No abstract

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS.(CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm-1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer.

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X ) D, T, Q), and aug-cc-pVDZ] were carried out to study the (OCS) 2 ‚CO 2 van der Waals trimers. Three barrel-like structures [C 1 (two) and C 2 symmetry] and three planar (C s ) structures were located on the potential energy surface. Their CBS-MP2/cc-pVXZ (X ) D, T, Q) stabilization energies are 1760 (C 1 ), 1514 (C 2 ), 1660 (C 1 ), 1325 (C s ), 1556 (C s ), and 1398 (C s ) cm -1 , respectively. The most stable structure (one of the C 1 barrel-like isomers) has bond lengths, angles, rotational constants, and dipole moment that agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the rest of the isomers strongly suggests that the experimentally unobserved structures might also be present in the supersonic adiabatic expansion of the gas in the microwave spectroscopic studies as in the case of the (CO 2 ) 3 trimer where both barrel-like and planar isomers have been reported to exist. The many-body symmetry-adapted perturbation theory helps to shed some light on the nature of the interactions leading to the formation of the different isomers. While the dispersion forces make the most important attractive contributions to the interaction energies of the (OCS) 2 ‚CO 2 isomers, the induction forces make contributions similar in magnitude to the electrostatic forces. The three-body contributions are small and stabilizing for the barrel-like structures and are less important for the cyclic isomers.

The effect of correlation energy, basis set size, zero-point energy (ZPE) correction, and solvation on the reaction mechanism of the ketene-imine cycloaddition reaction has been investigated. The electrostatic solvent effect was studied with a self-consistent reaction field method in which the solvation energy is obtained using a multipole expansion of the molecular charge distribution. The ab initio results have been analyzed by means of a theoretical method based on the expansion of the MOs of the supermolecule in terms of those of the reactants and the performance of the configuration analysis. In gas phase, due to the correlation energy and/or the ZPE corrections, the reaction is predicted to be a one-step process. In solution, the stabilization of the charge-transferred configurations results in the occurrence of a very stable, Zwitterionic intermediate giving a two-step mechanism.

No abstract

Ab initio calculations at the MP2/6-31G*//HF/6-31G* level have been carried out to study both thermal and Lewis acid-catalyzed [A + Asl-cheletropic and [As + Asl-cycloaddition reactions of sulfur dioxide with 1,3-dienes (1,3-butadiene and isoprene). The ab initio results have been analyzed by means of a theoretical method based on the expansion of the MOs of the supermolecule in terms of those of the fragments and the performance of the configuration analysis. While the thermal cheletropic reaction proceeds in a way which closely resembles a a donation/jr back-donation mechanism, the alternative Diels-Alder cycloaddition mostly involves charge transfer in one direction, HOMO(diene) -LUMO(dienophile). Although a Diels-Alder adduct is the kinetic product of reactions of sulfur dioxide with conjugated dienes, the cheletropic reaction gives rise to a thermodynamically more stable five-membered ring adduct, in agreement with the experimental facts.

The bifunctional catalysis by HF of the hydrogenation of ethylene has been studied by means of an extension to three-molecule interactions of a theoretical method for bimolecular interactions. A comparative theoretical analysis of ab initio molecular orbital calculations on the transition states for the direct addition of molecular hydrogen to ethylene and for the HF-catalyzed hydrogenation of ethylene has been done. Both in the least motion and the reduced symmetry reactions pseudoexcitation is very important in the formation of the two new C-H bonds. The presence of the catalyst reduces the importance of pseudoexcitation and provides an alternative mechanism for electron rearrangement along the reaction coordinate more favorable energetically by acting as an electronic bridge.The splittings between the two lowest energy cation states and the two lowest energy anion states are calculated in both the Koopmans' theorem and ASCF approximations for a series of rigid nonconjugated dienes, with the two double bonds separated by 4-12 C-C u bonds. To determine the role of long-range through-bond coupling on the splittings, the calculations are carried out using several basis sets-STO-3G, 3-21G, 6-311G, 6-31+G, and D95v-which differ in the radial extent of the outermost basis functions. The dependence of the various splittings on n, the number of C-C u bonds separating the ethylenic groups, is examined. The u+,u-splittings for the dienes with bridges containing 8, 10, and 12 C-C bonds are consistent with an exponential dependence on n, independent of the basis set employed. Similarly, when the STO-3G basis set is used, the x+*,T-* splittings for these compounds are also consistent with an exponential n dependence. However, with more flexible basis sets, the u+*,u-* splittings for the longer dienes show small, but significant, deviations from an exponential n dependence. Moreover, when the results for the dienes with the four-and six-bond bridges are included, single exponentials no longer provide good fits to the *+,A-or the a+*,*-* splittings for the entire series of compounds. The deviations from exponential behavior are greater with the nonminimal basis sets and at the Koopmans' theorem than at the ASCF level of theory. It is suggested that the nonexponential distance dependence is due to the importance of multiple through-bond pathways.

Hetero Diels-Alder reactions have attracted considerable attention in the last years due to their interesting role in organic synthesis.1 This methodology allows for the preparation of several types of six-membered-ring heterocycles and acyclic structures and has been used in key steps of the total synthesis of natural products.1•2 *Despite their synthetic appeal, it has been only recently that hetero Diels-Alder reactions have been studied using theoretical methods, in contrast to the case of all-carbon Diels-Alder reactions.3-7 According to the ab initio calculations, hetero Diels-Alder reactions are predicted to take place through concerted but slightly asynchronous transition structures in a similar way to the all-carbon version.5-7In a very recent communication, Deguin and Vogel reported that 1,3-dienes react with sulfur dioxide to give Diels-Alder adducts.8 It is well known that sulfur dioxide undergoes cheletropic reactions with conjugated dienes to give sulfolenes, but only a few examples of such Diels-Alder reactions have been reported to date.9 In this communication we report the results of ab initio calculations10 on the Lewis acid-catalyzed (BH3 was used as a model for the Lewis acid catalyst) Diels-Alder reaction of isoprene with sulfur dioxide (Scheme 1). Geometry optimizations were performed at the restricted Hartree-Fock (RHF) level using GAUSSIAN 92.11 Structures were fully optimized with the 3-21G * basis set,12 followed by vibrational frequency calculations. Schlegel's algorithm13 was used to locate the transition structures.Single-point RHF/6-31G* calculations were carried out on the RHF/3-21G* geometries.14 Four transition structures, correct) Boger, D. L.; Weinreb, S. N. Hetero Diels-Alder Methodology in Organic Synthesis', Academic Press; San Diego, 1987.