Anharmonic Vibrational Analysis of Ethylene, Butadiene, and Longer Conjugated Alkenes by Double-Hybrid Density-Functional Theory Calculations

The anharmonic vibrational wavenumbers of ethylene, trans-1,3-butadiene, and conjugated all-trans alkenes (1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene) were calculated by using the double-hybrid density-functional method. The coefficients of terms constituting the energy in the default double-hybrid density-functional (B2PLYP) were carefully optimized for ethylene. Calculations using this modified double-hybrid density-functional (B2ePLYP) gave anharmonic vibrational wavenumbers for the above-mentioned molecules in satisfactory agreement with the published data on fundamentals, overtones, and combinations. Isotope effects observed for deuterated species were mostly well reproduced.