THE TORSIONAL ROTATIONAL SPECTRUM AND STRUCTURE OF THE FORMALDEHYDE DIMER

The microwave spectra of (H2CO)2 and (D 2CO)2 have been observed with a poised beam, Fabry-Perot cavity, Fourier transform microwave spectrometer. Both species exhibit a-type spectra which are split by internal rotation of each monomer unit and an interchange of donor-acceptor bonding roles analogous to the water dimer. Rotational analysis of each spectrum provides the constants A = 18583.(54) MHz,1/2(B + C) = 3272.105(34) MHz, and B - C = 503.92(17) MHz for (H 2CO)2 and A = 14 862.1(35) MHz, 1/2(B + C) = 3030.2366(37) MHz, and B - C = 490.977(18) MHz for (D2CO)2. Stark effect measurements result in derived electric dipole components μa = 0.858(4) D and μb = 0.027(10) D for (H 2CO)2 and μa = 0.908(4) D and μb = 0.095(4) D for (D2CO)2. The geometry obtained from fitting the derived moments of inertia has the planes of the two monomer units perpendicular in a nearly antiparallel orientation of the CO groups with a center-of-mass distance of 3.046(17) Å. The shortest carbon to oxygen distance (2.98 Å) and hydrogen to oxygen distance (2.18 Å) between the monomer units are indicative of a dual bond interaction to form a ring structure. © 1980 American Institute of Physics.