Determining the Intermolecular Structure in the S₀ and S₁ States of the Phenol Dimer by Rotationally Resolved Electronic Spectroscopy

Abstract

The rotationally resolved UV spectra of the electronic origins of five isotopomers of the phenol dimer have been measured. The complex spectra are analyzed using a fitting strategy based on a genetic algorithm. The intermolecular geometry parameters have been determined from the inertial parameters for both electronic states and compared to the results of ab initio calculations. In the electronic ground state, a larger hydrogen-bond length than in the ab initio calculations is found together with a smaller tilt angle of the aromatic rings, which shows a more pronounced dispersion interaction. In the electronically excited state, the hydrogen-bond length decreases, as has been found for other hydrogen-bonded clusters of phenol, and the two aromatic rings are tilted less toward each other.

Determination of the Geometry Change of the Phenol Dimer upon Electronic Excitation

“…[1]. The COM distances for all isotopomers in both states are very well described by the structures obtained by the Franck-Condon fit, cf.…”

“…The second column shows the results for the geometry changes from the Franck-Condon fit to 547 fluorescence emission bands and to the changes of the rotational constants of five isotopomers. The third column gives the experimentally determined rotational constant changes and the results for the geometry changes from the fit to the rotational constants of five isotopomers of PH2 using pKrFit by Schmitt et al [1] Both phenol rings remain planar upon electronic excitation. The donor ring expands nearly symmetrically whereas the acceptor ring changes are almost zero indicating a local excitation in the donor moiety.…”

“…In ref. [1] this assumption was made as basis for "model 2". In Table 5 the experimental values, using "model 1" from ref.…”

“…S 0 state geometry parameters of the phenol dimer, calculated at the MP2/cc-pVTZ, MP2/6-311GA C H T U N G T R E N N U N G (d,p) counterpoise-corrected, RIMP2/TZVPPP, RICC2/TZVP level of theory, and from a structural fit to the ground state rotational constants of 5 isotopomers published in ref. [1]. In parentheses: relative deviations from the experimental values in %.…”

“…[1] While in principle the determination of molecular structures in different electronic states from inertial parameters is a straightforward procedure, one has to pay considerable attention to possible model errors, when the number of geometry parameters exceeds the number of rotational constants. For a complete fit of the phenol dimer geometry in internal coordinates 72 parameters (25 distances, 24 angles and 23 dihedral angles) have to be determined, while only 15 rotational constants (from five isotopomers) are known.…”