High-resolution rotational coherence spectroscopy of the phenol dimer

We report on an investigation of the phenol dimer by high-resolution rotati onal coherence spectroscopy (RCS) using the method of time-resolved fluores cence depletion (TRFD). With this technique we determined with high precisi on the rotational constants of the ground and electronically excited states . The phenol dimer is an ideal model system to study aromatic-aromatic inte raction under the constraints of an intermolecular hydrogen bond, which lea ds to its unique "V-shaped" structure. The TRFD investigation was complemen ted by an (1 + 1 ') pump-probe ionization (PPI) experiment in order to uneq uivocally assign ground and excited-state transients. Seven different types of RCS transients have been observed in the RCS spectrum and assigned to H "-, H '-, J "-, J '-, C-, K-, and A-type transients. From a detailed analy sis by a grid search procedure based on numerical simulations of RCS spectr a and a nonlinear least-squares fitting routine, the following values for t he rotational constants have been obtained: A " = 1414.4 +/- 0.6 MHz, B " = 313.7 +/- 0.8 MHz, C " = 287.5 +/- 0.7 MHz, A ' = 1425.7 +/- 2.3 MHz, (B ' + C ') = 590.6 +/- 2.7 MHz. Furthermore, information about the alignment o f the transition dipole moment in the molecular frame was obtained from the fit procedure. We report a geometry of the O-H . . .O hydrogen bonded phen ol dimer as determined by a fit of the intermolecular parameters to the rot ational constants. The ground-state results confirm the gross geometry of a former RCS investigation of Felker and co-workers [Connell, L. I,.; Ohline , S. M.; Joireman, P.W.; Corcoran, T. C.; Felker, P. M. J. Chem. Phys. 1992 , 96, 2585]. Moreover, it was found that upon electronic excitation of the donor molecule the center of mass distance of the monomer moieties increase s slightly from 5.25 Angstrom +/- 0.01 Angstrom to 5.31 Angstrom +/- 0.05 A ngstrom. On the basis of assumptions for structural changes of the hydrogen bond and the donor monomer moiety upon electronic excitation, we propose a modification of the intermolecular structure of the phenol dimer, which is consistent with the experimental data. However, although the changes in th e rotational constants are small, larger changes of intermolecular paramete rs cannot be excluded, e.g., a decrease of the wagging angle by several ten s of degrees. The ground-state results are compared with structures obtaine d from calculations on different levels of theory. In particular, the resul ts from semiempirical calculations based on atom-atom pair potentials and a b initio calculations at the MP2/6-31G(d) level of theory are examined.