Halide–Nitrogen Gas-Phase Clusters: Anion Photoelectron

Aug 24, 2015 - The gas phase anion photoelectron spectra are presented for the halide–nitrogen clusters X–···(N2)n, where X = Br and I and n â‰...
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Halide−Nitrogen Gas-Phase Clusters: Anion Photoelectron Spectroscopy and High Level ab Initio Calculations Kim M. L. Lapere, Marcus Kettner, Peter D. Watson, Allan J. McKinley, and Duncan A. Wild* School of Chemistry and Biochemistry, The University of Western Australia, Crawley, Western Australia, 6009, Australia S Supporting Information *

ABSTRACT: The gas phase anion photoelectron spectra are presented for the halide−nitrogen clusters X−···(N2)n, where X = Br and I and n ≤ 5. Electron binding energies for each cluster in the halide series are determined, with no evidence observed for first solvation shell closure in either series. High level ab initio calculations at the CCSD(T) level of theory are presented for the anion and neutral halogen−nitrogen complexes. For the anion species, two minima are predicted corresponding to a loosely bound C2v “T-shaped” species and to a higher energy covalently bound “triangle” C2v symmetry geometry. For the neutral species, three stationary points were located, two of which display similar form to the anion minima and a third which is linear, i.e., C∞v symmetry. The “T-shaped” geometry is a transition state linking equivalent C∞v symmetry minima. Cluster dissociation energies (D0) were determined, for both anion and neutral global minima at the CCSD(T) complete basis set limit, to be 7.8 kJ mol−1 and 7.0 kJ mol−1 and 3.5 kJ mol−1 and 5.0 kJ mol−1 for the bromine and iodine species, respectively.

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INTRODUCTION Spectroscopic investigations of gas phase ion−molecule clusters offer the opportunity to gain insight into the driving forces behind ion solvation. The beauty of these pursuits lies in the combination of mass spectrometry with various flavors of spectroscopy, whereby the mass selection gives great confidence that the correct species is being targeted and additionally allows one to follow the inception of solvation by probing each cluster size in turn. Mass selective anion photoelectron spectroscopy is one such avenue whereby UV−vis radiation detaches an electron that carries with it information on the electronic, vibrational, and possibly rotational energy levels of the analogous neutral complex. This has been an active area of study, and we provide recent illustrative examples in refs 1−8. In this contribution photoelectron spectra are presented for the halide−N2 anion gas phase clusters X−···(N2)n and accompanying high level ab initio calculations at the CCSD(T) level for the 1:1 X−···N2 anion and X•···N2 neutral complexes. Despite the long history of ion−molecule cluster research, the halides interacting with molecular N2 have been scarcely studied. Hiraoka et al. perfectly summed up the situation in their investigation of the fluoride anion clustering with N2, O2, and CO when they commented that “it is surprising that these fundamental cluster ions have not been studied yet.”9 Indeed aside from the Hiraoka study, only one other theoretical study on F−···N2 clusters was found in a search of the literature, by Matsubara and Hirao.10 There have, however, been numerous studies conducted on the collisions between halides and N2 © 2015 American Chemical Society

toward understanding the cross sections for ion−molecule reactions.11−16 In their study, Hiraoka et al. utilized pulsed electron beam mass spectrometry and MP2 calculations to probe the fluoride−nitrogen clusters F−···(N2)n with n = 1, 2. Hybrid basis sets for fluoride and nitrogen were chosen for the geometry optimizations and frequency calculations at the MP2 level of theory, while MP4 single point energies were used for computing cluster binding energies. The authors determined that the binding energy of F−···N2 was 14.3 kJ mol−1 and was more strongly bound than F−···O2 (11.9 kJ mol−1). The difference in these binding energies cannot be explained in terms of the polarizability of the two molecules, as they are of the same order (1.6 Å3 for O2, 1.7 Å3 for N2); however, if one considers that the quadrupole moment of N2 is three times that of O2, it is perhaps not so surprising, as the charge−quadrupole interaction will dominate the inductive interaction.17 The geometry optimization resulted in C2v symmetry structure for the singly solvated complex. Another finding of the Hiraoka study was a large change in the van’t Hoff plots of n = 1 and n = 2. The −ΔH0n=1,2 for a second N2 molecule binding to form F−···(N2)2 is given as 11.7 kJ mol−1, while for −ΔH0n=0,1 it was 18.8 kJ mol−1. The authors suggested that there is a slight charge transfer of the fluoride to the nitrogen indicative of the formation of an incipient covalent Received: July 2, 2015 Revised: August 22, 2015 Published: August 24, 2015 9722

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The Journal of Physical Chemistry A bond for n = 1. Once a second nitrogen solvates the fluoride, the interaction becomes electrostatic and the symmetry was determined to be D2d. Although long-range interactions were demonstrated, the F−−N distance decreased slightly with the second nitrogen molecule complexing. Some time after the work of Hiraoka et al. a DFT study on F−···N2 (alongside other anions and neutral molecules) was conducted by Matsubara and Hirao using the B3LYP functional.10 They concluded that the charge transfer from the fluoride to nitrogen occurs through the π* orbital, thereby overcoming any electronic repulsion. The calculated dissociation energy of 18.4 kJ mol−1 is close to that of Hiraoka et al., although no indication is made that the zero point vibrational energy differences between the cluster and separated moieties have been included, and therefore the value is assumed to be De. There is, however, a complete discord between the predicted geometries of the two studies, as Matsubara and Hirao calculated a structure of Cs symmetry, with much shorter F−−N distance of 2.501 Å vs 2.948 Å by Hiraoka et al. One would assume that the MP2 geometry would be more trustworthy considering B3LYP does not describe weak intermolecular interactions well, as highlighted in refs 18−20. The work presented in this article extends the experimental study of Hiraoka et al. on the F−···(N2)n clusters to the analogous Br− and I−···(N2)n clusters. We have applied time-offlight mass spectrometry to first identify the anion−molecule clusters in the gas phase for the first time and follow this with anion photoelectron spectroscopy to determine electron binding energies and cluster size dependent stabilization energies. In addition we accompany the experimental results with high level ab initio calculations at the CCSD(T) level of theory with triple-ζ basis sets. This approach incorporates geometry optimizations of anion and neutral halogen−N2 clusters, harmonic vibrational frequency calculations, and predicted electron detachment energies. We have also employed a complete basis set (CBS) extrapolation technique, aiming for more accurate calculated energies.

converted to kinetic energy (eKE), and subsequently converted to electron binding energy (eBE) using the following relation: eBE = hν − eKE

(1)

where hν is the photon energy being 266 nm, i.e., 4.66 eV. Spectra for each cluster size were recorded over several days and subsequently averaged, and each individual spectrum was collected over 10 000 laser shots. Calibration was achieved by utilizing the known eBE of the bare halide anions versus the experimental electron TOF. Following on, a Jacobi transform (dt → dE) corrected for the conversion from time-of-flight binning to energy binning of the photoelectrons, i.e., multiplying the intensities by t3. The resolution of the photoelectron spectrometer is most influenced by the velocity of the anion, with the spread in photoelectron energies given by22 dEe = 4

me EeE I mI

(2)

with Ee being the electron kinetic energy in the center of mass, EI is the ion kinetic energy, while me and mI are the masses of the electron and ion, respectively. The beam energy is around 1000 eV, and therefore, an electron with 1.05 eV kinetic energy emitted from the chloride ion following absorption of a 4.66 eV photon would feature an energy spread of dEe = 0.51 eV. The resolution does improve for heavier species with subsequent lower velocities; however, at present we are limited to a minimum beam energy of 1000 eV. Computational Methods. CCSD(T) ab initio calculations were employed for the halogen−nitrogen anion and neutral 1:1 complexes. Dunning’s triple-ζ augmented correlation consistent basis set was used for nitrogen (aug-cc-pVTZ),23 while for bromine and iodine the aug-cc-pVTZ PP basis sets24,25 were used which reduced the time needed for the calculations and additionally accounted for relativistic effects, as they consist of small-core relativistic pseudopotentials adjusted to multiconfiguration Dirac−Hartree−Fock data based on the Dirac− Coulomb−Breit Hamiltonian. A series of MP2 calculations were also undertaken, with basis sets up to quadruple-ζ quality. The results from this work are provided in ref 26 and were entirely consistent with the higher level CCSD(T) calculations presented here. The CCSD(T) calculations were based on either a restricted or unrestricted Hartree−Fock reference wave function, RHF for the closed shell anion species, and UHF for the open shell radicals. Only the valence electrons were treated. For the geometry optimizations, tighter convergence criteria were applied being 1 × 10−8 hartree/bohr due to the low binding energies of the complexes and the associated flat and shallow potential energy surfaces. For each stationary point, a vibrational frequency analysis was performed to determine the nature of the point, i.e., whether they represented minima, transition states, or high order stationary points. The electronic energies of the bare halide anions, neutral radicals, and the bare nitrogen molecule were also calculated in order to determine the cluster binding energy De, while the harmonic zero point energies were used to determine D0. Finally, in order to improve the cluster binding energies, we have employed a complete basis set limit extrapolation similar to W1 and W2 theory.27 Single point energy calculations at the cluster geometries are performed with quadruple and quintuple-ζ basis sets, and the two-point extrapolation formula applied was E(L) = E∞ + A/Lα (α = 5 for SCF extrapolation and α = 3 for

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METHODOLOGY Experimental Methods. The apparatus used for the anion photoelectron spectroscopy experiments at UWA comprises a TOF mass spectrometer after the Wiley and McLaren design21 in combination with a magnetic bottle photoelectron spectrometer.22 The anion−molecule clusters were synthesized by bombarding a pulsed supersonic expansion of a prepared gas mixture with electrons. The constituents of the gas determine the nature of the ion−molecule clusters formed, and for the halide−nitrogen clusters the mixture comprises nitrogen and argon in a 1:10 ratio, with the source of halogen being trace amounts of CH3I or CH2Br2. The total pressure of the gas mixture was 400 kPa. The specific X−···N2 cluster to be interrogated was intersected with a 5 ns pulse of 266 nm radiation (4.66 eV, fourth harmonic of a Nd:YAG laser, Spectra Physics Quanta Ray Pro) while the ions were located in a strongly divergent magnetic field. Ejected photoelectrons are guided, by application of a second magnetic field, along a 1.8 m flight tube and ultimately sensed by a microchannel plate detector. Improved detection efficiency of low energy electrons is achieved by application of a potential between the biased front face of the detector (+200 V) and an electrically grounded mesh situated 2 cm in front of the detector. The time of flight of the photoelectrons referenced to the laser pulse is recorded, 9723

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The Journal of Physical Chemistry A CCSD and (T) extrapolations). The quantum chemical calculations were performed using the Gaussian 09 package.28

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RESULTS AND DISCUSSION Computational Results. Electrostatic and Induction Modeling. Before considering the high level ab initio calculations, it is instructive to consider which long-range interactions dominate in the formation of a loosely bound complex between an atomic anion and neutral nitrogen molecule. The interaction, where the anion is defined as a point negative charge, can be cast in terms of the multipole expansion of the molecule and consists of the charge− quadrupole and charge−hexadecapole electrostatic interactions, combined in Velec in eq 3a. In addition, there is the chargeinduced dipole interaction, Vind in eq 3b, and hence the total interaction is the sum of these two parts. Velec(R ,θ ) =

q ⎛ Θ(3 cos2 θ − 1) ⎜ 4π ϵ0 ⎝ 2R3 +

Vind(R ,θ ) = −

Φ(35 cos 4 θ − 30 cos2 θ + 3) ⎞ ⎟ 8R5 ⎠

(3a) Figure 1. Sum of the charge-induced dipole, charge−quadrupole, and charge−hexadecapole terms for an anion point charge interacting with molecular nitrogen. The charge−quadrupole interaction is shown to be dominant, as the minimum is around 90°. Darker shading represents lower energy, with contours separated by 100 cm−1.

q2(α cos2 θ + α⊥ sin 2 θ ) 2(4π ϵ0)2 R4

(3b)

In the equations, q is the charge on the anion, Θ the quadrupole moment, Φ the hexadecapole moment, R is the distance between the anion and the midpoint of the diatomic N2, and θ the angle between the line defined by R and the internuclear axis. Values for the quadrupole and hexadecapole moment are taken from the work of Maroulis, with Θ = −4.994 × 10−40 C m2 and Φ = −8.493 × 10−60 C m4.29 The values used for nitrogen’s dipole polarizabilities parallel (α ∥ ) and perpendicular (α⊥) to the internuclear axis were 2.436 × 10−40 C2 m2 J−1 and 1.670 × 10−40 C2 m2 J−1, respectively. A contour plot for the total interaction, i.e., the sum of the charge−quadrupole, charge−hexadecapole, and charge-induced dipole, is presented in Figure 1. The plot axes correspond to the angular and radial coordinates, and the analysis suggests a minimum would occur for an angle of 90°. The anisotropy of the molecular polarizability (α∥ > α⊥) would result in a linear complex; however, this interaction is dominated by the charge− quadrupole interaction, thereby leading to a “T-shaped” structure. CCSD(T) Calculations. We now turn our attention to the high level ab initio CCSD(T) calculations, beginning with the anion systems (singlet multiplicity). An extensive search of the potential energy surface of the Br−···N2 and I−···N2 species revealed two stationary points for each halide−nitrogen complex, corresponding to a “T-shaped” C2v geometry featuring a rather large intermolecular separation, and a higher energy covalently bound “triangle” C2v structure. Geometrical parameters for the global minimum, being the “T-shaped” complex, are presented in Table 1 alongside cluster binding energies De and D0, and we have provided additional data as Supporting Information including electronic energies, vibrational mode frequencies and assignments, and zero point energies. The form of the cluster geometries are represented in Figure 2. The covalently bound “triangle” C2v symmetry complex lies at a much higher energy than the other C2v van der Waals style complex (837 and 797 kJ mol−1 for the bromide and iodide species, respectively), and indeed they both lie above the

Table 1. Geometrical Parameters of the C2v Bromide−N2 and Iodide-N2 Gas Phase Anion Complexes from CCSD(T) Calculationsa

Br···N2 I···N2 N2

rX···|||b

rX−N

rNN

∠X−N−N

De

D0

[Å]

[Å]

[Å]

[deg]

[kJ mol−1]

[kJ mol−1]

3.732 4.019

3.773 4.057

1.104 1.104 1.104

81.6 82.2

8.5 7.6

7.8 7.0

a De values are derived from CCSD(T)/CBS results, while D0 utilizes the CCSD(T)/aPVTZ computed zero point energies. b||| is the midpoint of the NN bond.

energy of separated ground state halide and nitrogen dissociation products. These complexes were located following a potential energy surface scan whereby the intermolecular separation between the halide and nitrogen moieties was reduced, which led to a jump to the excited state potential energy surface. The dissociation asymptote of the complex corresponds to the separated products being the ground state halide anion (1S0) and excited state nitrogen in the 1Πg state. Attempts to locate a linear stationary point showed that the potential energy surface was dissociative along this coordinate. This is entirely consistent with the results from the electrostatic and inductive modeling discussed earlier, as the interaction is repulsive at an angle of 0° whereas it is attractive at an angle of 90° (see Figure 1). We note that the predicted minima for the bromide and iodide−N2 complexes, i.e., “T-shaped” C2v symmetry, are consistent with the MP2 results of Hiraoka et al. for the fluoride−N2 system9 and with the MP2 predictions presented in ref 26. However, our CCSD(T) predictions are not consistent with the geometry put forward by Matsubara and Hirao who used the B3LYP functional.10 The reasons for the discrepancy have been explained earlier in the Introduction of this paper, i.e., that the B3LYP functional does not describe 9724

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With reference to Tables 1 and 2, the perturbation on the molecular structure of nitrogen by the halogen radical or anion is negligible, which is evident when one compares the NN bond length in the free and the complexed N2 computed at the same level of theory. There is essentially no difference in bond length, which indicates that the intermolecular interaction is rather weak, supported by the predicted cluster binding energies being all below 5 kJ mol−1 for the neutral complexes, while for the anion species the binding energies are below 8 kJ mol−1. The cluster binding energies (De) were predicted from the two-point extrapolation technique discussed earlier, while estimates for D0 employed zero point energies calculated at the CCSD(T)/aug-cc-pVTZ level of theory (the aug-cc-pVTZ PP basis sets were used in the cases of Br and I). The complex binding energies (D0) of the neutral linear complexes were predicted to increase for increased halogen size, being 3.5 and 5.0 kJ mol−1 for Br and I, respectively. This can be rationalized if one considers that the attraction between the halogen and nitrogen is due to dispersion and therefore is driven by the magnitude of the polarizabilities of the species, whereby for the neutral halogen atoms the polarizabilities are 21.8au and 34.6au (where au ≡ e2a02Eh−1) for bromine and iodine, respectively,30 while there is a large polarizability anisotropy for nitrogen whereby α⊥ = 2.436 × 10−40 C2 m2 J−1 and α∥ = 1.670 × 10−40 C2 m2 J−1, which would lead to a linear geometry being favored. The trend in the binding energies of the anion−N2 species can be rationalized in terms of the size of the anion, i.e., a steric bulk argument whereby the distance to which the N2 can approach the anion is increased for iodide compared with bromide. Considering that the interaction is dominated by the charge−quadrupole interaction, which has a 1/R3 dependence, this leads to the lower binding energy of 7.8 and 7.0 kJ mol−1 for Br and I, respectively, from CCSD(T)/CBS calculations. While there is no prior experimental or theoretical data on the cluster binding energies for the bromide or iodide−N2 clusters, we can comment on the trend from fluoride to bromide to iodide. Hiraoka determined the fluoride complex binding energy to be 14.3 kJ mol−1 from MP2 calculations, and we have briefly readdressed the C2v “T-shaped” anion complex at the CCSD(T) level with aug-cc-pVTZ basis sets for fluoride and nitrogen. Our computed D0 using the same procedure as described previously for the bromide and iodide cases (i.e., CCSD(T)/CBS with CCSD(T)/aug-cc-pVTZ zpe) is 15.7 kJ mol−1, which is in line with Hiraoka’s MP2//MP4 calculations. We should also mention that the optimized geometry for this fluoride complex is also in agreement with the MP2 calculations for the bromide and iodide clusters presented in ref 26. Predicted Electron Detachment Energies. To conclude the computational results, the electronic and zero point energies predicted for the anion and neutral complexes allow us to predict the adiabatic electron detachment energy (ADE). To begin with, we calculated the vertical detachment energies for the anion “T-shaped” complexes by performing single point energy calculations on the neutral complex (doublet multiplicity) at the geometry of the anion complex (singlet multiplicity). For both the bromide and iodide complexes the predicted vertical detachment energy is lower than adiabatic detachment energy, and hence the adiabatic detachment energy is compared with the experimental spectra. The predicted adiabatic detachment energy from the anion to each of the neutral minima is provided in Table 3 and will be discussed later with reference to the experimental spectra.

Figure 2. Stationary points predicted from CCSD(T) calculations of the neutral and anion halogen−nitrogen complexes. Structural parameters are provided in Tables 1 and 2 for the global minimum for both the anion and neutral. Anion: T and Tri complexes are both minima. Neutral: Tri and Lin are minima, while T is a transition state.

weak intermolecular interactions well, as highlighted in refs 18−20. For the neutral X···N2 complexes (doublet multiplicity) three stationary points were located on the potential energy surface corresponding to a linear C∞v symmetry geometry, a “Tshaped” C2v symmetry geometry, and a higher energy covalently bound C2v symmetry geometry. Data for all three species are provided in the Supporting Information, and as for the anion species while it was determined that the covalently bound stationary point for both the bromide and iodide complexes are minima, they are again predicted to lie to higher energy 512 and 432 kJ mol−1 above the global minimum for the bromine and iodine complexes, respectively, and hence will not be discussed further. Of the remaining two stationary points located on the neutral potential energy surface, the linear C∞v complex is confirmed to be a minimum whereas the “T-shaped” complex is a transition state featuring an imaginary mode that corresponds to internal rotation of the N2 toward the linear structure. Geometrical parameters for the X····N2 C∞v complexes and cluster binding energies (De, D0), are presented in Table 2, and additional data on all three neutral stationary points are provided in the Supporting Information, including absolute electronic energies, vibrational mode frequencies and assignments, and zero point energies. Again, the form of the linear, “T-shaped”, and covalently bound geometries are represented in Figure 2. Table 2. Geometrical Parameters of the C∞v Bromine−N2 and Iodine−N2 Gas Phase Neutral Complexes from CCSD(T) Calculationsa

Br···N2 I···N2 N2

rX···N

rNN

De

D0

[Å]

[Å]

[kJ mol−1]

[kJ mol−1]

3.230 3.456

1.104 1.104 1.104

4.3 5.8

3.5 5.0

a

De values are derived from CCSD(T)/CBS results, while D0 utilizes the CCSD(T)/aPVTZ computed zero point energies. 9725

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The Journal of Physical Chemistry A Table 3. Experimental Photoelectron Band Positions for the Halide−Nitrogen Clusters, in eV, and Stabilization Energies in meVa Br−···N2 n

2

2

0 1

3.39 3.43 3.41 3.50 3.58 3.64 3.70

3.86 3.92 3.86 3.98 4.08 4.14 4.20

2 3 4 5

P3/2

P1/2

I−···N2 Estab 40 70 80 60 30

2

P3/2

3.05 3.07 3.08 3.13 3.15 3.19

2

P1/2

3.87 3.92 4.02 3.95 4.00 4.03

Estab 20 60 20 40

a

The band positions have a conservative error of 0.05 eV. Italicized values are predicted ADEs.

It is important to note that the CCSD(T) calculations do not take into account the two spin−orbit states of the neutral halogen, so in order to better compare the predictions with experiment, the known spin−orbit coupling constants of bromine and iodine were employed to produce transitions from the anion to these two states, i.e., the 2P3/2 ← 1S0 and 2 P1/2 ← 1S0 transitions.31,32 In addition, a second shift was applied that accounts for any inadequacies of the CCSD(T) level of theory and was determined by taking the differences between the well-known experimental electron detachment energies of the bare anions and the value predicted from the current CCSD(T) calculations. The correction is seen to decrease as the basis set size is increased and is quite small when the CBS limit extrapolated single point energies are used in the comparison with experiment. The corrections for bromide and iodide are 0.006 and 0.000 eV, respectively. We provide the data for each basis set in the Supporting Information. Because of the large change in geometry between the anion and neutral complexes, i.e., “T-shaped” for the anion and linear for the neutral, we were not able to simulate the photoelectron spectrum, with vibrational structure, using ezSpectrum33 as we recently did for the halide−acetylene complexes.34 Experimental Results. Mass Spectrometry. We begin our discussion of the experimental results by presenting a representative mass spectrum for the bromide−nitrogen anion clusters, shown in Figure 3. While we were able to produce clusters with up to seven nitrogen monomers attached to the bromide core, the number densities of the larger clusters were too low to allow for photoelectron spectra to be recorded. In addition to the highlighted bromide−nitrogen series, we direct the reader’s attention to the triplet of peaks at 172, 174, and 176 amu with intensity ratio of 1:2:1. Because of both the masses of these peaks and the characteristic peak intensities, the features are assigned to the isotopologues of the CH2Br2•− radical anion. This is an interesting species and represents an example of a radical anion that should have undergone dissociative electron attachment to form Br− and CH2Br• products. Indeed it is this reaction that is the source of the bromide anions essential for the synthesis of the Br−···(N2)n clusters. There is precedence in the literature for the production of radical anion complexes that survive the dissociative attachment process, and their existence is attributed to the stabilizing effect of argon in the supersonic gas expansion. Johnson and co-workers showed evidence of the CH3I•− species through mass spectrometry and photoelectron spectroscopy, and the form of the photoelectron spectrum

Figure 3. Mass spectrum of the Br−···(N2)n clusters. Also shown are peaks attributed to residual chloride and iodide.

implied that the complex is a loosely bound I−···CH3• anionradical complex.35 We have recorded the photoelectron spectrum of the CH2Br2•− complex, and this will form the basis of a future publication, accompanied by high level ab initio calculations on the anion−radical molecule complex. Other peaks present in the mass spectrum are adducts involving chloride: Cl−···H2O at 53 and 55 amu, Cl−···N2 at 63 and 65 amu, Cl−···Ar at 75 and 77 amu, and Cl−···(N2)2 at 91 and 93 amu. Other adducts of bromide are observed, i.e., Br−···H2O at 97 and 99 amu, Br−···Ar at 119 and 121 amu, and Br−···Ar·N2 at 147 and 149 amu. The multiplets of peaks at 205 and 231 and 251 amu correspond to isotopologues of Cl−···CH2Br2, Br−···CCl4, and Br−···CH2Br2 clusters, respectively. Photoelectron Spectroscopy. Returning now to the halide− nitrogen clusters that are the focus of this article, the experimental photoelectron spectra recorded using 266 nm radiation are shown in Figure 4. Data derived from the spectra

Figure 4. Anion photoelectron spectra of the bromide and iodide− nitrogen gas phase complexes, recorded with photon energy of 4.66 eV. Noise at high electron binding energy is magnified because of the Jacobi transform discussed in the section Methodology. 9726

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by considering that the predicted D0 values of 9.9 and 7.3 kJ mol−1 for the bromide and iodide−carbon monoxide complexes are close to those predicted for bromide and iodide−N2. The stabilization energies are also very similar, being 140 and 90 meV for bromide and iodide−CO, respectively.36,37

are provided in Table 3 and consist of electron binding energies for each cluster and stabilization energies. Because of the low resolution of the spectrometer, as discussed in the section Methodology, the spectra do not show vibrational progressions. We can, however, determine the shift in the electron binding energies upon complex formation and a concomitant increase in this shift for each nitrogen added to the anion core (with respect to the bare halide anion). As the spectrum for each cluster resembles the bare halide anion, it is clear that the anions and nitrogen molecules are tethered by noncovalent interactions. Each spectrum is best considered to be that of the halide anion perturbed by solvating nitrogen ligands. The noise at higher electron binding energy is a consequence of the Jacobi transform discussed in the section Methodology. The formation of a gas phase complex between the halogen anion and nitrogen molecule leads to the shift to higher electron binding energy (eBE) for the two spin−orbit states when compared with the bare anion, whereas the separation between the states is largely unaffected. The shift to higher energy results from the stabilization that the nitrogen molecule affords to the anion from forming the complex and alternatively can be explained as the disparity in the dissociation energies (D0) of the anion and neutral complexes. If the dissociation energies were similar for the anion and neutral complexes, then one would not observe a shift of the photoelectron bands from those of the bare X− anions. From the ab initio calculations the neutral complexes feature smaller D0 values, as they are bound by dispersion forces only, whereas the anion complexes are bound by additional electrostatic charge−quadrupole and charge-induced dipole forces. Referring to Tables 1 and 2, one can see the differences in the cluster binding energies, for example, for the Br−···N2 complex, D0 = 7.8 kJ mol−1 and is 3.5 kJ mol−1 for the C∞v for the neutral complex (CCSD(T)/CBS results). Looking to Table 3, the comparison between the predicted adiabatic detachment energies and experiment is very good, giving us confidence that we are producing van der Waals anion complexes in the ion source, whereby the anion is loosely bound to the N2 monomer. When one considers the stabilization energies as the cluster sizes increase, there is no dramatic jump which indicates that the first solvation shell has not closed. This puts a lower limit on the first solvation size of 4 and 5 for iodide and bromide, respectively. Halide−Nitrogen Complexes Compared with Similar Species. We can now compare the properties of the halide− nitrogen complexes with other weakly bound halogen− molecule species, in terms of the ab initio cluster binding energies, experimental electron affinities, and stabilization energies. Before doing so, we note that the effect of increased halide size is a decrease in the stabilization energies which is linked to the smaller dissociation energies for the anion complexes for larger halogen size, accompanied by an increase in the cluster binding energy for iodine neutral complex compared with the bromine complex. From Table 2, the CCSD(T)/CBS limit predicted binding energies are 7.8 and 7.0 kJ mol−1, while from Table 3 the stabilization energies are 40 and 20 meV. A correlation is observed between the experimental stabilization energy and the ab initio dissociation energy (D0) of the anion−molecule complex, which can be explained by D0 being larger for the anion compared with the neutral. Compared with the halide−carbon monoxide species reported previously by our group, the halide−nitrogen set displays similar stabilization energies which can be rationalized

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SUMMARY We have presented experimental photoelectron spectra for the halide−nitrogen clusters, with up to five nitrogen monomers surrounding the anion core. The larger binding energy of the anion cluster when compared with its neutral analogue leads to a shift of the spectral features to higher electron binding energy. The instrumental resolution meant that no vibrational resolution was observed; however, electron detachment energies were reported and therefore the electron affinities for the neutral complexes. Additionally, ab initio calculations performed at the CCSD(T) level of theory were used to predict structures for the anion and neutral dimer complexes, i.e., X··· N2. For the neutral radical species three stationary points were located, corresponding to a “T-shaped” geometry of C2v symmetry (transition state), a linear C∞v symmetry complex (the minimum), and a high energy “triangle” geometry of C2v symmetry lying some 800 kJ mol−1 above the linear stationary point. The binding energies of the neutral minimum (D0) is predicted to be 3.5 and 5.0 kJ mol−1 for the bromine and iodine species, respectively. For the anion systems two stationary points were located corresponding to a “T-shaped” geometry of C2v symmetry (minimum) and a high energy “triangle” geometry of C2v symmetry (also lying some 800 kJ mol−1 above the minimum). The predicted cluster binding energies for the anion species were 7.8 and 7.0 kJ mol−1 for the bromide and iodide, respectively. There was a very good agreement between the experimentally determined electron binding energies and the predicted adiabatic electron detachment energies from the “T-shaped” C2v symmetry anion complex to the linear C∞v symmetry neutral complex.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b06348. Results from ab initio calculations, including electronic energies, vibrational mode frequencies and assignments, zero point energies, and Cartesian coordinates for all stationary points (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 (8) 6488 3178. Fax: +61 (8) 6488 1005. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.M.L.L. and M.K. acknowledge the support of an Australian Postgraduate Award (APA). We acknowledge financial support from the Faculty of Science and the School of Chemistry and Biochemistry, and the Australian Research Council is acknowledged for funding the laser installation under the LIEF scheme (Grant LE110100093). 9727

DOI: 10.1021/acs.jpca.5b06348 J. Phys. Chem. A 2015, 119, 9722−9728

Article

The Journal of Physical Chemistry A

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(21) Wiley, W. C.; McLaren, I. H. Time of Flight Mass Spectrometer with Improved Resolution. Rev. Sci. Instrum. 1955, 26, 1150−1157. (22) Cheshnovsky, O.; Yang, S. H.; Pettiette, C. L.; Craycraft, M. J.; Smalley, R. E. Magnetic Time-of-Flight Photoelectron Spectrometer for Mass-Selected Negative Cluster Ions. Rev. Sci. Instrum. 1987, 58, 2131−2137. (23) Dunning, T. H., Jr. Gaussian Basis Sets for use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (24) Peterson, K. A.; Figgen, D.; Goll, E.; Stoll, H.; Dolg, M. Systematically Convergent Basis Sets With Relativistic Pseudopotentials. II. Small-Core Pseudopotentials and Correlation Consistent Basis Sets for the Post-d Group 16−18 Elements. J. Chem. Phys. 2003, 119, 11113. (25) Peterson, K. A.; Shepler, B. C.; Figgen, D.; Stoll, H. On the Spectroscopic and Thermochemical Properties of ClO, BrO, IO, and Their Anions. J. Phys. Chem. A 2006, 110, 13877−13883. (26) Lapere, K. M. Anion Photoelectron Spectroscopy of Halide Complexes and Clusters. Ph.D. Thesis, University of Western Australia, 2015. (27) Karton, A.; Martin, J. M. Explicitly Correlated Wn Theory: W1F12 and W2-F12. J. Chem. Phys. 2012, 136, 124114. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01; Gaussian Inc.: Wallingford, CT, 2009. (29) Maroulis, G.; Thakkar, A. J. Multipole Moments, Polarizabilities, and Hyperpolarizabilities for N2 From Fourth-Order Many-Body Perturbation Theory Calculations. J. Chem. Phys. 1988, 88, 7623− 7632. (30) Fleig, T.; Sadlej, A. J. Electric Dipole Polarizabilities of the Halogen Atoms in 2P1/2 and 2P3/2 States: Scalar Relativistic and TwoComponent Configuration-Interaction Calculations. Phys. Rev. A: At., Mol., Opt. Phys. 2002, 65, 032506. (31) National Institute of Standards and Technology, 2002, NIST Chemistry WebBook NIST Standard Reference Database Number 69, NIST,Gaithersburg, MD. http://webbook.nist.gov/ (accessed July 3, 2015). (32) National Institute of Standards and Technology, 2003, Handbook of Basic Atomic Spectroscopic Data, NIST Standard Reference Database 108, NIST,Gaithersburg, MD. http://www.nist. gov/pml/data/handbook/index.cfm (accessed July 3, 2015). (33) Mozhayskiy, V. A.; Krylov, A. I. ezSpectrum 3.0. 2009. http:// iopenshell.usc.edu/downloads (accessed July 3, 2015). (34) Beckham, D. A. R.; Conran, S.; Lapere, K. M.; Kettner, M.; McKinley, A. J.; Wild, D. A. Anion Photoelectron Spectra and High Level Ab Initio Calculations of the Halide-Acetylene Dimer Complexes. Chem. Phys. Lett. 2015, 619, 241−246. (35) Kim, J.; Kelley, J. A.; Ayotte, P.; Nielsen, S. B.; Weddle, G. H.; Johnson, M. A. Preparation and Photoelectron Spectrum of the CH3I− Anion: Rare Gas Cluster Mediated Synthesis of an Ion-Radical Complex. J. Am. Soc. Mass Spectrom. 1999, 10, 810−814. (36) Lapere, K. M.; LaMacchia, R. J.; Quak, L. H.; Kettner, M.; Dale, S. G.; McKinley, A. J.; Wild, D. A. The Bromide-Carbon Monoxide Gas Phase Complex: Anion Photoelectron Spectroscopy and Ab Initio Calculations. Aust. J. Chem. 2012, 65, 457−462. (37) Lapere, K. M.; LaMacchia, R. J.; Quak, L. H.; Kettner, M.; Dale, S. G.; McKinley, A. J.; Wild, D. A. Anion Photoelectron Spectra and Ab Initio calculations of the Iodide Carbon Monoxide Clusters: I−··· (CO)n, n = 1−4. J. Phys. Chem. A 2012, 116, 3577−3584.

REFERENCES

(1) Opoku-Agyeman, B.; Case, A. S.; Lehman, J. H.; Lineberger, W. C.; McCoy, A. B. Nonadiabatic Photofragmentation Dynamics of BrCN−. J. Chem. Phys. 2014, 141, 084305. (2) Martin, J. P.; Case, A. S.; Gu, Q. L.; Darr, J. P.; McCoy, A. B.; Lineberger, W. C. Photofragmentation Dynamics of ICN−(CO2)n Clusters Following Visible Excitation. J. Chem. Phys. 2013, 139, 064315. (3) Kim, J. B.; Weichman, M. L.; Neumark, D. M. Structural Isomers of Ti2O4 and Zr2O4 Anions Identified by Slow Photoelectron VelocityMap Imaging Spectroscopy. J. Am. Chem. Soc. 2014, 136, 7159−7168. (4) Weichman, M. L.; Kim, J. B.; Neumark, D. M. Rovibronic Structure in Slow Photoelectron Velocity-Map Imaging Spectroscopy of CH2CN− and CD2CN−. J. Chem. Phys. 2014, 140, 104305. (5) Graham, J. D.; Buytendyk, A. M.; Zhang, X.; Collins, E. L.; Kiran, B.; Gantefoer, G.; Eichhorn, B. W.; Gutsev, G. L.; Behera, S.; Jena, P.; et al. Alanate Anion, AlH 4 − : Photoelectron Spectrum and Computations. J. Phys. Chem. A 2014, 118, 8158−8162. (6) Buytendyk, A.; Graham, J.; Wang, H. P.; Zhang, X. X.; Collins, E.; Ko, Y. J.; Gantefoer, G.; Eichhorn, B.; Regmi, A.; Kiran, B.; Bowen, K. H. Photoelectron Spectra of the MgH− and MgD− Anions. Int. J. Mass Spectrom. 2014, 365−366, 140−142. (7) Popov, I. A.; Li, W. L.; Piazza, Z. A.; Boldyrev, A. I.; Wang, L. S. Complexes Between Planar Boron Clusters and Transition Metals: A Photoelectron Spectroscopy and Ab Initio Study of CoB12− and RhB12−. J. Phys. Chem. A 2014, 118, 8098−8105. (8) Erdogdu, Y.; Jian, T.; Lopez, G. V.; Li, W. L.; Wang, L. S. On the Electronic Structure and Chemical Bonding of Titanium Tetraauride: TiAu4 and TiAu4−. Chem. Phys. Lett. 2014, 610, 23−28. (9) Hiraoka, K.; Nasu, M.; Katsuragawa, J.; Sugiyama, T.; Ignacio, E. W.; Yamabe, S. How is the Fluoride Ion Bound to O2, N2, and CO Molecules? J. Phys. Chem. A 1998, 102, 6916−6920. (10) Matsubara, T.; Hirao, K. Density Functional Study of the Interaction of H2, N2, O2, CO, and NO Diatomic Molecules with H−, Li−, and F− Anions. Prediction of a New Type of Anion Cluster. J. Phys. Chem. A 2003, 107, 2505−2515. (11) Annis, B. K.; Datz, S.; Champion, R. L.; Doverspike, L. D. Role of Negative-Molecular-Ion Resonances in Collisional Detachment: Cl− + N2. Phys. Rev. Lett. 1980, 45, 1554−1557. (12) Doverspike, L. D.; Smith, B. T.; Champion, R. L. Total Electron Detachment Cross Sections for Collisions of Negative Halogen Ions with Various Molecules for Energies Around Threshold. Phys. Rev. A: At., Mol., Opt. Phys. 1980, 22, 393−398. (13) Vedder, M.; Doverspike, L. D.; Champion, R. L. Collisional Excitation in Negative-IonMolecule Systems. Phys. Rev. A: At., Mol., Opt. Phys. 1981, 24, 615−618. (14) Huq, M. S.; Doverspike, L. D.; Champion, R. L.; Esaulov, V. A. Total Electron Detachment Cross Sections for Collisions of H− and He and F− with Atomic and Molecular Targets. J. Phys. B: At. Mol. Phys. 1982, 15, 951−959. (15) Tuan, V. N.; Esaulov, V. A. Electron Detachment via Direct Coupling and Charge Exchange to Shape Resonances in Negative-IonMolecule Collisions. J. Phys. B: At. Mol. Phys. 1982, 15, L95−L100. (16) Tuan, V. N.; Esaulov, V. A.; Grouard, J. P.; Hall, R. I.; Montmagnon, J. L. Electron Detachment and Charge Exchange to Shape Resonances in F−-Molecule Collisions. J. Phys. B: At. Mol. Phys. 1984, 17, 2897−2912. (17) Greenhow, C.; Smith, W. V. Molecular Quadrupole Moments of N2 and O2. J. Chem. Phys. 1951, 19, 1298−1300. (18) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry. Acc. Chem. Res. 2008, 41, 157−167. (19) Grimme, S. Accurate Description of Van Der Waals Complexes by Density Functional Theory Including Empirical Corrections. J. Comput. Chem. 2004, 25, 1463−1473. (20) Goerigk, L.; Grimme, S. A Thorough Benchmark of Density Functional Methods for General Main Group Thermochemistry, Kinetics, and Noncovalent Interactions. Phys. Chem. Chem. Phys. 2011, 13, 6670−6688. 9728

DOI: 10.1021/acs.jpca.5b06348 J. Phys. Chem. A 2015, 119, 9722−9728