Conformational isomerization and collisional cooling dynamics of bis(2-hydroxyphenyl)methane.

Stimulated emission pumping-population transfer spectroscopy (SEP-PTS) has been used to directly measure the energy threshold to isomerization between the two conformational isomers of bis(2-hydroxyphenyl)methane. These conformers have been shown in the preceding paper (DOI 10.1021/jp8098686) to be an OH...O H-bonded structure (conformer A) and a doubly OH() ...pi H-bonded conformer (conformer B). Lower and upper bounds on the energy threshold for A-->B isomerization are at 1344 and 1399 cm(-1), respectively, while the corresponding bounds on the B-->A isomerization are 1413 and 1467 cm(-1). The difference between these thresholds provides a measure of the relative energies of the two minima, with DeltaE(AB) = E(A) - E(B) = 14-123 cm(-1). The transition-state structure responsible for this energy threshold has been identified using DFT B3LYP, DFT M05-2X, and MP2 calculations, all with a 6-31+G* basis set. Only the DFT M05-2X calculations correctly reproduce both the energy ordering of the two minima and the magnitude of the barrier separating them. Below the energy threshold to isomerization, we have used the extensive Franck-Condon progressions present in the SEP spectrum of conformer A to undertake an SEP-PT study of its vibrational relaxation rate, as a function of internal energy over the 0-1200 cm(-1) region. The position of SEP excitation in the expansion was systematically varied in order to change the rate and number of cooling collisions that occur between SEP excitation and probe steps and the initial temperature at which SEP occurs. From this data set, three energy regimes were identified, each with a unique value of the average energy lost per collision with helium (region 1: 13 cm(-1)/collision for E = 300-1200 cm(-1), region 2: 0.6 cm(-1)/collision for E = 200-300 cm(-1), and region 3: 7 cm(-1)/collision for E < 200 cm(-1)). In region 1, the vibrational density of states is sufficient to support efficient loss of energy via Deltav = -1 collisions, involving the lowest-frequency vibrations of the molecule (with a frequency of 26 cm(-1)). In region 2, the vibrational energy levels are sufficiently sparse that energy gaps exist, reducing the efficiency of relaxation. In region 3, a combination of the quantum nature of the helium, attractive forces, and orbiting resonances may be responsible for the increased efficiency at lowest-energy regime.