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The homodirners of singly fluorine-substituted phenylacetylenes Were investigated :using electronic and vibrational spectroscopic methods in combination with density functional theory calaitations. The IR spectra in the aCetytenic 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 dieters is minimally perturbed relative to the. corresponding monomer. The observed spectra were assigned to a set of pi-stacked structures within an energy-range of 1.5 kJ mol(-1), 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 dieters 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 pi-pi stacking with appreciable contribution of electrostatics. The stabilization energies of fluorophenylacetylene &melt follow the,same ordering as their dipole moments, which suggests that dipole moment enhances the ability to form pi-stacked structures.

Elucidation of host-guest chemistry of 2-(4'pyridyl)benzimidazole (4PBI) and 2-(2'-pyridyl)benzimidazole (2PBI) with cucurbiturils in aqueous solution is rather challenging. The guest molecules are capable of binding in three different states of protonation: cation C, tautomer T, and normal N. Charge distribution on the species governs the formation of the inclusion complexes. Binding modes of the guest molecules with cucurbiturils have been investigated by proton NMR spectroscopy. 4PBI binds with its benzimidazole ring pointing inward,, while the binding mode is opposite for 2PBI. This difference is governed by the position of substitution of the pyridyl ring. Energetics of complexation has been studied by quantum chemical calculations for 4PBI-CB as 2PBI-CB type complexes, for each form (C, T, and N) of both the molecules. Release of high energy water from cucurbituril cavity is the main driving force for complexation. Excited state proton transfer (ESPT) in 2PBI and 4PBI are affected strongly upon complexation, as is manifested in steady state and time-resolved fluorescence experiments.

The red-shifts in the acetylenic C-H stretching vibration of C-H center dot center dot center dot X (X = O, N) hydrogen-bonded complexes increase with an increase in the basicity of the Lewis base. Analysis of various components of stabilization energy suggests that the observed red-shifts are correlated with the electrostatic component of the stabilization energy, while the dispersion modulates the stabilization energy.

The C-H⋅⋅⋅N hydrogen-bonded complexes of several fluorophenyacetylenes with ammonia and methylamine were characterized by a redshift in the acetylenic C-H stretching vibration of the phenylacetylene moiety. These redshifts were linearly correlated with the stabilization energies calculated at the CCSD(T)/CBS//MP2-aug-cc-pVDZ level. Analysis of various components of the interaction energy indicated that the observed redshifts were weakly correlated with the electrostatic component. The weaker linear correlation between the frequency shifts and the electrostatic component between two data sets can perhaps be attributed to the marginal differences in the Stark tuning rate and zero-field shifts. The induction and exchange-repulsion components were linearly correlated. However, the dispersion component depends on the nature of the hydrogen-bond acceptor and shows a quantum jump when the hydrogen-bond acceptor is changed from ammonia to methylamine. The observed linear correlation between the redshifts in the C-H stretching frequencies and the total stabilization energies is due to mutual cancellation of deviations from linearity between various components.

The hydrogen-bonded complexes of phenylacetylene, 4-fluorophenylacetylene, 2-fluorophenylacetylene, and 2,6-difluorophenylacetylene with ammonia are investigated using IR-UV double resonance spectroscopy in combination with high-level ab initio calculations at the CCSD(T)/CBS level of theory. The C-H···N hydrogen-bonded complex, which involves an interaction of ammonia with the acetylenic CH group is the global minimum and is observed in all four cases investigated. In addition, phenylacetylene and 4-fluorophenylacetylene form a quasi-planar cyclic complexes with ammonia incorporating N-H···π and C-H···N hydrogen bonds, wherein the π-electron density of the acetylenic C≡C bond acts as an acceptor to the N-H group of ammonia. A third ammonia complex is observed for 4-fluorophenylacetylene in which ammonia interacts with the fluorine atom once again, leading to the formation of a quasi-planar cyclic complex. The substitution of the fluorine atom on the phenyl ring of phenylacetylene modulates the intermolecular potentials, which are dependent on the position of the substitution.