The hydration shell of Na+ ion in the gas phase (∼4) is smaller than that in solution (6). The energetics of various solvated Na+ clusters indicate that lowering the pairwise interaction between the ligand and the ion can increase the coordination number in the gas phase. We report here the first hexa-coordinated Na+ cluster ion in the gas phase, which can be formed by substituting H2O with 1,4-difluorobenzene. Infrared-photodissociation spectroscopy of the O–H stretch was used to infer the structure.

The structure of the phenylacetylene-dimer has been elucidated using IR-UV double resonance spectroscopy in combination with high level ab initio calculations at the CCSD(T)/CBS level. The IR spectra in the acetylenic and the aromatic C-H stretching regions indicate that the two phenylacetylene moieties are in identical environments and very similar to the phenylacetylene monomer. Calculated stabilization energies and the free energies at the CCSD(T)/CBS level favor the formation of an anti-parallel π-stacked structure. The DFT-SAPT energy decomposition analysis points out that the anti-parallel π-stacked structure maximizes electrostatic as well as the dispersion components of energy. The observed IR spectra are consistent with the anti-parallel π-stacked structure.

The electric field experienced by an acid molecule in the acid-water cluster depends on its local environment comprising of surrounding water molecules. A critical field of about 193 and 163 MV cm is required for the dissociation of HCl and HBr, respectively, and is associated with the arrangement of water molecules around the acid. The critical field required for dissociation of isolated HCl and HBr is 510 and 462 MV cm, respectively. Hence the solvation of the proton and the halide anion by water molecules substantially lowers the critical electric field by about 300 MV cm, relative to vacuum.

The structure of the phenylacetylene-water complex has been elucidated based on spectral shifts in electronic and vibrational transitions. Phenylacetylene forms a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds, which is different from both the benzene-water and acetylene-water complexes, even though phenylacetylene combines the features of both benzene and acetylene. Formation of such a complex can be rationalized on the basis of cooperativity between the two sets of hydrogen bonds.

The homodimers of singly fluorine-substituted phenylacetylenes were investigated using electronic and vibrational spectroscopic methods in combination with density functional theory calculations. The IR spectra in the acetylenic C-H stretching region show a marginal red shift for the dimers relative to the monomers. Further, the marginal red shifts indicate that the acetylenic group in all the dimers is minimally perturbed relative to the corresponding monomer. The observed spectra were assigned to a set of π-stacked structures within an energy range of 1.5 kJ mol, which differ in the relative orientation of the two monomers on the basis of M06-2X/aug-cc-pVTZ level calculation. The observed red shift in the acetylenic C-H stretching vibration of the dimers suggests that the antiparallel structures contribute predominantly based on a simple coupled dipole model. Energy decomposition analysis using symmetry-adapted perturbation theory indicates that dispersion plays a pivotal role in π-π stacking with appreciable contribution of electrostatics. The stabilization energies of fluorophenylacetylene dimers follow the same ordering as their dipole moments, which suggests that dipole moment enhances the ability to form π-stacked structures.

The infrared spectra in the acetylenic C-H stretching region for the complexes of phenylacetylene with water, methanol, ammonia, and methylamine are indicative of change in the intermolecular structure upon substitution with a methyl group. High-level ab initio calculations at CCSD(T)/aug-cc-pVDZ level indicate that the observed complexes of water and ammonia are energetically the most favored structures, and electrostatics play a dominant role in stabilizing these structures. The ability of the pi electron density of the benzene ring to offer a larger cross-section for the interaction and the increased polarizability of the O-H and N-H groups in methanol and methylamine favor the formation of pi hydrogen-bonded complexes, in which dispersion is the dominant force. Further, the observed phenylacetylene-methylamine complex can be tentatively assigned to a kinetically trapped higher energy structure. The observed methyl group-induced hydrogen bond switching in the phenylacetylene complexes can be attributed to the switching of the dominant interaction from electrostatic to dispersion.

No abstract

The electric field experienced by the OH group of phenol embedded in the cluster of ammonia molecules depends on the relative orientation of the ammonia molecules, and a critical field of 236 MV cm −1 is essential for the transfer of a proton from phenol to the surrounding ammonia cluster. However, exceptions to this rule were observed, which indicates that the projection of the solvent electric field over the O−H bond is not a definite descriptor of the proton transfer reaction. Therefore, a critical electric field is necessary, but it is not a sufficient condition for the proton abstraction. This, in combination with an adequate solvation of the acceptor ammonia molecule in a triple donor motif that energetically favors the proton transfer process, constitutes necessary and sufficient conditions for the spontaneous proton abstraction. The proton transfer process in phenol-(ammonia) n clusters is statistically favored to occur away from the plane of the phenyl ring and follows a curvilinear path which includes the O−H bond elongation and out-ofplane movement of the proton. Colloquially, this proton transfer can be referred to as a "bend-to-break" process.