Theoretical studies of the (alpha)-clevage and hydrogen abstraction reactions of some ketones and thiones.

Abstract

Carbonyl and thiocarbonyl compounds undergo a variety of photochemical reactions. The Norrish type I cleavage reaction (?-cleavage reaction) and the intermolecular hydrogen abstraction reactions have been considered in the present study.

When an unsaturated system is excited, it leads to the cleavage of ?-bonds situated in the position ? to the chromophore. In the case of a carbonyl compound, the primary step is the cleavage of the ?-bond to produce a radical pair comprising an alkyl and an acyl radical.

Ketones in their lowest triplet state (usually n, ?*) also abstract a hydrogen atom from a hydrocarbon chain by an intermolecular as well as an intramolecular (Norrish type II) process. Like ketones, thiones also undergo hydrogen abstraction reactions. While ketones normally abstract hydrogen in their lowest n, ?* states, thiones undergo photoreduction from both the n, ?* and n, ?* states.

A brief account of the ?-cleavage reactions of ketones and thioketones is given in the first few sections of Chapter I. Correlation diagrams that have been used to understand the ?-cleavage processes are discussed. The correlation diagrams often fail to reveal the smaller energy barriers in the potential energy surfaces. Reliable data on the activation energies of most photochemical unimolecular reactions are scarce. A scrutiny of the values of the activation parameters for the ?-cleavage processes reported in the literature show large discrepancies for the same molecule and unexpectedly large contrasts between different molecules. The later sections describe the intermolecular hydrogen abstraction reactions of ketones and thioketones. The perturbation procedure employed to understand this reaction is briefly outlined. The aim and scope of the present investigation are highlighted.

Ab initio studies of the radical decomposition of some carbonyl and thiocarbonyl compounds are described in Chapter II. Results of the calculations already reported on some ketones and thioketones are compiled. In the present work, calculations are done on acetaldehyde and formaldehyde using the Gaussian-70 program with the STO-3G and the 4-31G basis sets. It is observed that the ab initio calculations using the small basis set Gaussian functions overestimate the activation barriers for the photofragmentation reactions. If these values are to be believed, then none of these molecules can undergo photodissociation from their lowest triplet or the first excited singlet states. Reliable and more accurate values can be obtained only if large basis set ab initio calculations with extensive configuration interaction (C.I.) and geometry optimization at the C.I. level, at all points on the potential energy surface are performed. However, such calculations are highly labour-intensive and costly even for simple organic molecules. Thus, our next step has been to apply suitably modified semi-empirical methods (which need relatively less computer time) to these processes in a series of ketones and thioketones, in order to predict reliable trends in the heights of the activation barriers.

In Chapter III, an account is given of these semi-empirical molecular orbital studies of the radical decomposition of some symmetric and non-symmetric ketones in their lowest excited states. The CND0/2-CI (Complete Neglect of Differential Overlap) and the MIND0/3-CI (Modified Intermediate Neglect of Differential Overlap) methods are used to calculate the Potential Energy Surfaces (PES) of the reactions in a few ketones. The Self-Consistent Field (SCF) procedures used in the semi-empirical theories are described. A brief discussion is given on the use of the SCF procedures and the method of calculation of the PES. The calculated activation barriers for the photodissociation into radicals in the lowest n, ?* surfaces obtained by the CND0/2-CI procedure are approximately an order of magnitude higher than that obtained by the MIND0/3-CI procedure. However, both these methods lead to approximately similar trends.

In an ?-cleavage process, when an ?-bond is elongated along its bond direction, the perpendicular motion of the carbonyl carbon with respect to the planar geometry of the reactant is found to be an important component of the reaction coordinate. In a non-symmetrical ketone, the selectivity of cleavage of the ?-bond is related to the amount of the bonding and antibonding characters in the highest occupied and the lowest unoccupied orbitals (MO’s) respectively of the ketone.

Chapter IV discusses the ?-cleavage processes in thiones. The theoretical PES for the ?-cleavage processes in some thiocompounds including thioformaldehyde are studied. The semi-empirical procedures outlined in the previous chapter are followed. Although the activation barrier for the ?-cleavage processes in thioformaldehyde is within the upper limit required for a unimolecular reaction to compete with phosphorescence, the non-observation of photocleavage is probably due to the effective quenching of the triplet state by the ground state. It is seen that, in the case of cyclopropenethiones, the ?-cleavage process can take place in its lowest triplet state with an activation barrier of about 52 kJ/mole. In p-dithiolactones, a similar process can take place in its lowest excited singlet state as its activation barrier is still lower.

The intermolecular hydrogen abstraction reaction of a photoexcited ketone and a thioketone is dealt with in Chapter V. The standard perturbation procedure is employed within the framework of a semi-empirical method (MIND0/3) to understand this process. The perturbation theory is discussed in brief. The reaction coordinate involves the motion of the two molecules and the stretching of the donor C–H bond toward the atom to which the hydrogen atom is transferred. While neither the activation barriers nor the geometries of the transition states can be predicted accurately from this analysis, the different photochemical reactivities of a ketone and a thione toward a common hydrogen donor can be understood satisfactorily. This is possible by a qualitative study of the relative positions of the energy levels of the lone pair MO of the ketone or thione H2CX (X = O, S) and the Highest Occupied Molecular Orbital (HOMO) of the hydrogen donor for the reaction in the n, ?* state. If the reaction is from the n, ?* state, the energy levels of the highest occupied n-MO of H2CX and the HOMO of the hydrogen donor become important. Results show that hydrogen abstraction by the heteroatom is observed in ketones and thiones in their n, ?* state by an in-plane process. The thiocarbonyl carbon atom in thiones in their n, ?* state can, however, abstract a hydrogen atom in a plane perpendicular to the molecular plane.