Dramatic effect of the nature of R on the intrinsic acidity and basicity of potential astrochemical R-C COH and R-C CSH compounds

International audience ; The effect of changing the nature of the R substituent from the first row (H, Li, BeH, BH2, CH3, NH2, OH and F) to second row (Na, MgH, AlH2, SiH3, PH2, SH and Cl) on the intrinsic acidity and basicity of R-C COH and R-C CSH compounds was investigated through the use of G4 high-level ab initio calculation. The variation of the acidity and basicity of the R-C CSH derivatives as a function of R is practically parallel to that found for the corresponding R-C COH analogs; though the basicities of the former are 9-14% higher than those of the latter, the acidity gap being very small (similar to 2%). When this analysis is extended to the derivatives in which the triple CC bond is replaced by a double or single bond, it is found that the acidity gap increases systematically as the CC bond goes from triple to single; whereas, as expected for the basicity, the trend is the opposite. Quite surprisingly, however, the variation of the basicity of R-C CX (X = OH, SH) compounds with the nature of the first-row substituents, R, is remarkably different from that produced by the second-row analogs. The same is observed as far as intrinsic acidities are concerned. These dissimilarities reflect the rather different changes in the strength of the CC and the CX (X = OH, SH) bonds when a first-row substituent is replaced by the second-row analog, as reflected in the atoms in molecules (AIM), natural bond orbital (NBO) and the electron localization function (ELF) analyses of the corresponding species.