Complex hydrogen bonded cations. The benzimidazole benzimidazolium cation

Abstract

B. BORAH and J. L. WOOD. Can. J. Chem. 54, 2470 (1976). The complex salts (Bz2H)+Y-, BZ = benzimidazole, Y = PFs-or BF4-have been identified in solution. and the molecularitv of the fluoroborate. which has been crvstallised. confirmed gravimetrically. Infrared and ~i m a n spectra, together with those of thideuterated systems, indicate a similar structure in solution and the crystal, viz., an asymmetric single minimum bridging proton potential, in conformity with the X-ray structure, proving that the pronounced doublet observed in the V, band of the H bridged system is not due to proton tunnelling. The origin of the asymmetric structure is discussed.On a identifiC en solution les sels complexes (Bz2H)+Y-, Bz = benzimidazole, Y = PF6-.ou BF4-et on a confirm6 gravimktriquement la mol6cularit6 du fluoroborate qui a kt6 cristallisC. Les spectres infrarouges et Raman de ces complexes de mCme que ceux des systkmes deutCrCs correspondants indiquent que la structure est similaire en solution et a l'Ctat cristallin savoir la prCsence d'un potentiel asymktrique, comportant un pont de proton et un seul minimum; ceci est en accord avec la structure dCduite de rayon-X; ces rksultats prouvent que le doublet prononce observe dans la bande v, du systkme hydrogkne pontC n'est pas due h un effet tunnel du proton. On discute de I'origine de la structure asymktrique.[Traduit par le journal] Introduction tained. The spectra of the crystalline complex The vibrational spectra of solutions containing and of the complex in solution are discussed in both benzimidazole and its cation show many this report, and the crystal structure reported in indications of complexing between these species. the accompanying paper. A preliminary account In particular the V, (NH stretching) infrared of the present results has been given (3). band has a notable doublet contour, which is replaced by a single band in the D bridged species, a behaviour similar to that in many other (BHB)+ complexes involving heterocyclic bases. However, although this has often been taken as a characteristic consequence of proton tunnelling in a double minimum potential (1), such a cause of the v, band doubling in the complexes previously examined was rejected, since several other spectroscopic features do not tally with this model. Instead, an asymmetric single minimum potential for the hydrogen bond (on the vibrational time scale) was proposed (2). Previous attempts to obtain a direct check on this inference by crystallisation of the complexes and consequent structure determination have met the difficulty that solutions containing the complex yielded crystals of either the base or the free salt. However, crystals of the complex benzimidazole salt suitable for X-ray analysis have been ob-

Reassignment of the Vibrational Spectrum of Benzimidazole

“…2,4,11,12 The previous optical assignments were made on the basis that the greatest deuteration shift should be demonstrated by γ(NH), and in the absence of the 918 cm -1 band, the greatest shift was given by the 628 cm -1 feature. This value was also consistent with the assignment of γ(NH) in indole, 512 cm -1 .…”

“…The assignment of γ(NH) at 918 cm -1 in the solid is in complete disagreement with the optical spectroscopic assignments that place it about 628 cm -1 . 2,4,11,12 The previous optical assignments were made on the basis that the greatest deuteration shift should be demonstrated by γ(NH), and in the absence of the 918 cm -1 band, the greatest shift was given by the 628 cm -1 feature. This value was also consistent with the assignment of γ(NH) in indole, 512 cm -1 .…”

“…Due to the pseudo-C 2V symmetry of benzimidazole the vibrations of the benzenoid moiety are not simply those of isolated benzene (for the form of benzene modes see ref 19). The γ(CH) ben mode is composed (50:50) of the benzene modes ν 8 and ν 11 . In turn this γ(CH) ben mixes with γ(CH) im to generate in-and out-of-phase components; these are calculated at (γ(CH) ben + γ(CH) im ) 853 and (γ(CH) ben -γ(CH) im ) 864 cm -1 , respectively, see Table 2.…”

Formation of acid–water hydrogen bonds with large proton polarizability and molecular processes with dissociation of acids in the pKa range 0–4

“…With long N+H...N N-.-H+N bonds, band splitting and band-like structures of the continua were explained by various authors (28,(32)(33)(34)(35) as being caused by Fermi resonance of the fundamental transitions of the hydrogen bonds with combinational vibrations. Finally, Wolff and co-workers (36, 37) explained the fine structure of bands observed with carboxylic acids on the same basis.…”

Normal Vibrational Mode Analysis and Assignment of Benzimidazole by ab Initio and Density Functional Calculations and Polarized Infrared and Raman Spectroscopy

“…Several vibrational studies of BZI were performed on its KBr disk, solid and solution, KBr mull, melt and single crystal, and its gaslike phase in the Ar matrix to settle its modes' assignments. However, there is still some ambiguity in the assignment of certain fundamental modes due to their low intensities or extensive band overlapping.…”