Ozonolysis of 2-Methyl-2-pentenal: New Insights from Master Equation Modeling.

Experimental and theoretical studies were carried out to investigate the ozonolysis of trans-2-methyl-2-pentenal. The experiments were conducted in atmospheric simulation chambers coupled to a Fourier transform infrared (FTIR) spectrometer and a gas chromatograph-mass spectrometer at room temperature and atmospheric pressure in the presence of an excess of cyclohexane in dry conditions (RH < 1%). The ozonolysis reaction was investigated theoretically from the results of accurate density functional (M06-2X) and ab initio [CCSD(T)] computations, employing the AVTZ basis set. The sequence of reaction steps was established, and the system of kinetics equations was modeled using MESMER. In the first step, a primary ozonide is formed, which then decomposes along two pathways. The principal ozonolysis products are propanal, methylglyoxal, ethylformate, and a secondary ozonide. An interesting competition between sequential reaction steps and well-skipping is found, which leads to an inversion of the expected methylglyoxal/propanal product ratio at temperatures below 210 K. The mechanism of the "hot ester" reaction channel of the Criegee intermediate was revisited. The computed ozonolysis rate constant and product branching ratio are in excellent agreement with the experimental data that are also reported in the present work.

Lifetimes and decay mechanisms of isotopically substituted ozone above the dissociation threshold: matching quantum and classical dynamics.

Energies and lifetimes of vibrational resonances were computed for 18O-enriched isotopologue 50O3 = {16O16O18O and 16O18O16O} of the ozone molecule using hyperspherical coordinates and the method of complex absorbing potential. Various types of scattering resonances were identified, including roaming OO-O rotational states, the series corresponding to continuation of bound vibrational resonances of highly excited bending or symmetric stretching vibrational modes. Such a series become metastable above the dissociation limit. The coupling between the vibrationally excited O2 fragment and rotational roaming gives rise to Feshbach type resonances in ozone. Different paths for the formation and decay of symmetric 16O18O16O and asymmetric species 16O16O18O were also identified. The symmetry properties of the total rovibronic wave functions of the 18O-enriched isotopologues are discussed in the context of allowed dissociation channels.

UV-spectrum and photodecomposition of peroxynitrous acid in the troposphere.

The UV spectrum of peroxynitrous acid, HOONO, was computed at the B3LYP/AVTZ and MCSCF/AVTZ levels using the fewest switches surface hopping algorithm. Due to large-amplitude vibrational motions of this molecule, the maxima in the simulated spectra are displaced from the positions of vertical excitations. The three lowest excited electronic singlet states, which are all repulsive, can be reached by UV absorption. The photolysis products are determined, and the photolysis rate constant is provided for the first time. We found that near the tropopause the photolysis rate constant J ≈ 6 × 10-4 s-1, exceeds that for thermal decomposition by two orders of magnitude. The photolysis lifetime is about 30 minutes. Thus, photolysis is an important process and should be included in atmospheric models.

Localized and delocalized bound states of the main isotopologue 48O3 and of 18O-enriched 50O3 isotopomers of the ozone molecule near the dissociation threshold.

Knowledge of highly excited rovibrational states of ozone isotopologues is of key importance for modelling the dynamics of exchange reactions, for understanding longstanding problems related to isotopic anomalies of the ozone formation, and for analyses of extra-sensitive laser spectral experiments currently in progress. This work is devoted to new theoretical study of high-energy states for the main isotopologue 48O3 = 16O16O16O and for the family of 18O-enriched isotopomers 50O3 = {16O16O18O, 16O18O16O, 18O16O16O} of the ozone molecule considered using a full-symmetry approach. Energies and wave functions of bound states near the dissociation threshold are computed in hyperspherical coordinates accounting for the permutation symmetry of three identical nuclei in 48O3 and of two identical nuclei in 50O3, using the most accurate potential energy surface available now. The obtained vibrational band centers agree with observed ones with the root-mean-squares deviation of about 1 cm-1, making the results appropriate for assignments and analyses of future experimental spectra. The levels delocalized between the three potential wells of ozone isomers are computed and analyzed. The states situated deep in the three (for 48O3) or two (for 50O3) equivalent potential wells have similar energies with negligible splitting. However, the states situated just below the potential barriers separating the wells, are split due to the tunneling between the wells resulting in the splitting of rovibrational sub-bands. We evaluate the amplitudes of the corresponding effects and consider possible perturbations in vibration-rotation bands due to interactions between three potential wells. Theoretical predictions for the splitting of observable band centers are provided for the first time.

Dichlorine peroxide (ClOOCl), chloryl chloride (ClCl(O)O) and chlorine chlorite (ClOClO): very accurate ab initio structures and actinic degradation.

The structural parameters of the three most stable isomers with formula Cl2O2, dichlorine peroxide, chloryl chloride and chlorine chlorite, were determined by high-level ab initio theory. The effects of core-valence electronic correlation as well as relativistic corrections were included in the calculations, and vibrational averaging of the so-obtained structures was performed. The bond distances agree with experimental data, where available, to within 0.01 Å, an unprecedented accuracy in particular for the floppy dichlorine peroxide molecule. The UV spectra of the three molecules were computed and decay pathways investigated. Under actinic UV radiation at 248 nm dichlorine peroxide decomposes principally according to ClOOCl → 2Cl + O2, in a synchronous concerted mechanism. In the low-energy tail region of this signal, the decay proceeds in a non-synchronous manner. There is also a low probability of the decay channel towards ClOO + Cl, whereas ClO molecules were not found. Chloryl chloride absorbs strongly around 300 nm. It disintegrates into ClO2 + Cl, where ClO2 seems to be formed mainly in excited electronic states which decompose further into Cl + O2. Hence chloryl chloride, though much less abundant in the stratosphere than dichlorine peroxide, also contributes to ozone depletion.

Non-adiabatic effects in the H3+ spectrum.

The effect of non-adiabatic coupling on the computed rovibrational energy levels amounts to about 2 cm-1 for H3+ and must be included in high-accuracy calculations. Different strategies to obtain the corresponding energy shifts are reviewed in the article. A promising way is to introduce effective vibrational reduced masses that depend on the nuclear configuration. A new empirical method that uses the stockholder atoms-in-molecules approach to this effect is presented and applied to H3+. Furthermore, a highly accurate potential energy surface for the D3+ isotopologue, which includes relativistic and leading quantum electrodynamic terms, is constructed and used to analyse the observed rovibrational frequencies for this molecule. Accurate band origins are obtained that improve existing data. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Accurate theoretical characterization of dioxygen difluoride: a problem resolved.

Dioxygen difluoride is a tough molecule that has defied accurate theoretical description for many decades. In the present work we have identified the reason for this resistance: the flatness of the OO, and more important OF, stretching potential energy curves, which make it difficult to localise the global minimum. It is not related to the weak multi-reference character. Using high-level CCSD(T)-F12/VTZ-F12 ab initio theory, the global minimum has been properly located and vibrationally averaged bond lengths obtained. These vibrationally averaged parameters agree with experimental data to within 0.01 Å. Averaging was found essential to achieve this unprecedented accuracy. We have then simulated the IR and UV spectra, which compare well with experimental data and permit identification of the observed transitions.

Tropospheric Reactions of Triazoles with Hydroxyl Radicals: Hydroxyl Addition is Faster than Hydrogen Abstraction.

We present the results of a systematic investigation, at the BHandHLYP/AVTZ density functional (DFT) level, of the tautomeric equilibria of 1,2,3- and 1,2,4-triazoles and their reactions with hydroxyl radicals in the gas phase. A total of twenty-six chemical reactions has been studied, and thermodynamical data and rate constants are reported. The reactions can be classified in two categories: hydrogen abstraction and OH addition. Nine of these reactions are favourable at room temperature. It was found that OH addition proceeds more rapidly than hydrogen abstraction, by several orders of magnitude. For the most stable tautomers, which presumably dominate in the gaz-phase, the fastest reactions are OH addition to 2H-1,2,3-triazole and site-specific OH addition to carbon atom 5 of 1H-1,2,4-triazole. In absolute values, however, the rate constants are rather small, k=5.82×10-20  cm3 s-1 and k=4.75×10-18  cm3 s-1 , respectively, at room temperature. Therefore, under the conditions of the troposphere, triazoles cannot be eliminated by reactions with OH. The accuracy of the computational approach has been demonstrated by studying the tautomeric equilibria of 1,2,3- and 1,2,4-triazoles in the gas phase. The computed equilibrium constants are in excellent agreement with those derived from spectroscopic observations. Additional assessment of the quality of the computed data was made by comparison with the results from high-level CCSD(T)-F12/AVTZ ab initio calculations for three exemplary structures. This comparison demonstrates the high accuracy of our DFT results of 0.29 kcal mol-1 for energy differences between stable isomers.

The gas-phase structure of dimethyl peroxide.

There has been a disagreement amongst experimentalists and between experimentalists and theoreticians as to the gas-phase structure of dimethyl peroxide. We have investigated this problem with high-level CCSD(T)-F12 and MRCI procedures. There can be no doubt anymore that, at the minimum of the potential energy surface, the COOC fragment has a trans-structure. The dynamical structure of the molecule can, however, be different and be explained by the very slow torsional motion. We have analysed the dynamical structure using numerical wavefunctions of the torsional motion and a fully optimized potential curve of MP2/aug-cc-pVTZ quality. Computational and all experimental results are shown to be in complete agreement. The problem that has persisted for more than thirty years, highlighted in a recent review article by Oberhammer titled "Gas phase structures of peroxides: experiments and computational problems", has been resolved.

CO2 diffusion in champagne wines: a molecular dynamics study.

Although diffusion is considered as the main physical process responsible for the nucleation and growth of carbon dioxide bubbles in sparkling beverages, the role of each type of molecule in the diffusion process remains unclear. In the present study, we have used the TIP5P and SPC/E water models to perform force field molecular dynamics simulations of CO2 molecules in water and in a water/ethanol mixture respecting Champagne wine proportions. CO2 diffusion coefficients were computed by applying the generalized Fick's law for the determination of multicomponent diffusion coefficients, a law that simplifies to the standard Fick's law in the case of champagnes. The CO2 diffusion coefficients obtained in pure water and water/ethanol mixtures composed of TIP5P water molecules were always found to exceed the coefficients obtained in mixtures composed of SPC/E water molecules, a trend that was attributed to a larger propensity of SPC/E water molecules to form hydrogen bonds. Despite the fact that the SPC/E model is more accurate than the TIP5P model to compute water self-diffusion and CO2 diffusion in pure water, the diffusion coefficients of CO2 molecules in the water/ethanol mixture are in much better agreement with the experimental values of 1.4 - 1.5 × 10(-9) m(2)/s obtained for Champagne wines when the TIP5P model is employed. This difference was deemed to rely on the larger propensity of SPC/E water molecules to maintain the hydrogen-bonded network between water molecules and form new hydrogen bonds with ethanol, although statistical issues cannot be completely excluded. The remarkable agreement between the theoretical CO2 diffusion coefficients obtained within the TIP5P water/ethanol mixture and the experimental data specific to Champagne wines makes us infer that the diffusion coefficient in these emblematic hydroalcoholic sparkling beverages is expected to remain roughly constant whathever their proportions in sugars, glycerol, or peptides.

Unveiling the Interplay Between Diffusing CO2 and Ethanol Molecules in Champagne Wines by Classical Molecular Dynamics and (13)C NMR Spectroscopy.

The diffusion coefficients of carbon dioxide (CO2) and ethanol (EtOH) in carbonated hydroalcoholic solutions and Champagne wines are evaluated as a function of temperature by classical molecular dynamics (MD) simulations and (13)C NMR spectroscopy measurements. The excellent agreement between theoretical and experimental diffusion coefficients suggest that ethanol is the main molecule, apart from water, responsible for the value of the CO2 diffusion coefficients in typical Champagne wines, a result that could likely be extended to most sparkling wines with alike ethanol concentrations. CO2 and EtOH hydrodynamical radii deduced from viscometry measurements by applying the Stokes-Einstein relationship are found to be mostly constant and in close agreement with MD predictions. The reliability of our approach should be of interest to physical chemists aiming to model transport phenomena in supersaturated aqueous solutions or water/alcohol mixtures.

Vibrating H3(+) in a uniform magnetic field.

Potential energy surfaces are obtained for singlet H3(+) in magnetic fields of up to 2350 T. The magnetic interaction was treated by first-order perturbation theory and the interaction terms computed ab initio. They were then fitted to a functional form and added to a recent, highly accurate adiabatic potential energy surface. In its most stable orientation, the molecule is arranged such that the magnetic field vector is in the molecular plane. The most stable configuration is no longer D3h as in the field-free case, but C2v, though the stabilization energy is extremely small, of the order of 0.01 cm(­1) for a 2350 T field. Finally, we have calculated, for a range of magnetic field strengths and orientations, all the vibrational eigenvalues that are below the barrier to linearity in the field-free case.

Core-mass nonadiabatic corrections to molecules: H2, H2+, and isotopologues.

For high-precision calculations of rovibrational states of light molecules, it is essential to include non-adiabatic corrections. In the absence of crossings of potential energy surfaces, they can be incorporated in a single surface picture through coordinate-dependent vibrational and rotational reduced masses. We present a compact method for their evaluation and relate in particular the vibrational mass to a well defined nuclear core mass derived from a Mulliken analysis of the electronic density. For the rotational mass we propose a simple, but very effective parametrization. The use of these masses in the nuclear Schrödinger equation yields numerical data for the corrections of a much higher quality than can be obtained with optimized constant masses, typically better than 0.1 cm(-1). We demonstrate the method for H(2), H(2)(+), and singly deuterated isotopologues. Isotopic asymmetry does not present any particular difficulty. Generalization to polyatomic molecules is straightforward.

Spectroscopy of H3+ based on a new high-accuracy global potential energy surface.

The molecular ion H(3)(+) is the simplest polyatomic and poly-electronic molecular system, and its spectrum constitutes an important benchmark for which precise answers can be obtained ab initio from the equations of quantum mechanics. Significant progress in the computation of the ro-vibrational spectrum of H(3)(+) is discussed. A new, global potential energy surface (PES) based on ab initio points computed with an average accuracy of 0.01 cm(-1) relative to the non-relativistic limit has recently been constructed. An analytical representation of these points is provided, exhibiting a standard deviation of 0.097 cm(-1). Problems with earlier fits are discussed. The new PES is used for the computation of transition frequencies. Recently measured lines at visible wavelengths combined with previously determined infrared ro-vibrational data show that an accuracy of the order of 0.1 cm(-1) is achieved by these computations. In order to achieve this degree of accuracy, relativistic, adiabatic and non-adiabatic effects must be properly accounted for. The accuracy of these calculations facilitates the reassignment of some measured lines, further reducing the standard deviation between experiment and theory.

Calibration-quality adiabatic potential energy surfaces for H3(+) and its isotopologues.

Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H(3)(+). The underlying Born-Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born-Oppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41,655 ab initio points is presented which gives a standard deviation better than 0.1 cm(-1) when restricted to the points up to 6000 cm(-1) above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The ro-vibrational transition frequencies for H(3)(+), H(2)D(+), and HD(2)(+) are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute ro-vibrational energy levels, which makes explicit allowance for the inclusion of non-adiabatic effects, reproduces all the known ro-vibrational levels of the H(3)(+) isotopologues considered to better than 0.2 cm(-1). This represents a significant (order-of-magnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of non-adiabatic effects in the H(3)(+) isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of H(3)(+) resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16,000 cm(-1), and (c) results suggest that we can predict accurately the lines of H(3)(+) towards dissociation and thus facilitate their experimental observation.

Precision measurements and computations of transition energies in rotationally cold triatomic hydrogen ions up to the midvisible spectral range.

First-principles computations and experimental measurements of transition energies are carried out for vibrational overtone lines of the triatomic hydrogen ion H(3)(+) corresponding to floppy vibrations high above the barrier to linearity. Action spectroscopy is improved to detect extremely weak visible-light spectral lines on cold trapped H(3)(+) ions. A highly accurate potential surface is obtained from variational calculations using explicitly correlated Gaussian wave function expansions. After nonadiabatic corrections, the floppy H(3)(+) vibrational spectrum is reproduced at the 0.1 cm(-1) level up to 16600 cm(-1).

H4(+): what do we know about it?

The potential energy surface of H(4)(+) has been analyzed and stationary points and minima of intersections characterized by benchmark multireference configuration interaction calculations with basis sets as large as augmented septuble zeta. No evidence for minima other than those of the well established stable C(2v) configuration has been found. Some of the results obtained previously at a lower level of ab initio theory had to be revised.

Hyperspherical nuclear motion of H3 + and D3 + in the electronic triplet state, a 3Sigmau +.

The potential energy surface of H(3) (+) in the lowest electronic triplet state, a (3)Sigma(u) (+), shows three equivalent minima at linear nuclear configurations. The vibrational levels of H(3) (+) and D(3) (+) on this surface can therefore be described as superimposed linear molecule states. Owing to such a superposition, each vibrational state characterized by quantum numbers of an isolated linear molecule obtains a one- and a two-dimensional component. The energy splittings between the two components have now been rationalized within a hyperspherical picture. It is shown that nuclear motion along the hyperangle phi mainly accounts for the splittings and provides upper bounds. This hyperspherical motion can be considered an extension of the antisymmetric stretching motion of the individual linear molecule.

Accurate ab initio based multisheeted double many-body expansion potential energy surface for the three lowest electronic singlet states of H3+.

The authors present diabatic and adiabatic potential energy surfaces for the three lowest electronic singlet states of H3+. The modeling of the surfaces is based on the multi-sheeted double many-body expansion method which consists of dressing the various matrix elements of the diatomics-in-molecules potential matrix with three-body terms. The avoided crossing between the two lowest states and the conical intersection between the second and the third state are accurately represented by construction.

H3+ in the electronic triplet state: current status.

We review the theoretical work carried out on the tri-hydrogen ion in the electronic triplet state 1(3)E', which is split into a3Sigma+u and 2(3)A' by vibronic interaction. We begin with an overview on analytical potential energy surfaces and calculations of rovibrational states by focusing on our own results, which are based on the most accurate potential energy surfaces available so far. This is followed by an examination of the selection rules and predictions of infrared transition frequencies. Finally, we discuss the Slonczewski resonance states supported by the upper sheet of the potential energy surface. Theoretical work reported here may be of interest for future experiments on the title ion.