Theodore S. Dibble

Evaluating the accuracy of density functional methods for ClOO

We present results of density functional calculations on ClOO using the local density approximation (LDA) and the Becke–Lee–Yang–Parr functional (BLYP). The accuracy of computed geometries and vibrational frequencies appears comparable to that achieved with very high quality single‐determinant methods [QCISD(T)].

FNO: Another challenging molecule for correlation methods

Although FNO has long been considered a computationally challenging molecule, no thorough study of this species has been performed with methods of electron correlation beyond second‐order Mo/ller–Plesset perturbation theory (MP2) and the singles and doubles configuration interaction (CISD) method. We have investigated the equilibrium structure, vibrational spectra, and relative energetics of FNO using high‐levels of ab initio theory. Methods include third and fourth‐order Mo/ller–Plesset perturbation (MP3 and MP4), singles and doubles coupled‐cluster (CCSD) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The quadratic configuration interaction method including single, double, and triple excitations [QCISD(T)] method is also used. Computations using these methods are performed with medium and large basis sets. Geometries calculated at the Hartree–Fock level are grossly in error, while going to the CISD, MP3, MP4, and CCSD methods ...

COMPLETE ACTIVE SPACE SELF-CONSISTENT FIELD AND DENSITY FUNCTIONAL STUDY OF FNO

The molecular structure and bond dissociation energy of FNO are studied using complete active space self‐consistent field (CASSCF) and density functional methods (DFT), and the results are compared to experimental. Nitrogen–fluorine bond lengths obtained from DFT using the local density approximation are fairly accurate but N–F bond energies are overestimated by more than 40 kcal mol−1. Application of gradient corrections reduces the error in the bond dissociation energy to ∼15 kcal mol−1 but yields bond lengths that are significantly too long. Basis set effects in density functional results are compared to those observed previously in FNO2. The CASSCF method produces accurate bond lengths only with very large (10 in 8 or 12 in 9) active spaces; smaller active spaces yield N–F bond lengths that are short by 0.07 A or more. The 12 in 9 active space underestimates the bond energy by ∼14 kcal mol−1. The inclusion of certain orbitals in the active space leads to much improved structures and significantly lowe...

Dissociation of acetyl bromide: an experimental and theoretical study

Abstract We examine the ground state dissociation pathways of acetyl bromide, CH 3 C(O)Br, using infrared multiphoton dissociation and ab initio molecular orbital theory. Experiments reveal stable products C 2 H 6 , CH 3 Br, and CO which are consistent with the CBr bond fission pathway. However, production of C 2 H 4 is consistent with ketene (CH 2 CO) formation, which has been a suggested intermediate in the dissociation of CH 3 C(O)Br. Calculations indicate that the activation energy for the molecular dissociation of CH 3 C(O)Br into ketene and HBr exceeds the CBr bond energy of 64 kcal mol −1 by about 30 kcal mol −1 , implying that ketene may be formed by secondary chemistry rather than by molecular dissociation of acetyl bromide.

Structure and Transformation. Clusters as Models for Condensed Phases

The special advantages and characteristic limitations of molecular clusters in investigations of the properties of condensed phases are evaluated. Clusters naturally lend themselves to the exploration of (1) the possible ways molecules can pack while undergoing various degrees of thermal agitation, and (2) the cooperative behavior of molecules in transitions between phases. Clusters can be studied experimentally, in expanding supersonic jets, or computationally, in molecular dynamics simulations. Of particular interest are clusters of polyatomic molecules because their structural diversity is much richer than that of atomic clusters. Novel phases not seen in the bulk have been generated, and explained. Extraordinarily rapid nucleation rates have been observed at the extremely high supercoolings that can be attained. Interfacial free energies of boundaries between liquid and solid phases have been inferred. Clusters are versatile and effective model systems of great promise in research on condensed matter.

Peroxy and alkoxy radicals from 2-methyl-3-buten-2-ol.

Quantum mechanical calculations were used to determine the structure and energetics of peroxy radicals (P1 and P2) and alkoxy radicals (A1-A3) formed in the atmospheric degradation of 2-methyl-3-buten-2-ol. At the level of theory employed (B3LYP/6-31G(d,p)) low energy conformers were identified with zero, one, or two hydrogen bonds. The beta C-C scission (decomposition) reactions are computed to occur with low barriers, and the 1,5 H-shift (isomerization) reaction of A2 is computed to be of negligible importance. Scission 2 of A2 is computed to be about 93% of the fate of A2, with the balance being scission 1. The new BB1K functional of Truhlar was employed to investigate activation barriers for single intramolecular H-atom transfers across the OH...O* hydrogen bonds, but the barriers to these reactions appear to be too high for these reactions to be important. Extensive searches for transition states for simultaneous double intramolecular H-atom transfer across OH...OH...O* hydrogen bond pairs were unsuccessful.