An extension of density functional theory to situations with significant nondynamical correlation is presented. The method is based on the spin–flip (SF) approach which is capable of describing multireference wave functions within a single reference formalism as spin–flipping, e.g., α→β, excitations from a high-spin (Ms=1) triplet reference state. An implementation of the spin–flip approach within the Tamm–Dancoff approximation to time-dependent density functional theory (TDDFT) is presented. The new method, SF-TDDFT/TDA or simply SF-DFT, describes target states (i.e., closed- and open-shell singlets, as well as low-spin triplets) by linear response from a reference high-spin triplet (Ms=1) Kohn–Sham state. Contrary to traditional TDDFT, the SF-DFT response equations are solved in a subspace of spin–flipping operators. The method is applied to bond-breaking (ethylene torsional potential), and equilibrium properties of eight diradicals. The results demonstrate significant improvement over traditional Kohn–...

The spin–flip approach within time-dependent density functional theory: Theory and applications to diradicals

Molecular Rods. 1. Simple Axial Rods.

The spin-flip approach has been applied to calculate vertical and adiabatic energy separations between low-lying singlet and triplet states in diradicals. The spin-flip model describes both closed- and open-shell singlet and (low-spin) triplet states within a single reference formalism as spin-flipping, e.g., α→β, excitations from a high-spin triplet (Ms=1) reference state. Since both dynamical and nondynamical correlation effects are much smaller for the high-spin triplet states than for the corresponding singlet states, the spin-flip models yield systematically more accurate results than their traditional (non-spin-flip) counterparts. For all the diradicals studied in this work, the spin-flip variant of the coupled-cluster model with double excitations yields energy separations which are within less than 3 kcal/mol of the experimental or the highly accurate multireference values. In most cases the errors are about 1 kcal/mol.

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Singlet-triplet gaps in diradicals by the spin-flip approach: A benchmark study

The 351 nm photoelectron spectra of the negative ions of o-, m-, and p-benzyne (1,2-, 1,3-, and 1,4-dehydrobenzene, respectively) and their perdeuterated isotopomers have been obtained. The o-benzyne ions were generated by the reaction of benzene and benzene-d6 with O-, while the m- and p-benzyne ions were prepared by the gas-phase reaction between the corresponding 3- and 4-(trimethylsilyl)phenyl anions and molecular fluorine, F2. The photoelectron spectra of the benzyne anions each contain two features, corresponding to formation of the singlet and triplet states of the biradicals. The electron affinities of o- and p-benzyne are found to be 0.564 ± 0.007 and 1.265 ± 0.008 eV, respectively, while the electron affinities of deuterated o- and p-benzyne are found to be 8 and 5 meV lower, respectively. The electron affinity of m-benzyne could not be determined from the photoelectron spectrum because the origin peak could not be assigned unequivocally. For o- and p-benzyne, the singlet−triplet energy splittin...

Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p- Benzyne Negative Ions. Electron Affinities and Singlet−Triplet Splittings for o -, m -, and p -Benzyne

Cascade Radical Cyclizations via Biradicals Generated from Enediynes, Enyne-Allenes, and Enyne-Ketenes.

Ab Initio Calculation of Hydrogen Abstraction Reactions of Phenyl Radical and p-Benzyne

The photoelectron spectrum of dichlorocarbene, CCl 2 , is reported. Simulation of the partially resolved vibrational structure is used to extract the adiabatic ionization potential, IP ad [CCl 2 ]=9.27±0.04 eV, and the heat of formation, ΔH o f,298 [CCl 2 ]=51.0±2.0 kcal/mol. A protocol for calculating polyatomic Franck-Condon factors is given and compared to two less general approaches

Cameron F. Logan

Papers

Ab Initio Calculation of Hydrogen Abstraction Reactions of Phenyl Radical and p-Benzyne

Photoelectron spectrum, ionization potential, and heat of formation of dichlorocarbene

The photoelectron spectrum of the .alpha.,3-dehydrotoluene biradical

Syntheses of 2-substituted benzocyclopropenes: Functional group transformations on the benzocyclopropene nucleus

Photoelectron Spectrum, Ionization Potential, and Heat of Formation of CCl2.