An exploration of electronic structure and nuclear dynamics in tropolone: II. The A (1)B2 (pi* pi) excited state.

The first excited singlet state of tropolone (A (1)B(2)) and the attendant pi(*)<--pi electronic transition have been examined computationally by applying several quantum chemical treatments built upon the aug-cc-pVDZ basis set, including time-dependent density functional theory (TDDFT/B3LYP), configuration interaction singles with perturbative corrections [CIS and CIS(D)], and equation-of-motion coupled-cluster schemes [EOM-CCSD and CR-EOMCCSD(T)]. As in the case of the X (1)A(1) ground state [L. A. Burns, D. Murdock, and P. H. Vaccaro, J. Chem. Phys. 124, 204307 (2006)], geometry optimization procedures and harmonic force-field calculations predict the electronically excited potential surface to support a global minimum-energy configuration of rigorously planar (C(s)) symmetry. Minimal Hartree-Fock (HF/CIS) and density-functional (DFT/TDDFT) approaches yield inconsistent results for the X (1)A(1) and A (1)B(2) manifolds; however, coupled-cluster (CCSD/EOM-CCSD) methods give fully relaxed proton-transfer barrier heights of DeltaE(pt) (X)=3296.1 cm(-1) and DeltaE(pt) (A)=1270.6 cm(-1) that are in accordance with the experimentally observed increase in vibrationless tunneling splitting upon electronic excitation. Detailed analyses show that this reduction in DeltaE(pt) stems from a variety of complementary factors, most notably an overall contraction of the proton-transfer reaction site (whereby the equilibrium O...O donor-acceptor distance decreases from 2.53 to 2.46 A) and a concomitant shortening of the intramolecular hydrogen bond. Further refinement of A (1)B(2) energies through single-point perturbative triples corrections [CR-EOMCCSD(T)] leads to 1316.1 cm(-1) as the best current estimate for DeltaE(pt) (A). Direct comparison of the lowest-lying out-of-plane torsional mode [nu(39)(a(2))] for X (1)A(1) and A (1)B(2) tropolone reveals that its disparate nature (cf. nu(39) (X)=101.2 cm(-1) and nu(39) (A)=42.0 cm(-1)) mediates vibrational-averaging effects which can account for inertial defects extracted by rotationally resolved spectroscopic measurements.