Bond dissociation energies and the Hammett correlation, Part 1: Remotely substituted aromatic compounds and vitamin E

Gas-phase alkyl and N-alkylamino cation affinities of anionic alpha-oxygen nucleophiles (H n XO-; X = N, P, As, O, S, Se, F, Cl, Br; n = 0-2): a theoretical G2(+)M study

In this work, we studied anionic alpha-oxygen nucleophiles in which there was an atom of with one or more unshared lone-pair electrons from groups 15-17 and periods 2-4 of the periodic table positioned adjacent to the negative attacking-oxygen (i.e., H _n XO^-; X = N, P, As, O, S, Se, F, Cl, Br; n = 0-2); these nucleophiles were termed "alpha-agents." Specifically, we investigated the gas-phase simple alkyl cation affinities (ACAs) and the simple N-alkylamino cation affinities (NAAMCAs) of these alpha-agents theoretically via the modified G2(+)_M method. Our calculations indicate that the O-C bond of CH₃OXH _n is somewhat similar to that of iPrOXH _n (n = 0, 1, 2; X = N, P, As, O, S, Se, F, Cl, Br). Also, steric repulsion is an important influence on these cation affinities. It appears that the kinetic gas-phase alpha effect in the S_N2 reactions of interest cannot be explained by the thermodynamic proton affinity or the affinity (SuA) of the anionic or neutral nucleophile (Nu) for the substrate cation, as the gas-phase ACA (NAAMCA) was found to be linearly correlated with the PA for the alpha-agents, similar to the previously reported linear correlation of the gas-phase ACA (NAAMCA) with the PA for normal nucleophiles.

Substituent effect on the radical scavenging activity of 6-chromanol derivatives

A series of 6-chromanol derivatives were synthesized, and their second order rate constants (k) for the reaction with galvinoxyl radical were determined. The reaction constants from the slope, which plotted with the log k versus the Hammett sigma or Taft sigma constants for the four substituents at each, were obtained.

Assessment of Experimental Bond Dissociation Energies Using Composite ab Initio Methods and Evaluation of the Performances of Density Functional Methods in the Calculation of Bond Dissociation Energies

Composite ab initio CBS-Q and G3 methods were used to calculate the bond dissociation energies (BDEs) of over 200 compounds listed in CRC Handbook of Chemistry and Physics (2002 ed.). It was found that these two methods agree with each other excellently in the calculation of BDEs, and they can predict BDEs within 10 kJ/mol of the experimental values. Using these two methods, it was found that among the examined compounds 161 experimental BDEs are valid because the standard deviation between the experimental and theoretical values for them is only 8.6 kJ/mol. Nevertheless, 40 BDEs listed in the Handbook may be highly inaccurate as the experimental and theoretical values for them differ by over 20 kJ/mol. Furthermore, 11 BDEs listed in the Handbook may be seriously flawed as the experimental and theoretical values for them differ by over 40 kJ/mol. Using the 161 cautiously validated experimental BDEs, we then assessed the performances of the standard density functional (DFT) methods including B3LYP, B3P86, B3PW91, and BH&HLYP in the calculation of BDEs. It was found that the BH&HLYP method performed poorly for the BDE calculations. B3LYP, B3P86, and B3PW91, however, performed reasonably well for the calculation of BDEs with standard deviations of about 12.1-18.0 kJ/mol. Nonetheless, all the DFT methods underestimated the BDEs by 4-17 kJ/mol in average. Sometimes, the underestimation by the DFT methods could be as high as 40-60 kJ/mol. Therefore, the DFT methods were more reliable for relative BDE calculations than for absolute BDE calculations. Finally, it was observed that the basis set effects on the BDEs calculated by the DFT methods were usually small except for the heteroatom-hydrogen BDEs.

Remote substituent effects on homolytic bond dissociation energies

In the study we tried to answer two questions. First, does X-Z homolytic bond dissociation energy (BDE) of Y-C6H4-X-Z obey the Hammett relationship? Second, if it does what factors determine the magnitude and sign of the slope (rho+) of Hammett regression against substituent sigma(p)(+) constants? We collected a large number of X-Z BDEs for over one-thousand Y-C6H4-X-Z systems using the RMP2/6-311++G**//UB3LYP/6-31G* method. We found that remote substituent effects on X-Z BDEs are determined by both the ground effect (i.e. stabilization/destabilization of X-Z by the substituents) and the radical effect (i.e. stabilization/destabilization of X. by the substituents). The ground or radical effect is determined by the electron demand of X-Z or X. in the same way as the deprotonation enthalpy of HOOC-C6H4-X-Z or HOOC-C6H4-X. is affected by X-Z or X. . As a result, rho+ (BDE) for X-Z bond homolysis can be quantitatively predicted by using the change in deprotonation enthalpy from HOOC-C6H4-X-Z to HOOC-C6H4-X. .