The changes in charge and momentum distributions upon forming a hydrogen bond in the water dimer are examined. The computed Compton profile anisotropies show the same oscillations as were observed for solid ice. These oscillations are already found when the unperturbed orbitals of the water monomers are used to construct a Slater determinant for the dimer. Hence we conclude that the oscillations are irrelevant to the discussion of the covalent character of the bond. Rather they just reflect the result of antisymmetrizing the product of monomer wave functions. In fact, at the oxygen-oxygen distance in ice, the calculations indicate a net antibonding contribution to energy from overlap effects.

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We have studied the variation of hardness, polarizability, and valency of molecules during the course of a chemical reaction. For isomerization reactions, the quantities are calculated through Kohn−Sham version of spin-polarized density functional theory while for other types of reactions, available data on polarizability are used for the study. It is observed that a state of minimum polarizability usually can be associated with higher stability or maximum hardness.

Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.

In an attempt to explore how electrolyte ions influence the friction experienced by solutes with different functional groups, rotational diffusion of two structurally similar coumarins, coumarin 343 (C343) and coumarin 334 (C334), has been studied in dimethyl sulfoxide (DMSO) at several concentrations of LiNO 3 . The two coumarins are almost identical except for the different functional groups in the 3-position; C343 has -COOH whereas C334 has -COCH 3 . Because of the presence of the -COOH functional group, C343 exists as both anionic and neutral species in DMSO. Although both anionic and neutral forms of C343 exist in the ground state, C343 predominantly exists as neutral species in the excited state. The measured reorientation time of C343 in DMSO is slower by a factor of 2 compared to that of C334 due to the hydrogen bonding between the -COOH group of the probe and sulfoxide group of the solvent. However, the viscosity normalized reorientation times (τ r /η) of C343 in LiNO 3 /DMSO solutions increase by 60% to 70% compared to that in pure DMSO. Addition of electrolyte ions shifts the equilibrium toward the anionic form of C343, which in turn forms ion pairs with Li + , and the large increase in τ r /η values is due to the association of DMSO solvent molecules with these ion pairs. On the other hand, τ r /η values of the neutral solute, C334, remain invariant in the entire range of the electrolyte concentration. The C343 -Li + ion pairs have been characterized using ab initio molecular orbital methods. Two approaches have been employed to model the friction experienced by the C343 -Li + ion pairs. In the first approach, the complex formed between C343 -Li + and the solvent molecules is treated as a rigid entity and the increase in the viscosity normalized reorientation time has been accounted for solely as an enhancement in the mechanical friction. Alternatively, the solvent association with the C343 -Li + ion pairs has been modeled as dielectric friction using the extended charge distribution theory.