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C O M M U N I C A T I O N S TO THE E D I T O R A Case €or Solvated Electrons Publication costs assisted by Professor 144. C. R. Symons
Sir: Tuttle and Graceffal have drawn a comparison between solutions of alkali metals in amines (and ammonia) with those of sodium naphthalenide in ethernapththalene mixtures. They seem to suggest that a common model may well be able to accommodate the similar esr results for these two systems and since, obviously, the “solvated electron’’ model is quite inapplicable to the naphthalenide system, they seem to favor the concept that the metal solutions are best described as comprising the alkali metal salts of the solvent (ie., Na+NH3-, Na+RNH2- etc.). If this analogy is accepted, it should be taken one step further. As was stressed,l IYa+N- (N- = naphthalenide) and N- in dilute solution in inert solvents have esr spectra quite characteristic of the molecular system, the unpaired electron being in a highly localized 7r* 110 of the naphthalene. All attempts to reproduce this behavior for ammonia and amines in dilute solutions of inert solvents have failed. Even hexamethylphosphoramide, which is a very good solvent for metals, does not interact with sodium-ether systems despite the presence of relatively low-lying empty ,110’s for this compound2 and despite the fact that “solvated electrons” are formed in such media. Thus the analogy breaks down. True, nmr results indicate a large contact interaction with 14N, but this could be spread out over several solvent molecules : there is nothing in the results to suggest that it is confined to one per electron. The reason why most workers in this field accept that two distinct models are required for the systems under consideration is that only naphthalene has a Iowlying MO into which the extra electron can move. Anions such as NH3- can be formed, in principle, by reaction between NH2- and hydrogen atoms. Indeed, when hydrogen atoms are trapped in polar media, it seems very likely that there is a real interaction with one or more “solvent” molecules3 which has the effect of hardly modifying their esr spectra, but introducing a characteristic optical absorption band. Perhaps the nearest example is that of HF-, which is structurally comparable with NH,-. Such units are thought to be formed in CaF2-H systems, and again, the proton COUpling is close to that for normal hydrogen atomsa5 Thus experimental evidence and calculations4 support the statement that the esr spectra of species such as NHB- are likely to be characterized by very large, positive, 1H hyperfine coupling constants. I n fact, however, the coupling detected by nmr is invariably The Journal of Physical Chemistry, Vol. 76, N o . 26, 1071
small and negative. This implies a spin polarization of (presumably) the N-H c electrons by the unpaired electron, rather than a direct occupancy of a suitable antibonding orbital. In the gas phase, anions such as NHs- would spontaneously lose an electron. If NH3- is formed in liquid or solid ammonia, in the absence of distortions, the electron would become delocalized over all the molecules as a conduction electron. This is certainly not the case in the systems concerned. If, however, the electron is momentarily confined to one ammonia molecule, and other solvent molecules re-orient to suit the negative change, such delocalization mould be prevented. Since, however, the electron is in an antibonding state, even greater stabilization (kinetic) would result if the central molecule were removed, leaving the electron in a cavity. There are plenty of analogies for this, perhaps the simplest being the conversion of an optically excited halide ion in an alkali halide crystal into an F center by removal of the central halogen atom. There is a large volume of evidence in favor of the cavity-solvated electron model from the solid states6 For example, glassy ethanol on irradiation gives trapped electrons, but crystalline ethanol does not. The former contain many cavities, the latter very few. At 4.2 K the former has a narrow esr singlet indicating weak COUpling to solvent molecules, but on warming to 77 K i t broadens markedly because of specific coupling to several (probably 4) OH protons. The link is now via the optical spectra. That a t 4.2 I< is probably comparable with electrons trapped in ether cavities: that at 77 K is characteristic of alcohols and links well with results for electrons in fluid alcohols. The contrast between molecules with and without acceptor orbitals is again clear: alcohols, amines, ammonia, water, etc., do not give the corresponding negative ions, but methyl cyanide, benzene, etc., do. On the other hand, the former do give trapped and solvated electrons; the latter do not. (1) T . R. Tuttle and P. Graceffa, J . P h y s . Chem., 75,843 (1971). (2) R. Catterall, L. P. Stodulski, and M. C. R. Symons, J . Chem. Soc., A , 437 (1968). (3) P. W.Atkins, N. Keen, M .C. R. Symons, and H. W.Wardale, ibid.,5594 (1963). (4) T. A. Claxton and M. C. R. Symons, Chem. Commun., 379 (1970). (5) J. L. Hall and R. T. Schumacher, P h y s . Rev., 127,1892 (1962). (6) M. C. R. Symons, P u r e A p p l . Chem., 309 (1970).
&I.c. R.S Y M O N S DEPARTMENT OF CHEMISTRY THEUNIVERSITY LEI 7RH, ENGL.?ND LEICESTER, RECEIVED M A Y 28, 1971
