Hydrogen bonding of N-methylacetamide in CDCl3 solution studied by NMR and IR spectra

The amide proton NMR chemical shifts have been widely used for the determination of the population fraction of the non-hydrogen-bonded states in studies of hydrogen bonding in peptides and proteins. However, in such works the determination of the limiting chemical shifts for the fully hydrogen-bonded state and for the non-hydrogen-bonded state is quite problematic, because they cannot be measured directly. In the present study, we carried out variable-temperature ¹H NMR and IR measurements for N-methylacetamide in CDCl₃ solution. We derived the expression for the limiting chemical shift for the fully hydrogen-bonded state as a function of IR band intensity and chemical shift. According to the equation, we determined the values of the limiting shifts at various temperatures. The present method may be applicable to other hydrogen-bonding systems.     

Intermolecular hydrogen bonding of chloro-substituted acetic and propionic acids in carbon tetrachloride

A calorimetric enthalpy of dilution technique has been applied to the investigation of the self-association of several chloro-substituted carboxylic acids in dilute solutions in anhydrous carbon tetrachloride at 25°C. Calorimetric data are expressed in terms of relative apparent molar enthalpies, Φ_L. The values of Φ_L are shown to be consistent with the monomer-dimer model. Values of the equilibrium constant, standard enthalpy, and standard entropy of dimerization are given for seven acids. Results are discussed in terms of the effect of substitution on the electrostatic nature of the hydrogen bond.     

Cooperative hydrogen bonding in solution: Influence of molecule structure

Influence of cooperative interactions on IR stretching vibration frequencies of complexes CCl₃H⋯ROH⋯B (B-base), CCl₃H⋯ROH⋯O(C₂H₅)₂ and CCl₃H⋯ROH⋯ROH in proton donor solvent CCl₃H were studied.Solvent effects on OH stretching vibration frequencies of CF₃CH₂OH⋯B, ROH⋯O(C₂H₅)₂ and ROH⋯ROH complexes in aprotic solvents were calculated using earlier proposed method. Cooperativity factors in CCl₃H⋯CF₃CH₂OH⋯B complexes were determined. Obtained values were significant smaller than for CCl₃H⋯CH₃OH⋯B complexes. Also cooperativity factors for CCl₃H⋯ROH⋯O(C₂H₅)₂ and CCl₃H⋯ROH⋯ROH complexes were determined. It was demonstrated that obtained values slightly depend on length and embranchment of alkyl group in alcohol molecules. The main role in cooperative interactions plays electronic effect.     

Evaluation of the individual hydrogen bonding energies in N-methylacetamide chains

The individual hydrogen bonding energies in N-methylacetamide chains were evaluated at the MP2/6-31+G** level including BSSE correction and at the B3LYP/6-311++G(3df,2pd) level including BSSE and van der Waals correction. The calculation results indicate that compared with MP2 results, B3LYP calculations without van der Waals correction underestimate the individual hydrogen bonding energies about 5.4 kJ mol^?1 for both the terminal and central hydrogen bonds, whereas B3LYP calculations with van der Waals correction produce almost the same individual hydrogen bonding energies as MP2 does for those terminal hydrogen bonds, but still underestimate the individual hydrogen bonding energies about 2.5 kJ mol^?1 for the hydrogen bonds near the center. Our calculation results show that the individual hydrogen bonding energy becomes more negative (more attractive) as the chain becomes longer and that the hydrogen bonds close to the interior of the chain are stronger than those near the ends. The weakest individual hydrogen bonding energy is about ?29.0 kJ mol^?1 found in the dimer, whereas with the growth of the N-methylacetamide chain the individual hydrogen bonding energy was estimated to be as large as ?62.5 kJ mol^?1 found in the N-methylacetamide decamer, showing that there is a significant hydrogen bond cooperative effect in N-methylacetamide chains. The natural bond orbital analysis indicates that a stronger hydrogen bond corresponds to a larger positive charge for the H atom and a larger negative charge for the O atom in the N-H?O=C bond, corresponds to a stronger second-order stabilization energy between the oxygen lone pair and the N-H antibonding orbital, and corresponds to more charge transfer between the hydrogen bonded donor and acceptor molecules.     

Conformational analysis of 2-halocyclopentanones by NMR and IR spectroscopies and theoretical calculations

The conformational isomerism of 2-chlorocyclopentanone and 2-bromocyclopentanone has been determined through the solvent dependence of the ¹H NMR ³J_HH coupling constants, theoretical calculations and infrared data, using the solvation theory for the treatment of NMR data. In 2-chlorocyclopentanone, the energy difference (E_Ψ-e − E_Ψ-a), in the isolated molecule at B3LYP level of theory, between the pseudo-equatorial (Ψ-e) and pseudo-axial (Ψ-a) conformers is 0.42 kcal mol⁻¹, which decreases in CCl₄ and in acetonitrile solutions, in good agreement with infrared data (ν_CO), despite the uncertainties of the latter method. The conformational equilibrium for 2-bromocyclopentanone is also between the Ψ-e and Ψ-a conformations, with an energy difference (E_Ψ-e − E_Ψ-a), in the isolated molecule at B3LYP level of theory, is 0.85 kcal mol⁻¹ which decreases in CCl₄ and in acetonitrile solutions, also in good agreement with infrared data.     

IR and NMR spectra,intramolecular hydrogen bonding and conformations of para-tert-butyl-aminothiacalix[4]arene in solid state and chloroform solution

It is demonstrated that dissolution of aminothiacalix[4]arene in chloroform results in transformation of 1,3-alternate conformation, adopted in single-crystal and bulk polycrystalline solids, to the pinched-cone form. This conformer is stabilised by the intramolecular hydrogen bonds of two distal amino-groups acting as H-donors with another two amino moieties that appear as H-acceptors. The H-bonds cause quite small (ca. 10–20 cm^?1) red shift of the IR bands of the NH₂ stretching vibrations, which suggests rather weak NH?N hydrogen bonding. This latter is sufficient to stabilize the pinched-cone conformation in the chloroform solution, but the energy gap between the pinched-cone and other conformations is small, and solid-state intermolecular forces easily overcome it, leading to realisation of the 1,3-alternate conformer. The comparison of the DFT computed and experimental vibrational and NMR spectra demonstrates good quality of present quantum-chemical computations, allows complete interpretation of the spectra and reveals simple IR and NMR spectroscopic markers of the conformers of aminothiacalix[4]arenes.     

Study of hydrogen bonds in N-methylacetamide by DFT calculations of oxygen, nitrogen, and hydrogen solid-state NMR parameters

Solid-state nuclear magnetic resonance (NMR) parameters of ¹⁷O, ¹⁴N/¹⁵N, and ²H/¹H nuclei were evaluated in two available neutron crystalline structures of N-methylacetamide (NMA) at 250 and 276 K, NMA-I and NMA-II, respectively. Density functional theory calculations were performed by B3LYP method and 6-311++G** and IGLO-II type basis sets to calculate the electric field gradient (EFG) and chemical shielding (CS) tensors at the sites of mentioned nuclei. In order to investigate hydrogen bonds (HBs) effects on NMR tensors, calculations were performed on four-model systems of NMA: an optimized isolated gas-phase, crystalline monomers, crystalline dimers, and crystalline trimers. Comparing the calculated results reveal the influence of N–H···O=C and C–H···O=C HB types on the NMR tensors which are observable by the evaluated parameters including quadrupole coupling constant, C _Q, and isotropic CS, σ _iso. Furthermore, the results demonstrate more influence of HB on the EFG and CS tensors of NMA at 276 K rather than that of 250 K.     

Infrared studies on hydrogen bonding interaction between acrylic esters with 1-dodeconal and phenol in carbon tetrachloride

The influence of the hydroxyl group in 1-dodeconal and phenol on the carbonyl vibration of representative compounds belonging to methyl methacrylate, ethyl methacrylate and butyl methacrylate in carbon tetrachloride has been studied by Fourier transform infrared spectroscopic method. The integrated intensities and change in dipole moments for O–H and C=O bonds were calculated. The formation constants of the 1 : 1 complexes have been calculated using Nash's method. The values of the formation constant and Gibbs energy vary with the ester chain length, which suggests that the strengths of the intermolecular hydrogen bonds (O–H ··· O=C) are dependent on the alkyl group of the acrylic ester and the results show that the proton accepting ability of acrylic esters is in the order methyl methacrylate < ethyl methacrylate < butyl methacrylate. The strength of the intermolecular C=O: HO bonds is also shown to be dependent on the basicity of the C=O group of acrylic esters and the acidity of the proton donor.     

Thermodynamic data for hydrogen bonding of thiols with Lewis bases in carbon tetrachloride. Weak association by the PMR method

Data for 30 hydrogen bonding pairs taken from the alkanethiols, i-C₃H₇SH, nC₃H₉SH and t-C₄H₉SH, and 16 bases have been obtained by a PMR method. Representative data for i-C₃H₇SH at 304 ± 2°K are (base, 10²K in M^?1, –ΔH° in kcal/mole): (CH₂)₄S, 3·1, 0·8; (CH₃)₂S, 3·0, 0·9; (CH₃)₂S₂, 3·7, 0·5; (CH₃)₂CO, 4·7, 0·9; CH₃COOC₂H₅, 5·7, 1·1; (CH₂)₄O, 6·1, 1·0; HCON(CH₃)₂, 12, 0·9; (CH₃ O)₂ SO, 12, 0·9; (C₂ H₅O)₃PO, 6·5, 1·0; CH₃ (CH₃ O)₂PO, 18, 1·0; ((CH₃)₂N)₂ CO, 5·9, 1·1; CH₃ CN, 13, 0·6. In essence, the problems and errors involved in obtaining equilibrium data for weak complexes stem from the limited concentration rangethat is accessible. This leads to large uncertainties in the quantities K, ΔH° and ΔS°. Structural effects on hydrogen bonding at the sulfur site, both as Lewis acid or base, are discussed. Two erroneous PMR methods in the literature used for assessing the strength of hydrogen bonds are pointed out.     

Cooperative hydrogen bonding in trimers involving HCN and HBO

A computational study of the cooperative effect of hydrogen bonding in linear trimers comprised of HCN and HBO molecules was undertaken at the MP2/6-311++G(2d,2p) level of theory. It was found that the third molecule leads to enhancement of the binding relative to that of the dimer species comprising these monomers. HBO is a better proton acceptor and a weaker proton donor than HCN.     

Reaction of carbon tetrachloride with hydrogen peroxide

Reaction of carbon tetrachloride with aqueous hydrogen peroxide in the presence of anhydrous iron(III) chloride was studied. Optimal conditions for the preparation of phosgene were found on the basis of analysis of the kinetic data and mechanism of the process. The reaction rate and yield (the latter reaching 95% in the stationary mode) are determined mainly by the amount of the heterogeneous catalyst. According to the experimental data, the reaction follows a radical mechanism.Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 10, 2004, pp. 1455–1458.Original Russian Text Copyright © 2004 by Tatarova, Trofimova, Gorban, Khaliullin.     

An NMR, IR and theoretical investigation of (1)H chemical shifts and hydrogen bonding in phenols

The change in (1)H NMR chemical shifts upon hydrogen bonding was investigated using both experimental and theoretical methods. The (1)H NMR spectra of a number of phenols were recorded in CDCl(3) and DMSO solvents. For phenol, 2- and 4-cyanophenol and 2-nitrophenol the OH chemical shifts were measured as a function of concentration in CDCl(3). The plots were all linear with concentration, the gradients varying from 0.940 (phenol) to 7.85 (4-cyanophenol) ppm/M because of competing inter- and intramolecular hydrogen bonding. Ab initio calculations of a model acetone/phenol system showed that the OH shielding was linear with the H...O=C distance (R) for R < 2.1 A with a shielding coefficient of - 7.8 ppm/A and proportional to cos(2)phi where phi is the H...O=C--C dihedral angle. Other geometrical parameters had little effect. It was also found that the nuclear shielding profile is unrelated to the hydrogen bonding energy profile. The dependence of the OH chemical shift on the pi density on the oxygen atom was determined as ca 40 ppm/pi electron. This factor is similar to that for NH but four times the value for sp(2) hybridized carbon atoms. The introduction of these effects into the CHARGE programme allowed the calculation of the (1)H chemical shifts of the compounds studied. The CHARGE calculations were compared with those from the ACD database and from GIAO calculations. The CHARGE calculations were more accurate than other calculations both when all the shifts were considered and also when the OH shifts were excluded. The calculations from the ACD and GIAO approaches were reasonable when the OH shifts were excluded but not as good when all the shifts were considered. The poor treatment of the OH shifts in the GIAO calculations is very likely due to the lack of explicit solvent effects in these calculations.     

Detecting hydrogen bonding by NMR relaxation of the acceptor nuclei

The formation of hydrogen bonds (HB) between phenol or N-methyltrifluoroacetamide and several acceptors (pyridine, carbonyl compounds, nitriles, amides) in CCl4 or CHCl3 been investigated through the analysis of NMR relaxation times (T1) of the heteronuclei (14N and 17O) directly involved in the HB interaction. Thus, a comparison is made between such T1 values (corrected for changes in molecular dynamics and motional anisotropy) and electric field gradients calculated by ab initio methods for the acceptor molecules, both isolated and in 1:1 or 2:1 hydrogen-bonded complexes. When other effects are properly accounted for, there is a good agreement between theoretical and experimental electric field gradient (efg) changes. The noticeable difference found between CCl4 or CHCl3 as solvents is discussed in relation to the presence of phenol oligomers, and the non-negligible HB donor power of CHCl3.     

Unusual hydrogen bonding in water-filled carbon nanotubes

We present the first experimental vibrational spectroscopy study providing direct evidence of a water phase inside single-walled carbon nanotubes that exhibits an unusual form of hydrogen-bonding due to confinement. Water adopts a stacked-ring structure inside nanotubes, forming intra- and inter-ring hydrogen bonds. The intra-ring hydrogen bonds are bulk-like while the inter-ring hydrogen bonds are relatively weak, having a distorted geometry that gives rise to a distinct OH stretching mode. The experimentally observed infrared mode at 3507 cm(-1) is assigned to vibrations of the inter-ring OH-groups based on detailed atomic-level modeling. The direct observation of unusual hydrogen bonding in nanotubes has potential implications for water in other highly confined systems, such as biological channels and nanoporous media.     

IR and NMR spectra, intramolecular hydrogen bonding and conformations of mercaptothiacalix[4]arene molecules and their para-tert-butyl-derivative

It is demonstrated that the introduction of p-tert-butyl groups dramatically influences the conformational behaviour of the mercaptothiacalix[4]arene molecules. Quantum-chemical computations in combination with IR and NMR spectroscopy prove that, in contrast to closely related calixarenes, the 1,3-alternate becomes a dominant conformer of p-tert-butyl-mercaptothiacalix[4]arene not only in crystal, but also in solutions and in vacuum. It is shown that the title molecules form essentially non-cooperative intramolecular hydrogen bonds: their SH groups are intramolecularly H-bonded solely to the sulfide groups bridging thiophenolic units. The enthalpy of this bonding, evaluated from Iogansen’s rule, amounts to ca. 1.5 kcal mol⁻¹ per one SH···S bond, which about four times smaller than the enthalpies of cooperative intramolecular H-bonds formed by related calixarenes and thiacalixarenes. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An NMR study of hydrogen bonding in some azo dyestuffs

The ¹H, ¹³C, and ¹⁵N NMR data reported for compounds 1–4 show that in DMSO solutions all of them exist in the azo form only and do not participate in the azo–hydrazoimine equilibrium. The NMR data for compounds 1 and 2 show the presence of a weak hydrogen bond for the non-protonated forms, between N10 and the 2-NHCH₃ proton. All compounds have also been studied in TFA solutions in which they are protonated. The site of protonation of 1, 2 and 3 is determined to be at N10 in TFA solutions. These results are supported by some ab initio GIAO-CHF molecular orbital calculations.     

Compatibilization of copolymers in solution through interchain hydrogen bonding

Low concentrations of polar units interacting through hydrogen bonds were introduced in polystyrene and polyvinylacetate chains by free radical copolymerization. Phase diagrams of copolymer mixtures in tetrahydrofuran were investigated. The influences on cloud-point isotherms of polar comonomer nature and concentration, and of copolymer molecular weight were studied. Viscometry appears to be reliable for evaluating the interactions betwen the different copolymers synthesized.