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
Ab initio and semi-empirical (MNDO, M I N D 0 / 3 ) studies of the interaction of NO,+ with ethylene and benzene are reported. 6-31 G**//4-31 G Calculations have been performed at selected points of the ethylene-nitronium ion energy hypersurface, as well as RMP2 4-31 G i 4 -3 1 G calculations, from which it is found that the non-classical x-complex is more stable than the classical nitro complex. Fully optimised STO-3G calculations are described for 5 stationary points of the benzene-NO,+ hypersurface; 3-21 G calculations at these geometries show that the nitrito o-complex and a 5-membered-ring structure have similar energies. 4-31 G//STO-3G Calculations however show the nitro form to be the next most stable to the nitrito form. Geometry optimisation at the 3-21G level of the nitrito and nitro forms shows this finding to persist at both the 3-21G and 4-31G levels. If correlation is introduced via GVB/1 calculations both nitro compounds are shown t o possess incipient biradicaloid character. Finally semi-empirical M IND0/3-solvaton calculations of solvation effects show that the nitro form is the most stabilised in both systems.This work seeks to extend ab initio work already reported on the gas-phase nitration of ethylene, and to provide a moderately complete low-level ab initio study of the nitration of benzene.The types of structures considered in the two studies are the same, as are the methods of study. Neither study is definitive, but interesting new aspects of the systems are discovered. In addition to the ab initio data semi-empirical MIND0/3 and MNDO data are reported.The paper is presented as two entirely separate introductions, but with a unifying discussion section.Ethylene.-Bernardi and Hehre originally reported STO-3G calculations on the ethylene-nitronium ion system, but they did not report the structures or absolute energies. These data now appear to be lost2 The essential conclusions of that work were that the 4and 5-membered-ring systems (I) and (11) are considerably more stable than the classical (open) nitroform (111).N , "
Complete structure determinations for the Wheland intermediates in the electrophilic substitution of benzene and toluene, as well as for the unprotonated species, have been carried out using the MIND0/2' method. Studies o f the potential energysurfaces about these energy minima afforded the frequencies of vibration(in the harmonic approximation), from which were constructed vibrational partition functions and vibrational contributions to the free energy and enthalpy functions. Thermodynamic functions for the various species are recorded. The results are compared with existing structural and thermodynamic data, and the relationship of the present calculations to the theory of aromatic substitution is discussed.DATA on gas-phase aromatic substitution reactions are becoming more readily available following the work of (for example) Cacace,l Olah,2 and Rice,3 using various contrasting techniques. Of considerable theoretical interest is the prototype electrophilic substitution involving the attack of a proton on an aromatic system. Experimental data for similar reactions have been accumulated by C a ~a c e , ~-~ using the helium tritiide cation as the electrophile. The reactions show little intramolecular or intermolecular selectivity, but the isomer proportions are not those corresponding to simple statistical attack of the electrophile. It is towards an understanding of the isomer distribution that this and subsequent work will be directed.The essential ideas of activated complex theory as applied to ion-molecule reactions were expounded by Eyring et a1.' in a study of the hydrogen moleculehydrogen molecule ion reaction, and the following features may be expected to apply to electrophilic aromatic substitution in the gas phase. If the proton and aromatic molecule have no angular momentum about their common centre of gravity there is no activation energy, i.e. the barrier is at infinity. The presence of relative angular momentum in the system produces a centrifugal barrier to reaction, the magnitude of which depends on the rotational quantum number. The varying magnitude of this barrier, in opposition to the attractive ion-molecule (rotation free) potential energy surface, causes the structure of the activated complex to be a function of the rotational quantum number.8As a preliminary study attention has been focused on the ionic reaction products ; the transition states themselves will be the subjects of further investigations. For the Wheland intermediates and related species the geometric structures have been optimised, and the vibrational frequencies determined. This information allows -f l'art I,
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