MOLECULAR QUADRUPOLE-MOMENT FUNCTION OF AMMONIA

The zz component of the molecular quadrupole moment Theta of ammonia i s computed at the restricted Hartree-Fock and coupled cluster theory w ith singles and doubles (CCSD) levels of approximation as a function o f the symmetric stretching and inversion motions, considering a wide r ange of values for the corresponding vibrational coordinates (z is the threefold symmetry axis). Using these results and the nonrigid invert or rovibrational wave functions, effective molecular quadrupole moment s are obtained for the low lying roinversional states of (NH3)-N-14. T he predicted values exhibit a sizable and nonmonotonic dependence on t he corresponding inversional (upsilon(2)) and rotational (J,k) quantum numbers. The calculated effective quadrupole moments for the vibratio nal (inversional) ground states (upsilon(2) = 0(+/-)) are found to be about 10% smaller in absolute value than the two available experimenta l results [-2.42 +/- 0.04 a.u.; J. G. C. M. van Duijneveldt-van de Rij dt and F. B. van Duijneveldt, J. Mel. Struct. (THEOCHEM) 89, 185 (1982 ) and -2.45 +/- 0.3 a.u.; S. G. Kukolich, Chem. Phys. Lett. 5, 401 (19 70); 12, 216 (1971) (E)]. Our CCSD value of the quadrupole moment pert aining to the equilibrium geometry is in a very close agreement with t he best theoretical values that are available in the literature. This fact and the high quality of the roinversional wave functions that wer e employed lead us to believe that our theoretical predictions may be more accurate than their experimental counterparts. In particular, the difference between the theoretical and experimental values casts doub t on the reliability of the error bars (+/- 0.04 a.u.) in one of the e xperimental results quoted above. (C) 1996 American Institute of Physi cs.