THE NUCLEAR-QUADRUPOLE COUPLING-CONSTANTS AND THE STRUCTURE OF THE PARA-PARA AMMONIA DIMER

Expressions are derived for the nuclear quadrupole splittings in the E 3 and E4 (para-para) states of (NH3) 2 and it is shown that these can be matched with the standard expressions for rigid rotors with two identical quadrupolar nuclei. The matching is exact only when the off-diagonal Coriolis coupling is neglected. However, the selection rules for rotational transitions are just opposite to those for the rigid rotor. Hyperfine splittings are measured for the J = 2←1 transitions in the E3 and E4 states with |K| = 1; the quadrupole coupling constants χaa 0.1509(83) MHz and χbb-χcc=2.8365(83) MHz are extracted from these measurements by the use of the above mentioned correspondence with the rigid rotor expressions. The corresponding results are also calculated, with and without the Coriolis coupling, from the six-dimensional vibration-rotation-tunneling (VRT) wave functions of (NH3) 2, which were previously obtained by Olthof et al. [E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. From the comparison of χaa with the measured value it follows that the semiempirical potential and the resulting VRT states of Olthof et al. are very accurate along the interchange (θA,θB) coordinate. From χbb - χcc it follows that this potential is probably too soft in the dihedral angle γ̄= γA - γB, which causes the torsional amplitude to be larger than derived from the experiment. © 1995 American Institute of Physics.