Rotationally resolved spectra in the 5 nu (1) region of (CH3OH)-C-13 reveal strong vibrational splittings, which by analogy to (CH3OH)-C-12, we assign to a resonance between 5 nu (1) and 4 nu (1)+nu (2). Accordingly, the vibrational dynamics on a subpicosecond time scale are similar for the two isotopomers. Comparison of the secondary structure of the first-order states resulting from this strong resonance shows a distinct difference in the two isotopomers. While the C-12 species exhibits sharp secondary structure for the lower energy band resulting from weak coupling to the remaining bath of dark states, the C-13 species shows an equally complex pattern of couplings for both first-order states. The difference between the two isotopic species arises from the relative position of key dark background states. Despite a vibrational density of states of 100 per cm(-1), only a small number of states seem to determine the secondary structure, and the difference in the positions of these states relative to the first-order states results in a difference in the vibrational dynamics on a picosecond time scale. What one might consider as statistical intramolecular energy transfer appears to occur on significantly longer time scales. (C) 2000 American Institute of Physics. [S0021-9606(00)02046-8].