State Potential Surface Research Articles

Geometry and Electronic Structure of CuCl(6)(4-) Polyhedra Doped into (3-Chloroanilinium)(8)[CdCl(6)]Cl(4)-An EPR and Structural Investigation.

The EPR single-crystal and powder spectra of mixed crystals of (3-chloroanilinium)(8)(Cd(1-x)Cu(x)Cl(6))Cl(4) are measured as a function of temperature and x and analyzed with respect to the geometry and bonding properties of the CuCl(6) polyhedra. These undergo strong distortions due to vibronic Jahn-Teller coupling, with the resulting tetragonal elongation being superimposed by a considerable orthorhombic symmetry component induced by a host site strain acting as a compression along the crystallographic a axis. This strain becomes apparent in the cadmium compound (x = 0), whose crystal structure is also reported [a = 8.701(2) Å, b = 13.975(2) Å, c = 14.173(2) Å, alpha = 81.62(1) degrees, beta = 72.92(1) degrees, gamma = 77.57(1) degrees, triclinic P&onemacr;, Z = 1]. A calculation of the ground state potential surface and its vibronic structure nicely reproduces the g values, Cu-Cl spacings, and ligand field data. At high copper concentrations (including x = 1), the CuCl(6) polyhedra are coupled elastically, with the long axes of neighboring polyhedra having perpendicular orientations. The elastic correlation presumably is not of the long-range antiferrodistortive type, however. Above about 55 K, the angular Jahn-Teller distortion component becomes dynamically averaged within the time scale of the EPR experiment, leading to local tetragonally compressed CuCl(6) octahedra.

A Theoretical Study of Reactions on the ClHCN Surface

RHF+1+2/cc-pvdz calculations are reported for 14 minima and 9 transition states on the ClHCN ground state potential surface. The calculations predict the lowest energy route for the reaction Cl + HCN → HCl + CN to be a simple, direct, collinear abstraction. A lower energy addition reaction leading to the HClCN adduct is predicted to exist, but no direct pathway from HClCN to HCl + CN could be found. For the reaction of H with ClCN the calculations predict three pathways. These are, in order of increasing barrier height, addition to the carbon, forming HClCN; addition to the nitrogen, forming cis-ClCNH, and abstraction via a slightly bent transition state, forming HCl + CN.

Photoisomerization Quantum Yield and Apparent Energy Content of the K Intermediate in the Photocycles of Bacteriorhodopsin, Its Mutants D85N, R82Q, and D212N, and Deionized Blue Bacteriorhodopsin

The quantum yield of photoisomerization and the energy content of the K intermediate in the photocycle of bacteriorhodopsin and its mutants D85N, R82Q, and D212N and deionized blue bR were measured. Transient optical absorption and photoacoustic spectroscopy with excitation using 400 fs laser pulse were combined to obtain results. The spectroscopic characteristics of the excited state, the J and K intermediates in the photocycle of the mutants, and deionized blue bR were determined. The presence of both 13-cis and all-trans isomers in the ground state of light-adapted D85N, R82Q, and D212N and deionized blue bR makes extraction of the quantum yield for each isomer difficult. Thus, only average values of the quantum yield for these samples were determined. The replacement of charged groups in the vicinity of the retinal Schiff base was found to decrease the rate of the photoisomerization by up to 30 times, but with no signficant change in either the apparent quantum yield of the photoisomerization or the energy stored in the K intermediate. The results are discussed in terms of the different models for the excited and ground state potential surfaces of the retinal configuration in bacteriorhodopsin.

The molecular and electronic structure of s-tetrazine in the ground and first excited state: A theoretical investigation

The ground- and first excited state of s-tetrazine arising from a π*←n excitation (1Ag,1B3u) have been studied using the complete active space (CASSCF) and the second order multiconfiguration perturbation theory (CASPT2) ab initio methods. The focus of this study is on the effect of the electronic excitation on the molecular structure and on those electronic properties which are important to model the solvatochromatic behavior of the molecule in polymer matrices as used in permanent hole burning experiments. Since the accurate computation of excited state molecular properties represents a major challenge for today’s numerical quantum chemistry, some technical aspects are also considered. The present study shows that the change in geometry upon electronic excitation is small. This is in partial contradiction with the experimental studies which however disagree among themselves [see K. K. Innes, I. G. Ross, and W. R. Moomaw, J. Mol. Spectrosc. 132, 492 (1988), and R. E. Smalley, L. Wharton, and D. H. Levi, ibid. 66, 375 (1977)]. This study also confirms that the first excited state equilibrium structure is of D2h symmetry. In an earlier theoretical study it was found that the D2h symmetry structure may represent a saddle point rather than a minimum on the excited state potential surface [see A. C. Scheiner and H. F. Schaefer III, J. Chem. Phys. 87, 3539 (1987)]. In the first excited state, we observe an increase of the mean polarizability of s-tetrazine along with an enhanced anisotropy. The change in the polarizability is almost exclusively in the ‘‘in-plane’’ components of the tensor; the polarizability in the vertical direction is nearly unchanged. This observation questions recent experimental results reported for this molecule [see S. Heitz, D. Weidnauer, and A. Hese, J. Chem. Phys. 95, 7952 (1991)].

Molecular Dynamics Study of the Early Intermediates in the Bacteriorhodopsin Photocycle

The early stages of the bacteriorhodopsin photocycle, including the J625, K~w, and L550 intermediates and the role of water molecules within the protein interior, are studied by means of molecular dynamics simulations. Our calculations examine two models for the excited state potential surface governing the observed all-trans - 13-cis photoisomerization: one surface hindering a Cl4-CI5 single-bond corotation and the other surface allowing such corotation. The investigations use as a starting structure a model of bacteriorhodopsin based on electron-microscopy studies and subsequent molecular dynamics refinement. The following questions are addressed: How does the binding site guide retinal’s photoisomerization? How does the photoisomerization depend on features of the excited state potential surface? Can one recognize a J625 intermediate? How does water participate in the early part of the pump cycle? How is the initial photoreaction affected by a lowering of temperature? To model the quantum yield, i.e., the dependence of the dynamics on initial conditions, 50 separate isomerization teals are completed for each potential surface, at both 300 and 77 K, the trials being distinguished by different initial, random velocity distributions. From these trials emerge, besides all-transretinal, three different photoproducts as candidates for the K5w intermediate: (1) 13-cis-retinal, with the Schiff base proton oriented toward Asp-96; (2) 13-cis-retinal, highly twisted about the c6-c7 bond, with the Schiff base proton oriented perpendicular to the membrane normal; (3) 13,14-dicis-retinal, with the Schiff base proton oriented toward the extracellular side. Two candidates for the K~w intermediate, case 2 and case 3 above, were subjected to simulated annealing to determine corresponding L550 structures. We suggest that photoproduct 2 above most likely represents the true K~w intermediate. Water molecules near the Schiff base binding site are found to play a crucial role in stabilizing the K590 state and in establishing a pathway for proton transfer to Asp-85.

Coherent bacterial fluorescence dynamics: A semiclassical model

Oscillatory fluorescence in reaction centres of photosynthetic origin was assigned to coherent vibrational motion on the excited state potential surface by Vos and collaborators. We formalize this motion with a semiclassical wave packet consisting of several (intraprotein) modes. Our model indicates a dissipative (or collisional) to dephasing decay time ratio of 1 6 or less, and reproduces observed spectral features. We argue that the multi-mode semiclassical wave packet is more resistant to quantal interference than a single-mode one.

Quasicluster Electronic Structure Calculations of Point Defects in Crystals

AbstractAn earlier proposed quasicluster embedding scheme is applied to the calculations of the ground state potential surfaces, energies, and probabilities of electronic transitions in neutral and charged point defects in KCI, SiO2, and diamond crystals. The model is closely related to the periodic supercell approach (providing similar electronic structure characteristics) and the molecular cluster one. Two modified versions of the model are proposed, which improve results of calculations of ground state properties (especially for small quasiclusters) and transition energies. Ground state potential surfaces calculated for defects in ionic and semiionic crystals are better than those obtained using supercell and molecular cluster models.

New rovibrational data for MgOH and MgOD and the internuclear potential function of the ground electronic state

Using millimeter wave absorption spectroscopy we have recorded pure rotation spectra of MgOH in its ground vibrational state and in six excited vibrational states. We have also determined a large number of bending vibrational energies, and the MgO stretching frequency, using dispersed fluorescence studies of both MgOH and MgOD. By a weighted least-squares optimized fitting to this data, using the vibrational approach MORBID (Morse oscillator rigid bender for internal dynamics), we have derived the full ground state potential surface in an analytical form. The equilibrium structure is determined to be linear but with a flat predominantly quartic bending potential.

Thermal and photodesorption from a molecular surface: Ammonia on Ag2.

The one-to-one complex A${\mathrm{g}}_{2}$N${\mathrm{H}}_{3}$ was prepared by titrating disilver with ammonia. Its photodissociation along ground and excited state potential surfaces was studied. Binding energies and the ammonia umbrella frequency were determined to be in good accord with precise electronic structure calculations showing that the complex is linear. Complexes of this type are important members of a series that extend to infinite surfaces. They allow the dynamics of particular adsorbate excitations to be isolated. Here, an excitation at 3 eV involving the metal-metal and the chemisorption bond is revealed.

Current density investigation of tunneling in photodissociation resonances: CH 3ONO

The application is described of our recently developed current density analysis of photodissociation to the methyl nitrite (CH 3ONO) system. The spectrum exhibits a series of vibrational resonances, or predissociating states, arising from a local minimum in the excited state potential surface. The degree to which tunneling through the barrier plays a role in their dissociation is investigated by means of the current density at the transition state and a semi-classical quantization model. For vibrationally excited resonances, current density maps are considerably complicated by the presence of vortices.

Absorption by dissociative continua in condensed matter: H 2O in rare gas and ice matrices

The absorption spectrum of H 2O in Ne, Ar, Kr, Xe and ice matrices in the region of the first à 1B 1 continuum is reported. Substantial blue gas-to-matrix shifts are observed for the maximum, which increase with decreasing matrix cage size. The absorption thresholds show much weaker shifts. These features are interpreted in terms of Rydbergization of the (3s/4a 1) molecular orbital correlated to the à state. The photochemistry is strongly affected by the environmental effects on the excited state potential surface. An energy threshold to photodissociation is confirmed, consistent with a potential barrier due to the cage and a prompt exit of the H fragment. The photodissociation quantum yield shows a linear dependence with excess kinetic energy in Ar and Xe matrices, in agreement with recent MD simulations also favouring prompt exits as the dominant mechanism to permanent dissociation.

Evidence for quantization of the transition state for cis–trans isomerization

Cis–trans isomerization rates of trans,trans-1,3,5,7-octatetraene (OT) on the first excited singlet state (2 1Ag) potential surface have been obtained as a function of vibrational energy by measuring the fluorescence lifetimes. A stepwise increase in the isomerization rate with increasing energy has been observed, which indicates quantization of the vibrational levels of the transition state for the cis–trans isomerization of a double bond. The energy spacing of 80±10 cm−1 between the first two steps tentatively is assigned to an in-plane bending vibration of the transition state.

New experimental insight into the ground state potential surface of triatomic hydrogen.

Continuous UV spectra emitted by the triatomic hydrogen molecules ${\mathrm{H}}_{3}$, ${\mathrm{D}}_{3}$, ${\mathrm{H}}_{2}$D, and ${\mathrm{D}}_{2}$H have been measured between 200 and 400 nm and exhibit two pronounced maxima. The two maxima in the spectra are interpreted as arising from the decay to both sheets of the ground state potential surface which form a conical intersection at the point of equilateral symmetry. This is confirmed by a detailed computer simulation. The fact that the ${\mathrm{H}}_{3}$ ground state potential surface has two sheets is, to our knowledge, observed here experimentally for the first time.

Temperature Dependence of Fluorescence and Photoisomerization in Symmetric Carbocyanines.Influence of Medium Viscosity and Molecular Structure

The temperature dependence of fluorescence emission and photoisomerization of five symmetric carbocyanines was studied in a group of primary n-alcohols from 0 to 70 o C. The trans-cis isomerization on the excited state potential surface was studied by steady-state fluorescence emission and flash photolysis. The back isomerization rate constant on the ground-state potential surface from the photoisomer to the normal form was also studied by flash photolysis. In all cases fluorescence quantum yields were found to diminish with temperature and to increase with viscosity (η), while isomerization quantum yields showed the opposite behavior

Ground state dynamics of NO3: Multimode vibronic borrowing including thermal effects

The X 2A2′ band of the photoelectron spectrum of NO−3 is theoretically calculated in the framework of a multimode vibronic coupling model. Linear coupling between and within the X 2A2′ and the B 2E′ electronic states of NO3 is found to be the important mechanism governing the dynamics. The necessary coupling constants are obtained from the ionization potentials of NO−3 calculated at different geometries using ab initio Green’s functions. Comparison of the theoretical with experimental results [A. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Chem. Phys. 94(3), 1740 (1991)] for the PE spectrum shows good agreement if a temperature of the NO−3 anion of 455 K in the experiment is assumed. A general theoretical treatment of thermal effects in vibronically coupled electronic states is presented. The ground electronic state potential surface of NO3 is discussed in the framework of the vibronic coupling model. A shallow minimum at a C2v geometry with a barrier height of 0.006 eV relative to the minimum energy D3h configuration is found. It is too weak to deform the effective geometry of the molecule from D3h to C2v. Mode-mode coupling is found to play a relevant role in the ground electronic state of NO3, in general.

Ab initio investigation of the ground state potential surfaces of HeNO + and ArNO +

New experiments based on resonant multiphoton ionization spectroscopy have allowed the observation of ArNO and ArNO + ground states. We present a new theoretical method, using a CIPSI approach, to calculate the ground state potential surfaces of XNO + complexes (X = He, Ar), which gives results in good agreement with experiment.

(n,3s) Rydberg spectra of diethyl ether, diisopropyl ether, and methyl vinyl ether: Analysis of the torsional motion

Two-photon resonance mass resolved excitation spectra are obtained for diethyl ether-h10, diethyl ether-d10, diisopropyl either, and methyl vinyl ether cooled in a supersonic jet expansion. The spectra are assigned as due to 2p3s←(2p)2 Rydberg excitations: significant progressions in a low energy vibrational mode are observed for both diethyl and diisopropyl ethers, but not for methyl vinyl ether. The transition energies for the vibronic progressions are modeled by a double well potential function of polynomial form truncated at the quartic term. The transition intensities for the progressions are calculated based on a Franck–Condon analysis of the excited state potential surface and a harmonic ground state surface. Calculations identify the progression forming mode as the antigeared torsion of the C–O–C–C dihedral angles. Excitation of diethyl ether from its ground electronic state to its 2p3s Rydberg state leads to changes in the dihedral angles (τ1, τ2) from ±180° to ±157°. Similar electronic excitation of diisopropyl ether yields changes in (τ1,τ2) from ±157° in the ground state to ±146° in the Rydberg state. The torsional displacement active in the 2p3s←(2p)2 oxygen Rydberg transition is suggested to arise from the interaction between the diffuse 3s electron and the β-methyl groups on both diethyl and diisopropyl ethers.

Mechanisms for the reactions between methane and the neutral transition metal atoms from yttrium to palladium

Calculations including electron correlations have been performed for the oxidative addition reactions between methane and the whole sequence of second row transition metal atoms from yttrium to palladium. The lowest barrier for the C-H insertion reaction is found for the rhodium atom. Palladium has the lowest methane elimination barrier. The barrier height is governed by two factors. In the reactant channel low repulsion favors a low barrier, and in the product channel strong bond formation is important. The atomic state with lowest repulsion toward methane is the d{sup n+2} state and the strongest bonds are formed to the d{sup n+1} s state. For rhodium both these states are energetically low lying. Only palladium has a bound {eta}{sup 2} precursor state on the ground state potential surface. Another interesting result is that the potential surface for the reaction between methane and the rhodium atom is remarkably similar to the potential surface for the reaction between methane and ClRhL{sub 2}, which has been studied experimentally. 44 refs., 5 figs., 5 tabs.

Interference dips in molecular absorption spectra calculated for coupled electronic state potential surfaces

Interference effects in electronic absorption spectra caused by coupling between excited states are calculated and interpreted by using numerical integration of the time-dependent Schrödinger equation and the time-dependent theory of electronic spectroscopy. The interference between nearby spin-forbidden doublet and spin-allowed quartet states of chromium (III) and vanadium (II) metal complexes causes sharp decreases of intensity or dips in the envelopes of absorption bands. The states are coupled by spin–orbit coupling. The origin of the dips is explained in terms of interference between wave packets moving on potential energy surfaces representing the states. The importance of curve crossing and amplitude transfer between the states is analyzed quantitatively. The theory is applied to the absorption spectrum of a chromium (III) complex.

Ab initio studies of ground and excited electronic states of MgAr, CdAr, and BeAr

The ground state (X 1Σ+) and several excited state (A 3Π,c 3Σ+,C 1Π,D 1Σ+, andE 3Σ+) potential energy surfaces for the diatomic molecules MgAr, CdAr, and BeAr have been computed using complete active space self-consistent field (CASSCF) wavefunctions and valence double- and triplezeta quality basis sets augmented with polarization and diffuse functions. Pump-and-probe laser experiments have examined the quenching, of excited singlet states of metal-rare gas complexes such as CdXe to produce triplets that dissociate to3 P Jmetal atoms. This quenching, which is detected for CdXe but not for CdAr or MgAr, is thought to occur via a crossing or strong coupling of a repulsive triplet curve correlating to the underlying3 P state of the metal, with an attractive singlet curve that correlates to the higher1 P state of the metal. The present work indicates that the attractiveC 1Π and repulsivec 3Σ+ curves of MgAr and CdArdo not intersect in the energetically accessible region of theC 1Π surface, unlike the corresponding curves for the CdXe diatom. These data are consistent with the absence of3 P J Cd atoms in the MgAr and CdAr experiments, respectively. However, an alternative quenching mechanism involving “vibronic” coupling between theC 1Π vibrational eigenstates and the continuum eigenstates of the underlying repulsive3Σ+ surface may be operative; this possibility is examined qualitatively and predicted to be unlikely for MgAr (due to small spin-orbit coupling) and CdAr (due to unfavorable vibronic factors). BeAr, which has yet to be probed experimentally, is predicted to be bound by 770 and 900 cm−1 in theD 1Σ+ state (which has metal 2s2p character) and theE 3Σ+ state (which has Rydberg metal 2s3s character), respectively, and to display interesting potential curve intersections.