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

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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.