Force Constants and Frequency Assignments for Some Substituted Methylacetylenes

Force constants have been determined for methylacetylene using the Wilson FG matrix method. It was found that of the thirteen force constants required for the potential energy function for methylchloroacetylene, methylbromoacetylene, and methyliodoacetylene, eight could be transferred from the set obtained for methylacetylene. Since only four of the ten fundamental frequencies of the methylhaloacetylenes were used in establishing the force constants, all the frequencies were calculated to see if the sets of force constants for the methylhaloacetylenes were reasonable. With the exception of one frequency, the agreement between the calculated and observed values was within two percent. As the calculations served to identify the fundamental frequencies for the methylhaloacetylenes, it was found that previous assignments by Cleveland and Murray of the observed Raman frequencies for methylbromoacetylene and methyliodoacetylene had to be modified. New assignments are given for some of their observed values for these two molecules and assignments also are given for their observed Raman frequencies for a solution of methylchloroacetylene in ethylbromide for which they made no assignments. The fundamental frequencies for methyldeuteroacetylene also were calculated using the same force constants, interbond angles, and equilibrium values for the bond distances as for methylacetylene, in order to determine how a pure mass change affects the frequencies of methylacetylene. As a result of this investigation the C–C and C≡C stretching frequencies were found to be greater for the methylhaloacetylenes than for methylacetylene or methyldeuteroacetylene. The C — X (X = halogen atom) stretching frequencies for the methylhaloacetylenes were found to be less than for the corresponding methyl halides despite the fact that the C — X stretching force constant for the methylhaloacetylenes always was greater than for the corresponding methyl halides.