Orbital effects, computational studies

The structure of isolated R3M+ cations in the gas phase was the subject of several computational studies. Significant difficulties are associated with calculations for molecules containing the heavier elements, particularly Sn and Pb. NMR chemical shift calculations require consideration of relativistic effects (spin-orbit coupling). A discussion of these difficulties and of effective core potentials developed for these calculations is beyond the scope of this review. 

As shown by Clark [38] in a comprehensive computational study of (HnX XHn)+ radical cations (X= Li to C, Na to Si), one-electron bonds are already rather well described by simple Hartree-Fock theory. This is because the active system contains a single electron, so that the breathing orbital effect is ineffective in the active subspace, where each orbital is either empty or singly occupied as illustrated in 32, 33 for the OC bond. 

A prototype system for computational study of solvent effects is the SN2 reaction of Cl- + CH3CI, involving an anionic nucleophile and a neutral substrate. The chloride exchange reaction has been extensively studied previously by a variety of theoretical methods.17,45-50 In this system, as in the other two cases, there are four electrons and three orbitals that directly participate in bond forming and breaking during the chemical reaction. 

Increasingly complex basis sets are generated by further splitting of orbitals, and the addition of fnrther polarization and/or diffuse functions. As basis fnnctions are added, the nnmber of orbitals to be evaluated is effectively increased, resnlting in a simnltaneons and exponential (fourth power) increase in time reqnirements for a given system. As a conseqnence, a computational study must take into acconnt the size of the system, and compntational resources to establish the feasibility of a particular effort. 

By H-NMR spectroscopy and dipole measurements, the 3,7-dimethyl-3,7-diazabicyclo[3.3.1]nonane (dimethylbispidine) 88 was shown to adopt a cc conformation with flattened wings (108). This was later confirmed by C-NMR spectroscopy and semiempirical molecular orbital (MO) calculations (EH, CNDO/2 the double-chair form was found to be more stable than the cb isomer by --83.8kJmoP and the bb form is even more destablized (133). This preference for cc is not due to solvent or packing effects since 88 has the same conformational preference in the gas phase, as shown by a computational study, which involved a scan of the potential energy surface (PES) and the calculated... 

Common features of most of the important HAHA contributions appear to be the following a) they arise from the core shells of the heavy atom rather than from core tails of the valence orbitals like the dominant HALA SO-I effects discussed in section 3 (this has been confirmed also by a localized MO analysis of the FC/SZ-KE contributions to heavy-atom shieldings in various small hydrides in [59]). b) the contributions are predominantly isotropic [23,27a,58,59,60]. In agreement with our above discussion, this suggests that HAHA effects are essentially atomic in nature and should cancel to a large extent when looking at relative chemical shifts. This appears to be bourne out by the available computational studies. 

Abstract The agostic bond defines an intramolecular interaction where a a bond is geometrically close to an electron deficient centre (often a transition metal). The computational studies on this energetically weak interaction are reviewed and discussed. Various types of a bonds have been considered (C-H, C-C, Si-H, Si-C, B-H). It is suggested that a C-X bond in which X carries a lone pair should preferably not be viewed as agostic. The factors that contribute to his occurrence are discussed. In particular, the agostic interaction is very sensitive to steric effects. Explanations based on molecular orbital analysis, electron delocalization and topological analysis of the electron density are presented. 

Keywords Agostic interaction Weak interaction Bond activation Computational studies DFT QM/MM calculations Molecular orbitals Topological analysis Steric effects... 

Zeng T (2010) Development and applications of model core potentials for the studies of spin-orbit effects in chemistry. Ph.D thesis, University of Alberta Krause D, Klobukowski M (1996) Can J Chem 74 1248 Lovallo CC, Klobukowski M (2003) J Comput Chem 24 1009... 

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