The S∴π hemibond and its competition with the S∴S hemibond in the simplest model system: infrared spectroscopy of the [benzene-(H₂S)_n]⁺ (n = 1–4) radical cation clusters

Abstract

IR spectroscopy of [benzene-(H2S)n]+ (n = 1–4) elucidates the change of the positive charge accommodation motif from the S∴π hemibond to the S∴S hemibond.

Hidden Hemibonding in the Aqueous Hydroxyl Radical

“…First described by Pauling, a two-center, three-electron (2c–3e) “hemibond” is formed when a doubly occupied frontier orbital of one molecule overlaps favorably with the half-filled orbital of a radical, resulting in a stabilizing interaction with a bond order of 1/2. Computational evidence for the existence of hemibonds has been put forward in a variety of systems, − and hemibonding has long been discussed in the context of aqueous-phase pulse radiolysis experiments involving sulfur-containing species. − Definitive experimental evidence for hemibonds exists only in a few cases, however, including (H 2 S) n + , , (CH 3 SH) 2 + , (CH 3 S) 2 + , [(CH 3 ) 2 S] 2 + , Cl···NH 3 , and a few other systems. , …”

“…Computational evidence for the existence of hemibonds has been put forward in a variety of systems, [2][3][4][5][6][7][8][9] and hemibonding has long been discussed in the context of aqueous-phase pulse radiolysis experiments involving sulfur-containing species. [10][11][12][13] Definitive experimental evidence for hemibonds exists only in a few cases, however, including (H 2 S) + n , 14,15 (CH 3 SH) + 2 , 16 (CH 3 S) + 2 , 17 [(CH 3 ) 2 S] + 2 , 18 Cl• • • NH 3 , 19 and a few other systems. 20,21 Hemibonding in OH(aq) was predicted in some of the earliest ab initio molecular dynamics (ai MD) simulations using density functional theory (DFT), [22][23][24][25][26] but these predictions were quickly rejected as artifacts of selfinteraction error (SIE) associated with the use of semilocal functionals, 26 since SIE is known to overstabilize 2c-3e bonds.…”

Infrared Spectroscopy and Anharmonic Vibrational Analysis of (H2O–Krn)+ (n = 1–3): Hemibond Formation of the Water Radical Cation

“…While the bonding orbital is fully occupied, the antibonding orbital is half occupied, and such an electron configuration results in an effective bond with the bond order of 1/2. The hemibond has attracted much interest in radiation chemistry, especially of sulfur-containing systems, − and recently its roles in biochemistry have also been extensively discussed. − …”

An interesting possibility of forming special hole stepping stones with high-stacking aromatic rings in proteins: three-π five-electron and four-π seven-electron resonance bindings

“…45,46 More recently, Fujii group have conrmed the formations of SrS and Srp two-center hemibonds, Srp:S multicenter hemibond in the [benzene-(H 2 S) n ] + cluster by the IR spectroscopy and DFT calculations. 47 The essential features of the instant stepping stones forming during the hole transfer processes include that the each constituent moiety is electron-rich with lone pair-or p-orbitals, the formations of them can signicantly decrease the ionization energy of the associative complex relative to the each moiety, the stabilization of them is moderate, which ensures the effective formation and dissociation of stepping stone to promote hole migration in proteins. In spite of the great efforts to investigate the possible relay stones, however, the accurate pathway of hole transfer in proteins remains incomprehensible owing to the complexity of proteins.…”

“…Motivated by these issues and inspired by the previous work, [46][47][48][49][50][51] we are inquisitive about whether the interactions of three, four or more close aromatic rings may generate a new type of stepping stone to prompt hole transportation in proteins. Therefore, we carry out a DFT computational investigation on the electronic structures and the potential conductance of higher p-stacking structures in the present work.…”