Studies of cyclization reactions of linear cumulenes and heterocumulenes using the neutralization-reionization procedure and/or ab initio calculations.

Abstract

A number of linear cumulenes and heterocumulenes have been made by charge stripping of anions of known bond connectivity in the source of a mass spectrometer. Some of these reactive molecules have been identified in interstellar molecular clouds. The structures of these neutrals may be investigated by reionization to a decomposing positive ion [the neutralization-reionization technique ((-)NR(+))], and/or by ab initio calculations. Energized linear cumulenes and heterocumulenes may undergo cyclization to form stable cyclic isomers. To cite a selection of the examples described in this review: (i) four-atom systems CCCC and some heterocumulenes CCCX (X=B, N, Al, Si, P) involve the formation of stable four-membered ring rhombic (also called kite and fan) structures. One of the cyclic molecules, cyclo-C(3) Si, has been detected in interstellar molecular clouds, (ii) five-atom cumulene and heterocumulene systems are more complex. Linear CCCCC rearranges the carbon skeleton by forming a C substituted rhomboid system, CCCCO forms a three-membered cyclic isomer, while nitrogen containing five-atom cumulenes effect nitrile to isonitrile interconversion via three-centered cyclized intermediates, and (iii) CCCCCC and CCCCBO cyclize to give unique six-membered ring systems.

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STUDIES OF CYCLIZATION REACTIONS OF LINEAR
CUMULENES AND HETEROCUMULENES USING THE
NEUTRALIZATION–REIONIZATION PROCEDURE AND/OR
AB INITIO CALCULATIONS
Tianfang Wang and John H. Bowie*
Department of Chemistry, The University of Adelaide, South Australia,
Australia
Received 22 September 2010; revised 14 November 2010; accepted 15 November 2010
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/mas.20328
A number of linear cumulenes and heterocumulenes have been
made by charge stripping of anions of known bond connectivity
in the source of a mass spectrometer. Some of these reactive
molecules have been identified in interstellar molecular
clouds. The structures of these neutrals may be investigated by
reionization to a decomposing positive ion [the neutralization–
reionizationtechnique(?NRþ)],and/orbyabinitiocalculations.
Energized linear cumulenes and heterocumulenes may undergo
cyclizationtoformstablecyclicisomers.Tociteaselectionofthe
examples described in this review: (i) four-atom systems CCCC
and some heterocumulenes CCCX (X ¼ B, N, Al, Si, P) involve
the formation of stable four-membered ring rhombic (also called
kite and fan) structures. One of the cyclic molecules, cyclo-C3Si,
has been detected in interstellar molecularclouds, (ii) five-atom
cumuleneandheterocumulenesystemsaremorecomplex.Linear
CCCCC rearranges the carbon skeleton by forming a C substi-
tuted rhomboid system, CCCCO forms a three-membered cyclic
isomer, while nitrogen containing five-atom cumulenes effect
nitrile to isonitrile interconversion via three-centered cyclized
intermediates, and (iii) CCCCCC and CCCCBO cyclize to give
unique six-membered ring systems. # 2011 Wiley Periodicals,
Inc., Mass Spec Rev
Keywords: negative ions; neutralization–reionization; charge
reversal; neutral cumulenes; neutral heterocumulenes; cycliza-
tion reactions
I. INTRODUCTION
Aristotle considered thatlife originated onacosmic ratherthan a
terrestrial level because of the then considered five primary
elements and because he thought that ‘‘cells’’ came to Earth
from external sources (Lloyd, 1968). This matter was addressed
by Hoyle et al. in the 1950s: they proposed that the biological
molecules of life could originate from interstellar regions
(Burbidge et al., 1957; Hoyle & Wickramasinghe, 1981).
There seems no doubt that (i) such molecules could have been
formedonprebioticearth(Miller,1953;Johnsonetal.,2008),but
also (ii) some biomolecules and their precursors are present
in circumstellar envelopes, interstellar molecular clouds,
interstellar ice and comets. Molecules that have been detected
in such regions in space (using telescopes with infrared and/or
microwave measuring capability) are listed in Table 1. Some
examples of precursors to biologically important molecules that
have been detected in molecular clouds range from the glycine
precursor amino acetonitrile (NH2CH2CN) (Belloche et al.,
2008) and glycolaldehyde (HOCH2CHO) (a protosugar)
(Hollis, Lovas, & Jewell, 2000; Halfen et al., 2006), to possible
pyrimidineandpurinebaseprecursorslikeHCN(Snyder&Buhl,
1971), HNCO (Johansson et al., 1984) and NH2CONH2(Snyder
et al., 2009).
The list of currently identified interstellar molecules
(Table 1) includes a number of cumulenes, heterocumulenes,
and cumulene nitriles. Strictly speaking, cumulenes have
=C=C=C= functionality, but in the context of this article, poly-
acetylenic systems (–C:C–C:C–) are also classified as cumu-
lenic. These molecules together with small unsaturated
molecules (like, e.g., HCN and other nitriles) constitute some
30% of the molecules listed in Table 1. Cumulenes are not
only of astrochemical interest: they are also involved in the
chemistry of flames (Kroto & McKay, 1988; Frenklach,
2002), and small cumulenes, particular those containing
hydrogen and/or hetero substitution can be used in synthetic
reactions(Ulrich,2009).Interstellarcumulenetypemolecules
and related species have fascinating structures with a high
degree of multiple bond character. Many of these are highly
reactive and can only be isolated under low-temperature
conditions or detected within a short time frame (usually
microseconds) in a collision cell of a mass spectrometer.
Such species can be involved in condensation and polymeri-
zation reactions to form a variety of larger molecules
(Baddour & Timmins, 1967; Kroto et al., 1985; Jones et
al., 1996). For example, the reaction between the interstellar
molecules CCCO and urea (NH2CONH2) should form
uracil (Wang & Bowie, unpublished observations). Reviews
on carbon clusters (including cumulenes) are available
(Weltner & VanZee, 1989; Van Orden & Saykally, 1998).
Interstellar andrelated cumulenesand heterocumuleneshave
also been reviewed (Blanksby & Bowie, 1999).
During the last two decades many cumulenes and cumulene
derivatives have been made using collisional induced neutraliz-
ation of cations or anions of known bond connectivity in tandem
collision cell assemblies of reverse sector mass spectrometers.
Someofthesecorrespondtoknowninterstellarcumulenes,while
*Correspondence to: John H. Bowie, Department of Chemistry, The
University of Adelaide, South Australia 5005, Australia.
E-mail: john.bowie@adelaide.edu.au
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# 2011 by Wiley Periodicals, Inc.

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TABLE 1. Interstellar molecules
Diatomic
AlCl, AlF, AlO, CC, CH, CH+, CF+, CN, CO, CO+, CP, CS, CSi, FeO, HCl, HF,
H2, HN, HO, HS, KCl, LiF, MgH+, N2, NO, NP, NS, NSi, NaCl, NaI, O2, OP, OS,
OSi, SH, SSi.
Triatomic
AlNC, AlOH,
C3, C2H, CH2, C2O, CO2, C2P, C2S, cyclo-C2Si, HCN, HCO,
HCO+, HCP, HCS, HCS+, HDO, H3+, HNC, HN2+, HNO, H2N, H2O, H2S, HOC+,
KCN, MgCN, MgNC, NaCN, NaOH, N2O, OCN-, OCS, O3, SO2, SiCN, SiNC.
4-atomic
CH2O, CH2S, C2H2, C2HN, CH2N, C3H, cycloC3H, C3N, C3O, C3S, C3Si, CH3,
C4, HCNH+, HCNO, HNCO, HNCS, HSCN, HOCO+, H3O+, NH3.
5-atomic
CH4, CH2CN, CH2CO, CH2NC, CH2NH, CH2OH+, C3H2, cycloC3H2, HC2CN,
HC2NC, C4H, C4H-, C4Si, C5, HCO2H, HCOCN, NH2CN, SiH4.
6-atomic
C2H4, CH2CHO, CH2CNH, CH2OH+, cycloC3H2O, CH3CN, CH3NC, CH3NH2,
CH3OH, CH3SH, C3H2N, C4H2, C4HN, C5H, C5H-, C5N, HC2CNH+, HCONH2,
NH2CHO.
7-atomic
cycloC2H4O, CH2CHCN, CH2CHOH, CH3CHO, CH3NH2, CH3C2H, HC4CN, C6H,
C6H-.
8-atomic
CH2OHCHO, CH2CHCHO, CH2CCHCN, CH3CO2H, CH3C2CN, C6H2, C7H,
HCO2CH3, NH2CONH2, NH2CH2CN.
9-atomic
CH3CH2CN, CH3CH2OH, CH3CHCH2, CH3CONH2, CH3OCH3, CH3C4H, C7HN,
C8H.
10 or more atoms
CH3CH2CHO, CH3COCH3, CH3C4CN, HOCH2CH2OH, CH3C6H, HC8CN,
HCO2C2H5, nC3H7CN, C6H6, HOCH2COCH2OH, HC10CN, HC11N, C14H10, C60,
C70.
Deuterated molecules
HD, H2D+, HD2+, HDO, D2O, DCN, DCO, DNC, N2D+, NH2D, NHD2, ND3,
HDCO, D2CO, CH2DC2H, CH3C2D.
Molecules reported but not confirmed
HOCN, NH2CH2CO2H, CO(CH2OH)2, C2H5COCH3, C10H8+, SiH, PH3.
________________
________________
?
* This list was compiled from data listed in:-
http://physics.nist.gov/PhysRefData/Micro/Html/tab1.html
http://www.astrochymist.org/astrochymist_ism.html
http://en.wikipedia.org/wiki/List_of_molecules_in_interstellar_space. Original references may
be found in these articles.
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TABLE 2. Cyclization processes of cumulenes and related neutrals
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others are related to those molecules. The structures of such
neutrals have been probed by reionization (generally to positive
ions) in the second collision cell of a tandem collision cell
assembly, and the mass spectra of the resulting decomposing
ions provides an insight into the structure of the neutral. When
negative ion precursors are used the charge reversal spectrum
[?CRþ, a one step loss of two electrons from the precursor anion
to produce a positive ion spectrum (Bowie & Blumenthal, 1975;
Szulejko et al., 1980; Bursey, 1990)] and the neutralization–
reionizationspectrum[?NRþ,sequentialelectronlosstoconvert
theaniontoaneutralandthentoacationinatandemcollisioncell
assembly [(Goldberg & Schwarz, 1994; Zagorevskii & Holmes,
1994); see also Turecek, 2003] are compared. Differences
between the two spectra indicate decomposition or rearrange-
ment of the neutral(s) produced (Goldberg & Schwarz, 1994;
McAnoy&Bowie,2004a).Inthecaseofapositiveionprecursor,
the CID MS/MS andþNRþspectra are compared, and differ-
ences between the two spectra indicate decomposition and/or
rearrangement of the neutral (Goldberg & Schwarz, 1994).
The neutralization procedure approximates to a vertical
Franck–Condon process, and some of the neutral cumulenes
produced are stable for the microsecond lifetime of the neutral
in the second collision cell prior to reionization. Other cumulene
neutrals produced by the neutralization procedure may be ener-
gized,presumably followingadditionalcollision(s) inacollision
cell involving either the precursor ion or the neutral product.
Energized cumulenes may either dissociate and/or rearrange in
the collision cell prior to reionization. Some energized cumu-
lenesundergocyclizationtoformcyclicspecies,whichhavebeen
studied (i) both experimentally (by the neutralization–reioniza-
tion method as outlined above) and theoretically by high-level
ab initio calculations, or (ii) by theoretical methods only, when
theneutralwasnotabletobeformedbecausetheprecursorionof
the correct bond connectivity was not available. Cyclization
processes of cumulenes and heterocumulenes are the subject
of this review. Topics dealt with in the earlier review
(Blanksby & Bowie, 1999), are not covered.
II. DISCUSSION
Cyclizationoflinear,orbentcumulenescontainingahighdegree
of bond multiplicity seems a counter intuitive concept, since it
might be expected that the energy barriers to the transition states
for such cyclization processes should be high, and the cyclic
products may be of higher energy than the linear precursor
because of the considerable strain energy of the cyclic form.
However, it is now known that energized cumulenes and hetero-
cumulenes commonly undergo cyclization reactions as summar-
izedinTable2,andthatcertainofthesecyclicformsareasstable,
or on occasions more stable, than their linear counterparts.
Indeed, often, the formation of a cyclic isomer from a linear
cumulene is energetically more favorable than dissociation of
that cumulene involving loss of an atom or a neutral molecule.
The data contained in Table 2 show those linear and bent
cumulenes which undergo reactions to form cyclic isomers (i)
which are either more stable or of comparable energy to the
precursor cumulene or (ii) are intermediates along reaction
coordinate pathways to the formation of some other stable iso-
mer. Considering the cited examples in Table 2, the ratio of
cyclized systems formed is in the order 3 > 4 >> 5 > 6.
Normally, for cyclization reactions of mainly saturated
precursors,theratiowouldbe expected tobe inthe reverse order.
What is special about cumulenes and their cyclic isomers? A
selection of the systems shown in Table 2 will now be discussed.
A. Three-Atom Systems
LinearCCCismadebychargestrippingof(CCC)?.whichinturn
maybeproducedintheionsourceofamassspectrometer (inour
case a modified Micromass ZAB 2HF mass spectrometer
equipped with a tandem collision cell assembly) by the negative
ion fragmentation shown in Equation (1) (McAnoy et al., 2002),
whichisbasedonthedoubledesilylationproceduredevelopedby
Squires and colleagues (Wenthold, Hu, & Squires, 1994).
ð1Þ
In view of the comments made in the previous section, it
might be expected that the prototypical cumulene, linear CCC,
should readily convert to cyclic C3. This is not so. Linear CCC is
aninterstellarmoleculewhichhasbeenwidelystudied(McAnoy
et al., 2002, and references cited therein), whereas cyclic C3
has not been detected in the interstellar media, although cyclo-
C3H has (Mangum & Wootten, 1990). The singlet1Sgþstate
(electronic configuration: {core}3sg22su24sg23su21pu4) is the
groundstateoflinearCCC;thecorrespondingtriplet({core}3sg2
2su24sg21pu23su22pu11pg1) lies 48.8 kcal mol?1above the
ground state [at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d)
level of theory]. Singlet cyclo-C3is not a stable minimum due to
theJahn–Tellereffect:theD3hstructurewilldistorteitherbackto
linear CCC, or give a C2vstructure, whose second derivative
matrixshowsanegativeeigenvalue.Thisstructureactsasathree-
memberedtransitionstateinthedegeneraterearrangement(atom
scrambling) of CCC (see Fig. 1).?In contrast, triplet cyclo-C3is
stablewiththedoublydegenerateHOMO(3e0)equallyoccupied,
lying 28.4 kcal mol?1below triplet linear CCC. However, there
is a barrier of 37.5 kcal mol?1to be overcome to effect inter-
conversion through the unsymmetrical transition state (see Fig.
1). Triplet cyclo-C3is symmetrical (D3hpoint group) with bond
lengths of 1.368 A˚and bond angles of 60.08. The probability of
the formation of cyclo-C3is small in a neutralization–reioniza-
tion experiment, because intersystem crossing between singlet
and triplet CCC will be of small probability because of the
significant energy difference between the two linear isomers.
Even if intersystem crossing was successful, the barrier to the
triplet transition state shown in Figure 1 is large, so the cycliza-
SCHEME 1
*ThestructuresdrawninallFiguresofreactioncoordinateprofilesshow
bond connectivity only, they do not indicate bond multiplicity.
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tioniskineticallyunfavorable.Thetripletstructureofcyclo-C3is
rationalized in Scheme 1 using a simple valence bond treatment.
The bond order of each CC bond is 1.667. The two electrons
shown in each contributing structure are in orbitals orthogonal to
eachother,sotheycannotformabondtoproduceastablesinglet
structure.
The only other cyclizing ‘‘cumulene like’’ three atom sys-
tems that have been considered in detail are the cyanate (CNO)
and fulminate (ONC) (Dua & Bowie, 2003), and thiocyanate
(SCN) and thiofulminate (SNC) radicals (Fitzgerald & Bowie,
2004).Neutralization–reionizationexperimentsindicatethatany
interconversion between the CNO and ONC radicals is minimal
during the microsecond lifetime of the experiment. This is sup-
ported by the reaction coordinate of this process, with the stable
cyclic intermediate being73.8 kcal mol?1
(0 kcal mol?1). The other isomer ONC (61.5 kcal mol?1)
requires 30.2 kcal mol?1to convert to the cyclic intermediate.
In contrast, some interconversion of SNC to SCN is likely since
the barrier between SNC and the cyclic intermediate is only
12.8 kcal mol?1.
above OCN
B. Four-Atom Systems
Asearlyas1977,atheoreticalstudysuggestedthatC4mighthave
a stable cyclic state (Slanina & Zahradnik, 1977): this study
indicated that the cyclic state was rhomboid and that there are
both stable singlet and triplet forms that are close in energy.
Further theoretical work suggested that linear and rhomboid C4
structures have similar energies (Blanksby et al., 2000a, and
references cited therein). CCCC has been tentatively identified
as an interstellar molecule (Cernicharo, Goicoechea, & Benilan,
2002)buttherehasbeennoreportofaninterstellarrhomboidC4.
Inthissection possibletrends inactivityandcyclizationin a
number of C3X systems have been compared for periods 2 and 3
of the periodic classification, namely: C3B, C3Al [theoretical
(Wang & Bowie, 2009)]; C4 [experimental and theoretical
(Blanksby et al., 2000a; Wang, Buntine, & Bowie, 2009)],
C3Si [theoretical (Maclean et al., 2008)]; C3N [experimental
and theoretical (Maclean, Fitzgerald, & Bowie, 2007)], C3P
[theoretical (Maclean et al., 2008)]; C3O and C3S [experimental
and theoretical (Tran, McAnoy, & Bowie, 2004a)]. Of the
systems studied, only one, C4, can be a true rhombus. When
a C3X system is considered, there is the possibility of two
‘‘rhomboidal’’ forms: these are shown in Scheme 2, and have
beennamedkiteandfanisomersbyBatesandcolleagues(Bates,
Rittby, & Graham, 2006). The structures shown in Scheme 2
show bond connectivities only; they do not indicate bond multi-
plicities. This section will start with the prototypical C4system
and compare this with the other systems mentioned above.
1. Rhombus C4and the Associated C3Si System
a. C4
Neutral13CCC13C can be made in the mass spectrometer from
(13CCC13C)?.by a vertical charge-stripping process in the first
cell of a tandem collision cell. The anion radical is produced in
the ion source of the mass spectrometer by the process shown in
Equation (2).
ðCH3Þ3Si13CCC13CSiðCH3Þ3
? !
F?
ð13CCC13CÞ??þ 2ðCH3Þ3SiF (2)
Comparison
(13CCC13C)?.indicatesthatneutral13CCC13Cundergoescarbon
scrambling prior to or during fragmentation [i.e. it decomposes
by loss of both12C and13C (see Fig. 2)] (Blanksby et al., 2000a).
TheCCCCsystemiscomplexbecausethegroundstatetripletlies
only8.9 kcal mol?1belowthesingletstate[attheCCSD(T)/aug-
cc-pVTZ//B3LYP/6-311 þ G(3df) level of theory] and the two
forms may interconvert by intersystem crossing (Wang, Buntine,
& Bowie, 2009).
The carbon scrambling noted for energized
was explained by an equilibrium reaction between linear and
rhomboid C4(Blanksby et al., 2000a), and the reaction coordi-
nates for the both triplet and singlet reactions are shown in
Figures 3 and 4 (Wang, Buntine, & Bowie, 2009). Conversion
from triplet CCCC to triplet rhombus C4is synchronous through
a three-membered ring transition state to produce the triplet
of the
?CRþ
and
?NRþ
spectra of
13CCC13C
SCHEME 2
FIGURE 1. Reaction coordinate profiles for the cyclization of CCC. A:
singlet rearrangement. Transition state C1C2¼ 1.260 A˚; C2C3¼ C1C3¼
1.471 A˚; C1C2C3¼ 64.78. B: Triplet rearrangement. Cyclo-C3. Bond len-
gths ¼ 1.370 A˚; bond angles 60.08. CCSD(T)/aug-cc-pVTZ//B3LYP/6-31-
1G(d)leveloftheory.AlltransitionstateswereconfirmedbyIRCcalculations
(McAnoy et al., 2002).
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rhombus (þ17.0 kcal mol?1) (Fig. 3), whereas singlet CCCC
proceeds to the singlet rhombus (?10.5 kcal mol?1) via a three-
membered ring intermediate which lies 25.9 kcal mol?1above
the rhombus. The singlet rhombus is the ground state, lying
18.6 kcal mol?1below the triplet. The spin orbit coupling
constant between singlet/triplet CCCC is 19.8 cm?1[at the
CASSCF/6-311 þ G(3df) level of theory]: this large value
indicates that crossing from the bound triplet to the repulsive
singlet state should be rapid onthe appropriate reaction potential
surfaces.
It is of interest first, to compare the structures of cyclic C4
species using the simple valence bond model. The three-mem-
bered ring triplet is symmetrical as shown in Scheme 3. It does
not lie on the reaction coordinate shown in Figure 3, but is
20.0 kcal mol?1abovethetripletrhombicstructureonthetriplet
potential surface. The three-membered ring singlet isomer is
similar to the structure shown in Scheme 3, but it is slightly
unsymmetrical and lies 25.9 kcal mol?1above ground state
rhombus C4on the linear to rhomboid C4reaction coordinate
(Fig. 4).
FIGURE 4. Reaction coordinate profile for the rearrangement of singlet
CCCC. CCSD(T)/aug-cc-pVTZ//B3LYP/6-311 þ G(3df) level of theory
(Wang, Buntine, & Bowie, 2009).
FIGURE 2.
mass spectrometer (Technical University, Berlin) (Blanksby et al., 2000a).
?NRþspectrum of (13CCC13C)?.. Four sector ZAB AMD 604
SCHEME 3
FIGURE 3. Reaction coordinate profile for the rearrangement of triplet
CCCC. CCSD(T)/aug-cc-pVTZ//B3LYP/6-311 þ G(3df) level of theory
(Wang, Buntine, & Bowie, 2009).
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The dimensions of the ground state singlet and triplet of
planar and symmetrical rhombus C4 at [at the B3LYP/6-
311 þ G(3df)leveloftheory]areshowninAandB,respectively
(Scheme 3). Four possible contributing structures to the reson-
ance hybrid are shown in Scheme 3. The singlet structure is best
represented by a combination of C, D, and E. In the triplet state
there is less double bond character in the outer bonds (1.426 A˚)
with the central bond essentially a single bond (1.569 A˚). This
structure is best represented by a mixture of C, D, and F.
The molecular orbital treatment of rhombus C4is compli-
cated and selected data are shown in Figure 5. Singlet
rhombus C4of D2hsymmetry is the ground state of this system
with triplet
18.6 kcal mol?1abovethegroundstate. The bonds in the singlet
rhombus have more double bond character than those in the
tripletstructure.Theelectronicconfigurationofthetripletdiffers
considerably from that of the singlet structure (see Fig. 5). For
example the 5agorbital is located at a higher energy than 3b1u.
Theelectronic structureofthesinglet showsthat5ag(NHOMO),
mainly includes bonding pyorbitals of C1and C3, and is only
0.001 hartree more stable than 3b1u(HOMO) at the B3LYP/6-
311 þ G(3df) level of theory. Thus, the electrons in either of
these two orbitals can be almost equally promoted to the
LUMO (1b2g), which consists of empty pxorbitals perpendicular
to the molecular plane. The formation of the triplet increases
the bonding density in the pxorbitals, resulting in an increase
in bond lengths. In particular, an electron from 5ag is
promoted to 1b2g, weakening the bond between C1and C3and
1Ag
3B3urhombus C4(also of D2hsymmetry) lying
increasing in bond length by 0.219 A˚(Wang, Buntine, & Bowie,
2009).
b. Kite and fan C3Si
The C3Si radical potential surface has been studied extensively
(Maclean et al., 2008, and references cited therein), and is of
particular interest because there is experimental evidence that
there are three stable isomers: a linear triplet together with a kite
(‘‘rhombic’’)andafansystem(McCarthy,Gottleib,&Thaddeus,
1999;McCarthy,Apponi,&Thaddeus,1999).KiteC3Sihasbeen
identified in the evolved carbon star system IRC þ 10216
(Apponi et al., 1999), which raises the distinct possibility that
rhombicC4isalsoaninterstellarmolecule.LinearCCCSihasnot
beenidentifiedasaninterstellarmolecule,eventhoughthehigher
homologue CCCCSi has (Apponi et al., 1999).
The geometries of the kite and fan structures of C3Si are
shown in Scheme 4. The fan structures show shorter CC bonds
and longer CSi bonds than the corresponding kite structures.
Triplet CCCSi is the ground state by 8.0 kcal mol?1at the
CCSD(T)/aug-cc-pVDZ//B3LYP/6-31 þ G(d) level of theory,
while the kite and fan structures have singlet ground states
22.3 and 8.1 kcal mol?1below the respective triplets. The re-
arrangements of triplet and singlet C3Si are shown in Figures 6
and 7. No intersystem crossing calculations were carried out for
theC3Sipotentialsurfaces.ThereactioncoordinatesinFigures6
and7shouldbecomparedwiththeformationofrhombusC4from
linear C4shown in Figures 3 and 4: there are significant sim-
ilarities between the two systems.
FIGURE 5. Electronic configurations and 3D HOMOs of the singlet and triplet rhombus C4(Wang, Buntine, &
Bowie, 2009).
SCHEME 4
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2. The Isoelectronic Systems C4, (C3B)Sand (C3N)þ
The structures and formation of rhombus C4from linear C4have
also been compared theoretically with those cyclic systems
derived from the isoelectronic linear species (CCCB)?and
(CCCN)þ. The isoelectronic systems all form stable cyclic
four-membered rings (Wang, Buntine, & Bowie, 2009). Stable
kite and fan structures are formed for the singlet ground states of
both (C3B)?and (C3N)þ; the two structures being interconver-
tible but with significant barriers to transition states connecting
the two cyclic structures. Like rhombus C4, the singlet states are
the ground states of the kite and fan structures. The singlet
rhombus of C4and the singlet kite and fan structures of singlet
(C3B)?are more negative in energy (more stable) than the
appropriate linear precursor. Other cyclic states are less stable
thanthelinearprecursors.LikesingletrhombicC4,theformation
of most of the kite and fan structures are preceded by the
intermediacy of higher energy three-membered ring isomers
(Wang, Buntine, & Bowie, 2009).
3. Kite and Fan C3B and C3Al
There
the C3B and C3Al systems. Consider first, the C3B system
(Wang & Bowie, 2009). The quartet is the ground state of
CCCB by 11.9 kcal mol?1at the UCCSD(T)/aug-cc-pVTZ/
UB3LYP/6-311 þ G(3df) level of theory, whereas the kite and
fan cyclic structures have the doublet as the ground state by 31.5
and 10.1 kcal mol?1, respectively. The geometries of the cyclic
states are shown in Scheme 5. The doublet and quartet kite
structures are similar as are those of the corresponding fan
structures. The fan structures show more double bond character
for CC bonds and more single bond character for CB bonds [the
aresimilaritiesand differences between
FIGURE 6. Reaction coordinate profile for the rearrangement of triplet
CCCSi. CCSD(T)/aug-cc-pVDZ//B3LYP/6-31 þ G(d) level of theory
(Maclean et al., 2008).
FIGURE 7. Reaction coordinate profile for the rearrangement of singlet
CCCSi. CCSD(T)/aug-cc-pVDZ//B3LYP/6-31 þ G(d) level of theory
(Maclean et al., 2008).
SCHEME 5
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bond length of the CB single bond is reported to be 1.56 A˚
(Weast, 1975)]. Both quartet and doublet CCCB rearrange to
the cyclic forms of C3B, but the rearrangement of the doublet is
themorefacile.Thereactioncoordinateofthedoubletrearrange-
ment is shown in Figure 8. Intersystem crossing may occur
between the quartet and doublet of CCCB (spin orbit coupling
constant is 10.4 cm?1) but not between the doublet and quartet
of either kite C3B (3.9 cm?1) or fan C3B (0.3 cm?1). The
doublet fan and kite structures lie below CCCB by 14.5 and
23.7 kcal mol?1, respectively, and ring opening of each of these
structures can lead to CCCB and CCBC.
The C3Al system is different from the C3B system in that
(i) the doublet is the ground state of linear CCCAl and the cyclic
kite and fan structures, and (ii) there is no intersystem crossing
between doublet and quartet CCCAl. The geometries of the
cyclic systems are shown in Scheme 5. Like the C3B system,
thefanstructuresshowmoredoublebondcharacterforCCbonds
and more single bond character for CAl bonds (the CAl single
bond distance is 2.24 A˚). The doublet CCCAl rearrangement
reaction coordinate is shown in Figure 9. Kite C3Al lies below
CCCAl (?2.9 kcal mol?1) while the fan structure is less stable
than CCCAl (þ4.5 kcal mol?1). Unlike the C3B system
(in which CCCB and CCBC are interconvertible through the
cyclic forms) neither kite nor fan C3Al can effect facile ring
opening to CCAlC (see Fig. 9). The kite and fan systems for
both C3B and C3Al are interconvertible (see Figs. 8 and 9).
4. Kite Structures of C3N and C3P
The neutral radical CCCN has been identified in regions around
thecarbonrichstarIRC þ 10206,intheTaurusdarkcloudsTM1
and TM2 and in other interstellar regions (Fuchs et al., 2004,and
references cited therein). This system has been studied exper-
imentally by the neutralization–reionization procedure. The
anion (CCCN)?may be made in the source of a mass spec-
trometer by deprotonation of cyanoacetylene, but this synthesis
doesnotlenditselftothepreparationofthe13Clabeledanalogues
required to determine whether carbon scrambling is effected by
energizedCCCN.Anumberof13ClabeledformsofCCCNwere
made from several13C labeled acrylonitriles. This is illustrated
FIGURE 8. Reaction coordinate profile for the rearrangement of doublet
CCCB. UCCSD(T)/aug-cc-pVTZ//UB3LYP/6-311 þ G(3df) level of theory
(Wang & Bowie, 2009).
FIGURE 9. Reaction coordinate profile for the rearrangement of doublet
CCCAl. UCCSD(T)/aug-cc-pVTZ//UB3LYP/6-311 þ G(3df) level of the-
ory (Wang & Bowie, 2009).
FIGURE 10. (A)?CRþand(B)?NRþmassspectraof(CCCN)?.Modified
VG ZAB 2HF mass spectrometer (The University of Adelaide) with tandem
collision cell assembly (Maclean, Fitzgerald, & Bowie, 2007).
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for (CC13CN)?in Equation (3) (Maclean, Fitzgerald, & Bowie,
2007).
CH2¼ CH13CN þ HO?! CH2¼?C?13CN
! ½H?ðHCC13CNÞ? ! ðCC13CNÞ?þ H2
(3)
CCCN ! CCN þ Cðþ139:5kcalmol?1Þ
The?CRþand?NRþmass spectra of (CCCN)?are shown
in Figure 10. The peak corresponding to loss of carbon in the
?NRþspectrum is larger than that shown in the?CRþspectrum
(compared with the abundances of other fragment ions), indi-
cating that some energized radicals CCCN are decomposing by
loss of carbon. This is a high energy process [see, e.g.,
Eq. (4); calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-
31 þ G(d) level of theory], and the?CRþand?NRþspectra of
13ClabeledanaloguesofCCCNshowlossof12Cand13C:inother
words energized CCCN is undergoing a rearrangement which is
(partially) scrambling the carbon atoms.
The reaction coordinate profile which defines the process
whichcausescarbonscramblinginCCCNisshowninFigure11.
This defines the interconversion of CCCN to CCNC via
a kite (‘‘rhombus’’) structure, which lies 41.6 kcal mol?1
above CCCN. There is a three-membered ring intermediate
(þ43.4 kcal mol?1) preceding the formation of the C3N
kite (see Fig. 11): there is an analogous intermediate
(þ15.4 kcal mol?1) in the singlet C4rearrangement shown in
Figure 4. Although there are analogies between the CCCC and
CCCN systems, there are also some differences, namely: (i)
CCCC is linear whereas CCCN and CCNC are bent (CCC is
163.68forCCCN;CCNis165.28forCCNC),(ii)theC3Nprocess
needs significantly more energy (59.9 kcal mol?1; see Fig. 11)
than does the C4 rearrangement (27.2 kcal mol?1; Fig. 3),
and (iii) the rhombus structure is significantly less stable
for C3N than C4.
(4)
The geometry of the kite (‘‘rhombus’’) CCCN is shown in
Scheme 6 [C–C, 1.54 A˚; C=C, 1.34 A˚; C–N, 1.47 A˚; C=N,
1.30 A˚(Aylward & Findlay, 2008)]. The CC and CN bonds in
this structure all have some double bond character.
NeutralradicalCCCPhasnotbeendetectedasaninterstellar
molecule although CP has (Gue ´lin et al., 1990). Neither has
CCCP been studied experimentally by the neutralization–reio-
nization procedure (because of the difficulty of unequivocally
making the precursor anion). The C3P system been investigated
theoretically at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31 þ G(d)
level of theory (Maclean et al., 2008). The rearrangement of
doubletCCCPtokiteC3PisshowninFigure12,andissimilar to
that shown in Figure 11 for the analogous doublet CCCN. The
energies along the reaction coordinate are similar in Figures 11
and 12 and the geometry of the kite structure of C3P is shown in
Scheme6.ApartfromthedifferencesinCNandCPbonds[single
bond CP is 1.84 A˚(Aylward & Findlay, 2008)] and the CNC and
CPC angles (65.68 and 49.08, respectively) the two structures
shown in Scheme 6 show similarities. A major difference
between the rearrangements shown in Figures 11 and 12 is that
whereas doublet CCNC is formed from kite C3N, kite C3P does
not decompose to CCPC.
5. CCCO and CCCS
CCCO and CCCS are amongst the most abundant heterocumu-
lenes found in interstellar regions (Ohishi et al., 1991, and
referencescitedtherein).Theanionsofthe13Clabeledanalogues
CC13CO and CC13CS have been formed in the source of a mass
FIGURE 11. Reaction coordinate profile for the interconversion of CCCN
and CCNC. CCSD(T)/aug-cc-pA˚VDZ//B3LYP/6-31 þ G(d) level of theory
(Maclean, Fitzgerald, & Bowie, 2007).
SCHEME 6
FIGURE 12. Reaction coordinate profile for rearrangement of doublet
CCCP. CCSD(T)/aug-cc-pVDZ//B3LYP/6-31 þ G(d) level of theory. Lin-
earCCPCisnotstableatthisleveloftheory(Macleanetal.,2008).Thereisa
nonlinearCCPCisomer(CPCis125.28)lying77.4 kcal mol?1aboveCCCP:
nonlinear CCPC is formed directly from the three-membered ring intermedi-
ate at þ13.4 kcal mol?1(Wang & Bowie, unpublished observations).
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spectrometer as shown in Equation (5) (X ¼ O or S) (Tran,
McAnoy, & Bowie, 2004a).
½ðCH3Þ3SiCCð13C ¼ XÞSiðCH3Þ3?
? !
The?CRþand?NRþspectra of each of (CC13CO)?.and
(CC13CS)?.are very similar indicating that both neutrals are
stable for the microsecond duration of the neutralization–reio-
nization experiment. No carbon scrambling is detected. Ab initio
calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d)
level of theory show that to scramble the carbons of singlet
CCCO,therewouldneedtobethree-memberedringcyclizations
withatransitionstateof114 kcal mol?1:singletkiteC3Oisnota
stable species. In the case of the singlet CCCS reaction coor-
dinate, there is a cyclic kite structure lying 51.6 kcal mol?1
above CCCS, but to reach the transition state effecting this
rearrangement requires 80.3 kcal mol?1. These rearrangements
requiresignificantlymoreenergythanthoseconsideredearlierin
this section, data consistent with the experimental evidence
indicating no carbon scrambling of either system.
F?
ðCC13CXÞ??þ 2ðCH3Þ3SiF (5)
6. The (CCBO)þto (OCCB)þRearrangement
The (CCBO)?anion was made in the mass spectrometer source
by the sequence shown in Equation (6) (McAnoy et al., 2003)
F?þ ðCH3Þ3SiCCBðOisoPrÞ2
!?CCBðOisoPrÞ2þ ðCH3Þ3SiF?
?CCBðOisoPrÞ2! ½ð: C ¼ C ¼ BOisoPrÞ?OisoPr?
! ðCCBOÞ?þ ðisoPrÞ2O
The?CRþand?NRþspectra of (CCBO)?are identical,
suggesting that doublet CCBO is stable for the microsecond
time frame of the
by the significant barrier (63.8 kcal mol?1) in the reaction co-
ordinate profile for the doublet rearrangement (see Fig. 13A).
Both the?CRþand?NRþspectra show pronounced loss of CO
whichmustmeanthatsome(CCBO)þionshaverearrangedtoan
isomer which may lose CO; perhaps (OCCB)þ? Both the triplet
ground state and singlet of (CCBO)þrearrange to (OCCB)þvia
the intermediacy of three-membered ring and kite structures, an
interesting result since (CCBO)þand CCCC are isoelectronic.
Thereactioncoordinateprofileofthetripletcationrearrangement
is shown in Figure 13B (and compare with data for the triplet C4
system shown in Fig. 3). The triplet cation rearrangement
is a favorable process where the maximum barrier is only
21.0 kcal mol?1
at the UMP4STDQ/aug-cc-pVTZ//UMP2
(full)/6-31G(d) level of theory.
(6)
?NRþexperiment: a proposal supported
C. Five-Atom Cumulene Systems
1. C5
The C5molecule detected in star system IRC þ 10216 was
reported to be the linear isomer (Bernath, Hinkle, & Keady,
1989). Can energized CCCCC scramble its carbons analogous
to CCCC, and if so does it do this via cyclic intermediates? To
investigatethisquestion,(CC13CCC)?.wasmadeinthesourceof
a mass spectrometer by the SF6modification (Blanksby et al.,
2000a) of the Squires double desilylation procedure (Wenthold,
Hu, & Squires, 1994), as shown in Equation (7) (Dua & Bowie,
2002a).
ðCH3Þ3SiCC13Cð¼ N2ÞCCSiðCH3Þ3þ SF6
! ðCC13CCCÞ??þ SF4þ N2þ 2ðCH3Þ3SiF
The?CRþand?NRþspectra of (CC13CCC)?.are complex
but indicate that some of the energized neutrals are fragmenting
by loss of CC and C13C, which can only occur accompanying
or following rearrangement of the initially formed neutral
CC13CCC (the process CCCCC ! CCC þ CC is unfavorable
by 162 kcal mol?1). Ab initio calculations at the CCSD(T)/aug-
cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that singlet
CCCCC is the ground state by 63.9 kcal mol?1from the corre-
sponding triplet. The triplet state is therefore unlikely to be
involved in the vertical charge stripping of (CC13CCC)?.. The
reaction coordinate profile of the singlet CCCCC rearrangement
is shown in Figure 14. Equilibration occurs through rhomboid
structureslying54.8 kcal mol?1aboveCCCCC,aprocesswhich
(7)
FIGURE 13. Reaction coordinate profiles for the rearrangements of (A)
doublet neutral CCBO and (B) triplet (CCBO)þat the UMP4STDQ/aug-cc-
pVTZ//UMP2(full)/6-31G(d) level of theory (McAnoy et al., 2003).
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requires significantly more energy than the formation of rhom-
boid C4from linear CCCC (cf. Fig. 14 with Figs. 3 and 4).
Geometricfeaturesofthecyclicformareindicatedinthecaption
to Figure 14.
There is no stable singlet five-membered cyclo-C5at the
level of theory used for this study, but there is a stable
triplet cyclo-C5 (lying 50.2 kcal mol?1above triplet linear
CCCCC) on the triplet potential surface (Dua & Bowie,
2002a). This is similar to the situation with cyclo-C3, and may
be rationalized using a valence bond theory explanation as for
cyclo-C3(see Scheme 1).
2. Miscellaneous Five-Atom Heterocumulene
Rearrangements
a. CCOCC and CCCCO; NCCCO and NCCCN
Experimental and theoretical results indicate that bent CCOCC
rearranges to linear cumulene oxide CCCCO via a three-mem-
bered ring. This rearrangement is summarized in Seq. (8) of
Scheme7.SomeoftheproductmoleculesCCCCOareenergized
and decompose to CCC and CO (Fitzgerald et al., 2005). The
isonitrile–nitrile rearrangement of NCCCO to CNCCO occurs
via the three-membered ring intermediate shown in Seq. (9) of
Scheme 7 (McAnoy & Bowie, 2004a). A comparison of the
?CRþand?NRþspectra of (NCCCN)?.and some13C labeled
analogues shows that energized NCCCN rearranges to an isomer
that decomposes by loss of carbon. Theoretical studies indicate
that this decomposition is preceded by an initial nitrile/isonitrile
reaction (NCCCN ! CNCCN) by a mechanism similar to that
shown in Seq. (9) of Scheme 7 (Blanksby et al., 2000b).
b. The C4B system
Arguably the most interesting of the five-atom heterocumulene
rearrangements is that which involves the rearrangement of
doublet CCBCC to doublet CCCCB: this involves the interme-
diacy of two stable and unusual cyclic structures; namely a
symmetrical five-membered ring and an unsymmetrical substi-
tuted‘‘rhombus.’’Thereactioncoordinatediagramofthissystem
is shown in Figure 15 (calculations at the CCSD(T)/aug-cc-
pVTZ//B3LYP/6-31G(d) level of theory) (McAnoy, Bowie, &
Blanksby, 2003).
TherearethreeunusualstructuresshowninFigure15.These
are summarized in Scheme 8. The CCBCC structure is bent
aroundCCB,withtheCCandCBbondsapproximatingtodouble
and partial double bonds, respectively. The five-membered ring
system is symmetrical and planar with C1C2and C3C4being
double bonds, C2C3having some double bond character, and the
CB bonds showing significant single bond character. Perhaps the
stability of this structure is a consequence of the multi-centered
bonding involving boron. The substituted kite structure is the
most unusual of all such structures uncovered in these studies. It
is unsymmetrical butplanar, with the external carbon attempting
to form a single bond (1.656 A˚) with the adjacent carbon of the
ring system.
It was not possible to make CCBCC by an unequivocal
synthesisin themass spectrometer,
(HCCBCCH) (a seven atom system) was formed by electron
capture of (HCCBCCH)þ, made by the cation reaction
[B(CCH)3]þ! (HCCBCCH)þþ HC2.]. Comparison of the
CID MS/MS andþNRþspectra of (HCCBCCH)þshows that
some of the neutrals formed by electron attachment to
(HCCBCCH)þare energized, decomposing by loss of boron, a
butthe analogue
FIGURE 14. Reaction coordinate profile for the scrambling of singlet
CCCCC through equilibrating rhombus C5intermediates. CCSD(T)/aug-
cc-pVDZ//B3LYP/6-31G(d) level of theory. Geometric data for rhomboid
structure: C1C2(1.427 A˚), C4C5(1.316), C4C1(1.427), C2C3(1.439), C1C3
(1.482), C5C4C1(149.0o), C4C1C2(117.0) (Dua & Bowie, 2002a).
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reaction which must be preceded or accompanied by rearrange-
ment. A selection of the ab initio calculations of a complex
rearrangement reaction potential profile are summarized in
Figure 16, suggesting that boron could be lost from a five-
membered ring cyclic intermediate analogous to that shown
for C4B in Figure 15 (McAnoy & Bowie, 2004b).
D. Six-Atom Cumulenes
1. C6
Although C6has not been detected in interstellar regions, it is
possible that it may be present, since hexatriynyl (C6H) (Ziurys,
2006) and hexapentaenylidene (C6H2) (Cernicharo et al., 2001)
are interstellar molecules. To determine whether energized
CCCCCC rearranges its carbon skeleton a number of doubly
labeled13Canaloguesof(CCCCCC)?.areneededbutthishasnot
been done.
Could rearrangement of the carbon skeleton occur for
energized CCCCCC and if so how? The triplet and singlet
isomers of C6have been the subject of a number of theoretical
papers: the first (Raghavachari & Binkley, 1987) and the most
recent(Masso ´ &Senenst,2009;seealsoreferencescitedtherein).
This complex system was also investigated using the
ROCCSD(T)/aug-cc-pVTZ//B3LYP/6-311 þ G(3df) level of
theory (Wang & Bowie, unpublished data). Nineteen stable
isomers were identified at this level of theory; nine singlet and
ten triplet structures. All except five of these structures are high
energy and as such are unlikely to be involved in any carbon
chain rearrangements. The structures that remain are singlet
cyclo-C6(0 kcal mol?1), triplet CCCCCC (þ9.3 kcal mol?1),
singlet CCCCCC (þ14.6 kcal mol?1), singlet C2-cyclo-C4
(þ23.4 kcal mol?1), singlet C3- cyclo-C3(þ29.5 kcal mol?1),
and tripletcyclo-C6(þ26.7 kcalmol?1). These data are in agree-
mentwithresultsreportedinthelatestpublicationonthissubject
(Masso ´ & Senenst, 2009). The small triplet-singlet gap
(5.3 kcal mol?1) between the triplet ground state of CCCCCC
and the corresponding singlet suggests facile intersystem cross-
ing between these linear forms [as was the case with triplet and
linear C4(Wang, Buntine, & Bowie, 2009)]. There are then
three possible intermediates which, when formed, might effect
rearrangement of the backbone of CCCCCC: these are the
3, 4, and 6 membered ring isomers. The reaction coordinate
calculation for the rearrangement of CCCCCC is shown in
Figure 17.
The lowest energy rearrangement route is likely to involve
intersystem crossing of ground state triplet CCCCCC to the
singlet form which transforms to the singlet ground state of
thecyclo-C6isomer(Wang&Bowie,unpublishedobservations).
The structure of the C6cyclic isomer has been controversial for
several decades. Singlet ground state cyclo-C6has an unusual
structure (see Fig. 17). Why this structure should have D3hrather
thanD6hsymmetryisnotfullyunderstoodat thistime(cf.Masso ´
& Senenst, 2009).
SCHEME 8
FIGURE 16. Reaction coordinate profile for the rearrangement of HCC-
BCCH to cyclo-C4H2B. CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of
theory. Geometry of cyclo-C4H2B: (HC)B, (1.493 A˚); (HC)C, (1.357); CC,
(1.421); (HC)B(CH), (149.38); C(CH)B, (72.6); (HC)CC, (119.7) [cf. cyclo-
C4B (Scheme 8)] (McAnoy & Bowie, 2004b).
FIGURE 15. Reaction coordinate profile for the rearrangement of doublet
CCBCC to doublet CCCCB. CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d)
level of theory (McAnoy, Bowie, & Blanksby, 2003).
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2. The Interconversion of CCCCBO to OCCCCB
In Section II.B.6 above, it was shown that CCBO was stable, and
could only rearrange to OCCB via a four-centered kite inter-
mediate provided CCBO was energized [63.8 kcal mol?1is
required (see Fig. 13A)]. What will happen if the linear system
is CCCCBO? The anion precursor (CCCCBO)?was formed in
the source of the mass spectrometer by a process directly
analogous to that shown in Equation (6) (McAnoy et al.,
2004). A comparison of the
(CCCCBO)?indicates that some of the neutrals formed by the
charge-stripping process decompose by loss of CO, a process
which cannot occur from CCCCBO. Reaction coordinate calcu-
lations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31 þ G(d)
level of theory indicate that this is due to the rearrangement
shown in Seq. (10) of Scheme 9 which proceeds through an
unusualunsymmetricalandplanarsix-memberedringintermedi-
ate [see Seq. (10)]. The loss of CO from OCCCCB to yield
CCCCB requires 54.6 kcal mol?1, an energy similar to that
required for the rearrangement of CCCCBO.
?CRþand
?NRþspectra of
E. Miscellaneous Processes
1. A joint experimental and theoretical study shows that
doublet CCC(CO)CN rearranges to OCCCCN via a
three-membered ring intermediate as shown in Seq. (11)
of Scheme 9, and that the energized species OCCCCN so
formed,maydecomposetoCCCNandCO:aprocesswhich
requires only 38.5 kcal mol?1(Dua & Bowie, 2002b).
2. Iminoethenethione (HNCCS) must surmount a barrier
of 69 kcal mol?1before it may effect a 1,3-hydrogen re-
arrangement to produce thioformyl cyanide (H-CS-CN) in
a reaction favorable by 32 kcal mol?1(Flammang et al.,
1994).
3. Joint experimental and theoretical studies indicate that
CCCHO and CCCHS interconvert to HCCCO and
HCCCS by H transfer and cyclization processes (Peppe,
Blanksby, & Bowie, 2000; Peppe et al., 2003; Tran et al.,
2004b). The reaction coordinate profile of the rearrange-
ment of CCCHO to HCCCO is shown in Figure 18 for
illustrative purposes. In contrast, energized neutrals
CCCCHO, CCCCCHO, and NCCCHO rearrange to
FIGURE 17. Reaction coordinate profile for the degenerate rearrangement
of linear and cyclo-C6[(A) triplet and (B) singlet]. ROCCSD(T)/aug-cc-p-
VTZ//B3LYP/6-311 þ G(3df) (Wang and Bowie, unpublished data).
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HCCCCO, HCCCCCO, and NCCHCO, respectively, by
low-energy 1,2-H transfer processes rather than by more
energy-demandingbackbonecyclizations(Fitzgeraldetal.,
2003; Fitzgerald, Bowie, & Dua, 2003;McAnoy, Bowie, &
Dua, 2004).
4. An experimental and theoretical study shows that
NCCCCCN rearranges to the isonitrile CNCCCCN (prior
to decomposition by loss of carbon), by a three-membered
ring rearrangement similar to those already discussed for
OCCCN(Seq.9,Scheme7)andNCCCN,ratherthanbythe
four-centered ‘‘rhomboid’’ rearrangement of CCCN to
CCNC (Wang, Dua, & Bowie, 2010).
III. CONCLUSIONS
1. A number of linear cumulenes and heterocumulenes have
been made by charge stripping of anions of known bond
connectivity in the source of a mass spectrometer. Some of
these reactive molecules have been identified in interstellar
molecular clouds. The structures of these neutrals may be
investigated by reionization to a decomposing positive ion
[theneutralization–reionizationtechnique(?NRþ)],andby
ab initio calculations. When the required neutral cannot be
madeinthiswaythestructuremaybestudiedusingabinitio
calculations.
2. Energized linear cumulenes and heterocumulenes may
undergo cyclization to form stable cyclic isomers, in spite
of the fact that the precursors are generally linear with
significant bond multiplicity. One of the cyclic molecules,
cyclo-C3Si, has been detected in interstellar molecular
clouds.
3. Four-atom systems CCCC and heterocumulenes CCCX
(X ¼ B, N, Al, Si, P) involve the formation of four-mem-
beredringrhombic(kiteandfan)structures.Sometimesthe
rhombic form is formed directly from the linear precursor,
in other cases, the formation of the four-membered cyclic
system is preceded by cyclization of the linear cumulene to
a three-membered cyclic precursor of the rhombic system.
The only CCCX system studied where the singlet ground
state rhombus is not stable is C3O, which either cyclizes to
form a three-membered ring, or decomposes to yield CC
and CO.
4. Five-atomcumuleneandheterocumulenesystemsaremore
complex. CCCCC rearranges the carbon skeleton by form-
ing a carbon substituted rhomboid system, CCCCO forms
three and four-membered cyclic isomers, while nitrogen
containing cumulenes like OCCCN and NCCCN effect
nitriletoisonitrileinterconversionviathree-centercyclized
intermediates. The only systems studied which give stable
cyclic and planar five-centered cyclic systems are triplet C5
and doublet C4B.
5. CCCCCC and CCCCBO cyclize to give unique six-
membered ring systems.
6. CCCHO and CCCHS rearrange via H transfer or backbone
cyclization to yield HCCCO and HCCCS, respectively,
whereas the homologues CCCCHO and CCCCCHO con-
vert to HCCCCO and HCCCCCO by sequential 1,2-H
migration processes rather than higher-energy backbone
cyclization.
ACKNOWLEDGMENTS
We thank the Australian Research Council for financing our
negative ion and interstellar research. TW thanks the ARC for
a research associate stipend.
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FIGURE 18. ReactioncoordinateprofilesoftherearrangementofCCCHO
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(Peppe, Blanksby, & Bowie, 2000).
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