An investigation of weak CH...O hydrogen bonds in maltose anomers by a combination of calculation and experimental solid-state NMR spectroscopy

Two-dimensional (1)H-(13)C MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The experimental (1)H chemical shifts of the CH and CH(2) protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudopotential approach. Further calculations show that the calculated change in the (1)H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C-H...O weak hydrogen bonding. Notably, a clear correlation between a large chemical shift change (up to 2 ppm) and both a short H...O distance (<2.7 A) and a CHO bond angle greater than 130 degrees is observed, thus showing that directionality is important in C-H...O hydrogen bonding.     

C(Ar)-H···O hydrogen bonds in substituted isobenzofuranone derivatives: geometric, topological, and NMR characterization

Substituted isobenzofuranone derivatives 1a-3a and bindone 4 are characterized by the presence of an intramolecular C(Ar)-H···O hydrogen bond in the crystal (X-ray), solution ((1)H NMR and specific and nonspecific IEF-PCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the C(Ar)-H···O interaction weakens in 1a-3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl(3) solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a-3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating (1)H NMR chemical shifts (δ(calcd) - δ(expl) < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a-3a and 1b-3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C-H moiety involved in the hydrogen bond, the increase of the spin-spin coupling constant (1)J((13)C-(1)H) by about 7.5 Hz is in good agreement with calculations for C-H bond shortening and for blue shifts of C-H stretching vibrations (by 55-75 cm(-1)).     

Microenvironmental and conformational structure of triblock copolymers in aqueous solution by 1H and 13C NMR spectroscopy

1H and 13C nuclear magnetic resonance (NMR) spectra of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers in D2O solutions have been systematically investigated. The detailed assignments of various 1H and 13C NMR signals are presented. The hyperfine structure of PO -CH2- protons was clearly assigned, the arising reason of this hyperfine structure was attributed to the influence of the chiral center of -CHCH3- groups and the direct coupling between the PO -CH2- and -CH3 protons. The external standard 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) was firstly applied in this system. Accurate chemical shift values referenced to the external standard DSS were obtained. 1H NMR chemical shift of PO -CH2- and -CH3 signals shows a larger decrease in ppm values than that of EO -CH2- signal with the increase of PPO/PEO ratio or temperature indicating that PO segments exist in a more hydrophobic microenvironment. A new resonance signal assigned to the PO -CH2- protons appeared when the temperature is above the CMT, which is attributed to the breakdown of the intra-molecular (C-H)...O hydrogen bond between the PO -CH2- protons and the ester oxygens. The breakdown of this intra-molecular hydrogen bond may result in a decrease of gauche conformers of the PPO chain. The increase of 13C NMR chemical shift of block copolymers validates this conformational change assumption. It can be inferred that the amount of gauche conformers decreases whereas that of trans conformers increases in both PO and EO chains when elevating the PPO/PEO ratio or temperature. The observed 13C NMR chemical shifts of PO segments show a bigger increase than those of EO segments, supporting the formation of a nonpolar microenvironment around PO segments.     

Quantifying weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl: a combined computational and experimental investigation of NMR chemical shifts in the solid state

Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH...O=C and C[triple bond]CH...N[triple bond]C distances, is investigated in a study that combines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at natural abundance and with 13C[triple bond]13CH and 15N[triple bond]C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH...O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH...O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 A, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1 ppm. For the C[triple bond]CH...N[triple bond]C weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. In uracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized, respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength.     

NMR investigation of methyl-2,4-dimethoxysalicylate: effect of solvent and temperature

(1)H NMR and (13)C NMR of methyl-2,4-dimethoxysalicylate 2 was measured in chloroform-d at the temperature range of 220-330 K, in dimethyl sulfoxide-d(6) at the temperature range of 300-400 K and in a polar protic solvent (CD(3)OD) at 300 K. The structure of 2 in liquid phase (solvent) is compared with those in solid phase (X-ray) and in the gas phase (quantum mechanical calculations). The relationship between molecular geometry, (1)H NMR chemical shift and W coupling of involved protons has a complex nature, but hydrogen bonds [C=O...H-O and C=O...H-CH(2)O] strength is the principle factor.     

Modeling of hydrogen bonds in monohydrated 2,4-dithiothymine: an ab initio and AIM study

Twelve tautomers of 2,4-dithiothymine are calculated at the MP2/6-31+G(d) level, and the most stable one is referred to the di-keto form (P12). Then four H-bonded complexes between P12 and water are optimized at the MP2/6-31+G(d) level of theory. The calculation of vibrational frequencies and natural bond orbital analysis are also carried out at the same level to investigate the hydrogen bonds involved in all the systems. Within all the four complexes, three types of hydrogen bonds are formed, in which the O-H...S and N-H...O bonds are the normal bonds with the X-H bond elongation and red shift of the corresponding stretch frequencies, while the C-H...O interaction is an improper, blue-shifting hydrogen bond accompanied with the contraction of the C-H bond and a blue shift of the C-H stretch frequency. The topological properties are investigated with the atoms-in-molecules (AIM) theory. The NMR chemical shielding for the isolated and the four monohydrated 2,4-dithiothymine are calculated using the "gauge-including atomic orbital" (GIAO) method. The 1H chemical shifts are influenced by the formation of hydrogen bonds.     

Comparison of the effects of the intra- and intermolecular hydrogen bonds C-H...X (X = O,N) in1H and13C NMR spectra

A similarity between manifestations of the effects of the intra- and intermolecular hydrogen bonds C-H...X (X = O, N) in¹H and¹³C NMR spectra has been shown. A correlated increase in the direct spin-spin coupling constant¹³C—¹H and the chemical shifts of the proton participating in the interaction has been observed.Translated fromIzvestiya Akademii Nauk. Seriyo Khimicheskaya, No. 5, pp. 1205–1207, May, 1996.     

1H NMR studies of maltose,maltoheptaose, alpha-, beta-, and gamma-cyclodextrins,and complexes in aqueous solutions with hydroxy protons as structural probes

The (1)H NMR chemical shifts, coupling constants, temperature coefficients, and exchange rates have been measured for the hydroxy protons of aqueous solutions of alpha-, beta-, and gamma-cyclodextrins, maltose, and maltoheptaose. In cyclodextrins (CDs), the high chemical shift of the O(3)H signal and its small (3)J(OH,CH) value suggest that O(3)H is involved in a hydrogen bond. The small temperature coefficients and rate of exchange values of O(2)H and O(3)H confirm the involvement of O(3)H in hydrogen bonding and indicate that O(2)H is the hydrogen bond partner. In maltose, two distinct NMR signals with two different vicinal coupling constants are found for O(2')H. A cross-peak in the ROESY spectrum indicates chemical exchange between the O(2')H and O(3)H protons. The existence of two distinct NMR signals with different J values for O(2')H shows the influence of anomeric configuration on the O(2')H-O(3)H interaction. The effect of complexation with methyl benzoate, adamantane-1-carboxylic acid, adamantane-1-ol, and l- and d-tryptophane on the NMR spectra of the hydroxy protons of alpha-, beta-, and gamma-cyclodextrins and of maltose has been investigated. No significant spectral changes were observed upon addition of methyl benzoate and adamantane-1-carboxylic acid. The addition of adamantane-1-ol resulted in an upfield shift and a strong broadening of the O(2)H signal from alpha-CD, and a small temperature coefficient was measured upon complexation. The O(2)H and O(3)H signals in beta-CD were broadened and shifted downfield upon addition of l- and d-tryptophane.     

Pseudopeptide foldamers: the homo-oligomers of pyroglutamic acid

As a part of a program evaluating substituted gamma-lactams as conformationally constrained building blocks of pseudopeptide foldamers, we synthesized the homo-oligomers of L-pyroglutamic acid up to the tetramer level by solution methods. The preferred conformation of this pseudopeptide series in structure-supporting solvents was assessed by FT-IR absorption, 1H NMR and CD techniques. In addition, the crystal structure of the N alpha-protected dimer was established by X-ray diffraction. A high-level DFT computational modeling was performed based on the crystallographic parameters. In this analysis, we demonstrated that an alpha C-H...O=C intramolecular hydrogen bond is responsible for the stabilization of the s-trans L-pGlu-L-pGlu conformation by 1.4 kcal mol-1. This effect can be easily detected by 1H NMR spectroscopy, owing to the anomalous chemical shifts of the alpha CH protons present in all of the oligomers. In summary, we have developed a new polyimide-based, foldameric structure that, if appropriately functionalized, has promise as a rigid scaffold for novel functions and applications.     

A solid state 13C NMR, crystallographic, and quantum chemical investigation of chemical shifts and hydrogen bonding in histidine dipeptides

We report the first solid-state NMR, crystallographic, and quantum chemical investigation of the origins of the 13C NMR chemical shifts of the imidazole group in histidine-containing dipeptides. The chemical shift ranges for Cgamma and Cdelta2 seen in eight crystalline dipeptides were very large (12.7-13.8 ppm); the shifts were highly correlated (R2= 0.90) and were dominated by ring tautomer effects and intermolecular interactions. A similar correlation was found in proteins, but only for buried residues. The imidazole 13C NMR chemical shifts were predicted with an overall rms error of 1.6-1.9 ppm over a 26 ppm range, by using quantum chemical methods. Incorporation of hydrogen bond partner molecules was found to be essential in order to reproduce the chemical shifts seen experimentally. Using AIM (atoms in molecules) theory we found that essentially all interactions were of a closed shell nature and the hydrogen bond critical point properties were highly correlated with the N...H...O (average R2= 0.93) and Nepsilon2...H...N (average R2= 0.98) hydrogen bond lengths. For Cepsilon1, the 13C chemical shifts were also highly correlated with each of these properties (at the Nepsilon2 site), indicating the dominance of intermolecular interactions for Cepsilon1. These results open up the way to analyzing 13C NMR chemical shifts, tautomer states (from Cdelta2, Cepsilon1 shifts), and hydrogen bond properties (from Cepsilon1 shifts) of histidine residue in proteins and should be applicable to imidazole-containing drug molecules bound to proteins, as well.     

Inefficiency of specific intramolecular C-H...X interactions (X=F,Cl, Br) in N-vinylazoles as indicated by1H and13C NMR spectroscopy

¹H and¹³C NMR spectroscopy indicates the low efficiency of specific intramolecular C-H...Hal (Hal=F, Cl, Br) interactions in comparison with analogous C-H...Y (Y=N, O, S) interactions despite their formal similarity. This is seen in the absence of anomalous downfield PMR shifts of the hydrogen atom and increase in the corresponding direct¹³C-¹H coupling.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1442–1446, June, 1991.     

All-atom molecular dynamic simulations and relative NMR spectra study of weak C-H...O contacts in amide-water systems

Amide-water mixtures are studied by all-atom molecular dynamics (MD) simulations and the relative temperature-dependent NMR experiment. The weak C-H...O contacts are found in the amide-water systems theoretically and experimentally. The statistical results of the average numbers of hydrogen bonds indicate that the methyl groups in amide molecules represent different capabilities in forming the weak C-H...O contacts. The statistics also imply that the C-H...O contacts are more obvious in the amide-rich region than those in the water-rich region. The temperature-dependent NMR spectra are also adopted to investigate the weak C-H...O contacts in the amide-water systems. The relative chemical shifts of the methyl groups are in good agreement with the MD simulations.     

Strongly blue-shifted C-H stretches: interaction of formaldehyde with hydrogen fluoride clusters

The equilibrium structures, binding energies, and vibrational spectra of the cyclic, hydrogen-bonded complexes formed between formaldehyde, H(2)CO, and hydrogen fluoride clusters, (HF)(1< or =n < or =4), are investigated by means of large-scale second-order M?ller-Plesset calculations with extended basis sets. All studied complexes exhibit marked blue shifts of the C-H stretching frequencies, exceeding 100 cm(-1) for n = 2-4. It is shown that these blue shifts are, however, only to a minor part caused by blue-shifting hydrogen bonding via C-H...F contacts. The major part arises due to the structural relaxation of the H(2)CO molecule under the formation of a strong C=O...H-F hydrogen bond which strengthens as n increases. The close correlation between the different structural parameters in the studied series of complexes is demonstrated, and the consequences for the frequency shifts in the complexes are pointed out, corroborating thus the suggestion of the primary role of the C=O...H-F hydrogen bonding for the C-H stretching frequency shifts. This particular behavior, that the appearance of an increasingly stronger blue shift of the C-H stretching frequencies is mainly induced by the formation of a progressively stronger C=O...H-F hydrogen bond in the series of H(2)CO...(HF)(1< or =n < or =4), complexes and only to a lesser degree by the formation of the so-called blue-shifting C-H...F hydrogen bond, is rationalized with the aid of selected sections of the intramolecular H(2)CO potential energy surface and by performing a variety of structural optimizations of the H(2)CO molecule embedded in external, differently oriented dipole electric fields, and also by invoking a simple analytical force-field model.     

Hydrogen bond stabilization in 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate probed by high pressure: the role of charge-enhanced C-H...O interactions in the room-temperature ionic liquid

The hydrogen bonding structures of room-temperature ionic liquids 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate have been studied by infrared spectroscopy. High-pressure infrared spectral profiles and theoretical calculations allow us to make a vibrational assignment of these compounds. The imidazolium C-H bands of 1,3-dimethylimidazolium methyl sulfate display anomalous non-monotonic pressure-induced frequency shifts. This discontinuity in frequency shift is related to enhanced C-H...O hydrogen bonding. This behavior is in contrast with the trend of blue shifts in frequency for the methyl C-H stretching mode at ca. 2960 cm(-1). Our results indicated that the imidazolium C-H groups are more favorable sites for hydrogen bonding than the methyl C-H groups in the pure 1,3-dimethylimidazolium methyl sulfate. Nevertheless, both methyl C-H and imidazolium C-H groups are favorable sites for C-H...O hydrogen bonding in a dilute 1,3-dimethylimidazolium methyl sulfate/D(2)O mixture. Hydrogen bond-like C-H...F interactions were observed between PF(6)(-) and H atoms on the alkyl side chains and imidazolium ring for 1-butyl-3-methylimidazolium hexafluorophosphate.     

醇与极性有机溶剂间氢键的多核NMR研究

醇是一类重要的有机溶剂,对其结构和性质的研究已有很长历史。由于OH的存在,醇分子间存在着较强的氢键缔合作用,使其结构变得复杂,因而较难对它得到一个很清楚的认识。用NMR方法研究氢键也有几十年历史。早在五十年代,Arnold,Becker等就用~1HNMR研究了EtOH在CCl_4中的行为,测量了化学位移随浓度的变化。Becker认为当醇浓度很稀时,体系中只存在单体-二聚体平衡,井结合IR数据求得了平衡常数及缔合物位移。有     

Influence of N-H...O and C-H...O hydrogen bonds on the 17O NMR tensors in crystalline uracil: computational study

We report a computational study for the 17O NMR tensors (electric field gradient and chemical shielding tensors) in crystalline uracil. We found that N-H...O and C-H...O hydrogen bonds around the uracil molecule in the crystal lattice have quite different influences on the 17O NMR tensors for the two C=O groups. The computed 17O NMR tensors on O4, which is involved in two strong N-H...O hydrogen bonds, show remarkable sensitivity toward the choice of cluster model, whereas the 17O NMR tensors on O2, which is involved in two weak C-H...O hydrogen bonds, show much smaller improvement when the cluster model includes the C-H...O hydrogen bonds. Our results demonstrate that it is important to have accurate hydrogen atom positions in the molecular models used for 17O NMR tensor calculations. In the absence of low-temperature neutron diffraction data, an effective way to generate reliable hydrogen atom positions in the molecular cluster model is to employ partial geometry optimization for hydrogen atom positions using a cluster model that includes all neighboring hydrogen-bonded molecules. Using an optimized seven-molecule model (a total of 84 atoms), we were able to reproduce the experimental 17O NMR tensors to a reasonably good degree of accuracy. However, we also found that the accuracy for the calculated 17O NMR tensors at O2 is not as good as that found for the corresponding tensors at O4. In particular, at the B3LYP/6-311++G(d,p) level of theory, the individual 17O chemical shielding tensor components differ by less than 10 and 30 ppm from the experimental values for O4 and O2, respectively. For the 17O quadrupole coupling constant, the calculated values differ by 0.30 and 0.87 MHz from the experimental values for O4 and O2, respectively.     

溴代1-己基-3-甲基咪唑离子液体与丙酮的相互作用

1H and 13C NMR chemical shifts were determined to investigate the interactions of acetone with a room temperature ionic liquid 1-hexyl-3- methylimidazolium bromide C6mimBr at various mole fractions. Changes in chemical shifts of hydrogen nuclei and of carbon nuclei with the acetone concentration indicated the formation of hydrogen bond between anion of the ionic liquid and methyl protons of acetone. The NMR results were in good agreement with the ab initio computational results.     

Solution and solid-state models of peptide CH...O hydrogen bonds

Fumaramide derivatives were analyzed in solution by (1)H NMR spectroscopy and in the solid state by X-ray crystallography in order to characterize the formation of CH...O interactions under each condition and to thereby serve as models for these interactions in peptide and protein structure. Solutions of fumaramides at 10 mM in CDCl(3) were titrated with DMSO-d(6), resulting in chemical shifts that moved downfield for the CH groups thought to participate in CH...O=S(CD(3))(2) hydrogen bonds concurrent with NH...O=S(CD(3))(2) hydrogen bonding. In this model, nonparticipating CH groups under the same conditions showed no significant change in chemical shifts between 0.0 and 1.0 M DMSO-d(6) and then moved upfield at higher DMSO-d(6) concentrations. At concentrations above 1.0 M DMSO-d(6), the directed CH...O=S(CD(3))(2) hydrogen bonds provide protection from random DMSO-d(6) contact and prevent the chemical shifts for participating CH groups from moving upfield beyond the original value observed in CDCl(3). X-ray crystal structures identified CH...O=C hydrogen bonds alongside intermolecular NH...O=C hydrogen bonding, a result that supports the solution (1)H NMR spectroscopy results. The solution and solid-state data therefore both provide evidence for the presence of CH...O hydrogen bonds formed concurrent with NH...O hydrogen bonding in these structures. The CH...O=C hydrogen bonds in the X-ray crystal structures are similar to those described for antiparallel beta-sheet structure observed in protein X-ray crystal structures.     

The influence of solvent molecules on NMR spectrum of barbituric acid in the DMSO solution

This work shows the modification of barbituric acid (BA) chemical shifts by dimethylsulphoxide (DMSO) molecules. The discussed changes are caused by creation of the H-bonded associates formed by barbituric acid with DMSO in solution. Free molecule of barbituric acid, the cluster of BA with two DMSO molecules and two different clusters of BA with four DMSO units are taken into consideration. The chemical shifts of these systems have been calculated and the obtained results have been compared with experimental data. Theoretical calculations predict a significant downfield shift for imino protons of barbituric acid involved in intermolecular-N-H...DMSO hydrogen bonds. The influence of the solvent molecules on other nuclei chemical shifts, especially protons of barbituric acid methylene group, is also reported. The calculations have involved Hartree-Fock and several Density Functional Theory methods. All methods correctly describe experimental ¹H and ¹³C NMR spectra of barbituric acid. The best consistence between experiment and theory is observed for the BLYP functional. Four approximations of magnetic properties calculations embedded in the Gaussian’98 package have been tested. The results of the performed calculations indicate that from a practical point of view the GIAO method should be preferred.     

Weak C-H...O and C-H...F-C hydrogen bonds in the oxirane-trifluoromethane dimer

The oxirane-trifluoromethane dimer generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. The rotational spectra of the parent species and of its two (13)C isotopomers in combination with ab initio calculations have been used to establish a C(s)() geometry for the dimer with the two monomers bound by one C-H.O and two C-H.F-C hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-H.O and C-H.F hydrogen bonds, r(O.H) and r(F.H), are 2.37 and 2.68 A, respectively. The C-H.F-C interactions give rise to the HCF(3) internal rotation motion barrier of 0.55(1) kJ/mol, which causes the A-E splittings observed in the rotational spectra. The analysis of the structural and energetic features of the C-H.O and C-H.F-C interactions allows us to classify them as weak hydrogen bonds. Ab initio calculations predict these weak interactions to produce blue shifts in the C-H vibrational frequencies and shortenings of the C-H lengths.