General Force Field of NF3, PF3, and AsF3 by the Combined Use of Vibrational Frequencies, Centrifugal Stretching, and Coriolis Coupling Constants

Abstract

Possible sources of the discrepancies between zeta parameters and centrifugal distortion constants found in the attempt to determine the general force field of NF₃, PF₃, and AsF₃ have been analyzed. Large anharmonicities on the ν₂ bending modes cannot be responsible for the results obtained by other authors; the discrepancies become smaller only by assuming that the ν₄ bending modes are considerably anharmonic. On the other hand, for smaller values of the bond angle in the case of PF₃ and AsF₃ it is possible to obtain consistency between infrared and microwave data. For a bond angle of 98° for PF₃, in agreement with recent results of electron diffraction measurements, it is possible to determine a force field which gives zeta parameters and centrifugal distortion constants within their experimental accuracy even by using anharmonic vibrational frequencies. Consistency between infrared and microwave data for AsF₃ is obtained for ∠FAsF=99°±1°. In the case of NF₃ the structure is very accurately known and a force field consistent with all the experimental data has been determined even by using anharmonic vibrational frequencies.