Random-Orientation High-Spin Electron Spin Resonance Spectroscopy and Comprehensive Spectral Analyses of the Quintet Dicarbene and Dinitrene with meta-Topological Linkers: Origins of Peculiar L

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• 作者：Teruaki Koto ; Kazunobu Sato ; Daisuke Shiomi ; Kazuo Toyota ; Koichi Itoh ; Edel Wasserman ; Takeji Takui
• 刊名：Journal of Physical Chemistry A
• 出版年：2009
• 出版时间：August 27, 2009
• 年：2009
• 卷：113
• 期：34
• 页码：9521-9526
• 全文大小：204K
• 年卷期：v.113,no.34(August 27, 2009)
• ISSN：1520-5215

文摘

In high-spin chemistry, random-orientation fine-structure (FS) electron spin resonance (ESR) spectroscopy entertains advantages as the most facile and convenient method to identify high-spin systems, as frequently reported in the literature. Random-orientation ESR spectroscopy applicable to organic high-spin entities can date back to the Wasserman and co-workers’ attempt on the first spin-quintet dicarbene, m-phenylenebis(phenylmethlene) (m-PBPM), in the 2-MTHF glass in 1963 and 1967, following their successful work on randomly oriented triplet-state ESR spectroscopy. The FS ESR spectrum of m-PBPM in the 2-MTHF glass, however, has never fully been analyzed due to a peculiar line-broadening appearing at many canonical peaks. Organic high-spin spectra from most quintet dinitrenes also suffer from similar phenomena. Seemingly intrinsic line-diffusing or -broadening phenomena adversely affect the reliable determination of FS parameters for organic high-spin entities in rigid glasses. In high-spin chemistry, the line-broadening has been an obstacle that masks key FS transitions in many cases. Thus, both the origin of the broadening and the comprehensive spectral analysis have been a long-standing issue. In this report, we examine the origin of the line-broadening appearing in the FS ESR spectra, illustrated by a comprehensive spectral analysis for m-PBPM in the quintet ground state and the first-documented quintet-state dinitrene, m-phenylenebis(nitrene) (m-PBN) in the 2-MTHF glass. A complete analysis of the random-orientation FS spectra from m-PBPM diluted in the benzophenone crystal has shown that the g-anisotropy of m-PBPM is not prominent. Also the higher-order FS terms such as S_i²S_j² group-theoretically allowed for S = 2 are not necessary in spite of the argument for a hydrocarbon-based tetraradical (S = 2) in the ground state. Our new approach to the line-broadening analysis invokes both exact analytical solutions for the resonance fields of canonical peaks and the magnetic-parameters gradient method. The D- and E-values of m-PBPM acquired by the spectral simulation in this study give different molecular structures of the quintet dicarbene in the benzophenone crystal lattice (D = +0.0703₀ cm⁻¹, E = +0.0212₀ cm⁻¹) and in the 2-MTHF glass (D = +0.0780₀ cm⁻¹, E = +0.0221₀ cm⁻¹). Microscopic origins of the line-broadening observed for high-spin oligocarbenes or oligonitrenes generated by photolysis in organic glasses have been proposed.