Potential Energy Hypersurface Research Articles

Conformational Relaxation Paths in Tryptamine

The relative fluorescence intensities of three conformers of tryptamine have been determined as a function of stagnation pressure and nozzle temperature in a supersonic expansion. The relative intensities of the conformers that are connected by different direct and indirect interconversion paths on the potential-energy hypersurface differ considerably depending on the experimental conditions. The energies of the local minima and some transition states interconnecting them are studied on the ab initio level of theory. From the energies of the barriers and vibrational frequencies at the stationary points, conformer interconversion rates k(T) and k(E) have been obtained using statistical theories. In the cases experimentally observed here, vibrational cooling rates and interconversion rates must be of the same order of magnitude, while RRKM theory considerably overestimates k(E) values, most probably due to an insufficient consideration of anharmonic coupling of the vibrational modes in tryptamine.

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0001)-(√3×√3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0001)-(√3×√3)R30° surface was exposed to oxygen molecules with translational energy of 0.5eV at 300K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si–Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.

Global analysis of the high resolution infrared spectrum of methane 12CH4 in the region from 0 to 4800 cm−1

We report the global analysis of methane (12CH4) lines from high resolution rovibrational spectra including accurate line positions and intensities in the region 0–4800cm−1. This covers four polyads: The Ground State Monad (rotational levels), the Dyad (940–1850cm−1, 2 vibrational levels, 2 sublevels), the Pentad (2150–3350cm−1, 5 vibrational levels, 9 sublevels) and the Octad (3550–4800cm−1, 8 vibrational levels, 24 sublevels) and some of the associated hot bands (Pentad–Dyad and Octad–Dyad). New Fourier transform infrared (FTIR) spectra of the Pentad and Octad regions have been recorded with a very high resolution (better than 0.001cm−1 instrumental bandwidth, unapodized) at 78K using the Bruker IFS 125 HR Zürich prototype (ZP2001) spectrometer in combination with a long optical path collisional cooling system [S. Albert, S. Bauerecker, M. Quack, A. Steinlin, Mol. Phys. 105 (2007) 541]. Existing spectra previously recorded with the FTIR spectrometer at the National Solar Observatory on Kitt Peak in Arizona were remeasured selectively to provide new intensities and positions of weaker lines above 4400cm−1. These were combined with previously reported absorption data from FTIR and laser absorption, as well as high-resolution stimulated Raman and microwave spectra. The effective hamiltonian was expanded up to order 6 for the Ground State, order 6 for the Dyad, order 5 for the Pentad and order 5 for the Octad. A total of 16,738 line positions were used in the least squares adjustment characterized by the following global root mean square deviations dRMS for line positions: 1.3×10−4cm−1 for the Dyad, 6.0×10−4cm−1 for the Pentad, and 3.5×10−3cm−1 for the Octad. Absolute intensities were also analyzed for all the cold bands and some of the hot bands in the region under consideration and we obtained dRMS=9.6% including 3262 experimental line intensities for the Octad. This analysis represents a large improvement over the previous one [J.-C. Hilico, O. Robert, M. Loëte, S. Toumi, A.S. Pine, L.R. Brown, J. Mol. Spectrosc. 208 (2001) 1] with dRMS=0.041cm−1 for positions and 15.6% for intensities in the Octad for a smaller data set. The new results are discussed as benchmarks in relation to accurate potential energy hypersurfaces and for atmospheric and planetary spectra.

Dynamic Quantum Isotope Effects on Multiple Proton-Transfer Reactions

Abstract In order to investigate static quantum isotope effects on the stability of proton-transferred structures, we defined an effective quantal potential energy hypersurface (EQPES), which includes mass-dependent quantum effects. We demonstrated the difference between the ordinary potential energy surface and EQPES of the double well potential as a simple example. The minimum energy path on EQPES describes the zero-point energy-corrected structures and tunneling motion, which is characterized as a classically forbidden motion. We also performed the EQPES analysis of model proton-transfer reactions in DNA base pairs. It was found that the double proton-transferred structure of an adenine–thymine (AT) pair is quantum mechanically unstable and that of a guanine–cytosine (GC) pair is stable within a least uncertainty regime. In order to investigate dynamic quantum isotope effects on the stability of the proton-transferred structures, we performed quantal cumulant dynamics (QCD) simulations of the GC pair. Results show that the proton-transferred structure of the protonated isotopomer is dynamically unstable though the EQPES analysis predicts its stability. It is relevant to include dynamic effects in the investigation of the quantum isotope effects on geometric stability of systems with a small energy gap between global and metastable structures.

13C, 18O, and D Fractionation Effects in the Reactions of CH3OH Isotopologues with Cl and OH Radicals

A relative rate experiment is carried out for six isotopologues of methanol and their reactions with OH and Cl radicals. The reaction rates of CH2DOH, CHD2OH, CD3OH, (13)CH3OH, and CH3(18)OH with Cl and OH radicals are measured by long-path FTIR spectroscopy relative to CH3OH at 298 +/- 2 K and 1013 +/- 10 mbar. The OH source in the reaction chamber is photolysis of ozone to produce O((1)D) in the presence of a large excess of molecular hydrogen: O((1)D) + H2 --> OH + H. Cl is produced by the photolysis of Cl2. The FTIR spectra are fitted using a nonlinear least-squares spectral fitting method with measured high-resolution infrared spectra as references. The relative reaction rates defined as alpha = k(light)/k(heavy) are determined to be: k(OH + CH3OH)/k(OH + (13)CH3OH) = 1.031 +/- 0.020, k(OH + CH3OH)/k(OH + CH3(18)OH) = 1.017 +/- 0.012, k(OH + CH3OH)/k(OH + CH2DOH) = 1.119 +/- 0.045, k(OH + CH3OH)/k(OH + CHD2OH) = 1.326 +/- 0.021 and k(OH + CH3OH)/k(OH + CD3OH) = 2.566 +/- 0.042, k(Cl + CH3OH)/k(Cl + (13)CH3OH) = 1.055 +/- 0.016, k(Cl + CH3OH)/k(Cl + CH3(18)OH) = 1.025 +/- 0.022, k(Cl + CH3OH)/k(Cl + CH2DOH) = 1.162 +/- 0.022 and k(Cl + CH3OH)/k(Cl + CHD2OH) = 1.536 +/- 0.060, and k(Cl + CH3OH)/k(Cl + CD3OH) = 3.011 +/- 0.059. The errors represent 2sigma from the statistical analyses and do not include possible systematic errors. Ground-state potential energy hypersurfaces of the reactions were investigated in quantum chemistry calculations at the CCSD(T) level of theory with an extrapolated basis set. The (2)H, (13)C, and (18)O kinetic isotope effects of the OH and Cl reactions with CH3OH were further investigated using canonical variational transition state theory with small curvature tunneling and compared to experimental measurements as well as to those observed in CH4 and several other substituted methane species.

A Computational Study on Some Viable Targets for Gas-Phase Synthesis of Metal Complexes of the Cyclic (B6C)−2and Their Bonding Pattern

In this account, a detailed computational study is conducted to verify the geometric, energetic, and electronic properties of the planar cyclic (B 6C) (-2) (as the simplest carrier of hexacoordinate carbon) within some metal complexes. The [M(B 6C)] ((-)) (M = Li, Na, K) and [M(B 6C)] (M = Be, Mg, Ca) series are employed for this purpose. Relevant ab initio calculations at both DFT and post-HF levels vividly demonstrate that this dianion is stabilized considerably in the electric field generated by cations, whereas the geometrical and electronic properties of this ring remain almost intact in these complexes. The complementary topological analysis of charge densities confirms that cyclic (B 6C) (-2) within these complexes exhibits the same topological patterns as the naked dianion, thus confirming the presence of an unusual charge density distribution in this dianion. An electrostatic model is proposed that not only qualitatively but also quantitatively explains the observed computational trends in these complexes. This model successfully traces the polarization of the central carbon atom of the ring in the presence of a hard, multiply charged cation. To facilitate experimental detection, the photoelectron spectra of the [M(B 6C)] ((-)) (M = Li, Na, K) series are computed and the dominant features are extracted. Although considered species are not global minima on their potential energy hypersurfaces, their kinetic stabilities are verified and demonstrated unequivocally.

Calculation of the transport and relaxation properties of methane. I. Shear viscosity, viscomagnetic effects, and self-diffusion

Transport properties of pure methane gas have been calculated in the rigid-rotor approximation using the recently proposed intermolecular potential energy hypersurface [R. Hellmann et al., J. Chem. Phys. 128, 214303 (2008)] and the classical-trajectory method. Results are reported in the dilute-gas limit for shear viscosity, viscomagnetic coefficients, and self-diffusion in the temperature range of 80-1500 K. Compared with the best measurements, the calculated viscosity values are about 0.5% too high at room temperature, although the temperature dependence of the calculated values is in very good agreement with experiment between 210 and 390 K. For the shear viscosity, the calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization are small, less than 0.7%, in the temperature range considered. The very good agreement of the calculated values with the experimental viscosity data suggests that the rigid-rotor approximation should be very reasonable for the three properties considered. In general, the agreement for the other measured properties is within the experimental error.

DFT study of a heterofullerene molecule containing fifty-eight carbon atoms and one sulphur atom

A novel heterofullerene C 58S has been reported here. It has odd number of atoms assembling a heterofullerene-cage and a near planar tetracoordinate sulphur atom on the skeleton of the cage. Density functional calculations and minimization techniques have been employed to characterize the structural and electronic properties of the molecule. Geometry, electronic properties, and vibrational frequencies of the molecule have been calculated at the B3LYP/6-31G ∗ level of theory. The absence of imaginary vibrational frequency confirms that the molecule corresponds to a true minimum on the potential energy hypersurface. Sixteen 13C nuclear magnetic resonance (NMR) spectral signals of C 58S are characterized, and its heat of formation was estimated in this work.

Triplet germylenes with separable minima at ab initio and DFT levels

Six ground state triplet germylenes, with HGeX and GeX 2 formulae, are reached at QCISD(T)/6-311++G** and B3LYP/6-311++G** levels of theory (X = Li, Na, and K). Their corresponding less stable electronic isomers appears as singlet germylene minima. Scanning Ф as a function of energy for every triplet species gives two energy minima, separated by significant energy barriers. The same scanning gives merely one minimum for most singlet species, at the potential energy hyper-surface. Thus, only the open-shell (σ 1π 1) triplet states exhibit bond-angle isomerism, since no such isomers could be found for the (σ 2π 0) singlet states except for singlet GeLi 2 which shows a possible covalent bonding interaction between the two Li atoms. All triplet X Ge X species appear more linear than their corresponding triplet H Ge X. As the electro-positivity of X increases, the Ge X bond distances in all species lengthen, while their singlet–triplet energy separations (Δ E s–t) decrease, with the lowest Δ E s–t given for H Ge K. The Δ E LUMO–HOMO for singlet HGeX and GeX 2 species appear inversely proportional to the size of X, showing a trend of K > Na > Li. Interestingly, for both singlet HGeX and GeX 2 species, linear correlations are found between the Δ E LUMO–HOMO and their corresponding Δ E s–t,X.

Ab initio intermolecular potential energy surface and second pressure virial coefficients of methane

A six-dimensional potential energy hypersurface (PES) for two interacting rigid methane molecules was determined from high-level quantum-mechanical ab initio computations. A total of 272 points for 17 different angular orientations on the PES were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory with basis sets of aug-cc-pVTZ and aug-cc-pVQZ qualities. The calculated interaction energies were extrapolated to the complete basis set limit. An analytical site-site potential function with nine sites per methane molecule was fitted to the interaction energies. In addition, a semiempirical correction to the analytical potential function was introduced to take into account the effects of zero-point vibrations. This correction includes adjustments of the dispersion coefficients and of a single-parameter within the fit to the measured values of the second virial coefficient B(T) at room temperature. Quantitative agreement was then obtained with the measured B values over the whole temperature range of the measurements. The calculated B values should definitely be more reliable at very low temperatures (T<150 K) than values extrapolated using the currently recommended equation of state.

The Temperature and Pressure Dependence of the Reactions H + O2 (+M) → HO2 (+M) and H + OH (+M) → H2O (+M)

The reactions H + O2 (+M) --> HO2 (+M) and H + OH (+M) --> H2O (+M) have been studied using high-level quantum chemistry methods. On the basis of potential energy hypersurfaces obtained at the CASPT2/aug-cc-pVTZ level of theory, high-pressure limiting rate coefficients have been calculated using variable reaction coordinate transition state theory. Over the temperature range 300-3000 K, the following expressions were obtained in units of cm(3) molecule(-1) s(-1): k(infinity)(H + O2) = (25T(-0.367) + (7.5 x 10(-2)) T(0.702)) x 10(-11) and k(infinity)(H + OH) = (4.17 x 10(-11)) T(0.234)exp (57.5/T). Available experimental data on the pressure dependence of the reactions were analyzed using a two-dimensional master equation. The following low-pressure limiting rate coefficients were obtained over the temperature range 300-3000 K in units of cm(6) molecule(-2) s(-1): k0(H + O2 + Ar) = (9.1x10(-29)) T(-1.404)exp (-134/T), k0(H + O2 + N2) = (2.0x10(-27)) T(-1.73)exp (-270/T), k0(H + OH + Ar) = (8.6x10(-28)) T(-1.527)exp (-185/T), and k0(H + OH + N2) = (1.25x10(-26)) T(-1.81)exp (-251/T). For the H + O2 reaction system, F(cent)(Ar) = 0.67 and F(cent)(N2) = 0.72 were obtained as center broadening factors, whereas F(cent)(Ar) = 0.72 and F(cent)(N2) = 0.73 were obtained for the H + OH reaction system. The calculations provide a good description of most of the experimental data, except the room temperature measurements on the H + OH (+M) --> H2O (+M) reaction.

Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates

The existence of a broad, mid-infrared absorption ranging from 1000 to 3000 cm(-1) is usually interpreted as a signature for the existence of protonated water networks. Herein, we use cryogenic mixtures of water and hydrogen fluoride (HF) and show experimental and computational evidence that similarly wide absorptions can be generated by a broad distribution of proton-shared and ion pair complexes. In the present case, we demonstrate that the broadening is mainly inhomogeneous, reflecting the fact that the topology of the first solvation shell determines the local degree of ionization and the shared-proton asymmetric stretching frequency within H2O x HF complexes. The extreme sensitivity of the proton transfer potential energy hypersurface to local hydrogen bonding topologies modulates its vibrational frequency from 2800 down to approximately 1300 cm(-1), the latter value being characteristic of solvation geometries that yield similar condensed-phase proton affinities for H2O and fluoride. By linking the local degree of ionization to the solvation pattern, we are able to propose a mechanism of ionization for HF in aqueous solutions and to explain some of their unusual properties at large concentrations. However, an important conclusion of broad scientific interest is our prediction that spectral signatures that are normally attributed to protonated water networks could also reveal the presence of strong hydrogen bonds between un-ionized acids and water molecules, with important consequences to spectroscopic investigations of biologically relevant proton channels and pumps.

The photophysics of flavins: What makes the difference between gas phase and aqueous solution?

The ground and low-lying excited electronic states of isoalloxazine, 10-methylisoallox-azine and lumiflavin, three flavin-related compounds, were investigated by means of quantum chemical methods. Minimum structures were determined employing (time-dependent) Kohn–Sham density functional theory. Spectral properties were computed utilizing a combined density functional and multi-reference configuration interaction (DFT/MRCI) method. Solvent effects were mimicked by a conductor like screening model and micro-hydration with four explicit water molecules. At selected points along a linearly interpolated path connecting the Franck–Condon region and the S 1minimum, spin–orbit interaction was computed employing a nonempirical mean-field Hamiltonian. For isoalloxazine, intersystem crossing (ISC) rate constants were computed, taking both direct and vibronic spin–orbit coupling into account. On the basis of these calculations we suggest the following photo relaxation model. In the vacuum, efficient ISC ( k ISC ≈ 1 0 9 s − 1 ) takes place between the primarily excited ( π → π ∗ ) 1 state (S 1) and the lowest ( n → π ∗ ) 3 state (T 2). The energetic proximity of the ( n → π ∗ ) 1 state (S 2) enhances the nonradiative relaxation of S 1 by internal conversion (IC). In aqueous solution these ISC and IC channels are energetically not accessible due to the blue shift of the ( n → π ∗ ) states. The high triplet quantum yield observed in experiment [J.T.M. Kennis, S. Crosson, M. Gauden, I.H.M. van Stokkum, K. Moffat, R. van Grondelle, Biochemistry 42 (2003) 3385–3392] is explained by the intersection between the ( π → π ∗ ) 1 state (S 1) potential energy hypersurface (PEH) and the second ( π → π ∗ ) 3 (T 2) PEH along the relaxation pathway and the strong enhancement of their spin–orbit coupling by vibronic interactions. The calculated ISC rate for this channel ( k ISC ≈ 1 0 8 s − 1 ) is in good agreement with experimental results. According to our model, lack of an efficient IC channel leads to an increased fluorescence quantum yield in aqueous solution.

Oxidation of Ethyne and But-2-yne. 2. Master Equation Simulations

The aim of this study is to improve understanding of the tropospheric oxidation of ethyne (acetylene, C2H2) and but-2-yne, which takes place in the presence of HO and O2. The details of the potential energy hypersurface have been discussed in a previous article [Maranzana et al., J. Phys. Chem. A 2008, 112, XXXX]. For both molecules, the initial addition of HO radical to the triple bond is followed by addition of O2 to form peroxyl radicals. In both reaction systems, the peroxyl radicals take two isomeric forms, E1 and E2 for ethyne and e1 and e2 for but-2-yne. Energy transfer parameters (alpha = 250 cm-1) for the ethyne system were obtained by simulating laboratory data for N2 buffer gas, where O2 was not present. In simulations of C2H2 + HO when O2 is present, E1 reacts completely and E2 reacts almost completely, before thermalization. Radical E1 produces formic acid ( approximately 44%) and E2 gives glyoxal ( approximately 53%), in quite good agreement with experiments. For but-2-yne, pressure-dependent laboratory data are too scarce to obtain energy transfer parameters directly, so simulations were carried out for a range of values: alpha = 200-900 cm-1. Excellent agreement with the available experimental yields at atmospheric pressure was obtained with alpha = 900 cm-1. Two reaction channels are responsible for acetic acid formation, but one is clearly dominant. Biacetyl is produced by reactions of e1 and, to a minor extent, e2. The peroxyl radical e2 leads to less than 8% of all products. Vinoxyl radical (which has been reported in experiments involving C2H2 + HO) and products of its reactions are predicted to be negligible under atmospheric conditions.

The Limitations of Certain Density Functionals in Modeling Neutral Water Clusters

We have used density functional theory (DFT) with four different functionals and with basis sets optimized for studying water clusters to calculate the structures and energies for selected water dimer, trimer, tetramer, pentamer, hexamer, and octamer structures. We compare the results to the CBS‐APNO and G3 model chemistry methods, and with highly accurate MP2 complete basis set limit energies. We find that while all the DFT methods capture the minimum energy structures for tetramers and pentamers, and reproduce the interaction energies well, they fail to find certain structures where London dispersion forces are critical to the interaction. Specifically structures that are not composed of cyclic rings, such as the tetramer pyramid and the pentamer cage structures, are not minima on the DFT potential energy hypersurfaces.

Toward Calculations of the 129Xe Chemical Shift in Xe@C60 at Experimental Conditions: Relativity, Correlation, and Dynamics

We calculate the 129Xe chemical shift in endohedral Xe@C60 with systematic inclusion of the contributing physical effects to model the real experimental conditions. These are relativistic effects, electron correlation, the temperature-dependent dynamics, and solvent effects. The ultimate task is to obtain the right result for the right reason and to develop a physically justified methodological model for calculations and simulations of endohedral Xe fullerenes and other confined Xe systems. We use the smaller Xe...C6H6 model to calibrate density functional theory approaches against accurate correlated wave function methods. Relativistic effects as well as the coupling of relativity and electron correlation are evaluated using the leading-order Breit-Pauli perturbation theory. The dynamic effects are treated in two ways. In the first approximation, quantum dynamics of the Xe atom in a rigid cage takes advantage of the centrosymmetric potential for Xe within the thermally accessible distance range from the center of the cage. This reduces the problem of obtaining the solution of a diatomic rovibrational problem. In the second approach, first-principles classical molecular dynamics on the density functional potential energy hypersurface is used to produce the dynamical trajectory for the whole system, including the dynamic cage. Snapshots from the trajectory are used for calculations of the dynamic contribution to the absorption 129Xe chemical shift. The calculated nonrelativistic Xe shift is found to be highly sensitive to the optimized molecular structure and to the choice of the exchange-correlation functional. Relativistic and dynamic effects are significant and represent each about 10% of the nonrelativistic static shift at the minimum structure. While the role of the Xe dynamics inside of the rigid cage is negligible, the cage dynamics turns out to be responsible for most of the dynamical correction to the 129Xe shift. Solvent effects evaluated with a polarized continuum model are found to be very small.

On the Performance of Ligand Field Molecular Mechanics for Model Complexes Containing the Peroxido-Bridged [Cu2O2]2+Center

The ability of ligand field molecular mechanics (LFMM) to model accurately the structures and relative conformer energies of complexes containing the [Cu2O2](2+) unit found in oxidized copper type 3 (T3) enzymes is investigated. The consequences of ignoring the coupling between the metal centers are analyzed and predicted to be unimportant with respect to computing molecular geometries. Angular overlap model (AOM) parameters for the peroxido bridge in [Cu2O2](2+) are derived on the basis of the mononuclear model compound [(NH3)3CuO2] for which good ligand field and density functional theory (DFT) calculations are also possible. Metal-ligand pi-bonding parameters are shown to play an important role with the in-plane AOM pi-bonding parameter value being significantly larger than that for the out-of-plane parameter. The LFMM treatment is then extended to the model dinuclear species [(H3N)3CuO2Cu(NH3)3](2+). The planarity of the [Cu2O2](2+) moeity is implicitly obtained by defining the directions of the local Cu-O pi-bonding interactions with respect to the other copper atom, rather than the other oxygen. In conjunction with the force field parameters based on the Merck molecular force field, the model, as implemented in our DommiMOE program (Deeth, R. J. ; Fey, N. ; Williams-Hubbard, B. J. J. Comput. Chem. 2005, 26, 123- 130), is applied to a set of crystallographically characterized small-molecule mimics of the T3 active site. Extensive LFMM conformational searches are carried out for these compounds, and the quality of the LFMM potential energy hypersurface is assessed by comparison with results using DFT. We find that the description of the geometries does not in fact suffer from the neglect of explicit coupling between the metal centers. Moreover, the structures and relative energies obtained by the LFMM conformational searches agree well with both experiment and the DFT values for all systems except one where the LFMM structure which is in best agreement with experiment is about 10 kcal mol(-1) higher than the lowest energy conformer. However, this discrepancy is traced to generic shortcomings in the "organic" force field rather than the LFMM.

Deactivation via ring opening: A quantum chemical study of the excited states of furan and comparison to thiophene

Minimum nuclear arrangements of the ground and low-lying excited electronic states of furan were obtained by means of (time dependent) Kohn–Sham density functional theory. A combined density functional/multi-reference configuration interaction method (DFT/MRCI) was employed to compute the spectral properties at these points. Multiple minima were found on the first excited singlet (S 1) potential energy hypersurface with electronic structures S1, S2, S3 corresponding to the 1 1A 2 (π → 3s-Ryd), 1 1B 2 (π → π ∗), and 2 1A 1 (π → π ∗) states in the vertical absorption spectrum, respectively. In analogy to recently published studies in thiophene [S. Salzmann, M. Kleinschmidt, J. Tatchen, R. Weinkauf, C.M. Marian, Phys. Chem. Chem. Phys. 10 (2008) 380] a deactivation mechanism for electronically excited furan was detected that involves the opening of the pentacyclic ring. We found a nearly barrierless relaxation pathway from the Franck–Condon region along a C–O bond-breaking coordinate. Hereby the initially excited 1B 2 (π → π ∗) state undergoes a conical intersection with a 1B 1 (π → σ ∗) state. The system can return to the electronic ground state through a second conical intersection of the 1(π → σ ∗) state before the minimum of that B 1 state is reached.

Structural, electronic, and magnetic properties of heterofullerene C 58Si with odd number of atoms and a near planar tetracoordinate Si atom

Density functional calculations and minimization techniques have been employed to characterize the structural and electronic properties of [5,6]-heterofullerene-C 58Si- C 2v. Since it has odd number of atoms and a near planar tetracoordinate Si atom on the skeleton of the cage, it has odd number of atoms assembling a cage and is a novel molecule. Vibrational frequencies of the molecule have been calculated at the B3LYP/6-31G* level of theory. The absence of imaginary vibrational frequency confirms that the molecule corresponds to a true minimum on the potential energy hypersurface. Sixteen 13C nuclear magnetic resonance (NMR) spectral signals of C 58Si are characterized, and its heat of formation was estimated in this work.

A Three-State Model for the Photophysics of Guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated 1(pi pi* L(a)) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L(a))CI. On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, 1(pi pi* L(b)) and, in particular, 1(n(O) pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.