Marek Ilczyszyn

Classification and nature of hydrogen bonds to betaine. X-ray, 13C CP MAS and IR description of low barrier hydrogen bonds

Abstract Twenty two crystalline complexes of betaine [(CH 3 ) 3 N + –CH 2 –COO − ] with different acids are considered. The resulting hydrogen bonds to the betaine oxygen atoms are characterised by X-ray, 13 C NMR and IR data. On this basis five categories of the betaine–acid complexes and their hydrogen bonds are described: (1) MO — the molecular complexes with the bonding protons do not transferre to the betaine; (2) IP — the hydrogen bonded ion pairs with the bonding proton at the betaine oxygen atom, (3) BHB — the homoconjugated betaine complexes with the bonding proton at the central region of the hydrogen bonds, (4) PPD — the MO, IP and BHB complexes with the bonding proton position disorder and (5) LBHB — the PPD complexes with low barrier hydrogen bonds. The results indicate conditions and possible inversion mechanisms of the bonding proton positions. For the betaine phosphoric acid complex the proton disorder, inside one of its hydrogen bonds, was confirmed.

Sarcosine-maleic acid (1:1) crystal: structure, 13C NMR and vibrational properties, protonation character

The crystal structure of sarcosine-maleic acid (1:1) complex has been determined by X-ray diffraction method at 293 K as monoclinic, space group C2/c, Z=8. The crystal unit consists of semi-maleate and sarcosinium ions with two strong hydrogen bonds: the intramolecular one in the maleic part of the complex and the intermolecular one between sarcosinium and semi-maleate carboxylic groups. Phase transitions in this crystal are excluded. Its structure is in accordance with FT-IR, FT-Raman and 13C NMR study. Properties of the titled crystal and a family of similar amino acid-maleic acid systems are compared and discussed. Special attention is paid for relations between different structural and spectroscopic parameters as well as for character of hydrogen bonds formed in these crystalline complexes.

Proton-exchange mechanisms in phenol–tertiary amine–aprotic solvent systems

1 H NMR experiments at different temperatures exhibit perturbations of the phenol–amine 1:1 complexes by the phenol or amine molecules, in solutions with a molar excess of either the acid or base. In the former, (phenol)2–amine 2:1 complexes with OHO and OHN hydrogen bonds are formed. The exchange between these bridging protons require the rupture and reformation of the OHN bonds. In the latter, the phenol–amine 1:1 complexes weakly interact with the base molecules. The exchange between the complexing and surrounding base molecules is achieved by the rupture and reformation of the OHN bonds when the phenols are less acidic. For more acidic systems, phenol–(amine)2 transition complexes with bifurcated hydrogen bonds are formed. This phenomenon is accompanied by the reversible proton transfer inside the hydrogen bridges, O—H⋯N ⇄ O–⋯H—N+, of the complexes dissolved in halogenated solvents at low temperatures.

Reversible proton transfer phenomenon in the 2,4-dichlorophenol-triethylamine hydrogen-bonded complex studied by low-temperature 1H NMR spectroscopy

Abstract Low-temperature 1 H NMR studies of the bridging OHN signal in the hydrogen-bonded complex formed between 2,4-dichlorophenol and triethylamine in C 2 H 5 Cl solution have demonstrated for the first time that separate signals for the molecular and ion-pair forms of this type of system can be observed. The temperature dependence of these signals leads to thermodynamic and kinetic quantities which suggest that the potential energy profile of the hydrogen bond is roughly symmetric with a double minimum.

Reversible proton transfer in the 2,3,5,6-tetrachlorophenol-N,N-dimethylaniline hydrogen-bonded complex studied by low-temperature 1H NMR spectroscopy

Abstract Low temperature 1 H NMR studies of the bridging OHN signal in the hydrogen-bonded complex formed between 2,3,5,6-tetrachlorophenol and N,N -dimethylaniline in C 2 H 5 Cl solution have shown that separate signals for the molecular and ion-pair forms of the complex can be observed below −135°C (138 K). Analyses of the observed lineshapes have yielded values for the thermodynamic quantities Δ H °, Δ S ° as well as for the activation quantities Δ H ‡ , Δ S ‡ .

Protonation of pyridines by phenols, thiophenol, trifluoroacetic acid and methanesulfonic acid in aprotic solvents

At moderately low temperatures, 293–213 K, phenols and pyridines form complexes linked by hydrogen bridges as a way for proton migration: A—H⋯B ⇄ A–⋯H—B+. The enthalpy and entropy variations induced by this intracomplex proton transfer have been evaluated for the 2,6-dichloro-4-nitrophenol-3,5-lutidine complex from the temperature dependences of the phenol 13C chemical shifts. At lower temperatures, down to 110 K, 1H NMR results provided direct evidence for the homoconjugation equilibrium: A–⋯H—B++ B ⇄(A–)(B2—H+). Separate 1H signals of the B—H+ and B2—H+ species were recorded for solutions of pyridines with thiophenol, trifluoroacetic acid or methanesulfonic acid with molar excess of the base. The hydrogen-bridge protons in the homoconjugated cations were strongly deshielded, at ca. 20.5 ppm, and this observation suggests their central or near-central position in the bonds: δ+B⋯H⋯Bδ+. The thermodynamic quantities of the homoconjugation equilibrium were calculated for selected systems.

Participation of Xenon Guest in Hydrogen Bond Network of β-Hydroquinone Crystal

In the β-hydroquinone (β-HQ)-Xe crystal, the Xe guest is placed between two hexagonal rings of coupled [···O-H···O-](6) H-bonds. This clathrate is treated as the model for monitoring the H-bonding system with the Xe participation. Three kinds of isotope effects due to the H/D substitution in the [···O-H···O-](6) bonds are considered: (i) structural changes in the clathrate (X-ray diffraction), (ii) variations of (129)Xe NMR signal of the guest (CP MAS), and (iii) variations of selected vibrations of the host (IR). This study predicts subtle inclination of every other hydroxyl group of the [···O-H···O-](6) rings into the Xe atom and formation of six Xe···H-O pairs in every cage, the frequency shift of the γOH mode due to these contacts, -ΔγOH(Xe···H) > 74 cm(-1), as well as the enthalpy formation, -ΔH(Xe···H) > 6-8 kJ mol(-1). Our IR results reveal a tendency of the Xe atom to form the H-bond-like network inside its cage and much weaker Xe···D-O interactions in the H/D substituted crystal. The (129)Xe NMR results do not reflect this kind of interactions due to averaging of the (129)Xe shielding phenomena, probably. We also predict elongation of the O···O distances due to the β-HQ-Xe crystal heating and the Xe escape.

β-Alanine–oxalic acid (1:1) hemihydrate crystal: structure, 13C NMR and vibrational properties, protonation character

The crystal structure of beta-alanine-oxalic acid (1:1) hemihydrate complex has been reinvestigated by X-ray diffraction method at 293 K. Formation of monoclinic crystal system belonging to C2/c space group and consisting of semi-oxalate chains, diprotonated beta-alanine dimers and water molecules bonded to both these units is confirmed. New results are obtained for distances in the carboxylic groups and hydrogen bonds. These structural observations are used for protonation degree monitoring on the carboxylic oxygen atoms. They are in accordance with our vibrational study. The 13C NMR spectra provide insights into the solid structure of this complex, character of its hydrogen bonds and the beta-alanine protonation.

Nature of hydrogen bonds formed by phenol derivatives and N,N-dimethylaniline in aprotic solvents : low-temperature NMR studies

Phenol interact with N,N-dimethylaniline to produce complexes with molecular (O—H⋯N) and ion-pair (O–⋯H—N+) hydrogen bonds. The degree of association and the character of the hydrogen bonds depends strongly on solution temperature and ΔpKa[= pKa(protonated base)– pKa(phenol)] in water. The tautomeric equilibrium, O—H⋯N⇄O–⋯H—N+, which is slow on the NMR timescale has been recorded in the 0.4 to –0.5 ΔpKa range below 140 K. Thermodynamic and kinetic quantities of the equilibrium were calculated from 1H NMR spectra of the 2,3,5,6,-tetrachlorophenol complex. The thermodynamic quantities for the 2,6-dichloro-4-nitrophenol complex were obtained from 13C NMR results.

Character of the hydrogen bond in the thiophenol-triethylamine complex

Abstract Low temperature 1 H NMR studies of the hydrogen-bonded complex formed between thiophenol and triethylamine in CHF 2 Cl solutions have shown that only one hydrogen bridge proton signal appears up to 113 K. It was assigned to the hydrogen bond with the ion-pair structure (S − ⋯HN + on the basis of additional 13 C NMR investigations and by comparison with the results obtained for phenol-triethylamine complexes under similar conditions.