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. Botha-type andb-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 μa= 3.379(13) × 10−30C·m (μa= 1.043(4) D) and μb= 2.54(20) × 10−30C·m (μb= 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) Å with theC2vsymmetry axes of the monomers oriented at 19.3° from parallel with the dipole moments opposed. The complex is quite strongly bound with a harmonic pseudodiatomic stretching force constant,ks= 8.93 N/m, and hydrogen bond lengths of approximately 2.68 Å between the water oxygen atom and a CH2hydrogen atom, and 2.03 Å between a water hydrogen atom and the oxygen atom of H2CO. Expanded uncertainties (coverage factork= 2, i.e., two standard deviations) are shown in parentheses for each experimental value reported above.