Nikolai S. Golubev

NUCLEAR SCALAR SPIN - SPIN COUPLING REVEALS NOVEL PROPERTIES OF LOW-BARRIER HYDROGEN BONDS IN A POLAR ENVIRONMENT

The structure of the hydrogen bridge 19 F· ·· 1 H· ·· 15 N in the acid - base complex A ··· H ··· B formed by HF and ( 15 N)2,4,6-trimethylpyridine in CDF3/ CDF2Cl has been studied between 112 K and 200 K by low-temperature, multinuclear NMR spectroscopy. For the first time scalar spin - spin coupling between all three nuclei of a hydrogen bridge is observed. This bridge exhibits a two-bond coupling constant 2 J19F15N of about 96 Hz, which is larger than the one-bond coupling constants 1 J1H15N and 1 J19F1H. The latter are strongly dependent on temperature. The function 1 J1H15Na f( 1 J19F1H) cannot be described in terms of a conventional equilibrium between the molecular and the zwitterionic form, but only with the intermediate forma- tion of very strongly hydrogen-bonded complexes of the type A dˇ ··· H· ·· B da that exhibit a vanishing or very small barrier for the proton motion. Here, the difference between the covalent bond and the hydrogen bond disappears even in the case of a polar solvents, as indicated by the large value of 2 J19F15N. Implications for the mechanism of pro- ton transfer and of acid - base catalyzed enzyme reactions in a locally aprotic but polar environment are discussed.

H/D isotope effects on the low-temperature NMR parameters and hydrogen bond geometries of (FH)2F− and (FH)3F− dissolved in CDF3/CDF2Cl

Using liquid state 1H, 2H and 19F NMR spectroscopy in the temperature range 110–130 K we have studied the hydrogen-bonded anions (FH)2F− and (FH)3F− and their partially and fully deuterated analogs dissolved in the low-freezing freon mixture CDF3/CDF2Cl, in the presence of (C4H9)4N+ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)2F−, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)3F− have been resolved and assigned. Thus, we were able to determine the zero-, one- and two-bond H/D isotope effects on the hydrogen and fluorine NMR chemical shifts as well as isotope effects on the scalar spin–spin hydrogen–fluorine and fluorine–fluorine coupling constants. Using the valence bond order model these NMR data are related to H/D isotope effects on the hydrogen bond geometries. A semi-quantitative interpretation of the observed long range isotope effects is proposed in terms of an anti-cooperative coupling between the hydrogen bonds within each anion. The experimental data can be rationalized in terms of an empirical NMR isotope sum rule, which is analogous to a similar rule for the vibrational frequencies.

Localization and moving of a proton inside hydrogen-bonded complexes in aprotic solvents

Abstract The results of the study of a number of molecular and ionic H-bonded complexes in freon solutions by 1 H NMR at 100–150 K are reported. It is shown that under these conditions the signals of OH(NH) protons belonging to various complexes, self-associates and free molecules are observed separately. The spin-spin coupling of the signals is frequently discernible. The fine structure makes it possible to distinguish between complexes with fast proton migration between two wells on the potential surface and those with the proton localized in one well (in particular, the central one). Several complexes with slow (in the NMR scale) proton migration have also been found. The results of the study of the non-catalytic proton exchange kinetics between various molecules containing OH and NH groups in dilute solutions in aprotic solvents are considered. The exchange between the RCOOH and ROH molecules goes on via the intermediate formation of a cyclic ionic pair with two equivalent H-bonds even in non-polar solvents such as cyclohexane. For exchange between two RCOOH or ROH molecules a synchronous transfer of two protons in a cyclic molecular complex is likely.

Study of mutual influence of hydrogen bonds in complicated complexes by low-temperature 1H NMR spectroscopy

Abstract 1 H NMR spectra of various acid-base complexes of different stoichiometry at 100–120K in freon mixtures have been obtained. The separate signals of non-equivalent OH-protons, involved in different H-bonds, have allowed us to consider the problem of the mutual influence of these bonds, using a correlation between the δ OH chemical shift and the AΔ H H-bond enthalpy. The mutual strengthening of H-bonds in complexes of the AH⋯AH⋯B type and their weakening in AH⋯B⋯HA complexes have been found, the value of the effect being about 10–30%

Effect of intermolecular hydrogen bonding and proton transfer on fluorescence of salicylic acid

Abstract Effects of intermolecular interactions, in particular the influence of intermolecular hydrogen bonds formed by salicylic acid (SA) as a proton donor with proton acceptors of different strength, on fluorescence spectra of SA in non-aqueous solutions have been investigated. Infrared spectra of studied systems have been analyzed in order to elucidate the ground state structure of the complexes formed. It has been found that at the room temperature in dilute solutions in non-polar or slightly polar aprotic solvents, where the SA molecule is not involved in intermolecular hydrogen bonding, the position of the main (blue) fluorescence component is determined by the excited state intramolecular proton transfer (ESIPT) in the lowest singlet excited state S 1 . With increasing proton acceptor ability of the environment, when formation of weak or middle strength intermolecular H-bonds is possible, the emission band shifts gradually to lower frequency, the quantum yield falls and poorly resolved doublet structure becomes more pronounced, especially in the solvents containing heavy bromine atoms. As a possible reason for these effects, coupling between the S 1 and closely lying triplet term is considered. With the strongest proton acceptors like aliphatic amines, intermolecular proton transfer with ionic pair formation in the ground state and double (intra- and intermolecular) proton transfer in the excited state take place, resulting in a blue shift of the emission band. Similar emission is typical for the SA anion in aqueous solutions. The p K a value of SA in S 1 state has been found to be 3.1. Such a small value can be explained taking into account the ESIPT reaction following the excitation. The SA complex with pyridine exhibits emission spectrum containing both molecular-like and anion-like bands with relative intensities strongly dependent on the temperature and solvent properties. The most probable origin of this dual emission is the molecular-ionic tautomerism caused by the existence of two potential minima and reversible intermolecular proton transfer in the excited state.

Geometries and tautomerism of OHN hydrogen bonds in aprotic solution probed by H/D isotope effects on (13)C NMR chemical shifts.

The (1)H and (13)C NMR spectra of 17 OHN hydrogen-bonded complexes formed by CH(3)(13)COOH(D) with 14 substituted pyridines, 2 amines, and N-methylimidazole have been measured in the temperature region between 110 and 150 K using CDF(3)/CDF(2)Cl mixture as solvent. The slow proton and hydrogen bond exchange regime was reached, and the H/D isotope effects on the (13)C chemical shifts of the carboxyl group were measured. In combination with the analysis of the corresponding (1)H chemical shifts, it was possible to distinguish between OHN hydrogen bonds exhibiting a single proton position and those exhibiting a fast proton tautomerism between molecular and zwitterionic forms. Using H-bond correlations, we relate the H/D isotope effects on the (13)C chemical shifts of the carboxyl group with the OHN hydrogen bond geometries.

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. In particular, primary H/D isotope effects on the hydrogen bond proton signals as well as secondary H/D isotope effects on the (13)C signals of the carboxylic groups are reported and analyzed. We demonstrate that in most of the studied systems there is a degenerate proton tautomerism between O-H···O(-) and O(-)···H-O structures which is fast in the NMR time scale. The stronger is the proton donating ability of the acid, the shorter and more symmetric are the H-bonds in each tautomer of the homoconjugate. For the maleate and phthalate anions exhibiting intramolecular hydrogen bonds, evidence for symmetric single well potentials is obtained. We propose a correlation between H/D isotope effects on carboxylic carbon chemical shifts and the proton transfer coordinate, q(1) = ½(r(OH) - r(HO)), which allows us to estimate the desired OHO hydrogen bond geometries from the observed (13)C NMR parameters, taking into account the degenerate proton tautomerism.

The role of short hydrogen bonds in mechanisms of enzymatic action

Abstract 1 H and 13 C NMR spectra of trypsin and ribonuclease, stabilized by chemical modification with a hydrophilic polymer, have been obtained over a wide pH range (1–11). The spectral features, referred to some nuclei of the catalytic sites (the “catalytic triad” for trypsin and the His-12—His-119 pair for ribonuclease), have been identified using different NMR techniques as well as chemical modification with selective reagents. It is found that monoprotonation of these systems leads to symmetrical (or quasi-symmetrical) H-bonds formed between the basic groups. This allows us to explain the discrepancies between experimental data obtained by different authors on the protonation sites in these catalytic systems. The simulation of the catalytic triad by a 15 N labeled low molecular weight model has led us to the conclusion that external agents do not cause any discrete proton transfers but do cause a smooth shift of the bridging protons from one basic atom to another, with the quasi-symmetrical H-bonds being formed in intermediate cases. On the basis of these experimental data, a new concept has been proposed for the mechanism of acid—base catalysis performed by the pairs of weak basic groups like His—Im and Asp(Glu)—COO − (p K a 3—7) which are not capable of proton abstraction from alcoholic or water OH groups (p K a > 13). This catalysis may consist on the one hand of changing the charge densities on reacting groups due to strong H-bonding and, on the other hand, of facilitating the free movement of a proton in the field of several basic atoms when going along the reaction coordinate. The energy of the very strong H-bonds thus formed diminishes the activation energy of the reaction.

Excited state intramolecular proton transfer and dual emission of the cyclic homo- and heterodimers of 2-hydroxy and 2,6-dihydroxy benzoic acids

Abstract The fluorescence and excitation spectra of homo- and heterodimers formed by salicylic and 2,6-dihydroxy benzoic acids (SA and DHBA respectively) with various carboxylic acids were obtained in the crystalline state, liquid and frozen glassy solutions within the temperature range 300-90 K. In all the cases, dual emission has been found with the low frequency band attributed to the excited state with intramolecular proton transfer (ESIPT) and the high frequency one to a state without ESIPT. The relative intensities of these bands depend strongly on the temperature, excitation wavelength and the pK a value of the second acid molecule involved in a cyclic dimer. The dual emission of molecules like methyl salicylate is conventionally explained in terms of the ground state tautomeric equilibrium with the OH ⋯ OH intramolecularly hydrogen bonded isomer incapable of ESIPT. Since in the case of 2,6-dihydroxy benzoic acid the only stable isomer with two hydrogen bonds can exist, this explanation seems to be at least insufficient. Other explanations of the dual emission origin are discussed. An attempt is made to explain the available experimental data using the hypothesis involving double well potential surface of the cyclic dimer in the excited state. In the framework of this hypothesis, the barrier between two wells emerges from the reorganization of the whole hydrogen bonded system of a dimer.

Solvent induced temperature dependencies of NMR parameters of hydrogen bonded anionic clusters

Abstract The solvent induced temperature dependence of NMR parameters (proton and fluorine chemical shifts, the two-bond scalar spin coupling constant across the hydrogen bridge, 2h J FF ) for dihydrogen trifluoride anion, (FH) 2 F − , in a polar aprotic solvent, CDF 3 /CDF 2 Cl, is reported and discussed. The results are interpreted in terms of a simple electrostatic model, accounting a decrease of electrostatic repulsion of two negatively charged fluorine atoms on placing into a dielectric medium. The conclusion is drawn that polar medium causes some contraction of hydrogen bonds in ionic clusters combined with a decrease of hydrogen bond asymmetry.