Our research revolves around highly reactive organic molecules. These unstable and elusive intermediates, such as carbenes, nitrenes, and biradicals, are especially important in photochemistry, but their chemistry and properties are poorly understood. Moreover, these molecules are related to searches for organic conducting and magnetic materials. Much of the organic synthesis that we carry out involves making previously unknown compounds, and we spend a considerable amount of our time developing new synthetic methods to tackle these challenging molecules. A specialized technique that we use to study reaction intermediates involves matrix isolation photochemistry. In this method, organic molecules are frozen into glasses of inert gas at extremely low temperatures (10 Kelvin). The samples are then irradiated with UV light to generate highly reactive intermediates. The low temperatures and high dilution in inert surroundings protect these otherwise unstable species from reaction. IR and UV spectra of the samples, acquired at low temperature, tell us a great deal about the bonding and structures of the products. Finally, we carry out a variety of ab initio and DFT electronic structure calculations to model the structures, spectra, and electronics of these novel molecules. Our recent work has focused on three major areas:
I. Conjugated Reactive Intermediates
There is considerable interest world-wide in developing high-spin, highly conjugated, molecular arrays that may be technologically useful for organic magnetic materials, high-density information storage, etc. Our contributions toward these goals have been the investigation of the intimate details of electronic communication in small sub-units based on conjugated polyreactive intermediates. For example, we have found that the carbene centers in the para-system shown below are strongly coupled, resulting in a diradical which can be directly characterized spectroscopically.
II. Highly Strained Organic Molecules
A common theme through much of our work is the study of molecules which do not follow the conventional rules of organic structure and bonding. The use of matrix isolation spectroscopy at cryogenic temperatures allows us to generate highly unstable and unusual molecules, and to directly study their properties. For example, we have recently been able to characterize the remarkably strained and reactive cumulenes shown below.
III. Quantum Mechanical Tunneling in Reactive Intermediates
Most chemical reactions display kinetics that are consistent with the simple-minded picture of thermally activated molecules surmounting an energy barrier. It is becoming increasingly clear, however, that quantum mechanical tunneling (QMT) can play an important role in reaction dynamics, particularly in reactions with energetically low and narrow reaction barriers. Such "through-barrier" contributions to reaction rates have recently been implicated in systems as diverse as interstellar space and enzyme catalyzed processes. We have found that, in certain cases, we can directly observe highly reactive organic molecules rearranging to stable products even at temperatures approaching absolute zero - specifically through tunneling. For example, recently we reported that the carbene shown below rearranges to methylfluorocyclopentene even at 8 Kelvin.
Geometry changes that occur along the tunneling reaction pathway from 1-methylcyclobutylfluorocarbene to 1-fluoro-2-methylcyclopentene. For the portion of the reaction pathway in which the reactant is tunneling through the barrier that separates it from the product, the carbenic carbon turns purple (Thanks go to David Hrovat, University of Washington).
Further details of this reaction may be found in Zuev, P.S.; Sheridan, R.S.; Albu, T.V.; Truhlar, D.G.; Hrovat, D.A.; Borden, W.T. Science 2003, 299, 867-870.
- Sheridan, R.S. Heteroarylcarbenes. Chem. Rev. (Washington, DC, U.S.) 2013, 113, 7179-7208.
- Song, M.-G.; Sheridan, R.S. Regiochemical Substituent Switching of Spin States in Aryl(trifluoromethyl)carbenes. J. Am. Chem. Soc. 2011, 133, 19688-19690.
- Song, M.G.; Sheridan, R.S. Effects of CF3 Groups and Charged Substituents on Singlet Carbene Stabilities: A Density Functional Theory Study. J. Phys. Org. Chem. 2011, 24, 889-893.
- Sheridan, R.S. Exploring Matrix Isolated Carbenes and Biscarbenes with Electronic Spectroscopy as a Guide. J. Phys. Org. Chem. 2010, 23, 326-332.
- Wang, J.; Sheridan, R.S. A Singlet Aryl-CF3 Carbene: 2- Benzothienyl(trifluoromethyl)carbene and Interconversion with a Strained Cyclic Allene. Org. Lett. 2007, 9, 3177-3180.
- Sheridan, R.S. Quantum Mechanical Tunneling in Organic Reactive Intermediates. In Reviews in Reactive Intermediate Chemistry; Moss, R.A., Platz, M.S., Jones, M.J., Jr., Eds.; John Wiley & Sons, 2007; pp 415-463.
- Zuev, P.S.; Sheridan, R.S.; Sauers, R.R.; Moss, R.A.; Chu, G. Conformational Product Control in the Low-Temperature Photochemistry of Cyclopropylcarbenes. Org. Lett. 2006, 8, 4963-4966.
- Moss, R.A.; Tian, J.; Sauers, R.R.; Sheridan, R.S.; Bhakta, A.; Zuev, P.S. Kinetic Studies of the Cyclization of Singlet Vinylchlorocarbenes. Org. Lett. 2005, 7, 4645.
- Nikitina, A.; Sheridan, R.S. Geometry and Aromaticity in Highly Strained Heterocyclic Allenes: Characterization of a 2,3-Didehydro-2H-thiopyran. Org. Lett. 2005, 7, 4467.
- Chu, G.; Moss, R.A.; Sauers, R.R.; Sheridan, R.S.; Zuev, P.S. Activation Energies for the 1,2-Carbon Migration of Ring-Fused Cyclopropylchlorocarbenes. Tetrahedron Lett. 2005, 46, 4137.
- Zuev, P.S.; Sheridan, R.S. Singlet Vinylcarbenes: Spectroscopy and Photochemistry. J. Am. Chem. Soc. 2004, 126, 12220.
- Moss, R.A.; Sauers, R.R.; Sheridan, R.S.; Tian, J.; Zuev, P.S. Carbon Tunneling in the Ring Expansion of Noradamantylchlorocarbene. J. Am. Chem. Soc. 2004, 126, 10196.
- Zuev, P.S.; Sheridan, R.S.; Albu, T.V.; Truhlar, D.G.; Hrovat, D.A.; Borden, W.T. Carbon Tunneling from a Single Quantum State. Science (Washington, DC, U.S.) 2003, 299, 867.
For commentary on this work, also see:
- Berger, R. Do Heavy Nuclei See Light at the End of the Tunnel? Angew. Chem., Int. Ed. 2004, 43, 398.
- McMahon, R.J. Chemical Reactions Involving Quantum Tunneling. Science (Washington, DC, U.S.) 2003, 299, 833.
- Carbon Can Tunnel, Too. Chem. Eng. News, Feb 10, 2003, p 24.