Zero-point Vibrational Effects Research Articles

Bonding in the Heavy Analogue of Hydrogen Cyanide: The Curious Case of Bridged HPSi

The bonding of firstand second-row elements differ dramatically. The simplest unsaturated silicon hydrides Si2H2 and Si2H4 exhibit quite unusual geometries [1] compared to the analogous hydrocarbon molecules. For example, the most stable form of Si2H2 is nonplanar with C2v symmetry and two bridging H atoms, in sharp contrast to linear acetylene, HC! CH. Phosphorus and nitrogen share many of the same bonding characteristics, but P prefers single over multiple bonds. For these reasons, it may be difficult to predict the most stable isomeric arrangement, even for a small molecule with a single Por atom and especially when it contains both. Silicon–phosphorus bonds are important in materials science and organometallic chemistry. Geometries for several-hundred larger molecules containing a phosphorus– silicon single bond have been determined by X-ray crystallography, but the uncertainties are typically 0.05!. With respect to phosphorus–silicon multiple bonds, it appears that sterically stabilized phosphasilenes R2Si=PR’ are the only compounds that have been isolated to date. The precise structures have been determined for only two of these species, using single-crystal X-ray crystallography. Our knowledge of the SiP bond is therefore quite inadequate. Gas-phase investigations of phosphorus–silicon compounds have been hampered by their high reactivity; consequently the structures of only a few species with SiP bond have been accurately characterized, that is, with accuracies of at least 0.05 !. Nevertheless, it should be noted that small molecules containing both elements are also of astronomical interest, because siliconand phosphorus-bearing molecules[8j have been detected in the interstellar space by radio astronomy. HPSi is perhaps the simplest unsaturated compound to have a chemical bond between silicon and phosphorus. Quantum-chemical calculations and experimental investigations have shown that linear structures are by far the most stable arrangements for HCN, HNC, and the heavier analogues HNSi and HCP. Until the present investigation, no experimental data were available for HPSi, but ab initio calculations concluded that a bridged structure with a Si P double bond (hereafter denoted HPSi) is more stable than linear HSiP. These calculations also predict an energy difference, of the order of 10 kcalmol1, between these forms; a transition state lying 13 kcalmol1 above HSiP connects the two isomers. Although hydrogen bonding occurs in silicon hydrides, no phosphorus-bearing molecules with this type of bonding are known. To determine the existence, geometry, and bonding of HPSi, a study has been undertaken to measure its rotational spectrum. Rotational spectroscopy is an ideal technique to study HPSi and other polar molecules because rotational frequencies are directly related to the moments of inertia along the three principal axes of the molecule. With sufficient isotopic substitutions, it is then possible to determine highly accurate molecular structures, that is, with bond lengths to an uncertainty on the order of 0.010 !. When the experimental rotational constants are corrected for zero-point vibrational effects, it is possible to derive bond lengths to even higher accuracy, on the order of # 0.001 ! or better. The experimental investigations presented herein were guided by highlevel coupled-cluster calculations at the same level of theory as that reported recently for silanethione, H2SiS. Details concerning the approach used to treat electron-correlation and basis-set effects in quantum-chemical calculations in a quantitative manner are given below and can also be found in Ref. [14]. HPSi was produced in the gas phase through a discharge of silane and phosphine in neon and then detected at high[*] Dr. V. Lattanzi, Prof. P. Thaddeus, Dr. M. C. McCarthy Harvard-Smithsonian Center for Astrophysics 60 Garden Street, Cambridge, MA 02138 (USA) and School of Engineering and Applied Sciences, Harvard University Cambridge, MA 02138 (USA) Fax: (+1)617-495-7013 E-mail: mccarthy@cfa.harvard.edu Dr. S. Thorwirth I. Physikalisches Institut, Universit!t zu Kln 50937 Kln (Germany) and Max Planck Institut f#r Radioastronomie 53121 Bonn (Germany) Dr. D. T. Halfen, Prof. L. M. Ziurys Departments of Chemistry and Astronomy and Steward Observatory, University of Arizona Tucson, AZ 85721 (USA) Dipl.-Chem. L. A. M#ck, Prof. Dr. J. Gauss Institut f#r Physikalische Chemie, Universit!t Mainz 55099 Mainz (Germany) Fax: (+49)6131-39-23895 E-mail: gauss@uni-mainz.de [**] Financial support for the work in Cambridge and Tucson is provided by the National Science Foundation (NSF) Center for Chemical Innovation (CCI) grant CHE 08-47919. D.T.H. is supported by a NSF Astronomy and Astrophysics Post-doctoral Fellowship (AST 0602282). S.T. and J.G. gratefully acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through grants TH1301/ 3-1 and GA370/5-1. L.A.M. is supported by a fellowship by the Graduate School of Excellence “MAINZ”. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201001938. Angewandte Chemie

Quantitative prediction of gas-phase N15 and P31 nuclear magnetic shielding constants

High-level ab initio benchmark calculations of the (15)N and (31)P NMR chemical shielding constants for a representative set of molecules are presented. The computations have been carried out at the Hartree-Fock self-consistent field (HF-SCF), density functional theory (DFT) (B-P86 and B3-LYP), second-order Moller-Plesset perturbation theory (MP2), coupled cluster singles and doubles (CCSD), and CCSD augmented by a perturbative treatment of triple excitations [CCSD(T)] level of theory using basis sets of triple zeta quality or better. The influence of the geometry, the treatment of electron correlation, as well as basis set and zero-point vibrational effects on the shielding constants are discussed and the results are compared to gas-phase experimental shifts. As for the first time a study using high-level post-HF methods is carried out for a second-row element, we also propose a family of basis sets suitable for the computation of (31)P shielding constants. The mean deviations observed for (15)N and (31)P are 0.9 [CCSD(T)/13s9p4d3f] and -3.3 ppm [CCSD(T)/15s12p4d3f2g], respectively, when corrected for zero-point vibrational effects. Results obtained at the DFT level of theory are of comparable accuracy to MP2 for (15)N and of comparable accuracy to HF-SCF for (31)P. However, they are not improved by inclusion of zero-point vibrational effects. The PN molecule is an especially interesting case with exceptionally large electron correlation effects on shielding constants beyond MP2 which, therefore, represents an excellent example for further benchmark studies.

Quantitative prediction of gas-phase O17 nuclear magnetic shielding constants

Benchmark calculations of (17)O NMR chemical shifts for a series of 19 molecules with 22 chemical shifts are presented. This includes calculations at the HF-SCF, DFT (BP86 and B3-LYP), MP2, CCSD(T), and for a special case full CCSDT level of theory using basis sets of quadruple zeta quality and better. The effects of the quality of the geometry, electron correlation, basis set, and the inclusion of zero-point vibrational and temperature corrections are discussed in detail and the results are compared to gas-phase experimental values. Mean and standard deviations are 6 and 24 ppm for HF-SCF, -20 and 14 ppm for BP86, -20 and 13 ppm for B3-LYP, and 26 and 12 ppm for MP2. Results at the CCSD(T)/pz3d2f level of theory using geometries optimized at the CCSD(T)/cc-pVTZ level of theory exhibit a mean deviation of 16 ppm and a standard deviation of 6 ppm. A mean deviation of 6 ppm and a standard deviation of 4 ppm are obtained if these values are corrected for zero-point vibrational and temperature effects.

The accuracy of rotational constants predicted by high-level quantum-chemical calculations. I. molecules containing first-row atoms

A statistical analysis of the accuracy of theoretically predicted rotational constants is presented based on the data for a total of 16 molecules and 97 isotopologues. Special focus is given on the treatment of electron correlation by using coupled-cluster methods up to quadruple excitations, core correlation, basis-set effects, zero-point vibrational corrections, and the electronic contribution to the rotational constants. The high accuracy achieved in the present investigation is demonstrated by the fact that at our best theoretical level, termed as CCSD(T)cc-pV infinity Z+Delta core+DeltaT+DeltaQ+DeltaB vib+DeltaB el, the mean absolute error is 0.04% and the standard deviation is 0.07% in comparison with the available experimental data. The importance of higher excitations, core correlation, and zero-point vibrational effects is emphasized, while the electronic contribution is found to be less important.

Molecular structures of the two most stable conformers of free glycine

The equilibrium molecular structures of the two lowest-energy conformers of glycine, Gly-Ip and Gly-IIn, have been characterized by high-level ab initio electronic structure computations, including all-electron cc-pVTZ CCSD(T) geometry optimizations and 6-31G* MP2 quartic force fields, the latter to account for anharmonic zero-point vibrational effects to isotopologic rotational constants. Based on experimentally measured vibrationally averaged effective rotational constant sets of several isotopologues and our ab initio data for structural constraints and zero-point vibrational shifts, least-squares structural refinements were performed to determine improved Born-Oppenheimer equilibrium (r(e)) structures of Gly-Ip and Gly-IIn. Without the ab initio constraints even the extensive set of empirical rotational constants available for 5 and 10 isotopologues of Gly-Ip and Gly-IIn, respectively, cannot satisfactorily fix their molecular structure. Excellent agreement between theory and experiment is found for the rotational constants of both conformers, the rms residual of the final fits being 7.8 and 51.6 kHz for Gly-Ip and Gly-IIn, respectively. High-level ab initio computations with focal point extrapolations determine the barrier to planarity separating Gly-IIp and Gly-IIn to be 20.5 +/- 5.0 cm(-1). The equilibrium torsion angle tau(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 +/- 2) degrees. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.

Extension of the Lennard-Jones potential: Theoretical investigations into rare-gas clusters and crystal lattices of He, Ne, Ar, and Kr using many-body interaction expansions

The many-body expansion ${V}_{\mathrm{int}}={\ensuremath{\sum}}_{ilj}{V}^{(2)}({r}_{ij})+{\ensuremath{\sum}}_{iljlk}{V}^{(3)}({r}_{ij},{r}_{ik},{r}_{jk})+\ensuremath{\cdots},$ in terms of interaction potentials between rare-gas atoms converges fast at distances $rg{r}_{\mathit{HS}}$, with ${r}_{\mathit{HS}}$ being the hard-sphere radius at the start of the repulsive wall of the interaction potential. Hence, for the solid state where the minimum distance is always above ${r}_{\mathit{HS}}$, a reasonable accuracy is already obtained for the lattice parameters and cohesive energies of the rare-gas elements using precise two-body terms. All tested two-body potentials show a preference of the hcp over the fcc structure. We demonstrate that this is always the case for the Lennard-Jones potential. We extend the Lennard-Jones potential to obtain analytical expressions for the lattice parameters, cohesive energy, and bulk modulus using the solid-state parameters of Lennard-Jones and Ingham [Proc. R. Soc. London, Ser. A 107, 636 (1925)], which we evaluate up to computer precision for the cubic lattices and hcp. The inclusion of three-body terms does not change the preference of hcp over fcc, and zero-point vibrational effects are responsible for the transition from hcp to fcc as shown recently by Rosciszewski et al. [Phys. Rev. B 62, 5482 (2000)]. More precisely, we show that it is the coupling between the harmonic modes which leads to the preference of fcc over hcp, as the simple Einstein approximation of moving an atom in the static field of all other atoms fails to describe this difference accurately. Anharmonicity corrections to the crystal stability are found to be small for argon and krypton. We show that at pressures higher than $15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ three-body effects become very important for argon and good agreement is reached with experimental high-pressure density measurements up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, where higher than three-body effects become important. At high pressures we find that fcc is preferred over the hcp structure. Zero-point vibrational effects for the solid can be successfully estimated from an extrapolation of the cluster zero-point vibrational energies with increasing cluster size $N$. For He, the harmonic zero-point vibrational energy is predicted to be always above the potential energy contribution for all cluster sizes up to the solid state at structures obtained from the two-body force. Here anharmonicity effects are very large which is typical for a quantum solid.

The potential energy landscape of noradrenaline: An electronic structure study

The relaxed potential energy profiles for interconversion between the conformers in the two most stable noradrenaline families (AG1 and GG1, each containing four members) have been calculated at the density functional B3LYP/6-31 + G* level of theory. Rigid rotation (in which the molecule is kept frozen during variation of the torsional angle) is found to introduce large errors in the relative energies and geometries of the stationary points; it appears to be best to compute relaxed potential energy surfaces wherever possible, even if this can only be accomplished by using a lower level of theory. The barriers for interconversion between the noradrenaline conformers (from least to most stable structure, with inclusion of scaled zero-point vibrational effects) range from 7 to 15 kJ mol−1. The barrier for conversion between the two most stable noradrenaline conformers, AG1a and GG1a, is among the highest of those computed: 15.4 kJ mol−1 at the B3LYP/6-31 + G* level and 21.7 kJ mol−1 at the MP2/6-31 + G* level. Temperature effects further increase the barrier between AG1a and GG1a. The high transition barriers indicate that conformer conversion is unlikely to occur during collisional cooling of noradrenaline in a supersonic expansion.

Critical analysis of the spin-rotation constants of CF 2 and CCl 2: A theoretical investigation

Quantum chemical ab initio calculations for the spin-rotation constants of difluorocarbene (CF 2) and dichlorocarbene (CCl 2) were carried out using coupled-cluster techniques with sequences of correlation-consistent basis sets. Theoretical best estimates were obtained using extrapolation to the complete basis-set limit and taking into account corrections for core correlation, additional diffuse functions and zero-point vibrational effects. It is demonstrated that such accurate theoretical estimates can be used either to support or to challenge the analysis of the experimental spectra and the reliability of the resulting data.

Lattice dynamics ofNaAlH4from high-temperature single-crystal Raman scattering andab initiocalculations: Evidence of highly stableAlH4−anions

Polarized Raman scattering on single crystals of has been used to determine the symmetry properties and frequencies of the Raman-active vibrational modes over the temperature range from 300 to 425 K, i.e., up to the melting point . Significant softening (by up to 6%) is observed in the modes involving rigid translations of cations and translations and librations of . Surprisingly, the data indicate mode softening of less than 1.5% for the Al- stretching and Al- bending modes of the anion. These results show that the anion remains a stable structural entity even near the melting point. First-principles linear response calculations of phonon mode frequencies are in reasonably good agreement with the Raman results. The phonon mode Gr\"uneisen parameters, calculated using the quasiharmonic approximation, are found to be significantly higher for the translational and librational modes than for the Al- bending and stretching modes, but cannot account quantitatively for the dramatic softening observed near in the former two types of modes, suggesting an essentially anharmonic mechanism. The effect of zero-point vibrations on the calculated lattice parameters is found to be large (expansion by 1.2 and 1.5 % in the and parameters, respectively), as expected for a compound with many light elements. We discuss the implications of the observed mode softening for the kinetics of hydrogen release and hypothesize that breaking up the anions is the rate limiting step. The enhanced kinetics of absorption and desorption in Ti-doped powders is attributed to the effectiveness of Ti in promoting the breakup of the anions.

Point defects in NiAl: The effect of lattice vibrations

We investigate the effect of atomic vibrations on point defect free energies and equilibrium concentrations in the B2 NiAl compound using the quasiharmonic approximation in combination with a recently developed embedded-atom potential. The entropy term appears to be the dominant contribution to the Gibbs free energies of point defects. Vibrational entropies of the main defect complexes: triple-Ni defect, exchange defect, divacancy, and even of the interbranch-Al defect turn out to be positive in the whole range of temperatures studied here (0--1700 K). This leads to an increase in the concentrations of all four types of point defects in Ni-rich and stoichiometric NiAl. On the Al-rich side, the effect of lattice vibrations is to shift the minimum on the vacancy concentration versus temperature curve towards lower temperatures. The effect of zero-point vibrations is shown to be too small to affect the type of constitutional defects in NiAl. The constitutional defects remain nickel antisite atoms on the Ni-rich side and nickel vacancies on the Al-rich side.

Theoretical prediction of a linear isomer for NeBr2(X1Γg + an ab initio approach

A supermolecular approach is used to predict the stationary structures of the NeBr2(X1Γg +) van der Waals cluster. The intermolecular potential is calculated using the single and double excitation coupled-cluster method with non-iterative perturbation treatment of triple excitations [CCSD(T)]. Relativistic effects are included for the Br atoms using effective core potentials (ECPs) and their efficiency is tested for Br2 and NeBr2 molecules. Our results for NeBr2 show minima for both linear and T-shaped configurations, in accord with previous ab initio calculations for rare gas-Br2 species. The dependence of the interaction potential, as a function of the Br-Br bond, is also presented. Finally, vibrational analysis is carried out to examine the stability of the two isomers including zero-point vibrational energy effects.

An accurate semiclassical method to predict ground-state tunneling splittings

A new method for calculating the ground-state tunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The density-functional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the low-level energy surface is mapped onto the high-level points to get a good quantitative description of the high-level PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the high-level PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional consideration of zero-point vibrational effects. The method is applied to three molecular systems, namely hydrofluoric acid dimer, malonaldehyde, and tropolone. These systems were chosen because their energy barriers differ strongly (1 kcal/mol–7 kcal/mol). The predicted tunneling splittings agree very well with the experimental ones, therefore, we expect our method to be generally applicable, independent of the magnitude of the energy barrier.

Molecular Magnetizabilities: Zero-Point Vibrational Effects and the Breakdown of Pascal's Rule

We demonstrate, by ab initio calculations on more than 60 molecules, that zero-point vibrational corrections to isotropic magnetizabilities in general are negligible, being less than 0.5% for almost all molecules studied. The exceptions to this rule are aromatic and anti-aromatic ring systems where the effect may be as large as 1% due to a sizable vibrational contribution to the component of the magnetizability perpendicular to the molecular plane. For the magnetizability anisotropy, zero-point vibrational corrections are much more important, often contributing 5−10% of the total vibrationally averaged magnetizability anisotropy. We also demonstrate that the additivity of magnetizabilities (known as Pascal's rule) breaks down in the case of fluorine-containing molecules.

A converged calculation of the energy barrier to internal rotation in the ethylene–sulfur dioxide dimer

Geometrical parameters for the equilibrium (MIN) and lowest saddle-point (TS) geometries of the C2H4⋯SO2 dimer, and the corresponding binding energies, were calculated using the Hartree–Fock and correlated levels of ab initio theory, in basis sets ranging from the D95(d,p) double-zeta basis set to the aug-cc-pVQZ correlation consistent basis set. An assessment of the effect of the basis set superposition error (BSSE) on these results was made. The dissociation energy from the lowest vibrational state was estimated to be 705±100 cm−1 at the basis set limit, which is well within the range expected from experiment. The barrier to internal rotation was found to be 53±5 cm−1, slightly higher than the (revised) experimental result of 43 cm−1, probably due to zero-point vibrational effects. Our results clearly show that, in direct contrast with recent ideas, the BSSE correction affects differentially the MIN and TS binding energies and so has to be included in the calculation of small energy barriers such as that in the C2H4⋯SO2 dimer. Previous reports of positive MP2 frozen-core binding energies for this complex in basis D95(d,p) are confirmed. The anomalies are shown to be an artifact arising from an incorrect removal of virtual orbitals by the default frozen-core option in the GAUSSIAN program.

The Cotton–Mouton effect of gaseous CO2, N2O, OCS, and CS2. A cubic response multiconfigurational self-consistent field study

The hypermagnetizability and the hypermagnetizability anisotropy of CO2, N2O, OCS, and CS2 are computed at a wavelength of 632.8 nm using cubic response theory with multiconfigurational self-consistent field wave functions. The anisotropies of the electric dipole polarizability and of the magnetizability are also obtained. This allows us to study the temperature dependence of the Cotton–Mouton constant for all four molecules and thus to compare to the results of the experimental study by Kling and Hüttner [Chem. Phys. Lett. 90, 207 (1984)]. We also assess the importance of pure and zero-point vibrational effects on the relevant molecular properties. In particular, we show that for CO2, OCS, and CS2, the pure vibrational effects to the hypermagnetizability anisotropy can be even more important than the electronic contribution.

Zero-point vibrational effects on optical rotation

We investigate the effects of molecular vibrations on the optical rotation in two chiral molecules, methyloxirane and trans-2,3-dimethylthiirane. It is shown that the magnitude of zero-point vibrational corrections increases as the electronic contribution to the optical rotation increases. Vibrational effects thus appear to be important for an overall estimate of the molecular optical rotation, amounting to about 20–30% of the electronic counterpart. We also investigate the special case of chirality introduced in a molecule through isotopic substitution. In this case, the zero-point vibrational effects are the only source of optical activity, and we investigate the magnitude of these effects through calculations on mono-deuterated oxirane.

Structures, energies, vibrational spectra, and electronic properties of water monomer to decamer

The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve as sensitive indicators of the status of proton donation (“d”) and acceptance (“a”) (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O–H frequencies (ν) for each cluster can be arranged in the following order: ν3da (single donor–single acceptor) ≅ν3daa (single donor–double acceptor) >ν3dda (double donor–single acceptor) >ν1dda>ν1da> (or ≅) ν1daa. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν3da and ν3daa). The intensities of ν1daa and ν1da are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν1dda modes are diminished by more than half. Most of the above data have been compared to the available experimental data. Keeping in view the recent experimental reports of the HOH bending modes, we have also analyzed these modes, which show the following trend: ν2dda>ν2daa≅ν2da. The present study therefore would be useful in the assignments of the experimental O–H stretching and HOH bending modes.

Noble gas–metal chemical bonding? The microwave spectra, structures, and hyperfine constants of Ar–CuX(X=F, Cl, Br)

The rotational spectra of the complexes Ar–CuF, Ar–CuCl, and Ar–CuBr have been observed in the frequency range 5–22 GHz using a pulsed-jet cavity Fourier transform microwave spectrometer. All the complexes are linear and rather rigid in the ground vibrational state, with the Ar–Cu stretching frequency estimated as ∼200 cm−1. Isotopic data have been used to calculate an r0 structure for Ar–CuF, while for Ar–CuCl and Ar–CuBr partial substitution structures have also been obtained. To reduce zero-point vibrational effects a double substitution method (rd) has also been employed to calculate the structures of Ar–CuCl and Ar–CuBr. The Ar–Cu distance has been found to be rather short and to range from 2.22 Å in Ar–CuF to 2.30 Å in Ar–CuBr. Ab initio calculations at the MP2 level of theory model the geometries and stretching frequencies well and predict an Ar–Cu bond energy in Ar–CuF of ∼47.3 kJ mol−1. Large changes in the Cu nuclear quadrupole coupling constant on complex formation show that extensive charge rearrangement occurs upon formation of the complexes. This, in conjunction with the sizable dissociation energy, suggests that the Ar–Cu bonds in these complexes are weakly covalent. The rotational spectrum of CuF has also been reinvestigated to improve the hyperfine constants.

The microwave spectra and structures of Ar–AgX (X=F,Cl,Br)

The rotational spectra of the complexes Ar–AgF, Ar–AgCl, and Ar–AgBr have been observed in the frequency range 6–20 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. All the complexes are linear and rather rigid in the ground vibrational state, with the Ar–Ag stretching frequency estimated as ∼140 cm−1. Isotopic data have been used to calculate an r0 structure for Ar–AgF, while for Ar–AgCl and Ar–AgBr partial substitution structures have also been obtained. To reduce zero-point vibrational effects a double substitution method (rd) was employed to calculate the structures of Ar–AgCl and Ar–AgBr. The Ar–Ag bond distance has been found to be rather short and to range from 2.56 Å in Ar–AgF to 2.64 Å in Ar–AgBr. Ab initio MP2 and density functional theory calculations for Ar–AgF and Ar–AgCl model the geometries and stretching frequency well, and predict an Ar–Ag bond energy in Ar–AgF of ∼23 kJ mol−1. These results indicate that the Ar–AgX complexes are more strongly bound than typical van der Waals complexes. Analysis of the halogen nuclear quadrupole coupling constants was unable to confirm whether extensive electron rearrangement occurs upon formation of the complexes.

Behaviors of an excess proton in solute-containing water clusters: A case study of H+(CH3OH)(H2O)1–6

Behaviors of an excess proton in solute-containing water clusters were investigated using infrared spectroscopy and ab initio calculations. This investigation characterized the structures of protonated methanol-water clusters, H+(CH3OH)(H2O)n with n=2–6, according to their nonhydrogen-bonded and hydrogen-bonded OH stretches in the frequency range of 2700–3900 cm−1. Ab initio calculations indicated that the excess proton in these clusters can be either localized at a site closer to methanol, forming a methyloxonium ion core (CH3OH2+), or at a site closer to water, forming a hydronium ion core (H3O+). Infrared spectroscopic measurements verified the calculations and provided compelling evidence for the coexistence of two distinct structural isomers, CH3OH2+(H2O)3 and H3O+(CH3OH)(H2O)2, in a supersonic expansion. The spectral signatures of them (either CH3OH2+ or H3O+ centered) are the free-OH stretching absorption band at 3706 cm−1 of a single-acceptor-single-donor H2O, and the band at 3673 cm−1 of a single-acceptor CH3OH. At n=4–6, the clusters adopt structures similar to their pure water analogs with five-membered rings starting to form at n=5. The position of the excess proton in them varies sensitively with the number of solvent water molecules as well as the geometry of the clusters. To further elucidate the behaviors of the excess proton in these clusters, we analyze in detail the potential energy surface along the proton transfer coordinate for two specific isomers of n=2 and 4: MW2II and MW4I. It is found that the proton can be nearly equally shared by methanol and the water dimer subunit in the form of CH3OH–H+–(H2O)2, as substantiated by hydrogen bond cooperativity and zero-point vibrational effects.