Strong Exchange Interactions between Two Radicals Attached to Nonaromatic Spacers Deduced from Magnetic, EPR, NMR, and Electron Density Measurements

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• 作者：Raymond Ziessel ; Christophe Stroh ; Henrike Heise ; Frank H. Kö ; hler ; Philippe Turek ; Nicolas Claiser ; Mohamed Souhassou ; and Claude Lecomte
• 刊名：Journal of the American Chemical Society
• 出版年：2004
• 出版时间：October 6, 2004
• 年：2004
• 卷：126
• 期：39
• 页码：12604 - 12613
• 全文大小：318K
• 年卷期：v.126,no.39(October 6, 2004)
• ISSN：1520-5126

文摘

A nitronyl-nitroxide (NIT) biradical D-NIT2 linked by a single double bond has been engineeredand investigated in the solid state by a combination of X-ray diffraction, magnetic susceptibility measurement,EPR, as well as solid-state ¹H and ¹³C NMR spectroscopies, and experimental electron density distribution.All techniques reveal that a double bond is a very efficient coupling unit for exchange interactions betweentwo radical moieties. Using a Bleaney-Bowers model dimer (H = -JS₁S₂), a singlet-triplet energy gap ofJ = -460 K was found with the singlet state being the ground state. This very strong intramolecularinteraction was confirmed by EPR measurements in CH₂Cl₂ solution (6 10^-4 M) or dispersed in a polymermatrix at low concentration. In keeping with these unusual interactions, solid-state NMR signals of thebiradical were found to be considerably less shifted than those found for related monoradicals. Temperature-dependent solid-state ¹³C NMR spectra of D-NIT2 confirmed the very strong intramolecular coupling constant(J = -504 K). The electron density distribution of D-NIT2 was measured by high resolution X-ray diffraction,which also revealed that this biradical is an ideally conjugated system. The in-depth characterization includesthe deformation maps and the observed electron density ellipticities, which exhibit a pronounced -character of the O-N-C=C-N-O cores in keeping with an efficient electronic delocalization along thealkene spacer.