Photochemistry of CH3Cl: Dissociation and CH center dot center dot center dot Cl Hydrogen Bond Formation
Résumé
State-of-the-art electronic structure calculations (MR-CISD) are used to map five different dissociation channels of CH3Cl along the C-Cl coordinate: (i) CH3((X) over tilde (2)A(2)'') + Cl(P-2), (ii) CH3(3s(2)A(1)') + Cl(P-2), (iii) CH3+((1)A(1)' + Cl-(S-1), (iv) CH3(3p(2)E ') + Cl(P-2), and (v) CH3(3p(2)A(2)'') + Cl(P-2). By the first time these latter four dissociation channels, accessible upon VUV absorption, are described. The corresponding dissociation limits, obtained at the MR-CISD+Q level, are 3.70, 9.50, 10.08, 10.76, and 11.01 eV. The first channel can be accessed through n sigma* and n3s states, while the second channel can be accessed through n(e)3s, n(e)3p(sigma) and sigma 3s states. The third channel, corresponding to the CH3+ + Cl- ion-pair, is accessed through n(e)3p(e) states. The fourth is accessed through n(e)3p(e), n(e)3p(sigma), and sigma 3p(sigma), while the fifth through sigma 3p(e) and sigma(CH)sigma* states. The population of the diverse channels is controlled by two geometrical spots, where intersections between multiple states allow a cascade of nonadiabatic events. The ion-pair dissociation occurs through formation of CH3+center dot center dot center dot Cl- and H2CH+center dot center dot center dot Cl- intermediate complexes bound by 3.69 and 4.65 eV. The enhanced stability of the H2CH+center dot center dot center dot Cl- complex is due to a CH center dot center dot center dot Cl hydrogen bond. A time-resolved spectroscopic setup is proposed to detect those complexes.