Molecular-orbital calculations formation

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). 

Recently H. L. Jones and D. L. Beveridge have presented molecular orbital calculations on the electronic structure of 2,3-pyrid5me explaining the exclusive formation of 2-aminopyridine from this intermediate [Tetrahedron Letters No. 24, 1577 (1964)]. 

Molecular orbital calculations, whether by ab initio or semiempirical methods, can be used to obtain structures (bond distances and angles), energies (such as heats of formation), dipole moments, ionization energies, and other properties of molecules, ions, and radicals—not only of stable ones, but also of those so unstable that these properties cannot be obtained from experimental measurements." Many of these calculations have been performed on transition states (p. 279) this is the only way to get this information, since transition states are not, in general, directly observable. Of course, it is not possible to check data obtained for unstable molecules and transition states against any experimental values, so that the reliability of the various MO methods for these cases is always a question. However, our confidence in them does increase when (1) different MO methods give similar results, and (2) a particular MO method works well for cases that can be checked against experimental methods. ... 

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

There is considerable literature precedent for this reaction. In particular, Fotsch and Chamberlin (10) have reported that open chain y,8, 8,e and 6, -epoxy ketones and esters undergo cyclization in the presence of acids to form the corresponding dioxacarbenium ions. In addition, molecular orbital calculations were conducted to determine the heats of formation of the intermediates IX and X. Data from these calculations are given in Table 2. These calculations suggest that 1,6-attack (X) is... 

Hitherto, thio ether formation has clearly been proved only in the case of the ji-donor substituted 4-nitrosophenetol and the electron-rich l-methyl-2-nitrosoimidazole. The low yields of this adduct (about 2% at 1 1- and about 10% at 1 5-stoichiometry for 4-nitrosophenetol reacting with GSH56) may be the reason for its rare discovery. However, other nitrosoarenes should yield this family, too. Semiempirical molecular orbital calculations (MNDO) indicate a similar positive charge at the exposition of the N-(methylthiol-S -yl)-aniline cation and -4-anisole cation as well (Scheme 6). Furthermore, formation of l-(glutathion-S -yl)-2-naphthylamine was reported to occur in mixtures of 2-nitrosonaphthalene and GSH12. 

Only a few molecular orbital calculations regarding the structures, conformational changes and bonding of S molecules have been published. Fimdamental reactions of sulfur rings like interconversions are only poorly understood, and more examples for the formation of homocyclic derivatives directly from the corresponding S parent molecules are likely to be found. These few comments show that much more work has to be done before the chemistry of elemental sulfur can be regarded as well known . 

ASED (Atom Superposition and Electron Delocalization) molecular orbital calculations on the formation of monomeric 1-triazolylborane by the process BH3-I-triazole H2-f H2B(Tz) indicate... 

A theoretically estimated enthalpy of formation of 1,4-dithiane 1,1-dioxide was calculated from high-level ab initio molecular orbital calculations at the G2(MP2) level. The theoretical calculations appear to be in very good agreement with experiment (enthalpy of formation (T = 298.15 K) of 1,4-dithiane sulfone = —333.0kJ mol ) <2006JOC2581>. 

Orbital mapping analysis of thermal isomerization of aza- and diaza-bicyclo[2.2.0]hexa-dienes has been carried out (80MI51205). Molecular orbital calculations (MNDO) of a series of bisdehydropyridines showed that the 2-azabicyclo[2.2.0]hexa-l,3,5-triene (125) is the least stable form having a calculated heat of formation of 728.4 kJ mol-1 (77CC539). 

The formation of anion radicals by chemical or electrochemical reduction may provide information about electron delocalisation in an inorganic ring system through the determination of the EPR spectrum in conjunction with molecular orbital calculations. In practice, however, the LUMO of the homo- or heterocycle is often an antibonding orbital. Consequently, occupation of the LUMO... 

Molecular mechanics calculations have been carried out on a wide range of chemical compounds including hydrocarbons, heteroatomic molecules, steroids, carbohydrates and proteins. Furthermore, a variety of information has been obtained such as heats of formation, rotational barriers, and rates of reaction (18,2,19). The major advantage of this method over other computational methods is that it is reasonably fast to perform in comparison to formal molecular orbital calculations. 

The photoelectron spectrum is frequently discussed in terms of Koopmans theorem, which states that the ionization potentials (IPs) are approximately related to the energies of the canonical orbital found in molecular orbital calculations.106. The relationship is approximate because two factors are neglected the change in the correlation energy, and the reorganization energy, which is a consequence of the movement of electrons in response to the formation of a cation. The two quantities are approximately equal and opposite. 

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