AMI molecular orbital calculations

Density functional and semiempirical AMI molecular orbital calculations have been used to investigate substituent effects on site selectivity in heterocumulene-hetero-diene4 + 2-cycloadditions between ketene imines and acroleins.The new and novel heterocumulenes a, /3-unsaturated thioaldehyde S -oxides (97) behave as both diene... 

Imidazo[ 1.2-a]pyrazines (14) with activating alkyloxy or alkylamino substituents at the 8-position were mononitrated regiospecifically at the 3-position by nitric acid in sulfuric acid in accord with semiempirical (AMI) molecular orbital calculations.40... 

Further support for structure 7a is obtained by AMI molecular orbital calculations performed on the reaction of C o and silylenes (Ph2Si ). These calculations show that Ph2Si adds across the junction of two six-membered rings in Cgo to give the 1,2-addition silirane analogous to 7a with an exothermicity of 61.3 kcalmol-1. The analog of 7b was not located on the potential energy surface. Also, 7a was 19.4 and 10.7 kcalmol-1 more stable than the 1,6-adducts 11a and lib, respectively (Scheme 4). 

The silylene precursor, 2,2-bis(2,6-diisopropylphenyl)hexamethyltrisilane (5), was pho-tolyzed with a low-pressure mercury lamp in a toluene solution of C7012. The adduct 12 obtained (equation 2) contains two isomers of (Dip)2SiC7o (12a and 12b) which were separated by flash chromatography on silica gel. FAB mass spectrometry of 12 displays a peak for adduct 12 at 1190-1194 as well as for C70 at 840-843 which arises from the loss of diarylsilylene. The UV-vis absorption spectra of 12 are virtually identical to those of C70 with bands at 333, 381 and 471 nm. AMI molecular orbital calculations... 

Design of Transition State Analogs for Dihydroorotase. Semiempirical AMI Molecular Orbital Calculations. ... 

An intermediate role of cation radicals in cis-trans isomerisation of carotenoids has been considered [138]. AMI molecular orbital calculations show that the energy barrier of cis-trans isomerisation are much lower in cation radicals ( 20 kcal/mol) and dications ( 0 kcal/mol) than in neutral carotenoids ( 55 kcal/mol). HPLC analyses of the product mixture after bulk electrolysis of [3-carotene (1), canthaxanthin (16) and apocarotenoids showed the presence of 5-cis, 13-cis, 9-cis, 9,13-dicz s and all-trans isomers [138]. 

It was observed that electrochemical oxidation of all-trans P-carotene (1) and canthaxanthin (16) in CH2CI2 leads to significant trans-cis isomerisation [105]. It was suggested that the isomerisation mechanism involved cation radicals and/or dications which could easily undergo geometrical isomerisation. This proposal was supported by AMI molecular orbital calculations, which showed that the energy barrier from trans to cis is much lower in the cation radical and dication species than in the neutral carotenoid [105]. 

Maryanoff and Turchi pursued a detailed theoretical study of the reaction between 1,2-dicyanocyclobutene 279 and munchnone 280, prepared by cyclodehydration of 278 and acetic anhydride (Fig. 4.100). The results from these AMI molecular orbital calculations led to the conclusions that the transition state leading to the exo cycloadduct 281 is favored electrostatically and that azomethine ylide 282 is a discrete intermediate in the formation of dihydroazepine 283. More recently, Turchi reported cycloaddition reactions between munchnone 285 and 279 to afford dihydroazepine 286 in high yield. Further cyclization of 286 gave tricycle 287. Likewise, diester 288 reacts with munchnone 42 to give dihydroazepine 289. 

Brpnsted correlations for proton transfer and methyl transfer between pairs of mimicked 4-substituted pyridines have been simulated by means of AMI molecular-orbital calculations.The relationship to the Marcus equation is also considered. It is concluded that the Brpnsted coefficient fi provides an approximate measure of the position of the transition structure along the reaction coordinate between reactant and product encounter complexes. 

We have addressed the problem by MNDO and AMI molecular orbital calculations on the simplest system namely trans stilbene "in vacuo" by calculation of the total potential energy as function of the torsional angles about the Cj -C q d bonds. The two methods... 

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