VIBRATIONAL-ENERGY DYNAMICS OF HYDROGEN-BONDED PYRROLE COMPLEXES

Picosecond IR transient bleaching and double-resonance absorption expe riments were performed on the pyrrole (C4H5N) NH stretching vibration of solution phase hydrogen-bonded pyrrole:base complexes in room tempe rature CCl4. Population lifetime (T-1) measurements show that hydrogen -bonding interactions enhance the vibrational energy relaxation rate o f the NH functional group significantly, and the magnitude of this enh ancement increases monotonically with proton-accepting strength of the base suggesting it is independent of base vibrational structure. Valu es for the NH stretch (v = 1) T lifetime include 49 +/- 3 ps (1 sigma) for ''free'' pyrrole, 13 +/- 2 ps for pyrrole:acetonitrile, and 4.5 /- 0.4 ps for pyrrole:tetrahydrofuran complexes. Intramolecular vibrat ional energy transfer is the dominant relaxation pathway, but the resu lts suggest a weak pump-induced dissociation channel. Extracted T-1 li fetimes differ markedly from earlier values obtained by transient anti -Stokes Raman methods.