Ab Initio and Density Functional Studies of HOBr−H2O and BrONO2−H2O Complexes

A comparison between hydrogen and halogen bonding: the hypohalous acid–water dimers, HOX⋯H2O (X = F, Cl, Br)

Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds.

An ab initio correlated study of the potential energy surface for the HOBr.H[sub 2]O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order Møller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.

Geometrical and vibrational DFT studies of HOBr·(H2O)n clusters (n = 1–4)

The geometrical structures and the vibrational spectra of the HOBr·(H2O)n clusters (n = 1–4) have been calculated at the DFT level of theory, using the pBP method and the DN* and DN** numerical basis sets. The results showed that the interaction involving the H of the HOBr and the O of the water molecule represent the preferred arrangements for these hydrated compounds. Both HOBr·H2O and HOBr·(H2O)2 clusters presented stable structures with syn and anti conformations, the syn being the most stable. The HOBr·(H2O)3 and the HOBr·(H2O)4 clusters have presented stable cyclic structures. In the HOBr·H2O and HOBr·(H2O)2 clusters, low-frequency stretching values could be assigned to hydrogen bonds, but the same could not be done so clearly for the HOBr·(H2O)3 and the HOBr·(H2O)4 cyclic clusters. The binding energies were also determinated for these HOBr hydrated clusters, showing that the addition of a water molecule to the HOBr·H2O and HOBr·(H2O)2 clusters increases the binding energy by approximately 4 kcal mol–1, while the addition of a water molecule to the HOBr·(H2O)3 cluster decreases this value by 4 kcal mol–1.Key words: DFT, numerical basis, HOBr·(H2O)n, clusters.

Equilibrium geometry, vibrational frequencies, and heat of formation of HOBr, HBrO2, and HBrO3 isomers

The equilibrium geometries, vibrational frequencies, heat capacity, and heat of formation for compounds of general formula HBrOx were calculated by DFT (BP and pBP methods) with DN* and DN** numerical basis sets. The comparison of our HOBr calculated results with the HOBr experimental values points out that the BP and pBP methods are as good as other ab initio and DFT methods related in the literature employing extended basis sets. The calculated HBrOx total energy and heat of formation values, at 0 and 298.15 K, present the following order: HOBr < HBrO; HOOBr < HOBrO < HBrO2; HOOOBr < HOBrO2 < HOOBrO < HBrO3. The HBrOx heat of formation was calculated using isodesmic and homodesmic reactions and the results show that, in general, the use of these reactions gives similar results.Key words: HOBr, HBrO2, HBrO3, DFT, numerical basis.