HIGH-LEVEL AB-INITIO PREDICTION OF THE STRUCTURE AND INFRARED-SPECTRA OF FORMALDEHYDE WATER RADICAL-CATION COMPLEXES

In a previous work we have identified two possible structures for the radical cation obtained by ionization of hydrogen-bonded formaldehyde-water complexes [Coitiño et al., J. Am. Chem. Soc. 115, 9121 (1993)], a hydrogen-bonded and an addition-like complexes. We observed that the results were highly dependent on the method of calculation employed. Inclusion of correlation was crucial for obtaining the correct structures of some of the complexes. In this work we used high-level ab initio calculations in order to predict the equilibrium structure of these two complexes, the possibility of its existence in gas phase, and the infrared spectrum to be expected in each case. A series of progressively more sophisticated basis sets was used to assess the effect of the quality of the calculations on the expected results. Also, full geometry optimization with each basis set was performed at the second-order Møller-Plesset level, and correlation energy was calculated at the fourth-order Møller-Plesset level to assess the contribution of this factor to the global result. Confirming our previous results, we found that correlation affects the hydrogen-bonded radical-cation complex more than the addition one, due to the different bonding patterns in each of them. Both complexes are stable - toward decomposition to the fragments or to CO+H+H 3O+ - by several kcal/mol at all levels of theory. The hydrogen-bonded complex is more stable than the additional one by a respectable amount (13 kcal/mol at the highest level used here), lending support to our previous analysis of the reactions of the former as the main channels for evolution of the formaldehyde-water radical cation. The H-bonded complex [H 3O+⋯HCO.] presents two characteristics, very intense absorptions which should allow identification of this radical cation if present in the experimental setup. These transitions are also present in the HCO. radical but their intensity is enhanced by an order of magnitude due to the coupling with the proton in H3O+. We conclude that the combination of stability and characteristic infrared transitions should make this radical-cation complex a relatively easy target for experimental determination. © 1995 American Institute of Physics.