Post Hartree-Fock and density functional theory studies on structure and conformational stability of nitrosoethylene and substituted compounds of nitrosoethylene

Three decades of unveiling the complex chemistry of C-nitroso species with computational chemistry

This review revisits the complex reactivity of C-nitroso derivatives through the synergistic combination of computational and synthetic organic chemistry, with an emphasis on the rationalization of mechanisms and selectivities. 

Vibrational spectra and analysis of acetohydrazide CH3-CO-NH-NH2

The structural stability of acetohydrazide CH(3)-CO-NH-NH(2) was investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. The C-N rotational barrier in the molecule was calculated to be about 26 kcal/mol that suggested the planar sp(2) nature of the nitrogen atom of the central NH moiety. The N atom of the terminal NH(2) group was predicted to highly prefer the pyramidal sp(3) structure with an inversion barrier of about 7-8 kcal/mol. The molecule was predicted to have a trans-syn (N-H bond is trans with respect to CO bond and NH(2) moiety is syn to C-N bond) conformation as the lowest energy structure. The vibrational frequencies were computed at B3LYP level of theory and normal coordinate calculations were carried out for the trans-syn acetohydrazide. Complete vibrational assignments were made on the basis of normal coordinate analyses and experimental infrared and Raman data.

Infrared and Raman spectra and vibrational analyses calculated with Moeller-Plesset perturbation theory of second order of nitrosoethylene and its chloro-derivatives

The conformational and structural stabilities of nitrosoethylene CH2=CH-N=O, chloronitrosoethylene CH2=CCl-N=O, and Dichloronitrosoethylene CCl2=CH-N=O were investigated by ab initio Moeller-Plesset perturbation theory of second order (MP2) calculations using the 6-311+G** basis set to include electron correlation. From the calculations all three were predicted to exist predominantly in the planar trans structure (C=C and N=O bonds are trans to each other) with high trans-cis rotational barriers of about 9 kcal mol(-1) as a result of pronounced conjugation between C=C and N=O bonds. The vibrational frequencies were computed for the three molecules, and also the d1 and d2 deuterated variants for the parent molecule at the MP2 level. Normal coordinate analyses were carried out and the potential energy distributions (PED), among the symmetry coordinates of the normal modes of the molecule were computed. Complete vibrational assignments were made on the basis of normal coordinate analyses for the molecules. The two chlorinated derivatives of nitrosoethylene were also investigated in the same way. As expected, we then find high Raman and infrared intensities in all modes that contain a high content of chlorine movements because vibrations of C-Cl bonds lead to large changes in polarizability, as well as to a large change in dipole moment. However, modes involving double bonds also have quite large intensities. An appreciable number of modes in these molecules are more or less pure symmetry coordinates.

Normal coordinate analyses and barrier to internal rotation of nitroso- and nitroazides

The conformational and structural stability of nitrosoazide NNN-N=O and nitroazide NNN-NO2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. From the calculations, nitrosoazide was predicted to exist predominantly in the planar trans (NNN and N=O groups are trans to each other) structure with high trans-cis rotational barrier of about 11 kcal mol-1 as a result of pronounced conjugation between the azide group and the N=O bond. The NO2 rotational barrier in nitroazide was predicted from the symmetric potential function to be of about 7 kcal mol-1. The vibrational frequencies were calculated at the DFT-B3LYP level and the infrared and Raman spectra of the cis-trans mixture were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the stable conformers of both molecules. For nitrosoazide, the calculated wavenumbers were compared to the corresponding experimental values obtained from early reported Raman spectrum of the molecule.

Structure, conformation and NMR studies on 1,2-dioxane and halogen substituted 1,2-dioxane molecules

Molecular structure and conformational stability of chair and twist conformers of 1,2-dioxane and halogen substituted compounds of the 1,2-dioxane have been studied using ab initio and density functional theory (DFT) methods. The molecular geometries of 1,2-dioxane, 3,6-difluoro, 3,6-dichloro, 3,3,6,6-tetrafluoro and 3,3,6,6-tetrachloro 1,2-dioxane compounds were optimized at HF, MP2, B3LYP and B3PW91 levels of theory by implementing 6-31G* basis set. To study the effect of polar medium, self-consistent reaction field theory is used to optimize the conformers at B3LYP/6-31G* level of theory. The geometrical parameters of chair and twist conformers have been discussed in the light of interaction between lone pair electrons present in the oxygen and substituted halogen atoms. The relative stability of the conformers have been studied using relative energy, maximum hardness principle and thermodynamical quantities. The 13C-NMR chemical shift study for carbon atoms in the title compounds are calculated and the results have been discussed.

Quantum chemical studies on tautomerism of barbituric acid in gas phase and in solution

Ab inito and density functional theory (DFT) methods were used to study the tautomers of barbituric acid in the gas phase and in a polar medium. In the gas phase, the tautomers were optimized at the HF/6-31G*, MP2/6-31G* and B3LYP/6-31G*, B3PW91/6-31G* levels of theory. The self-consistent reaction field theory (SCRF) at the HF/6-31G* level of theory has been used to optimize the tautomers in a polar medium. The relative stability of the tautomers was compared in the gaseous and polar mediums. The ability of maximum hardness principle to predict the stable tautomer has been studied. The 13C-NMR chemical shift for carbon atoms in the tautomers was calculated and the results are discussed.