Structural Isomerization of the Gas Phase 2-Norbornyl Cation Revealed with Infrared Spectroscopy and Computational Chemistry Dept. of Chemistry University of Georgia 140 Cedar St, Athens, GA Daniel T. Mauney Jonathan D. Mosley Justin W. Young Jay Agarwal Henry F. Schaefer, III Paul v. R. Schleyer Michael A. Duncan

Carbocations Classical Have C−C, C=C, C≡C bonds with trivalent carbon Non-Classical Have bridging methyl or hydrogen moieties (3c-2e bonding) Leads to “hypervalent” carbon Multiple structural isomers possible, often with competing classical vs non-classical structures

Why 2-norbornyl (2NB)? Its structure was debated for decades – classical vs nonclassical Solvolysis rates suggested bridged nonclassical intermediate (shown here) Herbert Brown strongly opposed this view and suggested rapidly equilibrating classical structures and steric hindrance to account for different solvolysis rates Experimental and theoretical investigations into the structure of 2NB ensued Raman spectra, 1 NMR spectra, 2 and finally X-ray crystal structure 3 in superacid matrices confirm the nonclassical structure Theory at increasingly higher levels 4 favored the nonclassical symmetrically bridged structure The infrared spectrum has never been measured in the gas phase 1 Olah, G. A.; White, A. M.; Demember, J. R.; Commeyra, A.; Lui, C. Y., J. Am. Chem. Soc. 1970, 92, Yannoni, C. S.; Macho. V.; Myhre, P. C., J. Am. Chem. Soc. 1982, 104, Scholz, F.; Himmel, D.; Heinemann, F. W.; Schleyer, P. v. R.; Meyer, K.; Krossing, I., Science 2013, 341, Goddard, J. D.; Osamura, Y.; Schaefer, H. F., J. Am. Chem. Soc. 1982, 104,

5 μs Production of cold ionic and neutral molecules with electrical discharge in a supersonic expansion. IR photodissociation spectroscopy via argon tagging. Laser Vision OPO/OPA cm -1 Mass Selected IR photodissociation (IR-PD) In a Reflectron Time-of-Flight Mass Spectrometer Mass gate selects one ion Ethanol precursor in argon Full mass spectrum [C,H 3,O] + -Ar m/z=71 Loss of Ar

The Infrared Spectrum of Gas Phase C 7 H 11 + Experiment Theory

Some Structural Isomers of 2-Norbornyl IsomerΔE (CCSD(T)) A1,3-dimethyl- cyclopentenyl 0.0 E1-methylcyclohexenyl+11.0 F2-norbornyl+23.4 Structures and their relative energies computed at the MP2/cc-pVTZ level of theory 2-norbornyl is not the global minimum structure Mosley, J. D.; Young, J. W.; Agarwal, J.; Schaefer, H. F.; Schleyer, P. v. R.; Duncan, M. A., Angew. Chem. Int. Ed. 2014, in press.

The Infrared Spectrum of DMCP + Predicted frequencies in Figure obtained at the MP2/cc-pVTZ level Large band at 1525 cm -1 is allyl C-C-C stretch Lower frequency bands are carbon stretching and bending modes Broad band at 2910 cm -1 due to overlapping sp 3 C−H stretches x9

1,3-Dimethylcyclopentenyl Cation (DMCP + ) Global Minimum structure of the C 7 H 11 + potential surface Stabilized by symmetrical, optimally located methyl groups Methanol-to-Gasoline studies showed that small hydrocarbons like ethylene can lead to DMCP + Presence confirmed by 13 C NMR as an ion pair component in the acid catalyst 5-7 First gas phase study 5 Xu, T.;Haw, J. F. J. Am. Chem. Soc. 1994, 116, Haw, J. F.; Nicholas, J. B.;Song, W. G.;Deng, F.;Wang, Z. K.;Xu, T.;Heneghan, C. S., J. Am. Chem. Soc. 2000, 122, 4763– Chua, Y. T.;Stair, P. C.;Nicholas, J. B.;Song, W. G.;Haw, J. F., J. Am. Chem. Soc. 2003, 125, 866–867.

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