N–H···S Interaction Continues To Be an Enigma: Experimental and Computational Investigations of Hydrogen-Bonded Complexes of Benzimidazole with Thioethers

Competition between O-H and S-H Intermolecular Interactions in Conformationally Complex Systems: The 2-Phenylethanethiol and 2-Phenylethanol Dimers

Noncovalent interactions involving sulfur centers play a relevant role in biological and chemical environments. Yet, detailed molecular descriptions are scarce and limited to very simple model systems. Here we explore the formation of the elusive S-H···S hydrogen bond and the competition between S-H···O and O-H···S interactions in pure and mixed dimers of the conformationally flexible molecules 2-phenylethanethiol (PET) and 2-phenylethanol (PEAL), using the isolated and size-controlled environment of a jet expansion. The structure of both PET-PET and PET-PEAL dimers was unraveled through a comprehensive methodology that combined rotationally resolved microwave spectroscopy, mass-resolved isomer-specific infrared laser spectroscopy, and quantum chemical calculations. This synergic experimental-computational approach offered unique insights into the potential energy surface, conformational equilibria, molecular structure, and intermolecular interactions of the dimers. The results show a preferential order for establishing hydrogen bonds following the sequence S-H···S < S-H···O ≲ O-H···S < O-H···O, despite the hydrogen bond only accounting for a fraction of the total interaction energy.

Unveiling the underappreciated: The bonding features of C-H⋯S-S interactions observed from rotational spectroscopy

The C-H⋯S-S interactions are fundamentally important to understand the stability of biomacromolecules and their binding with small molecules, but they are still underappreciated. Herein, we characterized the C-H⋯S-S interactions in model molecular complexes. The rotational spectra of the complexes of diethyl disulfide with CH2CH2 and CH2CHF were measured and analyzed. All the detected structures are mainly stabilized by a C-H⋯S-S hydrogen bond, providing stabilization energies of 2.3-7.2 kJ mol-1. Incidental C-H⋯π or C-H⋯F interactions enhance the stabilization of the complexes. London dispersion, which accounts for 54%-68% of the total attractions, is the main driving force of stabilization. The provided bonding features of C-H⋯S-S are crucial for understanding the stabilizing role of this type of interaction in diverse processes such as supramolecular recognition, protein stability, and enzyme activity.

Hydrogen bond interactions between thioethers and amides: A joint rotational spectroscopic and theoretical study of the formamide⋯dimethyl sulfide adduct

The rotational spectrum of the binary adduct of formamide (HCONH₂) with dimethyl sulfide (DMS) has been investigated employing cavity-based Fourier transform microwave spectroscopy combined with theoretical computations. Experimentally, only one isomer of the adduct was unambiguously observed and assigned according to the theoretically predicted spectroscopic parameters, and its rotational spectrum displays the hyperfine splittings associated with the ¹⁴N nuclear quadrupole coupling effect. The observed isomer exhibits C_s symmetry, such that the ∠CSC angle of the DMS subunit is bisected by the ab-plane of the HCONH₂ moiety. The two moieties in the detected isomer are connected via one primary NH···S and two secondary CH···O hydrogen bonds. Quantum theory of atoms in molecules (QTAIM), non-covalent interaction (NCI), natural bond orbital (NBO) and symmetry-adapted perturbation theory (SAPT) approaches were utilized for characterizing the intermolecular interactions occurring in the titled adduct. Additionally, the adduct of HCONH₂ with dimethyl ether (DME) was also theoretically investigated to compare the difference in structure and energy characteristics between the NH···S and NH···O hydrogen bonds.

Selenium in Proteins: Conformational Changes Induced by Se Substitution on Methionine, as Studied in Isolated Model Peptides by Optical Spectroscopy and Quantum Chemistry

The side-chain of methionine residues is long enough to establish NH⋯S H-bonds with neighboring carbonyl groups of the backbone, giving rise to so-called intra-residue 6^δ and inter-residue 7^δ H-bonds. The aim of the present article is to document how the substitution of sulfur with a selenium atom affects the H-bonding of the Met system. This was investigated both experimentally and theoretically by conformation-resolved optical spectroscopy, following an isolated molecule approach. The present work emphasizes the similarities of the Met and Sem residues in terms of conformational structures, energetics, NH⋯Se/S H-bond strength and NH stretch spectral shifts, but also reveals subtle behavior differences between them. It provides evidence for the sensitivity of the H-bonding network with the folding type of the Sem/Met side-chains, where a simple flip of the terminal part of the side-chain can induce an extra 50 cm⁻¹ spectral shift of the NH stretch engaged in a 7^δ NH⋯S/Se bond.

Hydrogen Bond and Geometry Effects of Thioamide Backbone Modifications

Thioamide substitution of backbone peptide bonds can probe interactions along the main chain of proteins. Despite theoretical predictions of the enhanced hydrogen bonding propensities of thioamides, previous studies often do not consider the geometric constraints imposed by folded peptide secondary structure. This work addresses drawbacks in previous studies that ignored the geometry dependence and local dielectric properties of thioamide hydrogen bonding and identifies cases where thioamides may be either stronger or weaker hydrogen-bonding partners than amides.

Molecular Recognition, Transient Chirality and Sulfur Hydrogen Bonding in the Benzyl Mercaptan Dimer

The homodimers of transiently chiral molecules offer physical insight into the process of molecular recognition, the preference for homo or heterochiral aggregation and the nature of the non-covalent interactions stabilizing the adducts. We report the observation of the benzyl mercaptan dimer in the isolation conditions of a supersonic jet expansion, using broadband (chirped-pulse) microwave spectroscopy. A single homochiral isomer was observed for the dimer, stabilized by a cooperative sequence of S-H···S and S-H···π hydrogen bonds. The structural data, stabilization energies and energy decomposition describe these non-covalent interactions as weak and dispersion-controlled. A comparison is also provided with the benzyl alcohol dimer.

Characterization and Photofragmentation Studies of the Benzimidazole Homodimer: Evidence for Excited-State Charge-Coupled Proton Transfer

The spectroscopic characterization of the benzimidazole (BIM) homodimer was carried out in a molecular beam in the ground state as well as in the cationic state using the R2PI and RIDIR methods. Primarily, interest in the dimer was due to the observation of a proton-transferred BIM fragment at energies well below its thermodynamic threshold (i.e., barely above the ionization energy of the dimer where fragmentation was not expected). The detailed photofragmentation studies of the homodimer combined with spectroscopic observations and quantum chemical computations of the excited states established that the proton transfer from one subunit to the other occurs via conical intersections connecting the locally excited state, the charge-transfer state, and the ground state. In this study, we have also determined the N-H···N hydrogen bond dissociation energy in the ground state and in the cationic state to be 10.36 ± 0.14 and 27.55 ± 0.20 kcal mol^-1, respectively. Incidentally, this happens to be the first such report on the dissociation energy of the N-H···N hydrogen bond.

Participation of S and Se in hydrogen and chalcogen bonds

The heavier chalcogen atoms S, Se, and Te can each participate in a range of different noncovalent interactions. They can serve as both proton donor and acceptor in H-bonds. Each atom can also act as electron acceptor in a chalcogen bond.

A comprehensive infrared spectroscopic and theoretical study on phenol-ethyldimethylsilane dihydrogen-bonded clusters in the S0 and S1 states

To investigate microscopic characters of Si-H⋯H-O type dihydrogen bonds, we observed OH and SiH stretch bands in both the S₀ and S₁ states of phenol-ethyldimethylsilane (PhOH-EDMS) clusters by infrared (IR)-ultraviolet (UV) and UV-IR double resonance spectroscopies. Density functional theory (DFT) calculations and energy decomposition analysis were also performed. Structures of two isomers identified were unambiguously determined through the analysis of IR spectra and DFT calculations. To discuss the strength of dihydrogen bond in various systems, we performed theoretical calculations on clusters of EDMS with several acidic molecules in addition to PhOH. It was revealed that charge-transfer interaction energies from a bonding σ orbital of SiH bond to an anti-bonding σ^* orbital of OH bond well reflect strengths of dihydrogen bonds. Additionally, it was found that the red shift of SiH stretch frequencies can be used as a crude measure of the strength of dihydrogen bonds. Relationship between the red shifts of OH/SiH stretch frequencies and various electrostatic components of the interaction energy was examined. In the S₁ state, large increases in red shifts were observed for both the OH and SiH stretch frequencies. Since the EDMS moiety is not associated with the electronic excitation in a cluster, the strength of dihydrogen bonds in the S₁ and S₀ states was able to be directly compared based on the red shifts of the SiH stretch bands. A significant increase in the red shift of SiH stretch frequency indicates a strengthening of the dihydrogen bonds during the electronic excitation of the PhOH moiety.

Sulfur hydrogen bonding and internal dynamics in the monohydrates of thenyl mercaptan and thenyl alcohol

Water forms weak H-bonds with thenyl compounds, simultaneously retaining internal mobility in the dimer.