Hemibond complexes between H2S and free radicals (F, Cl, Br, and OH)

Abstract

as F [4][5][6][7][8] and Cl [9] and found that there often exists a pre-reaction complex between the two reactants. The existence of such complexes has also been reported in other ab initio studies [10][11][12][13]. Interestingly, the F⋯H 2 O complex has been shown to aid the F + H 2 O reaction by guiding the trajectories to the transition state, resulting in a significant enhancement of the reactivity, particularly at low collision energies [14]. More interestingly, it was demonstrated that such complexes are not van der Waals or electrostatic in nature, but due to covalent interactions based on a twocenter three-electron bond [15].Such a two-center three-electron bond was first discussed by Pauling [16]. Due to the fact that one electron is in the antibonding orbital, such a covalent bond is usually quite weak with a relatively long bond length and hence called a hemibond [17]. Hemibond bonding has been found in many branches of chemistry [18], particularly between molecules with lone pairs of electrons and a free radical with an unpaired electron [19-31]. As mentioned above, such hemibond complexes are not only of a fundamental interest, but might also play an important role in molecule-radical reactions. Like oxygen-containing molecules, sulfur compounds may also provide lone electron pairs on S, thus allowing the formation of hemibonds with free radicals. In the literature, however, most such complexes investigated before are charged [17,19,21,22,[24][25][26], which complicates the bonding with electrostatic interactions. To further understand the sulfur-containing hemibond complexes, we report here an extensive highlevel ab initio and density functional theory (DFT) study of the complexes formed by H 2 S and several free radicals. The major aim is to understand the similarities and differences between the H 2 O⋯X and H 2 S⋯X complexes with X = F, Cl, Br, and OH by examining the structural, energetic, and electronic aspects of these interesting systems. AbstractThe interaction of hydrogen sulfide (H 2 S) with F, Cl, Br, and OH is investigated using ab initio methods to identify the two-center three-electron hemibond responsible for their complexation. The binding energies are found to be stronger than those in the analogous water complexes, but follow the same trend of increasing strength: F > Cl > Br > OH. The radicals are located nearly perpendicular to the H 2 S plane forming an angle of about 90°. Analysis of molecular orbitals and natural bond orbitals are carried out to understand the energetics, structures, and bonding characteristics of these hemibonded complexes.

The Hydrogen Abstraction Reaction H2S + OH → H2O + SH: Convergent Quantum Mechanical Predictions

“…Another isomer RC-B has been called the hemibond complex (Figure ), ,, in which the S and O atoms are thought to share three electrons, making the S–O bond order 0.5. Note that the analogous bonding was also found in the water dimer cation (H 2 O) 2 + and the dihydrogen sulfide dimer cation (H 2 S) 2 + , in which a hemibond was between two heavy atoms. − The RC-B complex has only the 2 A′ electronic state, with the unpair electron necessarily in the reflection plane.…”

“…Note that the analogous bonding was also found in the water dimer cation (H 2 O) 2 + and the dihydrogen sulfide dimer cation (H 2 S) 2 + , in which a hemibond was between two heavy atoms. − The RC-B complex has only the 2 A′ electronic state, with the unpair electron necessarily in the reflection plane. The nature of the purported two-center three-electron hemibond was investigated by Alday et al The 2 A″ state for RC-B is not a stationary point, since no such a hemibond exists without the in-plane unpair electron, and it collapses to RC-A ( 2 A″).…”

“…28−32 The RC-B complex has only the 2 A′ electronic state, with the unpair electron necessarily in the reflection plane. The nature of the purported two-center three-electron hemibond was investigated by Alday et al 8 The 2 A″ state for RC-B is not a stationary point, since no such a hemibond exists without the in-plane unpair electron, and it collapses to RC-A ( 2 A″).…”

“…Recently, Alday et al 8 studied such a two-center three-electron hemibond complex H 2 S•••OH, and examined the electronic structure of the hemibond using the molecular orbital (MO) and natural bond orbital (NBO) analyses. Alday et al attempted to compare the hemibond complex to the hydrogen bond complex which was optimized with DFT, but they were unable to optimize the hydrogen-bond complex with the CCSD(T)-F12 method.…”

“…In 2005, however, ab initio and DFT investigations reported another kind of complex H 2 S···OH with a two-center three-electron S···O bond, based on experimental evidence for the two-center three-electron adduct of (CH 3 ) 2 S–OH. Recently, Alday et al studied such a two-center three-electron hemibond complex H 2 S···OH, and examined the electronic structure of the hemibond using the molecular orbital (MO) and natural bond orbital (NBO) analyses. Alday et al attempted to compare the hemibond complex to the hydrogen bond complex which was optimized with DFT, but they were unable to optimize the hydrogen-bond complex with the CCSD­(T)-F12 method.…”