Research Structure and Reactivity Solvation Variable Spin Molecules Phosphorus Project Dossiers

Interests and Future Plans

Interesting properties are often associated with molecules having two or more frontier molecular orbitals that are similar in energy; e.g., they may exhibit high- or low-spin ground states, unique reactivities, significant sensitivity to condensed-phase effects, etc. Beyond the chemically interesting aspects of specific molecular architectures, the modeling of such systems in general poses unique challenges to electronic structure methodologies. In addition to the development of new theoretical methodologies for the accurate treatment of different aspects of variable-spin systems, we are characterizing key chemical characteristics of carbenes (fundamental organic reactive intermediates), nitrenium ions (carcinogenic products from aromatic amine catabolysis), arynes (fundamental organic reactive intermediates enjoying a renaissance of interest owing to their intermediacy in certain anticancer therapeutic agents), non-Kekulé molecules (fundamental organic reactive intermediates and possible building blocks for organic ferromagnetic materials), and bimetallic model systems for copper-containing enzymes.

Accomplishments

Single- and multi-center diradicals have been a subject of particular interest to my group. Such molecules offer the intrinsic challenge of calculating multiplet splittings in systems having low-lying excited states--moreover, our selection of molecules has been influenced by their relevance either to carcinogenicity (nitrenium ions), organic ferromagnetism (non-Kekulé diradicals), bio-inorganic relevance (dicopper cores), or general organic reactivity (carbenes). More specifically, we have:

• performed high-level calculations of the singlet-triplet energy gaps for a number of prototypical carbenes and nitrenium ions, in particular considering substituent alkyl and aryl groups and the effect of incorporating the hypovalent atom into a ring structure. Based on our calculations, we have been able to rationalize the interplay of orbital hybridization, steric effects, conjugation, hyperconjugation, and aromatic-ring substitution on the magnitude of singlet-triplet gaps and on general diradical structure and reactivity. (See Publications 16, 34, 40, 45, 59, 60, 73, 76, 77, 84, 106, and 118.)

• characterized the energetics associated with the cracking of perfluorinated oxiranes to generate perfluoroalkyl carbenes and analyzed the stability of the latter with respect to unimolecular rearrangements. These studies have been carried out in collaboration with the experimental group of Professor Marc Hillmyer at the University of Minnesota. (See Publication 118.)

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  rationalized the vibrational spectra and reactivity of a number of experimentally studied arylnitrenium ions. These studies have been carried out in collaboration with the experimental group of Professor Dan Falvey at the University of Maryland. (See Publication 77--another publication on this subject is currently being drafted.)

  
  

• carried out detailed electronic structure calculations on non-Kekulé hydrocarbons, specifically trimethylenemethane, tetramethyleneethane, and isoelectronic heteroatom-substituted analogs. We have discovered DFT to be efficacious for the calculation of multiplet splittings, with the key exceptions of closed-shell singlets having frontier molecular orbitals degenerate in energy and intrinsically multi-determinantal states. In the latter instances, we have explored the utility of correlated multi-reference methods. (See Publications 62, and 89.)
  

  Figure: The four pi orbitals for planar trimethylenemethane are shown on right. A 90 degree rotation of one methylene transforms the 2b1 orbital to the b2 orbital shown at lower left. The possible wave functions (with the spin functions ignored) that can be formed using the two highest energy electrons are listed. The 3B2 state carries the label 3A2' in full D3h symmetry.

  
  
  
• developed a new method to calculate open-shell singlet energies from application of a spin-annihilation operator to Slater determinant wave functions formed from DFT one-electron orbitals. We have used this technique to accurately predict the near-UV spectra of 1,3-butadiene and diazene--these molecules have proven to be particular challenges for other electronic structure methods. (See Publications 52, and 59.)

• explored the effects of fluorine substitution on the reactivity of arylnitrenes, and discussed the implications for their use as photoaffinity labels. As part of a more general study aimed at characterizing similarities and differences between different monovalent diradicals, we have examined the effects of vinyl substitution on nitrenes and phosphinidenes. (See Publications 64, and 67.)

• extensively characterized the structures and energetics of aryne biradicals (e.g., benzynes) and the reaction coordinates for key electrocyclic reactions by which some of them may be formed (e.g., Bergman cyclization, Myers-Saito cyclization). As part of this work we have demonstrated the utility of economically calculated proton hyperfine coupling constants from aryl radicals as predictors for aryne singlet-triplet splittings, which are much more difficult to accurately calculate. We have also provided the first reliable estimate for the cyclization energetics associated with the neocarzinostatin chromophore. (See Publications 80, 85, 97, 101, 103, 120, and 122.)

  2,3-Didehydroquinone

  
  
  
• explained the electronic structure of recently discovered dicopper dioxygen compounds. We illustrated how molecular orbital effects control an apparent isomerization between two forms of the bimetallic molecule that have relevance to biological oxygen fixation in copper proteins, and we further provided the theoretical underpinnings for a detailed analysis of the resonance Raman spectroscopy of the bimetallic core. All of this work was in collaboration with the experimental groups of Professors Bill Tolman and sometimes Larry Que of the University of Minnesota. (See Publications 53, 68, and 69--another publication on this subject is currently under review.)