Search

We present a physical model that explains the dependence of the UV cross section of ClOOCl on temperature in the actinic wavelength region measured before by J.J. Lin, A.F. Chen and Y.T. Lee [Chemistry — An Asian Journal, 6, 1664–1678 (2011)]. It was unclear why there are frequency regions in which the cross section decreases with increasing temperature and others in which it increases. This interesting behaviour is now shown to be due to the non-rigidity of the molecule and can be described in terms of the Boltzmann populations of the lowest torsional states and small, non-systematic variations in their absorption cross sections. Combination of these two ingredients produces the non-trivial temperature effect.

International audience; We report on the structures of two conformers of 2-thiophenecarboxaldehyde as obtained using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemical calculations. The microwave spectrum was recorded using two spectrometers operating in the frequency ranges of 2.0 to 26.5 GHz and 26.5 to 40.0 GHz. The spectra of all singly-substituted heavy atom isotopologues 13C, 18O and 34S in their natural abundances could be measured and assigned to determine the gas-phase substitution rs and semi-experimental reSE structures of the most abundant conformer. The spectrum of the 33S isotopologue with its nuclear quadrupole coupling hyperfine structure was analysed, yielding the complete quadrupole tensor with high accuracy. The experimental results are used to map the observed rotational constants to the corresponding molecular structure obtained from quantum chemical calculations, which predicted two conformers with an energy difference of about 6 kJ mol−1 at the MP2/6-311++G(d,p) level of theory. Insight into the conformational stability of aromatic heterocyclic carboxaldehydes and bond situations of the sulfur atom extracted from the hyperfine structure of the 33S nucleus are discussed within the frame of the current literature. This work provides an important contribution to the study and characterisation of sulfur-containing volatile organic compounds.

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.

International audience; Internal rotation is a fundamental motion of end-standing methyl groups that provides important insight on the molecular physics of isolated molecules. The barrier heights of such large amplitude motions are highly sensitive to their molecular and electronic environment. Until now, it is still not possible to accurately determine barrier heights using quantum chemical calculations. To probe for the effect of molecular conformations on the barrier heights of substituted furan rings, the molecular jet Fourier transform microwave spectrum of 5-methyl furfural was recorded in the frequency range from 2 to 40 GHz. Quantum chemical calculations yielded two conformers with a trans and a cis orientation of the formyl group, which were both observed in the experimental spectrum. Torsional splittings due to the internal rotation of the methyl group were resolved and analyzed. The experimental spectra is reproduced with standard deviations close to the experimental accuracy, yielding sets of highly accurate rotational and internal rotation parameters. The results, especially the V3 potentials, are compared to quantum chemical calculations and discussed within the scope of the current literature of other methyl substituted furans, where the methyl group is in close proximity of the furan oxygen atom. Our work provides an accurate evaluation of the different case studies and highlights the bottle necks and future options of current available theoretical techniques.

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.

A systematic study on the magnetic property variations of Pd13 particles as a function of particle sizes and structures, and of Pd13 interactions with the (100) TiO2 anatase surface, was carried out with standard density functional theory (DFT) based method with periodic boundary conditions. The biplanar Pd13 structure with the spin state quintet is obtained to be the ground state in the gas-phase and on the anatase surface, being more stable than the compact icosahedral configuration by 0.067 and 0.865 eV, respectively. The preferred particle nucleation on the anatase surface is that with the largest possible contacts between Pd13 and the oxide surface oxygens. The supported Pd13 particles are better stabilized by 0.3 eV compared to the isolated ones. The profiles of the relative energies as a function of the spin state reveal that the Pd13 magnetic isomers, belonging to the same cluster topology, become closer in energy and nearly degenerate on the anatase surface compared to their gas-phase counterpart...

1 - Article; Raman scattering and Fourier-transform infrared (FT-IR) attenuated transmission reflectance (ATR) spectra of two alpha-amino acids (alpha-AAs), i.e., glycine and leucine, were measured in H2O and D2O (at neutral pH and pD). This series of observed vibrational data gave us the opportunity to analyze vibrational features of both AAs in hydrated media by density functional theory (DFT) calculations at the B3LYP/6-31++G* level. Harmonic vibrational modes calculated after geometry optimization on the clusters containing each AA and 12 surrounding water molecules, which represent primary models for hydration scheme of amino acids, allowed us to assign the main observed peaks.