A carbon-13 nuclear magnetic resonance study of a series of 5-substituted benzimidazoles

Abstract

Carbon-13 nmr chemical shift assignments are reported for a series of 5-substituted benzimidazoles, 2-methylbenzimidazole, and 5,6-dimethylbenzimidazole. By employing the three-parameter equation of Smith and Proulx, the chemical shifts of all the ring carbons of the 5-substituted compounds are correlated with F and R of Swain and Lupton and Q of Schaefer; only the ipso carbon, C-5, has chemical shifts with a correlation coefficient less than 0.953. These correlations are compared with those obtained from the Swain–Lupton and Taft relationships; in general those from the latter two approaches are found to be less satisfactory. Correlations of the same chemical shifts with excess charge density values, calculated at the INDO level of approximation, are also presented.

ChemInform Abstract: The Thermal Instability of 2,4 and 2,6‐N‐Alkylamino‐Disubstituted and 2‐N‐Alkylamino‐Substituted Nitrobenzenes in Weakly Alkaline Solution: sec‐Amino Effect.

“…From an examination of the IR spectrum ( Fig. 1a), the broad band centered around 3366 cm À1 illustrates the acidic nature of the hydrogen atom attached to one of the nitrogen atoms located in the ring of (5) and is confirmed further by the absence of an absorbance due to this hydrogen atom in the 1 H-NMR [11]. The absorbances due to the various carbon atoms in the 13 C spectrum are consistent with the calculated values from chemical shifts contributions [12].…”

The Thermal Instability of 2,4 and 2,6‐N‐Alkylamino‐disubstituted and 2‐N‐Alkylamino‐substituted Nitrobenzenes in Weakly Alkaline Solution: sec‐Amino Effect

“…From an examination of the IR spectrum (Fig. 1a), the broad band centered around 3366 cm −1 illustrates the acidic nature of the hydrogen atom attached to one of the nitrogen atoms located in the ring of ( 5 ), and is confirmed further by the absence of an absorbance due to this hydrogen atom in the 1 H NMR [11]. The absorbances due to the various carbon atoms in the 13 C spectrum are consistent with the calculated values from chemical shifts contributions [12].…”

Rapid and Efficient Synthesis of a New Series of 2-Aryl-5-Fluoro-6-(4-Phenylpiperazin-1-Yl)-1H-Benzimidazoles Using Microwave Heating

“…In the 13 C NMR spectra of the benzimidazole, the number of aromatic C-atoms are less than proposed because of tautomerism of imidazole hydrogen between the two nitrogen atoms. [25][26][27] The number of 13 C-signals of compounds 3-17 indicates two (or four) aromatic C-atoms. The C=N carbon of benzimidazole was observed at about 151 ppm.…”