Microwave and tunable far-infrared laser spectroscopy of the ammonia–water dimer

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Abstract
Microwave and far-infrared spectra of the H3N–HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a-type pure rotational microwave data extend the previous m=0, K=0 A symmetry manifold measurements of Herbine and Dyke [J. Chem. Phys. 83, 3768 (1980)] to higher frequency and also provide an additional set of microwave transitions in the mK=+1 E symmetry manifold. Two sets of five b-type rotation–tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V3=10.5±5.0 cm−1 for the barrier height to internal rotation for the NH3 monomer. A nonlinear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos−1[〈λz〉] determined from 〈λz〉, the projection of the ammonia’s angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of ∼700 cm−1 is derived for the water tunneling motion about its c axis.