The Force Constants for CF4 and C2F6

Abstract

Vibrational potential functions of the quadratic type for CF₄ and C₂F₆, which assume no interaction between the CF₃ groups in C₂F₆ and include only the interaction terms involving bond‐bending with bond‐stretching motions, are used with the observed normal frequencies to evaluate a set of force constants for the molecules. The force constants associated with the CF₃ group are used interchangeably between the two molecules. The reproduction of the observed values of the normal frequencies resulting from the use of the force constants is of the order of five percent or better except for three of the frequencies. The omission of terms in the potential function for the interaction of the CF₃ groups is largely responsible for the relatively poor reproduction of the type E_u frequencies of C₂F₆. Additional interaction terms, e.g., between bond‐bending motions, are necessary for the improvement of the agreement in the case of the type F₂ frequencies of CF₄.