Thermally induced gauche-trans isomerization and direct solute-solvent interactions of the solute, ethyl isocyanate (EIC), in the solvent, 2-methylpentane (2MP), are investigated using ultrafast infrared vibrational echo experiments and linear vibrational absorption spectroscopy of the isocyanate (N=C=O) antisymmetric stretching mode (2278 cm(-1)). Both the EIC vibrational echo measured pure vibrational dephasing and the absorption spectra show complex behavior as a function of temperature from room temperature to 8 K. The EIC data are compared to absorption experiments on the same mode of isocyanic acid (HNCO), which cannot undergo isomerization. To describe the observations, a model is presented that involves both intramolecular dynamics and intermolecular dynamical interactions. At room temperature, gauche-trans isomerization is very fast, and the isomerization dynamics contribution to the vibrational echo decay and the absorption line shape is small because it is motionally narrowed. The dominant contribution to both the vibrational echo decay and the absorption spectrum is from direct dynamical interactions of the solute with the solvent. As the temperature is lowered, the direct contribution to vibrational dephasing decreases rapidly, but the contribution from isomerization increases because the extent of motional narrowing diminishes. The combined effect is a very gradual decrease of the rate of pure dephasing as the temperature is initially lowered from room temperature. At very low temperature, below the 2MP glass transition, isomerization cannot occur. The absorption spectrum displays two peaks, interpreted as the distinct gauche and trans absorption bands. Even at 8 K, the pure dephasing is surprisingly fast. The direct solvent-induced dephasing is negligible. The dephasing is caused by motions of the ethyl group without isomerization occuring. At intermediate temperatures (150 K>T>100 K), isomerization takes place, but its contribution to the pure dephasing is not motionally narrowed. The absorption spectral shapes are complex. Dephasing arising from direct interaction with the solvent is small. Both isomerization and fluctuations on the gauche-trans surface contribute to the absorption line shape. The model that is used to describe the results involves a NMR type exchange calculation with additional contributions from the direct solvent interactions that are obtained from the temperature-dependent HNCO IR spectra. From the temperature dependence of the isomerization "jump" rate, the barrier height for the isomerization is found to be similar to400 cm(-1). (C) 2003 American Institute of Physics.