1H, 2H, and 13C NMR spectra of enriched CH3 13COOH acid without and in the presence of tetra-n-butylammonium acetate have been measured around 110 K using a liquefied Freon mixture CDF3/CDF2Cl as a ...solvent, as a function of the deuterium fraction in the mobile proton sites. For comparison, spectra were also taken of the adduct CH3 13COOH·SbCl5 1 and of CH2Cl13COOH under similar conditions, as well as of CH3 13COOH and CH3 13COO- dissolved in H2O and D2O at low and high pH at 298 K. The low temperatures employed allowed us to detect several well-known and novel hydrogen-bonded complexes in the slow hydrogen bond exchange regime and to determine chemical shifts and coupling constants as well as H/D isotope effects on chemical shifts from the fine structure of the corresponding signals. The measurements show that self-association of both carboxylic acids in Freon solution gives rise exclusively to the formation of cyclic dimers 2 and 3 exhibiting a rapid degenerate double proton transfer. For the first time, a two-bond coupling of the type 2 J(CH3 COOH) between a hydrogen-bonded proton and the carboxylic carbon has been observed, which is slightly smaller than half of the value observed for 1. In addition, the 1H and 2H chemical shifts of the HH, HD, and the DD isotopologues of 2 and 3 have been determined as well as the corresponding HH/HD/DD isotope effects on the 13C chemical shifts. Similar “primary”, “vicinal”, and “secondary” isotope effects were observed for the novel 2:1 complex “dihydrogen triacetate” 5 between acetic acid and acetate. Another novel species is the 3:1 complex “trihydrogen tetraacetate” 6, which was also characterized by a complex degenerate combined hydrogen bond- and proton-transfer process. For comparison, the results obtained previously for hydrogen diacetate 4 and hydrogen maleate 7 are discussed. Using an improved 1H chemical shift−hydrogen bond geometry correlation, the chemical shift data are converted into hydrogen bond geometries. They indicate cooperative hydrogen bonds in the cyclic dimers; i.e., widening of a given hydrogen bond by H/D substitution also widens the other coupled hydrogen bond. By contrast, the hydrogen bonds in 5 are anticooperative. The measurements show that ionization shifts the 13C signal of the carboxyl group to low field when the group is immersed in water, but to high field when it is embedded in a polar aprotic environment. This finding allows us to understand the unusual ionization shift of aspartate groups in the HIV-pepstatin complex observed by Smith, R.; Brereton, I. M.; Chai, R. Y.; Kent, S. B. H. Nature Struct. Biol. 1996, 3, 946. It is demonstrated that the Freon solvents used in this study are better environments for model studies of amino acid interactions than aqueous or protic environments. Finally, a novel correlation of the hydrogen bond geometries with the H/D isotope effects on the 13C chemical shifts of carboxylic acid groups is proposed, which allows one to estimate the hydrogen bond geometries and protonation states of these groups. It is shown that absence of such an isotope effect is not only compatible with an isolated carboxylate group but also with the presence of a short and strong hydrogen bond.
The low-temperature 1H, 19F, and 15N NMR spectra of mixtures of collidine-15N (2,4,6-trimethylpyridine-15N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the temperature range ...94−170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19F1H19F and 19F1H15N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on temperature and the polarity of the solvent. The 2:3 complex ColHFHCol+FHF- is a contact ion pair. Collidinium hydrogen difluoride ColH+FHF- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the FHF- anion. In this complex, the anion FHF- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride ColH+F(HF)2- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the F(HF) n - clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant 1 J FH when the F···H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1 J FH = 0, the heavy atom coupling constant 2 J FN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries.
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.
In this paper, equations are proposed which relate various NMR parameters of OHN hydrogen‐bonded pyridine–acid complexes to their bond valences which are in turn correlated with their hydrogen‐bond ...geometries. As the valence bond model is strictly valid only for weak hydrogen bonds appropriate empirical correction factors are proposed which take into account anharmonic zero‐point energy vibrations. The correction factors are different for OHN and ODN hydrogen bonds and depend on whether a double or a single well potential is realized in the strong hydrogen‐bond regime. One correction factor was determined from the known experimental structure of a very strong OHN hydrogen bond between pentachlorophenol and 4‐methylpyridine, determined by the neutron diffraction method. The remaining correction factors which allow one also to describe H/D isotope effects on the NMR parameters and geometries of OHN hydrogen bond were determined by analysing the NMR parameters of the series of protonated and deuterated pyridine‐ and collidine–acid complexes. The method may be used in the future to establish hydrogen‐bond geometries in biologically relevant functional OHN hydrogen bonds.
H/D isotope effects on NMR parameters and geometries of acid–base complexes (see scheme) reveal novel properties of strong OHN hydrogen bonds. The method may be used in the future to establish geometries of functional hydrogen bonds in biomolecules.
Ab initio calculations of the scalar coupling constants 1 J 15 N - 1 H ≡ J NH and 2 J 15 N··· 15 N ≡ J NN of the N−H···N hydrogen bonds in the anion C⋮15N···L···15N⋮C- (1), L = H, D, and of the ...cyclic hydrogen-bonded formamidine dimer (HCNHNH2)2 (2) have been performed using the density functional formalism as a function of the hydrogen bond and molecular geometries. The coupling constants are discussed in comparison with the experimental and calculated constants 1 J 19 F - 1 H ≡ J FH and 2 J 19 F - 19 F ≡ J FF reported previously as first set of examples of scalar couplings across hydrogen bonds for the hydrogen-bonded clusters of F(HF) n -, n = 1−4 by Shenderovich, I. G.; Smirnov, S. N.; Denisov, G. S.; Gindin, V. A.; Golubev, N. S.; Dunger, A.; Reibke, R.; Kirpekar, S.; Malkina, O. L.; Limbach, H. H. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 422. Using the valence bond order model, which has been successfully applied previously to explain hydrogen bond correlations in crystallography and solid-state NMR of hydrogen-bonded systems, the coupling constants are related to the hydrogen bond geometries and NMR chemical shifts. In terms of this model, there is no principal difference between FHF- and NHN hydrogen-bonded systems. Whereas the coupling constant values calculated using the DFT method for the fluorine case only reproduce the experimental trends, the agreement between theory and experiment is much better in the nitrogen cases, which allows one to determine the hydrogen bond geometries including the hydrogen bond angle from a full set of experimental coupling constants. It is found that the coupling constants J AB in A−H···B are proportional to the product of valence bond orders (p AH p HB) m , where m is an empirical parameter equal to 2 in the case of fluorine bridge atoms and close to 1 in the case of nitrogen bridge atoms. The coupling constants J AH depend on two terms, a positive term proportional to p AH and a negative term proportional to p AH(p HB)2 leading to vanishing or even negative values of J AH at larger A···H distances; in this region the constants J AB are larger than the absolute values of J AH. As a consequence, vanishing couplings between a hydrogen-bonded proton to a heavy nucleus across the hydrogen bond cannot be taken as an indication for a noncovalent character of this hydrogen bond. The existence of J AB is taken as a strong evidence for the covalent character of the hydrogen bonds studied. This is corroborated by a analysis of the molecular orbitals of (1) and their individual contributions to the coupling constants.
In this paper we describe H/D isotope effects on the chemical shifts of intermolecular hydrogen-bonded complexes exhibiting low barriers for proton transfer, as a function of the position of the ...hydrogen bond proton. For this purpose, low-temperature (100−150 K) 1H, 2H, and 15N NMR experiments were performed on solutions of various protonated and deuterated acids AL (L = H, D) and pyridine-15 N (B) dissolved in a 2:1 mixture of CDClF2/CDF3. In this temperature range, the regime of slow proton and hydrogen bond exchange is reached, leading to resolved NMR lines for each hydrogen-bonded species as well as for different isotopic modifications. The experiments reveal the formation of 1:1, 2:1, and 3:1 complexes between AH(D) and B. The heteronuclear scalar 1H−15N coupling constants between the hydrogen bond proton and the 15N nucleus of pyridine show that the proton is gradually shifted from the acid to pyridine-15 N when the proton-donating power of the acid is increased. H/D isotope effects on the chemical shifts of the hydrogen-bonded hydrons (proton and deuteron) as well as on the 15N nuclei involved in the hydrogen bonds were measured for 1:1 and 2:1 complexes. A qualitative explanation concerning the origin of these low-barrier hydrogen bond isotope effects is proposed, from which interesting information concerning the hydron and heavy atom locations in single and coupled low-barrier hydrogen bonds can be derived. Several implications concerning the role of low-barrier hydrogen bonds in enzyme reactions are discussed.
(1)H, (2)H, (19)F and (15)N NMR spectra of a strongly hydrogen-bonded anionic cluster, CNHF(-), as an ion pair with a tetrabutylammonium cation dissolved in CDF(3)-CDF(2)Cl mixture were recorded in ...the slow exchange regime at temperatures down to 110 K. The fine structure due to spin-spin coupling of all nuclei involved in the hydrogen bridge was resolved. H/D isotope effects on the chemical shifts were measured. The results were compared with those obtained earlier for a similar anion, FHF(-), and interpreted via ab initio calculations of magnetic shielding as functions of internal vibrational coordinates, namely an anti-symmetric proton stretching and a doubly-degenerate bending. The values of primary and secondary isotope effects on NMR chemical shifts were estimated using a power expansion of the shielding surface as a function of vibrational coordinates. A positive primary isotope effect was explained as a result of the decrease of the hydron stretching amplitude upon deuteration. We show that the proton shielding surface has a minimum close to the equilibrium geometry of the CNHF(-) anion, leading to the positive primary H/D isotope effect in a rather asymmetric hydrogen bond. We conclude that caution should be used when making geometric estimations on the basis of NMR data, since the shapes of the shielding functions of the internal vibrational coordinates can be rather exclusive for each complex.
Ten formally symmetric anionic OHO hydrogen bonded complexes, modeling Asp/Glu amino acid side chain interactions in nonaqueous environment (CDF3/CDF2Cl solution, 200–110 K) have been studied by 1H, ...2H, and 13C 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 13C 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 = 1/2(r OH – r HO), which allows us to estimate the desired OHO hydrogen bond geometries from the observed 13C NMR parameters, taking into account the degenerate proton tautomerism.
In this paper, empirical corrections for anharmonic ground-state vibrations of hydrogen and deuterium in the hydrogen bridges A–L⋯B, L=H, D are introduced into the geometric hydrogen bond correlation ...analysis based on the empirical Pauling valence bond orders. The method is verified using the examples of the hydrogen bonded anions in (CO)
5Cr–CN⋯H⋯NC–Cr(CO)
5
− As(Ph)
4
+ (
1h), in (CO)
5Cr–CN⋯H⋯NC–Cr(CO)
5
− N(
n-propyl)
4
+ (
2h), in the model system CN⋯H⋯NC
− Li
+ (
3h), and their deuterated isotopologs (
1d,
2d and
3d) studied previously by dipolar NMR and theoretical methods by H. Benedict et al. J. Am. Chem. Soc. 120 (1998) 2939. The new corrections are able to describe isotope effects on hydrogen bond geometries from the weak to the strong hydrogen bond regime, taking into account single and double-well situations.
