The high resolution, slit jet cooled infrared v=1<--0 methyl asymmetric stretch spectra of trans-2-butene and 1-butene an reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (rho(vib)approximate to 200 states/cm(-1)) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C-C-C skeletal torsional coordinate in 1-butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans-2-butene. The trans-2-butene asymmetric 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 asymmetric 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 of K-a’ and K-c’ as good quantum labels in the spectrum. The average density 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 stretch spectrum exhibits an essentially continuous absorption contour even at T-rot=6 K and 0.002 cm(-1) resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis 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-butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all rho(vib).(2J’+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1-butene and trans-2-butene behavior support 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 torsional mode which greatly enhances the available state density for IVR. Hence, 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.