Weak hydrogen bonds C–H⋯S and C–H⋯F–C in the thiirane–trifluoromethane dimer

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.

Exploring the conformational landscape, hydrogen bonding, and internal dynamics in the diallyl ether and diallyl sulfide monohydrates

The conformational spaces of the diallyl ether (DAE) and diallyl sulfide (DAS) monohydrates were explored using rotational spectroscopy from 6 to 19 GHz. Calculations at the B3LYP-D3(BJ)/aug-cc-pVTZ level suggested significant differences in their conformational behavior, with DAE-w exhibiting 22 unique conformers and DAS-w featuring three stable structures within 6 kJ mol-1. However, only transitions from the lowest energy conformer of each were experimentally observed. Spectral analysis confirmed that binding with water does not alter the conformational preference for the lowest energy structure of the monomers, but it does influence the relative stabilities of all other conformers, particularly in the case of DAE. Non-covalent interaction and quantum theory of atoms in molecules analyses showed that the observed conformer for each complex is stabilized by two intermolecular hydrogen bonds (HBs), where water primarily interacts with the central oxygen or sulfur atom of the diallyl compounds, along with secondary interactions involving the allyl groups. The nature of these interactions was further elucidated using symmetry-adapted perturbation theory, which suggests that the primary HB interaction with S in DAS is weaker and more dispersive in nature compared to the primary HB in DAE. This supports the experimental observation of a tunneling splitting exclusively in the rotational spectrum of DAS-w, as the weaker contact allows water to undergo internal motions within the complex, as shown based on calculated transition state structures for possible tunneling pathways.

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.

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.

The Characteristics of Disulfide-Centered Hydrogen Bonds

The disulfide-centered hydrogen bonds in the three different model systems of diethyl disulfide⋅⋅⋅H₂ O/H₂ CO/HCONH₂ clusters were characterized by high-resolution Fourier transform microwave spectroscopy and quantum chemical computations. The global minimum energy structures for each cluster are experimentally observed and are characterized by one of the three different S-S⋅⋅⋅H-C/N/O disulfide-centered hydrogen bonds and two O⋅⋅⋅H-C hydrogen bonds. Non-covalent interaction and natural bond orbital analyses further confirm the experimental observations. The symmetry-adapted perturbation theory (SAPT) analysis reveals that electrostatic is dominant in diethyl disulfide⋅⋅⋅H₂ O/HCONH₂ clusters being consistent with normal hydrogen bonds, whilst dispersion takes over in diethyl disulfide⋅⋅⋅H₂ CO cluster. Our study gives accurate structural parameters for the disulfide bond involved non-covalent clusters providing important benchmarking data for the theoretical evaluation of more complex systems.

C-HS interaction exhibits all the characteristics of conventional hydrogen bonds

This is a tale of a pair of a hydrogen bond donor and acceptor, namely the CH donor and sulphur acceptor, neither of which is a conventional hydrogen bond participant. Sulfur (S), being less electronegative (2.58) compared to its first row analogue oxygen (3.44), has not been considered as a potential HB acceptor for a long time. The C-HY (Y = HB acceptor) interaction has its own history of exhibiting omnidirectional shifts in the CH stretching frequency upon complex formation. Therefore, a systematic investigation of the C-HS interaction was the primary goal of the work presented here. Together with gas-phase vibrational spectroscopy and ab initio quantum chemical calculations, the nature and strength of the C-HS hydrogen bond (HB) have been investigated in the complexes of 1,2,4,5-tetracyanobenzene (TCNB) with various sulfur containing solvents. Despite the unconventional nature of both HB donor and HB acceptor (C-H and S, respectively), it was found that the C-HS hydrogen bond exhibits all the characteristics of the conventional hydrogen bond. The binding strength of the C-HS H-bond in these complexes was found to be comparable to that of the conventional hydrogen bonds. The unusual stabilities of these HBs have been mainly attributed to the attractive dispersion interaction.

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.

Remarkable shifts of Csp2 -H and O-H stretching frequencies and stability of complexes of formic acid with formaldehydes and thioformaldehydes

Thirty-six stable complexes of formic acid with formaldehydes and thioformaldehydes were determined on the potential energy surface, in which the XCHO···HCOOH complexes are found to be more stable than the XCHS···HCOOH counterparts, with X = H, F, Cl, Br, CH₃ , NH₂ . All complexes are stabilized by hydrogen bonds, and their contribution to the total stabilization energy of the complexes increases in going from C-H···S to C-H···O to O-H···S and finally to O-H···O. Remarkably, a significant blueshift of C_sp2 -H bond by 81-96 cm^-1 in the C_sp2 -H···O hydrogen bond has hardly ever been reported, and a considerable redshift of O-H stretching frequency by 206-544 cm^-1 in the O-H···O/S hydrogen bonds is also predicted. The obtained results in our present work and previous literatures support that a distance contraction and a stretching frequency blueshift of C-H bond involving hydrogen bond depend mainly on its polarity and gas phase basicity of proton acceptor, besides the rearrangement of electron density due to complex formation. Markedly, we suggest the ratio of deprotonation enthalpy to proton affinity (R _c ) as an indicator to prospect for classification of hydrogen bonds. The symmetry adapted perturbation theory results show a larger role of attractive electrostatic term in XO-n as compared to that in XS-n and the electrostatic interaction is overwhelming dispersion or induction counterparts in stabilizing XO-n and XS-n, with n = 1, 2, 3. © 2019 Wiley Periodicals, Inc.

Thiols as Hydrogen Bond Acceptors and Donors: Spectroscopy of 2-Phenylethanethiol Complexes

Evidence and understanding of sulfur-centered hydrogen bonding, especially where the donor is a thiol, lags far behind that for conventional OH interactions. To help address this deficiency, conformer specific IR spectra of 2-phenylethanethiol (PET) and associated 1:1 solvent complexes have been measured in SH, OH, and CH stretch regions using resonant-two-photon-ionization (R2PI) and IR-UV ion dip spectroscopic techniques. The aromatic and aliphatic CH stretch regions show signature differences between anti and gauche conformers. Supported by ab initio calculations, a PET-water cluster with an OH···S arrangement and a PET-diethyl ether cluster expressing an SH···O interaction were identified. The SH stretch band of the SH···O complex is red-shifted and undergoes significant intensity enhancement compared to the bare molecule, which is characteristic of hydrogen bonding. These findings offer insight into the nature of the thiol functional group as a potential hydrogen bond donor and acceptor.

Evaluation of N-H...S and N-H...π interactions in O,O'-diethyl N-(2,4,6-trimethylphenyl)thiophosphate: a combination of X-ray crystallographic and theoretical studies

In the crystal structure of O,O'-diethyl N-(2,4,6-trimethylphenyl)thiophosphate, C₁₃H₂₂NO₂PS, two symmetrically independent thiophosphoramide molecules are linked through N-H...S and N-H...π hydrogen bonds to form a noncentrosymmetric dimer, with Z' = 2. The strengths of the hydrogen bonds were evaluated using density functional theory (DFT) at the M06-2X level within the 6-311++G(d,p) basis set, and by considering the quantum theory of atoms in molecules (QTAIM). It was found that the N-H...S hydrogen bond is slightly stronger than the N-H...π hydrogen bond. This is reflected in differences between the calculated N-H stretching frequencies of the isolated molecules and the frequencies of the same N-H units involved in the different hydrogen bonds of the hydrogen-bonded dimer. For these hydrogen bonds, the corresponding charge transfers, i.e. lp (or π)→σ*, were studied, according to the second-order perturbation theory in natural bond orbital (NBO) methodology. Hirshfeld surface analysis was applied for a detailed investigation of all the contacts participating in the crystal packing.

The Cage Structure of Indan-CHF3 is Based on the Cooperative Effects of C-H⋅⋅⋅π and C-H⋅⋅⋅F Weak Hydrogen Bonds

The structural and energetic features of the C-H⋅⋅⋅π interaction and the internal dynamics of the CHF3 group change drastically in going from benzene-CHF3 to indan-CHF3 , according to the analysis of the rotational spectrum of the latter complex generated in a supersonic expansion.

Graphane/fluorographene bilayer: considerable C-H···F-C hydrogen bonding and effective band structure engineering

Systematic density functional theory (DFT) computations revealed the existence of considerable C-H···F-C bonding between the experimentally realized graphane and fluorographene layers. The unique C-H···F-C bonds define the conformation of graphane/fluorographene (G/FG) bilayer and contribute to its stability. Interestingly, G/FG bilayer has an energy gap (0.5 eV) much lower than those of individual graphane and fluorographene. The binding strength of G/FG bilayer can be significantly enhanced by applying appropriate external electric field (E-field). Especially, changing the direction and strength of E-field can effectively modulate the energy gap of G/FG bilayer, and correspondingly causes a semiconductor-metal transition. These findings open new opportunities in fabricating new electronics and opto-electronics devices based on G/FG bilayer, and call for more efforts in using weak interactions for band structure engineering.

On the weak O-H···halogen hydrogen bond: a rotational study of CH3CHClF···H2O

We measured the molecular beam Fourier transform microwave spectra of six isotopologues of the 1 : 1 adduct of CH(3)CHClF with water. Water prefers to form an O-H···F rather than an O-H···Cl hydrogen bond. This is just the contrary of what was observed in the chlorofluoromethane-water adduct, where an O-H···Cl link was formed (W. Caminati, S. Melandri, A. Maris and P. Ottaviani, Angew. Chem., Int. Ed., 2006, 45, 2438). The water molecule is linked with an O-H···F bridge to the fluorine atom, with r(F···H(w)) = 2.14 Å, and with two C-H···O contacts to the alkyl hydrogens with r(C(1)-H(1)···O(w)) = 2.75 Å and r(C(2)-H(2)···O(w)) = 2.84 Å, respectively. Besides the rotational constants, the quadrupole coupling constants of the chlorine atom have been determined. In addition, information on the internal dynamics has been obtained.

Rotational spectrum and structure of the carbonyl sulfide-trifluoromethane weakly bound dimer

Pure rotational spectra of five isotopomers of the 1:1 weakly bound complex formed between carbonyl sulfide and trifluoromethane (TFM) have been measured using Fourier transform microwave spectroscopy. The experimental rotational constants and dipole moment components are consistent with a structure of C(s) symmetry in which the dipole moment vectors of OCS and HCF(3) are aligned antiparallel and at an angle of about 40 degrees and with a center of mass separation of 3.965(26) A. The derived H...O distance is 2.90(5) A, which is up to 0.6 A longer than is seen in other similar TFM complexes exhibiting C-H...O interactions. Ab initio calculations at the MP2/6-311++G(2d,2p) level give a structure with rotational constants that are in reasonable agreement with those of the normal isotopic species.