Demetrios K. Papayannis

Ab initio and RRKM calculations for the decomposition channels of CH3OBr and BrCH2OH

Abstract Quantum mechanical and RRKM calculations are carried out to study the potential energy surface and the kinetics for the six most important decomposition channels of methyl hypobromite (CH 3 OBr). Optimized geometries, vibrational frequencies, and relative energies have been obtained for the various stationary points. The O–Br bond scission to CH 3 O+Br products and the 1,2 elimination pathway leading to HCHO+HBr appear to be the most important dissociation channels. Analogous paths from the isomeric BrCH 2 OH are also examined. The calculations are compared with the results for the other two similar systems, CH 3 OF and CH 3 OCl.

A quantum mechanical study of the structure, vibrational spectra and relative energetics of XOOI, XIO2 and XOIO (X=Cl, Br, I) isomers

Abstract Two different effective-core-potential methodologies, augmented by extra polarization functions were used at the MP2 and CCSD(T) levels of theory to investigate a number of iodine containing isomers of the type XOOI, XIO2 and XOIO (X=Cl, Br, I). The procedures, successfully tested first on the well studied Cl2O2 system, produced results for XIO2 isomers analogous to the corresponding Cl and Br species. The relative energetics however, indicates competing stability depending on the method, between the peroxide XOOI and the Y-shaped XIO2 form for ClIO2 and BrIO2 and between IOOI and IOIO for the I2O2 family.

Ab initio calculations for (BrO)2 system and quasiclassical dynamics study of BrO self-reaction

Abstract Ab initio quantum mechanical studies for BrOOBr peroxide and its isomers were carried out at the MP2 level of theory using the 6-311+G(2d) basis set. Three minima were determined which are in good consistency with previous density functional theory calculations. On the basis of the ab initio results a simple analytical potential energy surface (PES) was constructed and quasiclassical trajectory calculations (QCT) for the self-reaction of BrO radical were performed. Reactive cross-sections, rate coefficients and branching ratios for the two reactive channels are calculated for a series of initial relative kinetic energies which compare favourably with the experimental trends. The quantum mechanical calculations and the analysis of the trajectory results support the experimental evidence that the reaction proceeds through an energetically enriched conformer of BrOOBr peroxide. In addition, the increasing collision lifetime with decreasing collision energy indicates a possible weak stabilization of a short-lived collision complex at lower temperatures which must be responsible for the increasing importance of the rate coefficient of the secondary channel as the temperature decreases, in consistency with the experimental evidence.

Quantum mechanical studies of methyl bromoperoxide isomers and the CH3O+BrO reaction

Ab initio quantum mechanical methods are employed to study methyl bromoperoxide and its isomers which are interesting intermediates in the reaction between methoxy radicals and bromine monoxide. Structural parameters, harmonic frequencies and relative energetics are calculated for all isomeric and conformeric forms. The CH3OOBr isomer is found to be the lowest energy structure followed by CH3OBrO while CH3BrO2 lies much higher in energy. The role of these isomers in the mechanism of the reaction CH3O + BrO is examined. Several transition state structures for the most important reaction channels are also investigated.

Ab initio investigation of isomeric and conformeric structures of halogen nitrites, XONO (X = Cl, Br, I)

A comparative study of isomers and conformers of halogen nitrites, XONO (X = Cl, Br, I), with particular emphasis on the I derivatives, has been carried out using high levels of electronic structure theory. All isomeric and conformeric structures and cis- to trans- conformational barriers were determined for each family. The nitryl halide isomers, XNO2, were calculated to be the lowest energy compounds. Interesting variations in the structural parameters, harmonic vibrational frequencies and stabilization energies were obtained on halogen substitution, that are particularly pronounced in the iodine family.

Structural and relative stability studies of IOOX peroxides (X=Cl, Br, I) and their isomers

Abstract Two different effective-core-potential methodologies, augmented with extra polarization functions are used at the MP2 level of theory to investigate the isomers of I-containing peroxides of the type XOOI, XIO 2 , IXO 2 , XOIO and IOXO (X=Cl, Br, I). The reliability of the results is checked against the all-electron calculations of Misra and Marshall [J. Phys. Chem. A, 102 (1998) 9056] for the IOOI family. Contrary to what has been well established for the chlorine and bromine analogs, the present study confirms the interesting observation that the I-containing peroxide isomers, XOOI, are not the most stable structures in each subfamily.

Quantum mechanical and RRKM studies of the reactions CH3+ClO→CH3O+Cl and CH3O+Cl→HCHO+HCl

Abstract Bimolecular rate coefficients based on variational RRKM theory are calculated for the reaction of methyl radicals with chlorine monoxide and the reaction of methoxy radicals with chlorine atoms. The reaction pathways, established by ab initio calculations, are found to involve the intermediate formation of methyl hypochlorite. Our computations agree well with recent ab initio data on CH 3 OCl thermal decomposition channels and also the resulting rate coefficients are found in good agreement with the experimental measurements.

Structural and relative stability studies of (IClO3) and (IBrO3) polyoxides

An effective-core-potential (ECP) methodology, augmented with extra polarization functions has been used at the MP2 level of theory to study the isomers of (IXO3) polyoxides (X=Cl, Br). The reliability of the calculations has been checked by examining the well-characterized (ClClO3) system. Consistent with the relative stability trends observed in the case of chlorine polyoxides, the calculations confirm that the I-hypercoordinated structures present high stability, especially the compounds containing five-coordinated I.

Quantum mechanical and kinetic studies of the reaction of methyl radicals with molecular bromine

Abstract Quantum mechanical electronic structure calculations were carried out to determine equilibrium geometries, energetics and normal mode frequencies of stationary points along the minimum energy reaction path for the reaction of methyl radicals with bromine molecules. The results are used to calculate the reactive cross-section as a function of the initial energy and the rate coefficient at different temperatures, employing both extended RRKM theory and quasi-classical trajectory techniques. Both methods agree fairly well with the experimental measurements and describe sufficiently well the slight decline in reactivity with increasing temperature, which is observed experimentally.

Theoretical enthalpies of formation and structural characterisation of halogenated nitromethanes and isomeric halomethyl nitrites

The structural, energetic, and thermochemical properties of a number of halogenated nitromethanes, CHnX3−nNO2, and the isomeric nitrites, CHnX3−nONO, are investigated, using theoretical ab initio and density functional theory (DFT) electronic structure methods. Analysis of the results and comparison with the maternal species, nitromethane, CH3NO2, and methyl nitrite, CH3ONO, reveal strong dependence of the molecular properties on the halogen induction effect. Opposite trends are obtained in the C—N and C—O bond dissociation energies (BDE) upon halogenation and higher stabilities are calculated for the trans-nitrite isomers, in contrast with the plain alkyl families where the nitroalkanes are the most stable species. Formation enthalpies, ΔHfℴ, at 298 K are calculated for all halogenated isomers.