Core photoelectron spectroscopy of some acetylenic molecules

Abstract

Carbon 1s binding energies have been measured for CH 3CCH, CH 3CCCH 3, CF 3CCH and CF 3CCCF 3 and compared to a verified value for acetylene. Assignments are based on the application of a CNDO potential model with relaxation corrections which is quite successful in predicting binding energy shifts and upon qualitative considerations. Substitution of CF 3 groups shifts the acetylenic C 1s binding energy from 291.2 (HCCH) to 292.2 in CF 3CCH and 292.7 eV in CF 3CCCF 3. The unequal substitutional shifts are probably due to a saturation of substituent effect expected in competitive situations. With reservations arising from uncertainties in assignment due to lack of resolution, it appears that acetylenic C 1s binding energies decrease [to 290.7 (av.) in CH 3CCH and to 290.1 eV in CH 3CCCH 3] upon replacement of H by CH 3 groups. Although the decrease in acetylenic binding energies agrees with the chemical notion that CH 3 groups are electron donating with respect to unsaturated portions of the molecule, theoretical calculations available in the literature indicate that actual electron withdrawal or donation does not occur in these differently substituted molecules. The shifts of apparent binding energy correlate reasonably well with a ground state potential model which accounts for the effect of the charge on the adjacent atoms as well as on the photoionized atom. Even better correlation is obtained if the atomic potentials are corrected for electronic redistribution (relaxation) effects which occur during the photoionization process, and it is suggested that relaxation effects make a significant contribution to shifts of apparent binding energies. Surprisingly ground state potential and relaxation corrected potential calculations with the CNDO method suggest a large difference in C 1s binding energies of the two acetylenic carbon atoms in CH 3CCH which is not verified experimentally nor mirrored by calculations on CF 3CCH. The CH 3 binding energies are 291.8 eV in CH 3CCH and 291.3 eV in CH 3CCCH 3, both higher than values assigned to CH 4 or C 2H 6.