Investigation into the properties of physical chemistry of methane series by density functional theory

The density functional theory method using B3' exchange and Lee-Yang-Parr's correlation function (B3LYP) with 6-31G** basis set were used to study and optimize equilibrium structure and fundamental vibrational frequencies in the equilibrium for CH4, CH3D, CH2D2, CHD3 and CD4. The relative result error from 1.1% to 6.3% were obtained between the calculation and the observation values of fundamental vibrational frequencies. After scaled, the relative result error is between 0.1% and - 2.9%. This method is used to study period properties of methane that some hydrogen atoms are replaced by tritium atoms. Following deuterium or tritium atom number rising in methane, entropy increases, but zero-point vibrational energy, heat capacity and enthalpy reduces