Semiempirical MINDO

Not much information has been added in recent years to the earlier studies of tautomeric equilibria of benzimidazoles based on basicity measurements [76AHC(S1), p. 292]. For 5(6)- and 4(7)-substituted benzimidazoles and 2-methyl-5(6)-substituted benzimidazoles values are very close to 1, which indicates near equivalence in the stability of N1(H) and N3(H) tautomers. The tautomeric equilibria of 2-substituted (H, NH2, OMe, CN) 5-nitrobenzimidazoles and 4-nitrobenzimidazoles were analyzed with the use of semiempirical MINDO/3 and INDO methods. It was predicted that electron-releasing groups in position 2 shifted the equilibria to the 6-NO2 and 4-NO2 tautomers, respectively. 

Sporadic reports of the 2//-thiopyran transformation are still available. The electrocyclic ring-opening of 2-benzyl-2,4,6-triphenyl-2//-thiopyran (131, X = PhCH2) was detected by laser flash spectroscopy (86MI1). 2-Amino-2//-thiopyran was predicted to be more stable than its valence-bond isomer on the basis of semiempirical MINDO/3 and MNDO calculations (84JPR955). 

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

The spontaneous oxepin-benzene oxide isomerization proceeds in accordance with the Woodward-Hoffmann rules of orbital symmetry control and may thus be classified as an allowed thermal disrotatory electrocyclic reaction. A considerable amount of structural information about both oxepin and benzene oxide has been obtained from theoretical calculations using ab initio SCF and semiempirical (MINDO/3) MO calculations (80JA1255). Thus the oxepin ring was predicted to be either a flattened boat structure (MINDO/3) or a planar ring (SCF), indicative of a very low barrier to interconversion between boat conformations. Both methods of calculation indicated that the benzene oxide tautomer... 

In a recent important study, Tamon and coworkers [733-735] implicitly took the approach that orbital control is dominant. They analyzed the adsorption and desorption characteristics of a series of aromatic compounds at uncontrolled pH and used the semiempirical MINDO/3 method to determine the HOMO energy levels ( ii) for these adsorbates and the LUMO levels ( l) for several adsorbents (including chemically modified activated carbons). The range of calculated ii — El I values for adsorbates ranging from / -nitrobenzoic acid to aniline on a model activated carbon was 6.7-9.0 eV, as shown in Table 38. Low values (<7.3 eV) coincided with those systems for which irreversible adsorption was found those adsorbates that possess electron-donating groups (e.g.. aniline), i.e., with the highest HOMO levels (or lowest ionization potentials), could not be... 

TABLE 38 Results of Semiempirical (MINDO/3) Quantum Chemical Calculations of Energies of the Highest Occupied Molecular Orbitals (HOMO) and Lowest Unoccupied Molecular Orbitals (LUMO) for a Series of Aromatic Adsorbates and Model Carbon Clusters... 

The theoretical studies carried out to understand the photochemical behavior of P-r-butylstyrene, using open shell SCF methods involving semiempirical MINDO/3 approximation [44], indicate the highly polarized or zwitterionic nature of the singlet excited state. The model is also successful in explaining the P-j-butylstyrene photochemistry [45]. 

The triplet spin density distribution along the polyene chain of dodecapentaenal has been calculated using the semiempirical MINDO method (Kok and Groenen, 1996). The spin density at carbon 2 has been measured with ESEEM on deuterated molecules and agrees weU with these calculations and validates them (Kok and Groenen, 1996). 

Geometrical parameters have been calculated by an ab initio [9] and a semiempirical (MINDO, see [11]) SCF-MO method [10]. Distances r in A, bond angles a and dihedral angles in degrees (the point group is C2 [9,10]) ... 

Table 1.1. Ab initio (DZ-type basis set) [9], semiempirical (MINDO/3) [12] and experimental (gas phase) frequencies (cm" ) of normal vibrations of hydrogen cyanide...

On the PES of the reaction, obtained by the semiempirical MINDO/3 method, there are no minima of the biradical type, which is an evidence for the concerted character of the process [4] ... 

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