Structural Information from Liquid- and Solid-Phase ESR The example of triphenyl-substituted radicals (C6H&A' of Group IVB Elements M. Geoffroy Department of Physical Chemistry, University of Geneva. Sciences 11, 1211 Geneva 4, Switzerland
J. H. Hammons Swarthmore College. Swarthmore, PA 19081 In resonance spectroscopy most experiments are performed either on samples in the liquid phase (high-resolution NMR) or in the solid phase (NQR, broad-line NMR). In this paper, we intend to show that with an electron spin resonance (ESR) spectrometer, complementary chemical information can he obtained from solution spectra and from single-crystal suectra. In rreneral, s e w n ~IlI S ~ C C ~~Si t h iet r w n w oiorganic radirsls :trtt ofihentiral interrst. Thrie im411deuurstions of molecular conformation, atomic orbital hyhridization, and electron delocalization. Certain radicals are viewed in the valvn(e hmd apprmtm.iti~~tt a, having rhr itnp;tired electron t~ss~ntiallv localized 011 3 cenrral atmn If tht. central atom hai three ligaids, as, for example, in the alkyl and haloalkyl radicals, ESR spectroscopy can give information on the degree of planarity around the radical center and on the hyhridization of the atomic orbital containing the odd electron. Other radicals have the unpaired electron in a pi-molecular orhital which may be delocalized over the entire pi-system. In such cases. ESR snectrosco~vallows the determination of the distribution oiunpairede"1ectron density over the atoms of the pi-system and thus provides invaluable information about the nature of the half-filled molecular orhital. The objective of this article is to show in terms of a specific radical system how ESR data can he used to obtain structural information. For several reasons, the triphenylmethyl radical seemed to he an especially appropriate example. The structural questions which have arisen in the investigation of triphenylmethyl-hybridization and planarity around the central atom, delocalization of the unpaired electron over the three phenyl rings, and the twisting of the rings-are typical of those which commonly arise with organic radicals. Furthermore. the availahilitv of ESR data derived from studies of triphedylmethyl both in solution ( I ) and in oriented single crvstals (.2.)makes it oossible to illustrate the complementary nature of the two kiids of data. Finally, in the past few years the triphenylsilyl(3),triphenylgermyl(4,5), and triphenylstannyl(6) radicals have also been generated and investigated hv ESR. The trends observed for the series of Group IVB r:~clic.tliare 01' interest in relation t o ory;inm~eraiiirchemi~try. ;I tield in which t h t w i, currently i n t ~ n w wtivtly. General Theoretical Considerallons The general Hamiltonian used in the analysis of ESR spectra due to organic radicals is given by: ?f = PHoSgh
+ ~ N B N H O+II~T S ;
(1)
h is the unit vector along the external magnetic field Ho. The other svmbols have their conventional meanings: S a n d I are. reapertivrly, rhr rlrctrmir and ~ni,.lritrspin vtx.lori. g s n d p y nrr thr l.andi: 1:~forn for rhr .
The triohenvlstannvl radical has been ~ r o d u c e dbv X-ir. . railinrim of H single crystal of Ph,Sn ( 6 ) .Hew there are twu miwncllc isorooes ..I1-Sn11= ,, narl~ralabuodnnce = 76l'W and"%n ( I ;ll2, natural abundance = 8.58%)-causing hyperfine interaction. The analysis of the angular variation is made difficult by the fact that the radical can result from the scission of any one of four Ph-Sn bonds, and that there are two molecules in the unit cell. Nevertheless, the tensors have been determined. The parameters given in Table 3 demonstrate the high degree of localization of the unpaired electron on the Sn atom, as well as the substantial s character of the Sn orhital. Comparison of the Ph3A .Radicals of Group lVB In the radicals PhnA, the total spin densities on the central
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ESR studies of organic pi-radicals in solution yield the isotropic coupling constants of the magnetic nuclei. From these parameters, valuable information about the delocalization of the unpaired electron can be obtained. ESR investiaations of oriented single crystals permit determination of the anisotropic compon&s of the hiperfine coupling. These values can he used to determine the hybridization of the atom in question as well as the spin density on that atom. Application of both of these complementary techniques to a sinele radical can vield a wealth of detail about molecular confryrmation and eiectronic structure. Comparisons of ESR results for structurallv related radicals can lead to the elucidation of interesting trends within families of the Periodic Table.
Journal of Chemical Education
1 ~ u h l i s h i n g C o . . ~ & ~ t ~l967.p. ~ d ~ r n .257. (81 Schremr, K.. Rcrndr. L a n d Baer. F.,Molrc. Phys..26.9?9 110751
107.
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