Perturbational molecular orbital calculations

Naphthalene 1,2-oxide (136), a non-K-region epoxide, shows low thermal stability. Anthracene 1,2-oxide, on the other hand, is stable at ambient temperatures for several weeks. Preparation of (+ )-(lR,2S)-anthracene 1,2-oxide (137), using the above method, constitutes the first example of preparation of an optically pure arene oxide. However, the non-K-region oxides of phenanthrene, namely, its 1,2- and 3,4-oxides (47 and 48), obtained from chiral precursors, racemize fast.66 Perturbational molecular orbital calculations indicate that epoxide-oxepin valence tautomerism is possible. However, the oxepin could not be detected by NMR. 

The first section summarizes simply the essential features of the different types of molecular orbital calculations currently used to solve theoretical problems in organometallic chemistry. A critical comparison of these calculations is also given. The second section discusses the bonding in organometallic complexes and draws on recent computational results and develops the chemical and structural implications of bonding models based on perturbation theory arguments. 

Hilal (1994) calculated the pKa values of 214 dye molecules using the SPARC (SPARC Performs Automated Reasoning in Chemistry) computer program. SPARC computational methods use the knowledge base of organic chemistry and conventional Linear Free Energy Relationships (LFER), Structure/Activity Relationships (SAR), and Perturbed Molecular Orbital (PMO) methods. 

The static polarizabilities, a, of various xanthone analogues including seleno- and telluroxanthen-9-one Id and le and seleno- and telluroxanthen-9-thione 2d and 2e have been estimated by ab initio molecular orbital calculations using the coupled perturbed Hartree-Fock (CPHF) method <1996CPL125>. The results indicate that the introduction of heavy elements in 1 and 2 increases all components of a with a greater effect observed in the case of the thione derivatives 2. 

Ab initio molecular orbital calculations on these systems have been confined to the 1,2,3-triazolo[4,5-d]pyrimidines (7), the so-called 8-azapurines , and references to this subject may be found in the previously mentioned review <86AHC(39)ii7>. In 1989, quantum mechanical perturbation methods have been used to study the activity of 8-azapurine nucleoside antibiotics in transcription processes <89Mi 7i3-oi>. The l,2,3-thiadiazolo[5,4-d]pyrimidine derivative (51), a rearrangement product of 8-aza-6-thioinosine, has been used in a molecular modeling study of the antitumor activity of sugar derivatives of pyrimidopyrimidines <89PNA(86)8242>. 

Tel. 904-392-1597, fax 904-392-8722, e-mail bartlett qtp.ufl.edu Ab initio molecular orbital calculations using coupled-cluster and many-body perturbation theory methods. 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

Perturbation theoretical expressions 136 Molecular design and two- and three-level contributions Molecular orbital calculations for second-order polarizabilities of rr systems 141... 

Notice that, since e, < ej, SE is negative, that is, Ll is stabilized. The antibonding orbital 4 2 is destabilized by an amount A 2 This is set by an equation analogous to equation (5) except that the ordering of ei and Cj in the denominator is reversed. An important feature of perturbation theory, and indeed when one does a molecular orbital calculation at any level which that includes overlap, is that the antibonding combination ( 2) is destabilized more than the bonding combination (fl i) is stabilized, that is, I A 2 I > IA fi I Therefore the addition of two electrons to Ll in (1) is associated with stabilizing the formation of an M L... 

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