JOURNAL• FMOLECULARSPECTROSCOPY 154,72-82(1992) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO Determination of the Structure of C02-H&O zyxwvutsrqponmlkjihgfed THOMAS A. BLAKE ’ AND STEWART E. NOVICK zyxwvutsrqponmlkjihgfedcbaZY Department of Chemistry, Wesleyan University. Middletown. Connecticut 06459-0180 AND F. J. LOVAS AND R. D. SUENRAM Molecular Physics Division, National Institute of Standards and Technology. Gaithersburg, Maryland 20899 The structure of the weaklybound complex C02-H&IO has been determined by pulsed-jet Fourier-transform microwave spectroscopy. The spectroscopic constants of three isotopomers, C02-H2C0, “CO2-H2C0, and C02-D2C0. have been determined. Two distinct states of the complex are seen. These are caused by inversion doubling from “rotation” of the formaldehyde about its twofold axis. The complex is planar with the Cl axis of formaldehyde almost parallel to the Dab axis of carbon dioxide. The center-of-mass to center-of-mass distance is 3. I7 I A. Cl 1992 Academic Press. Inc. INTRODUCTION Formaldehyde is one of the simplest carbon-containing compounds which is com- monly thought of as a true organic molecule. In our ongoing attempt to understand the nature of the “van der Waals” bond we have studied a series of formaldehyde- containing species. Work on six other formaldehyde-containing complexes, from our laboratory and elsewhere, have been completed. Three of these complexes, HF-H&Z0 ( I, 2)) HCI-H$O (3), and HCN-H2C0 (4)) share an essentially common structure: they are planar molecules with the H of the hydrogen halide or cyanide pointing toward the oxygen atom of the formaldehyde. Thus it is reasonable to view these three complexes as bound via a hydrogen bond with the formaldehyde as the proton acceptor. The fourth complex, C2H2-H2C0 (5) is planar and appears to bond via two nonlinear hydrogen bonds, one between acetylene’s hydrogen and the oxygen and one between formaldehyde’s proton and the C-C bond. The structure of the formaldehyde dimer, ( H2CO)* (6)) has the two planes of the monomer units perpendicular to each other with a nearly antiparallel orientation of the CO groups. This complex, like the dimer of water, exhibits a set of rather interesting internal inversion motions. Finally, Ar- H2C0 ( 7) exhibits hindered internal rotation of the formaldehyde moiety about the C-O bond. The effective structure is one in which the Ar atom sits almost directly above the center of mass of the formaldehyde. We report here the seventh structure, that of the complex CO*-H&O. We have studied the rotational spectrum of the three isotopomers CO*-H*CO, ‘3C02-HzC0, and C02-D2C0. The complex is planar with the C’, axis of formaldehyde almost parallel to the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA D, ,, axis of carbon dioxide. I Present address: NASA/Ames Research Center, Mail Stop 245-4, Moffett Field. CA 94035 1000. 0022-2852192 $5.00 72 Copynghr 0 IW? by Academx Press. Inc. All rights of reproduclion in any Form reserved.