The reactivity and spectra of formaldehyde isomers and dimeric complexes between them are studied with ab initio methods. A large number of complexes between H,CO, trans-HeOH, cis-HeOH is calculated. Infrared and Raman spectra of (H 2 eO), are calculated with relatively simple methods using spectroscopic masses and scaled force constants. In this way, the structure of dimers in matrices can be deduced. Hydroxycarbene (HCOH) plays a key role in a model that explains a large number of experimental facts of the nonzero pressure photochemistry. Hydroxycarbene forms complexes with H,CO; the stabilization is due to classical hydrogen bonds. HCOH is a new example of an ambiphilic carbene. Addition products are formed from HCOH· .. H,CO complexes. The calculations show that, in agreement with matrix experiments, glycoaldehyde and methanol are easily formed. The formation of trans-HeOH -occurs through a dimeric interaction with the shifting hydrogen. This bimolecular process is 9.6 kcallmol (6-310') in favor of the unimolecular conversion. cis-HeOH -might be formed via a nonplanar transition state, where also stabilization at the carbenic center is possible. When higher concentrations of HCOHJare available, a hydrogen exchange mechanism easily transfers hydroxycarbene back to H,CO. Several experiments are suggested in this paper; notably, molecular beam and isotopic-mixture experiments will give useful information. The involvement of HCOH in the light-induced formose reaction is suggested.

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