A Strategy for Modeling Nonstatistical Reactivity Effects: Combining Chemical Activation Estimates with a Vibrational Relaxation Model.

The kinetics of many chemical reactions can be readily explained with a statistical approach, for example, using a form of transition state theory and comparing calculated Gibbs energies along the reaction coordinate(s). However, there are cases where this approach fails, notably when the vibrational relaxation of the molecule to its statistical equilibrium occurs on the same time scale as the reaction dynamics, whether it is caused by slow relaxation, a fast reaction, or both. These nonstatistical phenomena are then often explored computationally using (quasi)classical ab initio molecular dynamics by calculating a large number of trajectories while being prone to issues such as zero-point energy leakage.

Excited-state antiaromaticity relief drives facile photoprotonation of carbons in aminobiphenyls.

A combined computational and experimental study reveals that -, - and -aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer. Deuterium exchange experiments show that the -isomer undergoes a facile photoprotonation at a carbon atom excited-state intramolecular proton transfer (ESIPT). The -isomer undergoes water-assisted excited-state proton transfer (ESPT) and a photoredox reaction proton-coupled electron transfer (PCET).

Wavelength-dependent rearrangements of an α-dione chromophore: a chemical pearl in a bis(hypersilyl) oyster.

The symmetric bissilyl-dione 3 reveals two well-separated n → π* absorption bands at = 637 nm ( = 140 mol dm cm) and 317 nm ( = 2460 mol dm cm). Whereas excitation of 3 at = 360/365 nm affords an isolable siloxyketene 4 in excellent yields, irradiation at = 590/630 nm leads to the stereo-selective and quantitative formation of the siloxyrane 5. These remarkable wavelength-dependent rearrangements are based on the electronic and steric properties provided by the hypersilyl groups.

Inequivalent Solvation Effects on the N 1s Levels of Self-Associated Melamine Molecules in Aqueous Solution.

This work shows how the N 1s photoemission (PE) spectrum of self-associated melamine molecules in aqueous solution has been successfully rationalized using an integrated computational approach encompassing classical metadynamics simulations and quantum calculations based on density functional theory (DFT). The first approach allowed us to describe interacting melamine molecules in explicit waters and to identify dimeric configurations based on π-π and/or H-bonding interactions. Then, N 1s binding energies (BEs) and PE spectra were computed at the DFT level for all structures both in the gas phase and in an implicit solvent.

Excited State Intramolecular Proton Transfer (ESIPT) from -NH to the Carbon Atom of a Naphthyl Ring.

Excited state intramolecular proton transfer (ESIPT) has been documented from an amino NH group to a carbon atom of an adjacent aromatic ring. This finding changes the paradigm, as hitherto such processes have not been considered as plausible due to slow protonation of carbon and low (photo)acidity of the NH group. The ESIPT was studied by irradiation of 2-(2-aminophenyl)naphthalene in CHCN-DO, whereupon regiospecific incorporation of deuterium takes place at the naphthalene position 1, with a quantum yield of Φ = 0.

On the propensity of formation of cyclobutane dimers in face-to-face and face-to-back uracil stacks in solution.

UV irradiation of RNA leads to the formation of intra- and inter-strand crosslinks of cyclobutane type. Despite the importance of this reaction, relatively little is known about how the mutual orientation of the two bases affects the outcome of the reaction. Here we report a comparative nonadiabatic molecular dynamics study of face-to-back (F2B) and face-to-face (F2F) stacked uracil-water clusters.

A Computational Approach to Nontraditional Intrinsic Luminescence: Vibrationally Resolved Absorption and Fluorescence Spectra of DABCO.

Recently, so-called "nontraditional intrinsic luminescence" has been reported in several macromolecular systems. Although DABCO (1,4-diazabicyclo[2.2.

Ab Initio Quasiclassical Simulation of Femtosecond Time-Resolved Two-Dimensional Electronic Spectra of Pyrazine.

Two-dimensional (2D) electronic spectroscopy is a powerful nonlinear technique which provides spectroscopic information on two frequency axes as well as dynamical information as a function of the so-called waiting time. Herein, an ab initio theoretical framework for the simulation of electronic 2D spectra has been developed. The method is based on the classical approximation to the doorway-window representation of three-pulse photon-echo signals and the description of nuclear motion by classical trajectories.

Combined Surface-Hopping, Dyson Orbital, and B-Spline Approach for the Computation of Time-Resolved Photoelectron Spectroscopy Signals: The Internal Conversion in Pyrazine.

A computational protocol for simulating time-resolved photoelectron signals of medium-sized molecules is presented. The procedure is based on a trajectory surface-hopping description of the excited-state dynamics and a combined Dyson orbital and multicenter B-spline approach for the computation of cross sections and asymmetry parameters. The accuracy of the procedure has been illustrated for the case of ultrafast internal conversion of gas-phase pyrazine excited to the (ππ*) state.

Simulation of UV absorption spectra and relaxation dynamics of uracil and uracil-water clusters.

Despite many studies, the mechanisms of nonradiative relaxation of uracil in the gas phase and in aqueous solution are still not fully resolved. Here we combine theoretical UV absorption spectroscopy with nonadiabatic dynamics simulations to identify the photophysical mechanisms that can give rise to experimentally observed decay time constants. We first compute and theoretically assign the electronic spectra of uracil using the second-order algebraic-diagrammatic-construction (ADC(2)) method.

Toward Understanding Optical Properties of Amyloids: A Reaction Path and Nonadiabatic Dynamics Study.

Amyloids have unique structural, chemical, and optical properties. Although much theoretical effort has been directed toward understanding amyloid nucleation, the understanding of their optical properties has remained rather limited. In particular, the photophysical mechanisms leading to near-UV excitation and characteristic blue-green luminescence in amyloid systems devoid of aromatic amino acids have not been resolved.

Photoelimination of Nitrogen from Diazoalkanes: Involvement of Higher Excited Singlet States in the Carbene Formation.

Although diazoalkanes are important carbene precursors in organic synthesis, a comprehensive mechanism of photochemical formation of carbenes from diazoalkanes has not been proposed. Synergies of experiments and computations demonstrate the involvement of higher excited singlet states in the photochemistry of diazoalkanes. In all investigated diazoalkanes, excitation to S results in nonreactive internal conversion to S.

UV absorption spectra of DNA bases in the 350-190 nm range: assignment and state specific analysis of solvation effects.

The theoretical assignment of electronic spectra of polyatomic molecules is a challenging problem that requires the specification of the character of a large number of electronic states. We propose a procedure for automatically determining the character of electronic transitions and apply it to the study of UV spectra of DNA bases in the gas phase and in the aqueous environment. The procedure is based on the computation of electronic wave function overlaps and accounts for an extensive sampling of nuclear geometries.

Assessing the performance of trajectory surface hopping methods: Ultrafast internal conversion in pyrazine.

Trajectory surface hopping (TSH) methods have been widely used to study photoinduced nonadiabatic processes. In the present study, nonadiabatic dynamics simulations with the widely used Tully's fewest switches surface hopping (FSSH) algorithm and a Landau-Zener-type TSH (LZSH) algorithm have been performed for the internal conversion dynamics of pyrazine. The accuracy of the two TSH algorithms has been critically evaluated by a direct comparison with exact quantum dynamics calculations for a model of pyrazine.

Photochemistry of 1- and 2-Naphthols and Their Water Clusters: The Role of ππ*(L ) Mediated Hydrogen Transfer to Carbon Atoms.

The computational analysis of the isomer- and conformer-dependent photochemistry of 1- and 2-naphthols and their microsolvated water clusters is motivated by their very different excited state reactivities. We present evidence that 1- and 2-naphthol follow distinct excited state deactivation pathways. The deactivation of 2-naphthols, 2-naphthol water clusters, as well as of the anti conformer of 1-naphthol is mediated by the optically dark πσ* state.

Nonradiative Relaxation Mechanisms of UV Excited Phenylalanine Residues: A Comparative Computational Study.

The present work is directed toward understanding the mechanisms of excited state deactivation in three neutral model peptides containing the phenylalanine residue. The excited state dynamics of theγL(g+)folded form of N-acetylphenylalaninylamide (NAPA B) and its amide-N-methylated derivative (NAPMA B) is reviewed and compared to the dynamics of the monohydrated structure of NAPA (NAPAH). The goal is to unravel how the environment, and in particular solvation, impacts the photodynamics of peptides.

Photoionization of furan from the ground and excited electronic states.

Here we present a comparative computational study of the photoionization of furan from the ground and the two lowest-lying excited electronic states. The study aims to assess the quality of the computational methods currently employed for treating bound and continuum states in photoionization. For the ionization from the ground electronic state, we show that the Dyson orbital approach combined with an accurate solution of the continuum one particle wave functions in a multicenter B-spline basis, at the density functional theory (DFT) level, provides cross sections and asymmetry parameters in excellent agreement with experimental data.

Exploration of Excited State Deactivation Pathways of Adenine Monohydrates.

Binding of a single water molecule has a dramatic effect on the excited state lifetime of adenine. Here we report a joint nonadiabatic dynamics and reaction paths study aimed at understanding the sub-100 fs lifetime of adenine in the monohydrates. Our nonadiabatic dynamics simulations, performed using the ADC(2) electronic structure method, show a shortening of the excited state lifetime in the monohydrates with respect to bare adenine.

How does tetraphenylethylene relax from its excited states?

Tetraphenylethylene is a prototypical example of a molecule displaying aggregation-induced emission. Despite many studies on the optical properties of TPE and its derivatives, the origin of the non-emissive behavior in the gas phase or in dilute solutions has yet to be unravelled. Here, we identify the ultrafast deactivation mechanisms responsible for the fluorescence quenching in isolated TPE.

Timescales of N-H bond dissociation in pyrrole: a nonadiabatic dynamics study.

The excitation wavelength dependent photodynamics of pyrrole are investigated by nonadiabatic trajectory-surface-hopping dynamics simulations based on time dependent density functional theory (TDDFT) and the algebraic diagrammatic construction method to the second order (ADC(2)). The ADC(2) results confirm that the N-H bond dissociation occurring upon excitation at the origin of the first excited state, S1(πσ*), is driven by tunnelling [Roberts et al., Faraday Discuss.

Mechanisms of photostability in kynurenines: a joint electronic-structure and dynamics study.

Kynurenines are UV filters found in the human ocular lens which protect the retina from radiation damage. We report on ab initio investigations of the photochemistry of the cis and trans conformers of kynurenine and of an intramolecularly hydrogen-bonded conformer of 3-hydroxykynurenine O-β-D-glucoside. We have explored the excited-state reaction paths for several radiationless excited-state deactivation processes in kynurenines.

Non-radiative relaxation of UV photoexcited phenylalanine residues: probing the role of conical intersections by chemical substitution.

A conformation-selective photophysics study in phenylalanine model peptides, combining pump-probe gas phase experiments and excited state calculations, highlights for the first time the quenching properties of a primary amide group (through its nπ* excited state) along with the effect of vibrational energy that facilitates access to the conical intersection area.

Unraveling the mechanisms of nonradiative deactivation in model peptides following photoexcitation of a phenylalanine residue.

The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first ππ* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is ∼50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state.

Photoinduced dynamics of formic acid monomers and dimers: the role of the double hydrogen bond.

Nonadiabatic dynamics in the framework of time-dependent density functional theory was used to simulate gas-phase relaxation dynamics of pairs of conformations of formic acid monomers (cis and trans FAM) and dimers (acyclic aFAD and cyclic cFAD). In the early phase of the excited state dynamics, elongation of the C═O bond and pyramidalization of the carbon atom is observed in both FAM and FAD. Subsequently, the photodynamics of FAM is shown to be dominated by fragmentation processes occurring mostly in the excited state and resulting in HCO and OH radicals.

Excited state intramolecular proton transfer (ESIPT) from phenol to carbon in selected phenylnaphthols and naphthylphenols.

ESIPT and solvent-assisted ESPT in isomeric phenyl naphthols and naphthyl phenols 5-8 were investigated by preparative photolyses in CH3CN-D2O, fluorescence spectroscopy, LFP, and ab initio calculations. ESIPT takes place only in 5 (D-exchange Φ = 0.3), whereas 6-8 undergo solvent-assisted PT with much lower efficiencies.