Molecular orbital theory, ab initio

The continuous development and implementation of molecular orbital theory ab initio methods have enlarged the applications to this area too. Indeed, the impact of theoretical calculations in thermochemistry is substantial. Experimental groups often use calculations as a supplement to the interpretation of their results. In this section we will mention a few recent and representative studies that are directly associated with the bond dissociation energies of silanes. Early theoretical investigations of the Si—H bond strength in silanes have been summarized [13]. 

Li, J. Cramer, and Truhlar, D. G. 1999. Application of a Universal Solvation Model to Nucleic Acid Bases. Comparison of Semiempirical Molecular Orbital Theory, Ab Initio, Hartree-Fock Theory, and Density Functional Theory , Biophys. Chem.. 78, 147. 

Molecular orbital theories (ab initio methods (23)) were chosen and validated in this study to characterize the interaction energies between methane and water, while electron density functional theories (24) were tested, but found to be inadequate (see below). Four different ab initio methods were used in the validation MP2 (25,26), MP4(SDTQ) (27), QCISD(T) (28) and CCSD(T) (29). Three different DFT methods, BLYP (30Jl), B3LYP (32) and BPW91 (33), were used and the results compared with the ab initio methods. In addition, for each of the above methods, the effect of the size of different basis sets was investigated specifically, 6-31++G(2d,2p), cc-pVDZ, cc-pVTZ and cc-pVQZ were used. 6-31+-i-G(2d,2p) was chosen, because it was reported to yield reasonable results compared with that at near the basis set limit on this system (77). The others were chosen in order to observe the effect of systematically increasing the size of the basis set. Gaussian 94 (22) was used for all the above calculations. 

Fig. 2.7 The addition of a 3d orbital to 2p gives a distorted orbital. (Figure adapted from Hehre WJ, L Radom, p i)R Sdileycr and ] A Hehre 1986. Ab initio Molecular Orbital Theory. New York, Wiley.)...

I nple J A and D L Beveridge, 1970. Approximate Molecular Orbital Theory. New York, McGraw-Hill. Riduirds W G and D L Cooper 1983. Ab initio Molecular Orbital Calculations for Qieniists. 2nd Edition. Oxford, Clarendon Press. 

In this section, the conceptual framework of molecular orbital theory is developed. Applications are presented and problems are given and solved within qualitative and semi-empirical models of electronic structure. Ab Initio approaches to these same matters, whose solutions require the use of digital computers, are treated later in Section 6. Semi-empirical methods, most of which also require access to a computer, are treated in this section and in Appendix F. 

Hehre, W.J. Radom, L. Schleyer, P.v.R. Pople, J.A.Ab Initio Molecular Orbital Theory, John Wiley and Sons, New York, 1986... 

W J. Hehre, L. Radom, P. v. R. Schleyer, and J. Pople, Ab Initio Molecular Orbital Theory, Wiley-lnterscience, New Y k, 1986. 

Ab initio molecular orbital theory is concerned with predicting the properties of atomic and molecular systems. It is based upon the fundamental laws of quantum mechanics and uses a variety of mathematical transformation and approximation techniques to solve the fundamental equations. This appendix provides an introductory overview of the theory underlying ab initio electronic structure methods. The final section provides a similar overview of the theory underlying Density Functional Theory methods. 

A. Szabo and N. S. Ostlund, Modern Quantum Chemistry, McGraw-Hill, 1982 R. McWeeny, Methods of Molecular Quantum Mechanics, Academic Press, 1992 W. J. Hehre, L. Radom, J. A. Pople and P. v. R. Schleyer, Ab Initio Molecular Orbital Theory, Wiley, 1986 J. Simons, J. Phys. Chem., 95 (1991), 1017 R. J. Bartlett, J. F. Stanton, Rev. Comput. Chem., 5 (1994), 65. 

Details can be found in chapter 4 of the book by Hehre et al. as cited in the references. As a further aside, it is worth noting that despite of the title of this book which is "Ab Initio Molecular Orbital Theory", the authors admit on the very first page that most of the book is devoted to calibrated work ... 

Hehre, Warren, J, Radom, Leo, Schleyer, Paul, v.R., Pople, John, Ab Initio Molecular Orbital Theory. New York John Wiley, 1986. 

Substituent effects as evaluated on the basis of the Hammett equation and its extended forms, are - this has to be emphasized again — empirical results. Nevertheless, it is very soothing to know that theoretical approaches, i. e., calculations of substituent effects using ab initio molecular orbital theory (Topsom, 1976, 1981, 1983 Taft and Topsom, 1987, STO-3G and 4-31G level), give results that are consistent with the experimental data. However, it is not recommended to use only theoretically calculated substituent constants and values for F, R, and other parameters for the interpretation of experimental data. 

There are two complementary lines of approach to examining the part played by 3d orbitals in molecular orbital theory and to appreciating current doubts as to their role. On the one hand, there is the question of 3d orbitals in relation to the basic formulation of molecular orbitals by overlapping atomic orbitals on the other hand, there is the question of the effect of including or excluding 3d functions in molecular orbital calculations, particularly of the ab initio type. We shall consider each of these briefly in turn. 

Hehre, W.J. Radom, L. Schleyer, P. von R. Pople, J.A. Ab Initio Molecular Orbital Theory Wiley NY, 1986 Clark, T. A Handbook of Computational Chemistry Wiley NY, 1985, p. 233 Richards, W.G. Cooper, D.L. Ab Initio Molecular Orbital Calculations for Chemists, 2nd ed., Oxford University Press Oxford, 1983. 

Only the structures of di- and trisulfane have been determined experimentally. For a number of other sulfanes structural information is available from theoretical calculations using either density functional theory or ab initio molecular orbital theory. In all cases the unbranched chain has been confirmed as the most stable structure but these chains can exist as different ro-tamers and, in some cases, as enantiomers. However, by theoretical methods information about the structures and stabilities of additional isomeric sul-fane molecules with branched sulfur chains and cluster-like structures was obtained which were identified as local minima on the potential energy hypersurface (see later). 

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