J.L. Wood

Complex hydrogen bonded cations: I. The pyridinium-pyridine ion

Abstract The occurrence of the (py 2 H) + ion in solutions containing pyridinium ion and pyridine has been established spectroscopically. The infrared and Raman spectra of the four species , , , and have been examined in the range 80–6000 cm −1 . The spectra are independent of concentration, solvent, or counterion if this is inert. The NH stretching band is a strong, broad doublet with maxima at 2080 and 2530 cm −1 . In the bridge deuterated species there is a single strong, broad, ND stretching band, maximum at 1920 cm −1 . The N-N stretching is infrared active, at ~138 cm −1 . The internal modes of the base are all singlets in both the infrared and Raman spectra. The proton bridge is between the σ orbitais on the N atoms.

The vibrational spectra and origin of torsional barriers in some aromatic systems

Abstract The far-infrared, and in some cases the Raman spectra of 4-methoxy-, 4-hydroxy-, 4-nitro- and 4-N,N-dimethylamino-benzaldehyde have been obtained, and the CHO torsion mode identified. To assist the assignment, the low frequency spectra of anisole, N,N-dimethylaniline and N,N-dimethyl-4-nitroso-aniline were also examined. The effect of the 4-substituent on the torsional barrier shows a very good correlation with its influence on the 19 F shielding parameter. Previously reported torsional barriers for 4-fluoro-, 4-chloro-, 4-bromo- and 4-methylbenzaldehyde also fit the correlation. The influence of substituents on phenol torsional barriers is similarly accounted for. The barriers can be represented as arising from the contributions of the electronic structures respectively.

The intermolecular force field of the hydrogen bond

Abstract A detailed examination has been made of the force field of the 24 hydrogenbonded complexes 4X-phenol/4Y-pyridine where X = F, Cl, Br, I, H or Me and Y = H, Me, Et or Ph. Referring to a harmonic force field, the following conclusions result: (1) An intermolecular hydrogen bond stretching force constant σ is obtained, which is independent of uncertainties in the phenol and pyridine internal force fields or the relative orientation of these moieties. (2) These values of ƒ σ are linearly related to the p K a of the phenol, or Δ v s , through the series with common base. (3) The OH stretching mode v s is well localised, and thus Δ v s is a good measure of the strength of complexing. (4) The introduction of the intermolecular force field alone, together with changes in the phenol force field, account for all the more notable shifts as a result of hydrogen bonding of the internal mode frequencies of both the phenols and the pyridines. Although they may occur, it is not necessary to invoke any other changes in the internal force fields. (5) The internal mode frequency shifts of the phenol indicate the OH ⋯ N bending force constant as ≅0.07 mdyn A −1 . (6) The introduction of a bent hydrogen bond or of non-bonded interactions destroys the correlation between ƒ σ and Δ v s , suggesting that these do not occur. (7) A harmonic interaction force constant ƒ I between OH and H ⋯ N stretching can be introduced, and a set of ƒ OH ,ƒ σ and ƒ I , compatible with the observed frequencies, fit values from the Lippincott-Schroeder potential to within 2 %. For the phenol/pyridine complex, the best fit values are ƒ σ = 0.631, ƒ OH = 8.49, ƒ I = 1.94 (mdyn A −1 ). (8) A measure of the intermolecular stretching force field, the parameter ƒ R = (ƒ OH ƒ σ − ƒ I ) 2 / (ƒ OH − 2ƒ I + ƒ σ ) is developed. This “relaxation” force constant corresponds to dissociation of the complex, allowing the proton to adopt its minimum energy throughout. Values of ƒ R , fitted to the data, are almost independent of ƒ I , and show similar dependence on structure etc. to ƒ σ , including correlation with Δ v s . (9) The associated high frequency parameter K H = (ƒ OH - 2ƒ I + ƒ σ ) correlates closely with Δ v s . (10) All harmonic force fields imply the D-bond to be slightly weaker than the H-bond. This is probably a result of anharmonic terms. (11) The Lippincott-Schroeder potential does not satisfactorily represent the anharmonicity, if only cubic terms are used. Force fields and assignments for the uncomplexed phenols and pyridines have also been obtained.

The proton potential in complex hydrogen bonded cations

Abstract The effects of the proton potential on the following features in the vibrational spectra of hydrogen bonded systems are reviewed, and developed theoretically when necessary: (a) The intermolecular stretching mode (b) The internal modes of the systems complexed (c) The hydrogen bending mode (d) The hydrogen bending mode overtone (e) The hydrogen stretching mode (f) Fermi interaction between (d) and (e) Linear symmetric, bent symmetric and asymmetric systems are discussed. The theory is applied to the spectra of the complex hydrogen bonded cations presented in the preceding papers. An asymmetric potential with low barrier gives the best agreement with the observations, although the appearance of the internal modes indicates a single minimum potential.

Complex hydrogen bonded cations-IV further symmetric ions

Abstract Solutions containing the complex hydrogen bonded cations (NMe imidazole) 2 H + ; (thiazole) 2 H + and (benzothiazole) 2 H + have been prepared. The spectra display a pronounced double maximum in the v s infrared band, which is replaced by a single band in the D-bridged analogues. The activity of the far infrared v s band, the frequency of the NH bending mode, and the observation of internal modes characteristic of both donor and acceptor all indicate that the hydrogen bonding proton is not located at the mid point of the NβN bridge. Salts of the complex (benzothiazole) 2 H + cation have been obtained in the solid state, and have a similar infrared spectrum to the complex in solution.

Complex hydrogen bonded cations: II. Other symmetric ions

Abstract The occurrence of complex hydrogen bonded cations involving 4-methyl-pyridine, 4-ethylpyridine, 3-chloropyridine and quinoline is established. The similarity of their vibrational spectra to those of indicate a similar structure. 2,6-Dimethyl-, 3,5-dimethyl- and 2,4,6-trimethylpyridine salts are much less soluble in the respective bases, indicating weaker complexing.

The cytidinium—cytidine complex: infrared and Raman spectroscopic studies

Abstract Using IR and Raman spectra, it is shown that the sytidinium cation hydrogen bonds to cytidine to form a stable 1:1 complex, in both aqueous solution (pH ~ 3.3) and as a solid. The spectra indicate that the proton is located asymmetrically in the NH⋯N bond of the complex, on the vibrational time scale, in both solution and the solid. The perdeuterated systems were also examined; their spectra support these conclusions.

Complex hydrogen bonded cations-V further asymmetric ions

Abstract Solutions have been prepared containing the complex cations ( 1 BH 2 B) + , where the differing bases may be pyridine, 4-Me pyridine, NMe imidazole, thiazole, isooxazole, trimethylamine or NMe piperidine. The infrared spectra usually show a double maximum in the v s band, the intensity of the lower component decreasing as the difference in the p K as of the bases increases. In the D-bridged analogues only a single v s band occurs. Other features of the spectra indicate that the bridging proton potential is asymmetric. The lower frequency component of the v s band of the H-bonded system is attributed to Fermi resonance, not proton tunnelling. The proton potential in these and the related symmetric complex cations is reviewed.

Complex hydrogen bonded cations - VI cations involving aliphatic bases

Abstract Hydrogen bond complexing between aliphatic bases and their cations has been examined by infrared spectroscopy. The results are: Morpholinium/morpholine - strong complex - asymmetric single minimum bond (a.s.m.). Piperidinium/piperidine - strong complex (a.s.m.). Piperidinium/NMe morpholine - complex (a.s.m.). NMe piperidinium/morpholine - weaker complex (a.s.m.). NMe piperidinium/NMe piperidine - no complexing. NMe morpholinium/NMe morpholine - no complexing. Triethylenediamine (TED). Complexes of type (TEDH + ) n , and TEDH + /TED have been distinguished. Models indicate the Me group retains the equatorial position in the tertiary bases on complexing, and the N β N distance is ⩽ 2.5 A.

The strength of the deuterium bond

Abstract The fall in frequency on deuteration, by some 4 cm−1, of the intermolecular stretching mode vσ of hydrogen bonded complexes, is larger than can be accounted for by any reasonable harmonic force field. It is shown that this does not imply a different potential function for the deuterium bond; it may be quantitatively accounted for by anharmonicity in the vσ mode, together with anharmonic interaction with the AH stretching mode vs.