Martina Kieninger

The FO2 radical: a new success of density functional theory

Abstract The structure, heat of reaction and heat of formation of the FO 2 radical in its X 2 A″ ground state has been calculated using the three-parameter exchange functional of Becke and the Lee-Yang-Parr functional for correlation energy (B3LYP method). B3LYP geometrical parameters of FO 2 and FO are nearer to experiment than those obtained with conventional ab initio calculations. The calculated ΔH r,0 0 for F + O 2 → FO 2 at this level (−11.1 kcal/mol) is nearer to experiment (−11.68 kcal/mol) than that afforded by any other calculation, including QCISD(T), MP4(SDTQ) or G1 ab initio models. Calculation of the heat of reaction for previously considered isodesmic and isogyric reactions involving the FO 2 radical shows that the agreement is consistent. The root mean square error for the sixteen reactions considered, involving FO, FO 2 , FOH and FOOF, with heats of reaction varying from -149 to +56 kcal/mol, is 0.5 kcal/mol at the B3LYP level. The similar rms error for MP4(SDTQ)/6-311+ +G(2d, 2p) calculations is 5.2 kcal/mol. It is concluded that the B3LYP method is faster and more accurate than conventional ab initio methods for the study of reactions involving F-O bonds.

Equilibrium structure of the carbon dioxide-water complex in the gas phase: an ab initio and density functional study

High-level ab initio (MP2/6-311++G(2d,2p) geometry, Gaussian-2, MP4(SDTQ) and QCISD(T) binding energies) and density-functional (Becke3LYP/6-311++G(2df,2pd)) calculations have been performed on the charge-transfer complex between water and carbon dioxide. The complex appears to have two equivalent non-planar minima of Cs symmetry. Minima are separated by transition states with C1 symmetry, whereas the totally planar structure with C2v symmetry is a second-order transition state. All the critical points lie at approximately the same energy (less than 0.05 Kj mol−1 difference). Therefore, the experimentally observable structure should be planar. The best equilibrium intermolecular distance for this complex calculated at the MP2/6-311++G(2d,2p) level is 2.800 A. Our best estimate of the observable intermolecular distance (corrected for anharmonicity) is 2.84 A, in agreement with the experimentally derived value of 2.836 A. Our best estimate of the binding energy at the QCISD(T) level, taking into account the variation of the distance owing to anharmonicity and the use of more sophisticated theoretical treatments, is −12.0 ± 0.2 kJ mol−1. Our best estimate of the barrier to internal rotation, also at the MP2/6-311++G(2d,2p) level, is 4.0 kJ mol−1, outside the error limits of the experimental determination (3.64 ± 0.04 kJ mol−1). Density functional theory at the level employed here gives an equilibrium intermolecular distance that is too large (2.857 A), a binding energy that is too small (8.1 kJ mol−1), attributable neither to geometry nor to the basis set, and also a barrier to internal rotation that is slightly too small (3.39 kJ mol−1). The overall picture is, however, reasonably good.

Conformational and energetic properties of the ammonia dimer—comparison of post‐Hartree—Fock and density functional methods

The equilibrium structure of the ammonia dimer has been investigated with density functional and MP2 calculations. We used Slater‐ and Becke‐exchange functionals combined with correlation functionals as recommended by Vosko‐Wilk‐Nusair, by Perdew, and by Lee‐Yang‐Parr, respectively. The potential energy surfaces was investigated. The asymmetric cyclic “microwave” structure could be identified as a minimum. Optimization of the intermolecular parameters showed that this structure has nearly the same energy as the centrosymmetric cyclic structure. Full optimization transformed the asymmetric cyclic structure into the linear structure. The interaction energies in the dimer were corrected for the basis set superposition error using the Boys‐Bernardi counterpoise method and the a priori chemical Hamiltonian approach, respectively.

The chemical Hamiltonian approach in density functional theory

Abstract The chemical Hamiltonian approach (CHA) for handling the basis set superposition error problem in intermolecular interactions has been implemented within density functional theory (DFT) using Gaussian atomic basis sets. As test examples, the potential curves of the water dimer were calculated using the Vosko-Wilk-Nusair, Becke-Perdew and Perdew exchange-correlation functionals. Comparisons with the counterpoise correction method show that CHA within DFT performs as well as previously for Hartree-Fock.

Density functional and coupled-cluster calculations of isodesmic reactions involving fluorine oxides

Abstract Some isodesmic reactions, involving the fluorine oxide radical FO, have been studied employing density functional theory (DFT) and coupled-cluster (CC) calculations with an extended, uncontracted basis set. It is shown that CCSD(T) calculations can give more accurate enthalpies of reaction than DFT in some of the non-isodesmic reactions. DFT, however, gives more accurate results than CCSD(T) for the isodesmic reactions considered.

A discrepancy between experimental and theoretical thermochemical characterization of some oxygen fluorides

Abstract The recently recommended NIST-JANAF Thermochemical Table values for the thermochemical properties of oxygen fluorides are examined critically on the basis of high-level density functional and ab initio G2 calculations. Special attention is given to the enthalpies of formation and it is concluded that, although the proposed values for OF(g), FOO(g) and FOF(g) are reasonable and in agreement with our previously recommended values, the enthalpy of formation recommended for FOOF is too low. A value about 50% larger is proposed and it is recommended that the original 1959 experimental determination of this enthalpy of formation be repeated.

Density functional computational thermochemistry: solving the discrepancy between MO and DFT calculations on the enthalpy of formation of sulfine, CH2SO

Abstract The enthalpy of formation of sulfine is computed at the density functional (DFT) level to solve the discrepancy between previously recommended theoretical values. In agreement with the most recent CBS-QB3 calculations, which predict a value of −30±6 kJ/mol, DFT calculations on isodesmic reactions predict a value of −38±10 kJ/mol. Previous estimations of −9±14 kJ/mol (at the MO level) and −52±10 kJ/mol (at the DFT level) are discussed and shown to be artifacts of the methods of calculation employed.

Density functional computational thermochemistry: determination of the enthalpy of formation of sulfine, CH2SO, at room temperature

Abstract Density functional and coupled-cluster calculations using Poples basis sets up to 6-311++G(3df,2pd) have been employed to determine the heat of formation of sulfine, CH 2 SO, 1 , using the isodesmic reaction CH 2 S + SO 2 ⇌ CH 2 SO + SO . Other reactions, employed previously to determine the enthalpy of formation of sulfine at the CAS-SDCI/ CASSCF ab initio level, were used as well. The analysis of the results shows that: (a) the errors in the calculation of the enthalpies for the individual molecules do cancel reasonably only for the isodesmic reaction, and not for those used previously; (b) density functional methods produce smaller errors than CCSDT in the calculation of the enthalpies of formation of the molecules involved in this reaction; (c) the actual heat of formation of sulfine is determined as Δ f H o 298.15 ( 1 )=−52±10 kJ/mol, more in agreement with the prediction of Benson than with the ab initio value derived by Ruttink et al.; (d) the proton affinity of sulfine, calculated at the density functional level (792.0 kJ/mol) agrees reasonably well with the experimental result, 787.6±2.6 kJ/mol, but the enthalpy of formation of 1 derived from this proton affinity using the assumptions of Ruttink or Bouchoux is in disagreement with the value determined previously.

Density Functional Theory: A Useful Tool for the Study of Free Radicals

Abstract The application of density functional theory (DFT) to the study of the structure and reactivity of some molecules with unpaired electrons (radicals) performed by our group is presented. The results describe the application of LSD, gradient corrected and hybrid DFT methods to several small molecules. On average the results are as good as highly-correlated post-Hartree-Fock methods, but still some problems remain to be solved

Density functional studies of internal rotation: formamide as a prototype of the peptide bond

Abstract Different exchange-correlation potential combinations have been applied to the structural investigation of the planar form of formamide and of its two transition states. The rotational barrier of 18.3 kcal mol −1 obtained with the Becke3 exchange-Lee, Yang, Parr correlation potential compares well with the MP2 result of 18.7 kcal mol −1 . The geometrical structure obtained is in good agreement with both MP2 results and experiments.