C:...H-C hydrogen bonding shuts down H-tunneling in hydroxy- carbenes: a gas-phase study by tandem-MS, IR ion spectroscopy, and theory Mathias Paul 1 , Thomas Thomulka 1 , Wacharee Harnying 1 , Jörg-M. Neudörfl 1 , Charlie Adams 2 , Jonathan Martens 3 , Giel Berden 3 , Jos Oomens 3,4 , Albrecht Berkessel 1 , Mathias Sch fer 1 and Anthony J. H. M. Meijer 2 1 Department of Chemistry, Organic Chemistry, University of Cologne, Greinstraße 4, 50939 Cologne, Germany, 2 Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK, 3 Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands, 4 Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands An aliphatic hydroxycarbene formed by collision induced CO 2 loss from a (4-Carboxy-4-oxo-butyl)-trimethyl- ammonium precursor, was investigated by IR ion spectroscopy.[1,2] The recorded spectra match the computed ones of hydroxycarbene candidate structures with either a 5- or a 7-membered ring system held together by a peculiar C...H-C hydrogen bond. Additional to the predominantly formed hydroxycarbene a weak C=C stretching mode suggests the presence of the enol tautomer in low amounts. Time dependent investigations of the isomerization reactivity in the QIT revealed an outstanding stability of the hydroxycarbene ions. The recorded IR ion spectra show no evidence for the formation of the aldehyde isomer, although this is clearly the most stable tautomer of the three possible alternatives.[2] This finding contradicts the behavior of methyl-hydroxycarbene, which isomerizes cleanly to the acetaldehyde tautomer even at cryogenic temperatures.[3] We assume that the aliphatic model hydroxycarbene is substantially stabilized by the unique C: ... H-C hydrogen bond interaction, which also explains the absence of the typical hydrogen tunneling reactivity.[4] This set of fundamental results suggests a potent strategy for a conformation dependent reactivity switch of carbenes, based on the presence or absence of C: ... H-C interactions. These results are broadly confirmed by high level theory [5] and by an additional experimental data of tailor-made model compounds unable to establish the peculiar C: ... H-C interactions. References 1. M. Sch fer, K. Peckelsen, M. Paul, J. Martens, J. Oomens, G. Berden, A. Berkessel, A. J. H. M. Meijer, J. Am. Chem. Soc. 139 (2017) 5779-5786. 2. M. Paul, T. Thomulka, W. Harnying, J.-M. Neudörfl, C. Adams, J. Martens, G. Berden, J. Oomens, A. J. H. M. Meijer, A. Berkessel, M. Sch fer, J. Am. Chem. Soc. (2023) in preparation. 3. P.R. Schreiner, H.P. Reisenauer, D. Ley, D. Gerbig, Ch-H Wu, W.D. Allen, Science 322 (2011) 1300-1303. 4. A. J. Arduengo, S. F. Gamper, M. Tamm, J. C. Calabrese, F. Davidson, H. A. Craig, J. Am. Chem. Soc. 117 (1995) 572–573. 5. T. A. H. Burd, X. Shan, D. C. Clary, Phys. Chem. Chem. Phys. 22 (2020) 962-965.
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