Excited State Potential Surfaces Research Articles

Excited-state potential surfaces of ruthenium nitrosyl complexes: conical intersections and the Jahn-Teller effect

The Jahn-Teller effect in excited states of nitrosyl complexes was analyzed in terms of the density functional theory at the B3LYP level. This effect was shown to play a role in transitions between metastable isomers and their photoactivated relaxation to the ground state. In the compounds under study, the Jahn-Teller effect can be interpreted as the Renner-Teller effect. The characteristics of the Jahn-Teller potential energy surfaces were determined.

Modeling Structures and Spectra of Fluorescent Proteins in the Coordinate-Locking Cluster Approach: Application to the Photoswitchable Protein AsFP595

An interest in the fluorescent protein asFP595 is due to unexplained puzzles in its photophysical behavior. We report the results of calculations of structures, absorption, and emission bands in asFP595 by considering model molecular clusters in the coordinate-locking scheme. Both trans and cis conformations of the anionic chromophore are considered. Equilibrium geometry coordinates on the ground potential energy surface were optimized in the density functional theory approaches by considering both large- and reduced-size clusters. The cluster size was reduced to locate positions of the minimum energy points on the excited-state potential surface by using the configuration interaction singles approach. Vertical excitation energies and oscillator strengths were computed by using the ZINDO method. We show that consideration of large clusters mimicking the protein-containing pocket is an essential issue to calculate positions of absorption and emission bands with the accuracy compatible to experiments.

Trajectory surface-hopping study of methane photodissociation dynamics

We use the fewest switches nonadiabatic trajectory surface hopping approach to study the photodissociation of methane on its lowest singlet excited state potential surface (1 (1)T(2)) at 122 nm, with emphasis on product state branching and energy partitioning. The trajectories and couplings are based on CASSCF(8,9) calculations with an aug-cc-pvdz basis set. We demonstrate that nonadiabatic dynamics is important to describe the dissociation processes. We find that CH(3)(X (2)A(2) ("))+H and CH(2)(a (1)A(1))+H(2) are the major dissociation channels, as have been observed experimentally. CH(3)+H is mostly formed by direct dissociation that is accompanied by hopping to the ground state. CH(2)+H(2) can either be formed by hopping to the ground state to give CH(2)(a (1)A(1))+H(2) or by adiabatic dissociation to CH(2)(b (1)B(1))+H(2). In the latter case, the CH(2)(b (1)B(1)) can then undergo internal conversion to the ground singlet state by Renner-Teller induced hopping. Less important dissociation mechanisms lead to CH(2)+H+H and to CH+H(2)+H. Intersystem crossing effects, which are not included, do not seem essential to describe the experimentally observed branching behavior. About 5% of trajectories involve a roaming atom mechanism which can eventually lead to formation of products in any of the dissociation channels. Branching fractions to give H and H(2) are in good agreement with experiment, and the H atom translational energy distribution shows bimodal character which also matches observations.

Simulations of ICl−(CO2)nPhotodissociation: Effects of Structure, Excited State Charge Flow, and Solvent Dynamics

Experiments on the photodissociation of ICl(-) within size-selected CO(2) clusters [Nadal, M. E.; Kleiber, P. D.; Lineberger, W. C. J. Chem. Phys. 1996, 105, 504.] differ from analogous experiments on homonuclear systems in two major ways: an early onset of caged, ICl(-)-based products with cluster size peaking at n = 5 and a rapid decline of caging after n = 5 in favor of dissociated, Cl(-)-based fragments. Using an effective Hamiltonian model for the solute electronic structure and nonadiabatic MD trajectories, we have simulated the photodissociation of these clusters and found good agreement between experimental and simulated product distributions. The strong preference of the solvent for chlorine in the initial cluster structure, along with favorable kinematics, leads to the efficient caging at small n. At all cluster sizes, the Cl(-)-based fragments are formed by an adiabatic process and are accompanied by spin-orbit excited iodine. Caging of ICl(-) also occurs on the excited state potential surface, and electronic relaxation is not observed during the 100 ps trajectories. For n > 5, the initially caged products appear to be metastable, decaying to solvent-separated Cl(-)...I pairs that subsequently dissociate, leading to the falloff in the caging fraction in the larger clusters.

NeCl2 and ArCl2: Transition from Direct Vibrational Predissociation to Intramolecular Vibrational Relaxation and Electronic Nonadiabatic Effects

Pump-probe results are reported for NeCl(2) excited to the Cl(2) B state, undergoing vibrational predissociation, and then probed via E <-- B transitions. Intensities, lifetimes and product vibrational branching ratios are reported for 16 < or = v' < or = 19 Cl(2) stretching quanta. The intensity of the signal rapidly decreases above v' = 17. Detailed wave packet calculations of the vibrational predissociation dynamics are performed to determine if the experimental results can be explained by the onset of IVR dynamics. The calculations and the experiment are in close accord for low vibrational levels. For higher levels, some, but not all, of the loss of experimental signal can be attributed to IVR. To test whether electronic relaxation dynamics are important for NeCl(2) and ArCl(2), excited state potential surfaces that incorporate spin orbit coupling effects are calculated. These surfaces are then used in a wave packet calculation that includes both vibrational predissociation and electronic predissociation dynamics. The results show that electronic predissociation is important for ArCl(2) levels above v' = 12. For NeCl(2) the calculation suggests that the onset of electronic predissociation should occur for levels as low as v' = 13 but may not contribute markedly to the observed loss of signal above v' = 17. Suggestions are made for further studies of this puzzling problem.

A Benchmark Study of Different Methods for Calculating One- And Two-Dimensional Optical Spectra

The accuracy and robustness of several approximate methods for computing linear and nonlinear optical spectra are considered. The analysis is performed in the context of a benchmark model that consists of a two-state chromophore with shifted harmonic potential surfaces that differ in frequency. The exact one- and two-dimensional spectra for this system are calculated and compared to spectra calculated via the following approximate methods: (1) The semiclassical forward-backward initial-value representation (FB-IVR) method; (2) the linearized semiclassical (LSC) method; (3) the standard second-order cumulant approximation which is based on the ground-state equilibrium frequency-frequency correlation function (2OC); (4) an alternative second-order cumulant approximation which is able to account for nonequilibrium dynamics on the excited-state potential surface (2OCa). All four approximate methods can be shown to reproduce the exact results when the frequencies of the ground and excited harmonic surfaces are identical. However, allowing for the ground and excited surfaces to differ in frequency leads to a more meaningful benchmark model for which none of the four approximate methods is exact. We present a comparison of one- and two-dimensional spectra calculated via the above-mentioned approximate methods to the corresponding exact spectra, as a function of the following parameters: (1) The ratio of excited state to ground-state frequencies; (2) Temperature; (3) The horizontal displacement of the excited-state potential relative to the ground-state potential; (4) The waiting time between the coherence periods in the case of two-dimensional spectra. The FB-IVR method is found to predict spectra which are practically indistinguishable from the exact ones over a wide region of parameter space. The LSC method is found to predict spectra which are in good agreement with the exact ones over the same region of parameter space. The 2OC and 2OCa are found to be highly inaccurate unless the frequencies of the ground and excited states are very similar. These observations give credence to the use of the LSC method for modeling spectra in complex systems, where exact or even FB-IVR-based calculations are prohibitively expensive.

A comparison between different semiclassical approximations for optical response functions in nonpolar liquid solution. II. The signature of excited state dynamics on two-dimensional spectra

Optical response functions are known to reflect quantum dynamics in a superposition state and as such, lack a well-defined classical limit. In a previous paper we considered the importance of accounting for the quantum nature of the dynamics by comparing the linear absorption spectrum and homodyne-detected time-integrated two-pulse photon-echo signal as calculated via the semiclassical forward-backward approach, linearized semiclassical approach, and standard approach which is based on equilibrium ground state dynamics [Shi and Geva, J. Chem. Phys. 122, 064506 (2005)]. In the present paper, we extend the comparison to the case of heterodyne-detected and time-resolved nonlinear time-domain rephasing and nonrephasing signals generated in three-pulse experiments and the corresponding frequency-domain two-dimensional spectra. The comparison is performed in the context of a two-state chromophore solvated in a nonpolar liquid. It is shown that the inherent insensitivity of the standard method to the nonequilibrium dynamics on the excited state potential surface gives rise to two-dimensional spectra which are symmetrical relative to the diagonal. In contrast, accounting for the effect of nonequilibrium excited state dynamics, as is the case within the forward-backward and linearized semiclassical methods, is found to give rise to two-dimensional spectra that become increasingly asymmetrical relative to the diagonal as the waiting time between the second and third pulses becomes larger. It is argued that the emergence of the asymmetry provides a useful probe of nonequilibrium solvation on the excited state potential surface.

Time-resolved study of the photodissociation of Au2O−

Photodissociation of the Au2Oanion following absorption of a 3.1 eV photon is studied using time-resolved photoelectron spectro- scopy. A new spectral feature appearing after 1 ps indicates the existence of a long-lived excited state with a geometry different from that of the ground state. With further increasing delay the pump-probe feature shifts to higher binding energy. At 7 ± 2 ps after excitation no further shift is observed, but now fragmentation into AuO � + Au and Au � + AuO starts with a time constant of 110 ± 3 ps. The dissociation process is accompanied by a decrease of the excited state feature. The spectra indicate that the delayed onset of fragmen- tation is caused by the time the system needs to reach a new minimum on the excited state potential surface. Hence, the time-resolved photoelectron spectra mirror part of the motion of the anion on the excited state potential surface.

Influence of the Jahn‐Teller effect on absorption and photoluminescence spectra of Si nanocrystals

AbstractThe optical properties of H‐passivated silicon nanocrystals are determined by the energetics of the frontier orbitals and their dependence on the deformation in the relaxed excited state. In the present work, the geometry of H‐passivated tetrahedral silicon nanocrystals is optimized with density functional methods, and the deformation in the relaxed excited geometries is obtained from time‐dependent density functional theory. The Jahn‐Teller effect in the lowest excited state results in a distortion towards tetragonal symmetry, contributing significantly to the red shift of the photoluminescence spectra. For nanocrystals up to a diameter of 1.5 nm, we project the deformations at the minima of the excited state potential surface onto the different symmetries, allowing for a discrimination of the respective contributions to the total Stokes shift. For selected systems, the influence of the Jahn‐Teller effect can exceed the red shift related to a symmetryconserving deformation in the relaxed excited state. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Dynamical Calculations of Charge-Transfer-to-Solvent Excited States of Small I-(CH3CN)n Clusters

Relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I(-)(CH(3)CN)(n) (n = 2 and 3) clusters has been theoretically studied using electronic structure methods. First, we have calculated several lowest singlet and triplet potential energy surfaces using the multireference configuration interaction method. It was found that the character of the singlet CTTS excited-state potential surfaces is very similar to that of the triplet CTTS states. Due to a small singlet-triplet splitting, the lowest triplet potential energy surface was used as a good model to understand the dynamics of the photoexcited singlet CTTS states. We have carried out direct molecular dynamics simulations on the lowest triplet surface at the B3LYP level. When an I(-) anion is exteriorly solvated by CH(3)CN molecules, we found that the (CH(3)CN)(n)(-) anion cluster is effectively produced. In addition, when the I(-) anion is placed in the interior in I(-)(CH(3)CN)(n) clusters, photoexcitation gives an acetonitrile monomer anion plus neutral monomers. However, if the initial geometric configuration is distorted from the minimum structure, we also found that the (CH(3)CN)(2)(-) anion cluster, where an excess electron is internally trapped, is formed via I(-)(CH(3)CN)(2) + hnu --> I + (CH(3)CN)(2)(-) process.

Photoinduced Electron Transfer in 2-tert-Butyl-3-(Anthracen-9-yl)-2,3-Diazabicyclo[2.2.2]octane

Intramolecular photoinduced electron transfer from a hydrazine unit to an aromatic group is studied by resonance Raman spectroscopy and electronic absorption spectroscopy. Substituted hydrazine functional groups have played an important role in studies of electron-transfer reactions, photoinduced intramolecular electron transfer, and of mixed valence. A prototypical compound, 2-tert-butyl-3-(anthracen-9-yl)-2,3-diazabicyclo[2.2.2]octane, that has the hydrazine-to-anthracene charge-transfer band in a region of the visible spectrum suitable for detailed resonance Raman spectroscopy is studied in detail. Excitation profiles are obtained, calculated quantitatively by using time-dependent theoretical methods, and interpreted with the assistance of molecular orbital calculations. Excited-state distortions are calculated. The largest distortions occur on the hydrazine unit; the normal mode showing the largest distortion (659 cm(-1), calculated at 665 cm(-1)) involves an out-of-plane C-N-N-C bend consistent with removing an electron from the N-N pi antibonding orbital. Anthracene ring-centered C-C stretches also are enhanced, consistent with populating an antibonding pi orbital centered on the ring. Excellent fits to all of the excitation profiles and to the absorption band are obtained using one set of excited-state potential surfaces.

Potential energy surfaces and dynamics of Ni2+ ion aqueous solution: Molecular dynamics simulation of the electronic absorption spectrum

We develop a model effective Hamiltonian for describing the electronic structures of first-row transition metals in aqueous solutions using a quasidegenerate perturbation theory. All the states consisting of 3d(n) electronic configurations are determined by diagonalizing a small effective Hamiltonian matrix, where various intermolecular interaction terms such as the electrostatic, polarization, exchange, charge transfer, and three-body interactions are effectively incorporated. This model Hamiltonian is applied to constructing the ground and triplet excited states potential energy functions of Ni(2+) in aqueous solution, based on the ab initio multiconfiguration quasidegenerate perturbation theory calculations. We perform molecular dynamics simulation calculations for the ground state of Ni(2+) aqueous solution to calculate the electronic absorption spectral shape as well as the ground state properties. Agreement between the simulation and experimental spectra is satisfactory, indicating that the present model can well describe the Ni(2+) excited state potential surfaces in aqueous solution.

Ab Initio Study of the Photochemistry of c-C2H2Si

The photochemical reaction pathway of silacyclopropenylidene (c-C2H2Si) has been investigated using the eight electrons in eight orbitals complete active space with the 6-311++G(3df,3pd) basis sets. The mechanism of drastic structural change in the reaction of c-C2H2Si and the difference from the reaction of cyclyopropenylidene (c-C3H2) are elucidated. The photochemically active relaxation path of c-C2H2Si on the S1 excited-state potential surface leads to an S1/S0 conical intersection where the photoexcited system decays nonradiatively to S0. The relaxation of c-C2H2Si on the S1 surface causes the cleavage of the Si−C single bond, while that of c-C3H2 causes the cleavage of the C−C double bond. The difference in photochemical cleavage sites is well explained by the difference in the electronic nature of the S1 excited state. In the dark reaction following the relaxation on the S1 surface, hydrogen migration from carbon to silicon occurs together with ring opening at the Si−C bond.

Temperature-dependent behavior of protein-chromophore interactions: a theoretical study of a blue fluorescent antibody.

The unusual temperature-dependent excited-state dynamics of a stilbene-antibody complex reported by Simeonov et al. are explored using theoretical methods. The anomalous temperature-dependent fluorescence emission and lifetime are shown to be the result of interplay among temperature-modulated protein flexibility, the excited-state potential surface for the stilbene central twist, and changes in the stilbene charge distribution upon excitation. Stilbene is found to possess a similar geometry and orientation within the antibody binding pocket at all temperatures in the ground state and at low temperatures (approximately 240 K) in the excited state. At higher temperatures (approximately 260 K), the excited-state conformation twists around the central double bond and adopts an alternate orientation within the binding pocket. These changes result from protein side chain and loop motions that are frozen out at lower temperatures and account for the observed red shift of the fluorescence emission spectrum (a calculated shift of 3.8 kcal mol-1 compares favorably with the approximately 5 kcal.mol-1 observed experimentally). Approximately 3.0 kcal mol-1 of this stabilization is global in nature and is not attributable to specific local interactions. Local interactions between stilbene and Tyr-B39 contribute approximately 0.8 kcal mol-1 to the fluorescence shift. The primary structural change in simulations of the high temperature excited state involves a decrease in the stilbene-tyrosine distance and a relative change in orientation of the aromatic rings. We identify several nearby charged residues that contribute to the fluorescence emission shift and provide targets for mutagenesis to probe the temperature-dependent dynamics of protein-chromophore interactions.

Non-Adiabatic Direct Dynamics Study of Chromium Hexacarbonyl Photodissociation

The photodissociation of Cr(CO)6 has been studied using direct dynamics including trajectory surface hopping. The dissociation takes place on a steep excited-state potential surface leading to ejection of a CO ligand within 100 fs in reasonable agreement with time-resolved studies by Fuβ et al.1-3 Our dynamics studies of the Cr(CO)5 photoproduct allow us to determine mechanistic details of the relaxation of the excited state of this species. We find that Cr(CO)5 decays to the ground state via a Jahn−Teller (symmetry-induced) conical intersection, at a trigonal bipyramid geometry. The molecule then oscillates in the well of the “moat” surrounding this conical intersection. Our results provide evidence for the dissociation mechanism proposed by Fuβ et al.1-3 Both the computed frequency of the coherent oscillation and the delay time until this motion is observed are in good agreement with experimental results.

Photolysis of Nitrous Oxide Isotopomers Studied by Time-Dependent Hermite Propagation

Nitrous oxide (N2O) plays an important role in greenhouse warming and ozone depletion. Yung and Miller's zero point energy (ZPE) model for the photolysis of N2O isotopomers was able to explain atmospheric isotopomer distributions without invoking in situ chemical sources. Subsequent experiments showed enrichment factors twice those predicted by the ZPE model. In this article we calculate the UV spectrum of the key N2O isotopomers to quantify the influence of factors not included in the ZPE model, namely, the transition dipole surface, bending vibrational excitation, dynamics on the excited state potential surface, and factors related to isotopic substitution itself. The relative cross-sections are calculated as the Fourier transform of the correlation function of the initial vibrational wave function and the time-propagated wave function, using a Hermite expansion of the time evolution operator. The model uses the electronic structure data recently published by Balint-Kurti and co-workers and makes several predictions. (a) The absolute values of the enrichment factors decrease with increasing temperature. (b) Photolysis of N2O will produce “mass-independent” enrichment in the remaining sample. (c) Much of the enrichment is due to decreased heavy isotopomer cross-section over the entire absorption band, in contrast to the wavelength shift predicted by the ZPE model. Consequently, to within the error of the calculation, we predict only minor enrichments at λ < 182 nm. The smaller bending excursion of heavy isotopomers combines with the transition dipole surface to produce a smaller integrated cross-section. This effect is partially countered by the larger fraction of heavy isotopomers in excited bending states; the first three bending states have an integrated intensity ratio of ca. 1:3:6. The model agrees with available experimental enrichment factors and stratospheric balloon infrared remote sensing data to within the estimated error.

Nonequilibrium initial conditions of a Brownian oscillator system observed by two-dimensional spectroscopy

We study effects of a nonequilibrium initial condition of a Brownian oscillator system upon two-, three-, and four-time correlation functions of an oscillator coordinate as a subject of multidimensional spectroscopy. A nonequilibrium initial condition is set by a displacement of a Gaussian wave packet in an oscillator potential. Such situation may be found in a vibrational motion of molecules after a sudden bond breaking between a fragmental molecule and a targeting vibrational system or a movement of wave packet in an electronic excited state potential surface created by a laser pump pulse. Multitime correlation functions of oscillator coordinates for a nonequilibrium initial condition are calculated analytically with the use of generating functional from a path integral approach. Two-, three-, and four-time correlation functions of oscillator coordinates correspond to the third-, fifth-, and seventh-order Raman signals or the first-, second-, and third-order infrared signals. We plotted these correlation functions as a signal in multidimensional spectroscopy. The profile of the signal depends on the initial position and momentum of the wave packet in the fifth- and seventh-order Raman or the second and third order infrared measurement, which makes it possible to measure the dynamics of the wave packet directly in the phase space by optical means.

Photoinduced Electron Attachment to N2 Adsorbed on Platinum

Theoretical calculations on ground and excited electronic states of N2 adsorbed on Pt(100) are carried out to investigate the properties of photoinduced electron attachment states. The metal−adsorbate system is described by an embedded cluster method, and configuration interaction theory is used to resolve the electronic states. The N2 molecule is found to adsorb at an atop Pt atom site with an adsorption energy of 0.18 eV and with a vibrational frequency 70 cm-1 lower than in the gas phase. Only the perpendicular orientation is stable. An electron attachment state corresponding to excitation of an electron from the Fermi level of the metal to a π* orbital of N2 is found to occur at a vertical excitation energy of 4.2 eV (triplet state) and 4.3 eV (singlet state). The NN internuclear distance increases in the excited state and this leads to a range of excitation energies, from 3.6 eV to the minimum of the excited triplet state potential surface to 4.2 eV for the vertical excitation. The corresponding range for singlet states is 3.8 to 4.3 eV. The equilibrium distance of nitrogen from the surface is 2.09 Å for the ground state and 1.96 Å for the electron attachment state. Deexcitation from the excited state to the repulsive region of the ground-state potential can lead to desorption from the surface. Nitrogen does not desorb or dissociate in the excited electronic state.

Symmetry breaking effects in NO3−: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate–water complexes

Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenerate in the isolated D3h ion) and the resonance Raman intensity in this mode, suggesting that bulk solvent polarization enhances the asymmetry of the local environment for NO3− in water. Comparison of the ground-state vibrational frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures suggests that the rms difference among the three N–O bond lengths for nitrate anion in water probably exceeds 0.01 Å.

AM1 semiempirical calculated potential energy surfaces for the isomerization of symmetrical carbocyanines

Semiemipirical ground state (S 0) and first excited singlet state (S 1) potential surfaces were calculated for the isomerization of symmetrical carbocyanines such as 3,3′-diethylthiadicarbocyanine iodide (DTDCI), 1,1′-diethyl-2,2′-dicarbocyanine iodide (DDI), 1,1′-diethyl-2,2′-carbocyanine iodide (DCI), and cryptocyanine (1,1′-diethyl-4,4′-carbocyanine) iodide (CCI), as a function of the twist angle around a carbon–carbon bond of the polymethine chain. It was found that the isomerization potential surface depends on the polymethine chain length and its position on the quinoline moiety. The results of this study also indicate that steric hindrance associated with the rotation around a carbon–carbon bond plays an important role in the isomerization dynamics of cyanines, which correlates with the photoisomer lifetime that was measured from picosecond time-resolved fluorescence spectra.