Normal coordinates and potential energy distributions of methyl acetylene and some halogen substituted analogues

The normal coordinates and potential energy distributions of methyl acetylene, methyl-d₃ acetylene and some halogen substituted analogues (CH₃CCX; X = H, D, Cl, Br, I) have been calculated using previously determined force fields. For methyl chloro-acetylene, the chlorine-35,37 isotope frequency splitting has been calculated. In addition, for methyl acetylene, the harmonic frequencies were obtained and plotted for hypothetical molecules in which the mass of X varied from 1 to 1000 a.m.u. The frequencies for these molecules are compared and discussed.