Isotopic Dependence of Excited-State Proton-Tunneling Dynamics in Tropolone Probed by Polarization-Resolved Degenerate Four-Wave Mixing Spectroscopy

Abstract

The origin band of the Ã1B2-X1A1 (π* ← π) absorption system in monodeuterated tropolone (TrOD) has been probed with near-rotational resolution by applying the frequency-domain techniques of polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy under ambient, bulk-gas conditions. Judicious selection of polarization geometries for the incident and detected electromagnetic waves alleviated intrinsic spectral congestion and facilitated dissection of overlapping transitions, thereby enabling refined rotational-tunneling parameters to be extracted for the Ã1B2(π*π) manifold. A tunneling-induced bifurcation of Δ0Ã = 2.241(14) cm(-1) was measured for the zero-point level of electronically excited TrOD, reflecting the presence of a substantial barrier along the O-D···O ↔ O···D-O reaction coordinate and representing nearly a 10-fold decrease in magnitude over the analogous quantity in the parent (TrOH) isotopologue. Observed trends in hydron-migration rates are discussed in light of the changes in the potential-surface topology sustained from the π* ← π electron promotion and the dynamical effects incurred by selective isotopic modification of the nuclear framework, with similar considerations being applied to interpret rotational constants and inertial defects. Simultaneous analyses performed on an interloping sequence band built upon ν38(b1) gave an excited-state tunneling splitting of Δ(ν38)Ã = 1.217(61) cm(-1), highlighting the ability of this symmetric, out-of-plane normal mode to inhibit the unimolecular tautomerization process.

Hydrogen-Bonding Motifs and Proton-Transfer Dynamics in Electronically Excited 6-Hydroxy-2-formylfulvene

“…Further evidence for the putative quenching of HFF proton-transfer dynamics in the π*π excited state can be gleaned from the Coriolis-like coupling constant extracted for the tunneling components of the vibrationless à 1 B 2 level, F 0 à = 1.81(42) × 10 –3 cm –1 , which is nearly an order of magnitude larger than analogous parameters reported for the related tropolone system. − By invoking a one-dimensional reaction-coordinate ansatz with hydron migration presumed to take place along a single principal axis of a planar molecular framework, Baughcum and co-workers have proposed the following analytical expression for this quantity (in cm –1 ): where I a ≡ I aa , I b ≡ I bb , and I ab denote elements of the inertial (mass-moment) tensor with the effective mass along the tunneling coordinate, m 0 , being correlated to the Cartesian coordinates, x H and z H , of the shuttling hydron (the latter defined in a type- I r representation such that hydron migration occurs mainly along the b ≡ x axis of Figure ). While m 0 for the HFF ground state can be equated to the reduced mass of the lone imaginary frequency that characterizes the C 2 v transition-state (TS) configuration, μ = 1.23 amu (by CCSD/apVDZ), the aplanar reaction coordinate postulated for the electronically excited species presents additional complications, with the C 2 v saddle point (2SP) yielding two analogous quantities (by EOM-CCSD/apVDZ) ascribed to direct proton transfer, μ = 1.24 amu, and to heavy atom displacements, μ = 7.16 amu, needed to access the true TS of C 2 symmetry (where μ = 1.33 amu).…”

“…The solid curve superimposed on these data highlights the results of a spectral simulation built upon the rovibronic-energy levels and transition strengths calculated by numerical diagonalization of independent rotation-tunneling Hamiltonians, Ĥ v η , for the vibrationless (v = 0) ground (η = X̃ ) and excited (η = à ) electronic states. In particular, the forms of Ĥ 0 X̃ and Ĥ 0 à were adopted from those employed for previous ambient, bulk-gas studies of the kindred à 1 B 2 – X̃ 1 A 1 absorption system in tropolone , (with rotational quantum numbers constrained such that 0 ≤ J ≤ 60); however, the low effective temperatures attained in a supersonic expansion ( vide infra ) precluded the meaningful extraction of centrifugal-distortion information as well as the determination of J -/ K a -dependent corrections for the F 0 η terms that govern the Coriolis-like coupling between the v = 0 symmetric (+) and antisymmetric (−) manifolds. The rotational constants for the vibrationless ground state (0 + ) were held fixed at the values reported by prior microwave work while their excited-state counterparts were least-squares adjusted under the assumption that the 0 + and 0 – inertial parameters could be constrained to be equal ( viz ., A 0 + à = A 0 − à , B 0 + à = B 0 − à , and C 0 + à = C 0 − à as might be expected for the case of small tunneling-induced bifurcation).…”

“…The results emerging from the present study suggest several possible avenues for future research, all of which are designed to unravel the origins and ramifications of LBHBing in HFF and related species. To further elucidate the putative state specificity of proton-transfer dynamics in electronically excited manifolds, high-resolution probes of the rotational structure supported by individual vibronic bands (beyond the origin band) in the à 1 B 2 – X̃ 1 A 1 absorption system currently are in progress, with the unique capabilities of polarization-resolved degenerate four-wave mixing spectroscopy being deployed to dissect observed rovibronic features. , Similar investigations of the strongly perturbed ground electronic potential-energy surface can be envisioned by exploiting two-color variants of resonant four-wave mixing , in conjunction with lower-resolution measurements of dispersed fluorescence. Finally, ongoing efforts to interrogate weakly bound complexes formed between HFF and various “solvent” ligands under cryogenic molecular-beam conditions afford a unique opportunity to explore the influence of successive “microsolvation” on LBHBing motifs, thereby forging a tractable bridge to related work performed in condensed phases.…”

“…The solid curve superimposed on these data highlights the results of a spectral simulation built upon the rovibronic-energy levels and transition strengths calculated by numerical diagonalization of independent rotation-tunneling Hamiltonians, H ̂v η , for the vibrationless (v = 0) ground (η = X ̃) and excited (η = A ̃) electronic states. In particular, the forms of H ̂0 X ̃and H ̂0 A ̃were adopted from those employed for previous ambient, bulk-gas studies of the kindred A ̃1B 2 −X ̃1A 1 absorption system in tropolone 98,99 (with rotational quantum numbers constrained such that 0 ≤ J ≤ 60); however, the low effective temperatures attained in a supersonic expansion (vide infra) precluded the meaningful extraction of centrifugal-distortion information as well as the determination of J-/K a -dependent corrections for the Stationary-point geometries (EQ, TS, and 2SP) predicted by coupled-cluster analyses of X ̃1A 1 (CCSD/apVDZ) and A ̃1B 2 (EOM-CCSD/apVDZ) HFF were used to estimate the attendant rotational constants (A, B, C), asymmetry parameters (κ), and inertial defects (ΔI). The magnitude (|μ|) and inertial-frame components (μ a , μ b , μ c ) of the permanent and transition electric dipole moments (EDMs) also are tabulated, with the latter restricted to vertical transitions originating from the EQ configurations of the ground and excited electronic states.…”

“…Finally, to account properly for large-amplitude (proton-transfer) dynamics, corresponding nuclear-spin statistics must reflect classification of the total wavefunction with respect to the permutation element of the encompassing G 4 molecular-symmetry group. 98,99 For 6hydroxy-2-formylfulvene isotopologs having symmetric placement of 1 H, 12 C, and 16 O nuclei in the transition-state configuration (viz., HFF and HFF-d), this requires even:odd parities of the near-prolate K a quantum number to exhibit statistical weights of 6:10 (10:6) for the symmetric (antisymmetric) member of the vibrationless (v = 0) ground-state tunneling doublet.…”