Structure of organic paramagnetic nitroxyl radicals

Abstract

STRUCTURE OF ORGANIC PARAMAGNETIC NITROXYL RADICALS R. N. Shibaeva UDC 548.~/3~/ Results of the structural investigation of 25 stable nitroxyl radicals are presented in the review. Char- acteristics of geometry of )N--O fragments in radical molecules are given, and their mutual ar- rangement in the crystal, which determines exchange interactions in solid paramagnetic substances, is presented, Virtually all original structural papers published before October 1, 1974 are considered. 1. INTRODUCTION Interest in stable iminoxyl radicals has increased significantly lately in connection with their wide use as spin labels in the study of structures and properties of biologically active molecules. In addition, tminoxyl radicals are also used for inhibition of radical reactions in living organisms (in parricular, some of them are able to inhibit the development of malignant tumors); they also find important practical application in polymer investigations [1-3]. Compounds into the composition of which enters the )N--O group having one unpaired electron are called nitroxyl radicals. Iminoxyls are nitroxyls having aliphatic or other substituents on the N atom not participating in conjugation with the unpaired electron [1]. Together with monoradicals, in the molecule of which there is one N'-'O grouping having an unpaired electron, there also exist polyradicals having several unpaired electrons per mol- ecule: in particular, biradicals, already quite a broad class of compounds [1]. Data on the crystal structure of nitroxyl radicals are of interest from two points of view. First, knowledge of molecular conformation (particularly of the )N "--O) fragment) gives additional infor- marion on the electronic structure of the radical, which determines its properties. In addition, it makes it possible sometimes to compare conformation of the molecule in the solid state with its conformation in the liquid, when it is known on the basis of investigation by other methods. Second, knowledge of molecular packing in the crystal makes it possible to judge the mechanism of exchange interactions in solid paramagnetic substances, since the character of such interactions in the crystal is determined by the mutual arrangement of paramagnetic centers (unpaired electrons). It would be hoped that the structural investi- gations of nitroxyls with the simultaneous consecutive investigation ofmagneticinteractionsinthem [4-8] wouldmake it possible to establish experimentally the dependence of exchange energy on interelectronic distance I (r) (at pre- sent there are no reliable quantum-chemical methods of calculating this function). In addition, the knowledge of crystal structure for polyradicals makes it possible to separate intermolecular and intramolecular contributions to ex- change interactions. Formulas of all nitroxyl radicals, the structures of which will be discussed subsequently, are presented be- low. For each discussed radical structure we give the following: name, general characteristics, parameters of the elementary cell, space group, and number of molecules per cell [only two of the discussed radicals (I) and (II) are not crystalline]. In the description of molecular structure the main attention was given to the geometry of the )N~'O fragment, and in the description of crystal structure it was given to the mutual arrangement of paramag- netic centers. Branch of the Institute of Chemical Physics, Academy of Sciences of the USStL Translated from Zhumal Struktumoi Khimii, Vol. 16, No. 2, pp. 330-348, March-April, 1925. Original article submitted November 1, 1974.