Evidence for Dominant Role of Tunneling in Condensed Phases and at High Temperatures: Double Hydrogen Transfer in Porphycenes

Investigation of the double hydrogen
transfer in porphycene, its
2,7,12,17-tetra-tert-butyl derivative, and their
N-deuterated isotopologues revealed the dominant role of tunneling,
even at room temperature in condensed phase. Ultrafast optical spectroscopy
with polarized light employed in a wide range of temperatures allowed
the identification and evaluation of contributions of two tunneling
modes: vibrational ground-state tunneling, occurring from the zero
vibrational level, and vibrationally activated, via a large amplitude,
low-frequency mode. Good correspondence was found between the rates
of incoherent tunneling occurring in condensed phase and the values
estimated on the basis of tunneling splittings observed in molecules
isolated in supersonic jets or helium nanodroplets. The results provide
solid experimental insight into widely proposed quantum facets of
ubiquitous hydrogen-transfer phenomena.