THE ROLE OF RADICAL CATIONS IN THE RADIATION-CHEMISTRY OF ALDEHYDES -AN ESR STUDY AND A QUANTUM-CHEMICAL ANALYSIS

The electronic and geometrical structures of the radical cations of et hanal, propanal, butanal, pentanal, and their distonic isomers were ca lculated using the MNDO-UHF semiempirical method. The results revealed maximum positive charges and spin populations at the oxygen atoms, ad jacent carbon atoms, and also at the aldehydic H atoms. The C-H bond c orresponding to the aldehydic proton as well as the adjacent C-O and C -C bonds in the radical cations proved to be significantly weakened in comparison with those in neutral molecules. Accordingly, these bonds exhibit maximum tendency to rupture. The rupture of the bond results i n acyl radicals (products of deprotonation) and products of alpha- and beta-cleavage (the latter species were identified in the mass spectra of aldehydes). Distonic radical cations are considerably more stable than the corresponding classical isomers. The calculation results are supported by the ESR data for aldehydes irradiated in Freon matrices a t 77 K and in a liquid state. Both acyl radicals resulting from deprot onation of the radical cations and the (CH3CHOH)-C-. radical;species r esulting from H atom transfer tin the case of ethanal) were detected b y ESR. In addition, the products of rearrangement of ''normal'' radica l cations to distonic isomers were observed ((CH3CHCH)-C-.=OH+ and( .) CH(2)CH(2)CH=OH+ for propanal, and (CH2CH2CH2CH)-C-.=OH+ for butanal).