Study On Anharmonic Effect Of HM(:X)XH(M=C,Si;X=S,O) And Its Isotope Derivatives

According to the RRKM theory and TST theory,the rate constants of unimolecular dissociation of HM(:X)XH(M=C,Si;X=S,O)and its isotopic derivatives in the canonical and microcanonical systems are calculated with YL method.The anharmonic effect and isotope effect are investigated for the decomposition reaction.Geometric parameters,frequency and zero point energy of the reactants and TSs are obtained at B3LYP/6-311+G(3df,2p)level.Single point energy is computed at QCISD(T)/6-311++G(3d2f,2pd)level.The barrier height is obtained from zero-point-energy correction,and the total number of states,the density of state and the rate constants are calculated.1.Whether the canonical system or the microcanonical system,the harmonic and anharmonic rate constants of unimolecular dissociation of HM(:X)XH(M=C,Si;X=S,O)and its isotopic derivatives increase with the temperature(or energy)increasing.2.When temperature is 700K,the calculated anharmonic rate constant of trans-HC(:O)OH decomposition(dehydration reaction)is 1.69×10-7 s-1,which is closer to the other theoretical value 1.94×10-7 s-1,indicating that the anharmonic effect needs to be considered in the title dissociation reactions.3.The anharmonic effect of HC(:O)SH,DC(:S)SD,HSi(:S)SH and DSi(:S)SD dissociation reaction are more prominent in both the canonical and microcanonical cases.In the canonical system,whether it is a harmonic model or an anharmonic model,the rate constants of HM(:X)XH and DM(:X)XD are not much different,indicating the isotope effect nearly disappears.In the microcanonical system,the difference between rate constants of the HM(:X)XH and DM(:X)XD is relatively large,indicating the isotope effect.