Proton Transfer Reaction Coordinate Research Articles

Ultrafast proton transfer dynamics of 2-(2′-hydroxyphenyl)benzoxazole dye in different solvents

The excited-state intramolecular proton transfer of 2-(2′-hydroxyphenyl)benzoxazole dye in different solvents is investigated using ultrafast femtosecond transient absorption spectroscopy combined with quantum chemical calculations. Conformational conversion from the syn-enol configuration to the keto configuration is proposed as the mechanism of excited-state intramolecular proton transfer. The duration of excited-state intramolecular proton transfer is measured to range from 50 fs to 200 fs in different solvents. This time is strongly dependent on the calculated energy gap between the N-S0 and T-S1 structures in the S1 state. Along the proton transfer reaction coordinate, the vibrational relaxation process on the S1 state potential surface is observed. The duration of the vibrational relaxation process is determined to be from 8.7 ps to 35 ps dependent on the excess vibrational energy.

Reactive molecular dynamics models from ab initio molecular dynamics data using relative entropy minimization

We present two new multiscale molecular dynamics (MS-RMD) models for the hydrated excess proton in water developed directly from ab initio molecular dynamics (AIMD) simulation data of the same system. The potential of mean force along the proton transfer reaction coordinate and radial distribution functions for the MS-RMD models are shown to faithfully reproduce those of AIMD. The models are developed using an algorithm based on relative entropy minimization, thus demonstrating the ability of the method to rapidly generate accurate and highly efficient reactive MD force fields.

Multistate density function theory and the construction of diabatic and adiabatic potential energy surfaces

A multistate density function theory( MSDFT) based on valence bond theory was introduced. As an application,the MSDFT method was illustrated by the bond dissociation of H2 and the proton transfer between HNO3 and a water molecule. In the dissociation of H2,the MSDFT method yields a correct behavior as the two H atoms stretch to infinity,and gives a potential well in accord with second-order perturbation using complete active space( CASPT2). For the proton transfer process of HNO3,MSDFT can be used to yield both diabatic and adiabatic potential energy curves as a function of the proton transfer reaction coordinate. For the reaction barrier height,the inclusion of an ionic state in a three-state model can significantly improve the accuracy in barrier height in comparison with the high-level results.

A theoretical exploration of the nonradiative deactivation of hydrogen-bond complexes: isoindole–pyridine and quinoline–pyrrole

CC2 potential energy profiles of the ground and excited states of the isoindole–pyridine complex along the proton transfer reaction coordinate are studied.

Intramolecular excited proton transfer of 1-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol – A combined experimental and quantum chemical studies

Synthesis of the 1-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol [PPIN] is reported, spectral and fluorescent properties of the title compound are investigated. The feasibility of excited state intramolecular proton transfer (ESIPT) has been argued from the changeover of relative stability of the enol and the keto forms on photoexcitation from the S0-PES to the S1-PES. Critical evaluation on the modulations of geometrical parameters other than the proton transfer reaction coordinate has also been undertaken. The intramolecular hydrogen bonding (IMHB) interaction in PPIN has been explored by calculation of the hyperconjugative charge transfer interaction from the lone electron pair of ring nitrogen atom to the σ∗ orbital of O–H bond, under the provision of Natural Bond Orbital (NBO) analysis. However, DFT calculations together with the experimental results reveal that the excited species with the intramolecular N⋯HO hydrogen bond type undergoes rapid radiationless deactivation. This leads to a conclusion that the low-intensity dual-band fluorescence of the title compound in solution originates from the traces of the conformation with the –OH bond to azomethine nitrogen atom (ESIPT).

Heavy-Enzyme Kinetic Isotope Effects on Proton Transfer in Alanine Racemase

The catalytic effects of perdeuterating the pyridoxal phosphate-dependent enzyme alanine racemase from Geobacillus stearothermophilus are reported. The mass of the heavy perdeuterated form is ~5.5% greater than that of the protiated form, causing kinetic isotope effects (KIEs) of ~1.3 on k(cat) and k(cat)/K(M) for both L- and D-alanine. These values increase when Cα-deuterated alanine is used as the substrate. The heavy-enzyme KIEs of ~3 on k(cat)/K(M) with deuterated substrates are greater than the product of the individual heavy-enzyme and primary substrate KIEs. This breakdown of the rule of the geometric mean is likely due to coupled motion between the protein and the proton-transfer reaction coordinate in the rate-limiting step. These data implicate a direct role for protein vibrational motions in barrier crossing for proton-transfer steps in alanine racemase.

Ionization of dimethyluracil dimers leads to facile proton transfer in the absence of hydrogen bonds

Proton transfer is ubiquitous in chemistry and biology, occurring, for example, in proteins, enzyme reactions and across proton channels and pumps. However, it has always been described in the context of hydrogen-bonding networks ('proton wires') acting as proton conduits. Here, we report efficient intramolecular ionization-induced proton transfer across a 1,3-dimethyluracil dimer, a model π-stacked system with no hydrogen bonds. Upon photoionization by tunable vacuum ultraviolet synchrotron radiation, the dimethyluracil dimer undergoes proton transfer and dissociates to produce a protonated monomer. Deuterated dimethyluracil experiments confirm that proton transfer occurs from the methyl groups and not from the aromatic C-H sites. Calculations reveal qualitative differences between the proton transfer reaction coordinate in the π-stacked and hydrogen-bonded base pairs, and that proton transfer in methylated dimers involves significant rearrangements of the two fragments, facilitating a relatively low potential energy barrier of only 0.6 eV in the ionized dimer.

Sampling the Proton Transfer Reaction Coordinate in Mixed Quantum-Classical Molecular Dynamics Simulations

An umbrella sampling approach based on the vibrational energy gap is presented and examined for exploring the reaction coordinate for a proton transfer (PT) reaction. The technique exploits the fact that for a PT reaction the energy gap between the vibrational ground and excited states of the transferring proton reaches a minimum at the transition state. Umbrella sampling is used within mixed quantum-classical simulations to identify the transition state configurations and explore the reaction free energy curve and vibrationally nonadiabatic coupling. The method is illustrated by application to a model phenol-amine proton transfer reaction complex in a nanoconfined solvent. The results from this new umbrella sampling approach are consistent with those obtained from previous umbrella sampling calculations based on a collective solvent coordinate. This sampling approach further provides insight into the vibrationally nonadiabatic coupling for the proton transfer reaction and has potential for simulating vibrational spectra of PT reaction complexes in solution.

Excited state intramolecular proton transfer in 1-(trifluoroacetylamino)naphthaquinone: a CASPT2//CASSCF computational study†

The excited state intramolecular proton transfer (ESIPT) in 1-(trifluoroacetylamino)-naphthaquinone (TFNQ) has been investigated using the CASSCF and CASPT2 computational approaches with the 6-31G(d) basis set. The structures and relative energies of critical points along the proton transfer reaction coordinate were optimized and the associated spectroscopic and electrostatic properties obtained. Combined quantum mechanical and molecular mechanical (QM/MM) Monte Carlo simulations were performed to elucidate solvent effects on the vertical excitation S0 → S1. It was found that the ESIPT reaction is a barrierless process that takes place on a very flat potential energy surface (PES) and the tautomeric structure of the reaction product is the only minimum on the excited state surface. The PES for both the ground and excited state from accurate electronic structure calcualtions will be used to parameterize empirical force fields in subsequent molecular dynamics simulations of the reaction in solution. † This paper is dedicated to Professor Michael Robb on the occasion of his 60th birthday.

Proton motion in malonaldehyde: an ab initio molecular dynamics study

A Projector Augmented Wave (PAW) molecular dynamics study of the proton motion and proton transfer in malonaldehyde is presented. Molecular dynamics runs of malonaldehyde were performed at several temperatures between 100 K and 600 K. At elevated temperatures proton transfer is not associated with a well-defined C 2v symmetric transition state, but takes place at largely differing geometric situations. A short Otctdot;O distance is highly favourable for proton transfer to occur, but it is neither a sufficient nor a necessary criterion and a proper description requires many, if not all, 21 internal degrees of freedom. The energetics of the proton motion have been modelled by calculating potential energies along a properly chosen reaction coordinate for single time steps within a ‘heavy-light-heavy’ approximation. Four different extreme situations can reasonably well be distinguished: (1) ‘normal periods’, where the proton remains firmly located at one oxygen, and which are characterized by an approximately constant, strongly asymmetric single-minimum potential; (2) ‘isolated proton transfer transitions’, which start and end with strongly asymmetric single-minimum potentials, but pass through (near-) symmetric double- or single-minimum potentials; (3) ‘proton shuttling periods’, which include several consecutive transitions and correspond to a stationary proton motion within a symmetric single-minimum potential that remains approximately constant for a longer period of time; and (4) ‘near-transitions’ where the proton moves towards the opposite oxygen but due to kinetic reasons turns back again. It is shown that the proton motion is governed by both the potential energy along an appropriately chosen proton transfer reaction coordinate and by kinetic effects.

Ab initio study of the potential energy functions relevant for hydrogen transfer in formamide, its dimer and its complex with water

The potential energy (PE) functions of the electronic ground state and the lowest nπ ∗ and ππ ∗ excited singlet states of formamide, its dimer and its complex with water have been characterized theoretically along the proton transfer (PT) reaction coordinate. The calculations were performed using the ab initio complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory employing the CASSCF wavefunction as the reference state. The results suggest that the photoinduced PT reaction in an isolated molecule occurs via successive dissociation and association of the hydrogen atom, whereas in hydrogen-bonded complexes, the PE functions along the PT reaction coordinate are qualitatively the same as for the PT reaction along the intramolecular hydrogen bond. Some implications concerning biological molecules containing peptide linkages are discussed.

Theoretical investigation of potential energy surfaces relevant for excited-state hydrogen transfer in o-hydroxybenzaldehyde

The potential energy functions of the electronic ground state as well as the lowest n π * and ππ * excited single states of o-hydroxybenzaldedhyde (OHBA) have been theoretically characterized along the proton transfer (PT) reaction coridnate as well as the reaction coordinate leading to the prefulvenic form. The calculations have been performed with the ab initio complete-active-space self-consistent-field (CASSCF) method and with second-order perturbation theory, employing the CASSCF wave function as the reference state. It is found that the 1ππ * state is almost isoenergetic with the 1nπ * state after vertical excitation of OHBA. The 1ππ * potential energy function is found to be barrierless along the PT reaction coordinate, while the 1nπ * potential energy exhibits a significant barrier. An efficient non-radiative pathway has been identified resulting from strong non-adiabatic interactions between the ground and the excited singlet states along the reaction coordinate to the prefulvenic form of OHBA. It is argued that the consideration of multi-dimensional vibronic interactions is essential for the understanding of the excited state dynamics of OHBA.

Evidence for the need of a non-Born—Oppenheimer description of excited-state hydrogen transfer

The HOCHCHCHNH (enol from) to OCHCHCHNH 2 (keto form) excited-state isomerization of 1-amino-3-propenal is considered as a model system for photo-induced intramolecular hydrogen (or proton) transfer. The potential energy surfaces of the electronic ground state as well as of the lowest nπ* and ππ* excited singlet states have been characterized by ab initio compelte-active-space self-consistent-field and multireference configuration-interaction calculations. It is found that the nπ* potential energy function exhibits a significant barrier along the proton-transfer reaction coordinate, while the ππ* potential energy function is almost barrierless and intersects the nπ* surface, providing a pathway for ultrafast photo-induced proton transfer. It is argued that consideration of nπ*ππ* vibronic coupling is essential in understanding the photo-induced proton-transfer reaction.