Doo-Sik Ahn

Dynamic paths between neutral alanine–water and zwitterionic alanine–water clusters: single, double and triple proton transfer

Abstract Computations are presented for the alanine–(H 2 O) n ( n =1–3) and its zwitterion–(H 2 O) n clusters. We find that at least two water molecules need to bind to alanine to give stable alanine zwitterion–water cluster. Structures of the conformers are predicted, and their relative energies are compared. Detailed analysis is presented on the dynamic (proton transfer) pathways between the neutral alanine–(H 2 O) n and the zwitterionic alanine–(H 2 O) n ( n =2, 3) clusters, including the structures of the transition states. A variety of proton transfer pathways are predicted between alanine–(H 2 O) n and the zwitterion–(H 2 O) n clusters, depending on their structures: direct proton transfer, concerted double and triple proton transfer mechanism.

State-selective predissociation dynamics of methylamines: The vibronic and H∕D effects on the conical intersection dynamics

The photodissociation dynamics of methylamines (CH(3)NH(2) and CD(3)ND(2)) on the first electronically excited state has been investigated using the velocity map ion imaging technique probing the H or D fragment. Two distinct velocity components are found in the H(D) translational energy distribution, implying the existence of two different reaction pathways for the bond dissociation. The high H(D) velocity component with the small internal energy of the radical fragment is ascribed to the N-H(D) fragmentation via the coupling of S(1) to the upper-lying S(2) repulsive potential energy surface along the N-H(D) bond elongation axis. Dissociation on the ground S(0) state prepared via the nonadiabatic dynamics at the conical intersection should be responsible for the slow H(D) fragment. Several S(1) vibronic states of methylamines including the zero-point level and nnu(9) states (n=1, 2, or 3) are exclusively chosen in order to explore the effect of the initial quantum content on the chemical reaction dynamics. The branching ratio of the fast and slow components is found to be sensitive to the initial vibronic state for the N-H bond dissociation of CH(3)NH(2), whereas it is little affected in the N-D dissociation event of CD(3)ND(2). The fast component is found to be more dominant in the translational distribution of D from CD(3)ND(2) than it is in that of H from CH(3)NH(2). The experimental result is discussed with a plausible mechanism of the conical intersection dynamics.

On the stability of glycine-water clusters with excess electron: Implications for photoelectron spectroscopy

Calculations are presented for the glycine-(H(2)O)(n) (-) (n=0-2) anionic clusters with excess electron, with the glycine core in the canonical or zwitterion form. A variety of conformers are predicted, and their relative energy is examined to estimate thermodynamic stability. The dynamic (proton transfer) pathways between the anionic clusters with the canonical and the zwitterion glycine core are examined. Small barrier heights for isomerization from the zwitterion glycine-(H(2)O)(2) (-) anion to those with canonical glycine core suggest that the former conformers may be kinetically unstable and unfavorable for detection of neutral glycine zwitterion-(H(2)O)(n) (n=1,2) clusters by photodetachment, in accordance with the photoelectron spectroscopic experiments by Bowen and co-workers [Xu et al., J. Chem. Phys. 119, 10696 (2003)]. The calculated stability of the glycine-(H(2)O)(n) (-) anion clusters with canonical glycine core relative to those with zwitterion core indicates that the observation of the anionic conformers with the canonical glycine core would be much more feasible, as revealed by Johnson and co-workers [Diken et al. J. Chem. Phys. 120, 9902 (2004)].

Mode‐Dependent Fano Resonances Observed in the Predissociation of Diazirine in the S1 State

Resonance in chemical reactions is a very intriguing quantum mechanical phenomenon and provides essential information about the detailed shape of the potential-energy surfaces especially near the nuclear configuration that is critical for dynamical constraints. Fano-type resonance is particularly interesting since it results from the interference of two distinct reactive pathways leading to the same product channel. When an atom is excited to a super-excited state above the ionization threshold energy, and for instance, the isoenergetic continuum is coherently excited at the same time, this results in the interference of direct and indirect ionization processes to give the asymmetric line shape in the ionization cross section. Fano resonance has often been found in the ionization cross section of atoms, whereas its observation in the molecular systems is very rare, especially in gas-phase dynamics. Only a few cases have been reported to date in vibrational autoionization of H3 [4] or predissociation of H2, [5] Cs2, [6] and FNO. The multidimensionality of the internal coordinate of polyatomic molecules generally makes the observation of the Fano resonance more difficult since the coherent excitation of bound and continuum states is less probable when the number of internal degrees of freedom increases. The intrinsic spectral congestion of polyatomic molecules may also hamper the experimental observation of asymmetric resonance. Consequently, Fano resonances in polyatomic systems have rarely been interrogated in chemical reactions. Herein, we report the observation of Fano profiles in the partial photodissociation cross section of the diazirine. The yield of the nascent CH2(B̃) fragment from the S1 state of diazirine has been monitored as a function of the excitation energy by detecting the total fluorescence emitting in the CH2(B̃) to CH2(ffi) transition. All of the Franck–Condon active bands show asymmetric line shapes, giving the modedependent dynamic parameters, such as homogeneous line width, center of resonance frequency, and the asymmetry parameter. The symmetric and asymmetric C N stretching modes exhibit quite different Fano profiles, giving qualitative and yet essential information about the shape of the potentialenergy surface in the vicinity of the transition state. Especially since the interference in the reaction pathways could be further utilized to control the outcomes of chemical reactions, the experimental finding of the Fano resonance in a polyatomic molecule may open the possibility of controlling the reaction of complicated molecular systems in multidimensional coordinates through the manipulation of the phase involved in the excitation process. Diazirine (CH2N2), as a clean source of carbene and a precursor of diazomethane, has been intensively studied theoretically. However because of the lack of experimental studies, even the structure and dephasing mechanism of diazirine in the first electronically excited state (S1) are still the subject of controversy. The only molecular-beam experimental work on diazirine (S1) is the report that the excited singlet state of methylene, CH2(B̃), is produced from the S1–S0 origin band of diazirine. [10] In our current investigation, the total emission from CH2(B̃) was monitored as a function of the pump energy to give the photofragment excitation (PHOFEX) spectrum for the S1–S0 transition of diazirine. The PHOFEX signal reflects the energy-dependent absorption cross section multiplied by the yield of the CH2(B̃) fragment, representing the partial reaction cross section. The S1–S0 spectral origin was found at 30967 cm , Figure 1. All the spectral bands are found to be broad and a series of the progression bands with a fundamental frequency of 802 cm 1 is strongly observed. Remarkably, all the S1 vibronic bands show asymmetric line shapes, indicating that quantum interferences should be involved in the reaction pathway leading to the generation of CH2(B̃). The asymmetric line shape most likely indicates that the bound and continuum states are coherently excited in the S1– S0 transition. Therefore, the Fano profile function is used for the reproduction of the experiment is given by Equation (1), see Figure 1.

Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine

Predissociation dynamics of methylamines (CH(3)NH(2) and CH(3)ND(2)) on the first electronically excited states are studied using the slow-electron velocity imaging method to unravel the multi-dimensional nature of the N-H(D) chemical bond dissociation reaction which occurs via tunnelling. The nearly free internal rotation around the C-N bond axis is found to be strongly coupled to the reaction pathway, revealing nuclear motions actively involved in the tunnelling process on the S(1) potential energy surfaces. The vibrational state-resolved energy and angular distributions of photoelectron, ejected from the ionization mediated by the metastable intermediate S(1) state provide a unique way for mapping the predissociative potential energy surfaces.

Structure and dynamic role of conical intersections in the πσ*-mediated photodissociation reactions

Conical intersection as a dynamic funnel in nonadiabatic transition dictates many important chemical reaction outputs such as reaction rates, yields, and energy disposals especially for chemical reactions taking place on electronically excited states. Therefore, the energetics and topology of conical intersections have been subjected to intensive theoretical and experimental studies for decades as these things are the keys to understanding and controlling nonadiabatic transitions which are ubiquitous in nature. In this article, we focus on πσ*-mediated photodissociation reactions of thiophenols and thioanisoles. Interestingly, for these chemical systems, the nonadiabatic transition probability can be precisely measured as a function of the excitation energy, giving a great opportunity for spectroscopic characterization of the multi-dimensional conical intersection seam that governs the nonadiabatic transition dynamics of polyatomic molecules. The passage of the reactive flux in the proximity of the conical intersection gives rise to dynamic resonances corresponding to dramatic state-specific increases of the nonadiabatic transition probability. Accordingly, it is found that the electronic and nuclear configurations of the reactive flux and their evolution, coupled to the conical intersection seam, are critical in nonadiabatic transition dynamics. Nonadiabaticity is found to be extremely sensitive to the conformational molecular structure, and this has been demonstrated in the photodissociation dynamics of the chemical derivatives of thiophenol. Intramolecular vibrational redistribution, which is nontrivial in surmounting the reaction barrier, is found to wash out state-specific dynamic resonances, implying the importance of the dynamic interplay between vibrational energy flow and nonadiabatic transition. The experimental results on conical intersection dynamics presented in this review provide many interesting and important issues to be pursued in the near future by both theoreticians and experimentalists.

Conformationally Specific Vacuum Ultraviolet Mass-Analyzed Threshold Ionization Spectroscopy of Alkanethiols: Structure and Ionization of Conformational Isomers of Ethanethiol, Isopropanethiol, 1-Propanethiol, tert-Butanethiol, and 1-Butanethiol

Conformational isomers of alkanethiols are isolated in the molecular beam, and the conformer-specific ionization dynamics have been investigated using vacuum ultraviolet mass-analyzed threshold ionization (MATI) spectroscopy. Only a single conformer of ethanethiol is observed to give the adiabatic ionization potential (IP) of 9.2922 +/- 0.0007 eV for the gauche conformer. For isopropanethiol, IP is found to be 9.1426 +/- 0.0006 for the trans conformer and 9.1559 +/- 0.0006 eV for the gauche conformer. Only two major conformational isomers are identified for 1-propanethiol, giving an IP of 9.1952 +/- 0.0006 for the trans-gauche conformer and 9.2008 +/- 0.0006 eV for the gauche-gauche conformer. The tert-butanethiol, as expected, has a single conformer with an IP of 9.0294 +/- 0.0006 eV. For 1-butanethiol, there are a number of conformers, and the assignment of the MATI bands to each conformer turns out to be nontrivial. The spectral simulation using the Franck-Condon analysis based on the density functional theory (DFT) calculations has been used for the identification of each conformational isomer in the MATI spectrum. Each conformer undergoes its unique structural change upon ionization, as revealed in the vibration resolved MATI spectrum, providing the powerful method for the spectral identification of a specific conformational isomer. The conformer specificity in the ionization-driven structural change reflects the role of the electron of the highest occupied molecular orbital (HOMO) in the conformational preference.

Ionization spectroscopy of conformational isomers of propanal: the origin of the conformational preference.

Two different conformational isomers of propanal, cis and gauche, are investigated by the vacuum-UV mass-analyzed threshold ionization (VUV-MATI) spectroscopy to give accurate adiabatic ionization potentials of 9.9997 +/- 0.0006 eV and 9.9516 +/- 0.0006 eV, respectively. cis-Propanal, which is the more stable conformer in the neutral state, becomes less stable in the cation compared to gauche-propanal. Vibrational structures revealed in the MATI spectra indicate that cis and gauche isomers undergo their unique structural changes upon ionization. The ionization of gauche-propanal induces a geometrical change along the conformational coordinate, suggesting that the steric effect in the ground state is diminished upon ionization. Natural bonding orbital (NBO) calculations provide the extent of hyperconjugation in each conformational isomer of propanal.

Clustering Dynamics of the Metal−Benzene Sandwich Complex: The Role of Microscopic Structure of the Solute In the Bis(η6-benzene)chromium ·Arn Clusters (n = 1−15)

Ar clustering dynamics around the metal-benzene sandwich complex, bis(eta (6)-benzene)chromium: Cr(Bz) 2, is found to occur in two distinct regimes. The shift of the ionization potential (IP) upon the addition of Ar is measured to be 151 cm (-1), and it is constant until the number of Ar solvents ( n) becomes 6. The IP shift per Ar is found to be suddenly decreased to 82 cm (-1) for the clusters of n = 7-12. The cluster distribution indicates that the n = 6 cluster is most populated in the molecular beam. These experimental findings with the aid of ab initio calculation indicate that the first six Ar solvent molecules are attached to top and bottom of Cr(Bz) 2 to give the robust structure for the Cr(Bz) 2-Ar 6 cluster whereas the next six Ar molecules are gathered on the side of the solute core to give the highly symmetric structure of the Cr(Bz) 2-Ar 12 cluster.

A highly conformationally specific α- and β-Ala+ decarboxylation pathway

One-photon ionization of alanine and beta-alanine induces the decarboxylation reaction which occurs with the concomitant intramolecular hydrogen transfer in a highly conformationally specific manner.