Ground State Potential Energy Surface Research Articles

Path-integral molecular dynamics simulations of hydrated hydrogen chloride cluster HCl(H 2O) 4 on a semiempirical potential energy surface

Path-integral molecular dynamics simulations for the HCl(H 2O) 4 cluster have been performed on the ground-state potential energy surface directly obtained on-the-fly from semiempirical PM3-MAIS molecular orbital calculations. It is found that the HCl(H 2O) 4 cluster has structural rearrangement above the temperature of 300 K showing a liquid-like behavior. Quantum mechanical fluctuation of hydrogen nuclei plays a significant role in structural arrangement processes in this cluster.

Nuclear Quantum Tunneling in the Light-activated Enzyme Protochlorophyllide Oxidoreductase

In chlorophyll biosynthesis, the light-activated enzyme protochlorophyllide oxidoreductase catalyzes trans addition of hydrogen across the C-17-C-18 double bond of the chlorophyll precursor protochlorophyllide (Pchlide). This unique light-driven reaction plays a key role in the assembly of the photosynthetic apparatus, but despite its biological importance, the mechanism of light-activated catalysis is unknown. In this study, we show that Pchlide reduction occurs by dynamically coupled nuclear quantum tunneling of a hydride anion followed by a proton on the microsecond time scale in the Pchlide excited and ground states, respectively. We demonstrate the need for fast dynamic searches to form degenerate "tunneling-ready" configurations within the lifetime of the Pchlide excited state from which hydride transfer occurs. Moreover, we have found a breakpoint at -27 degrees C in the temperature dependence of the hydride transfer rate, which suggests that motions/vibrations that are important for promoting light-activated hydride tunneling are quenched below -27 degrees C. We observed no such breakpoint for the proton-tunneling reaction, indicating a reliance on different promoting modes for this reaction in the enzyme-substrate complex. Our studies indicate that the overall photoreduction of Pchlide is endothermic and that rapid dynamic searches are required to form distinct tunneling-ready configurations within the lifetime of the photoexcited state. Consequently, we have established the first important link between photochemical and nuclear quantum tunneling reactions, linked to protein dynamics, in a biologically significant system.

Photoinduced dissociation of water adsorbed on a Ag cluster

Theoretical electronic structure calculations are reported for the photoinduced dissociation of water adsorbed on a 31-atom silver cluster, Ag 31 to produce H 2 and adsorbed O. Both ground and excited electronic state processes are considered. The excited electronic state of interest is formed by photoemission of an electron from the Ag cluster and transient attachment of the electron to the adsorbed water molecule. A very large energy barrier for H 2 formation (3.51 eV) is found for the ground state process while the barrier in the excited state is much smaller (0.47 eV). In the excited state, partial occupancy of an OH antibonding orbital facilitates OH stretch and the electron hole in the metal cluster stabilizes OH and O adsorbates. The excited state pathway for dissociation of water begins with the formation of an excited electronic state at 3.49 eV. Stretch of the OH bond up to 3.44 a.u. (1.82 Å) occurs with little change in energy. In this region of OH stretch the molecule must return to the ground state potential energy surface to dissociate fully and to form H 2. Geometry optimizations are carried out using a simplex algorithm which allows the total energy to be calculated using configuration interaction theory.

Theoretical treatment of anharmonic effect on molecular absorption, fluorescence spectra, and electron transfer

It is well-known that the mirror image between absorption and fluorescence spectra is held for the displaced harmonic-oscillator system, and also this mirror image is independent to the chiral symmetry in which the excited-state potential energy surface is right-handed or left-handed with respect to the ground-state potential energy surface. As the first-order approximation of anharmonic correction is added into the displaced harmonic oscillator, this mirror image is broken down, and then the spectra can be depended on the chiral symmetry mentioned above. Both absorption and fluorescence coefficients are derived analytically within the first-order anharmonic approximation and numerical test is carried out to demonstrate the breaking down of the mirror image. Based on the same analysis, the electron transfer rate is derived analytically within the first-order anharmonic approximation. This rate might take the form of Arrhenius’s equation but not form of Marcus’s equation. Furthermore, it is found that this rate is also depending on the chiral symmetry.

A matrix isolation and computational study of the [C, N, F, S] isomers

The potential energy surface (PES) of the [C, N, F, S] system was investigated by quantum chemical and experimental methods. Seven minima were located on the ground state PES by density functional and ab initio electronic structure calculations. Four of these isomers, FSCN, FSNC, FCNS and FNCS, have an acyclic structure, while the other three, FC(NS), FS(CN) and FN(SC), form a three-membered fluorine-substituted ring. Out of these seven theoretically predicted isomers, FCNS and FC(NS) were successfully prepared in low-temperature Ar and Kr matrices by photochemical methods. The identification of these species was based on experimental considerations as well as on comparison of their IR spectra to computed anharmonic vibrational frequencies and infrared intensities. The present paper describes not only the first generation of both FCNS and FC(NS) species, but also reports the first time that a substituted CNS ring has been experimentally identified.

Quantum dynamics of the O+OH→H+O2 reaction at low temperatures

We report quantum dynamics calculations of the O + OH --> H + O(2) reaction on two different representations of the electronic ground state potential energy surface (PES) using a time-independent quantum formalism based on hyperspherical coordinates. Calculations show that several excited vibrational levels of the product O(2) molecule are populated in the reaction. Rate coefficients evaluated using both PESs were found to be very sensitive to the energy resolution of the reaction probability, especially at temperatures lower than 100 K. It is found that the rate coefficient remains largely constant in the temperature range of 10-39 K, in agreement with the conclusions of a recent experimental study [Carty et al., J. Phys. Chem. A 110, 3101 (2006)]. This is in contrast with the time-independent quantum calculations of Xu et al. [J. Chem. Phys. 127, 024304 (2007)] which, using the same PES, predicted nearly two orders of magnitude drop in the rate coefficient value from 39 to 10 K. Implications of our findings to oxygen chemistry in the interstellar medium are discussed.

Ground state potential energy surfaces and bound states of M–He dimers (M=Cu,Ag,Au): A theoretical investigation

We present an ab initio investigation on the ground state interaction potentials [potential energy surface (PES)] between helium and the group 11 metal atoms: copper, silver, and gold. To the best of our knowledge, there are no previous theoretical PESs proposed for Cu-He and Au-He, and a single one for Ag-He [Z. J. Jakubek and M. Takami, Chem. Phys. Lett. 265, 653 (1997)], computed about 10 years ago at MP2 level and significantly improved by our study. To reach a high degree of accuracy in the determination of the three M-He potentials (M=Cu,Ag,Au), we performed extensive series of test computations to establish the appropriate basis set, the theoretical method, and the computational scheme for these systems. For each M-He dimer we computed the PES at the CCSD(T) level of theory, starting from the reference unrestricted Hartree-Fock wave function. We described the inner shells with relativistic small core pseudopotentials, and we adopted high quality basis sets for the valence electrons. We also performed CCSDT computations in a limited set of M-He internuclear distances, adopting a medium-sized basis set, such as to define for each dimer a CCSD(T) to CCSDT correction term and to improve further the quality of the CCSD(T) interaction potentials. The Cu-He complex has minimum interaction energy (E(min)) of -28.4 microhartree at the internuclear distance of 4.59 A (R(min)), and the short-range repulsive wall starts at 4.04 A (R(E=0)). Quite interestingly, the PES of Ag-He is more attractive (E(min)=-33.8 microhartree) but presents nearly the same R(min) and R(E=0) values, 4.60 and 4.04 A, respectively. The interaction potential for Au-He is markedly deeper and shifted at shorter distances as compared to the lighter complexes, with E(min)=-69.6 microhartree, R(min)=4.09 A and R(E=0)=3.60 A. As a first insight in the structure of M-He(n) aggregates, we determined the rovibrational structure of the three M-He dimers. The Cu-He and Ag-He potentials support just few rotational excitations, while the Au-He PES admits also a bound vibrational excitation.

Aromaticity and Antiaromaticity in the Low-Lying Electronic States of Cyclooctatetraene

The levels of aromaticity of the most important geometries on the ground-state (S(0)), lowest triplet-state (T(1)), and first singlet excited-state (S(1)) potential energy surfaces (PESs) for cycloocta-1,3,5,7-tetraene (COT) are assessed using a wide range of magnetic criteria including nucleus-independent chemical shifts (NICSs), proton shieldings, and magnetic susceptibilities calculated using complete-active-space self-consistent-field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). It is shown that the ground state of D(8h) COT (transition state for the pi-bond-shift process on the S(0) PES) is markedly antiaromatic, even more so than the classical example of an antiaromatic system, the ground state of square cyclobutadiene. The CASSCF-GIAO magnetic properties of the ground state of D(4h) COT (transition state for the ring-inversion process on the S(0) PES) strongly suggest that it is much less antiaromatic than the ground state of D(8h) COT, whereas those of the ground state of D(2d) COT (local minimum on the S(0) PES) indicate that it is decidedly nonaromatic. The lowest triplet state and the first singlet excited state of D(8h) COT (local minima on the T(1) PES and the S(1) PES, respectively) exhibit surprisingly similar magnetic properties. These, in turn, are very close to the magnetic properties of benzene, which is a strong indication of a high degree of aromaticity.

Observation of geometric phase effect induced photodissociation dynamics in phenol

Dynamical effects attributable to the geometric phase have been observed in UV photo-induced O–H bond fission in jet cooled phenol. Phenoxyl radicals are formed with an odd quanta progression in a″ mode ν 16a. This mode specific product formation is reproduced by 2D wavepacket simulations and discussed in terms of quantum interference resulting from the geometric phase effect at the conical intersection between the ground and first excited state potential energy surfaces at extended O–H bond lengths.

Vibronic Effects in Cu(II)-Doped Ba2Zn(HCO2)6·4H2O

The temperature-dependent electron paramagnetic resonance (EPR) spectrum of approximately 1% Cu(II) ions doped into Ba 2Zn(HCO2)6 x 4 H2O was analyzed at the Q-band frequencies over the temperature range 100-350 K to obtain structural information about the local environment. It can be concluded that the host crystal imparts a large orthorhombic strain which mainly corresponds to a tetragonal compression imposed onto the Cu(II)O6 species. This results in a copper center which adopts an orthorhombically distorted elongated geometry with the elongated axis perpendicular to the direction of the tetragonal compression due to the host crystal. There are two possible axes of elongation, and these represent two conformers separated by approximately 320 cm(-1). The thermal population of the higher energy level averages the g values, giving the observed temperature-dependent EPR spectra. The averaging process is between vibronic levels that are localized at two different minima of a single ground-state potential energy surface. These vibronic levels correspond to vibrational levels having different electronic properties. The determination of the host lattice strain parameters from the Cu(II) EPR spectra means that the guest ion is used as a probe of the environment of the Zn(II) site. The structural data derived from the lattice strain parameters are correlated with those from the Ba 2Zn(HCO2)6 x 4 H2O crystal structure.

Ab initio and direct dynamics study of the reaction of Cl atoms with HOCO

The reaction of Cl with HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the reaction produces the HCl and CO(2) products as experimentally observed. The reaction occurs via a HOC(O)Cl intermediate with an estimated heat of formation of -97.8+/-2.0 kcal/mol. A direct ab initio dynamics method has been used to provide insight into the reaction mechanisms and to determine the thermal rate coefficients in the temperature range of 200-600 K. At room temperature, the thermal rate coefficient is predicted to be 3.0x10(-11) cm(3) molecule(-1) s(-1) with an activation energy of -0.2 kcal/mol. Two kinds of reactive trajectories are found. One kind proceeds through short-lived HOC(O)Cl complexes with a lifetime of 310 fs while the other kind occurs via longer-lived intermediates with a lifetime of 1.9 ps.

Classical Trajectory Study of the Dynamics of the Reaction of Cl Atoms with Ethane

We present an electronic-structure and dynamics study of the Cl + C2H6 --> HCl + C2H5 reaction. The stationary points of the ground-state potential energy surface have been characterized using various electronic-structure methods and basis sets. Our best calculations, CCSD(T) extrapolated to the complete basis limit, using geometries and harmonic frequencies obtained at the MP2/aug-cc-pVTZ level, are in agreement with the experimental reaction energy. Ab initio information has been used to reparameterize a semiempirical Hamiltonian so that the predictions of the improved Hamiltonian agree with the higher-level calculations in key regions of the potential energy surface. The improved semiempirical Hamiltonian is then used to propagate quasiclassical trajectories. Computed kinetic energy release and scattering angle distributions at a collision energy of approximately 5.5 kcal mol(-1) are in reasonable agreement with experiments, but no evidence was found for the low translational energy HCl products scattered in the backward hemisphere reported in recent experiments.

Photoinduced Dissociation of Water and Transport of Hydrogen between Silver Clusters

Theoretical electronic structure calculations are reported for the dissociation of water adsorbed on a 31-atom silver cluster, Ag31, and subsequent transfer of a H to a second Ag31 cluster leaving OH on the first cluster. Both ground and excited electronic state processes are considered for two choices of Ag cluster separation, 6.35 and 7.94 A, on the basis of preliminary calculations for a range of separation distances. The excited electronic state of interest is formed by photoemission of an electron from one Ag cluster and transient attachment of the photoemitted electron to the adsorbed water molecule. A very large energy barrier is found for the ground-state process (3.53 eV at a cluster separation of 6.35 A), while the barrier in the excited state is small (0.38 eV at a cluster separation of 6.35 A). In the excited state, partial occupancy of an OH antibonding orbital facilitates OH stretch and concomitant movement of the negatively charged OH toward the electron-hole in the metal cluster. The excited-state pathway for dissociation of water and transfer of H begins with the formation of an excited electronic state at 3.59-3.82 eV. Stretch of the OH bond occurs with little change in energy (0.38-0.54 eV up to a stretch of 1.96 A). In this region of OH stretch the molecule must return to the ground-state potential energy surface to fully dissociate and to transfer H to the other Ag cluster. Geometry optimizations are carried out using a simplex algorithm and a semigrid method. These methods allow the total energy to be calculated directly using configuration interaction theory.

On the Mechanisms of Degenerate Ligand Exchange in [M(CH3)]+/CH4 Couples (M=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as Explored by Mass Spectrometric and Computational Studies: Oxidative Addition/Reductive Elimination versus σ‐Complex‐Assisted Metathesis

The degenerate ligand exchange in [M(CH(3))](+)/CH(4) couples occurs in the gas phase at room temperature for M=Ni, Ru, Rh, Pd, and Pt, whereas the complexes containing Fe and Co are unreactive. Details of hydrogen-atom scrambling versus direct ligand switch have been uncovered by labeling experiments with CD(4) and (13)CH(4), respectively. The reactivity scale ranges from unreactive (M=Fe, Co) or inefficient (M=Ni, Pd) to moderately (M=Ru) and rather reactive (M=Rh, Pt). Quite extensive, but not complete, H/D exchange between the hydrogen atoms of the incoming and outgoing methyl groups is observed for M=Pt, whereas for M=Ni and Pd a predominantly direct ligand switch prevails. DFT calculations performed at the B3LYP level of theory account well for the thermal nonreactivity of the Fe and Co couples. For [Ni[CH(3))](+)/CH(4), a sigma-complex-assisted metathesis (sigma-CAM) is operative such that, in a two-state reactivity (TSR) scenario, two spin flips between the (3)A ground and (1)A excited states take place at the entrance and exit channels of the encounter complexes. For M=Ru and Rh, only oxidative addition/reductive elimination (OA/RE) is favored energetically, and the reaction is confined to the electronic ground states (3)A and (2)A. In contrast, for the [Pd(CH(3))](+)/CH(4) system, on the (1)A ground-state potential-energy surface both the OA/RE and sigma-CAM variants are energetically comparable, and the small reaction efficiency for the ligand switch is reflected in transition states located energetically close to the reactants. For the [M(CH(3))](+)/CH(4) complexes of the 5d elements, the sigma-CAM mechanism does not play a role. For M=Pt, the energetically most favored path proceeds in a spin-conserving manner on the (1)A potential-energy surface, which accounts for the extensive single and double hydrogen-atom exchange preceding ligand exchange. Although for M=Os and Ir the [M(CH(3))](+) complexes could not be generated experimentally, computational studies predict that both systems may undergo thermal reaction with CH(4), and an OA/RE mechanism will commence on the respective high-spin ground states; however, the bond-activation and ligand-exchange steps will occur on the excited low-spin surfaces in a TSR scenario.

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

The scattering dynamics leading to the formation of Cl (2P(3/2)) and Cl* (2P(1/2)) products of the CH(3)+HCl reaction (at a mean collision energy <E(coll)>=22.3 kcal mol(-1)) and the Cl (2P(3/2)) products of the CD(3)+HCl reaction (at <E(coll)>=19.4 kcal mol(-1)) have been investigated by using photodissociation of CH(3)I and CD(3)I as sources of translationally hot methyl radicals and velocity map imaging of the Cl atom products. Image analysis with a Legendre moment fitting procedure demonstrates that, in all three reactions, the Cl/Cl* products are mostly forward scattered with respect to the HCl in the center-of-mass (c.m.) frame but with a backward scattered component. The distributions of the fraction of the available energy released as translation peak at f(t)=0.31-0.33 for all the reactions, with average values that lie in the range <f(t)>=0.42-0.47. The detailed analysis indicates the importance of collision energy in facilitating the nonadiabatic transitions that lead to Cl* production. The similarities between the c.m.-frame scattering and kinetic energy release distributions for Cl and Cl* channels suggest that the nonadiabatic transitions to a low-lying excited potential energy surface (PES) correlating to Cl* products occur after passage through the transition state region on the ground-state PES. Branching fractions for Cl* are determined to be 0.14+/-0.02 for the CH(3)+HCl reaction and 0.20+/-0.03 for the CD(3)+HCl reaction. The difference cannot be accounted for by changes in collision energy, mass effects, or vibrational excitation of the photolytically generated methyl radical reagents and instead suggests that the low-frequency bending modes of the CD(3)H or CH(4) coproduct are important mediators of the nonadiabatic couplings occurring in this reaction system.

DFT study of metastable linkage isomers of six-coordinate ruthenium nitrosyl complexes

Abstract The linkage isomers of the [M(NO)(CN)5]2− (M = Fe, Ru, Os), [Ru(NO)Cl5]2−, trans-[Ru(NO)(NH3)4(L)]q+ (L = NH3, H2O, nicotinamide, imidazole, pyridine, pyrazine, NO2 −, OH−, Cl−), and trans-[Ru(NO)L4(OH)]q (L = pyridine, NO2 −) ions are studied by density functional theory. DFT calculations show that the electronic ground-state potential surface of these nitrosyl complexes has the local minima corresponding to η 2-NO and η 1-O linkage isomers, of which the former is characterized by lower energy. The stationary points on the ground state potential energy surface along the GS → MS2 → MS1 path for the [M(NO)(CN)5]2− (M = Fe, Ru, Os) and [Ru(NO)Cl5]2− complexes were found. The calculations of infrared spectra of linkage isomers [Ru(NO)Cl5]2− and [Ru(NO)(CN)5]2− are carried out. The analysis of quantum chemical bond orders indicates a noticeable delocalization of π-electron density along the Ltrans—M—NO axis and a possibility of direct interactions between NO and equatorial ligands in the structures with the bent {MNO} group.

Triplet and ground state potential energy surfaces of 1,4-diphenyl-1,3-butadiene: theory and experiment

Relative energies of the ground state isomers of 1,4-diphenyl-1,3-butadiene (DPB) are determined from the temperature dependence of equilibrium isomer compositions obtained with the use of diphenyl diselenide as catalyst. Temperature and concentration effects on photostationary states and isomerization quantum yields with biacetyl or fluorenone as triplet sensitizers with or without the presence of O(2), lead to significant modification of the proposed DPB triplet potential energy surface. Quantum yields for ct-DPB formation from tt-DPB increase with [tt-DPB] revealing a quantum chain process in the tt --> ct direction, as had been observed for the ct --> tt direction, and suggesting an energy minimum at the (3)ct* geometry. They confirm the presence of planar and twisted isomeric triplets in equilibrium (K), with energy transfer from planar or quasi-planar geometries (quantum chain events from tt and ct triplets) and unimolecular decay (k(d)) from twisted geometries. Starting from cc-DPB, varphi(cc-->tt) increases with increasing [cc-DPB] whereas varphi(cc-->ct) is relatively insensitive to concentration changes. The concentration and temperature dependencies of the decay rate constants of DPB triplets in cyclohexane are consistent with the mechanism deduced from the photoisomerization quantum yields. The experimental DeltaH between (3)tt-DPB* and (3)tp-DPB*, 2.7 kcal mol(-1), is compared with the calculated energy difference [DFT with B3LYP/6-31+G(d,p) basis set]. Use of the calculated DeltaS = 4.04 eu between the two triplets gives k(d) = (2.4-6.4) x 10(7) s(-1), close to 1.70 x 10(7) s(-1), the value for twisted stilbene triplet decay. Experimental and calculated relative energies of DPB isomers on the ground and triplet state surfaces agree and theory is relied upon to deduce structural characteristics of the equilibrated conformers in the DPB triplet state.

Application of mean-field and surface-hopping approaches for interrogation of the Xe3+ molecular ion photoexcitation dynamics

The dissociation dynamics of the excited Xe(3) (+) molecular ion through the Pi(12)(u) and Pi(12)(g) conical intersection was interrogated by computational simulation in which no adjustable parameters were used. The electronic ground and excited state potential energy surfaces were generated by the diatomics-in-molecules method, and the Ehrenfest mean-field and Tully surface-hopping approaches treated the nonadiabatic interactions. Reproduction of the experimental spectrum of the symmetric photofragmentation as a function of excitation energy was obtained within the region of interest (2.5-3.75 eV), with the exception of a 0.25 eV width on the red side of the spectral apex. Good agreement was obtained with the experimental dissociated photofragment kinetic energy spectra. It was determined that the greatest contribution to the nonadiabatic coupling between the two states originated from the bending vibrational mode of the molecule in the Sigma(12)(u), ground electronic state before excitation.

Interpreting ultrafast molecular fragmentation dynamics with ab initio electronic structure calculations

High-level ab initio electronic structure calculations are used to interpret the fragmentation dynamics of CHBr(2)COCF(3), following excitation with an intense ultrafast laser pulse. The potential energy surfaces of the ground and excited cationic states along the dissociative C-CF(3) bond have been calculated using multireference second order perturbation theory methods. The calculations confirm the existence of a charge transfer resonance during the evolution of a dissociative wave packet on the ground state potential energy surface of the molecular cation and yield a detailed picture of the dissociation dynamics observed in earlier work. Comparisons of the ionic spectrum for two similar molecules support a general picture in which molecules are influenced by dynamic resonances in the cation during dissociation.

The molecular structure of a curl-shaped retinal isomer

Computational studies of retinal protonated Schiff base (PSB) isomers show that a twisted curl-shaped conformation of the retinyl chain is a new low-lying minimum on the ground-state potential energy surface. The curl-shaped isomer has a twisted structure in the vicinity of the C(11)=C(12) double bond where the 11-cis retinal PSB isomerizes in the rhodopsin photoreaction. The twisted configuration is a trapped structure between the 11-cis and all-trans isomers. Rotation around the C(10)-C(11) single bond towards the 11-cis structure is prevented by steric interactions of the two methyl groups on the retinyl chain and by the torsion barrier of the C(10)-C(11) bond in the other direction. Calculations of spectroscopic properties of the 11-cis, all-trans, and curl-shaped isomers provide useful data for future identification of the new retinal PSB isomer. Circular dichroism (CD) spectroscopy might be used to distinguish between the retinal PSB isomers. The potential energy surface for the orientation of the beta-ionone ring of the 11-cis retinal PSB reveals three minima depending on the torsion angle of the beta-ionone ring. Two of the minima correspond to 6-s-cis configurations and one has the beta-ionone ring in 6-s-trans position. The calculated CD spectra for the two 6-s-cis configurations differ significantly indicating that the sign of the beta-ionone ring torsion angle could be determined using CD spectroscopy. Calculations of the CD spectra suggest that a flip of the beta-ionone ring might occur during the first 1 ps of the photoreaction. Rhodopsin has a negative torsion angle for the beta-ionone ring, whereas the change in the sign of the first peak in the experimental CD spectrum for bathorhodopsin could suggest that it has a positive torsion angle for the beta-ionone ring. Calculated nuclear magnetic resonance (NMR) shielding constants and infrared (IR) spectra are also reported for the retinal PSB isomers.