LARGE-AMPLITUDE SKELETAL ISOMERIZATION AS A PROMOTER OF INTRAMOLECULAR VIBRATIONAL-RELAXATION IN CH STRETCH EXCITED HYDROCARBONS

The high resolution, slit jet cooled infrared v=1<--0 methyl asymmetri c stretch spectra of trans-2-butene and 1-butene an reported. Both of these molecules are singly unsaturated butene chains, have 30 vibratio nal degrees of freedom, and yield nearly equivalent vibrational state densities (rho(vib)approximate to 200 states/cm(-1)) at CH stretch lev els of excitation. The key difference between these two molecules is t he presence of a large amplitude C-C-C skeletal torsional coordinate i n 1-butene corresponding to a low barrier, internal isomerization path way which is completely absent in trans-2-butene. The trans-2-butene a symmetric CH stretch (nu(16)) spectrum is fully discrete at 0.002 cm(- 1) resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J(K'aKc')(')<--J(Ka''Kc'')('')) in an asym metric top. Fragmentation of these zero order transitions into spetral ''clumps'' of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss o f K-a' and K-c' as good quantum labels in the spectrum. The average de nsity of coupled states is found directly from the spectrum to be 114 states/cm(-1), i.e., on the order of 0.5 rho(vib). In contrast to the behavior in trans-2-butene, the l-butene v=1<--0 methyl asymmetric str etch spectrum exhibits an essentially continuous absorption contour ev en at T-rot=6 K and 0.002 cm(-1) resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibr ational relaxation (IVR) induced fine structure limited by apparatus r esolution and characteristic of highly congested IVR coupling. Analysi s of this fine structure indicates a density of coupled states on the order of 1 000-10 000 states/cm(-1), i.e., 20-30-fold in excess of rho (vib), and 1-2 orders of magnitude larger than observed in trans-2-but ene. In order to model the degree of fine structure observed in the sp ectrum, this level of spectral congestion essentially requires complet e mixing of all rho(vib).(2J'+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively d ramatic differences between 1-butene and trans-2-butene behavior suppo rt a simple model for strong vibration-rotation (V-R) coupling in the bath states due to large amplitude skeletal motion in the C-C-C torsio nal mode which greatly enhances the available state density for IVR. H ence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.