Stability, properties and electronic g-tensors of H2CO- as stabilized in H2CO•Na complexes

Using HF and MP methods with high-level basis sets, the geometries, frequencies, electric dipole moments, electric field gradients, Fermi contact terms and other properties were calculated for three isomers of the H2CO . Na complex. Isomers I and II are ion-pair complexes of type H2CO-Na+, whereas III is of van der Waals type. The H2CO- moiety in isomers I and II is confirmed to be nonplanar. Isomer III is the most stable one, although energy differences between the isomers are very small (some less than 1 kcal/mol). The electron-spin magnetic moments (g-tensors) were calculated both by ROHF and multireference CI methods. Various basis sets and several gauge origins were used. The gauge dependence is small. At the ROHF level, Delta g(av) in ppm (Delta g = g-g(e)) is about 1300 for I, 800 for II, and -200 for III. Correlated values are expected to similar to 400 ppm higher for isomer I and similar to 250 ppm for isomer II. The large second-order contributions to Delta g of I and II result mainly from the sigma --> pi* and n --> pi* states, which are highly excited states of the complexes. Experimental studies on H2CO- in aqueous solution find Delta g(av) = 1400-1600 ppm, in good agreement with our results. (C) 1997 Elsevier Science B.V.