Anion Photoelectron Spectra and Ab Initio Calculations of the Iodide

Article pubs.acs.org/JPCA

Anion Photoelectron Spectra and Ab Initio Calculations of the Iodide−Carbon Monoxide Clusters: I−···(CO)n, n = 1−4 Kim M. Lapere, Robert J. LaMacchia, Lin H. Quak, Marcus Kettner, Stephen G. Dale, Allan J. McKinley, and Duncan A. Wild* School of Chemistry and Biochemistry, The University of Western Australia, M313 35 Stirling Highway, Crawley, 6009, Western Australia, Australia S Supporting Information *

ABSTRACT: Anion photoelectron spectra are reported for the iodide-carbon monoxide clusters, with supporting ab initio calculations for the 1:1 dimer anion and neutral complexes. A Cs minimum geometry is predicted for the anion complex, while for the neutral complex two linear van der Waals minima are predicted differing in the attachment point of the iodine, that is, I···CO and I···OC. The predicted adiabatic photodetachment energy agrees well with the experimental spectrum. The photoelectron spectra feature a vibrational progression in the CO stretching mode, which becomes more pronounced for the larger clusters.

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INTRODUCTION The spectroscopic investigation of ionic clusters of the form X±···(M)n allows for the elucidation of cluster structures, and the energetics of intermolecular interactions. These interactions have wide ranging influence, for example, in the driving force behind solute dissolution, the structure of biomolecules such as proteins, and in the direction and rate of chemical reactions. Because the targets are charged species, mass spectrometric techniques can be used to great effect to determine in the first instance the cluster identity and second to mass select a particular cluster ion for further spectroscopic interrogation. The various forms of spectroscopy for molecular species can be applied to ion clusters to provide intimate details on the electronic,1 vibrational,2 and rotational energy level structure.3−5 Furthermore, as mass selection is utilized, it follows that clusters of increasing size can be investigated in a stepwise manner to infer the dominant solvation structures as solvent ligands congregate around the solute anion core.6−8 The rationale behind such experiments is to bridge the gap between gas phase clusters and bulk contexts. In addition to aforementioned forms of spectroscopy, the technique of photoelectron spectroscopy applied to anion complexes and clusters is a powerful experimental avenue as it also provides insights into the geometries and energetics of the analogous neutral species by launching onto their potential energy surfaces following electron photodetachment from the anion complex. There has been a large volume of work produced in this area, notably from the groups of Lineberger, Neumark, Bowen, Wang, and Kaya, with some representative examples provided in references.9−13 To rationalize the direction and rates of chemical reactions, a crucial piece of information is the form of the potential energy © 2012 American Chemical Society

surface governing intermolecular interactions. This can be obtained through spectroscopic studies of significant regions of the surface, with the results used to improve upon surfaces produced from ab initio methods. Important regions of the PES often correspond to the weakly bound van der Waals minima that exist at the entrance and exit channels of reactions, as the reactive species interact, react, and then depart. This paper constitutes the first spectroscopic study of the I−···CO anion, which upon electron photodetachment accesses the van der Waals region of the I + CO reaction. The halogen-formyl radicals are recognized to be important species. The ClCO radical complex, in particular, has been proposed as an intermediate in the production of phosgene from Cl2 and CO,14 has a role in the reaction pathways of CO in our atmosphere,15 and is also postulated as an important intermediate in the chemistry of the Venutian atmosphere.16 The BrCO and ClCO species have received prior experimental and theoretical attention, via matrix and gas phase spectroscopy,17−24 kinetic studies,15,25−27 and via ab initio calculations.28−34 To the best of our knowledge, this paper is the first direct spectroscopic treatment of the anion I−···CO complex, and larger I−···(CO)n clusters, and the first full theoretical treatment of both the 1:1 anion and neutral complexes. The anion molecule complex has been observed indirectly from collision experiments between iodide anions and COS performed by Refaey.35 From this study, the electron affinity of the ICO neutral was determined to be 3.15 eV, a value that can be Received: January 15, 2012 Revised: March 1, 2012 Published: March 15, 2012 3577

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The Journal of Physical Chemistry A critically assessed from the photoelectron spectroscopy study undertaken by Vogt molecule reactions between for three reaction channels:

Article

(1)

eBE = hν − eKE

work reported here as we apply to the I−···CO complex. Another and Mischke36 followed the ion I− and CO and showed evidence

where hν is the energy of the incident photon. In this fashion, the spectra represent transitions from anion states to the various neutral states and resemble conventional absorption spectra. Multiple spectra were averaged for a given cluster size, with each spectrum collected over 10000 laser shots.

I− + CO → IO− + C

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I− + CO → O− + (I + C)

THEORETICAL METHODOLOGY The I−···CO anion and I···CO neutral 1:1 complexes were investigated by ab initio calculations at the MP2, CCSD, and CCSD(T) levels of theory with Dunning’s augmented correlation consistent basis sets up to quadruple-ζ quality for carbon and oxygen (aug-cc-pVXZ, where X = D, T, Q),41−43 while the aug-cc-pVXZ PP basis sets were employed for iodine.44 The PP basis sets are used, as they account for relativistic effects for the heavier iodine atom and consist of small-core relativistic pseudopotentials adjusted to multiconfiguration Dirac-Hartee-Fock data based on the DiracCoulomb-Breit Hamiltonian. For the open shell radical species we based the post-Hartree-Fock calculations on an unrestricted Hartree-Fock (UHF) reference wave function. We ensured that there was no spin contamination, and indeed the largest value of ⟨S2⟩ was 0.77 prior to annihilation of the contamination. The levels of theory were deemed appropriate for the complexes as they have been used successfully in the description of similar loosely bound species. Augmented basis sets were used as they contain diffuse functions which are essential in order to properly describe the nature of the anion orbitals. All calculations were performed using Gaussian 09 program suite.45 The geometries of the complexes were determined from the standard optimization routines, with “tight” or “very tight” convergence criteria employed. Vibrational frequency analyses were performed at the located stationary points to ensure that they represented minima on the global potential energy surface. Electronic energies were corrected for Basis Set Superposition Error (BSSE) using the familiar method of Boys and Bernardi.46 Energies of the anion and neutral species, corrected for zero point energy differences, were used to determine adiabatic detachment energies which can be compared to the experimental spectra. Finally, Natural Bond Order analyses were performed for both the anion and neutral complexes to assess the perturbation of the CO molecule by both the anion and neutral.47

I− + CO → CI− + O

The energy dependence of the reaction cross section for each reaction was measured, allowing prediction of the electron affinities of the CI and IO species. The ICO neutral complex has been addressed previously by Bridgeman who performed calculations using density functional theory on a series of main group carbonyl complexes.37 Structures, vibrational frequencies, and binding energies (Do) were predicted for various geometries of the FCO, ClCO, BrCO, and ICO radicals. The reliability of this methodology is questioned however, as DFT has the tendency to perform poorly for loosely bound species with shallow minima especially if the functionals chosen are not adequate for the task at hand. This issue has been recently highlighted in a benchmark study of density functional methods undertaken by Goerigk and Grimme.38 This article is an extension of our recent work on the analogous chloride-carbon monoxide complex also studied via photoelectron spectroscopy with supporting ab initio calculations.34 In this earlier work, we were able to determine the electron affinity and stabilization energy for the 1:1 complex, and rationalize the experimental spectrum by reference to predicted minima on the anion and neutral potential energy surfaces. We also located a new stationary point on the neutral ClCO potential energy surface, namely the Cl···OC linear geometry.

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EXPERIMENTAL SECTION The apparatus at UWA consists of a time-of-flight mass spectrometer for anion species, based on the design of Wiley and McLaren,39 coupled to a magnetic bottle photoelectron spectrometer similar to that introduced by Cheshnovsky et al.40 The spectrometer at UWA has been described previously,34 and hence only those conditions relevant to the experiments on I−···(CO)n clusters are described here. Ion clusters were formed in a plasma created by intersecting energetic electrons with a pulsed supersonic expansion of a gas in a vacuum chamber. The composition of the gas mixture is varied in order to produce the ion clusters of interest, and in this case consisted of a mixture of carbon monoxide and argon (1:5 ratio) seeded with traces of methyl iodide (iodide anion precursor). The I−···(CO)n clusters are selected using time-of-flight mass spectrometry and the cluster of choice is subsequently overlapped in the presence of a strongly divergent magnetic field by a 5 ns pulse of 266 nm radiation (4.66 eV, fourth harmonic of Continuum Surelite I Nd:YAG). The generated photoelectrons are guided to a detector at the end of a 1.5 m flight tube by means of a second homogeneous magnetic field subjected to the entire length of the flight tube. The time-offlight of the detached photoelectrons with respect to the laser pulse is recorded and initially converted to kinetic energy (eKE). The electron binding energy (eBE) is then obtained using the following expression:

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RESULTS AND DISCUSSION

Electrostatic and Induction Interactions in the Iodide−Carbon Monoxide Anion Complex. The intermolecular interaction between the iodide anion and carbon monoxide can be modeled in the first instance by considering the dominant electrostatic and induction terms:48 Velec(R , θ) =

q ⎛ μ cos θ Θ(3cos2 θ − 1) + ⎜ 2 4πε0 ⎝ R 2R3 +

Vind(R , θ) = − 3578

Ω(5cos3 θ − 3cos θ) ⎞ ⎟ 2R4 ⎠

(2)

q2(α cos2 θ + α⊥sin 2 θ) 2(4πε0)2 R4

(3)

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where q is the charge on the anion, while μ, Θ, and Ω, are the dipole, quadrupole, and octopole moments of CO, α∥, and α⊥ are the dipole polarizabilities parallel and perpendicular to the internuclear axis. R is the distance from the iodide to the center of mass of CO, and θ the angle formed between this vector and the CO bond vector. For the purpose at hand, values for the electric moments and polarizabilities were taken from CCSD(T) calculations of Maroulis as they constitute a set of parameters at a consistent level of theory.49,50 The total interaction potential, as a function of R and θ, is shown in Figure 1. At intermolecular separations typical of van der Waals

Figure 2. Geometries predicted from ab initio calculations of (a) the anion I−···CO complex and (b) the neutral I···CO/I···OC complexes. Bond lengths and angles are from CCSD(T)/aug-cc-pVTZ calculations. For the neutral complexes, numbers in parentheses are zero point energy corrected energy differences between minima, in kJ·mol−1.

∼98°. Attempts to optimize a linear structure invariably resulted in the geometry shown in Figure 2a, a result that was observed for all levels of theory and with all basis sets employed. There were the expected variations in the structural parameters upon increased basis set size, with the most noticeable change being the decrease in the intermolecular separation which, from the MP2 calculations, decreased from 3.791 → 3.667 → 3.627 Å on going from the aug-cc-pVDZ → aug-cc-pVTZ → aug-cc-pVQZ basis sets. The perturbation of the carbon monoxide by the iodide anion is rather small, as seen in a modest change of the CO bond length, and a small shift of around 100 cm−1 of the bare CO stretch. This is in agreement with the predicted binding energy of the complex, with Do of 7.3 kJ·mol−1 at the CCSD(T)/aug-cc-pVTZ level. While there are no previous calculations against which we can compare our findings for the I−···CO complex, the same methodology applied in our earlier work on the Cl−···CO complex produced satisfactory agreement with the work of Kryzhevoi et al. at the MP4/cc-pVTZ level.24 When comparing the Cl−···CO and I−···CO complexes, the former displays a shorter intermolecular bond (3.140 vs 3.652 Å) yet approximately the same X−C−O angle of around 98°. The shorter intermolecular bond reflects the larger binding energy of the Cl−···CO complex compared with I−···CO (14.6 kJ.mol−1 versus 7.3 kJ.mol−1). Ab Initio Calculations of the Neutral Complex. The neutral iodine−carbon monoxide complex was investigated using the same levels of theory as for the anion. Two minima were located for all methods trialled and are shown in Figure 2b. Both geometries have a linear structure differing in the orientation of the CO with respect to iodine. Again, structural parameters in Figure 2b are from CCSD(T)/aug-cc-pVTZ calculations. The I···CO geometry is lower in energy compared with I···OC, however, only by 1.6 kJ·mol−1, and the predicted dissociation energies (Do) are 3.2 and 1.6 kJ·mol−1 for I···CO and I···OC, respectively. The van der Waals style complexes have been predicted previously by our group,34 and others,28−33 for the chlorine and bromine−carbon monoxide analogues. In the case of the chlorine−carbon monoxide complex, a third minimum with a closer interaction between the chlorine and CO was located, the “covalently bound” ClCO radical. Attempts to optimize a similar structure for the iodine complex lead to the linear I···CO minimum shown in Figure 2b. The only other reported calculations for the neutral iodine− carbon monoxide complex were performed using DFT,37 and in Table 1 we compare these results with our CCSD(T)/aug-

Figure 1. Interaction energy between the iodide anion and CO, as a function of R and θ. Darker shading indicates lower energy, contours are separated by 250 cm−1. The coordinate system is shown as the inset (oxygen is red, carbon is green).

style complexes, the structure is predicted to be bent with an angle of around 100°. If one were to consider that the charge− dipole interaction is the dominant interaction, then a linear complex would be expected with the oxygen closest to the iodide anion, and this would be further reinforced by the charge-induced dipole interaction as α∥ > α⊥ (15.48 and 11.87 au, respectively, from ref 50). The charge−octopole interaction favors a linear complex, however, a local minimum also exists at around 115°. It is the charge−quadrupole interaction that is the driving force behind the potential well observed at 100°. This can be understood by considering the relative magnitudes of the first two electric moments of carbon monoxide, which features a rather small dipole moment μ = 0.0514 au (0.1307 D) yet has a large quadrupole moment Θ = 1.47 au (1.85 B).49,50 In the range of intermolecular separations in question, the charge−quadrupole attractive force exceeds the charge− dipole, and the charge−octopole interaction is weaker still. Ab Initio Calculations of the Anion Complex. The optimized geometry of the I−···CO complex is shown in Figure 2a, with geometric parameters provided at the CCSD(T)/augcc-pVTZ level of theory. Only one minimum was located for the anion complex, and a full data set for the complex in terms of geometries, vibrational frequencies, and energies at all levels of theory investigated (MP2, CCSD, and CCSD(T)) is provided as Supporting Information. The form of the complex agrees with the findings of the electrostatic and induction modeling, with the bent Cs symmetry geometry featuring the carbon of CO closer to the iodide anion, and an ICO angle of 3579

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shown in Figure 3, where the angle corresponds to that made between the iodine, the midpoint of the CO bond, and the

Table 1. Comparison between the CCSD(T)/aug-cc-pVTZ Calculations and DFT (BP86VWN/TZ) BP86VWN/TZa r(C−O)/Å r(C−I)/Å ∠ICO/deg. ω1/cm−1 ω2/cm−1 ω3/cm−1 Do/kJ·mol−1

CCSD(T)/apVTZb

ICO

I···CO

I···CO

1.158 2.396 131 1932 347 347 55

1.145 2.835 180 2105 155 542c 35

1.135 3.405 180.0 2155 63 47 3.2

a Taken from ref 37. bThis work. cIn addition, one imaginary frequency predicted.

cc-pVTZ calculations. The linear I···CO structure was predicted to be a transition state using DFT methods, however, is found to be a minimum at all levels of theory trialled here (MP2, CCSD, and CCSD(T)). The covalently bound ICO species was predicted to be a minimum from DFT calculations. Attempts to locate this stationary point using MP2, CCSD, and CCSD(T) methods resulted in convergence to the van der Waals I···CO linear complex. There also appears to be an issue with the reported vibrational frequencies, as from the DFT calculations for the Cs symmetry ICO complex the I−C stretch and the ICO bend are both reported to be 347 cm−1, and the intermolecular stretching and bending modes have vibrational wavenumbers of 155 and 542 cm−1, respectively. While the stretching mode is close to what one might expect for a loosely bound van der Waals complex, a value of 542 cm−1 for the bending mode is clearly too high. It appears likely that the problem may be due to inadequate DFT functionals for describing what is a loosely bound complex featuring shallow minima. The calculated Do values are also compared in Table 1. Again, the agreement is not satisfactory, as for the linear van der Waals I···CO complex, we predict Do to be 3.2 kJ·mol−1 compared with 35 kJ·mol−1 from the DFT calculations. We have calculated the adiabatic electron detachment energies from the I−···CO complex to the two neutral minima, that is, I···CO and I···OC. These data are provided in the Supporting Information for all levels of theory and correspond to the energy separation between the ground vibrational state of the anion complex and the ground vibrational state of the neutral complex in question. For comparison with the experimental spectrum, the calculated energies, which neglect spin−orbit coupling in the neutral I···CO complex and hence are the barycenter of the spin−orbit doublet, are split by the experimental splitting of the 2P3/2 and 2P1/2 states (7603 cm−1 or 0.943 eV).51 The 2P3/2 state therefore is shifted 0.315 eV below the CCSD(T) energy, while the 2P1/2 is shifted 0.628 eV above. A second shift was applied to the 2P3/2 and 2P1/2 energies, determined by the amount required to bring the predicted electron binding energies of the bare I− anion in line with the experimental photoelectron spectrum. This procedure yielded electron detachment energies of 3.11 and 4.06 eV to the 2 P3/2 and 2P1/2 states, that is, energy separations between the I−···CO anion and the spin orbit states of the I···CO neutral complex. Barrier to CO Internal Rotation in the I···CO Complex. To estimate the barrier for interconversion between the I···CO and I···OC minima, a relaxed potential energy scan was performed along the rotation coordinate at the CCSD(T) level of theory with aug-cc-pVTZ (PP) basis sets. The result is

Figure 3. Relaxed potential energy scan describing the internal rotation of CO. The dashed line indicates the geometry of the I−···CO anion complex.

carbon atom of CO. The barrier on going from I···CO to I···OC is predicted to be 6.3 kJ·mol−1 (525 cm−1). Also shown on the figure is the angle for the minimum energy geometry of the anion complex. One can see that photodetachment will access the region of the potential quite near to the barrier, and hence, an extended progression in the bending mode is expected. Natural Bond Order (NBO) Analyses. To assess the perturbation of the CO molecule from interaction with the anionic and neutral iodine, we have performed NBO analyses that reveal the populations in the molecular orbitals and the principal electron delocalization from the iodide/iodine to CO. The population in the CO antibonding orbitals increases by around 18 milli-electrons (me) for the anion complex and only 1 me for the neutral. To ensure that the effect seen for the anion complex is not solely due to polarization of the CO molecule resulting from the presence of the negatively charged iodide anion, we replaced the anion with a point negative charge and performed a further NBO analysis. In this case, the increase in the CO antibonding orbitals was less than 1 me. The differences in the CO antibonding orbital population for the anion and neutral I···CO complexes is reflected in a smaller CO bond length for the neutral complex. Photoelectron Spectra. Experimental photoelectron spectra of the iodide−carbon monoxide clusters, I−···CO, with n = 1−4, recorded with an excitation wavelength of 266 nm (4.66 eV), are shown in Figure 4. Positions of the spectral features are provided in Table 2. For comparison, the spectrum of the bare iodide anion is provided and shows transitions to the 2P3/2 and 2 P1/2 states of the bare iodine atom. Considering the degeneracy of these two states, the ratio of the peak intensities should be 2:1 (2P3/2/2P1/2), however, due to detection difficulties associated with low energy electrons in our setup, the 2P1/2 component has reduced intensity. Iodide−Carbon Monoxide Dimer Complex: I−···CO. Formation of the complex between the iodide anion and carbon monoxide results in the observed shift of the two spin orbit states (2P3/2 and 2P1/2) to higher electron binding energy (eBE) compared to the bare iodide anion, while the separation between the states remains constant. The shift to higher energy 3580

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excellent agreement with the value of 3.15 eV reported previously by Rafaey from ion−molecule collision data between I− and COS.35 The stick spectra associated with the n = 1 spectrum in Figure 4 (red lines) corresponds to the predicted adiabatic electron binding energies at CCSD(T)/aug-cc-pVTZ. Again, this is calculated by splitting the ab initio CCSD(T) result (which neglects spin−orbit coupling) by the experimental iodine spin−orbit splitting and shifting the energies by the same amount needed to bring the calculated eBE of the bare iodide anion in line with the experiment. The agreement is quite good and indicates that the spectrum is indeed due to photodetachment from a van der Waals style I−···CO complex to the analogous I···CO neutral complex. The predictions for detachment to the I···CO and I···OC neutral complexes are within 7 meV of each other, and hence, only the I···CO result is shown. Photoelectron Spectra of I−···(CO)n. The spectra of the larger clusters allow us to follow the solvation of the iodide by carbon monoxide. From Figure 4 it is clear that an increased number of attached CO molecules about the iodide anion results in a steady shift of the spectral features to higher electron binding energy. The positions of the 2P3/2 bands are provided in Table 2. Again, it is appropriate to think of the electron binding energy as the difference in the dissociation energies of the anion and neutral clusters. We were unfortunately constrained to a maximum cluster size of n = 4 due to difficulties in producing sufficient quantities of the larger clusters, and uncertainty in cluster identity as the ion source also produces I−···CH3I complex, which is only separated by 2 amu from the I−···(CO)5 cluster. As a result, we can only set a lower bound on the number of CO ligands that might be accommodated in the first solvation shell around the iodide anion. It would be desirable to extend this study if it is possible to make adequate quantities of the larger clusters, that is, n > 4, and from the spectra, the onset of a second solvation shell would be evident by a discontinuity in the incremental stabilization energy. Vibrational Structure. Vibrational structure associated with the 2P3/2 band is evident in the spectra and is more clearly illustrated in Figure 5, which is an expanded view of the

Figure 4. Photoelectron spectra of I−···(CO)n clusters, with n = 1−4. Spectra were recorded with an excitation energy of 4.66 eV. For n = 1, stick spectra are results of CCSD(T) calculations, see text for details.

Table 2. Photoelectron Spectra Band Positions of the I−···(CO)n Clustersa 2

n

0-0

0 1

3.06 3.16 3.11 3.21 3.28 3.33

2 3 4

2

P3/2

1-0

2-0

3.40

3.65

3.46 3.55 3.64

3.75 3.83 3.90

3-0

4.02

P1/2

0-0 4.01 4.11 4.06 4.16 4.25

Estab/meV 100 50 70 50

In units of eV. The uncertainty in the values is ±0.05 eV. Also provided are the incremental stabilization energies, Estab. Numbers in italics for n = 1 are results from CCSD(T) calculations. a

is a direct result of the additional stabilization of the anion afforded through complex formation with the carbon monoxide molecule. The shift is in fact due to the difference in the dissociation energies (Do) of the anion and neutral complexes, whereby if Do were the same for both the anion and the neutral complexes, then the result would be no shift of the photoelectron band compared with the bare I− anion. The neutral complex, however, has a smaller Do, as it is bound by dispersion forces only, while for the anion complex there are additional charge−dipole, charge−quadrupole, charge−octopole, and charge-induced dipole interactions. As shown earlier, from the ab initio calculations, Do = 7.3 and 3.2 kJ·mol−1 at the CCSD(T)/aug-cc-pVTZ level of theory for the anion and neutral complexes, respectively. The electron affinity of the I···CO complex can be taken directly from the spectrum and is defined as the energy difference (adiabatic) between the I−···CO and I···CO complexes. The EA is therefore (3.16 ± 0.05) eV, which is in

Figure 5. Photoelectron spectrum of the I−···(CO)4 cluster showing the vibrational progression in the CO stretching mode.

I−···(CO)4 spectrum. The spacing of the bands of the vibrational progression is approximately 0.25 eV (∼2000 cm−1) and is, therefore, assigned to a progression in the CO stretching mode, that is, νCO, 2νCO, 3νCO, and so on. A full list of the observed features for each cluster size is provided in Table 2. No such structure is evident for the 2P1/2 component, 3581

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size of the cluster increases, so too will the √n term in eq 4 and the progression will therefore be more pronounced as is observed in the experimental spectra. The enhancement of the progression is not as extreme as was observed in the case of the CO2 clusters, presumably due to the smaller change in the CO bond length compared with the change in the CO2 angle for the I−···(CO2)n clusters. Franck−Condon Simulations. Franck−Condon overlap simulations were performed using the calculated ab initio vibrational frequencies as input (at the CCSD(T)/aug-ccpVTZ level) in the PGopher program.53 We performed the simulation using only the carbon monoxide stretch, as the intermolecular stretching and bending vibrations were not resolved in the spectra. Stick spectra produced from the Franck−Condon analysis were convoluted with Gaussian functions to better approximate the form of the spectra, the results are shown in Figure 6. We note that the experimental

which is presumably due to the much smaller intensity of this band, and hence, any vibrational structure is below our detection limits. The progression becomes more intense for the larger clusters, as shown in Figure 5. Two important questions are raised by the appearance of the vibrational structure. First, how can the progression be rationalized in terms of the geometries of the anion and neutral complex? And second, what is the reason behind the increased intensity of the progression for the larger clusters? To answer the first question, we rely on the ab initio calculations that reveal that on going from the anion to the neutral geometry, the major transformation is internal rotation of the CO molecule. One might therefore expect that the dominant vibrational progression would involve the intermolecular bending mode. Unfortunately, with the resolution of the current setup, we are unable to see evidence of this progression with the mode predicted to be ∼45 cm−1 from CCSD(T) calculations. To observe such a progression, the resolution of the spectrometer would need to be better than 6 meV. Looking to the ab initio calculations, the CO bond length reduces by 0.004 Å on going from the anion to the neutral complex, and it is presumably this difference that leads to the progression observed in photoelectron spectra, as the peak spacing of the progression is appropriate for the CO stretch mode. As stated earlier, the reduction in the CO bond length and, hence, the progression, can be rationalized as a decrease in the electron population in the CO antibonding orbitals on going from the anion complex to the neutral complex. Naively, one might explain the enhanced vibrational progression as a result of a larger displacement along the normal coordinate of vibration on going to larger cluster sizes. This would suggest that the CO subunits are more distorted upon increased anion cluster size. This explanation fails, however, as the perturbation of the individual CO molecules in the I−···(CO)n cluster will be the same, or even reduced, upon increased cluster size. To rationalize the enhanced CO stretching progression, we can draw upon similar observations in the photoelectron spectra of iodide−carbon dioxide clusters reported by Arnold et al.,52 where an increased CO2 bending progression was observed for larger I−···(CO2)n clusters. The enhanced progression was rationalized by treating the n vibrating CO2 molecules of the cluster as an ensemble of equivalent one-dimensional oscillators. Photodetachment excites the collective vibration in which all CO2 molecules vibrate in phase, and the extent of the vibrational progression is determined by the displacement along the normal coordinate for this in-phase vibration. The key result is that ΔQ n ≈

n ΔQ 1

Figure 6. Franck−Condon simulations (solid line) vs experiment (points) for I−···(CO)1−4. See text for details.

bands are not Gaussian in profile, attributed to the use of magnetic fields to collect the photoelectrons which leads to asymmetry in the band profiles. An initial estimate of the displacement along the normal coordinate of vibration for the CO stretch, ΔQ, was taken from the CCSD(T)/aug-cc-pVTZ calculations (predicted CO bond length change of −0.004 Å); however, this drastically underestimated the intensity of the 1-0 band in the progression. In fact, it was found that a value of Q = −0.04 Å was more appropriate in fitting the intensity of the 1 ← 0 transition. An answer to this issue may be found in the fact that the intensities of the progression are artificially enhanced due to saturation of the 0 ← 0 transition in the spectra, thereby accounting for the fact that the displacement predicted from CCSD(T) calculations is an order of magnitude too low. In the first instance, rather than using eq 4, the simulations were run to provide the best fit with the 1 ← 0 band, and the

(4)

where ΔQn is the normal coordinate displacement for the size “n” cluster, ΔQ1 is the normal coordinate displacement for the 1:1 dimer, and n is the cluster size. In the case of CO2, if it is assumed that the OCO bond angle in the anion clusters is independent of n, and equal to 180° in the neutral clusters, then longer vibrational progressions are predicted as n increases. We can address the I−···(CO)n clusters in a similar fashion, although in this instance the normal coordinate displacement is associated with the CO stretching mode. The harmonic approximation used in the case of the CO2 bending mode also applies for the I···(CO)n clusters due to the fact that we are dealing with low quanta in the CO stretch where the potential can be well described by the harmonic approximation. As the 3582

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displacements were found to be ΔQ = 0.04, 0.055, 0.06, and 0.07 for the n = 1−4 clusters. However, in doing so, the intensities of the higher order members of the progression were not well modeled, with the intensity underestimated in the simulation compared to experiment. If we take the displacement of ΔQ = 0.04 for the n = 1 complex and apply this to eq 4, then the anticipated displacements for the n = 2, 3, and 4 clusters are ΔQ = 0.056, 0.069, and 0.080 respectively. The agreement is satisfactory, leading us to believe that the simple model of an ensemble of harmonic oscillators is appropriate. One way to proceed would be to include Duschinsky mixing of the vibrational modes in the analysis; however, this is problematic as there is a large geometry change between the anion and neutral complex (i.e., from bent in the anion, to linear in the neutral complex), leading to issues with developing the Duschinsky matrix between initial and final states. Ideally, it would be better to perform wavepacket dynamics simulations on the neutral potential energy surface to determine the energy level structure and to model the spectra. This requires the production of reliable multidimensional potential energy surfaces, which requires considerable computational effort. Comparison of I−···(CO)n with Other Halide−Molecule Complexes. With these spectra, we are now able to compare the electron affinity of the I···CO neutral, and the stabilization energy and binding energy of the I−···CO dimer with other similar halogen−molecule complexes (Table 3). In the first

CONCLUSIONS AND SUMMARY Experimental photoelectron spectra were recorded for the iodide−carbon monoxide clusters, with up to four carbon monoxide molecules attached to an iodide anion. Upon increased cluster size, the spectral features shift to higher electron binding energy which is a result of the increased stabilization of the anion cluster compared with its neutral analogue. The spectra show a well resolved vibrational progression assigned to the CO stretching mode. This progression becomes more pronounced for the larger clusters. No closure of the first solvation shell of CO molecules around the iodide anion was evident. The electron affinities of the I···(CO)n clusters were reported, and where possible, for the 1:1 dimer complex, the comparison with the previously reported result is excellent. Ab initio calculations performed at MP2, CCSD, and CCSD(T) levels of theory were used to predict structures for the anion and neutral dimer complexes, that is, I−···CO and I···CO. The complexes are loosely bound with predicted binding energies of 7.3 and 3.2 kJ·mol−1, respectively. A second neutral minimum was located, with I···OC linear structure, and featured a binding energy of 1.6 kJ·mol−1. Predicted adiabatic photodetachment energies based on the energies of the predicted minima agreed well with experiment confirming that the spectral features are due to transitions from a van der Waals I−···CO anion to the analogous van der Waals neutral.

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EA neutral/eV

Estab anion/meV

Do anion/kJ·mol−1

3.83 3.16 3.172 3.224

160 90 115 172

14.6 7.3 11 15

Cl···CO I···COb I···N2Oc I···CO2c a

b

ASSOCIATED CONTENT

S Supporting Information *

Table 3. Electron Affinities (EA) of I···M Neutral Complexes, Stabilization Energies (Estab), and Binding Energies (Do) of I−···M Anion Complexes a

Article

Ab initio data for the anion and neutral complexes are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +61 8 6488 1005. Tel.: +61 8 6488 3178. E-mail: duncan. [email protected].

c

From ref 34. This work. From ref 52.

Notes

The authors declare no competing financial interest.

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−

instance, we note that the stabilization energy of the I ···CO system is nearly half what was observed for the analogous Cl−···CO complex, with 90 and 160 meV, respectively. The difference is primarily due to the smaller dissociation energy for I−···CO compared with Cl−···CO, predicted to be 14.6 and 7.3 kJ·mol−1 at the CCSD(T)/aug-cc-pVTZ level of theory. The stablization energy correlates strongly with the dissociation energy (Do) of the anion−molecule complex. As discussed earlier, this is due to the larger Do for the anion complex compared with the neutral, driven by the nature of the intermolecular interaction. The attraction between the charged species and the various electric moments on the ligands will far surpass the dispersion interaction which operates in the neutral complexes. While we did not observe vibrational structure for the Cl−···CO complex, it is conceivable that the progression may become more pronounced for the larger Cl−···(CO)n clusters as it did for I−···(CO)n. Efforts are underway in our laboratory to record spectra for the larger Cl−···(CO)n clusters. As the binding energy of the chloride complex is larger than for the iodide complex, we are in a better position to produce the larger Cl−···(CO)n clusters in greater abundance. This will allow us to follow the trend in the vibrational progressions further than we were able to in the case of I−···(CO)n.

ACKNOWLEDGMENTS K.M.L. acknowledges the support of an Australian Postgraduate Award (APA). We acknowledge financial support from the Faculty of Life and Physical Sciences, the School of Biomedical, Biomolecular, and Chemical Sciences, and a UWA research development award\. D.A.W. thanks Prof. Evan Bieske (Melbourne University) for very useful discussions on the electrostatic and inductive modeling. The ab initio calculations reported in this publication were performed using time allocated under the Merit Allocation Scheme from the NCI National Facility, Project r65.

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