Photoelectron spectroscopy of multiply charged anions.

Abstract

Multiply charged anions (MCAs) are common in condensed phases but are challenging to study in the gas phase. An experimental technique, coupling photoelectron spectroscopy (PES) with electrospray ionization (ESI), has been developed to investigate the properties of free MCAs in the gas phase. This article reviews the principles of this technique and some initial findings about the intrinsic properties of MCAs. Examples include the observation of the repulsive Coulomb barrier that exists universally in MCAs and its effects on the dynamic stability and PES of MCAs. The solvation and solvent stabilization of MCAs have been studied in the gas phase and are also discussed. A second-generation low-temperature ESI-PES apparatus has been developed, which allows ion temperatures to be controlled from 10 to 350 K. New results from this low-temperature ESI-PES instrument are also reviewed, including doubly charged fullerene anions, inorganic metal complexes, and temperature-induced conformation changes of complex anions.

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ANRV373-PC60-06ARI 6 October 200817:7
R E V I E W S
I N
A D V A N C E
Photoelectron Spectroscopy
of Multiply Charged Anions
Xue-Bin Wang and Lai-Sheng Wang
Department of Physics, Washington State University, Richland, Washington 99354,
and Chemical and Materials Sciences Division, Pacific Northwest National Laboratory,
Richland, Washington 99352; email: ls.wang@pnl.gov
Annu. Rev. Phys. Chem. 2009. 60:105–26
The Annual Review of Physical Chemistry is online at
physchem.annualreviews.org
This article’s doi:
10.1146/annurev.physchem.59.032607.093724
Copyright c ? 2009 by Annual Reviews.
All rights reserved
0066-426X/09/0505-0105$20.00
Key Words
intramolecular Coulomb repulsion, repulsive Coulomb barrier, negative
electron binding energy, ion solvation, electronic structure, electrospray
ionization, low-temperature ion trap
Abstract
Multiply charged anions (MCAs) are common in condensed phases but are
challenging to study in the gas phase. An experimental technique, coupling
photoelectron spectroscopy (PES) with electrospray ionization (ESI), has
been developed to investigate the properties of free MCAs in the gas phase.
This article reviews the principles of this technique and some initial findings
about the intrinsic properties of MCAs. Examples include the observation of
the repulsive Coulomb barrier that exists universally in MCAs and its effects
on the dynamic stability and PES of MCAs. The solvation and solvent stabi-
lization of MCAs have been studied in the gas phase and are also discussed.
A second-generation low-temperature ESI-PES apparatus has been devel-
oped,whichallowsiontemperaturestobecontrolledfrom10to350K.New
results from this low-temperature ESI-PES instrument are also reviewed,
including doubly charged fullerene anions, inorganic metal complexes, and
temperature-induced conformation changes of complex anions.
105
Review in Advance first posted online
on October 13, 2008. (Minor changes may
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MCA: multiply
charged anion
RCB: repulsive
Coulomb barrier
ESI: electrospray
ionization
1. INTRODUCTION
Multiply charged anions (MCAs) are ubiquitous in nature and are important constituents in solu-
tions and solids. Their existence has been generally taken for granted in the teaching of chemistry
and in our understanding of materials. However, MCAs are usually rather fragile as isolated
species in the gas phase owing to the strong intramolecular Coulomb repulsion among the ex-
cess charges and the missing stabilization through solvation and other electrostatic interactions,
making many common textbook MCAs unstable and difficult to study (1–4). Despite a handful
of mass spectrometric observations of large organic molecular MCAs (5–8), our knowledge about
their properties had been limited previously. In 1990, Compton and colleagues’ (9) observation
of small carbon cluster dianions, Cn2−(n = 7–28), stimulated much experimental and theoretical
interest in gaseous MCAs.
Early theoretical considerations of MCAs derived from interest in searching for molecular
specieswithunusuallyhighelectronaffinities(10,11)andinunderstandingtheelectronicstructure
of condensed-phase species (12, 13). The groups of Cederbaum and Simons have made major
theoretical advances in our understanding of MCAs. Cederbaum and coworkers investigated a
series of gaseous MCAs (14–29) and proposed a construction principle for predicting gaseous
MCAs based on the alkali and alkali-earth halide systems (17, 18). Simons and coworkers (4, 30–
36) considered many MCAs that exist in the condensed phase and investigated their structure,
bonding,andstabilityinthegasphase.Theyproposedconceptsofmixedvalence–dipoleboundand
dipole-dipole bound dianions as well (35, 36). There have also been a number of other theoretical
studies on gaseous MCAs (37–45), in particular concerning small carbon clusters (15, 19, 37–
40) and those relevant to condensed-phase materials (44, 45). The electron-electron repulsion
and the resultant repulsive Coulomb barrier (RCB), which originates from the combination of
short-range binding and long-range Coulomb repulsion, have been pointed out in several early
theoreticalworksonmultiplychargedmetalclusters,fullereneanions(46–48),andlatertheoretical
models on several small MCAs (24, 25, 49, 50).
Early experimental efforts on gaseous MCAs were all made via mass spectrometric observa-
tions using a variety of ionization techniques. These include the observation of doubly charged
anions of small carbon clusters (9, 51–53), fullerenes (54–58), and other species (59–62) using
the ion-sputtering technique; doubly charged small organic, inorganic, and fullerene anions using
electrospray ionization (ESI) (63–72); and multiply charged metal clusters and fullerene anions
via sequential electron attachment in a Penning trap (73–76). Recently, several new experimental
findings have been reported, including the observation of the short-lived LiF32−species (77), a
series of new composite small dianions using a sputtering source along with gas flooding (78), and
the generation of doubly charged anions via a charge-transfer technique (79–83). The coordina-
tion numbers of ligand and solvent molecules required to make stable dianionic metal complexes
(84–86) and hydrated clusters (63, 87) have been pursued using size-selected collision-induced
dissociation techniques. The competition between ionic fragmentation versus electron detach-
ment and the counter-ion perturbation of MCAs has been investigated by collision excitation and
ab initio calculations (88–91). Lifetimes for a number of metastable dianions have been directly
measured (83, 92).
Beyond the above mass spectrometric investigations, spectroscopic studies of MCAs remained
achallenge.Althoughanumberofdoublychargedanionswereobservedusingionsputtering,laser
ablation, and other collisional techniques (9, 51–61), their intensities were too weak to allow for
spectroscopicinvestigations.Conversely,theESItechnique,pioneeredbyFennandcoworkers(93,
94)asasoft-ionizationmethodforbiomolecules,couldproducesmallgaseousMCAsdirectlyfrom
solutions(63–72),makingitapromisingtechniqueforspectroscopicstudiesoftheseexoticspecies.
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PES: photoelectron
spectroscopy
TOF: time of flight
In 1998, we developed an experimental technique combining ESI and photoelectron spec-
troscopy (PES) to investigate MCAs (95–97). PES is an ideal technique to probe the intrinsic
properties of MCAs in the gas phase. It provides direct measurements of the excess electron
binding energies in MCAs, thus allowing one to obtain information about their stability and in-
tramolecularCoulombrepulsion.Usingthistechnique,systematicstudieshavebeencarriedouton
avarietyoffreeMCAs,includinginorganicandorganicMCAs,redoxspecies,andbiologicallyrel-
evant molecules (95–120). Since these initial studies, several other spectroscopic techniques based
on ESI have been developed, including photodissociation (121), photodetachment (122–125), and
pump-probe experiments (126). Recently we have developed a second-generation ESI-PES appa-
ratus that can selectively trap ions under low-temperature conditions (127–129). Temperature is a
critical issue in gas-phase spectroscopy and ion chemistry. The low-temperature trap is valuable to
suppress thermal broadening in photoelectron spectra and to help stabilize weakly bound clusters.
Inaddition,temperature-dependentstudiesarepossible,whichcanbeusedtoinvestigateentropic
effects on ion conformations and isomer populations.
Several reviews and feature articles about gaseous MCAs are now available in the literature (1,
26, 91, 104, 109, 120) with emphasis on early theoretical and experimental advances, including
spectroscopic investigations of MCAs at room temperature. In this review, we first give a brief
summary of some of the early findings of free MCAs, such as the direct observation of the RCB
(96, 97), the experimental observation of negative electron binding energies (102, 103), and the
solvation and solvent stabilization of complex MCAs (106–111). Then we focus on progress made
using the low-temperature ESI-PES technique. One can effectively cool anions and completely
eliminate hot band transitions in PES (127). More accurate electron binding energies and detailed
electronic structure information are thus obtained from cold spectra for a number of transition-
metal complexes and fullerene dianions (130–134). Temperature-dependent studies on several
complex systems display drastic spectral changes as a function of temperature owing to entropy-
driven conformation changes, which allows energetic as well as thermodynamic information to be
obtained (128, 135).
2. ROOM- AND LOW-TEMPERATURE ELECTROSPRAY ION-TRAP
PHOTOELECTRON SPECTROSCOPY
The design and operation of a room-temperature ESI-PES apparatus have been described in
detail elsewhere (95). A schematic view of the instrument is given in Figure 1, and important
features are described below. Solutions containing the anions of interest (typically ∼1–0.1 mM in
CH3CN or 3:1 CH3OH:H2O) are sprayed through a syringe (∼0.04 mL h−1), which is biased at a
negativehighvoltage.Highlychargednegativedropletsfromthesyringearefedintoadesolvation
capillary maintained at ∼50–100◦C for desolvation. Anions that emerge from the heated capillary
are guided by a radiofrequency-only quadrupole ion guide into a quadrupole ion trap, where ions
are accumulated for 0.1 s before being ejected into a time-of-flight (TOF) mass spectrometer.
The desired anions are mass selected by a mass gate and decelerated before being detached by a
laserbeamintheinteractionzoneofamagnetic-bottlephotoelectronanalyzer.Thephotoelectron
energies are calibrated with the known spectra of I−, O−, or ClO2−(136).
The low-temperature ESI-PES apparatus has been described previously (127–129). A key
feature of the new apparatus is a temperature-controlled ion trap that is used for ion accumulation
and cooling (129). The ion trap is attached to the cold head of a closed-cycle helium refrigerator,
which can reach a low temperature of 5 K and can be controlled up to 350 K. The ESI source,
TOF mass spectrometer, and magnetic-bottle PES analyzer are similar to the room-temperature
instrument described above (95).
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7 9 10
8
11
12
5676
1
2
3
4
Figure 1
Schematic view of the electrospray photoelectron spectroscopy apparatus: (1) syringe, (2) heated
desolvation capillary, (3) radiofrequency quadrupole ion guide, (4) quadrupole ion trap, (5) time-of-flight
(TOF) mass spectrometer extraction stack, (6) vertical and horizontal deflectors, (7) Einzel lens assembly,
(8) 40-mm dual microchannel plate in-line ion detector, (9) three-grid mass-gate and momentum
decelerator assembly, (10) permanent magnet (NdFeB), (11) 4-m TOF tube with a low-field solenoid and
double-layer μ-metal shielding, and (12) 40-mm Z-stack microchannel plate photoelectron detector.
Figure reprinted from Reference 95.
ADE: adiabatic
detachment energy
3. UNIQUE PROPERTIES OF FREE MULTIPLY CHARGED ANIONS
3.1. Direct Observation of the Repulsive Coulomb Barrier, Intramolecular
Electron-Electron Repulsion, and Electron Tunneling Through
the Repulsive Coulomb Barrier
The universal existence of the RCB, which originates from the combination of the short-range
electron binding and long-range Coulomb repulsion between the outgoing detached photoelec-
tron and the remaining anion, was directly observed in the first PES experiment on MCAs: the
citratedianion(CA2−)(Figure 2)(96).The266-nmspectrum(Figure 2d)showsaverybroadand
asymmetric feature, whereas the 355-nm spectrum (Figure 2c) exhibits a relatively narrow and
symmetric band. Close examination of the 266-nm spectrum revealed that it actually contains two
overlapping bands, the X band observed in the 355-nm spectrum with an adiabatic detachment
energy (ADE) of 1.0 eV and another band at a higher binding energy, the A band, with an ADE
of 1.6 eV, as seen more clearly in Figure 2e. There was no photoelectron signal observed at 532
nm even though the photon energy was higher than the ADE of the X band. The photon-energy-
dependent spectra vividly demonstrated the existence and effects of the RCB (Figure 2b). The
266-nm photon must be above the RCB of both the X and A states, so both states were observable
at 266 nm. The 355-nm photon must be below the RCB of the A state, but higher than that of the
X state, so only the X state was accessible at 355 nm. The 532-nm photon must then be lower than
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6.14
CA–(A) + e–
1.0 eV
0.6 eV
RCB: 1.9 ~ 2.5 eV
RCA– – e–
355 nm
266 nm
X
X
X
A
A
532 nm
355 nm
266 nm
H HH H
CA–(X) + e–
a
b
c
d
e
Relative electron intensity
01234
Binding energy (eV)
COOH
C
C
C
C
O
O
O
109°
1.36
1.54
C
O
OH
Figure 2
(a) Structure of the citrate dianion with relevant bond lengths (˚A) and bond angles. (b) Schematic potential
energy curves showing the adiabatic electron binding energies and the repulsive Coulomb barrier (RCB) for
the detachment of CA2−, leading to the X and A asymptotic states of CA−. Note that the barrier heights
relative to the X and A states are assumed to be the same. Photoelectron spectra of citrate dianion (CA2−) at
(c) 355 nm (3.496 eV) and (d) 266 nm (4.661 eV). Note the increased spectral width in the 266-nm spectrum,
implying an additional detachment channel as shown in panel e. Figure reprinted from Reference 96.
the RCB of both the X and A states; thus no signal could be observed at this photon energy. With
the assumption that the RCBs of the X and A states are the same, its magnitude can be estimated
on the basis of the above photon-energy-dependent information as 1.9 < RCB < 2.5 eV. The
RCB effectively prevented photoelectrons with kinetic energies less than the height of the RCB
from being emitted, resulting in a cutoff in the high binding energy side in the PES spectra. This
cutoff has become a hallmark for PES of MCAs.
Subsequently, PES experiments were performed on a series of linear dicarboxylate dianions
−O2C-(CH2)n-CO2−,wherenindicatesthealiphaticchainlength(n = 3–10)(97).Thesedianions
are similar to the citrate dianion, and the two charges are localized on the terminal carboxylate
groups,butnowtheseparationbetweenthetwochargescanbesystematicallyvaried.Theelectron
bindingenergiesofthesedianionsincreaselinearlywithn,whereastheRCBdecreaseswithn.The
sumofthetwoisaconstantandclosetotheelectronbindingenergyofasinglychargedcarboxylate
anion. Therefore, the decrease of the binding energy in the dianions, owing to the presence of
the second charge (intramolecular Coulomb repulsion), is equal exactly to the RCB. Thus one
can quantitatively determine the amount of intramolecular Coulomb repulsion in MCAs through
measurements of the height of the RCB. A similar linear relationship between the proton affinity
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