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We performed here a systematic ab initio calculations on neutral gas-phase L-proline. A total of 8 local minima were located by geometry optimization of the trial structures using density functional theory (DFT) with B3LYP three parameter hybrid potential coupled with the 6-31G)d( basis set. The absolute minimum obtained will be subject to a rigid potential energy surface (PES) scan by rotating its carboxylic group using the same method with more accurate basis set B3LYP/6-311++G(d,p), to get a deeper idea about its conformational stability. The main aim of the present work was the study of the rigidity of the L-proline structure and the puckering of its pyrrolidine ring.
The reaction mechanism of NCO + NO was studied using density functional theory (DFT) method at 6-31G* level. Geometry optimization, vibrational frequency analysis, and kinetic and thermodynamic properties calculation were performed for reactants, transition state and products. These results indicate that the reaction branch NCO + NO rightwards arrow N2 + CO2 is a major path way of the reaction of NCO + NO on the potential energy surface.
Content available remote Some new studies on intermolecular interaction of C3-Ar complex
Theoretical studies of the intermolecular potential energy surfaces (PESs), the intermolecular vibrational states and the wavefunctions for the C3-Ar complex have been performed at the CCSD(T) level with four different basis sets,. Each PES can be characterized by two symmetrical T-shaped global minimum at ɵ = 75° and ɵ = 105°, a saddle point between them at ɵ = 90°, and two collinear saddle points at ɵ = 0° and ɵ = 180°. Moreover, we also calculated the intermolecular vibrational frequencies and the average structural parameters of the ground vibrational states for the C3-Ar complex. The vdw bending and stretching frequencies are calculated to be 8.682 and 34.511 cm-1, which are in agreement with the experimental results and a more accurate result was determined for the bending mode. Studies show that the larger the basis set is used, the more accurately the results are calculated.
In order to explain the discrepancies between the matrix isolated IR spectra of the products of the H2S + F2 reaction and recent computational studies of this system, we have analyzed the potential energy surface of monofluorinated hydrogen sulfide using high level post-Hartree-Fock methods. The molecular geometric and harmonic vibrational frequencies were calculated at the DFT, MP2, and CCSD levels conjunction with the 6-311++G(d,p) basis set. Besides the already proposed HSF isomer which corresponds to the global energetic minimum, a local minimum corresponding to the HFS isomer as well as a transition state structure between these minima have been also characterized. The calculated vibrational frequencies fit well with the experimental IR spectra and also complement previous ab initio calculations on the HSF isomer.
Effects of isotopic substitutions of "spectator" hydrogens on rate constants have been theoretically examined for the CH3 + H2 ' CH4 + H (I) and C2H + H2 ' C2H2 + H (II) reactions using variational transition state theory with the multidimensional semiclassical tunneling correction. A small but significant secondary isotope effect was found for reaction (I) but almost no isotope effect was found for reaction (II). This is because the potential energy surface for reaction (II) has an "early" character.
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