Elucidating the isotope effect in the Morse approximation of the ground state electronic term of hydrogen molecules H, n = 1 ÷ 7. Showcasing the Herzberg anomaly and anharmonicity.

The Morse function is the simplest anharmonic approximation of the potential of a diatomic molecule, for which the vibrational Schrödinger equation could be solved almost precisely. Despite its crudeness, the Morse function has been widely used in solving various problems in molecular spectroscopy. In recent years, special attention was paid to the existence of two Morse approximations for the electronic terms U(r) of diatomic molecules, M1(r) and M2(r), which differ by the selection of primary fitting parameters and satisfactorily reproduce different parts of U(r).

Modeling of the Response of Hydrogen Bond Properties on an External Electric Field: Geometry, NMR Chemical Shift, Spin-Spin Scalar Coupling.

The response of the geometric and NMR properties of molecular systems to an external electric field has been studied theoretically in a wide field range. It has been shown that this adduct under field approach can be used to model the geometric and spectral changes experienced by molecular systems in polar media if the system in question has one and only one bond, the polarizability of which significantly exceeds the polarizability of other bonds. If this requirement is met, then it becomes possible to model even extreme cases, for example, proton dissociation in hydrogen halides.

Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects.

Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene- and in CDCl. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups.

Combined NMR and UV-Vis Spectroscopic Studies of Models for the Hydrogen Bond System in the Active Site of Photoactive Yellow Protein: H-Bond Cooperativity and Medium Effects.

Intramolecular hydrogen bonds in aprotic media were studied by combined (simultaneous) NMR and UV-vis spectroscopy. The species under investigation were anionic and featured single or coupled H-bonds between, for example, carboxylic groups and phenolic oxygen atoms (COO···H···OC), among phenolic oxygen atoms (CO···H···OC), and hydrogen bond chains between a carboxylic group and two phenolic oxygen atoms (COO···H···(OC)···H···OC). The last anion may be regarded as a small molecule model for the hydrogen bond system in the active site of wild-type photoactive yellow protein (PYP) while the others mimic the corresponding H-bonds in site-selective mutants.

Modeling of Solute-Solvent Interactions Using an External Electric Field-From Tautomeric Equilibrium in Nonpolar Solvents to the Dissociation of Alkali Metal Halides.

An implicit account of the solvent effect can be carried out using traditional static quantum chemistry calculations by applying an external electric field to the studied molecular system. This approach allows one to distinguish between the effects of the macroscopic reaction field of the solvent and specific solute-solvent interactions. In this study, we report on the dependence of the simulation results on the use of the polarizable continuum approximation and on the importance of the solvent effect in nonpolar solvents.

Estimations of FH···X hydrogen bond energies from IR intensities: Iogansen's rule revisited.

In this work the possibility of using the IR intensity of the stretching vibration ν of proton donor group for estimation of hydrogen bond strength was investigated. For a set of complexes with FH···X (X = F, N, O) hydrogen bonds in the wide range of energies (0.1-49.

Sensitivity of P NMR chemical shifts to hydrogen bond geometry and molecular conformation for complexes of phosphinic acids with pyridines.

The results of the quantum-chemical investigation of a series of hydrogen-bonded 1:1 acid-base complexes formed by model phosphinic acids, Me POOH, and PhHPOOH, are reported. A series of substituted pyridines (pK range from 0.5 to 10) was chosen as proton acceptors.

Stretching force constants as descriptors of energy and geometry of F···HF hydrogen bonds.

In this work applicability of proton donor group stretching vibration force constants k and intermolecular stretching force constants k for evaluations of hydrogen bond strength and geometry are discussed. For a set of 30 complexes with F···HF hydrogen bonds in a wide range 0.5-48 kcal/mol by means of quantum chemical calculations equilibrium geometries, complexation energies, vibrational frequencies and corresponding force constants were calculated (MP2/aug-cc-pVTZ).

Lone pairs mapping by Laplacian of He NMR chemical shift.

Results of approbation of a new quantum mechanical approach of lone pairs (LPs) visualization, its optimization and testing on a range of model molecules are presented. The main idea of proposed methodology is using He atom as a probe for investigating electronic shells of species with LPs. As model objects, we consider "classical" examples of hydrogen cyanide, methanimine, ammonia, phosphine, formaldehyde, water, and hydrogen sulfide.

Adduct under Field-A Qualitative Approach to Account for Solvent Effect on Hydrogen Bonding.

The location of a mobile proton in acid-base complexes in aprotic solvents can be predicted using a simplified Adduct under Field (AuF) approach, where solute-solvent effects on the geometry of hydrogen bond are simulated using a fictitious external electric field. The parameters of the field have been estimated using experimental data on acid-base complexes in CDF/CDClF. With some limitations, they can be applied to the chemically similar CHCl and CHCl.

Unusual behaviour of the spin-spin coupling constant J upon formation of CHX hydrogen bond.

One-bond coupling constants J are usually used as a measure of the corresponding XY interatomic distances. However, the physical nature of this correlation is not well understood and, in some cases, a counterintuitive behaviour of J upon hydrogen bonded complex formation has been reported. In this work, the behavior of J upon formation and strengthening of complexes with CHX hydrogen bonds and upon a proton transfer process is investigated by means of H NMR spectroscopy and quantum chemical calculations.

Anticooperativity of FH···Cl hydrogen bonds in [FH) Cl] clusters (n = 1…6).

The change of cooperativity of FH···Cl hydrogen bonds upon sequential addition of up to six FH molecules to the Cl first coordination sphere is investigated. The geometry of clusters [(FH) Cl] (n = 1…6) was calculated (CCSD/aug-cc-pVDZ) and compared with [(FH) F] clusters. The geometry is determined by the symmetry-driven electrostatic requirements and also by the fact that formation of each new FH···Cl bond creates a depression in the chlorine's electron cloud on the opposite side of Cl (σ-hole), which limits the range of directions available for subsequent H-bond formation.

Conformational Mobility and Proton Transfer in Hydrogen-Bonded Dimers and Trimers of Phosphinic and Phosphoric Acids.

The monomers, H-bonded cyclic dimers, and trimers of five acids were studied by density functional theory calculations, such as hypophosphorous acid (HPOOH, ), dimethylphosphinic acid (MePOOH, ), phenylphosphinic acid (PhHPOOH, ), dimethylphosphoric acid ((MeO)POOH, ), and diphenylphosphoric acid ((PhO)POOH, ). Particular attention was paid to the conformational manifold existing due to the internal degrees of freedom: proton transfer (PT), puckering ("twist") within the ring of H-bonds, and mobility of the substituents (namely, -Ph, -OMe, and -OPh rotations). For acid , the number of conformers is additionally increased because of the varying relative orientation of nonequivalent substituents in cyclic complexes.

Solvent effects on acid-base complexes. What is more important: A macroscopic reaction field or solute-solvent interactions?

Can the geometry of an acid-base complex in solution be reproduced in calculations using an implicit accounting for the solvent effect in the form of a macroscopic reaction field? The answer is, "Yes, it can." Is this field equal to the real electric field experienced by the complex in solution? The answer is, "No, it is not." How can the geometry be correct under wrong conditions? This question is answered using density functional theory modeling of geometric and NMR parameters of pyridine⋯HF⋯(HCF) adducts in the absence and presence of an external electric field.

Influence of Hydrogen Bonds in 1:1 Complexes of Phosphinic Acids with Substituted Pyridines on H and P NMR Chemical Shifts.

Two series of 1:1 complexes with strong OHN hydrogen bonds formed by dimethylphosphinic and phenylphosphinic acids with 10 substituted pyridines were studied experimentally by liquid state NMR spectroscopy at 100 K in solution in a low-freezing polar aprotic solvent mixture CDF/CDClF. The hydrogen bond geometries were estimated using previously established correlations linking H NMR chemical shifts of bridging protons with the O···H and H···N interatomic distances. A new correlation is proposed allowing one to estimate the interatomic distance within the OHN bridge from the displacement of P NMR signal upon complexation.

Outer electronic shell visualization by NMR chemical shift laplacian of a helium probe.

A new method of visualization of lone pairs is proposed using fluorine atoms in fluoroacetylene, trifluoroethylene, and fluoroform (sp, sp , and sp hybridized carbons) as an example. The space around fluorines was probed by a helium atom and at each point He NMR chemical shift δ and its Laplacian ∇ δ were calculated. The ∇ δ isosurfaces have symmetrical toroidal shapes along CF axes; the maximum values of ∇ δ are reached at about 1.

NMR Study of Solvation Effect on the Geometry of Proton-Bound Homodimers of Increasing Size.

Hydrogen bond geometries in the proton-bound homodimers of quinoline and acridine derivatives in an aprotic polar solution have been experimentally studied using H NMR at 120 K. The reported results show that an increase of the dielectric permittivity of the medium results in contraction of the N···N distance. The degree of contraction depends on the homodimer's size and its substituent-specific solvation features.

Proton transfer in a short hydrogen bond caused by solvation shell fluctuations: an ab initio MD and NMR/UV study of an (OHO)(-) bonded system.

We present a joint experimental and quantum chemical study on the influence of solvent dynamics on the protonation equilibrium in a strongly hydrogen bonded phenol-acetate complex in CD2Cl2. Particular attention is given to the correlation of the proton position distribution with the internal conformation of the complex itself and with fluctuations of the aprotic solvent. Specifically, we have focused on a complex formed by 4-nitrophenol and tetraalkylammonium-acetate in CD2Cl2.

NMR study of CHN hydrogen bond and proton transfer in 1,1-dinitroethane complex with 2,4,6-trimethylpyridine.

The intermolecular complex with a CHN hydrogen bond formed by 1,1-dinitroethane (DNE) and 2,4,6-trimethylpyridine (collidine) dissolved in CD2Cl2 was studied experimentally by (1)H NMR spectroscopy at 180-300 K. Equilibrium between the molecular CH···N form and the zwitterionic C(-)/HN(+) form was detected in the slow exchange regime in the NMR time scale. No sign of a direct C(-)···HN(+) bond was observed; the ion pair is likely to be held by Coulomb interactions.

NMR studies of active-site properties of human carbonic anhydrase II by using (15) N-labeled 4-methylimidazole as a local probe and histidine hydrogen-bond correlations.

By using a combination of liquid and solid-state NMR spectroscopy, (15) N-labeled 4-methylimidazole (4-MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF3 /CDF2 Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active-site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4-MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy.

NMR studies of protonation and hydrogen bond states of internal aldimines of pyridoxal 5'-phosphate acid-base in alanine racemase, aspartate aminotransferase, and poly-L-lysine.

Using (15)N solid-state NMR, we have studied protonation and H-bonded states of the cofactor pyridoxal 5'-phosphate (PLP) linked as an internal aldimine in alanine racemase (AlaR), aspartate aminotransferase (AspAT), and poly-L-lysine. Protonation of the pyridine nitrogen of PLP and the coupled proton transfer from the phenolic oxygen (enolimine form) to the aldimine nitrogen (ketoenamine form) is often considered to be a prerequisite to the initial step (transimination) of the enzyme-catalyzed reaction. Indeed, using (15)N NMR and H-bond correlations in AspAT, we observe a strong aspartate-pyridine nitrogen H-bond with H located on nitrogen.

Solvent and H/D isotope effects on the proton transfer pathways in heteroconjugated hydrogen-bonded phenol-carboxylic acid anions observed by combined UV-vis and NMR spectroscopy.

Heteroconjugated hydrogen-bonded anions A···H···X(-) of phenols (AH) and carboxylic/inorganic acids (HX) dissolved in CD2Cl2 and CDF3/CDF2Cl have been studied by combined low-temperature UV-vis and (1)H/(13)C NMR spectroscopy (UVNMR). The systems constitute small molecular models of hydrogen-bonded cofactors in proteins such as the photoactive yellow protein (PYP). Thus, the phenols studied include the PYP cofactor 4-hydroxycinnamic acid methyl thioester, and the more acidic 4-nitrophenol and 2-chloro-4-nitrophenol which mimic electronically excited cofactor states.

NMR studies of solid pentachlorophenol-4-methylpyridine complexes exhibiting strong OHN hydrogen bonds: geometric H/D isotope effects and hydrogen bond coupling cause isotopic polymorphism.

We have studied the hydrogen bond interactions of (15)N labeled 4-methylpyridine (4-MP) with pentachlorophenol (PCP) in the solid state and in polar solution using various NMR techniques. Previous spectroscopic, X-ray, and neutron crystallographic studies showed that the triclinic 1:1 complex (4-MPPCP) exhibits the strongest known intermolecular OHN hydrogen bond in the solid state. By contrast, deuteration of the hydrogen bond gives rise to the formation of a monoclinic structure exhibiting a weaker hydrogen bond.

Hydrogen bond geometries and proton tautomerism of homoconjugated anions of carboxylic acids studied via H/D isotope effects on 13C NMR chemical shifts.

Ten formally symmetric anionic OHO hydrogen bonded complexes, modeling Asp/Glu amino acid side chain interactions in nonaqueous environment (CDF(3)/CDF(2)Cl solution, 200-110 K) have been studied by (1)H, (2)H, and (13)C NMR spectroscopy, i.e. intermolecularly H-bonded homoconjugated anions of acetic, chloroacetic, dichloroacetic, trifluoroacetic, trimethylacetic, and isobutyric acids, and intramolecularly H-bonded hydrogen succinate, hydrogen rac-dimethylsuccinate, hydrogen maleate, and hydrogen phthalate.

NMR study of conformational exchange and double-well proton potential in intramolecular hydrogen bonds in monoanions of succinic acid and derivatives.

We present a (1)H, (2)H, and (13)C NMR study of the monoanions of succinic (1), meso- and rac-dimethylsuccinic (2, 3), and methylsuccinic (4) acids (with tetraalkylammonium as the counterion) dissolved in CDF(3)/CDF(2)Cl at 300-120 K. In all four monoanions, the carboxylic groups are linked by a short intramolecular OHO hydrogen bond revealed by the bridging-proton chemical shift of about 20 ppm. We show that the flexibility of the carbon skeleton allows for two gauche isomers in monoanions 1, 2, and 4, interconverting through experimental energy barriers of 10-15 kcal/mol (the process itself and the energy barrier are also reproduced in MP2/6-311++G** calculations).