A NEW FORCE FIELD MODEL OF THE ANHARMONICITIES FOR THE $n\nu_{3}$ VAIBRATIONAL LADDER IN $SF_{6}$
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Date
1980
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Ohio State University
Abstract
We model the anharmonicities in $SF_{6}$ by superposing a Morse potential function in the S-F bond stretching coordinate, \[ V_{M} = D_{M} \{-1 + (1 -\exp[-a(r- r_{M})])^{2}\}, \] plus a Urey-Bradley interaction which assumes a Lennard-Jones potential between each of the 12 nearest-neighbor pairs of non-bonded fluorine atoms, \[ V_{UB} = V_{0} \left[\left(\frac{q_{0}}{q}\right)^{12} - 2\left(\frac{q_{0}}{q}\right)^{6}\right], \] with a quadratic potential determined from normal frequencies. When values of the two characteristic distance parameters $r_{M}$ and $q_{0}$ are selected for model calculations, the other parameters, $D_{M}$, a, and $V_{0}$, are determined from observed values of (a) the dissociation energy of the $F-SF_{5}$ $bond,^{1}$ (b) the length of the S-F $bond,^{2}$ and (c) the quadratic potential constant in the S-F stretching $coordinate.^{3}$ We use this model to calculate Hecht’s second-order coefficients$4 $X33, $G33, and $T33, of anharmonicity for the $nnU$3 ladder. A region is found in which pairs of values ($r_{M}$, qO) give $X_{33}$, $G_{33}$, and $T_{33}$, all within 2% of their experimental values.
Description
$^{1}$T. Kiang, R. C. Estler, and . N. Zare, J. Chem. Phys. 70. 5925-6 (1979). $^{2}$V. C. Ewing and L. E. Sutton, Trans. Faraday Soc. 59, 1241-7 (1963). $^{3}$R. S. McDowell, J. P. Aldridge, and R. F. Holland, J. Phys. Chem. 80, 1203-7 (1976). $^{4}$K.T. Hecht, J. Mol. Spectrosc. 5, 355-389 (1960)
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