Molecular-orbital calculations, for

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. 

HyperChem currently supports one first-principle method ab initio theory), one independent-electron method (extended Hiickel theory), and eight semi-empirical SCFmethods (CNDO, INDO, MINDO/3, MNDO, AMI, PM3, ZINDO/1, and ZINDO/S). This section gives sufficient details on each method to serve as an introduction to approximate molecular orbital calculations. For further details, the original papers on each method should be consulted, as well as other research literature. References appear in the following sections. 

A particularly useful property of the PX monomer is its enthalpy of formation. Conventional means of obtaining this value, such as through its heat of combustion, are, of course, excluded by its reactivity. An experimental attempt was made to obtain this measure of chemical reactivity with the help of ion cyclotron resonance a value of 209 17 kJ/mol (50 4 kcal/mol) was obtained (10). Unfortunately, the technique suffers from lack of resolution in addition to experimental imprecision. It is perhaps better to rely on molecular orbital calculations for the formation enthalpy. Using a semiempirical molecular orbital technique, which is tuned to give good values for heat of formation on experimentally accessible compounds, the heat of formation of /5-xylylene has been computed to be 234.8 kj/mol (56.1 kcal/mol) (11). 

Ab initio molecular orbital calculations for the model systems RCN3S2 (R = H, NH2) show that these dithiatriazines are predicted to be ground state singlets with low-lying triplet excited states (Section 4.4). The singlet state is stabilized by a Jahn-Teller distortion from C2v to Cj symmetry. In this context the observed dimerization of these antiaromatic (eight r-electron) systems is readily understood. 

Thus, Segura replaced ct with a scaled energy change, obtained by a molecular orbital calculation, for this proton transfer ... 

Molecular orbital calculations have been used to estimate equilibrium constants, although up to the present these attempts have not met with much success. Using calculations of this type, 2- and 4-hydroxypyridine 1-oxide were predicted to be more stable than 1-hydroxypyrid-2- and -4-one by ca. 20 kcal/mole, which corresponds to a ratio of ca. 10 between the forms. It was later shown experimentally that, at least in the series of 4-substituted compounds, there is very little energy difference between the forms and that the ratio between them is about unity. Molecular orbital calculations for... 

Molecular orbital calculations for transition metal compounds. D. R. Davies and G. A. Webb, Coord. Chem. Rev., 1971, 6, 95-146 (267). 

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. 

Molecular orbital calculations for the parent vinyl cation, Cj H3, were first reported by Hoffmann (161), who used the extended Hiickel method, and more recently by Yonezawa and co-workers (162), who used a semiempirical SCF procedure. Both treated the problem of classical, 172 (R = H), versus bridged structures, 173, but the methods suffered from their inability to account satisfactorily for bond-length changes, and neither discussed the question of linear, 172a, versus bent, 172b, structures. 

In this section, we will describe some experiments which we have performed using the above-mentioned nano-Raman microscope. Figure 2.6a shows the Raman spectmm of an adenine nanocrystal of height 7 nm and width 30 nm [19]. Several Raman bands are observed as the probe tip is near enough to the sample (AFM operation is made in contact mode). These bands, except the one appearing at 924 cm, are assigned as the vibrational modes, inherent to the adenine molecule, according to the molecular orbital calculation. For examples, two major bands, one at... 

Dixon SL, Merz KM Jr (1997) Fast, accurate semiempirical molecular orbital calculations for macromolecules. J Chem Phys 107(3) 879-893... 

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