Structural differences among methanol clusters (n=1–4) hydrogen-bonded to Coumarin 151

Clusters of up to four methanol molecules attached to Coumarin 151 exhibit large electronic red shifts ranging up to 3414 cm−1. Experiments involving infrared–optical double-resonance spectroscopy (3100–3800 cm−1) show that different cluster sizes involve different attachment sites on the coumarin molecule. The n=1, 3 and 4 clusters are evidently attached to the NH2 group, the 3 and 4 species being cyclic, whereas the n=2 species most likely attaches via proton donation to the 〉CO group. The structure assignments are supported by the results of semiempirical calculations.