Gyoosoon Park

Ab initio studies of the allylic hydroxylation: DFT calculation on the reaction of 2-methyl-2-butene with selenium dioxide

The mechanism of the allylic oxidation of 2-methyl-2-butene with selenium dioxide has been investigated by ab initio quantum mechanics. Transition states for two major steps (an ene reaction and a [2,3]-sigmatropic rearrangement) of this reaction have been optimized by the B3LYP/6-311+G(d,p) method. A comparison of the energies of the transition states shows that the anti-endo and syn-endo approaches are the efficient routes in the ene reaction and the methyl (C4) group is sited in a pseudo-equatorial environment in cyclic transition states during the [2,3]-rearrangement. Calculations also show the kind of the terminal alkyl (C4) substituents may control (E)-selectivity in the formation of the allylic alcohols.

Rotational barriers of disilane, hexafluorodisilane, and hexamethyldisilane: Ab initio, density functional, and molecular mechanics (MM3) studies

We investigated structures, vibrational frequencies, and rotational barriers of disilane (Si2H6), hexafluorodisilane (Si2F6), and hexamethyldisilane (Si2Me6) by using ab initio molecular orbital and density functional theories. We employed four different levels of theories (i.e., HF/6–31G*, MP2/6–31G*, BLYP/6–31G*, and B3LYP/6–31G*) to optimize the structures and to calculate the vibrational frequencies (except for Si2Me6 at MP2/6–31G*). MP2/6–31G* calculations reproduce experimental bond lengths well, while BLYP/6–31G* calculations largely overestimate some bond lengths. Vibrational frequencies from density functional theories (BLYP/6–31G* and B3LYP/6–31G*) were in reasonably good agreement with experimental values without employing additional correction factors. We calculated the ΔG‡(298 K) values of the internal rotation by correcting zero‐point vibration energies, thermal vibration energies, and entropies. We performed CISD/6–31G*//MP2/6–31G* calculations and found the ΔG‡(298 K) values for the internal rotation of Si2H6, Si2F6, and Si2Me6 to be 1.36, 2.06, and 2.69 kcal/mol, respectively. The performance of this level was verified by using G2 and G2(MP2) methods in Si2H6. According to our theoretical results, the ΔG‡(298 K) values were marginally greater than the ΔE‡(0 K) values in Si2F6 and Si2Me6 due to the contribution of the entropy. In Si2H6 the ΔE‡(0 K) and ΔG‡(298 K) values were coincidently similar due to a cancellation of two opposing contributions between zero‐point and thermal vibrational energies, and entropies. Our calculated ΔG‡(298 K) values were in good agreement with experimental values published recently. In addition, we also performed MM3 calculations on Si2H6 and Si2Me6. MM3 calculated rotational barriers and thermodynamic properties were compared with high level ab initio results. Based on this comparison, MM3 calculations reproduced high level ab initio results in rotational barriers and thermodynamic properties of Si2H6 derivatives including vibrational energies and entropies, although large errors exist in some vibrational frequencies. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1523–1533, 1997

Computational studies on the asymmetric induction in intramolecular 1,3-dipolar cycloaddition of (S)-5-phenyl-morpholin-2-one

The intramolecular 1,3-dipolar cycloaddition reactions of chiral morpholinone-derived azomethine ylides with tethered alkenes and alkynes have been investigated by semi-empirical methods and ab initio quantum mechanics. The observed efficient direction of the asymmetric induction in the 1,3-dipolar cycloaddition has been modelled from a comparison of the energies of the four possible transition states arising from addition to either face of E- or Z-configured ylides. In the reaction of morpholinone with hex-5-enal, the most favourable transition state is shown to be the anti-addition to the E-ylide (Eact ∼12 kcal/mol by HF/3-21G). By contrast, reaction with hex-5-ynal led to anti-addition from the Z-ylide (Eact ∼19 kcal/mol by HF/3-21G) exhibiting the lowest transition state energy. The resulting trends of relative transition state energy are in excellent agreement with the experimental observations.

An ab initio and X-ray study of morpholinone derivatives: the intermolecular 1,3-dipolar cycloaddition of a derived ylide with maleimide

Abstract Conformational analysis of N-acetyl 5-phenyl-morpholin-2-one, shows two main conformers. That which is also observed in the crystal structure determination contains the C(5)-phenyl substituent in a pseudo-axial position. The favored conformations of the paraformaldehyde-derived ylide, 1, were similarly established and two conformers were also found distinguished by the position of the C(5)-phenyl group either in a pseudo-axial or equatorial position. Four reaction pathways for the cycloaddition of maleimide with each ylide conformer, have been examined by ab initio methods (HF/6-31+G∗ and B3LYP/6-31+G∗) and our calculated transition state energies are in good agreement with the ratio of products observed experimentally.