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Unlocking a diazirine long-lived nuclear singlet state via photochemistry: NMR detection and lifetime of an unstabilized diazo-compound

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JournalJournal of the American Chemical Society
DateAccepted/In press - 8 Nov 2018
DateE-pub ahead of print - 8 Nov 2018
DatePublished (current) - 5 Dec 2018
Issue number48
Volume140
Number of pages10
Pages (from-to)16855-16864
Early online date8/11/18
Original languageEnglish

Abstract

Diazirines are important for photoaffinity labelling and their photoisomerization is relatively well known. This work shows how hyperpolarized NMR spectroscopy can be used to characterise an unstable diazo-compound formed via photoisomerization of a 15N2-labelled silyl-ether substituted diazirine. This diazirine is prepared in a nuclear spin singlet state via catalytic transfer of spin order from para-hydrogen. The active hyperpolarization catalyst is characterised to provide insight into the mechanism. The photochemical isomerisation of the diazirine into the diazo-analogue allows the NMR invisible nuclear singlet state of the parent compound to be probed. The identity of the diazo-species is confirmed by trapping with N-phenyl maleimide via a cycloaddition reaction to afford bicyclic pyrazolines that also show singlet state character. The presence of singlet states in the diazirine and the diazo-compound are validated by comparison of experimental nutation behaviour with theoretical simulation. The magnetic state lifetime of the diazo-compound is determined as 12 ± 1 s in CD3OD solution at room temperature whereas its chemical lifetime is measured as 100 ± 5 s by related hyperpolarized NMR studies. Indirect evidence for the
generation of the photoproduct para-N2 is presented.

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© 2018 American Chemical Society

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