Search

The reactivity and spectra of formaldehyde isomers and dimeric complexes between them are studied with ab initio methods. A large number of complexes between H2CO, trans-HCOH, cis-HCOH is calculated. Infrared and Raman spectra of (H2CO)2 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 H2CO; the stabilization is due to classical hydrogen bonds. HCOH is a new example of an ambiphilic carbene. Addition products are formed from HCOH···H2CO complexes. The calculations show that, in agreement with matrix experiments, glycoaldehyde and methanol are easily formed. The formation of trans-HCOH occurs through a dimeric interaction with the shifting hydrogen. This bimolecular process is 9.6 kcal/mol (6–31G*) in favor of the unimolecular conversion. cis-HCOH might be formed via a nonplanar transition state, where also stabilization at the carbenic center is possible. When higher concentrations of HCOH are available, a hydrogen exchange mechanism easily transfers hydroxycarbene back to H2CO. 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.

This paper reports an ab initio CI calculation of the radiationless decay of the formaldehyde 1A2 state. First we derive quantitative conditions, which a basis set must satisfy if it is to be used for describing radiationless decay. We checked these conditions for the formaldehyde molecule and found them to be satisfied for the adiabatic Born–Oppenheimer set used in the calculation. We then derive a general equation for the coupling elements resulting from this basis set. With the method used, rotational coupling could be treated completely equivalent with vibrational coupling; this rotational coupling turned out to be not important in formaldehyde however. The coupling elements for D2CO are a factor 10 smaller than the corresponding ones in H2CO. The results of the calculation show, that the internal conversion in formaldehyde is an example of the so-called resonance case. Therefore the decay cannot be described by the model proposed by Yeung and Moore, where S1¿S0 internal conversion is the rate determining step in the photodissociation. Finally, we discuss the applicability of the ’’golden rule’’ in describing radiationless decay.

This paper reports an ab initio CI calculation of the radiative 1A1¿1A2 transition of H2CO and D2CO. Throughout the calculation the electronic wavefunctions and transition moments are explicitly calculated as functions of the nuclear geometry, contrary to the conventional Herzberg–Teller approach. The evaluation of the vibrational wavefunctions and integrals was made numerically. The results show that the excited state frequency for mode 3 has to be reassigned and that the calculated vibrational structure agrees well with the experimental intensities.