SPECTROSCOPY OF C-H STRETCHING VIBRATIONS OF GAS-PHASE BUTENES - CIS-2-BUTENE, TRANS-2-BUTENE, 2-METHYL-2-BUTENE, AND 2,3-DIMETHYL-2-BUTENE

The C-H fundamental and overtone spectra of gas-phase cis-2-butene, trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene were measured. Three main bands were observed for each overtone for cis- and trans-2-butene and 2-methyl-2-butene that correspond to the out-of-plane C-H (a), the in-plane C-H (s), and the olefinic C-H (1) bonds. Two main bands were observed for 2,3-dimethyl-2-butene that correspond to C-H (a) and C-H (s) bonds. The highest energy absorption corresponds to the olefinic C-H (1) bond followed by the methyl in-plane C-H (s) and the methyl out-of-plane C-H (a) bonds, respectively. Computer deconvolution of the overtone absorptions was done to separate the bands corresponding to the different C-H bonds and to obtain information about the peak position, band profile, and line width of the absorption. Local mode harmonic frequencies and anharmonicities were calculated from the analysis of the spectra. Comparison was made with results obtained with infrared studies of partially deuterated cis- and trans-2-butene. The C-H fundamental local mode frequencies (omega(e)-2omega(e)x(e)) are very close to the corresponding isolated frequencies (nu(iso)) in trans-2-butene. The agreement is also very good for cis-2-butene except in the values of the methyl C-H (s) bond. The calculated dissociation enegy indicates that the olefinic C-H (1) and the in-plane methyl C-H (s) bonds are stronger in cis-2-butene than those in the trans-2-butene. The out-of-plane methyl C-H (a) bonds are very similar in strength in both compounds. By use of the fifth overtone peak absorption, isolated frequencies (nu(iso)) and bond lengths (r(CHo)) are predicted for 2-methyl-2-butene and 2,3-dimethyl-2-butene.