CIDNP as a tool for rapid rearrangements: New insights into cyclobutanoid 4‐center/3‐electron radical cations

Abstract

CIDNP has been often utilized as a mechanistic tool for radical reactions. Here, we show that detailed insights into the structure of carbon-centered radical cations with unusual structures, which, otherwise, are hardly detectable, can be obtained. Hydrocarbon cage compounds containing syn-periplanar cyclobutane moieties were oxidized by photoinduced one-electron transfer to chloranil at room temperature in CD 2 Cl 2 . The 1 H CIDNP polarization intensities of geminate radical pairs can be translated into isotropic hyperfine coupling constants and reveal radical cations with remarkable electronic structures. Density functional theory calculations are in line with the experimental results. a "Extended"/"Tight", the data for the tight isomer were determined by fluorescence-detected magnetic resonance [19] . b Only protons clearly distinguishable by different chemical shifts are given. c The number in parenthesis gives the average value for b protons resembling the CIDNP polarization intensities (see text). CIDNP OF CAGE COMPOUNDS

Spin Dynamics of Flavoproteins

“…There also have been works on the photo-CIDNP effect beyond 1 H liquid-state NMR using flavin dyes, including studies in the gas phase [71,72], on various nuclei as 19 F nuclei, using various fluorescent dyes [73], and studies of organic-chemical reaction mechanisms [74][75][76]. Protein structures have been investigated using the photo-CIDNP effect [77].…”

Photo-CIDNP in Solid State

“…For the interpretation of CIDEP spectra, this theoretic concept has been adapted and the term “spin-correlated radical pair” (SCRP) was introduced [ 24 , 25 ]. Some later break-throughs were studies in gas phase [ 26 , 27 ], 19 F photo-CIDNP NMR [ 28 ], usage of fluorescein as dye [ 29 ], studies to explore organic reaction mechanisms [ 30 – 32 ], photo-CIDNP-based studies of protein structures [ 33 ], time-resolved liquid-state photo-CIDNP studies on protein folding [ 34 ], the concept of isotropic mixing by Hans-Martin Vieth and co-workers [ 35 – 37 ] as well as field-cycling photo-CIDNP NMR of liquid samples [ 35 , 38 , 39 ]. Examples of molecular systems, which exhibit very strong liquid-state CIDNP effects, are given by confined SCRP, which is the case of radicals trapped in micelles [ 40 , 41 ] and, in particular, biradical systems [ 42 – 44 ].…”