Agnie M. Kosmas

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.

A classical trajectory study of the O(3P)+I2 reaction

Abstract A trajectory study of the O( 3 P)+ I 2 reaction at three initial translational energies, using a LEPS potential energy function is described. Comparison is made with the experimental measurements of Grice and co-workers. It is found that the symmetry of forward-backward scattering begins to deviate at high collision energies. This may be interpreted in terms of a gradual transition from a long-lived collision complex mechanism at very low initial translational energies to a two-component mechanism with prevailing contribution from backward scattering as the collision energy is increased.

Theoretical study on the mechanism of the reaction of CF(3)S with NO(2).

The singlet potential energy surface for the CF3S + NO2 reaction has been theoretically investigated using the B3LYP/6-311+G(3df) level of theory. The geometries, vibrational frequencies, and zero-point energies of all stationary points involved in the title reaction have been examined. More accurate energies of stationary points were obtained using ab initio G3//B3LYP and CBS-QB3 composite methods. The results show that the initial addition of CF3S with NO2 leads to CF3SNO2 or CF3SONO intermediates, which are formed without an electronic barrier. CF3SNO2 can easily isomerizes to CF3SONO, while CF3SONO readily isomerizes to CF3S(O)NO or dissociates to CF3SO + NO, which are the major products of the title reaction. Reaction channels leading to the formation the CF3O + SNO and CF2S + FNO2 products are highly improbable processes due to high energy barriers involved. We have also computed heats of formation for CF3SNO2, CF3SONO, and CF3S(O)NO intermediates. It was found that the most stable is the cis-perpendicular form of CF3SONO isomer with DeltaH(f,0)0 = -243.6 kcal mol-1.

Effect of Halogenation on the Mechanism of the Atmospheric Reactions between Methylperoxy Radicals and NO. A computational Study

The mechanism of the reactions between the halogenated methylperoxy radicals, CHX(2)O(2) (X = F, Cl), and NO is investigated by using ab initio and density functional quantum mechanical methods. Comparison is made with the mechanism of the CH(3)O(2) + NO reaction. The most important energy minima in the potential energy surface are found to be the two conformers of the halogenated methyl peroxynitrite association adducts, CHX(2)OONOcp and CHX(2)OONOtp, and the halogenated methyl nitrates, CHX(2)ONO(2). The latter are suggested to be formed through the one-step isomerization of the peroxynitrite adduct and may lead upon decomposition to carbonylated species, CX(2)O + HONO and CHXO + XNO(2). The ambiguous issue of the unimolecular peroxynitrite to nitrate isomerization is reconsidered, and the possibility of a triplet transition state involvement in the ROONOtp <--> RONO(2) rearrangement is examined. The overall calculations and the detailed correlation with the methyl system show the significant effect of the halogenation on the lowering of the entrance potential energy well which corresponds to the formation of the peroxynitrites. The increased attractive character of the potential energy surface found upon halogenation combined with the increased exothermicity of the CHX(2)O(2) + NO --> CHX(2)O + NO(2) reaction are suggested to be the important factors contributing to the enhanced reactivity of the halogenated reactions relative to CH(3)O(2) + NO. The calculated heat of formation values indicate the large stabilization of the fluorinated derivatives.

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 studies on the potential energy surface of the reactions CH3+OClO, CH3O+ClO and CH3O2+Cl

Abstract The potential energy surface for the reactions CH 3 +OClO (1), CH 3 O+ClO (2) and CH 3 O 2 +Cl (3) is investigated using ab initio quantum mechanical methods. The calculations indicate the intermediate formation of two important isomeric energy minima, CH 3 OClO and CH 3 OOCl. H 2 CO+HOCl and CH 3 O+ClO are shown to be the most probable products for reaction (1) and H 2 CO+HOCl and CH 2 O 2 +HCl for reaction (2). Reaction (3) most probably leads to either CH 2 O 2 +HCl or CH 3 O+ClO.