Abstract
The difluorocarbene radical (:CF2), used in organic synthesis and in photoaffinity labeling, can be generated by the pyrolytic or photolytic decomposition of 3,3-difluorodiazirine (CF2N2, DFD). DFD possesses no dipole moment in the ground electronic state S 0 but has an experimental dipole of 1.5 ± 0.2 debye (D) in its first singlet excited state S 1. These observations have been ascribed to the shift in electron population between orbitals (Frenking et al. in J Comp Chem 28:117–126, 2007). An alternative real-space explanation is presented, which shows that the vanishing dipole moment in S 0 results from a balance between a charge transfer contribution due to the flow of charge between atoms and an atomic polarization term due to the non-sphericity of atoms in molecules. This balance is altered in S 1. This orbital-free description is shown to be consistent with an incipient dissociation of DFD to :CF2 and N2 upon excitation. The Laplacian of the electron density and the molecular electrostatic potential exhibit significant reorganization on excitation, mirroring one another, with consequential changes in chemical reactivity. Conforming to Hund’s rule, the lowest excited state is a triplet state (T 1), and the next level, the one examined in this work, is the first singlet excited state (S 1) with vertical excitation energies of 2.81 and 3.99 eV, respectively. The calculated dipole moment magnitudes (in D) are 0.05 (S 0), 0.973 (T 1), and 0.969 (S 1) all pointing their negative end toward the nitrogens. The maximal average lifetime of S 1 (in absence of non-radiative de-excitation) is ca. 30 ps, sufficient for its slowest vibrational normal mode to complete 400 oscillations. From a comparison of Hartree–Fock, MP2, QCISD, CCSD, and TD-DFT/B3LYP calculations with experiment (all using an aug-cc-pVTZ basis set), for both the ground and excited states of DFD, the method of choice appears to be QCISD, the one used in this work.
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Acknowledgments
The authors thank the two anonymous reviewers especially Reviewer #1 who suggested the examination of the Laplacian scalar field and its change upon excitation. The authors also thank Professor Lou Massa (Hunter College, City University of New York), Professor Paul W. Ayers (McMaster University), and Drs. Jim Hess and Douglas J. Fox of Gaussian, Inc., for helpful comments. L.A.T. thanks CAPES for a doctoral fellowship and CNPq (Science without Borders Scholarship Program—205445/2014-4) and for a Visiting Graduate Studentship at Mount Saint Vincent University. R. L. A. H thanks FAPESP for financial support (Grants 2014/23714-1 and 2010/18743-1, São Paulo Research Foundation). C. F. M. acknowledges the funding of the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), and Mount Saint Vincent University for financial support.
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Published as part of the special collection of articles “CHITEL 2015 - Torino - Italy”.
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Terrabuio, L.A., Haiduke, R.L.A. & Matta, C.F. Difluorodiazirine (CF2N2): a quantum mechanical study of the electron density and of the electrostatic potential in the ground and excited electronic states. Theor Chem Acc 135, 63 (2016). https://doi.org/10.1007/s00214-015-1803-7
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DOI: https://doi.org/10.1007/s00214-015-1803-7