical study carried out by some of
the first paper authors, concluding that the H-
abstraction producing HCCH + OH is indeed
the most facile chemical reaction.82
In contrast, to the best of our knowledge,
no experimental works exist on the reactivity
of CCH with water ices. However, the addi-
tion of OH radicals to C2H2ices (isoelectronic
with CCH + H2O) at temperatures below 20
K, followed by H-additions, has been studied
by several authors, resulting with the formation
of several products, among them vinyl alcohol
and ethanol. This reactivity usually involves
an energetic pre-processing of the ice analogues
(i.e., irradiation with ions, electrons and pho-
tons) to generate the OH radicals. Among these
works, we can find: 1) ion radiolysis experi-
ments (MeV protons) that generate mainly CO,
CO2, methanol and ethanol;83 2) MeV protons
and far UV photon irradiation that yields vinyl
alcohol formation;84 3) UV irradiation and pro-
ton radiolysis of the ices that form vinyl alcohol,
acetaldehyde, ketene and ethanol;85 4) ice ir-
radiation with extreme UV photons that leads
to the formation of some iCOMs like ethane,
methanol and ethanol, together with some sim-
pler species (e.g. CO, CO2and methane);86 and
5) radiolysis of the C2H2:H2O ices with less en-
ergetic protons (200 keV) at 17 K that produce
several iCOMs like vinyl alcohol, acetaldehyde,
ketene and ethanol, and some other species such
as C2H4, C2H6, C4H2and C4H4.87 In this later
work, it was proposed that once vinyl alcohol is
formed by the attack of an OH radical to C2H2,
two possible situations may take place: either
an intramolecular isomerization step forming
acetaldehyde or successive H-additions on vinyl
alcohol to form ethanol. More recently, non-
energetic processes have also been explored, in
which C2H2:O2ices exposed to H atoms at 10 K
produce most of the products found in Chuang
et al.,87 including acetaldehyde, vinyl alcohol,
ketene and ethanol. Other experiments indicate
that vinyl alcohol and acetaldehyde can also
be formed through other chemical reactions.
That is, the O addition to C2H2mainly results
in ketene formation,30,85,88,89 while the O ad-
dition to more saturated hydrocarbons (acety-
lene, ethane and ethylene) leads to the forma-
tion of different iCOMs (ketene, ethanol and ac-
etaldehyde, and acetaldehyde and ethylene ox-
ide, respectively).30,90
Ethanol formation has recently received much
attention because it has been postulated to be
a parent molecule through which other iCOMs
can be formed by different gas-phase reactions
such as acetaldehyde, glycolaldehyde, formic
acid, acetic acid and formaldehyde (the so-
called genealogical tree of ethanol, see23,24,91).
Because of this significance, this work focuses
on the formation of this ancestor molecule fol-
lowing the reactions 1–3 on water ice surfaces
by means of quantum chemical simulations to
know if they are energetically feasible.
Methodology
All the calculations were based on the Den-
sity Functional Theory (DFT) and run with
the Gaussian16 software package.92 Geometry
optimisations and frequency calculations were
all performed by combining the DFT meth-
ods with the Pople-based 6-311++G(d,p) ba-
sis set.93,94 These energies were subsequently
5
refined at 6-311++G(2df,2pd)95 level by per-
forming single point energy calculations on the
optimised geometries. In order to identify the
DFT method that better describes our systems
(and hence to use it for the reaction simu-
lations on the water ice surface models), we
carried out a preliminary benchmarking study
using the CCH + H2O and CH2CHOH + H gas-
phase reactions as models. Five different hybrid
DFT methods were used, which were corrected
with Grimme’s D3 term or, if possible, with
the D3(BJ) version96–98 to account for disper-
sion interactions. The tested DFT-D meth-
ods were: BHLYP-D3(BJ),99,100 M062X-D3,101
MPWB1K-D3(BJ),102 PW6B95-D3(BJ)103 and
ωB97X-D3.104 By comparing the results with
those obtained with single energy points at
the CCSD(T)/aug-cc-PVTZ105 level of the-
ory, known as the "gold-standard" in quan-
tum chemistry, we found that the ωB97X-D3
method showed the best performance when
modeling the water addition to CCH, while
MPWB1K-D3(BJ) described better the hy-
drogenation of CH2CHOH (see § Results be-
low). Accordingly, the CCH + H2O and the
CH2CHOH + H reactions on the water ice
cluster models were computed, respectively, at
the ωB97X-D3/6-311++G(2df,2pd)//ωB97X-
D3/6-311++G(d,p) and the MPWB1K-
D3(BJ)/6-311++G(2df,2pd)//MPWB1K-
D3(BJ)/6-311++G(d,p) theory levels.
All the stationary points of the potential
energy surfaces (PESs) were characterized by
their analytical calculation of the harmonic fre-
quencies as minima (reactants, products, and
intermediates) and saddle points (transition
states). When needed, intrinsic reaction coor-
dinate (IRC) calculations at the level of the-
ory adopted in the geometry optimizations were
carried out to ensure that a given transition
state actually connects with the correspond-
ing minima. Thermochemical corrections to
the potential energy values were carried out
using the standard rigid rotor/harmonic oscil-
lator formulas to compute the zero point en-
ergy (ZPE) corrections.106 Since the systems
are open-shell in nature, calculations were per-
formed within the unrestricted formalism.
We additionally calculated the tunnelling
crossover temperatures, i.e., the temperature
below which quantum tunnelling becomes the
main mechanism for trespassing the potential
energy barriers. To this end, we used Eq. 4107,
where His the ZPE-corrected barrier height,
ωis the frequency associated to the TS, and
kBand ¯hare the Boltzmann’s and reduced
Planck’s constants. This temperature indicates
what reactions may actually have an important
role at low temperatures despite of having an
activation barrier.
Tc=¯H/kB
2πH¯ln(2) (4)
The surfaces of amorphous solid water (ASW)
ice coating interstellar grains were simulated
by two different cluster models: one consisting
of 18 water molecules, the other of 33 water
molecules (hereafter referred to as W18 and
W33, see Figure 1). While the former rep-
resents a compact, flat water ice surface, the
latter presents a cavity structure of about 6
Å. For more details, we refer the reader to
see.54,58,108
For the calculation of the binding energies
(BEs) of CCH interacting with the H2O ice
surface models (i.e., CCH/surf complexes)
we adopted the same electronic structure
methodology as for reactivity, namely, for
each CCH/surf complex and corresponding
isolated components, geometry optimizations
and frequency calculations (and hence ZPE
corrections) were computed at ωB97x-D3/6-
311++G(d,p), followed by single point energy
calculations at the improved ωB97x-D3/6-
311++G(2df,2pd) level. Basis set superposi-
tion error (BSSE) was corrected following the
Boys and Bernardi counterpoise method. The
final, corrected, adsorption energy (ECP
ads ) was
calculated as:
ECP
ads (CCH/surf)=∆Eads +BSSE(CCH)
+BSSE(surf)+∆Z P E
where Eads =E(CCH/surf)E(CCH)
E(surf )refers to the BSSE-non-corrected ad-
sorption energy. Note that we used the same
sign convention as in,54 namely, the adsorption
6
Figure 1: Structures of the 18 and 33 water molecules clusters, optimized at the ωB97X-D3/6-
311++G(d,p) level of theory.
energy is the negative of the binding energy:
ECP
ads =BE.
Results
Benchmarking study
As mentioned above, we carried out a prelim-
inary benchmarking analysis for the reactivity
using two gas-phase model reactions, CCH +
H2O and CH2CHOH + H, to find the DFT
method that describes better the reaction prop-
erties. For CCH + H2O, we found three pos-
sible reaction pathways, labelled as R’1,R’2
and R’3, the stationary points of which being
shown in Figure 2.
R’1 follows a stepwise mechanism, the first
step involving the formation of acetylene
(HCCH) and the hydroxyl radical (OH) as
intermediate species, and the second step con-
sisting of the condensation of these intermedi-
ates to form the HCCHOH radical. In contrast,
both R’2 and R’3 adopt a concerted mecha-
nism. These two reactions involve a C-O bond
formation followed by a H-transfer to the other
C atom to form the H2CCOH radical (isomer of
HCCHOH, the product of R’1). The difference
between R’2 and R’3 is that, in the former,
the H-transfer comes first, followed then by a
spontaneous C-O bond formation, while in the
latter the C-O bond formation and the proton
transfer evolve in a synchronized way.
The computed energetics of these reaction
pathways using the different quantum chemi-
cal methods are shown in Table 1. The opti-
mized geometries of the stationary points are
available in the supporting information (SI).
As it can be seen, the overall best functional
for these reactions of water addition to CCH
is ωB97X-D3, with an average unsigned er-
ror of 5% in the energy barriers and 6%
in the whole energetics. The performance of
the other functionals is, from better to worse:
MPWB1K-D3(BJ), PW6B95-D3(BJ), BHLYP-
D3(BJ) and M062X-D3.
For the hydrogenation steps (reactions 2–3),
we only considered the hydrogenation of vinyl
alcohol, namely, CH2CHOH + H, because it
is the unique step that can exhibit an energy
barrier due to involving a closed-shell molecule
(vinyl alcohol) with a radical (H atom). In
contrast, the other steps consist in the hydro-
genation of radical species, which are barri-
erless processes due to the spontaneous spin-
spin coupling. For this H-addition reaction, we
found two possible pathways (labelled as H’1
and H’2 in Figure 2), leading to two differ-
ent products (CH3CHOH and CH2CH2OH, re-
spectively), depending on which C atom the
H addition takes place. H’1 and H’2 share
a similar mechanism, in which the H atom in
the reactant structures is located at ca. 3.5 Å
from the C atom with which it will react. Re-
sults show that path H’1 is more favorable than
H’2, both for the stability of the product and
for presenting a lower activation energy barrier
(see Table 1). This is because the H’1 prod-
uct (CH3CHOH) exhibits a better delocaliza-
7
Figure 2: Stationary points identified in the benchmarking study. Reaction pathways R’1,R’2
and R’3 refer to gas-phase CCH + H2O reaction and are optimized at ωB97X-D3. H’1 and H’2
correspond to the two hydrogenation pathways of vinyl alcohol, optimized at MPWB1K-D3(BJ).
Distances are in Å.
tion of the unpaired electron with respect to the
H’2 product (CH2CH2OH). Among the used
DFT methods, the functional with the smallest
average unsigned error compared to CCSD(T)
single point energy calculations is MPWB1K-
D3(BJ). The performance of the other function-
als, from better to worse, is: PW6B95-D3(BJ),
ωB97X-D3 and BHLYP-D3(BJ). M062X-D3
was discarded for convergence problems of the
reactant structures. Therefore, according to
this benchmarking study, the ωB97X-D3 DFT
method has been chosen to simulate the addi-
tion of water to CCH on the W18 and W33
cluster models and MPWB1K-D3(BJ) has been
adopted for the hydrogenation step of vinyl al-
cohol.
Adsorption of CCH on water ice
and binding energies
The complexes formed when CCH adsorbs on
W18 and W33 are shown in the R structures of
Figures 3 and 4, respectively. Table 2 reports
the computed binding energies and the different
contributions, as detailed in § Methodology.
In most of the cases, a nonclassical hemibond
between the CCH species and a H2O of the ice
is established, due to the formation of a two-
center three-electron system between the un-
paired electron of CCH and a lone pair of H2O.
This interaction is highlighted by the computed
spin density values and maps, which clearly in-
dicate a delocalization of the unpaired electron
8
Table 1: Relative energies (in kJ mol1) of the stationary points involved in the reaction pathways
found for the gas-phase CCH + H2O reaction model (R’1,R’2 and R’3) and for the gas phase
hydrogenation (H’1 and H’2) for all the DFT-D methods and the CCSD(T) method used for
the benchmarking study. See the Methodology section for more details. Rows indicated as “TS
% RX” show the unsigned error (in percentage) relative to the predicted energy of the TS. Rows
indicated as “Avg % RX” show the averaged unsigned errors (in percentage and accounting for all
the stationary points) of each DFT-D method with with respect to the CCSD(T)//ωB97X-D3 for
R’1, R’2 and R’3 and CCDS(T)//MPWB1K-D3(BJ) for H’1 and H’2 reference values. The last
row indicates the global averaged unsigned error (in percentage).
Reaction Step BHLYP-D3(BJ) M062X-D3 MPWB1K-D3(BJ) PW6B95-D3(BJ) ωB97X-D3 CCSD(T)
R’1
H2O + CCH 0.0 0.0 0.0 0.0 0.0 0.0
TS1 28.5 8.9 25.6 19.6 29.0 29.1
OH + HCCH -91.6 -57.0 -72.5 -63.0 -62.3 -69.0
TS2 -68.5 -62.8 -60.2 -61.1 -53.5 -51.5
HCCHOH -217.0 -222.0 -217.7 -203.6 -205.9 -193.8
TS1 %R’1 2.1 69.5 11.8 32.6 0.4 - -
TS2 % R’1 33.2 22.1 17.0 18.8 4.0 - -
Avg % R’1 20.0 30.9 11.5 16.3 5.1 - -
R’2
H2O + CCH 0.0 0.0 0.0 0.0 0.0 0.0
TS1 117.4 107.5 96.4 90.8 108.0 99.5
H2CCOH -249.3 -246.8 -266.3 -253.2 -247.8 -241.11
TS % R’2 18.0 8.1 3.1 8.7 8.5 - -
Avg % R’2 10.7 5.2 6.8 6.9 5.6 - -
R’3
H2O + CCH 0.0 0.0 0.0 0.0 0.0 0.0
TS 120.3 87.7 98.6 123.5 113.0 105.4
H2CCOH -240.3 -244.8 -241.9 -228.8 -229.7 -214.1
TS % R’3 16.7 16.8 6.5 17.1 7.2 - -
Avg % R’3 14.5 15.6 9.7 12.0 7.2 - -
Global Avg % 15.1 17.2 9.3 11.7 6.0 - -
H’1
CH2CHOH + H 0.0 0.0 0.0 0.0 0.0 0.0
TS 0.3 - 4.1 2.3 11.4 6.5
CH3CHOH -192.6 - -183.3 -177.1 -188.1 -176.0
TS % H’1 116.4 - 43.6 78.5 93.0 - -
Avg % H’1 62.9 - 23.9 39.6 49.9 - -
H’2
CH2CHOH + H 0.0 0.0 0.0 0.0 0.0 0.0
TS 8.0 - 14.2 12.5 21.6 15.9
CH2CH2OH -157.9 - -143.0 -177.1 -149.0 -142.0
TS % H’2 54.2 - 11.9 23.7 38.5 - -
Avg % H’2 32.7 - 6.3 24.1 21.7 - -
Global Avg % 47.8 - 15.1 31.8 35.8 - -
between the two centers (see data in SI). The
distances of these hemibonds vary between 2.1–
2.3 Å (see the R structures of the R2,R3,R2-1
and R2-2 sequences of Figures 3 and 4). The
only exception not presenting an hemibonded
complex is the structure R of R1 (Figure 3), in
which classical, weak hydrogen bond (H-bond)
interactions are established between CCH and
the W18 ice model. Interestingly, any attempt
to find a pure H-bonded complex (i.e., with-
out any hemibonded interaction) on W33 failed,
the geometry optimizations collapsing into the
hemibonded structures. This reinforces the
idea that the more surface water molecules, the
higher the possibility to form hemibonded com-
plexes, although most of the hemibonded com-
plexes also present H-bond interactions. This is
because the outermost water molecules of the
cluster are unsaturated in terms of H-bonds,
presenting H- and O-dangling atoms ready to
establish H-bond and hemibonded interactions,
respectively. Remarkably, hemibonded systems
also form in the reactant structures of the gas-
phase reactions (see above). However, these
gas-phase systems present a lower hemibonded
character than the complexes on W18 and W33
9
because the spin is less delocalized between the
two centers (see spin density values and maps
in SI).
As expected, hemibonded complexes present
larger BE values than H-bonded ones (see Ta-
ble 2). The difference in the BEs between the
complexes shown in R2 and R3 (i.e., on W18)
arises from the orbital occupied by the CCH
unpaired electron. In the former, the unpaired
electron belongs to the πsystem of the CCH
(the spin density is shared between the two C
atoms and the linearity of CCH becomes broken
upon hemibond formation), while in the latter
to a σorbital of the C-end of CCH, in both
cases interacting with a lone pair of H2O to form
the hemibond. As the orbital overlap is more
efficient in σorbitals than in πones, the lat-
ter complex presents a larger BE than the for-
mer (49.9 and 37.9 kJ mol1, respectively). On
W33, both hemibonded complexes have similar
BE values (89.7 and 86.0 kJ mol1) as in both
systems the unpaired electron occupies a πor-
bital. Remarkably, the values on W33 are no-
tably higher than those on W18, and we investi-
gated which might be the cause. We checked for
a correlation between the BE and the number
of water molecules forming the cluster, carving
several structures from our W33 model, that is,
by removing water molecules from the edges of
the model in order to built a set of water clus-
ters with decreasing dimensions (and blocking
some O atoms of the cluster to prevent the cav-
ity from collapsing). By proceeding this way,
we found evidence that the increasing number
of interactions between CCH and the ice, to-
gether with the cooperativity of H-bonds, is re-
sponsible for the increment of the BE. Data of
this analysis are provided in SI.
Finally, it is worth mentioning that we per-
formed a preliminary benchmarking study on
the binding energies of the dimeric CCH/H2O
system. Results (available in SI) indicate that
while the H-bonded dimer is very well described
by most of the DFT methods, this is not the
case for the hemibonded one. However, for
the particular case of ωB97X-D3, the computed
binding energies are somewhat overestimated,
belonging to the group of the functionals that
better compare with CCSD(T). Accordingly,
the computed binding energies of the hemi-
bonded complexes arising from CCH in inter-
action with W18 and W33 should be consid-
ered overestimated by some amount. Despite
this drawback, we would like to stress that the
main scope of the work is on the reactivity be-
tween CCH and H2O followed by H-additions
and that the used DFT methods are actually ac-
curate enough for this purpose, as shown above.
In this case, the error in the binding energies
self-cancelled when deriving the energy features
(energy barriers and reaction energies) of the
reactions.
Reactivity between CCH and H2O
on the ice surface models
On the W18 ASW ice cluster model
Following the three reaction types found in
the benchmark study, we tried to reproduce
them on top of the W18 ice cluster model.
However, significant differences in the mecha-
nisms have been found, precisely because they
occur on the ice model. Indeed, the larger
number of water molecules infers that i) there
are no concerted mechanisms to work, and ii)
water-assisted proton-transfer reactions are op-
erative. Water-assisted proton transfer mecha-
nisms have been elucidated theoretically in the
latest of the last century109 and can induce im-
portant decrease in the energy barriers with re-
spect to the non-water-assisted analogues. This
is because the assisting water molecules bridge
the accepting/donating proton processes with
their neighboring molecules and, at the same
time, reduce the strain of the rings in the tran-
sition state structures. Hence that this mech-
anism is also called proton-transport catalysis.
Such catalytic effects have also been observed
in processes of interstellar interest on icy sur-
faces.58,110,111 It is worth mentioning that the
water-assisted proton-transfers, to be catalyti-
cally effective, need a proper H-bond connectiv-
ity among the bridging water molecules, from
the first to the last proton transfer. This means
that, if the H-bonding network is truncated due
to the presence of interstitial unpurities, the
10
Table 2: Binding energy (BE) values (in kJ mol1) of CCH on the W18 and W33 cluster models
according to the computed complexes shown in Figures 3 and 4 (the R structures of the reactions
(Rx) R1,R2,R3,R2-1 and R2-2). The contributions from the pure potential energy values
(Eads), the dispersion corrections (Edisp), the zero point energy corrections (Z P E), and the
BSSE corrections (BSSE) are also shown.
Surface Rx Eads Edisp ZP E BSSE BE
W18
R1 -21.2 -3.3 -0.6 1.4 23.7
R2 -32.8 -10.1 3.0 2.1 37.9
R3 -32.2 -22.1 2.2 2.2 49.9
W33 R2-1 -97.1 -7.2 11.2 3.3 89.7
R2-2 -81.6 -19.0 11.2 3.5 86.0
mechanism is not operative. This is an aspect
not to overlook in interstellar ices as they con-
tain minor volatile species in the ices that can
obstruct the chained proton relays. Remark-
ably, it is worth stressing that the catalytic
transfers involve H atoms with a proton char-
acter and not atomic radicals. This is because
the H species exchanged during the transfers
are proton-like atoms chemically bonded to a
more electronegative O atom.
Since the identified reaction channels on W18
differ significantly from the gas-phase ones, we
redefine the R’1–3 model channels into R1–3,
which adopt the following simplified mechanis-
tic steps:
R1 H2O + CCH OH + HCCH HC-
CHOH,
R2 H2O + CCH CC(H)–OH2HCCHOH,
R3 H2O+ CCH H2O–CCH H2CCOH.
The stationary points and the energy profiles
of these reaction pathways are shown in Figure
3.
The R1 path remains the same as the R’1
one, namely, formation of HCCH and OH as
intermediate species by H-transfer from the re-
active H2O molecule to CCH, and then coupling
of the OH to HCCH to form the HCCHOH
radical as vinyl alcohol precursor. Both R2
and R3 starts with the formation of the hemi-
bonded systems (see SI for their spin densities).
However, since the involved C atoms are differ-
ent, the pathways proceed in a different way as
well. The hemibonded structure of R2 evolves
towards the formation of the CC(H)-OH2inter-
mediate, which is followed by a proton transfer,
from the OH2moiety and adopting a water-
assisted proton-transfer mechanism, to form the
final HCCHOH radical. In contrast, the hemi-
bonded structure of R3 transforms into H2O-
CCH as the intermediate species. From this
intermediate, two different paths are possible,
namely, from the OH2moiety, a proton trans-
fer to i) the central C atom (TS2-1) or ii) the
terminal C atom (TS2-2), forming HCCHOH
or H2CCOH, respectively. Interestingly, both
paths proceed through a water-assisted proton-
transfer mechanism. The fact that R3 exhibit
these two paths is because there are well ori-
ented H-bonds in the water cluster allowing for
the water-assisted mechanism in the two direc-
tions, while this is not possible in R2 due to
the geometry of the intermediate structure.
In relation to the energetics (see also Figure
3), the first steps (TS1) of paths R1 and R2
become submerged below the energy of reac-
tants once they are ZPE-corrected. The same
happens for the second step (TS2) of path R2.
This means that R2 is an overall effectively
barrierless reaction on W18. In contrast, R1
has a low energy intermediate (associated with
the formation of HCCH) and, consequently, the
second step (TS2) encounters a relatively high
energy barrier of 34.6 kJ mol1(with respect
to the intermediate). For R3, the first step al-
ready presents a non-negligible energy barrier,
even when it is ZPE-corrected (14.1 kJ mol1).
For the second steps, TS2-1 has a lower energy
than that of TS2-2 because the geometry as-
sociated with the latter saddle point is more
11
strained, as the cycle created by the water as-
sistant molecules is smaller. All the reaction
paths present negative and very large reaction
energies, indicating that the formed vinyl alco-
hol radicals are more stable than the reactant
states. We observed that, in some cases, the
ZPE corrections added to the potential energies
are very large (see, for instance, TS1 in R1, TS2
in R2, and TS2-1 and TS2-2 in R3). Interest-
ingly, these transition states involve the motion
of H atoms of the water molecules, either as a
direct proton-transfer (case of TS1 in R1) or
as a water-assisted proton-transfer mechanism
(cases of TS2 in R2, and TS2-1 and TS2-2 in
R3). In contrast, when a C-O bond is formed
(namely, without involving any proton motion)
it results in a slight ZPE-correction. In order to
understand this behaviour, we simulated the IR
spectra of R, TS1, I and TS2-1 of R3 (see SI).
The spectra of R and TS1, where only the C–
O bond forms, show mostly common features,
with the exception of a couple of bands around
3000 cm1, due to O-H stretching of the water
molecules surrounding the CCH. On the other
hand, comparing the spectrum of TS2-1 with
R highlights the presence of a number of fea-
tures at a shorter wavenumber, arising from the
water-assisted proton-transfer. Given that the
ZPE arises from the sum of the energy of all the
vibrational modes, the shorter wavelength fea-
tures of TS2-1 explain why its ZPE is smaller
than R and hence that the resulting ZPE is
negative.
As a final comment, we would like to stress
the influence of the saturated state (in terms
of H-bonding) of the reacting water molecule in
the energy barriers. Indeed, according to the
potential energy values, the TS1 barrier in R2
is actually lower than that in R3, 0.7 and 16.0
kJ mol1, respectively (see Figure 3). This is
because, in the former, TS1 is more reactant-
like than in the latter due to the fact that, in the
R structure of R2, the reacting water molecule
is not fully saturated by H-bonds (i.e., 2 H-
bonds as donor + 1 H-bond as acceptor) while
this is the case in R3 (i.e., 2 H-bonds as donor +
2 H-bonds as acceptor). The lack of the H-bond
in R of the R2 reaction allows the water "unsat-
urated" lone pair to form the hemibonded sys-
tem with CCH, in which the C-O bond is half-
formed. Thus, to reach the TS1 structure, no
significant energy requirements and geometrical
changes are needed, and hence the low energy
barrier. In contrast, in R3, to form TS1 from
the R structure, the reacting water molecule has
to break one of the H-bonds to form the newly
C-O bond, implying a more energetic cost and
relevant geometrical changes. Nevertheless, the
associated quantum tunnelling crossover tem-
perature is rather high, of 244 K, indicating
that this channel could be relevant at the in-
terstellar temperatures.
On the W33 ASW ice cluster model
After modeling the reaction on the flat surface
of W18, we set out to investigate the possible ef-
fects of a cavity structure like that of W33. We
tried to reproduce the same three reaction paths
as those taking place on W18 but we only found
two mechanisms, both similar to R2, hence the
name R2–1 and R2–2. The stationary points
and the energy profiles of these reaction path-
ways are shown in Figure 4. It is worth men-
tioning that, to model the reaction path R2–
2it was necessary to fix the position of some
of the oxygen atoms placed at the edge of the
model, since full geometry relaxation lead to
the collapse of the cavity.
In both cases, CCH is located in the cavity,
with the central C atom interacting with the
oxygen atom of the reactant water molecule of
the surface, forming the aforementioned hemi-
bonded systems (see SI for spin densities).
Along the reactions, these structures evolve to
form the CC(H)-OH2intermediate. For both
cases, this step has a very low ZPE-corrected
energy barrier (2.1 and 0.8 kJ mol1for TS1
and TS2, respectively). The second step in-
volves, for both paths, the proton transfer from
the OH2moiety to the terminal C atom form-
ing the HCCHOH radical, also by means of a
water-assisted proton-transfer mechanism. The
two paths lead to the same radical because
the H-bond network enables that the water-
assisted proton-transfer connects with the ter-
minal C atom and not the central one. These
second steps are energetically below the cor-
12
Figure 3: Computed potential energy surfaces (PESs) of the reactions R1–3 between CCH and the W18 ASW ice cluster model. Bare
energy values correspond to relative ZPE-corrected values, while values in parenthesis to those missing this correction. The miniature
panels sketch the ZPE-corrected PESs. Energy units are in kJ mol1and distances in Å.
13
responding intermediates (by including ZPE-
corrections) and accordingly the water-assisted
proton-transfers proceed in a barrierless fash-
ion. Since the reaction energies are very large
and negative, the computed reaction on W33
are energetically very favourable, similarly to
the R2 reaction occurring on W18.
Towards the formation of ethanol:
hydrogenation of vinyl alcohol
As shown above, reaction of CCH with an
icy water molecule leads to the formation of
CHCHOH and H2CCOH. From these species,
to reach ethanol, a set of hydrogenation steps
are necessary, as sketched in reactions 5–7.
HCCHOH/H2CCOH + H CH2CHOH (5)
CH2CHOH + H CH3CHOH/CH2CH2OH
(6)
CH3CHOH/CH2CH2OH + H CH3CH2OH
(7)
The first hydrogenation forms vinyl alcohol
(VA). As this H-addition is a radical-radical
coupling, we consider it as barrierless. The sec-
ond hydrogenation step is the H-addition to
the VA. It involves the reactivity between a
closed-shell species (namely, VA) with a radi-
cal (H) and, accordingly, it is expected to have
an activation barrier. Interestingly, depend-
ing on the C in which the H-addition takes
place, the species formed is either CH3CHOH
or CH2CH2OH. The third and final hydrogena-
tion step leads to the formation of ethanol, ir-
respective of the initial radical species. This
H-addition is, again, a radical-radical coupling
and accordingly it is expected to be barrierless
in a similar fashion as the first hydrogenation.
According to this reactive scheme, to investi-
gate on ethanol formation, we simulated only
the reaction 6 on the W18 ice model.
We identified two reaction paths, named as
H1 and H2 (see Figure 5), the difference of
which being the C atom that undergoes the
H-addition, in analogy to the gas-phase H’1
and H’2 reactions (see Figure 2). Both re-
actions start from a pre-reactant structure in
which the H atom is at ca. 3.3 Å from the
VA due to the weak interactions between the
two partners. The computed potential energy
surfaces indicate that the two hydrogenation
reactions present a non-negligible energy bar-
rier. In agreement with gas-phase results, the
path leading to the formation of CH3CHOH
is more favourable than that resulting with
CH2CH2OH, both in terms of energy barriers
and reaction energies. Remarkably, the com-
puted energy barriers on W18 are higher (3–4
kJ mol1) than those obtained in the gas-phase.
This is probably because the adsorption of VA
with the surface induces an enhanced stabiliza-
tion of the former due to the intermolecular
forces between VA and the icy surface. Thus,
chemically strictly speaking, the surface does
not act as a chemical catalyst but slightly in-
hibits the process. However, it is worth high-
lighting that the H1 reaction presents a very
low potential energy barrier and, because of
the involvement of an H atom and the very low
temperatures of the ISM, tunneling effects can
operate as well. Indeed, its associated quan-
tum tunnelling crossover temperature is 118 K.
For reaction H2, the crossover temperature is
slightly higher, 174 K, therefore quantum tun-
neling could also play a role.
Isomerization between vinyl alco-
hol and acetaldehyde
Vinyl alcohol and acetaldehyde are tautomers
(i.e. structural isomers), and therefore we have
here considered the conversion from one to the
other according to reaction 8. This isomeriza-
tion reaction has been computed at MPWB1K-
D3(BJ)//6-311++G(2df,2pd)//MPWB1K-
D3(BJ)//6-311++G(d,p) for consistency with
the methodology applied to the hydrogenation
of vinyl alcohol.
CH2CHOH CH3CHO (8)
Among them, acetaldehyde is the most stable
14
Figure 4: Computed potential energy surfaces (PESs) of the reactions R2-1 and R2-2 between
CCH and W33 ASW ice cluster model. Bare energy values correspond to relative ZPE-corrected
values, while values in parenthesis to those missing this correction. The miniature panels sketch
the ZPE-corrected PESs. Energy units are in kJ mol1and distances in Å.
15
Figure 5: Computed potential energy surfaces (PESs) of the hydrogenation reactions H1 and H2
on W18 ASW ice cluster model. Bare energy values correspond to relative ZPE-corrected values,
while values in parenthesis to those missing this correction. The miniature panels sketch the ZPE-
corrected PESs. Energy units are in kJ mol1and distances in Å.
isomer (by 41 kJ mol1). In the gas phase,
the barrier connecting vinyl alcohol with ac-
etaldehyde is very high (237.9 kJ mol1, see SI).
This is because the mechanism involves an in-
tramolecular H-transfer from the OH of vinyl
alcohol to the terminal C atom, with a highly
strained transition state.
If we now consider this reaction to take place
through the water molecules of W18, as shown
in Figure 6, we can see that a water-assisted
proton-transfer mechanism can take place, low-
ering the activation energy barrier down to 73.5
16
kJ mol1when the reaction starts from vinyl al-
cohol (I1), and to 57.7 kJ mol1when it starts
from its less hydrogenated precursor (I2). The
latter reaction produces CH2CHO, which can
be successively hydrogenated to form acetalde-
hyde. The difference in the two activation barri-
ers is probably due to the structure of the ring
through which the proton is transferred, that
is, in I1 it is more strained. This is a great ex-
ample of how interstellar ices act as chemical
catalysts. However, computed energy barriers
are very high to be surmountable under inter-
stellar conditions and accordingly these chan-
nels seem unlikely. Nevertheless, we would like
to point out that the barrier of I2 is narrower
and lower than that of I1 (with barriers widths
of about 1.6 and 2.2 Å, respectively, assuming
the asymmetric Eckart barrier model, see112.
Therefore, I2 would be the most efficient mecha-
nism among the two, if assuming that quantum
tunneling does play a role.
Discussion and Astrophysical
Implications
CCH is a highly reactive species that easily re-
acts with water. As it is a C-centered radical
and contains both donor and acceptor H-bond
atoms (the H and the C-end atoms, respec-
tively), it tends to form H-bonded and hemi-
bonded complexes with water ice molecules.
The most relevant computed energetics of CCH
reactivity on our ASW ice models are summa-
rized in Table 3. On W18, R3 presents an ac-
tivation barrier of 14.4 kJ mol1and, accord-
ingly, it is not a priori an efficient path to form
vinyl alcohol, although it could still be relevant
if quantum tunnelling effects work. The first
step of R1 is barrierless to form HCCH + OH
but the reaction stops at this stage. This is be-
cause the second step has an intrinsic energy
barrier of 34.6 kJ mol1(the energy difference
between TS2 and I, see Figure 3). Moreover,
the energy released by the first reaction step
will no longer be available for the second step,
as water ices tend to efficiently dissipate chem-
ical energy fast.113,114 Therefore, R1 will lead
to the formation of a highly stabilized acety-
lene, which will hardly react with OH remain-
ing stuck on the ice. However, this reaction can
be an effective channel towards the formation
of OH radicals on the ice surfaces without the
need of a direct energy processing. This can be
of importance because the generated OH could
participate in further surface reactions in the
form of OH additions. R2, at variance with
the other two reactions, is an effective barri-
erless reaction path towards vinyl alcohol for-
mation. Because of that, this reaction has also
been modeled on W33 to study the effects of the
cavity on the reactivity. On W33, we found that
the reactions are no more barrierless, although
the two identified paths present low activation
barriers (2.1 and 0.8 kJ mol1for R2-1 and
R2-2, respectively). Because these two path-
ways are equivalent to R2 on W18, the effect
of the cavity is almost insignificant for the ener-
getics of the path. Therefore, we can consider
that the mechanistic steps represented by the
R2,R2–1 and R2–2 pathways constitute the
most likely channel to form vinyl alcohol.
The hydrogenation of vinyl alcohol on grains
leads to the formation of ethanol, as occur-
ring in methanol formation from CO115–119 and
ethane formation from C2H2and C2H4.120–122
The H-addition to vinyl alcohol is the only
hydrogenation step not involving a barrierless
radical-radical coupling and presents an energy
barrier of 7.4 and 17.3 kJ mol1for the H1
and H2 channel (see Table 3). Thus, H1 is
the most energetically favoured pathway for the
formation of the ethanol radical precursor. De-
spite the computed relatively high energy barri-
ers considering very low temperatures, these re-
actions can proceed through tunneling and ac-
cordingly operative in the reaction chain start-
ing from the adsorption of CCH on water ice.
These quantum chemical results can partly
be related to the experimental findings men-
tioned in the § Introduction. Indeed, part of the
experimental synthetic routes have been simu-
lated here, providing an atomistic interpreta-
tion (including the energetics) for the formation
of vinyl alcohol followed by it hydrogenation to
form ethanol. The main difference between the
experiments and our computations is that, in
the experiments, the C2H2/H2O ices need to be
17
Figure 6: Computed potential energy surfaces (PESs) of the isomerization between vinyl alcohol
and acetaldehyde (I1) and the analogue reaction involving their precursor, i.e., the product P2 of R3
(I2) on W18 ice cluster model at MPWB1K-D3(BJ)/6-311++G(2df,2pd)//MPWB1K-D3(BJ)/6-
311++G(d,p). Bare energy values correspond to relative ZPE-corrected values, while values in
parenthesis to those missing this correction. The miniature panel sketches the ZPE-corrected PESs.
Energy units are in kJ mol1and distances in Å.
processed to trigger reactivity (probably due to
generating the radical reactive species like CCH
and OH), while in our simulations the assump-
tion is that the reaction does not require ener-
getic processing of ices, since the CCH is readily
available in the gas phase in a wide variety of
18
environments.
Table 3: Summary of the energetics of the sim-
ulated reactions and their products.
Reaction Barrier Product
W18
R1 NO HCCH + OH
R2 NO HCCHOH
R3 14.4 kJ mol1H2CCOH/HCCHOH
W33 R2-1 2.1 kJ mol1HCCHOH
R2-2 0.8 kJ mol1HCCHOH
W18 H1 7.4 kJ mol1CH3CH2OH
H2 17.3 kJ mol1CH3CH2OH
We also considered the isomerization of vinyl
alcohol into acetaldehyde (and the same for
their less hydrogenated precursors), since in
the experiments these reactions were suggested
to explain the presence of both vinyl alcohol
and acetaldehyde. According to our calcula-
tions, however, these reactions, although being
catalyzed by the surfaces thanks to a water-
assisted proton-transfer mechanism, present
high energy barriers, rendering them poorly
competitive to the final H additions. How-
ever, experimental authors pointed out that the
isomerization could take place thanks to the
exothermicity of the previous steps, in which
the energy released along the reaction steps can
be used to overcome the isomerization energy
barrier. Our computed energetic data is consis-
tent with this view. The very favorable reaction
energies shown by the reactions make that the
energy barriers of the isomerization processes
lay below the pre-reactive asymptotic states
and therefore, they can be overcome by mak-
ing use of the nascent reaction energies. How-
ever, one should bear in mind that water ice sur-
faces are extraordinary third bodies113,114 and
accordingly, the direct transfer of the previous
reaction energies to surmount the isomerization
energy barriers is doubtful. To shed some light
onto this aspect, dedicated ab initio molecular
dynamics simulations are compulsory, which is
out of the scope of the present work.
Finally, results presented in this work are very
relevant in the framework of cold Astrochem-
istry. The presence of abundant (109
108) gaseous CCH radicals in cold (20 K)
regions, where water ices envelope the refrac-
tory cores of the interstellar dust grains (see
§ Introduction, can lead to the formation of
vinyl alcohol and ethanol, in addition to HCCH
and OH through a competitive reaction chan-
nel. Remarkably, at variance with most of
the experimentally proposed mechanisms (see
§ Introduction), the formation of the aforemen-
tioned products does not require the energetic
processing of interstellar ices. Regarding the
formation of iCOMs, our proposal complements
the non-energetic reaction scheme of Chuang
et al.123 , in which OH radicals attack C2H2to
form iCOMs. We want to point out that CCH
(and the other intermediate radicals) could also
be destroyed by other competitive surface reac-
tions, e.g., by H-abstraction reactions with H2,
or H-additions. This will be taken care of in the
future.
More in general, the astrochemical processes
in cold regions such as the ones described in this
work are important to improve our understand-
ing of the presence of complex species detected
in the cold (<20 K) outskirts of prestellar cores
during the last decade (e.g.16,124–128). In this
vein, ethanol has recently gained some atten-
tion, as it has been advocated to be the precur-
sor of several iCOMs formed by cold gas-phase
reactions (the genealogical tree of ethanol23,91).
Indeed, the correlation of glycoladehyde and
acetaldehyde abundances observed towards a
number of interstellar sources has been shown
to follow very well the theoretical predictions
when their synthesis takes place through this
gas-phase scheme.24 In the present work, we
showed that an efficient paths exists for the for-
mation of ethanol on the surfaces of interstellar
ices. However, the non-thermal desorption of
ethanol (and of the other products) remains as
a crucial missing step in the sequential events
linking the chemistry of iCOMs occurring on
the surface of grains and in the gas phase in cold
regions, this issue undoubtedly being a central
matter of further investigation.
Conclusions
Interstellar complex organic molecules (iCOMs)
have been detected in different astrophysical
19
environments. However, the chemistry lead-
ing to their formation is not unambiguously
known. Two prevailing paradigms have been
largely used to rationalize their presence in the
interstellar medium: one advocating reaction in
the gas-phase, the other on the surfaces of icy
grains. In this work, we focused on the lat-
ter by computing the reaction of CCH with a
H2O molecule forming part of the ice structure,
leading to vinyl alcohol (CH2CHOH), which
upon hydrogenation is converted into ethanol
(CH3CH2OH). This reaction is proposed as
an alternative synthetic route for iCOMs be-
yond the commonly assumed radical-radical
couplings. Investigations have been performed
by means of DFT quantum chemical simula-
tions and adopting cluster models of 18 and
33 water molecules (W18 and W33) to mimic
the icy surfaces. For the reaction of CCH with
H2O, three different reaction pathways have
been elucidated, leading to the formation of
HCCH + OH, CHCHOH and H2CCOH. Some
cases present small activation barriers but in
others the reactions are barrierless when zero-
point energy corrections are accounted for. Hy-
drogenation of vinyl alcohol on the W18 clus-
ter has been found to present activation barri-
ers of a certain significance but, for these reac-
tions, quantum tunneling is likely to be at work,
speeding them up. Isomerization between vinyl
alcohol and acetaldehyde have also been sim-
ulated on W18, results indicating that, despite
the strong catalytic role played by the water ice,
they have a barrier of significant height. Addi-
tionally, the direct H-abstraction from the wa-
ter molecule to CCH, leading to the formation
of HCCH and the OH radical on the surface has
been found to be an energetically competitive
channel. It is worth mentioning that a chemical
kinetics treatment of these results are under-
way in order to compute the rate constants (in-
cluding tunneling effects explicitly) for each of
the proposed reactive channels and hence eluci-
date branching ratios and formation efficiencies
of the simulated paths.
In summary, results from our calculations in-
dicate that the reaction of CCH with water ice
can lead to the formation of vinyl alcohol and,
lately, to the production of ethanol, and likely
acetaldehyde, without the need of ice energy
processing. This conclusion is of relevance in
the context of iCOM formation because, ac-
cording to the genealogical tree of ethanol, this
species is the parent molecule through which
different iCOMs (e.g., formic acid, formalde-
hyde, glycolaldehyde, and others ) can form by
means of gas-phase processes.23,24 Thus, in this
work, we provide a quantum chemical evidence
on the feasibility of our mechanistic proposal,
in which ethanol can be formed on interstel-
lar icy grain surfaces, hence linking the two
paradigms in the synthesis of iCOMs, at least in
this particular case. The missing link between
on-grain and gas-phase chemistry stands in the
non-thermal desorption of ethanol and its pre-
cursors, which should be a subject for further
investigation.
Acknowledgement This project has re-
ceived funding within the European Union’s
Horizon 2020 research and innovation pro-
gramme from the European Research Coun-
cil (ERC) for the projects “The Dawn of Or-
ganic Chemistry” (DOC), grant agreement No
741002 and “Quantum Chemistry on Inter-
stellar Grains” (QUANTUMGRAIN), grant
agreement No 865657. The authors acknowl-
edge funding from the European Union’s Hori-
zon 2020 research and innovation program
Marie Sklodowska-Curie for the project “Astro-
Chemical Origins” (ACO), grant agreement
No 811312. AR is indebted to “Ramón y Ca-
jal" program. MINECO (project CTQ2017-
89132-P) and DIUE (project 2017SGR1323)
are acknowledged. Finally, we thank Prof.
Gretobape for fruitful and stimulating discus-
sions. Most of the quantum chemical cal-
culations presented in this paper were per-
formed using the GRICAD infrastructure
(https://gricad.univ-grenoble-alpes.fr), which
is partly supported by the Equip@Meso project
(reference ANR-10-EQPX-29-01) of the pro-
gramme Investissements d’Avenir supervised
by the Agence Nationale pour la Recherche.
Additionally, this work was granted access to
the HPC resources of IDRIS under the alloca-
tion 2019-A0060810797 attributed by GENCI
(Grand Equipement National de Calcul Inten-
20
sif). CSUC supercomputing center is acknowl-
edged for allowance of computer resources.
Supporting Information Avail-
able
The following files are available free of charge:
Structures and errors of the benchmark-
ing study; energetics relative to the CCH
+ H2O reactions on W18 and W33; con-
tributions to the binding energies of CCH
on W18 and W33; Mulliken spin densities
and spin density maps of the complexes
CCH + H2O in the gas phase and on W18
and W33; energetics relative to the hydro-
genation of vinyl alcohol; energetics rela-
tive to the isomerization of vinyl alcohol
into acetaldehyde,
Jessica Perrero Joan Enrique-Romero
Berta Martínez-Bachs Cecilia Ceccarelli
Nadia Balucani Piero Ugliengo Albert Ri-
mola
21
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