Methyl Substituent Effects in [H_nX∴XH_n]⁺ Three-Electron-Bonded Radical Cations (X = F, O, N, Cl, S, P; n = 1−3). An ab Initio Theoretical Study

Abstract

The effects of methyl substitution on the geometries and bonding energies of a systematic series of three-electron-bonded radical cations of the type [H(n)X...XH(n)](+), covering all possible symmetrical three-electron bonds that may take place between atoms of the second and third rows of the periodic table, have been investigated at the level of Møller-Plesset perturbation theory. Methyl substitution leads to significant weakening and lengthening of the X...X bond when X is a second-row atom. The effects increase with the number of substitutions and are more and more important in the series X = N, O, F. By contrast, methyl substitution leaves the bonding energies between third-row atoms practically unchanged but leads to a surprising bond shortening in the S...S and P...P cases. These seemingly contradictory effects are rationalized through a qualitative analysis based on an elementary molecular orbital description of three-electron bonding.

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

“…The binding energy decreases in the order F ≫ Cl > Br > OH. These binding energies are consistent with those found previously for hemibond complexes involving sulfur [19,22,[24][25][26].…”

“…The long bond lengths are consistent with previous theoretical results for hemibond complexes involving sulfur [17,19,21,22,[24][25][26]. For example, the S-O distance in the H 2 S⋯OH complex is 2.62 Å, which can be Table 1 Geometric parameters of H 2 S and its complexes with various free radicals obtained at the UCCSD(T)-F12a/AVTZ level and at the MPW1K/AVTZ level (in parentheses)…”

“…While the angle θ is larger than 90° in the H 2 O⋯X (X = Cl, Br) complexes, the corresponding angle in the H 2 S⋯X complexes is near 90°. Similar right-angled hemibond complexes have been reported before between sulfur-containing molecules and free radicals [17,19,21,22,[24][25][26]. As shown in Fig.…”

“…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.…”

What can we learn from two‐center three‐electron bonding with the topological analysis of ELF?

“…This method appears to be the only candidate for calculating chemical properties of hemibonded species, since it includes the essential dynamical correlation at a rather economical cost, provided the molecules are symmetric and the geometries that are considered are not too stretched relative to equilibrium [50,70]. Numerous other methodological analyses have been published that provided better insight into the nature of this peculiar type of bonding either in cationic [51,52,71,73], anionic [74][75][76], or neutral species [68,69,78,79]. The crucial problem of competition that often occurs between 2c-3e bonded and hydrogen-bonded species and may determine the stability of the former has also been addressed in several studies [80][81][82].…”