Younghi Kwon

Theoretical studies of the equilibria between two enolic forms of asymmetric β‐diketones

17O NMR chemical shifts of two frozen enols in hydrogen tautomerizations for asymmetric β‐diketones were calculated at the Hartree–Fock and density functional B3LYP levels of theory using various basis sets. From the theoretical 17O chemical shifts of frozen enols, the equilibrium constants for the enol–enol equilibria were estimated and used for comparison with previous experimental results based on model shifts for the pure enol forms. The results show that our theoretical method can complement some inadequacies in the experimental NMR techniques in evaluating equilibrium constants of systems undergoing rapid dynamic equilibrium. Copyright

Vibrational spectroscopy and conformational studies of 1,4-benzodioxan: ab initio and density functional calculations

Abstract The gas-phase infrared and pure-liquid Raman spectra of 1,4-benzodioxan have been recorded and analyzed. The infrared vibrational frequencies, absolute intensities, and ring-inversion potential energy profile of the molecule have been also predicted using Beckes three-parameter hybrid (B3LYP) method in the density functional theory (DFT) method, as well as the Hartree–Fock (HF) and molecular mechanics (MM3) methods. The vibrational frequencies calculated at the B3LYP levels agree much better with the observed frequencies than those predicted by the HF or MM3 methods. The enlargement of basis sets at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. The ring-inversion process of 1,4-benzodioxan between the twist and bent conformers has also been investigated using the DFT, HF, and MM3 methods. The calculated results at the B3LYP/6-31G* level indicate that the twisted conformer has a lower energy than the bent conformer and that the energy difference between the two forms is 7.5 kcal/mol. This value is 1.3 kcal/mol lower than the barrier of nonbenzene-fused ring 1,4-dioxene.

A theoretical investigation into the conformational changes of dibenzo-p-dioxin, thianthrene, and selenanthrene

Abstract Theoretical ab initio calculations using the HF and B3LYP methods have been carried out to investigate the conformational differences of three cyclic rings, dibenzo-p-dioxin (DPD), thianthrene (THT), and selenanthrene (SET). The physical origin for the conformational preference of each molecule has been studied using the natural bond orbital (NBO) analysis. The NBO results indicate that DPD exists in a planar form due to strong electron delocalization caused by the specific orbital interaction, n π → π CC ∗ , around the X atom. On the other hand, THT and SET exist as puckered forms with high inversion barriers due to less effective electron delocalization. The NBO analysis also shows that the conformational stabilization in DPD is caused by a more effective overlap of the 2 p z – π CC ∗ orbitals, compared with the overlap of the 3 p z – π CC ∗ orbitals in THT.

Conformational stabilization of phthalan: physical origin of tiny ring-puckering barrier

Abstract The conformational property of phthalan has been investigated using ab initio calculation and natural bond orbital (NBO) analysis methods. Geometry optimizations for the planar ( C 2 v ) and puckered ( C s ) conformers have been carried out using the HF, B3LYP, and MP methods, and the results indicate that this molecule has a tiny ring-puckering barrier. This barrier appears to be in good agreement with the previous experimental result. NBO analysis shows that the tiny ring-puckering barrier is closely related to the molecular orbital interactions around the C–O bonds of the five-membered ring. The gas-phase infrared and liquid-phase Raman spectra of phthalan and 1,3-benzodioxole have been recorded and analyzed in terms of C 2 v symmetry. Vibrational frequency calculations using the B3LYP method have also been performed to compare with the spectroscopic data. The B3LYP frequency calculations do a reasonable job of estimating the frequencies.

Molecular structures, conformations, and vibrational spectra of bicyclo(3.1.0)hexane and its oxygen analogues

Abstract The molecular structures and ring-puckering potential energy profiles of bicyclo[3.1.0]hexane and its three oxygen analogues—6-oxabicyclo[3.1.0]hexane, 3-oxabicyclo[3.1.0]hexane, and 3,6-dioxa[3.1.0]hexane—have been reexamined using both ab initio (HF and MP2) and density functional theory (B3LYP) methods. The calculated structural parameters and ring-puckering potential profiles of those molecules have been compared to the previously reported microwave, electron diffraction, and far-infrared data. Our computational results show that the inclusion of electron correlation effects is crucial for the precise prediction of geometrical parameters of such bicyclic systems. The calculated ring-puckering potential energy profiles using the B3LYP method reproduce the experimental profiles more accurately than those predicted by MM3 force-field methods. Vibrational frequency calculations of 6-oxabicyclo[3.1.0]hexane have been also performed to compare with those measured from the infrared and Raman spectroscopy. Comparison of the calculated and experimental results indicates that the B3LYP method has led to the prediction of more accurate vibrational frequencies than the HF and MP2 methods.

Vibrational spectra and conformations of 1,4-cyclohexadiene and its oxygen analogues: ab initio and density functional calculations

Abstract The vibrational spectra and ring-puckering potential energy functions of 1,4-cyclohexadiene, 4 H -pyran and 1,4-dioxin have been examined using a density functional theory (DFT) method as well as the Hartree–Fock (HF) and second-order Moller–Plesset (MP2) methods. The calculated vibrational frequencies and potential energy functions of those molecules have been compared with previously reported experimental data and MM3 results. For all three molecules, the DFT method using Beckes three-parameter functional (B3LYP) has led to the prediction of more accurate vibrational frequencies than the HF and MP2 methods. The enlargement of the basis set at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. In particular, the C–O–C C torsional force field parameters obtained from the B3LYP method have correctly predicted the ring-puckering potential energy functions of the oxygen-containing analogues, 4 H -pyran and 1,4-dioxin, which could not be done by the MM3 method.

Theoretical molecular structures for partially bonded complexes of trimethylamine with SO2 and SO3: ab initio and density functional theory calculations

Abstract The self-consistent reaction field (SCRF) method based on Onsagers reaction field theory is applied to investigate the effect of polar media on molecular structures of complexes of trimethylamime (TMA) with SO x ( x =2,3). The calculated SCRF N–S bond lengths at the MPW1PW91/6-311+G(3df) level are in satisfactory agreement with the experimental N–S bond lengths for the TMA–SO x upon crystallization. The results are enough to demonstrate the usefulness of the reaction field theory in providing qualitative understanding of the medium effect on the partially bonded system such as TMA–SO x .

Keto–enol systems of some fluorinated cyclopentanones and cyclobutanones: density functional theory calculations

Abstract Density functional theory is applied to investigate the stability of keto–enol equilibrium systems for perfluorocyclopentanones and perfluorocyclobutanones: 2H-Perfluorocyclopentanone ( 1k ) and Perfluorocyclopentene-1-ol(e), 2,2H-Perfluorocyclopentanone ( 2k ) and 2H-Perfluorocyclopentene-1-ol ( 2e ), 2H-Perfluorocyclobutanone ( 3k ) and Perfluorocyclobutene-1-ol- ( 3e ), 2,2H-Perfluorocyclobutanone ( 4k ) and 2H-Perfluorobutene-1-o1 ( 4e ). The calculations at the B3LYP/6-311++G**//B3LYP/6-31+G* level show that some fluorinated enols such as 1e and 3e are a little bit more stable than their ketone tautomers 1k and 3k , respectively, whereas enol tautomers 2e and 4e are less stable than their respective keto forms. Our calculational predictions are well consistent with experimental results.

Molecular structures of gauche and anti conformers for oxalyl bromide: ab initio and DFT calculations

Abstract The molecular structure and conformational nature of oxalyl bromide are investigated by the ab initio and DFT methods. Both the MP2 and B3LYP optimized structures are better consistent with the experimental result in the gas phase. The CC bond torsional angle of the gauche conformation is quite sensitive to the choice of the calculational level. The B3LYP potential energy surface around the anti conformation is so flat that the existence of the anti conformation is not certain. It is suggested that the geometry calculations at the MP2 level can explain the nature of conformation in oxalyl bromide better than those at the B3LYP level.

The molecular structures and conformation of o-selenobenzyl fluoride derivatives, ArSeX (Ar=C6H4CH2F; X=CN, Cl, Me): ab initio and DFT calculations

Abstract Ab initio and density functional theory methods are applied to investigate the molecular structures, intramolecular orbital interactions, and 19 F and 77 Se NMR chemical shifts of o -selenobenzyl fluoride derivatives, ArSeX ( Ar = C 6 H 4 CH 2 F ; X = CN , Cl , Me ) , at both RHF and B3LYP levels with the basis sets 6-311G ∗∗ and 6-311+G ∗∗ . There are two stable rotational conformers for ArSeX. The energy differences between both conformers for each compound are small (within 2 kcal/mol) at various levels.