Hydrogen Tunneling above Room Temperature Evidenced by Infrared Ion Spectroscopy

While
hydrogen tunneling at elevated temperatures has, for instance, often
been postulated in biochemical processes, spectroscopic proof is thus
far limited to cryogenic conditions, under which thermal reactivity
is negligible. We report spectroscopic evidence for H-tunneling in
the gas phase at temperatures around 320–350 K observed in
the isomerization reaction of a hydroxycarbene into an aldehyde. The
charge-tagged carbene was generated in situ in a tandem mass spectrometer
by decarboxylation of oxo­[4-(trimethylammonio)­phenyl]­acetic acid upon
collision induced dissociation. All ion structures involved are characterized
by infrared ion spectroscopy and quantum chemical calculations. The
charge-tagged phenylhydroxycarbene undergoes a 1,2-H-shift to the corresponding
aldehyde with an half-life of about 10 s, evidenced by isomer-selective
two-color (IR-IR) spectroscopy. In contrast, the deuterated (OD) carbene
analogue showed much reduced 1,2-D-shift reactivity with an estimated
half-life of at least 200 s under the experimental conditions, and
provides clear evidence for hydrogen atom tunneling in the H-isotopologue.
This is the first spectroscopic confirmation of hydrogen atom tunneling
governing 1,2-H-shift reactions at noncryogenic temperatures, which
is of broad significance for a range of (bio)­chemical processes, including
enzymatic transformations and organocatalysis.