1H and 13C nmr studies of salicylalanilines

Several substituted salicylanilines (I) are studied by ¹H (chemical shifts) and ¹³C (chemical shifts and T₁ relaxation times) NMR in order to obtain information on molecular geometry changes and the transmission of the electronic effects due to substituents, as well as on the relative rates of the overall molecular tumbling and of the flipping of the phenyl rings. In particular, a good linear correlation of the OH proton shifts (affected by intramolecular hydrogen bonding) with Hammett's σ constants for p-substitution in the aniline moiety is found. Changes in rigidity of the rings expected as a result of the OH...N interaction and of p-substitution are reflected on the phenyl carbon relaxation times.     

Study of hydrogen bonds of hypophosphorous acid by 1H, 2H, 31P, and 15N NMR spectroscopy under slow exchange conditions

The NMR spectra of solutions containing partially deuterated anhydrous hypophosphorous acid (H₂POOH) and its complexes with organic bases as proton acceptors were obtained in CD₂Cl₂ in the temperature range 183–253 K. Under these conditions, the state of slow exchange is achieved, as evidenced by the fine spin-spin and isotope (H/D) structure of the NMR signals. The formation and strengthening of the hydrogen bond by the OH group result in strong shielding of the ³¹P nucleus and decrease the spin-spin coupling constants of nuclei in the PH₂ group. Saturation of these effects occurs in going from proton to base. Direct and long-range effects of H/D substitution in the OH and PH groups on the H, ³¹P, and ¹⁵N chemical shifts in complexes were measured. The signs of these effects were explained in terms of a simplified model of dynamic interaction of covalent and hydrogen bonds. The kinetics of the interconversion of a cyclic H₂POOH dimer and a zwitterionic complex with pyridine were studied by dynamic ¹H NMR, and thermodynamic and kinetic parameters of the process were measured. A hypothetical mechanism of the reaction with the transition state close to an open-chain dimer with one hydrogen bond was proposed.     

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

A theoretical study on two series of electron‐rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen‐bonded adducts of [(PP₃)MH₂] and [Cp*MH(dppe)] (M=Fe, Ru, Os; PP₃=κ⁴‐P(CH₂CH₂PPh₂)₃, dppe= κ²‐Ph₂PCH₂CH₂PPh₂) with CF₃CH₂OH (TFE) as proton donor. The results are compared with those of adduct [Cp₂NbH₃] ? TFE featuring a “pure” dihydrogen bond, and classical hydrogen bonds in pyridine ? TFE and Me₃N ? TFE. Deviation of the H ??? H? A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH ??? HA interaction to be described as a bifurcate hydrogen bond. The impact of the M ??? HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton‐transfer reactions via MH ??? HA intermediates. Strengthening of the M ??? H interaction in the case of electron‐rich 5d metal hydrides leads to direct proton transfer to the metal atom.     

The conformations of allylic alcohols in solution from FT-i.r. OH stretching vibration measurements

The distribution of conformations of allylic alcohols in CCl₄ differs from that in the vapour phase and from ab initio calculations. FT-i.r. measurements of the OH stretching vibrations show two peak maxima which can be resolved by band splitting techniques. The predominant conformations of allylic alcohols are intramolecularly hydrogen bonded, with a conformation gauche (G or G′) with respect to rotation about the CO bond and eclipsed (E or E′) with respect to rotation about the CC bond. In contrast with vapour phase data and ab initio calculations, no other hydrogen bonded conformations have been identified.For primary alcohols, the shoulder to the higher wavenumber side can be unequivocally assigned to conformations in which the OH is free, with a trans (T) conformation with respect to rotation about the CO bond.The secondary allylic alcohols exhibit no bands that can be attributed to free OH conformations except in the case of severe steric interaction in the eclipsed (E) conformation.In accordance with our previous work on solvent interactions with the OH group, the contrast with the vapour state assignments are interpreted in terms of an interaction of the solvent with the OH proton which destabilizes the hydrogen bond.     

A time-dependent density functional theory study on the excited state behavior of a novel T2(OH)B molecule

A novel pH-sensitive fluorescent probe T₂(OH)B was selected to theoretically investigate its excited state hydrogen bonding effects and excited state intramolecular proton transfer (ESIPT) process. First, it was verified that one intramolecular hydrogen bond is formed spontaneously in T₂(OH)B itself. Given the geometrical changes, we further confirm that the hydrogen bond should be strengthened in the first excited state. When it comes to the photoexcitation process, we present the charge redistribution around hydrogen bonding moieties facilitate the ESIPT tendency. The increased electronic densities around acceptor promote the attraction of hydrogen protons. The potential energy barrier in the constructed potential energy curves reveals that the ESIPT process of the T₂(OH)B system should be ultrafast. And comparing several nonpolar solvents, we deem solvent polarity plays little role in the ESIPT reaction. Furthermore, we also search the S₁-state transition state structure along with the ESIPT path, based on which we simulate the intrinsic reaction coordinate path. We not only confirm the ESIPT mechanism presented in this work but also clarify the ultrafast excited state process and explain previous experiment. We sincerely hope that our theoretical work could guide novel applications based on the T₂(OH)B system in future.     

Molecular modeling simulation and experimental measurements to characterize chitosan and poly(vinyl pyrrolidone) blend interactions

Blends of chitosan and poly(vinyl pyrrolidone) (PVP) have a high potential for use in various biomedical applications and in advanced drug‐delivery systems. Recently, the physical and chemical properties of these blends have been extensively characterized. However, the molecular interaction between these two polymers is not fully understood. In this study, the intermolecular interaction between chitosan and PVP was experimentally investigated using ¹³C cross‐polarization magic angle‐spinning nuclear magnetic resonance (¹³C CP/MAS NMR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). According to these experimental results, the interaction between the polymers takes place through the carbonyl group of PVP and either the OH? C₆, OH? C₃, or NH? C₂ of chitosan. In an attempt to identify the interacting groups of these polymers, molecular modeling simulation was performed. Molecular simulation was able to clarify that the hydrogen atom of OH? C₆ of chitosan was the most favorable site to form hydrogen bonding with the oxygen atom of C?O of PVP, followed by that of OH? C₃, whereas that of NH? C₂ was the weakest proton donor group. The nitrogen atom of PVP was not involved in the intermolecular interaction between these polymers. Furthermore, the interactions between these polymers are higher when PVP concentrations are lower, and interactions decrease with increasing amounts of PVP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1258–1264, 2008     

A detailed chemical kinetic model for high temperature ethanol oxidation

A detailed chemical kinetic model for ethanol oxidation has been developed and validated against a variety of experimental data sets. Laminar flame speed data (obtained from a constant volume bomb and counterflow twin‐flame), ignition delay data behind a reflected shock wave, and ethanol oxidation product profiles from a jet‐stirred and turbulent flow reactor were used in this computational study. Good agreement was found in modeling of the data sets obtained from the five different experimental systems. The computational results show that high temperature ethanol oxidation exhibits strong sensitivity to the fall‐off kinetics of ethanol decomposition, branching ratio selection for C₂H₅OH + OH ↔ Products, and reactions involving the hydroperoxyl (HO₂) radical. The multichanneled ethanol decomposition process is analyzed by RRKM/Master Equation theory, and the results are compared with those obtained from earlier studies. The ten‐parameter Troe form is used to define the C₂H₅OH(+M) ↔ CH₃ + CH₂OH(+M) rate expression as k^∞ = 5.94E23 T^−1.68 exp(−45880 K/T) (s⁻¹) k^o = 2.88E85 T^−18.9 exp(−55317 K/T) (cm³/mol/sec) F_cent = 0.5 exp(−T/200 K) + 0.5 exp(−T/890 K) + exp(−4600 K/T) and the C₂H₅OH(+M) ↔ C₂H₄ + H₂O(+M) rate expression as k^∞ = 2.79E13 T^0.09 exp(−33284 K/T) (s⁻¹) k^o = 2.57E83 T^−18.85 exp(−43509 K/T) (cm³/mol/sec) F _cent = 0.3 exp(−T/350 K) + 0.7 exp(−T/800 K) + exp(−3800 K/T) with an applied energy transfer per collision value of <ΔE_down> = 500 cm⁻¹. An empirical branching ratio estimation procedure is presented which determines the temperature dependent branching ratios of the three distinct sites of hydrogen abstraction from ethanol. The calculated branching ratios for C₂H₅OH + OH, C₂H₅OH + O, C₂H₅OH + H, and C₂H₅OH + CH₃ are compared to experimental data. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 183–220, 1999     

A study of the phase transitions and proton dynamics of the superprotonic conductor Cs5H3(SO4)4·0.5H2O single crystal with H and Cs nuclear magnetic resonance

The phase transitions and proton dynamics of Cs₅H₃(SO₄)₄·0.5H₂O single crystals were studied by measuring the NMR line shape, the spin-lattice relaxation time, T₁, and the spin-spin relaxation time, T₂, of the ¹H and ¹³³Cs nuclei. The “acid” protons and the “water” protons in Cs₅H₃(SO₄)₄·0.5H₂O were distinguished. The loss of water protons was observed above T_C1, whereas the content of water protons was found to recover above T_C2. Therefore, the water protons play a special role in the stability of the superprotonic phase at high temperatures. The mechanism of fast proton conduction was found to consist of hydrogen-bond proton transfer involving the breakage of the weak part of the hydrogen bond and the formation of a new hydrogen bond. Thus, these structural phase transitions probably involve significant reorientation of the SO₄ tetrahedra and dynamical disorder of the hydrogen bonds between them.     

Hydrate structures,intermolecular interactions and proton conducting mechanism in polyelectrolyte membranes — infrared results

5 μm thick membranes of polyelectrolytes were studied by IR spectroscopy. p]Salts at low degree of hydration (about two water molecules per fixed ion): It is shown that in the case of salts of polystyrene sulfonic acid the cations are present at the —SO₃^? groupings. They are attached asymmetrically to these groupings which they crosslink in the cases of di- and trivalent cations. The water molecules are attached with their lone pairs to the cations and are bound via hydrogen bonds to the anions. The strength of these hydrogen bonds is increased, due to both the polarization of the OH groups of the water molecules caused by the cation fields, as well as the interaction of the lone pairs with d-holes of transition element ions. With cations with hydrophobic character, and with monovalent cations in the series Li⁺ → Cs⁺, increasing clustering of water molecules is observed. p]With increasing degree of hydration the cation—anion interaction is increasingly weakened. The same is true with regard to the hydrogen bonds formed by the OH groups of the hydration water molecules. The attachment of a second layer of hydration molecules and the proteolytic splitting of water molecules in the membranes is discussed. p]Acidic forms of the membranes: In thoroughly dried membranes, the acid groups are always crosslinked via hydrogen bonds. In the weak acids, these groupings also remain crosslinked at high degrees of hydration. With the strongly acidic polystyrene sulfonic acid, these hydrogen bonds are already broken at low degrees of hydration. H₅O₂⁺ groupings are formed. The hydrogen bond in this grouping shows great proton polarizability, as indicated by an intense continuum. The nature of the H₉O₄⁺ grouping is discussed on the basis of these results. Finally, it is shown that the great proton polarizability of the hydrogen bond in H₅O₂⁺ is the ultimate reason of the Grotthus conductivity.     

Vibrational spectra and structure of some oxalic acid/substituted pyridine (methyl-, amino-, and dihalogeno-) complexes: Hydrogen bond features

The Raman and infrared spectra of some polycrystalline substituted pyridine/oxalic acid complexes have been investigated and assignments in terms of group frequencies are given. Various hydrogen bonds (NH?O, OH?O, OH?N) are distinguished and crystal structures are proposed. For the stronger bases (methyl- or aminopyridines with pk_a ≈ 6) proton transfer occurs. The 1/1 complex contains infinite chains of hydrogen oxalate ions linked by strong OH?O hydrogen bonds with vOH between 2000 and 800 cm⁻¹. R_OH?O distances are 2.47–2.62 Å). The substituted pyridinium cations are linked to the chain backbone by medium NH?O hydrogen bonds with NH?O lengths of 2.71–2.81 Å. The 3,5-dichloropyridine forms a 2/1 adduct without proton transfer, in accordance with its pk_a (0.6), and strong OH?N hydrogen bonds occur (vOH about 2000 cm⁻¹ and ). Finally, the 2,6-dihalogenopyridine derivatives do not form complex with oxalic acid, presumably because of steric hindrance.     

Proton affinities of phenylalkylamines by the kinetic method

The kinetic method was used to determine the proton affinities of phenylalkylamines of general formula R¹R²C₆H₃CHR³(CH₂)_nNR⁴R⁵ where R¹ = H or OH; R² = H, F, NO₂, OH or OCH₃; R³ = H or OH; R⁴ and R⁵ = H and/or CH₃; n = 1−3. Amines were used as reference bases and the proton affinities of the phenylalkylamines were bracketed by a pair of reference bases that give rise, in the MIKE spectra of the heterodimer, to more or less intense signals than the compound under study. The influence of the aliphatic chain length and of the substituents on the aromatic ring, on the proton affinities of the phenylalkylamines is presented and discussed. The formation of an hydrogen bond between the amino group and the aromatic ring is proposed to explain the results obtained.     

A DFT study of proton transfers for the reaction of phenol and hydroxyl radical leading to dihydroxybenzene and H2O in the water cluster

Reactions of phenol and hydroxyl radical were studied under the aqueous environment to investigate the antioxidant characters of phenolic compounds. M06‐2X/6‐311 + G(d,p) calculations were carried out, where proton transfers via water molecules were examined carefully. Stepwise paths from phenol + OH^• + (H₂O)_n (n = 3, 7, and 12) to the phenoxyl radical (Ph O^•) via dihydroxycyclohexadienyl radicals (ipso, ortho, meta, and para OH‐adducts) were obtained. In those paths, the water dimer was computed to participate in the bond interchange along hydrogen bonds. The concerted path corresponding to the hydrogen atom transfer (HAT, apparently Ph OH + OH^• → Ph O^• + H₂O) was found. In the path, the hydroxyl radical located on the ipso carbon undergoes the charge transfer to prompt the proton (not hydrogen) transfer. While the present new mechanism is similar to the sequential proton loss electron transfer (SPLET) one, the former is of the concerted character. Tautomerization reactions of ortho or para (OH)C₆H₅=O + (H₂O)_n → C₆H₄(OH)₂(H₂O)_n were traced with n = 2, 3, 4, and 14. The n = 3 (and n = 14) model of ortho and para was calculated to be most likely by the strain‐less hydrogen‐bond circuit.     

Chiral Transformation in Protonated and Deprotonated Adipic Acids through Multistep Internal Proton Transfer

Protonated and deprotonated adipic acids (PAA: HOOC? (CH₂)₄? COOH₂⁺ and DAA: HOOC? (CH₂)₄? COO^?) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H₂O???H???OH₂)⁺ Zundel‐like symmetric hydrogen bonding, whereas that of DAA has H₃O⁺ Eigen‐like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel‐like ions for PAA and Eigen‐like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH???O short hydrogen‐bond stretching peaks are predicted in the range of 1000–1700 cm^?1 in the Car–Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen‐bond peaks. The O? H? O stretching peaks in the range of 1800–2700 cm^?1 become insignificant above around 150 K and are almost washed out at about 300 K.     

On the importance of intramolecular hydrogen bond cooperativity in d‐glucose – an NMR and QTAIM approach

The idea that hydrogen bond cooperativity is responsible for the structure and reactivity of carbohydrates is examined. Density functional theory and gauge‐including atomic orbital calculations on the known conformers of the α and β anomers of d ‐glucopyranose in the gas phase are used to compute proton NMR chemical shifts and interatomic distances, which are taken as criteria for probing intramolecular interactions. Atom–atom interaction energies are calculated by the interacting quantum atoms approach in the framework of the quantum theory of atoms in molecules. Association of OH1 in the counterclockwise conformers with a strong acceptor, pyridine, is accompanied by cooperative participation from OH2, but there is no significant change in the bonding of the two following 1,2‐diol motifs. The OH6 ^... O5 (G?g+/cc/t and G+g?/cc/t conformers) or OH6 ^... O4 (Tg+/cc/t conformer) distance is reduced, and the OH6 proton is slightly deshielded. In the latter case, this shortening and the associated increase in the OH6–O4 interaction energy may be interpreted as a small cooperative effect, but intermolecular interaction energies are practically the same for all three conformers. In most of the pyridine complexes, one ortho proton interacts with the endocyclic oxygen O5. Analogous results are obtained when the clockwise conformer, G?g+/cl/g?, detected for the α anomer, and a hypothetical conformer, Tt/cl/g?, are complexed with pyridine through OH6. Generally, the cooperative effect does not go beyond the first two OH groups of a chain. Copyright © 2017 John Wiley & Sons, Ltd.     

Kinetics and Products of the Reactions of Ethyl and n‐Propyl Nitrates with OH Radicals

The kinetics of the reactions of ethyl (1) and n‐propyl (2) nitrates with OH radicals has been studied using a low‐pressure flow tube reactor combined with a quadrupole mass spectrometer. The rate constants of the title reactions were determined under pseudo–first‐order conditions from kinetics of OH consumption in high excess of nitrates. The overall rate constants, k₁ = 1.14 × 10^?13 (T/298)^2.45 exp(193/T) and k₂ = 3.00 × 10^?13 (T/298)^2.50 exp(205/T) cm³ molecule^?1 s^?1 (with conservative 15% uncertainty), were determined at a total pressure of 1 Torr of helium over the temperature range (248–500) and (263–500) K, respectively. The yields of the carbonyl compounds, acetaldehyde and propanal, resulting from the abstraction by OH of an α‐hydrogen atom in ethyl and n‐propyl nitrates, followed by α‐substituted alkyl radical decomposition, were determined at T = 300 K to be 0.77 ± 0.12 and 0.22 ± 0.04, respectively.     

Quantum-chemical calculation of a spillover model on a graphite support

The simplest quantum-chemical models of hydrogen spillover over a graphite-like surface as a proton or radical have been considered. The condensed planar C₂₄H₁₂ molecule was used as a model surface. Theab initio calculations of the interaction of hydrogen with the model surface were carried out by the restricted Hartree-Fock (HF) method in the STO-3G and 6-31 G^* basis sets. The radical hydrogen can not bind to such a surface, whereas the proton binds to it with an energy release of 186 kcal mol⁻¹. The activation energy of the transfer of the proton between two neighboring carbon atoms (10 kcal mol⁻¹) has been determined. The simplest model of the hydrogen migration as a proton over the model surface can be used for describing the spillover of hydrogen over the graphite surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 428–430, March, 1997.     

Pulsed NMR study of proton mobility in a hydrogen tungsten bronze

Crystalline hydrogen tungsten bronze H_0.46WO₃ was prepared by reduction of WO₃ single crystals. NMR relaxation times T₂, T₁, and T_1? were measured for 80 K < T < 450 K at 16 MHz and second moments for 160 K < T < 450 K at 100 MHz. The relaxation data were analyzed in terms of proton diffusion to give an activation energy of about 16 kJ mole^?1 and a correlation time preexponential factor of about 70 nsec for the process.     

Morphology and Temperature Phase Transitions in α,ω-Alkanediols with Different Chain Lengths

A comparative analysis of phase transitions in diols with various chain lengths [(CH₂)₄₄(OH)₂ and (CH₂)₂₂(OH)₂] and changes in their absorption spectra with temperature have been investigated by DSC and FTIR. Analysis of the DSC data has led to the conclusion that the low-temperature phase transition of (CH₂)₂₂(OH)₂ in a solid state (T_s-s = 367.1 K) is a phase transition of the first order, while the high-temperature phase transition (T_m = 376.3 K) is of the second order, i.e., a transition of the order-disorder type. Splitting of the IR absorption bands into doublets at 720-730 cm⁻¹ and 1463-1473 cm⁻¹ indicates that crystalline subcells in the lamellae of both diols are orthorhombic lattices with the parameters typical of hydrocarbons. IR spectra showed that at the phase transition temperature T_s-s transformation of an orthorhombic subcell into a hexagonal one occurs. This type of molecular chain packing remains the same up to the melting temperature T_m. In a (CH₂)₄₄(OH)₂ diol, the ortho-hexagonal subcell transition occurs only at the melting temperature (390.0 K). The wide IR band in the region from 3000 cm⁻¹ to 3600 cm⁻¹ shows that end hydroxyl groups of diol molecules form, on the surfaces of lamellar crystals, long (polymer) regular sequences consisting of intermolecular hydrogen bonds.     

Cooperativity in alkane-1,2- and 1,3-polyols: NMR,QTAIM, and IQA study of O─H…OH and C─H…OH bonding interactions

Proton nuclear magnetic resonance chemical shifts and atom–atom interaction energies for alkanepolyols with 1,2-diol and 1,3-diol repeat units, and for their 1:1 pyridine complexes, are computed by density functional theory calculations. In the 1,3-polyols, based on a tG'Gg' repeat unit, the only important intramolecular hydrogen bonding interactions are O─H^…OH. By quantum theory of atoms in molecules analysis of the electron density, unstable bond and ring critical points are found for such interactions in 1,2-polyols with tG'g repeat units, from butane-1,2,3,4-tetrol onwards and in their pyridine complexes from propane-1,2,3-triol onwards. Several features (OH proton shifts and charges, and interaction energies computed by the interacting quantum atoms approach) are used to monitor the dependence of cooperativity on chain length: This is much less regular in 1,2-polyols than in 1,3-polyols and by most criteria has a higher damping factor. Well defined C─H^…OH interactions are found in butane-1,2,3,4-tetrol and higher members of the 1,2-polyol series, as well as in their pyridine complexes: There is no evidence for cooperativity with O─H^…OH bonding. For the 1,2-polyols, there is a tenuous empirical relationship between the existence of a bond critical point for O─H^…OH hydrogen bonding and the interaction energies of competing exchange channels, but the primary/secondary ratio is always less than unity.     

A Reversible Proton Relay Process Mediated by Hydrogen‐Bonding Interactions in [FeFe]Hydrogenase Modeling

A reversible and temperature‐dependent proton‐relay process is demonstrated for a Fe₂ complex possessing a terminal thiolate in the presence of nitrogen‐based acids. The terminal sulfur site (S^t) of the complex forms a hydrogen‐bond interaction with N,N‐dimethylanilinium acid at 183 K. The Fe₂ core, instead, is protonated to generate a bridging hydride at 298 K. Reversibility is observed for the tautomerization between the hydrogen‐bonded pair and the Fe–hydride species. X‐ray structural analysis of the hydrogen‐bonded species at 193 K reveals a short N(H)???S^t contact. Employment of pyridinium acid also results in similar behavior, with reversible proton–hydride interconversion. DFT investigation of the proton‐transfer pathways indicates that the pK_a value of the hydrogen‐bonded species is enhanced by 3.2 pK_a units when the temperature is decreased from 298 K to 183 K.