Molecular orbital calculations solvent effect

Marriott and Topsom have recently developed theoretical scales of substituent field and resonance parameters. The former correspond to the traditional inductive parameters but these authors are firm believers in the field model of the so-called inductive effect and use the symbol The theoretical substituent field effect scale is based on ab initio molecular orbital calculations of energies or electron populations of simple molecular systems. The results of the calculations are well correlated with Op values for a small number of substituents whose Op values on the various experimental scales (gas-phase, non-polar solvents, polar solvents) are concordant, and the regression equations are the basis for theoretical Op values of about 50 substituents. These include SOMe and S02Me at 0.37 and 0.60 respectively, which agree well with inherent best values in the literature of 0.36 and 0.58. However, it should be noted that a, for SOMe is given as 0.50 by Ehrenson and coworkers . 

Kikuchi, O., T. Matsuoka, H. Sawahata, and O. Takahashi. 1994. Ab Initio Molecular Orbital Calculations Including Solvent Effects by Generalized Born Formula. Conformation of Zwitterionic Forms of Glycine, Alanine and Serine in Water. J. Mol. Struct. (Theochem) 305, 79-87. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

Chevrier et al., 1983), solvent effects (Bensaude et al., 1979), and the effect of added salt on the rate of reaction (Bensaude et al., 1978) have been studied to provide information about this process. Molecular-orbital calculations confirm that a suitable transition state for the reaction is one involving bridging water molecules (Field et al., 1984). 

The characteristic features of hydroboration of alkenes—namely, regioselec-tivity, stereoselectivity, syn addition, and lack of rearrangement—led to the postulation of a concerted [2 + 2] cycloaddition of borane353,354 via four-center transition state 37. Kinetic studies, solvent effects, and molecular-orbital calculations are consistent with this model. As four-center transition states are unfavorable, however, the initial interaction of borane [or mentioned monobridged dimer, Eq. (6.56)] with the alkene probably involves an initial two-electron, three-center interaction355,356(38, 39). 

O. Takahashi, H. Sawahata, Y. Ogawa and O. Kikuchi, Incorporation of solvent effects into ab initio molecular orbital calculations by the generalized Born formula. Formulation, parameterization, and applications, J. Mol. Struct. (THEOCHEM), 393 (1997) 141-150. 

If the activities of the laboratory in this field are said to be at the borders of quantum chemistry and statistical thermodynamics, these two disciplines are declared to be techniques." The problems raised by molecular liquids and solvent effects can be solved, or at least simplified by these techniques. This is firmly stated everywhere the method of calculation of molecular orbitals for the o-bonds was developed in the laboratory (Rinaldi, 1969), for instance, by giving some indications about the configuration of a molecule. The value and direction of a dipolar moment constitutes a properly quantum chemistry method to be applied to the advancing of the essential problems in the laboratory. In the same way, statistical mechanics or statistical thermodynamics constitute methods that were elaborated to render an account of the systems studied by chemists and physicists. In Elements de Mecanique Statistique, these methods are well said to constitute the second step, the first step being taken by quantum chemistry that studies the stuctures and properties of the constitutive particles. [53]... 

On the other hand, there is the well-known fact that sulfur is an excellent carbanion stabilizer. But not for those reasons popularly ascribed to dir-pir backbonding of the carbanion lone pair into the vacant 3d orbital of sulfur, as suggested earlier. Recent molecular orbital calculations appear to challenge the validity of this model. These calculations, for example, predict the order of gas phase (no solvent effects to worry about) carbanion stabilization as sulfur > oxygen > carbon whether or not the 3d orbitals of sulfur participate in the calculations. Besides, these theoretical considerations surprisingly show that the C-S bond in the thiomethylene carbanion is longer than in the neutral thio-methane parent. Until recently, the prevailing mechanism of stabilization was centered around the classical concept of polarizability of the electron cloud around the sulfur nucleus. 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

A radical solution to all of the above-mentioned difficulties is to eliminate the solvent medium entirely and to measure structural effects on heteroaromatic reactivity in the gas phase. During the last decade, a revolution has occurred in the experimental and theoretical approaches to understanding gas-phase ion chemistry. This has occurred as the result of the simultaneous development of several experimental methods for studying organic ion-molecule kinetics and equilibria in the gas phase with precision and range of effects equivalent to or even better than that normally obtained in solution and by very sophisticated molecular orbital calculations. The importance of reactivity studies in the gas phase is twofold. Direct comparison of rates and equilibria in gaseous and condensed media reveals previously inaccessible effects of ion solvation. In addition, reactivity data in the gas phase provide a direct evaluation of the fundamental, intrinsic properties of molecules and represent a unique yardstick against which the validity of theoretical estimates of such properties can be adequately assayed. 

Molecular orbital calculations,80 performed for each step of the catalytic cycle (solvent effects were neglected and L = PH3), verified that the mechanism postulated by Halpem is reasonable. The caveat to bear in mind with the Halpem mechanism or the mechanism of any catalytic process is that variations in alkene, solvent, and phosphine ligands may change the pathway or the rate-determining step (see above for the discussion regarding steps a and b). 

Molecular orbital calculations for the complex formation between propagating radical and solvent indicate that the effect of anisole is not anomalous, and that the likelihood of the complex formation is in the following order vinyl acetate > vinyl benzoate > phenyl methacrylate methyl methacrylate. 

Figure 8. Theoretical Jcc values for 4-methylimidazole based on a finite perturbation theory self-consistent field molecular orbital calculation, a-c correspond to varied protonation states of the molecule d-f include the effects of hydrogen bonding interactions with the solvent, and with the deprotonated and protonated amino group, respectively. (Based on calculations given in Ref. 40.)...

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