A METHOD OF DETERMINING THE POTENTIAL BARRIERS RESTRICTING INVERSION IN AMMONIA, PHOSPHINE, AND ARSINE FROM VIBRATIONAL FORCE CONSTANTS

Author Institution: Randall Laboratory of Physics, University of MichiganThe height of the potential barrier restricting inversion in the ammonia molecule has been determined by several investigators using data on the hyperfine splitting of certain lines in the infrared and microwave spectra of that molecule. None of these methods can be applied to phosphine and arsine since no corresponding experimental data are, available. By assuming that the ammonia molecule can be inverted by gradually increasing the amplitude of the symmetrical deformation vibration, the force constant controlling this vibration can be used to plot a parabolic potential of the form $V = K (\Delta \alpha)^{2}$ on either side of the planar configuration. For ammonia the resulting calculated barrier height differs by only 2 percent from that derived from inversion splittings in the spectrum by the Manning potential function, viz. $2070 cm^{-1}$ (5.9 kcal/mole). This indicates that a similar method can be applied to phosphine and arsine, and when this is done the corresponding barrier heights are computed to be close to $6000 cm^{-1}$ (17.1 kcal/mole) and $17,000 cm^{-1}$ (48.4 kcal/mole). Such values are consistent with the absence of inversion splittings in the infrared and microwave spectra of phosphine and arsine since barriers of such a height would yield splittings considerably less than $10^{-5} cm^{-1}$