Solvent control of the soft angular potential in hydroxyl-pi hydrogen bonds: inertial orientational dynamics

π-Hydrogen Bonding Probes the Reactivity of Aromatic Compounds: Nitration of Substituted Benzenes

The shifts of phenol O-H stretching vibration frequencies [Δν(OH)_exp] upon π-hydrogen bonding with aromatic compounds is proposed as a spectroscopic probe of the reactivity of aromatic substrates toward electrophiles. A single infrared spectrum reflecting the Δν(OH)_exp shift for an aromatic species in a reference solvent (CCl₄ in this study) provides a good estimate of reactivity. The methodology is applied in rationalizing reactivity trends for the BF₃ catalyzed nitration by methylnitrate in nitromethane of 20 aromatic reactants, including benzene, 11 methylbenzenes, several monoalkyl benzenes, the four halobenzenes, and anisole. Literature kinetic data are employed in the analysis. Very good correlations between relative rates of nitration and Δν(OH)_exp are obtained. The approach is best applied to reactions, where the initial interactions between the reactants controls the rates. A new theoretical quantity, the shifts (with respect to benzene) of the molecular electrostatic potential at 1.5 Å over the centroid of the aromatic ring [Δ V(1.5)] is defined and shown to provide a good description of substituent effects on properties of the aromatic species. B3LYP density functional and MP2 ab initio methods combined with the 6-311++G(3df,2pd) basis set are employed in evaluating the Δ V(1.5) values.

Unexpected solvent effects on the UV/Vis absorption spectra of o-cresol in toluene and benzene: in contrast with non-aromatic solvents

Cresol is a prototype molecule in understanding intermolecular interactions in material and biological systems, because it offers different binding sites with various solvents and protonation states under different pH values. It is found that the UV/Vis absorption spectra of o-cresol in aromatic solvents (benzene, toluene) are characterized by a sharp peak, unlike the broad double-peaks in 11 non-aromatic solvents. Both molecular dynamics simulations and electronic structure calculations revealed the formation of intermolecular π-complexation between o-cresol and aromatic solvents. The thermal movements of solvent and solute molecules render the conformations of o-cresol changing between trans and cis isomers. The π-interaction makes the cis configuration a dominant isomer, hence leading to the single keen-edged UV/Vis absorption peak at approximately 283 nm. The free conformation changes between trans and cis in aqueous solution rationalize the broader absorption peaks in the range of 260-280 nm. The pH dependence of the UV/Vis absorption spectra in aqueous solutions is also rationalized by different protonation states of o-cresol. The explicit solvent model with long-ranged interactions is vital to describe the effects of π-complexation and electrostatic interaction on the UV/Vis absorption spectra of o-cresol in toluene and alkaline aqueous (pH > 10.3) solutions, respectively.

Insights into Thiol-Aromatic Interactions: A Stereoelectronic Basis for S-H/π Interactions

Thiols can engage favorably with aromatic rings in S-H/π interactions, within abiological systems and within proteins. However, the underlying bases for S-H/π interactions are not well understood. The crystal structure of Boc-l-4-thiolphenylalanine tert-butyl ester revealed crystal organization centered on the interaction of the thiol S-H with the aromatic ring of an adjacent molecule, with a through-space Hthiol···Caromatic distance of 2.71 Å, below the 2.90 Å sum of the van der Waals radii of H and C. The nature of this interaction was further examined by DFT calculations, IR spectroscopy, solid-state NMR spectroscopy, and analysis of the Cambridge Structural Database. The S-H/π interaction was found to be driven significantly by favorable molecular orbital interactions, between an aromatic π donor orbital and the S-H σ* acceptor orbital (a π → σ* interaction). For comparison, a structural analysis of O-H/π interactions and of cation/π interactions of alkali metal cations with aromatic rings was conducted. Na+ and K+ exhibit a significant preference for the centroid of the aromatic ring and distances near the sum of the van der Waals and ionic radii, as expected for predominantly electrostatic interactions. Li+ deviates substantially from Na+ and K+. The S-H/π interaction differs from classical cation/π interactions by the preferential alignment of the S-H σ* toward the ring carbons and an aromatic π orbital rather than toward the aromatic centroid. These results describe a potentially broadly applicable approach to understanding the interactions of weakly polar bonds with π systems.

Hydrogen bonds between methanol and the light liquid olefins 1-pentene and 1-hexene: from application to fundamental science

Hydrogen bonding is the critical component of a new, sustainable ‘Extractive Refining’ technology for reducing the olefin content of commercial FCC gasoline.

Anisotropic structure and dynamics of the solvation shell of a benzene solute in liquid water from ab initio molecular dynamics simulations

The anisotropic structure and dynamics of the hydration shell of a benzene solute in liquid water have been investigated by means of ab initio molecular dynamics simulations using the BLYP (Becke–Lee–Yang–Parr) and dispersion corrected BLYP-D functionals.

Experimental measurement and theory of substituent effects in π-hydrogen bonding: complexes of substituted phenols with benzene

IR spectroscopic experiments and theoretical DFT computations reveal the effects of aromatic substituents on π-hydrogen bonding between monosubstituted phenol derivatives and benzene. Simultaneous formation of two π-hydrogen bonds (red-shifting O-H···π and blue-shifting ortho-C-H···π) contribute to the stability of these complexes. The interaction of the acidic phenol O-H proton-donating group with the benzene π-system dominates the complex formation. The experimental shifts of O-H stretching frequencies for the different phenol complexes vary in the range 45-74 cm(-1). Strong effects on hydrogen-bonding energies and frequency shifts of electron-withdrawing aromatic substituents and very weak influence of electron-donating groups have been established. Experimental quantities and theoretical parameters are employed in rationalizing the properties of these complexes. The acidities of the proton-donating phenols describe quantitatively the hydrogen-bonding process. The results obtained provide clear evidence that, when the structural variations are in the proton-donating species, the substituent effects on π-hydrogen bonding follow classic mechanisms, comprising both resonance and direct through-space influences. The performance of three alternative DFT functionals (B3LYP, B97-D, and PBE0 combined with the 6-311++G(2df,2p) basis set) in predicting the O-H frequency shifts upon complexation is examined. For comparison, O-H frequency shifts for several complexes were also determined at MP2/6-31++G(d,p).

Experimental quantification of electrostatics in X-H···π hydrogen bonds

Hydrogen bonds are ubiquitous in chemistry and biology. The physical forces that govern hydrogen-bonding interactions have been heavily debated, with much of the discussion focused on the relative contributions of electrostatic vs quantum mechanical effects. In principle, the vibrational Stark effect, the response of a vibrational mode to electric field, can provide an experimental method for parsing such interactions into their electrostatic and nonelectrostatic components. In a previous study we showed that, in the case of relatively weak O-H···π hydrogen bonds, the O-H bond displays a linear response to an electric field, and we exploited this response to demonstrate that the interactions are dominated by electrostatics (Saggu, M.; Levinson, N. M.; Boxer, S. G. J. Am. Chem. Soc.2011, 133, 17414-17419). Here we extend this work to other X-H···π interactions. We find that the response of the X-H vibrational probe to electric field appears to become increasingly nonlinear in the order O-H < N-H < S-H. The observed effects are consistent with differences in atomic polarizabilities of the X-H groups. Nonetheless, we find that the X-H stretching vibrations of the model compounds indole and thiophenol report quantitatively on the electric fields they experience when complexed with aromatic hydrogen-bond acceptors. These measurements can be used to estimate the electrostatic binding energies of the interactions, which are found to agree closely with the results of energy calculations. Taken together, these results highlight that with careful calibration vibrational probes can provide direct measurements of the electrostatic components of hydrogen bonds.

Hydrogen bond migration between molecular sites observed with ultrafast 2D IR chemical exchange spectroscopy

Hydrogen-bonded complexes between phenol and phenylacetylene are studied using ultrafast two-dimensional infrared (2D IR) chemical exchange spectroscopy. Phenylacetylene has two possible pi hydrogen bonding acceptor sites (phenyl or acetylene) that compete for hydrogen bond donors in solution at room temperature. The OD stretch frequency of deuterated phenol is sensitive to which acceptor site it is bound. The appearance of off-diagonal peaks between the two vibrational frequencies in the 2D IR spectrum reports on the exchange process between the two competitive hydrogen-bonding sites of phenol-phenylacetylene complexes in the neat phenylacetylene solvent. The chemical exchange process occurs in approximately 5 ps and is assigned to direct hydrogen bond migration along the phenylacetylene molecule. Other nonmigration mechanisms are ruled out by performing 2D IR experiments on phenol dissolved in the phenylacetylene/carbon tetrachloride mixed solvent. The observation of direct hydrogen bond migration can have implications for macromolecular systems.