On the Normal Vibration Frequencies of NH3, PH3 and AsH3

Abstract

It is shown that the assumption of valence forces for NH₃, PH₃ and AsH₃ leads to the conclusion that the high frequency, degenerate normal vibration, although theoretically active, does not appear as a fundamental in the infrared spectra of these compounds. This conclusion follows from the fact that the remaining three fundamentals, all of which have been observed, can be accounted for satisfactorily with proper values of the two constants of the potential function for valence forces. The values calculated for this absent frequency with the same force‐constants are 3450 cm^—1, 2340 cm^—1 and 2125 cm^—1 for NH₃, PH₃ and AsH₃, respectively. The frequency for NH₃ may be fixed more precisely at 3470 cm^—1 if we interpret the 4417 cm^—1 band as a combination band involving this frequency and the 950 cm^—1 frequency. Other evidence is given in support of a value for the frequency in the neighborhood of 3450 cm^—1. There is, however, the difficulty of explaining why the predicted frequencies are not observed. The 4417 cm^—1 band is probably to be ascribed to the fourth fundamental in case this fundamental does actually appear. If this is the case, then there is an extreme departure from valence forces in NH₃ and probably likewise in PH₃ and AsH₃. Frequencies are computed for NH₂D, ND₂H and ND₃ to provide a means of testing the value predicted upon the assumption of valence forces for the fourth fundamental in NH₃.