The red shift of the X-H stretching frequency, with a significant increase in intensity of the corresponding spectral band and a downfield chemical shift of hydrogen (deshielding) in nuclear magnetic resonance (NMR) spectroscopy, has traditionally been used as a criterion for identifying X-H···Y hydrogen bonds (HBs) where X is the hydrogen donor and Y is the acceptor. However, over the past two decades, it has become evident that certain HBs can exhibit a blue shift in the X-H stretching frequency and may also show a decrease in IR intensity, diverging from classical expectations. In this study, we investigate a wide array of HBs, encompassing both red-shifted and blue-shifted systems, as well as protonic and hydridic HB systems. We focus on understanding the underlying electronic conditions behind the reverse chemical shift effects─upfield shifts (shielding) upon HB formation, challenging the view that hydrogen bonding (H-bonding) typically leads to deshielding. We employ state-of-the-art quantum chemical methods, integrating computed NMR shielding tensors and electron deformation density, in combination with experimental NMR, to probe that phenomenon. The computational findings are thoroughly validated against experimental results. Our research confirms that shielding is also possible upon HB formation, thereby broadening the conceptual framework of H-bonding.

• Publication type
• Journal Article MeSH

Previously studied complexes with protonic and hydridic hydrogen bonds exhibit significant similarities. The present study provides a detailed investigation of the structure, stabilization, electronic properties, and spectral characteristics of protonic and hydridic hydrogen bonds using low-temperature infrared (IR) spectroscopy and computational methods. Complexes of pentafluorobenzene with ammonia (C₆F₅H⋯NH₃) and triethylgermane with trifluoroiodomethane (Et₃GeH⋯ICF₃) were analyzed using both experimental and computational tools. Additionally, 30 complexes with protonic hydrogen bonds and 30 complexes with hydridic hydrogen bonds were studied computationally. Our findings reveal that, despite the opposite atomic charges on the hydrogens in these hydrogen bonds, and consequently the opposite directions of electron transfer in protonic and hydridic hydrogen bonds, their spectral manifestations - specifically, the red shifts in the X-H stretching frequency and the increase in intensity - are remarkably similar. The study also discusses the limitations of the current IUPAC definition of hydrogen bonding in covering both types of H-bonds and suggests a way to overcome these limitations.

• Publication type
• Journal Article MeSH

Spectroscopic characteristics of Me3Si-H···Y complexes (Y = ICF3, BrCN, and HCN) containing a hydridic hydrogen were determined experimentally by low-temperature IR experiments based on the direct spectral measurement of supersonically expanded intermediates on a cold substrate or by the technique of argon-matrix isolation as well as computationally at harmonic and one-dimensional anharmonic levels. The computations were based on DFT-D, MP2, MP2-F12, and CCSD(T)-F12 levels using various extended AO basis sets. The formation of all complexes related to the redshift of the Si-H stretching frequency upon complex formation was accompanied by an increase in its intensity. Similar results were obtained for another 10 electron acceptors of different types, positive σ-, π-, and p-holes and cations. The formation of HBe-H···Y complexes, studied only computationally and again containing a hydridic hydrogen, was characterized by the blueshift of the Be-H stretching frequency upon complexation accompanied by an increase in its intensity. The spectral shifts and stabilization energies obtained for all presently studied hydridic H-bonded complexes were comparable to those in protonic H-bonded complexes, which has prompted us to propose a modification of the existing IUPAC definition of H-bonding that covers, besides the classical protonic form, the non-classical hydridic and dihydrogen forms.

• Publication type
• Journal Article MeSH

The pi-pi interactions between benzene and the aromatic nitrogen heterocycles pyridine, pyrimidine, 1,3,5-triazine, 1,2,3-triazine, 1,2,4,5-tetrazine, and 1,2,3,4,5-pentazine are systematically investigated. The T-shaped structures of all complexes studied exhibit a contraction of the C--H bond accompanied by a rather large blue shift (40-52 cm(-1)) of its stretching frequency, and they are almost isoenergetic with the corresponding displaced-parallel structures at reliable levels of theory. With increasing number of nitrogen atoms in the heterocycle, the geometries, frequencies, energies, percentage of s character at C, and the electron density in the C--H sigma antibonding orbital of the complexes all increase or decrease systematically. Decomposition analysis of the total binding energy showed that for all the complexes, the dispersion energy is the dominant attractive contribution, and a rather large attraction originating from electrostatic contribution is compensated by its exchange counterpart.

• MeSH
• Benzene chemistry   MeSH
• Models, Chemical * MeSH
• Nitrogen chemistry   MeSH
• Electrons MeSH
• Heterocyclic Compounds chemistry   MeSH
• Models, Molecular MeSH
• Energy Transfer MeSH
• Stereoisomerism MeSH
• Hydrogen Bonding MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Benzene MeSH
• Nitrogen MeSH
• Heterocyclic Compounds MeSH

Aromatic amino acid residues are often present in carbohydrate-binding sites of proteins. These binding sites are characterized by a placement of a carbohydrate moiety in a stacking orientation to an aromatic ring. This arrangement is an example of CH/pi interactions. Ab initio interaction energies for 20 carbohydrate-aromatic complexes taken from 6 selected ultra-high resolution X-ray structures of glycosidases and carbohydrate-binding proteins were calculated. All interaction energies of a pyranose moiety with a side chain of an aromatic residue were calculated as attractive with interaction energy ranging from -2.8 to -12.3 kcal/mol as calculated at the MP2/6-311+G(d) level. Strong attractive interactions were observed for a wide range of orientations of carbohydrate and aromatic ring as present in selected X-ray structures. The most attractive interaction was associated with apparent combination of CH/pi interactions and classical H-bonds. The failure of Hartree-Fock method (interaction energies from +1.0 to -6.9 kcal/mol) can be explained by a dispersion nature of a majority of the studied complexes. We also present a comparison of interaction energies calculated at the MP2 level with those calculated using molecular mechanics force fields (OPLS, GROMOS, CSFF/CHARMM, CHEAT/CHARMM, Glycam/AMBER, MM2 and MM3). For a majority of force fields there was a strong correlation with MP2 values. RMSD between MP2 and force field values were 1.0 for CSFF/CHARMM, 1.2 for Glycam/AMBER, 1.2 for GROMOS, 1.3 for MM3, 1.4 for MM2, 1.5 for OPLS and to 2.3 for CHEAT/CHARMM (in kcal/mol). These results show that molecular mechanics approximates interaction energies very well and support an application of molecular mechanics methods in the area of glycochemistry and glycobiology.

• MeSH
• Hydrocarbons, Aromatic chemistry   metabolism   MeSH
• Models, Chemical * MeSH
• Carbohydrate Metabolism * MeSH
• Computer Simulation * MeSH
• Carbohydrates chemistry   MeSH
• Thermodynamics * MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Hydrocarbons, Aromatic MeSH
• Carbohydrates MeSH