Semiempirical AMI

These findings accord with the semiempirical AMI calculations [92JCS(P1)2779] of the heats of formation of all theoretically possible tautomeric forms of 4-aminoimidazole 53. The most stable are the tautomers 53a AHf = 213 kJ mol ) and 53d AHf = 215 kJ mol ) (Scheme 29). All... 

Semiempirical AMI and ab initio calculations using GAUSSIAN 86 with the STO-3G basis set were used to explain the site of alkylation in compounds 1... 

Bond orders and charge densities of 4//-pyrido[l, 2-u]pyrimidin-4-one and its protonated form were calculated by the semiempirical AMI method with full optimization of geometry (97MI22). 

Bond orders, charges on the atoms in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one and its protonated form were calculated by quantum chemical calculations by the semiempirical AMI method. According to the results, the equilibrium conformation of the ring in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one is planar, while l//-pyrimido[l,2-u]quinolin-1-one adopts a conformation close to a half-chair due to the unfavorable interactions between the oxygen atom of the carbonyl group and the ring C-10 atom in the pen-position (97MI22). 

Intramolecular reactions usually dominate intermolecular addition for favorable ring sizes. Semiempirical (AMI) calculations found the intramolecular TS favorable to a comparable intermolecular reaction.68 (See Figure 4.1) The intramolecular TS, which is nearly 4 kcal/mol more stable, is quite productlike with a C—O bond distance of 1.6 A, and a bond order of 0.62. The bromonium ion bridging is unsymmetrical and fairly weak. The bond parameters for the intra- and intermolecular TSs are quite similar. 

Ferenczy, G. G., C. A. Reynolds, and W. G. Richards. 1990. Semiempirical AMI Electrostatic Potentials and AMI Electrostatic Potential Derived Charges A Comparison with ab initio Values. J. Comp. Chem. 11, 159. 

Using the semiempirical AMI method with full structure optimization, the bond lengths and bond angles of 4- and 5-aminoimidazoles (179) and (180) have been calculated [92JCS(P 1)2779] and the results are summarized in Table X. 

Experimental dipole moments and acidities of azoles, including 1,2,3-triazole, show linear correlations with their Jt-electron excess calculated by the semiempirical AMI method <2003CHE71>. Experimental dipole moments of azoles agree well with those calculated by the DFT program ALLCHEM <2003PCA4172>. Calculated dipole moments t (in units of Debye,D) of a few selected azoles are listed below ... 

Cycloaddition of 3-methylenephthalide with ot./V-diphenylnitrone gave two diastereoisomers of 2,3-diphenyl-2,3-dihydrospiro 1,3-oxazole-5(47/ )l (3 H)-2-benzoluran]-3 -one (805). The 1,3-dipolar cycloaddition reaction of /V-benzyl-C-(2-furyl)nitrones with electron-rich alkenes gave preferentially trans-3,5-disubstituted isoxazolidines (endo approach). These experimental results are in good qualitative agreement with those predicted from semiempirical (AMI and PM3) and ab initio (HF/3-21G) calculations (806). 

Fig. 2 Chemical structures of fluorescent ligands (anion coordination sites are indicated in blue) and tentative reaction schemes on interaction of 6 with chloride and fluoride in dichloromethane according to the color patterns observed in [41]. The wavelengths of absorption given in brackets have been calculated for the geometry optimized species 6, 6-C1 and 6 in the gas phase by semiempirical AMI calculations (Ampac V6.55, Semichem)...

The 5 n2 reactions of the radical anions (CHCr and CHBr ) and the closed-shell anions (CH2CF and CH2Br ) with CH3CI and CHsBr have been studied by using density functional theory. The closed-shell anions were found to be more reactive than the radical anions, in agreement with experiment. Other details of the systems were also elucidated. Ab initio and semiempirical (AMI) methods were used to study the gas-phase S 2 reactions between methyl nitrate and various nucleophiles. ... 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

Nucleophilic addition of phenolic nucleophiles to l,l-dicyano-2-arylethenes in the gas phase and in water has been studied theoretically" using the semiempirical AMI method and the Cramer-Truhlar solvation model SM2.1. The difference between the Brpnsted coefficients (a" = 0.81 and P" =0.65) determined for the gas-phase reaction is indicative of a small positive transition state imbalance of / = 0.16. For reaction in water the estimates (a" = 0.61 and P" = 0.36, giving I = 0.25) are close to the experimental values (a" = 0.55 and P" = 0.35) obtained with amine bases, and the small imbalance is as expected for a reaction involving no hybridization change at the incipient carbanion site. 

Thermochemical parameters estimated by semiempirical AMI calculations have been found to support the proposal that isobutene formation on gas-phase thermolysis of iV-methyl-A-phenyl-fert -butylsulfenamide and morpholinyl-ferf -butylsulfenamide occurs by a unimolecular mechanism involving a four-centre cyclic transition state and co-formation of the corresponding thiohydroxylamines." ... 

A semiempirical AMI study of the inverse-electron-demand Diels-Alder reaction of 4-substituted 6-nitrobenzofurans with enol ethers and enamines favours a stepwise mechanism involving short-lived diradical intermediates. The inverse-electron-demand intermolecular Diels-Alder reactions of 3,6-bis(trifluoromethyl)-l,2,4,5-tetra-zine with acyclic and cyclic dienophiles followed by the elimination of N2 produce 4,5-dihydropyridazines, which cycloadd further to yield cage compounds. The preparation of jS-carbolines (90) via an intramolecular inverse-electron-demand Diels-Alder... 

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... 

Computational studies in the field of imidazo[2,l-f>]thiazoles are still lacking. The results of some semiempirical (AMI, PM3) (90MI1), ab initio (93MI1), and density functional theoretical (95MI1) investigations (96UP2) are also given in Table VI. 

On the basis of NMR results, an intriguing anion within the trianion structure was proposed for S ". Such a structure would require 3.0 ee excess in the rim, and 1.0 ee excess in the hub, as compared to neutral 8. Analysis of the charge densities calculated at both semiempirical AMI and ab initio levels, however, predict the hub atoms to gain only about 0.5 ee, while the rim atoms bear more of the charge at 3.5 ee. ... 

In the benzofuroxan series ab initio calculations at the MP2 level give qualitatively correct predictions of structure, whereas semiempirical AMI, PM3 treatment is less satisfactory showing a preference for the dinitroso tautomer <94jpci2933>. The most likely intermediate in the interconversion of (9a) and (9b) is considered to be anri-l,2-dinitrosobenzene which is predicted to have an energy 50 kJ mol above that of benzofuroxan itself <94JOC643i>. Other calculated properties include diagmagnetic susceptibilities and IR, UV, and PE data. 

The mechanism of the dissociation of 1,2,3-trioxolanes to carbonyl oxides and carbonyl compounds has been studied by theoretical methods. Earlier ab initio calculations concluded that a biradical pathway and a concerted pathway were equally probable <81JA3619, 8UA3627), though it has been suggested that the transition state was not sufficiently characterized in these calculations to enable firm conclusions to be drawn from this work <91JA735>. More recent semiempirical AMI... 

---