Vibrational anharmonicity and harmonic force fields for dichloromethane from quantum-chemical calculations

Anharmonic and related constants have been calculated for CH2Cl2, CD2Cl2, and CHDCl2 by using the program Gaussian03 and B3LYP and MP2 models. Bases used were 6-311++G** and cc-pVTZ. The size of grid used in the B3LYP/6-311++G** model had a noticeable effect on resulting data. Features of the MP2/6-311++G** calculations suggested a deleterious effect of the absence off functions in this basis set. The need for the replacement of second-order terms in the perturbation theory formulas for the vibrational anharmonic constants x(ij) in the presence of Fermi resonance was explored, and minor resonances were found associated with the cubic constants phi(122) and phi(299) (d(0) isotopomer), phi(122) and phi(849) (d(2)) and phi(278) (d(1)). Computed x(ij) values for nu CH and nu CD motions agree quite well with earlier experimental data. Observed anharmonic frequencies, nu(obsd), were corrected to "observed" harmonic frequencies, omega(obsd), by using computed differences Delta = omega(QC) - nu(QC). These differences Delta are larger for the antisymmetric nu asCH2 mode than for symmetric nu sCH2 motion. This fact made it necessary to use differing scale factors for the two kinds of CH stretching force constants in a subsequent scaling of the harmonic force field to nu(obsd). Force field scaling was also carried out by refining to omega(obsd). In both approaches, the B3LYP models required differing scale factors for symmetric and antisymmetric CCl stretching force constants, indicating a failure to compute an accurate C-Cl stretch-stretch interaction force constant. The MP2/cc-pVTZ force field was preferred. Both scaled and unscaled harmonic force fields were used to calculate centrifugal distortion constants (CDCs) and contributions to the vibrational dependence of the rotational constants (alphas). Variations in the CDCs can, in part, be explained by the magnitudes of the frequencies used in the scaling process.