High-level Composite Methods Research Articles

Proton-transfer tautomerism and enthalpies of formation of some isoquinoline derivatives

Ab initio molecular orbital theory has been used to calculate enthalpies of formation of the keto tautomers of 1-hydroxyisoquinoline, 5-hydroxyisoquinoline, and 1,5-dihydroxyisoquinoline. The high-level composite method G3//B3LYP has been used for this study, and the results have been compared with available experimental values. The keto tautomer is more favourable for 1-hydroxyisoquinoline and 1,5-dihydroxyisoquinoline, and the experimental enthalpies of formation are in better agreement with the theoretical enthalpies of formation of the keto forms.

Calculation of the enthalpies of formation and proton affinities of some isoquinoline derivatives

Ab initio molecular orbital theory has been used to calculate enthalpies of formation of isoquinoline, 1-hydroxyisoquinoline, 5-hydroxyisoquinoline, and 1,5-dihydroxyisoquinoline as well as some pyridine and quinoline derivatives. The proton affinities of the four isoquinoline derivatives were also obtained. The high-level composite methods G3(MP2), G3(MP2)//B3LYP, G3//B3LYP, and CBS-QB3 have been used for this study, and the results have been compared with available experimental values. For six of the eight studied compounds, the theoretical enthalpies of formation were very close to the experimental values (to within 4.3 kJ · mol −1); where comparison was possible, the theoretical and experimental proton affinities were also in excellent agreement with one another. However, there is an extraordinary discrepancy between theory and experiment for the enthalpies of formation of 1-hydroxyisoquinoline and 1,5-dihydroxyisoquinoline, suggesting that the experimental values for these two compounds should perhaps be re-examined. We also show that popular low cost computational methods such as B3LYP and MP2 show very large deviations from the benchmark values.

Accurate Calculation of Absolute One-Electron Redox Potentials of Some para-Quinone Derivatives in Acetonitrile

Standard ab initio molecular orbital theory and density functional theory calculations have been used to calculate absolute one-electron reduction potentials of several para-quinones in acetonitrile. The high-level composite method of G3(MP2)-RAD is used for the gas-phase calculations and a continuum model of solvation, CPCM, has been employed to calculate solvation energies. To compare the theoretical reduction potentials with experiment, the reduction potentials relative to a standard calomel electrode (SCE) have also been calculated and compared to experimental values. The average error of the calculated reduction potentials using the proposed method is 0.07 V without any additional approximation. An ONIOM method in which the core is studied at G3(MP2)-RAD and the substituent effect of the rest of the molecule is studied at R(O)MP2/6-311+G(3df,2p) provides an accurate low-cost alternative to G3(MP2)-RAD for larger molecules.

Kinetics and mechanism of the C6H5 + CH3CHO reaction: experimental measurement and theoretical prediction of the reactivity toward four molecular sites.

The kinetics and mechanism of the reaction of C6H5 with CH3CHO have been investigated experimentally and theoretically. The total rate constant for the reaction has been measured by means of the cavity ring-down spectrometry (CRDS) in the temperature range 299-501 K at pressures covering 20-75 Torr. The overall bimolecular rate constant can be represented by the expression k = (2.8 +/- 0.2) x 10(11) exp[-(700 +/- 30)/T] cm3 mol-1 s-1, which is slightly faster than for the analogous C6H5 + CH2O reaction determined with the same method in the same temperature range. The reaction mechanism for the C6H5 + CH3CHO reaction was also explored with quantum-chemical calculations at various hybrid density functional theories (DFTs) and using ab initio high-level composite methods. The theories predict that the reaction may occur by two hydrogen-abstraction and two addition channels with the aldehydic hydrogen-abstraction reaction being dominant. The rate constant calculated by the transition state theory for the aldehydic hydrogen-abstraction reaction is in good agreement with the experimental result after a very small adjustment of the predicted reaction barrier (+0.3 kcal mol-1). Contributions from other product channels are negligible under our experimental conditions. For combustion applications, we have calculated the rate constants for key product channels in the temperature range of 298-2500 K under atmospheric-pressure conditions; they can be represented by the following expressions in units of cm 3mol-1 s-1: k1,cho = 8.8 x 10(3)T2.6 exp(-90/T), k2,ch3 = 6.0 x 10(1)T3.3 exp(-950/T), k3a(C6H5COCH3 + H) = 4.2 x 10(5)T0.6 exp(-410/T) and k3b(C6H5CHO + CH3) = 6.6 x 10(9)T-0.5 exp(-310/T).

Performance of the RB3-LYP, RMP2, and UCCSD(T) Procedures in Calculating Radical Stabilization Energies for •NHX Radicals

N-H bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with the •NHCF3, •NHCHO, •NHCOCH3, and •NHCONH2 radical have been calculated at a number of theoretical levels. These include UHF, RHF, UB3-LYP, RB3-LYP, UMP2, RMP2, UCCSD(T), and URCCSD(T) with a variety of basis sets, as well as the high-level composite methods W1, CBS-QB3, andG3X(MP2)-RAD. For these systems, particular care must be taken to ensure convergence to the lowest-energy solution of the self-consistent-field (SCF) equations. We have assessed the performance of the various levels of theory in calculating the BDEs and RSEs of the •NHX radicals and find that, although there are somewhat larger errors for the simpler methods, the performance generally parallels that observed previously for •CH2X radicals. In particular (and in contrast to a recent report), RB3-LYP and UCCSD(T) consistently produce very good RSEs for •NHX radicals, provided that the lowest-energy solutions are correctly identified. The RMP2 RSEs, while not as good as those for •CH2X radicals, do not show the previously claimed large errors.

Methyl Radical Addition to C═S Double Bonds: Kinetic versus Thermodynamic Preferences

Barriers and enthalpies for methyl radical addition to both the C- and S-centers of CH2S, CH3CHS, and (CH3)2CS, and for the methyl-transfer reactions that interconvert the S-centered and C-centered radical products have been calculated via a variety of high-level ab initio molecular orbital procedures, including variants of the CBS, G3, G3-RAD, and W1 methods. An extensive assessment of the performance of the various theoretical procedures has been carried out. One of the important conclusions of this assessment is that the B3-LYP geometries, prescribed for several of these high-level composite methods, greatly overestimate the forming bond length in the addition transition structures, leading to a significant underestimation of the reaction barriers. The addition reactions are found to be highly exothermic and have relatively low barriers that are increased somewhat on methyl substitution. The reactions are also contra-thermodynamicthat is, despite a clear thermodynamic preference for the S-centered radical product, the barriers for the production of the C-centered radical via addition to S are lower. Interconversion of the C-centered and S-centered radical products via a methyl-transfer reaction is a high-energy process.