MNDO semiempirical

In parallel with the ab initio calculations, also semiempirical smdies on the thermolysis of 1,2-dioxetane were performed. Most computations have been conducted by the PM3 method because it is the best semiempirical method for describing lone electron pairs on adjacent atoms . As an illustration, only the PM3 method reveals that in the dioxetane molecule the 0-0 bond is longer and weaker compared with the C—C one, as manifested by the computed values of bond lengths [rf(0—O) = 1.600 > d(C—C) = 1.522 A] and bond orders [n(0—O) = 0.973 < w(C—C) = 0.989] . In contrast, the AMI and MNDO semiempirical methods exhibit the opposite trends, i.e. AMI gives d 0—0) = 1.334 A, d(C-C) = 1.539 A, n(O-O) = 0.995 and n(C-C) = 0.976, whereas MNDO furnishes d(0-0) = 1.316 A, d(C-C) = 1.558 A, n(O-O) = 0.996 and n(C-C) = 0.9622 f-8. Nevertheless, despite the quantitative differences in the computed bond lengths, bond orders and bond angles, both the AMI and PM3 methods disclosed qualitatively similar reaction trajectories . 

The transition-state geometries for the cyclization of a range of electrophilic radicals have been obtained by MNDO semiempirical calculations.42 The regioselectivities observed were rationalized using a frontier orbital approach. Semiempirical calculations were not suited for describing the cyclization of the pent-4-en-l-oxyl radical.43 Instead, results indicated that ab initio (UHF/6-31G ) and (UBP/DZVP) were more reliable. 

The conformational properties of the naphtho[ l 1,5-dithiocin were studied by the MNDO semiempirical SCF MO method. The most stable conformation was the chair conformation. The plane-symmetrical boat, which has Cs symmetry, was calculated to be 0.17 kcal mol-1 was less stable than chair conformation. Both were separated by a low-energy barrier (4.84 kcal mol-1). The twist-boat conformation is 4.53 kcal mol-1 higher than that of the boat conformation. The barrier for chair-to-chair ring inversion in this compound was 12.37 kcal mol-1 (Scheme 164) < 1999J ST(489)67>. 

MNDO semiempirical method based on the modified neglect of diatomic... 

Section IV,A The structures of a number of iodonitroimidazoles " have been shown to be incorrect, and doubt has also been cast on the assigned structures of some iodoimidazoles. Calculations, using the recently developed MNDO semiempirical SCF-MO method, for the cyclization of 2-azidoimidazole and its anion to the tetrazole forms show that the bicyclic anion is more stable than the neutral bicyclic molecule. Such a shift of the equilibrium toward the tetrazole form in the anion is thought to be due mainly to delocalization of the negative charge on the tetrazole moiety of the bicyclic system. ... 

We have calculated those values by following the reaction coordinate during alkylation of toluene by zeolite-CHj and have observed that the densities of frontier orbital in the atoms which intervene directly in the reaction have strongly increase, and C(.j (HOMO) = 0.54 q7m-ch3 (lUMO) = 0.89 values have been calculated by means of PM3 and MNDO semiempirical methods. Furthermore, the population of the frontier orbitals show a larger electronic transfer from the HOMO to the LUMO, while PMO calculations show that the effect is larger for the attack in para than in ortho position [35]. 

Tubert-Brohman, I., Guimaraes, C.R.W., Repasky, M.P., Jorgensen, W.L. Extension of the PDDG/ PM3 and PDDG/MNDO semiempirical molecular orbitial methods to the halogens. J. Comput. Chem. 2003, 25,138-50. 

The photoelectron spectra of aziridine (1) <74PMH(6)l, 74PMH(6)53> and 2//-azirine (2) have been measured <83JA768i, 87CB1971). The lone-pair ionization potential was found to be 10.58 eV, while the C=N 7t-electron ionization potential was measured as 11.56 eV. These values compared favorably with those obtained from MNDO semiempirical MO calculations. 

Section IV,A The structures of a number of iodonitroimidazoles have been shown to be incorrect, and doubt has also been cast on the assigned structures of some iodoimidazoles. Calculations, using the recently developed MNDO semiempirical SCF-MO method, for the cyclization of... 

The summary of theoretical studies of oxo -hydroxy tautomerism of various heterocycles by AMI, MNDO-PM3, and MNDO semiempirical methods has been published (91JCC17). 

MNDO semiempirical calculations of 2-thiocytosine showed that tautomers 242a and 242c should be predominant species at ambient and low temperature, whereas at... 

Figure 4 Results of finite field calculation on the second hyperpolarizability (7) of biphenyl obtained with the MNDO semiempirical method.

Molecular Orbital Studies. Conformational energy calculations on [-SiH2-] and [-Si (0113)2"] model compounds have also been carried out using MNDO semiempirical MO techniques. These studies also included some preliminary results for a model compound of the analogous germanium-containing polymer [-Ge(CH3)2"]. 

Ab initio electronic structure calculations not only confirm the validity of the aforementioned theory, but also afford accurate estimates of properties of the endohedral complexes. The presence of several interesting phenomena, such as lengthening of the bonds in polar molecules upon encapsulation is exposed by the calculations. The present results clearly show that the MNDO semiempirical calculations [44,45] are incapable of predicting properties of the complexes correctly. One may expect other approximate models [46,47] to be also of a doubtful value. 

Lennard-Jones interactions needed to be computed for the QM/MM system as for the equivalent MM system on the condition that at least one MM atom was involved in the MM term. Or, in other words, all MM terms that involved only QM atoms were omitted. The electrostatic interactions were slightly more complicated to administer but the scheme that was found appropriate for the AMI and MNDO semiempirical methods was one in which all electrostatic interactions between the QM and MM atoms at the covalent interface were calculated, even those between QM and MM atoms separated by only one or two covalent bonds. 

The work described in Refs. 5 and 17 indicated that hybrid potentials could describe reaction processes in solution. However, it is only with the work of Gao that the real utility of this approach has become apparent. He and his co-workers have developed a hybrid potential that combines the AMI/MNDO semiempirical method with a force field similar to the OPLS force field of Jorgensen and with it they have studied a very wide range of solution phenomena in combination with Monte Carlo simulation techniques. As Gao has recently published two excellent reviews of hybrid potentials which include discussion of his own work only a few brief details will be given here. 

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