Quantum Tunneling Tautomer of N,N-Dimethyl-p-toluidine Dehydrogenates Identified by Deep-UV Laser Ionization Mass Spectroscopy

[Image: see text] Utilizing customized deep-ultraviolet laser ionization mass spectroscopy, here we report a finding of remarkable dehydrogenation product of N,N-dimethyl-p-toluidine (DMT). The DMT dehydrogenates find comparable mass abundance with the DMT molecule ions showing decent stability at the loss of one electron and one H atom from the DMT molecule. First-principles calculation reveals that the dehydrogenation most readily occurs at the N-connected methyl group. Furthermore, at the removal of a hydrogen atom, a neighboring hydrogen atom on the other methyl come close and interact with the dehydrogenated methylene group, pertaining to C–H···C weak interactions which give rises to a resonant structure (C···H–C) on a basis of hydrogen atom quantum tunneling effect. The quantum tunneling tautomer of DMT dehydrogenates displays reversible donor–acceptor charge-transfer interactions as demonstrated by natural bonding orbital analysis and vibrational spectroscopy. It is worth noting that the novel dehydrogenation product was also observed for another small organic molecule o-phenylenediamine, which bears two neighboring amino groups and the subsequent dehydrogenation product pertains to resonant structures of N–H···N and N···H–N. The deep ultraviolet laser not only produces fragmentation-free mass spectra for such small organic molecules but also tailors the interesting quantum tunneling tautomer from such specific molecules.