The microwave spectrum and structure of CH2O-H2O

The microwave spectrum of the formaldehyde-water complex (CH2O-H2O) has been studied with a pulsed-beam Fourier transform Fabry-Perot cavity spectrometer. Both a-type and b-type transitions were observed for each of the isotopic species studied. To provide additional structural information, the spectra of H2O, HDO, and D2O substituents combined with H2CO and D2CO were assigned. Measurement of the dipole moment components yielded the values mu(a) = 3.379(13) X 10(-30) C . m (mu(a) = 1.043(4) D) and mu(b) = 2.54(20) X 10(-30) C . m (mu(b) = 0.76(6) D) and indicated that the dipole moment vectors are anti-aligned. The molecular structure derived from the moments of inertia has a center of mass separation of 3.00(3) Angstrom with the C-2v symmetry axes of the monomers oriented at 19.3 degrees from parallel with the dipole moments opposed. The complex is quite strongly bound with a harmonic pseudodiatomic stretching force constant, k(s) = 8.93 N/m, and hydrogen bond lengths of approximately 2.68 Angstrom between the water oxygen atom and a CH2 hydrogen atom, and 2.03 Angstrom between a water hydrogen atom and the oxygen atom of H2CO. Expanded uncertainties (coverage factor k = 2, i.e., two standard deviations) are shown in parentheses for each experimental value reported above. (C) 1996 Academic Press, Inc.