Microwave spectrum of the CH 3OH-NH 3 complex

Abstract

Microwave spectra of CH 3OH-NH 3 and 13CH 3OH-NH 3 have been obtained using a pulsed-nozzle Fourier-transform microwave spectrometer. The spectra, which are complicated by the internal rotation of the CH 3 and NH 3 groups, exhibit five K=0 states at the ≈ 1 K rotational temperature of the expansion. Four of these are metastable, excited internal rotor states and correlate to E states of free CH 3OH or NH 3. For the two states in which the NH 3 top is in its ground internal rotor state, Δ J=1, K=0 progressions are observed and fit to linear-molecule-type frequency expressions to obtain effective spectroscopic constants. For CH 3OH-NH 3 these constants are: ( B+C)/2=3690.119(2) and 3686.114(2) MHz; eQq aa =−3.184(10) and −3.182(9) MHz; and μ a =2.854(9) and 2.956(14) D. The complexes are hydrogen bonded with an OH---N bond length of 2.02 Å. This bond length is similar to that found by Herbine and Dyke in the related HOH-NH 3 complex, 1.97 Å. Ab initio SCF calculations have also been carried out for the CH 3OH-NH 3 and HOH-NH 3 complexes in order to compare interaction-energy components and origins of the dipole moment enhancements.