Structural and Topological Characterization of the Three-Electron Bond:  The SO Radicals

Abstract

Although a two-center three-electron (2c-3e) bond between homonuclear atoms is well characterized, this is not the case for the S∴O bond, especially in neutral radicals resulting from the addition of a hydroxyl group on various sulfur substrates. A structural, energetical, and topological study is presented for prototypical radicals, ionic and neutral, RSOH -, RR′SOH 2 + , and RR′SOH, with R, R′ ) H, CH 3 . Three calculation methods have been applied, BH&HLYP, MP2, and CCSD(T), with different basis sets to determine the domains of accuracy of the more approximate ones to use them for larger systems. Qualitative and quantitative criteria, defined from the topological analysis of the electron localization function, are proposed to characterize such a 2c-3e bond. They specify the number and type of basins and their hierarchy of bifurcation, the global charge transfer between the fragments, the localization of the integrated spin density, and the electron delocalization between the lone pairs of the interacting atoms. Surprisingly, the neutral radicals show an intermediate behavior between the pure 2c-3e S∴O bond in anions and the electrostatic interaction in cations, despite the low energy of bond formation. As in the radical anions, the substitution favors the formation of a 2c-3e bond.

Methionine one-electron oxidation: Coherent contributions from radiolysis, IRMPD spectroscopy, DFT calculations and electrochemistry

“…An examination of the DFT methods led to the conclusion that similar results were obtained both with BH&HLYP as with the CCSD(T) approach [18][19][20]. We used also the PBE0 model, based on the Perdew-Burke-Erzenrhof exchange-correlation functional [21,22] and we showed that the results were totally in agreement with those of BH&HLYP.…”

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

“…It is worthy to note that, except for the sign, it is identical to the delocalization index we introduced in our previous studies [77,79]. As the total variance in a certain basin can be written in terms of covariance,…”

“…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].…”

“…Belonging to the group of topological theories of chemical bonding pioneered by Bader in his atoms-in-molecules (AIM) method [88], the topological analysis of ELF aims to partition the molecular space into chemically significant regions, thus providing a mathematical model of the Lewis theory. During the last 10 years, it has been mainly developed in our laboratory [89][90][91][92][93][94][95] and has been extensively applied by numerous groups to the understanding of the chemical structure of molecules [77,79,96, and solids [123,124]. It has also been combined with the catastrophe theory, a theory of chemical reactions, in which the evolution of ELF is studied as a function of control parameters [125][126][127][128][129].…”

“…The nature of the X O bond (X = N,F,Cl) is discussed into the framework of the ELF analysis and the resonant schemes which are inferred are compared to the ones of Pauling. Subsequent sections are devoted to the most striking results previously obtained into two different works; the first one deals with the process of electron attachment anion H n X І YH − m series (X,Y = heteroatom of the second period and n= 0,1,2,3) [77], whereas the second one is a comparative study of the SO bond in prototypical radicals, ionic and neutral, RSOH − , RR SOH + 2 , and RR SOH with R,R = H,CH 3 [79]. These complementary studies allow us to determine unambiguous topological signatures of the 2c-3e bond.…”