Symmetry force field calculations have been performed for the planar vibrations of urea using CNDO/Force method. The CNDO/Force calculations predict well the signs and the magnitudes of bending and interaction force constants; the stretching force constants are found to be higher in magnitude. The bending and interaction constants obtained from these calculations and the stretching force constants obtained from the literature are considered for the initial force field. Using the observed frequencies for urea and its isotopic analogues, urea-D₄, urea-¹⁵N₂, urea-¹⁵N₂D₄ and urea-¹⁸O in the solid as well as in the solution phases, the force field is refined by carrying out iterations over the diagonal force constants. In the final stages of the refinement iterations are carried out over all the force constants keeping the signs of the interaction constants unchanged. It is found that the agreement between the calculated and the observed frequencies is excellent. The final force fields in terms of symmetry as well as redundancy free internal valence coordinates are reported. On the basis of the potential energy distribution the vibrational assignments are discussed.

Various methods, employing molecular orbital calculations of varying approximations, for evaluation of force fields of polyatomic molecules have been reviewed. Applications ofcndo/force method for the force field calculations are specially dealt with in detail because of its ease of operation and being economically more viable in terms of computer time. The calculated C=O stretching force constants for a series of organic molecules are shown to have linear relationship with substituent constants.

Raman spectra of solutions ofdmso (dimethyl sulphoxide) are studied in the S=O stretching region by second derivative analysis. Broad bands observed in the normal Raman spectra show well resolved components in the derivative plots. On the basis of the various components obtained in these plots for solutions ofdmso in carbon tetrachloride and water, existence of equilibria between various associated species, involving dipolar and hydrogen bonding interactions, is suggested. It is concluded that solutions ofdmso in CCl₄ may have monomers, cyclic and linear dimers and polymers, whereas its aqueous solutions show the existence of 1:1 and 1:2 hydrogen bonded complexes of water withdmso in addition to smaller concentrations of monomers, dimers and polymers.

The geometry optimization of monomeric acetonitrile and its dimers is carried out using CNDO/force method. The results are compared with the experimental and already reported theoretical parameters.

CNDO/force calculations on the complexes of nitromethane and acetonitrile with H⁺, Li⁺, F⁻ and Cl⁻ have been carried out to determine their geometrical parameters and stretching force constants. The results are discussed in terms of specific interactions between the ions and the molecules.

CNDO/force calculations have been employed to calculate the O-H stretching force constants for various structures of associated water species such as water-dimethylether, water-acetonitrile, water-lithium fluoride, linear and cyclic water polymers. The variation in the O-H stretching force constant in A_m… O-H… D_n species where A_mand D_n represent, respectively,m number of electron-acceptor andn number of electron-donor molecules, is explained on the basis of the cooperativity effect. With increasing electron-acceptor power of A and electron-donor power of D, the hydrogen-bonded O-H stretching force constant is significantly reduced. The results obtained in these studies are in general agreement with experimental observations reported earlier.

Near infrared (NIR) spectra in the CH₃ stretching first-overtone region for methyl iodide, acetonitrile, nitromethane and dimethyl sulphoxide are discussed in terms of normal mode and local mode descriptions. NIR spectra in this region for solutions of several alkali metal halides and tetraalkyl ammonium halides in these solvents are also reported. It is found that the relative intensity of the cooperative excitation band with respect to the pure overtone excitation band reduces considerably on increase in the concentration of the electrolytes in solution. The results are discussed in terms of the formation of anion-molecular complexes as well as intermode coupling due to kinetic energy and anharmonicity effects.

Ab initio MO calculations are performed on a series of ion-molecular and ion pair-molecular complexes of H₂O + MX (MX = LiF, LiCl, NaCl, BeO and MgO) systems. BSSE-corrected stabilization energies, optimized geometrical parameters, internal force constants and harmonic vibrational frequencies have been evaluated for all the structures of interest. The trends observed in the geometrical parameters and other properties calculated for the mono-hydrated contact ion pair complexes parallel those computed for the complexes of the individual ions. The bifurcated structures are found to be saddle points with an imaginary frequency corresponding to the rocking mode of water molecules. The solventshared ion pair complexes have high interaction energies. Trends in the internal force constant and harmonic frequency values are discussed in terms of ion-molecular and ion-pair molecular interactions.

3-21G, 6–31G and 6–31 +G calculations have been performed on Cr…H₃CCl and LiCl…H₃CCl complexes with two different configurations each. Optimized geometries, stabilization energies, CH force constants and harmonic vibrational frequencies for CH₃Cl and its complexes are reported. Comparison of the calculated frequency shifts of CH-stretching bands of CH₃C1, on complexation, with experimental results of related systems indicate that the interaction of Cl^- with CH₃ group takes place in a linear manner with the CH bond. A lower frequency shift for LiCl…H₃CX with reference to the C1^-…H₃CX complexes is explained on the basis of the reduction of the basicity of Cl^- ion in the presence of counter ion in the former complex considered.

Using local mode description for methyl chloride, the infrared intensities for the first four CH₃ stretching overtone bands are used to calculate the dipole function for the C-H bond. Detailed expressions for overtone transitions for diatomic molecules derived by Bouanich are employed for the calculation of dipole moment derivatives.