Yi-gui Wang

Comparison of localization and delocalization indices obtained with Hartree–Fock and conventional correlated methods: Effect of Coulomb correlation

Atomic populations and localization [λ(A)] and delocalization [δ(A,B)] indices (LIs and DIs) are calculated for a large set of molecules at the Hartree–Fock (HF), MP2, MP4(SDQ), CISD, and QCISD levels with the 6‐311++G(2d,2p) basis set. The HF method and the conventional correlation methods [MP2, MP4(SDQ), CISD, and QCISD] yield distinct sets of LIs and DIs. Yet, within the four conventional correlation methods the differences in atomic populations and LIs and DIs are small. Relative to HF, the conventional correlation methods [MP2, MP4(SDQ), CISD, QCISD] yield virtually the same LIs and DIs for molecules with large charge separations while LIs and DIs that differ significantly from the HF values—the LIs are increased and DIs decreased—are obtained for bonds with no or small charge separations. Such is the case in the archetypal homopolar molecules HCCH, H2CCH2, CH3CH3, and “protonated cyclopropane” C3H  7+ , in which case the bonding may be atypical. Relative to HF, the typical effect of the conventional correlation methods is to decrease the DI between atoms.

Conformational effects on optical rotation. 2-Substituted butanes.

The specific rotations of 2-substituted butanes (X = F, Cl, CN, and HCC) were calculated at the B3LYP/aug-cc-pVDZ level as a function of the C-C-C-C torsion angle. The results for the four compounds are remarkably similar, despite large differences in the electronic transition energies. The temperature dependence of the specific rotations for 2-methylbutyronitrile and for 2-chlorobutane was studied to give experimental information about the effect of the torsion angle on the specific rotation. The results were in good accord with B3LYP/aug-cc-pVDZ calculations. The specific rotations derived from the study of 2-chlorobutane are similar to those previously obtained for 3-chloro-1-butene, indicating that the double bond does not have a large effect on the optical rotations, but it did lead to a large difference between calculated and observed specific rotations.

A practical and efficient method to calculate AIM localization and delocalization indices at post‐HF levels of theory

A practical and efficient method is proposed for calculating localization and delocalization indices at post‐Hartree‐Fock levels, and the method is tested at the CISD/6‐311G++(2d, 2p) level for a large set of molecules. Our method, which utilizes wave functions written in the natural molecular orbital format and obtained with GAUSSIAN 94 or GAUSSIAN 98, convincingly extends the concepts established at the HF level.

Optical rotatory dispersion of 2,3-hexadiene and 2,3-pentadiene.

The specific rotation of (P)-2,3-hexadiene (1) was measured as a function of wavelength for the gas phase, the neat liquid, and solutions. There was a surprisingly large difference between the gas phase and condensed phase values. The specific rotation was calculated using B3LYP and CCSD, and the difference in energy between the three low energy conformers was estimated at the G3 level. The Boltzmann-averaged CCSD-calculated rotations using the gauge independent velocity gauge representation, as well as the B3LYP values, are in agreement with the gas-phase experimental values. In order to avoid possible problems associated with the conformers of 1, 2,3-pentadiene (2) also was examined. Here again, there was a large difference between the gas-phase and condensed-phase specific rotations, with the CCSD velocity gauge (and B3LYP) results being close to the gas-phase experimental values. The possibility that 2,3-pentadiene could be distorted on going from the gas to liquid phase, thereby accounting for the effect of phase on the specific rotation, was examined via a Monte Carlo statistical mechanics simulation. No effect on the geometry was found. Specific rotations of 1 found in solutions were similar to those for the liquid phase, indicating that the phase difference was not due to association.

Towards the Accurate and Efficient Calculation of Optical Rotatory Dispersion Using Augmented Minimal Basis Sets

It has been recognized that quantum-chemical predictions of dispersive (nonresonant) chiroptical phenomena are exquisitely sensitive to the periphery of the electronic wavefunction. To further elaborate and potentially exploit this assertion, linear-response calculations of specific optical rotation were performed within the framework of density-functional theory (DFT) by augmenting small basis sets (e.g., STO - 3G and 3 - 21G) for the core and valence electrons with diffuse functions taken from substantially larger bases (e.g., aug-cc-pVXZ where X = D, T, or Q). Of particular interest was the ability of such computationally efficient (augmented small-basis) model chemistries to reproduce results derived from more expensive (canonical large-basis) schemes. The results appear to be quite promising, with the augmented minimal-basis ansatz often yielding wavelength-resolved rotatory powers close to those deduced from standard DFT(B3LYP)/aug-cc-pVXZ treatments. Analogous linear-response analyses were performed by means of coupled-cluster singles and doubles (CCSD) theory, once again leading to augmented small-basis estimates of specific rotation in reasonable accord with their large-basis counterparts. Although CCSD predictions were deemed to be slightly worse than those obtained from DFT, they still were of sufficient quality for such reduced-basis calculations to be considered viable for exploratory work.

Disparate Behavior of Carbonyl and Thiocarbonyl Compounds: Acyl Chlorides vs Thiocarbonyl Chlorides and Isocyanates vs Isothiocyanates

The reaction of benzoyl chloride with methanol catalyzed by pyridine is 9 times more rapid than is the same reaction with thiobenzoyl chloride. The difference in reactivity, as well as the dealkylation reactions that occur when the reaction of thiobenzoyl chloride is catalyzed by bases such as Et(3)N, can be understood in terms of the charge distributions in the intermediate acylammonium ions. The reaction of PhNCO with ethanol occurs at a much higher rate (4.8 x 10(4)) than that of PhNCS, corresponding to a difference in activation free energies for the additions of 6 kcal/mol. Transition states for each of these reactions were located, and each involves two alcohol molecules in a hydrogen bonded six-membered ring arrangement. Information concerning differences in reactivity was derived from analysis of Hirshfeld atomic charge distributions and calculated hydrogenolysis reaction energies.

Intramolecular Nonbonded Attractive Interactions: 1-Substituted Propenes

Whereas cis-substituted alkenes are normally significantly less stable than the trans-isomers, there is a group of 1-substituted propenes (X = F, OMe, Cl, Br, SMe) where the cis-isomers are the more stable. The calculated structures show that there is steric repulsion with the cis-isomers. However, this is overcome by attractive Coulombic interactions when X = F or OMe and by attractive dispersive interactions when X = Cl or Br. It was possible to calculate the magnitude of the latter term via the summation of the appropriate MP2 pair energies. The calculated and observed energy differences could be reproduced by a summation of steric, electrostatic, and dispersive interactions.

Excited States and Photochemistry of Bicyclo(1.1.0)butane

Calculations on the excited states of bicyclo[1.1.0]butane in the gas phase by different theoretical methods using several basis sets were performed. In general, the agreement between calculated and experimental excitation energies for bicyclo[1.1.0]butane in the gas phase is very good. Reviews of the solution-phase photochemistry of bicyclo[1.1.0]butane as well as previous calculations on the ground and excited states of bicyclo[1.1.0]butane are given to provide a necessary perspective of the photochemistry of bicyclo[1.1.0]butane in solution. To simulate the solution-phase photochemistry of bicyclo[1.1.0]butane, a well potential is added to the Kirkwood-Onsager model for obtaining solvation energies of molecules in solution. The addition of the well potential gives rise to a blue-shift of all gas-phase excitation energies in solution. However, there is also the very important added effect of providing an increase in Rydberg-valence mixing of solution-phase excited states. It is this mixing of antibonding valence character into the solution-phase excited states that is necessary to explain the solution-phase photochemistry of bicyclo[1.1.0]butane through bond-breaking and the formation of a conical intersection intermediate.

Conformational energies for 2-substituted butanes

The conformational free energies for some 2‐substituted butanes where X = F, Cl, CN, and CCH were calculated using G3‐B3, CBS‐QB3, and CCSD(T)/6‐311++G(2d,p) as well as other theoretical levels. The above methods gave consistent results with free energies relative to the trans conformers as follows: X = CCH, g+ = 0.77 ± 0.05 kcal/mol. g− = 0.88 ± 0.05 kcal/mol; X = CN, g+ = 0.85 ± 0.05 kcal/mol, g− = 0.75 ± 0.05 kcal/mol; X = Cl, g+ = 0.70 ± 0.05 kcal/ml, g− = 0.80 ± 0.05 kcal/mol; and X = F, g+ = 0.53 ± 0.05 kcal/mol, g− = 0.83 ± 0.05 kcal/mol. The conformational free energies also were estimated using the observed liquid phase IR spectra and intensities calculated using B3LYP/6‐311++G** and MP2/6‐311++G**. The rotational free energy profiles for all of the compounds were estimated at the G3‐B3 level.