Vibrational Spectra of Pyridinium Salts

Abstract

'The vibrational spectrum of the pyridiniuln ion, CjHjI\TH+, and the N-deuterated species has been studied in several pyridinium salts. By comparison with benzene and deuterobenzene (with which the p y r i d i n i~~~n ion is isoelectronic) a fairly complete assignment has been made. The N-H bonds, which are hydrogen bonded to the anions, have stretching vibrations which show systematic variations depending on the nature of the anion. The N-H in-plane deformation vibrations show little variation while the out-of-plane deformation vibrations, some of which do vary, are probably affected, in addition, by other factors. INTRODUCTIONPyridine forms salts with many strong mineral and organic acids. In these salts it is assumed that complete proton transfer taltes place between the acid and the pyridine molecule, giving the pyridinium ion I and an anion.Little spectroscopic data on such salts is available in the published literature." Greenwood and Wade (I) assigned a few bands from their published spectra of C5H6NH+Cl-and C5HSNH+BC14-, while Kynaston and co-workers (2) listed all the bands in the latter salt. Spinner published a list of frequencies in the infrared and Raman spectrum (aqueous solution) of C6H6T\TI-I+C1-(3) and made a few assignments.In these papers emphasis was placed on sinlilarities between these salts and C5HsN. I t is believed that a more pertinent and helpful co~npariso~l is between the pyridiniurn ion and C61-16. These n~olecules contain the same number of atoms, are isoelectronic, and, apart from the mass and electronegativity of the nitrogen atom, would be identical. This physical similarity brealts down, however, when the synlrnetry of the two ~nolecules is compared. C6Ho has D6h, and C6H6NH+ is expected t o have Cz, symmetry. Thus C6H6 has only four (strictly) infrared-active fundamentals and seven Ranlan fundamentals. C6H6NH+ will have 30 Raman-active fundamentals and 27 infrared-active fundamentals. (The selection rules are shown in Table I.) The conlparison between C6HBD and C6HSND+, however, reveals a much closer similarity, since both molecules now have Cz, symmetry, with identical selection rules.

Interaction of Fluazifop with Al-, Fe3+-, and Cu2+-Saturated Montmorillonite

“…guishing features of protonated Fluazifop are: (1) the occurrence of a weak absorption in the N-H stretching region centered at 3065 cm -l due to the protonated pyridinic nitrogen, and (2) the shift of the v8a ring vibration from 1580 (Wilmshurt and Bernstein, 1957) to 1655 cm -1, the latter value being typical of pyridinium compounds (Cook, 1961;Farmer and Mortland, 1966).…”

Protonation Site in Organic Bases From Infrared X—h Deformation Modes

“…Previously published work (20) has established that the in-plane hydrogen-bending mode in pyridinium salts gives rise to an absorption band near 1240 cm-l. No splitting was observed in the HC1 salt (or others), which is in accordance with the selection rules using the details of the crystal structure determined by RCrat (21). Spinner (22) assigned a band near 1250 cin-I in the infrared spectra of the hydrochlorides of several 2-, 3-, and 4-substituted pyridines to the NH in-plane bending mode, though this assignment was not confirmed by deuterium substitution.…”

Humic substances reactions involving bipyridylium herbicides in soil and aquatic environments

“…These bands are assigned to the outof-plane vibration mode of C-H (BELLAMY 1955). It has been reported (COOK 1961) that the pyridinium ion shows characteristic changes in the out-of-plane C-H deformation modes on charge-transfer complex formation with anions and that the shifts are a function of the electronegativity values of the anions. KROSS et al (1956) observed a shift in the C-H out-of-plane bending vibration frequency in mono-substituted and para-substituted benzenes.…”