Self-consistent spectrophotometric basicity scale in acetonitrile coveringthe range between pyridine and DBU

A self-consistent spectrophotometric basicity scale in acetonitrile, includ ing DBU, ten (arylimino)-tris(1-pyrrolidinyl)phosphoranes, two (arylimino)t ris(dimethylamino)phosphoranes, 2-phenyl-1,1,3,3-tetramethylguanidine, 1-(2 -tolyl)biguanide, benzylamine, two substituted benzimidazoles, pyridine, an d ten substituted pyridines, has been created. The span of the scale is alm ost 12 pK(a) units. Altogether, 29 different bases were studied and 53 inde pendent equilibrium constant measurements were carried out, each describing the relative basicity of two bases. The scale is anchored to the pK(a) val ue of pyridine of 12.33 that has been measured by Coetzee et al. Comparison of the basicity data of phenyliminophosphoranes and phenyltetramethylguani dines implies that the P=N bond in the (arylimino)tris(1-pyrrolidinyl)phosp horanes involves contribution from the ylidic (zwitterionic) structure anal ogous to that found in phosphorus ylides.