Infrared spectra are reported for (C2H2)-C-12, (C2H2)-C-13, (CCH2)-C-12-C-13, and (C2HD)-C-12 solvated in superfluid helium nanodroplets, corresponding to excitation of the C-H stretches. For the lowest rotational states (J=0 and J=1), molecular rotation is only weakly hindered by the liquid and the associated transitions are sharp, having small frequency shifts relative to the gas phase. In contrast, the R(1) transitions of (C2H2)-C-12 and (C2H2)-C-13, which access the corresponding J=2 rotational states, are much broader and more strongly shifted from the gas phase. The linewidths of the R(0) transitions show a strong isotopic dependence, which we take as evidence that vibrational resonances present in some of the isotopomers enhance the vibrational relaxation rate.