Ke-Sheng Song

PM3 studies on the complexation of α-cyclodextrin with benzaldehyde and acetophenone

Abstract PM3 was applied to the complexation of α-cyclodextrin with benzaldehyde and acetophenone satisfactorily. The results, in agreement with the experimental observations, suggest that the orientation in which the substituent groups of the guest compounds located near the secondary hydroxyls of the α-cyclodextrin cavity was favorable in energy for both α-cyclodextrin–benzaldehyde and α-cyclodextrin–acetophenone complexes.

Charge-transfer Interaction: A Driving Force for Cyclodextrin Inclusion Complexation

PM3 calculations were performed on the complexation of α-cyclodextrin (α-CD) with nitrobenzene, benzoic acid, benzoate anion, 4-nitrophenol, and 4-nitrophenolate anion. The results, in agreement with the experimental observations, indicated that the complex α-CD-benzoic acid was more stable than α-CD-nitrobenzene, and α-CD-4-nitrophenolate was more stable than α-CD-4-nitrophenol. Frontier orbital analysis suggested that charge-transfer interaction led to such behaviors, and hence constituted a nontrivial driving force in the molecular recognition of α-CD.

A quantum-chemical study on the molecular recognition of β-cyclodextrin with ground and excited xanthones

Abstract PM3 and density function theory B3LYP/3-21G(d) calculations in vacuo and in water were performed on the inclusion complexation of β-cyclodextrin (CD) with the ground singlet and excited triplet xanthones. It revealed that the complex of β-CD with the singlet xanthone was significantly more stable than that with the triplet one, which agreed with the experimental observation. Calculations on the model system at the level of B3LYP/6-311G(p, d) supported the above result, which indicated that the repulsion between the oxygens of xanthone and the oxygens of the secondary hydroxyls of β-CD constituted the origin for the above behavior. Hence, caution should be given when extrapolating excited state behavior to the supramolecular systems in their ground state.

PM3 calculations on the complexation of α-cyclodextrin with the ground and excited quinone

PM3 calculations were performed on the inclusion complexation of α-cyclodextrin (α-CD) with the ground and excited quinones from a complete and unrestricted geometry optimization and conformational analysis. It was found that the complexation of α-CD with the triplet quinone was significantly more favourable than that with the singlet one. Hence, caution should be given when extrapolating excited state behaviour to the supramolecular systems in their ground state.

A theoretical study on the homolytic dissociation energies of H–N+ bonds

Various levels of theoretical calculations were performed to study the N+–H bond dissociation energies (BDEs) of protonated amines in order to check the experimental results and to investigate the substituent effects. It was found that the reported experimental N+–H BDEs in the gas phase are possibly not accurate. Our best predictions on the basis of CBS-Q and G3 calculations for the N+–H BDEs of NH4+, CH3NH3+, (CH3)2NH2+, (CH3)3NH+, PhNH3+, and pyridinium are 125 ± 1, 110 ± 1, 107 ± 1, 95 ± 1, 75 ± 2, and 124 ± 1 kcal mol−1, respectively. In agreement with a previous study, it was also found that the solvent effects on the N+–H homolysis in acetonitrile are large, which significantly increases the N+–H BDEs compared to the gas phase. Further studies on the N+–H BDEs of protonated para-substituted anilines indicated that the substituent effects should have a slope of about 8.7 kcal mol−1 against the substituent σp+ constants. This value is larger than that for the O–H BDEs of phenols (6.7–6.9 kcal mol−1) and N–H BDEs of neutral anilines (3.0 kcal mol−1). The pattern of substituent effects is also completely different from that for the C–H BDEs of toluenes, as the C–H BDEs of toluenes are reduced by both the electron-withdrawing and -donating groups. Thus, we concluded that it is the electron demand of the system that dictates the substituent effects on BDEs. For the protonated aniline case, the origin of the substituent effects was found to be that an electron-withdrawing group destabilizes X–C6H4–NH2+˙ more than X–C6H4–NH3+, whereas an electron-donating group stabilizes X–C6H4–NH2+˙ more than X–C6H4–NH3+.

Substituent effects on the S–H bond dissociation energies of thiophenols

Density function UB3LYP/6-311++g(d,p) and perturbation theory ROMP2/6-311++g(d,p) calculations were performed on 4-substituted thiophenols and their corresponding radicals. It was found that although UB3LYP and ROMP2 methods underestimated the absolute S–H bond dissociation energies, they could predict almost as good relative S–H bond dissociation energies as a method of a considerably higher level, UCCSD(T)/6-311++g(d,p). From the calculation results it was determined that the S–H bond dissociation energies of thiophenols should have a positive correlation with the substituent σp+ constants whose slope was ca. 2.5 kcal mol−1. Such a slope indicated that the experimental S–H bond dissociation energies obtained from a previous solution phase measurement were reasonably accurate for para H, CH3, OCH3, Cl, and NO2 substituted thiophenols. However, the solution phase bond dissociation energy for 4-aminothiophenol was too low, which was found by the calculation in this study to be caused by the hydrogen bonding between the amino group and the solvent molecules. Finally, through the studies on the isodesmic reactions it was found that the substituent effects on the stability of neutral thiophenols had a fair and positive correlation with the substituent σp+ constants; the slope was 0.5 kcal mol−1. On the other hand, the substituent effects on the stability of thiophenol radicals had an excellent and negative correlation with the substituent σp+ constants and gave a slope of −1.8 kcal mol−1. Therefore, the major source of the substituent effects on S–H bond dissociation energies of thiophenols was the stability of the homolysis products, namely, thiophenol radicals.

A quantum chemistry study on oligo-para-phenylacetylenes and their inclusion complexes

Abstract B3LYP/6-31g(d)//PM3 and B3LYP/6-31g(d)//B3LYP/3-21g calculations were performed on oligo[ n ]- para -phenylacetylenes (O[ n ]PA, n =4–12), which afforded their geometries, strain energies, and frontier orbital energies. Dependences of these quantities on the size of O[ n ]PA were also obtained, which could be used to estimated the properties of larger O[ n ]PAs. It was found that with an appropriate interwall distance, mutilwall O[ n ]PA associations could be formed in an energetically favorable way. It was also found that O[4]PA could effectively bind with 1,4-disubstuted benzenes whereas O[6]PA was too large to bind with the same substrates. Hammett analyses revealed that electrostatic interaction was one of the most important driving forces leading to the above binding. Thus, the present study offered an additional example that curved π systems could be used as acceptors for molecular recognition.

Molecular orbital and DFT studies of the alimemazine radical cation

Semiempirical molecular orbital methods including CNDO, MNDO, AM1 and PM3, and density function theory method B3LYP/3-21G(d) were employed in the study of the alimemazine radical cation. It was found that PM3 was much better than CNDO, MNDO and AM1 in the structural optimization. The bond lengths and bond angles by PM3 were close to the experimental data, and comparable with the results by the density function theory method.