Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ∗-πσ∗ energy gap propensity and ortho-substitution effect

Time-dependent density functional theory assessment of UV absorption of benzoic acid derivatives

Benzoic acid (BA) derivatives of environmental relevance exhibit various photophysical and photochemical characteristics. Here, time-dependent density functional theory (TDDFT) is used to calculate photoexcitations of eight selected BAs and the results are compared with UV spectra determined experimentally. High-level gas-phase EOM-CCSD calculations and experimental aqueous-phase spectra were used as the references for the gas-phase and aqueous-phase TDDFT results, respectively. A cluster-continuum model was used in the aqueous-phase calculations. Among the 15 exchange-correlation (XC) functionals assessed, five functionals, including the meta-GGA hybrid M06-2X, double hybrid B2PLYPD, and range-separated functionals CAM-B3LYP, ωB97XD, and LC-ωPBE, were found to be in excellent agreement with the EOM-CCSD gas-phase calculations. These functionals furnished excitation energies consistent with the pH dependence of the experimental spectra with a standard deviation (STDEV) of ∼0.20 eV. A molecular orbital analysis revealed a πσ* feature of the low-lying transitions of the BAs. The CAM-B3LYP functional showed the best overall performance and therefore shows promise for TDDFT calculations of processes involving photoexcitations of benzoic acid derivatives.

Direct Observation of Hydrogen Tunneling Dynamics in Photoexcited Phenol

The excited-state dynamics of phenol following ultraviolet (UV) irradiation have received considerable interest in recent years, most notably because they can provide a model for understanding the UV-induced dynamics of the aromatic amino acid tyrosine. Despite this, there has been some debate as to whether hydrogen tunneling dynamics play a significant role in phenol's excited-state O-H bond fission when UV excitation occurs below the (1)ππ*/(1)πσ* conical intersection (CI). In this Letter, we present direct evidence that (1)πσ*-mediated O-H bond fission below the (1)ππ*/(1)πσ* CI proceeds exclusively through hydrogen tunneling dynamics. The observation of hydrogen tunneling may have some parallels with proton tunneling dynamics from tyrosine residues (along the O-H bond of the phenol moiety) in a wide range of natural enzymes, potentially adding further justification for utilizing phenols as model systems for investigating tyrosine-based dynamics.

Tunnelling under a conical intersection: application to the product vibrational state distributions in the UV photodissociation of phenols

When phenol is photoexcited to its S(1) (1(1)ππ∗) state at wavelengths in the range 257.403 ≤ λ(phot) ≤ 275.133 nm the O-H bond dissociates to yield an H atom and a phenoxyl co-product, with the available energy shared between translation and well characterised product vibration. It is accepted that dissociation is enabled by transfer to an S(2) (1(1)πσ∗) state, for which the potential energy surface (PES) is repulsive in the O-H stretch coordinate, R(O-H). This S(2) PES is cut by the S(1) PES near R(O-H) = 1.2 Å and by the S(0) ground state PES near R(O-H) = 2.1 Å, to give two conical intersections (CIs). These have each been invoked-both in theoretical studies and in the interpretation of experimental vibrational activity-but with considerable controversy. This paper revisits the dynamic mechanisms that underlie the photodissociation of phenol and substituted phenols in the light of symmetry restrictions arising from torsional tunnelling degeneracy, which has been neglected hitherto. This places tighter symmetry constraints on the dynamics around the two CIs. The non-rigid molecular symmetry group G(4) necessitates vibronic interactions by a(2) modes to enable coupling at the inner, higher energy (S(1)/S(2)) CI, or by b(1) modes at the outer, lower energy (S(2)/S(0)) CI. The experimental data following excitation through many vibronic levels of the S(1) state of phenol and substituted phenols demonstrate the effective role of the ν(16a) (a(2)) ring torsional mode in enabling O-H bond fission. This requires tunnelling under the S(1)/S(2) CI, with a hindering barrier of ∼5000 cm(-1) and with the associated geometric phase effect. Quantum dynamic calculations using new ab initio PESs provide quantitative justification for this conclusion. The fates of other excited S(1) modes are also rationalised, revealing both spectator modes and intramolecular vibrational redistribution between modes. A common feature in many cases is the observation of an extended, odd-number only, progression in product mode ν(16a) (i.e., the parent mode which enables S(1)/S(2) tunnelling), which we explain as a Franck-Condon consequence of a major change in the active vibration frequency. These comprehensive results serve to confirm the hypothesis that O-H fission following excitation to the S(1) state involves tunnelling under the S(1)/S(2) CI-in accord with conclusions reached from a recent correlation of the excited state lifetimes of phenol (and many substituted phenols) with the corresponding vertical energy gaps between their S(1) and S(2) PESs.

Hole-burning spectra of m-fluorophenol/ammonia (1:3) clusters and their excited state hydrogen transfer dynamics

Hole-burning spectra of m-fluorophenol/ammonia (1:3) clusters are measured by four-color UV-near IR-UV-UV hole-burning spectroscopy. Cis and trans isomers of the cluster are clearly distinguished in the (1:3) cluster. Picosecond time evolutions of the excited state hydrogen transfer (ESHT) reaction in the (1:3) clusters are measured by the ion depletion due to 3p-3s Rydberg transition of reaction products ⋅NH(4)(NH(3))(2) lying in the near infrared region. From the wavelength dependence of the time evolution, we have concluded 1) the initial formation of a metastable ⋅NH(4)-NH(3)-NH(3) radical and 2) successive isomerization to the most stable NH(3)-⋅NH(4) -NH(3) radical in both cis and trans isomers. The reaction lifetimes of ESHT are determined by the rate equation analysis as 32.4 and 31.8 ps for the cis and trans isomer, respectively, and the isomerization and its back-reaction lifetime of both isomers are determined to be 3.3 ps and 11.2 ps. The almost same reaction rates are consistent with the similarity of the hydrogen bond networks in both clusters.