Theoretical studies orbital phase theory

Matrix isolation spectroscopy showed that the major product of the condensation of A1 and CO was not AlCO but AlfCOj [61, 62], Theoretical studies suggested that the C-Al-C angle in AlfCOj [63] and the C-Si-C angle in SifCOj [64] should be unusually acute (Scheme 29a). The orbital phase theory accounts for the acute coordination angles and the stabihty of AlfCOj relative to AlCO [65],... 

In conclusion, these gas-phase measurements provide new elues to the role of solvation in ion-moleeule reaetions. For the first time, it is possible to study intrinsie reactivities and the extent to which the properties of gas-phase ion-moleeule reaetions relate to those of the eorresponding reactions in solution. It is clear, however, that gas-phase solvated-ion/moleeule reaetions in which solvent moleeules are transferred into the intermediate elusters by the nucleophile cannot be exaet duplieates of solvated-ion/ molecule reactions in solution in which solvated reactants exchange solvent molecules with the surrounding bulk solvent [743]. For a selection of more recent experimental [772] and theoretical studies of Sn2 reactions in gas phase and solution by classical trajectory simulations [773], molecular dynamics simulations [774, 775], ab initio molecular orbital calculations [776, 777], and density functional theory calculations [778, 779], see the references given. For studies of reactions other than Sn2 ion-molecule processes in the gas phase and in solution, see reviews [780, 781]. 

Theoretical studies are also done to interpret the synthesis reactions and mechanism of reactions. The regioselectivity of 1,3-dipolar cycloaddition reaction between substituted trimethylstannyl-ethynes and nitrile oxides yielding isoxazoles, was interpreted by the application of frontier electron theory <93CPB478>. By the combination of experimental and molecular orbital (ab initio) studies, a multistep mechanism is proposed for unimolecular radical chemistry of isoxazoles in the gas phase <920MS(27)317>. 

The symposium upon which this book is based included contributions from several leading theorists because of the obvious impact that theory (especially molecular orbital theory) has had on the study of nucleophilic reactions. Pioneering contributions in this area have come from Klopman (10b) and Hudson (11), and Hudson has reviewed his work in Chapter 13. These theoretical works are especially pertinent to the studies introduced in the present section because most of the theoretical studies have been for the gas phase and reproduce many of the key features described here. Unfortunately, several of the theorists participating in the symposium were not able to contribute to this volume, so we provide here a brief introduction to the recent literature in this area. 

Remko, M. (1997) Gas-phase binding of Li" ", Na and Mg to formaldehyde, acetaldehyde and their silicon and sulfur analogs. A theoretical study by means of ab initio molecular orbital methods at the G2 level of theory. Chem. Phys. Lett., 270, 369-375. 

These compounds have been the subject of several theoretical [7,11,13,20)] and experimental[21] studies. Ward and Elliott [20] measured the dynamic y hyperpolarizability of butadiene and hexatriene in the vapour phase by means of the dc-SHG technique. Waite and Papadopoulos[7,ll] computed static y values, using a Mac Weeny type Coupled Hartree-Fock Perturbation Theory (CHFPT) in the CNDO approximation, and an extended basis set. Kurtz [15] evaluated by means of a finite perturbation technique at the MNDO level [17] and using the AMI [22] and PM3[23] parametrizations, the mean y values of a series of polyenes containing from 2 to 11 unit cells. At the ab initio level, Hurst et al. [13] and Chopra et al. [20] studied basis sets effects on and y. It appeared that diffuse orbitals must be included in the basis set in order to describe correctly the external part of the molecules which is the most sensitive to the electrical perturbation and to ensure the obtention of accurate values of the calculated properties. 

In this chapter, we review recent theoretical results of studies of radiation damage to DNA obtained in gas phase and in solution with the use of ab initio molecular orbital theory which has the advantage of being free of any empirical parameters. In the first part, we discuss results from works performed on species that are predominant in the direct effect, that is the natural DNA bases and their radical ions in various environments (i.e. base pairs, stacked systems, solvent), focusing on electron affinities and ionization potentials. We then review theoretical data obtained for species which result from OH and H attack in the indirect effect (sugar radicals, hydroxyl and hydrogen base adducts). In a third part, we discuss the fate of the hydroxyl base adducts, which upon H atom addition and subsequent dehydration lead to the regeneration of the natural DNA bases. Finally, we focus on the radioprotective roles of selected thiols and the possible mechanisms by which they act. Since an excellent and comprehensive review of the tools and methods currently used in molecular orbital theory has recently appeared, [15] it will not be further discussed here. 

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