The photochemistries of methanol and methylamine are computationally rationalized using ab initio methods. It is shown that the lowest excited singlet states of these and related materials are n,3s Rydberg in character. These states are computationally shown to evolve adiabatically to the valence ground states of the various radical products along the NH, CN, CO, and O H bond rupture pathways in methylamine and methanol, respectively. The N H and CN n,3s bond rupture surfaces display minima in the region of the Franck-Condon excitation geometry. The N H bond ruptures in n,3s singlet ammonia and methylamine are shown to be identical in having small activation energies. The CN excited state bond rupture shows a much larger activation energy, indicating that trialkylamines should display some photostability in the region of the 0-0 transition. In methanol, neither CO nor OH excited-state bond rupture coordinates show minima. The observed preference for O H bond rupture in the UV Photochemistry of methanol is rationalized as resulting from the lighter mass of the H atom as well as the computed more repulsive nature of the OH bond rupture. In methanol, both 1,2-and l,l-Hz molecular elimination excited-state pathways are examined. 1,2-H2 elimination is found to have a small activation energy while the 1,l-elimination is difficult. The concept of de-Rydbergization is fully developed in order to rationalize the change in electronic character occurring along these various excited state pathways.The goal of this article is to characterize theoretically the absorption threshold photochemistry of methanol, methylamine, and related small molecules. We will show that the excited states generated in the absorption threshold region are all singlet and Rydberg (n,3s) in character. We will also show that there are adiabatic surfaces which allow these Rydberg states to evolve directlv to the valence states of the fragmentation products.'Centre de Mkanique Ondulatoire AppliquEe.Pre;ious emphasis on the properties oT small-molecule excited *University of Lancaster.states has been largely spectroscopic.' Standard photochemical

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-A review is given of a relatively unknown area of the electronic structures of Rydberg radicals. The primary examples of such structures are H , NH4 and H30. The condensed phase existence of the latter 8wo has long been proposed. Mercury amalgams of organic ammonium radicals, R4N, have been reported. Theoretical studies are reported on some of these species.in liquid ammonia vis-a-vis NH4 + e .An estimate is made of the-relative stability of NH4

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Portions of the ground and 3s,3px,y,z Rydberg excited state C–H and C–H2 fragmentation pathways are computed for the CH3 radical at the 4-31G+Rydberg+CI level. It is concluded that the B̃ state photochemistry of CH3 should give principally C–H bond rupture in analogy with the N–H bond rupture occurring in the à state of NH3. Higher energy photolyses should give a mixture of products whose mechanisms of formation will be difficult to establish. Rydberg extended molecular orbital and state correlation diagrams are developed for these transformations. The theoretical relationship between the behavior of the B̃ state C–H bond rupture surface in CH3 and the fragmentation occurring in the à states of NH3, H2O, and CH4 is established. Finally, an analysis is made as to the probable adiabatic photochemistry of the 3s Rydberg states of alkyl radicals.