Anharmonic Vibrational Analysis of Dihalogenomethanes and Dihalogenoethanes by Density-Functional Theory Calculations

The anharmonic vibrational wavenumbers of dihalogeno-methanes CH2X2 and 1,2-dihalogenoethanes CH2XCH2X (X: F, Cl, and Br) were calculated by using hybrid and double-hybrid density-functional methods. For comparison of the methods of calculation, the molecular-orbital method at the CCSD(T) level was also tested for CH2F2 and CH2Cl2. It was found that the hybrid-type B3LYP functional method was not satisfactory for calculating anharmonic vibrational wavenumbers. It was possible to improve the B3LYP functional method by altering the coefficients of the energy formula for CH2F2, but the set of obtained coefficients was not transferrable to CH2Cl2. The double-hybrid-type B2PLYP functional method with carefully chosen sets of basis functions gave molecular structures and anharmonic vibrational wavenumbers in good agreement with the published experimental values. The molecular-orbital method at the CCSD(T) level gave calculated results comparable with (or even better than) those by the B2PLYP functional method. However, the cost of computation by the B2PLYP functional method is far lower than that by the molecular-orbital method at the CCSD(T) level. The B2PLYP functional method, which has the advantage of giving satisfactory results with relatively low costs of computation, may become a realistic approach for anharmonic vibrational analyses of relatively large molecules in general.