Semiempirical computations

In a sense the tr-allyl compounds of the transition metals can be regarded as the simplest of the sandwich molecules. Bis(jr-allyl)nickel, the best known of such complexes, has been shown by x-ray crystallography (104,105) to have a staggered arrangement of tr-allyl moieties and hence a C2h molecular conformation. The electronic structure of the ground state of bis(jr-allyl)nickel has been investigated by both semiempirical (47) and ab initio (274,275) methods, and a semiempirical computation has been performed on bis(7r-allyl)palladium (47). 

Intercalation of cations into a framework of titanium dioxide is a process of wide interest. This is due to the electrochromic properties associated with the process (a clear blue coloration results from the intercalation) and to the system s charge storage capabilities (facilitated by the reversibility of the process) and thus the potential application in rocking-chair batteries. We have studied alkali-metal intercalation and ion diffusion in the Ti02 anatase and spinel crystals by theoretical methods ranging from condensed-phase ab initio to semiempirical computations [65, 66]. Structure relaxation, electron-density distribution, electron transfer, diffusion paths and activation energies of the ion intercalation process were modeled. 

As outlined below, molecular mechanics and semiempirical computations directly report heats of formation, a convenience that is not directly available in ab initio computations, which give absolute electronic energies (in atomic units, au). These have little meaning to... 

Holder AJ, Upadrashta SM. A semiempirical computational investigation of the antipsoriatic drug anthralin. J Pharm Sci 1992 81 1074-1078. 

In the benzo-fused series, synthetic efforts have given rise not only to new quinazolines, but also to new methods of preparing them. Quinazoline 34 has been obtained via deprotonation of A-phenyldiethylketenimine with excess strong base (LTMP/KOrBu) in rert-BuOMe followed by addition of another equivalent of the ketenimine and subsequent introduction of rBuCOCl [95T9031]. Semiempirical computational analyses were conducted to provide support for the... 

It is important to recognize that the low level computation, and the low and intermediate level computations in a three-layer ONIOM, does not have to be MM. Any computational method can be used for any layer. For example, a three-layer ONIOM might use a semiempirical computation for the real system (5q), a DPT computation for the intermediate layer (Si), and a CASSCF computation for the high layer (82). 

Experimental and theoretical studies have been performed with respect to the ions arising from protonation of spiro[2.2]pentane 72. Early semiempirical computations and mass spectrometric evidence pointed to cation 73 as the initial protonated spiro[2.2]pentane (Scheme 5.5). This would rearrange to form pyramidal intermediate 74, which would open up to yield cyclopentyl cation 75. Subsequently, using various experimental methods (nuclear decay, radiolysis, FT-ICR MS) in combination with ab initio calculations (HF/6-31G and MP2/6-31G levels), Schleyer and coworkers have found evidence that the corner-protonated cation 76 is a relatively long-lived intermediate in the gas phase. Edge-protonated isomeric cation 77, in turn, is 35.5 kcal mol less stable than ion 76. 

First, the original mechanism of the CIEEL process involves, as a key tenet, the chemically activated transfer of the electron from the donor (activator) to the acceptor (peroxide) functionality. Indeed, as estimated from electrochemical data, the electron transfer from an activator is endothermic at the equilibrium geometry of the peroxide bond. Thus, the chemical activation process involves stretching of the oxygen-oxygen bond to accommodate the transferred electron. Consistent with the semiempirical computations, the 0-0 bond in the radical anion is markedly elongated relative to the neutral dioxetane (cf. Fig. 1). 

It is hoped that future semiempirical computational attempts to explore the stereochemical problem will evaluate pentavalent phosphorus parameters in a setting where there are fewer unknowns. One relevant test would be to... 

It is too early to decide whether semiempirical computations can successfully evaluate TS models such as 117-124 given the current level of precision. This situation is likely to change as the theoretical methods refine their phosphorus parameters and their ability to mimic solvation effects. Table 22 provides many opportunities to test computational TS predictions. Selectivity in the Wittig reaction depends on substituents, not on some fundamental property of phosphorus ylides. Thus, any successful theoretical analysis of the selectivity issue will need to explain the substituent-induced differences in kinetic Z E ratios. 

Ab initio calculations (ST0-4G basis set) as well as semiempirical computations (CNDO/2) confirm the above analysis Specifically, it is found that a syn geometry is favored when the attacking nucleophile is neutral and this was shown to result from an increase in nonbonded attraction in the transformation anti - syn augmented by electrostatic interactions. For charged nucleophiles it was concluded that nonbonded attraction, although present, is dominated by electrostatic effects and, consequently, the anti geometry is lower in energy than the model syn transition state. 

Neglect of NDDO A semiempirical computational approach grounded in Hartree-Foch theory in which terms for interelectronic repulsion are specifically incorporated. 

Each semiempirical computational chemistry program has specific functions rather than use any specific program, we will consider a hypothetical program, although the bias will be toward the MOPAC series of programs. 

In principle, knowledge of an atom or molecule s electronic structure i.e., the quantum mechanical wavefunction) would enable one to predict both its physical properties and its chemical behavior, including the outcome of reactions with other atoms or molecules whose electronic structure are equally well known (cf Daudel, 1973 Daudel et al., 1982). But because the Schrddinger equation cannot be solved exactly for any system more complicated than the hydrogen atom, the wavefunction of atoms and molecules must be approximated. Spectroscopy provides us with an observational link between the macroscopic and microscopic realms of matter, and it has been both a guide to our conceptual understanding of matter and a means to approximate parameters that are used in semiempirical computational chemistry (cf. Segal, 1977). 

Ab initio and semiempirical computational methods have proved extremely useful. But also needed is a simple conceptual scheme that enables one to predict the broad outlines of a calculation in advance, or else to rationalize a coii5)uted result in a fairly simple way. Chemistry requires conceptual schemes, simple enough to carry around in one s head, with which new information can be evaluated and related to other information. Such a theory has developed alongside the mathematical methods described in earlier chapters. We shall refer to it as qualitative molecular orbital theory (QMOT). In this chapter we describe selected aspects of this many-faceted subject and illustrate QMOT applications to questions of molecular shape and conformation, and reaction stereochemistry. 

Semiempirical computational methods using some parameters determined by experiment were developed concurrently with ab initio methods. Although semiempirical methods have now been largely displaced by ab initio Gaussian methods using relatively large basis sets and corrections that can be handled by modem computers, the much smaller computational requirements for semiempirical methods still make them useful for simple and rapid initial structure optimizations. 

Cambridge Structural Database Crystal Structure Calculations 1 Intermolecular Interactions by Perturbation Theory X-Ray Crystallographic Analysis and Semiempirical Computations. 

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