Nelson Fuson

The NH Stretching Vibration and NH–N Hydrogen Bonding in Several Aromatic Compounds

The three‐micron infrared absorption bands assigned to the NH stretching vibration have been studied in pyrrole, indole, carbazole, diphenylamine, and aniline. Spectra have been obtained in solution in carbon tetrachloride, carbon disulfide, chloroform, and benzene over a wide range of concentrations. In pyrrole and indole with increasing concentration the sharp NH band fades out and a broader band to lower frequencies appears. While the results are not as clearcut for diphenylamine and aniline, for these four compounds hydrogen bonding satisfactorily explains the observed data. This conclusion, for pyrrole and indole, sets aside the infrared spectroscopic basis for Paulings hypothesis of two species of molecules. Carbazole was not soluble enough to make association studies possible. The relation of both the solvent and the solute molecular structure to the position of the associated and unassociated frequencies is discussed.

An Infrared Spectroscopic Study of the Carbonyl Stretching Frequency in a Group of Ortho and Para Quinones

This paper reports on an experimental study of the C=O stretching vibration frequency in a large number of quinonoid compounds, both substituted and unsubstituted. For nonsubstituted ortho and para quinones, relationships between the carbonyl frequency and (a) the number of fused rings, (b) the oxidation‐reduction potential, and (c) the index of free valence on the parent hydrocarbon are pointed out and discussed. It is shown that for a six‐atom quinonoid ring the C=O frequency is, to a first approximation, a function of the C=O bond order. For substituted quinones, the variation in the carbonyl frequency is related to the induction effect of the substituent in the same way as is the variation in oxidation‐reduction potential. Certain hydroxy‐substituted quinones show spectral anomalies which do not seem to fit into the usual category of hydrogen bonded compounds.

Infrared Study of the Effect of Solvent upon ν(NH) in Pyrrole

New data are presented on the NH group stretching frequency of pyrrole in dilute and concentrated solutions in fourteen different solvents and in several solvent mixtures. It is shown that in nonpolar solvents ν(NH) of pyrrole obeys the Kirkwood‐Bauer relationship, δν/ν=C(D—1)/(2D+1), where C is a constant, and D is the solvent dielectric constant. Furthermore, it is shown that C is only slightly affected as ν(HCl) of hydrochloric acid, ν(OD) of methanol‐d, or ν(OH) of methanol, ethanol, propanol, or phenol is substituted for the ν(NH) of pyrrole. The deviations from the Kirkwood‐Bauer relationship produced by polar solvents and aromatic hydrocarbon solvents are analyzed in terms of hydrogen bonding and the formation of molecular complexes. The effect of solvent upon band intensity is also briefly examined. A classification of types of OH and NH solution frequencies is suggested.

Infrared and Raman Spectroscopy Studies of Light and Heavy Trifluoroacetic Acids

An infrared and Raman spectroscopic study of light and heavy trifluoroacetic acids shows that their vapors at room temperature are composed of a mixture of monomeric and associated molecules. The principal bands corresponding to vibrations of the COOH and COOD groups have been identified for both the monomeric and the associated molecules by means of a comparative study of the two acids (a) in the vapor state at different pressures, (b) in CCl4 solutions at various concentrations, and (c) in the liquid state. The monomeric carbonyl frequencies, influenced by the presence of the perfluoromethyl group, are the highest yet reported in the literature; this is true not only for CF3COOH (1826 cm−1, vapor), CF3COOD (1823 cm−1, vapor), but also for the ethyl ester, CF3COOC2H5 (1802 cm−1, vapor), the sodium salt, CF3COONa (1687 cm−1, solid), and the anhydride (CF3COO)2O (1884 cm−1 and 1818 cm−1, vapor). On the basis of a comparative study of the spectra of all these compounds, as well as of other perfluoromethyl c...

Structure of the Associated OH Valence Vibration Band in Light and Heavy Acetic, Trichloroacetic, and Trifluoroacetic Acids*

This paper presents infrared spectral data in the three micron region for ordinary and deuterium-substituted acetic, trichloroacetic, and trifluoroacetic acids. Series regularities are pointed out among the many submaxima appearing in the broad associated hydroxyl vibration band. The spacing between the submaxima for the heavy acids appears to be only one half that for the light acids. A critical discussion is given of various combination band hypotheses which have been proposed to explain the structure of the associated band. An alternative hypothesis is then suggested. This explanation postulates a series of regularly spaced positions of equilibrium of the hydrogen atom between the oxygen atoms in the O—H…O chain, these different positions corresponding to different quantized ground-state energy levels of the dimer molecule. It is suggested that the different members of a series of absorption submaxima might then be correlated with transitions to the first-excited state from the corresponding different ground states.