Experimental and quantum chemical evidences for C-H center dot center dot center dot N hydrogen bonds involving quaternary pyridinium salts and pyridinium ylides

The rates of equilibrium of eight quaternary pyridinium salts containing ac tivated methylene groups with the corresponding ylides have been measured i n DMSO solution containing strong bases (DBU, MTBD and P1-tBu). The effect of the ring substituents on the deprotonation rate is greater than that of the methylene substituents. This observation is consistent with the energy of C-H . . .N hydrogen bonds and the elongation of the C-H bond participati ng in hydrogen bonds estimated by the PM3 and BLYP/6-31G(d,p) calculations. The UV spectra and H-I NMR spectra of selected ylides have been measured. The solvent effect an the proton chemical shifts of quaternary pyridinium h alides and ylides is discussed. Semiempirical and DFT calculations were car ried out to investigate the structure of ylides. In ylides, the alpha -pyri dine protons are more acidic than the methine proton. (C) 2000 Elsevier Sci ence B.V. All rights reserved.