High-Resolution Rotational Coherence Spectroscopy of the Phenol Dimer

Abstract

We report on an investigation of the phenol dimer by high-resolution rotational coherence spectroscopy (RCS) using the method of time-resolved fluorescence depletion (TRFD). With this technique we determined with high precision the rotational constants of the ground and electronically excited states. The phenol dimer is an ideal model system to study aromatic−aromatic interaction under the constraints of an intermolecular hydrogen bond, which leads to its unique “V-shaped” structure. The TRFD investigation was complemented by an (1 + 1‘) pump−probe ionization (PPI) experiment in order to unequivocally 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 analysis by a grid search procedure based on numerical simulations of RCS spectra and a nonlinear least-squares fitting routine, the following values for the 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 of 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 phenol dimer as determined by a fit of the intermolecular parameters to the rotational constants. The ground-state results confirm the gross geometry of a former RCS investigation of Felker and co-workers [Connell, L. L.; 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 increases slightly from 5.25 Å ± 0.01 Å to 5.31 Å ± 0.05 Å. 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 the rotational constants are small, larger changes of intermolecular parameters cannot be excluded, e.g., a decrease of the wagging angle by several tens of degrees. The ground-state results are compared with structures obtained from calculations on different levels of theory. In particular, the results from semiempirical calculations based on atom−atom pair potentials and ab initio calculations at the MP2/6-31G(d) level of theory are examined.