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.
To provide a deeper understanding of the kinetics of electron attachment to CF, the six-dimensional potential energy surfaces of both CF and CF were developed by fitting ∼3000 ab initio points per surface at the AE-CCSD(T)-F12a/AVTZ level using the permutation invariant polynomial-neural network (PIP-NN) approach. The fitted potential energy surfaces for CF and CF had root mean square fitting errors relative to the ab initio calculations of 1.2 and 1.8 cm, respectively. The main active mode for the crossing between the two potential energy surfaces was identified as the umbrella bending mode of CF in C symmetry. The lowest energy crossing point is located at R = 1.306 Å and θ = 113.6° with the energy of 0.051 eV above the minimum of the CF electronic surface. This value is only slightly larger than the experimental data 0.026 ± 0.01 eV determined by kinetic modeling of electron attachment to CF. The small discrepancy between the theoretical and experimentally measured values is analyzed.
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