Semiempirical MNDO Study of Phenyl-sydnone and its Protonated Forms

The electronic and geometric structure of the phenylsydnone and its conjugated acids is investigated by the semiempirical MNDO method. A comparison of the interatomic bond distances and bond angles with available experimental data for the parent compound reveals only qualitative accordance. It appears that the MNDO method is unable to reproduce finer details of the molecular geometry of sydnones. The gross changes caused by protonation are described, however, in agreement with chemical intuition .and available experimental data. The variation in geometric parameters is interpreted in terms of rehybridization and :n-electron bond orders. The most stable conjugated acid is that with a proton bonded to the keto-oxygen in accordance with the highest electron density concentration

Semiempirical MNDO Study of Phenyl-sydnone and its Protonated Forms

The electronic and geometric structure of the phenylsydnone and its conjugated acids is investigated by the semiempirical MNDO method. A comparison of the interatomic bond distances and bond angles with available experimental data for the parent compound reveals only qualitative accordance. It appears that the MNDO method is unable to reproduce finer details of the molecular geometry of sydnones. The gross changes caused by protonation are described, however, in agreement with chemical intuition .and available experimental data. The variation in geometric parameters is interpreted in terms of rehybridization and :n-electron bond orders. The most stable conjugated acid is that with a proton bonded to the keto-oxygen in accordance with the highest electron density concentration

Vitamin C and Its Radicals: Tautomerism, Electronic Structure and Properties

The biological importance and activity of ascorbic acid and its radicals are briefly reviewed. The quantum mechanical calculations performed on these remarkable compounds are presented in some detail. Particular attention is devoted to structural and electronic features offered by the semiempirical MINDO/3 and MNDO schemes. By making use of the self-consistent charge (SCC-MO) method, the ESCA spectra of the ascorbic acid tautomers are predicted. It is found that the radical anion is more stable than each of the four AA tautomers. This is of importance because the unusual biological protective property of ascorbate against free radical damage is most likely related to the stability of its radical, Ortgins of the enhanced stability of the radical anion are analyzed by the energy partitioning technique

Point-charge Description of some Molecular Properties

The notion of formal atomic charges in molecules is probably the most debated issue in quantum chemistry. Although atomic charge can not be rigorously defined in a unique way, conclusive evidence is given here which shows that it provides semiquantitative information about a number of molecular properties in a very simple and transparent way. In particular, the calculations of ESCA chemical shifts, diamagnetic shielding of nuclei and diamagnetic part of the molecular magnetic susceptibility are well described and thoroughly discussed. Finally, a relation between the effective atomic charges and total molecular SCF energies is illustrated by numerical examples. The point-charge description of the mentioned molecular properties is particularly useful if it is employed within the framework of semiempirical theories .because the computational costs are then negligible. The most successful semiempirical scheme in this respect seems to be the SCC- MO (self-consistent charge MO) method

Point-charge Description of some Molecular Properties

The notion of formal atomic charges in molecules is probably the most debated issue in quantum chemistry. Although atomic charge can not be rigorously defined in a unique way, conclusive evidence is given here which shows that it provides semiquantitative information about a number of molecular properties in a very simple and transparent way. In particular, the calculations of ESCA chemical shifts, diamagnetic shielding of nuclei and diamagnetic part of the molecular magnetic susceptibility are well described and thoroughly discussed. Finally, a relation between the effective atomic charges and total molecular SCF energies is illustrated by numerical examples. The point-charge description of the mentioned molecular properties is particularly useful if it is employed within the framework of semiempirical theories .because the computational costs are then negligible. The most successful semiempirical scheme in this respect seems to be the SCC- MO (self-consistent charge MO) method

Symmetry, hybridization and bonding in molecules

AbstractBrief historical review of the applications of symmetry arguments in interpreting the electronic and geometric structures of molecules is given. The hybridization of atomic orbitals is thoroughly discussed. It was shown that hybridization model describes directional features of covalent bonding and a number of other properties which are not directly related otherwise. This versatility in rationalizing a large number of experimental data of different kinds and a high interpretative power give to the model some semblance of truth. The model's apparently good reflection of molecular properties could be traced to the fact that hybrids conform with the local symmetry of an atom in a molecular environment. An analogy between the hybridization model and crystal field theory is found. The epistemological role of the hybridization is stressed. It provides a bridge between the most elementary first principles of quantum mechanics and the concept of a chemical bond, which is a basic tenet of the phenomenological theory of molecules