THE MICROWAVE-SPECTRUM AND STRUCTURE OF CH3NO2-H2O

The microwave spectrum of the nitromethane-water complex (CH3NO2-H2O) has been studied with a pulsed-beam Fourier-transform Fabry-Perot-cavi ty spectrometer. Both a-type and b-type transitions were observed for the A state of the complex with the b-type transition being more inten se by a factor of 2. Critical to the rotational assignments were well resolved N-14 nuclear electric quadrupole transitions, and the incorpo ration of the pulsed nozzle in one of the mirrors which provided a bea m coaxial with the cavity axis to attain linewidths of about 2 kHz (fu ll-width at half maximum-FWHM). To provide additional structural infor mation, the spectra of the HDO, D2O, and CD3NO2 substituents were assi gned. The molecular structure derived from the moments of inertia has a center of mass separation of 3.506(7) Angstrom. The moments of inert ia can not distinguish between two possible forms of the complex, one with the dipole moment vectors aligned and the other with them antiali gned. Measurement of the dipole moment components, mu(a) = 4.15(4) X 1 0(-30) C . m (mu(a) = 1.243(11) D) and mu(b) = 5.96(4) X 10(30) C . m (mu(b) = 1.706(12)D), resolves this difficulty and shows that the dipo le moment vectors are antialigned. The molecular complex is quite stro ngly bound with a harmonic pseudodiatomic stretching force constant, k (s) = 9.30 N/m, and hydrogen bond lengths of approximately 2.1 Angstro m between the water O atom and a CH3 H atom, and between a water H ato m and an oxygen atom of NO2. One standard deviation uncertainty is sho wn in parentheses for each experimental value reported. (C) 1995 Acade mic Press, Inc.