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 of f functions in this basis set. The need for the replacement of second-order terms in the perturbation theory formulas for the vibrational anharmonic constants xij in the presence of Fermi resonance was explored, and minor resonances were found associated with the cubic constants ϕ122 and ϕ299 (d0 isotopomer), ϕ122 and ϕ849 (d2), and ϕ278 (d1). Computed xij values for νCH and νCD motions agree quite well with earlier experimental data. Observed anharmonic frequencies, νobsd, were corrected to “observed” harmonic frequencies, ωobsd, by using computed differences Δ = ωQC − νQC. These differences Δ are larger for the antisymmetric νasCH2 mode than for symmetric νsCH2 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 νobsd. Force field scaling was also carried out by refining to ωobsd. 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.


American Chemical Society

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Journal of Physical Chemistry A


Chemistry and Biochemistry

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