200
COMMUNICATIONS TO THE EDITOR
Electron Paramagnetic Resonance of Adsorbed Nitroxide Pubiication costs assisted by the Petroleum Research Fund
Sir: We have performed epr studies of di-tert-butyl nitroxide (DTBN) adsorbed on silica-alumina. As is also true for 0 2 - and N 0 , I the parameters of the DTBN spin Hamiltonian (SH)
H
=
+ AySzIz +
PS*g*H
+
B~(Sx1x
SYIU)
are sensitive to the nature and strength of surface interactions. In the powder spectra of DTBN on surfaces or in frozen solutions the parallel component of the anisotropic I4N hyperfine splitting (hfs), AN, gives well-resolved splittings (Figure 1A). Complexing a nitroxide with an electron pair acceptor (A) stabilizes the resonance structure with the unpaired spin on nitrogen, I, shifting spin density
t
-
(~-Bu),N-O:A I
-
(~-Bu),N-O:A I1
toward N and resulting in an increase in A N . This change in AN increases with the strength of the interaction, and thus A N may be used as a measure of electron pair acceptor strength. When the nitroxide is complexed with a strong Lewis acid, additional information may be available from the observation of hfs by the coordinating nucleus of the acid.2 An advantage to the utilization of DTBN as a surface probe, as opposed to 0 2 - or NO, is that it also forms complexes with molecular Lewis acids in free solution;2 thus for the first time we use a radical to compare the properties of a surface site with those of a well-defined solution species. Silica-alumina (13% alumina) is first calcined in oxygen at 500" to remove impurities, rehydrated by exposure to water vapor at room temperatures, and then activated in uacuo to a selected temperature (125-500'). Exposure to DTBN vapor followed by equilibration at a fixed temperature between 50 and 125" can produce two distinct types of epr spectra, I (Figure 1A) and I1 (Figure 1B). The adsorbed nitroxides which give these signals are chemically unaltered, for they can be completely removed intact from the surface by exposure to a stronger base such as pyridine. Following such treatment, a spectrum of DTBN in pyridine is observed. In addition, quantitative epr shows that depending upon conditions of activation, some fraction of the applied DTBN is decomposed into diamagnetic products. In the type I spectrum (Figure lA), A N is increased over that in an inert solvent, indicative of perturbation of the nitroxide by interaction with the surface. The value of A N is within the range found for DTBN in hydrogen-bonding media.2 Since at the activation temperatures employed, adsorbed water has been removed, we associate this spectrum with nitroxide hydrogen bonded to surface hydroxThe Journal of Physical Chemistry, Voi. 78, No. 2, 1974
y l ~ A. ~similar spectrum is seen for DTBN on pure silicae4 In particular, from Table I we see that the value of A N on silica-alumina and in phenol are similar, demonstrating that the interactive strength of the hydroxyls in both systems is similar. As with DTBN in phenol, no splittings are resolved for the hydrogen-bonded proton. The type I1 spectrum shows A N to be further increased, and also exhibits a well resolved six-line hyperfine pattern arising from interaction with a single aluminum nucleus ( I = 5/2), indicating that the nitroxide is complexed to a coordinatively unsaturated surface aluminum a t ~ m . ~ a , b Figure 1 and Table I show the great similarity between the spectrum for DTBN coordinated to surface aluminum and that for the radical complexed with AlC13 in solut i ~ n In . ~ contrast, the S H parameters for DTBN complexed with, for example, aluminum alkyls, are quite different from those of Table I.5 Thus, the electron pair acceptor strength of the surface aluminum is similar to that of AlC13, greater than that of AlR3, and considerably greater than that of the surface hydroxyls or phenol. The type of site observed could be controlled by variation in temperature of activation and/or surface coverage.4 Type I1 spectra become increasingly important at the higher activation temperatures (200" and above). For a given temperature type I1 spectra tend to dominate at low surface coverage, presumably, because the more strongly interacting (larger AN) surface aluminum ion sites "fill up" first. However, computer simulations suggest a finite amount (-10%) of type I spectrum as a background even in Figure lB.4 At all activation temperatures the contribution from nitroxides hydrogen bonded to surface hydroxyls (type I) increases with increasing surface coverage until the type I1 spectrum is a small or even unobservable fraction of the signal. Finally, when the amount of DTBN approaches monolayer quantities, the type I spectrum begins to exhibit spin-spin interactions between nitroxides. With a low temperature of activation (T.4 = 125') a surface aluminum should be left with a bound water, which would then act as a strong proton d ~ n o r . By ~~,~ analogy to the behavior of the H20:AlC13 complex such a TABLE I: Hfs (Gauss) for DTBN o n Silica Alumina and i n Solution (T = 77°K) Ref
Medium
ANb
Toluene Silica-alumina,
34.6c
This work
38.6O
This work 2c
~ A l d
B
A
~
~
~~
type Io
Phenol Silica-alumina, type IIQ AICls complex
38 .3c
46, I d 47 .Od
16.6
14.6
17.1 16.2
This work 5
Hfs constants from Figure 1. Values vary somewhat with catalyst B N ( = 6 i 1 G) is not directly observable: it may treatment (ref 4). Error limits, 60.2 G. be estimated from computer simulation (ref 4). Error limits, f0.5 G.
Communications to the Editor
201
AN-
vestigations of activation conditions are in progress as well as a n extension to other surfaces of catalytic importance, such as silica, alumina, and zeolites.
r\
-1
\/ / -
T - l ,
-
-
A
;, i ,
References and Notes
,
i
H,
106
-r-r-' B " .'
Achnouledgrnent. We are deeply indebted to Professor Robert L. Burwell, Jr., for enlightening discussions. This work was supported by DA-ARO-D Grant No. 31-124 and by the donors of the Petroleum Research Fund, administered by the American Chemical Society.
,1
,
~
J
i ,J
, '
(1) (a) J. H. Lunsford, J. Catal., 14, 379 (1969); (b) P. H. Kasai and R. J. Bishop, Jr., J. Amer. Chem. SOC.. 94, 5560 (1972); ( c ) K. M . Waog and J. H. Lunsford, J. Phys. Chem., 74, 1512 (1970); (d) J. H. Lunsford, Advan. Catal., 22, 265 (1972); (e) B. M. Hoffman and N. J. Nelson, J. Chem. Phys., 50, 2598 (1969). (2) (a) T. B. Eames and B. M. Hoffman, J. Amer. Chem. SOC.. 93, 3141 (1971); (b) B. M. Hoffman and 1.B. Eames, /bid., 91, 2169 (19691, (c) A. H. Cohen and B. M. Hoffman, ibid., 95, 2061 (1973). (3) (a) J. J. Fripiat, A . Leonard, and J. B. Uytterhoeven, J. Phys. Chem., 69, 3274 (1965); (b) M. R. Basila and T. R . Kantner, ibid., 71, 467 (1967); (c) E. P. Parry, J. Cafal., 2, 371 (1963); (d) J. B. Peri, J. Phys. Chem., 70, 2937 (1966). (4) G. Lozos and B. M. Hoffman, to be submitted for publication. (5) A. H. Cohen, T. B. Eames, and B. M. Hoffman, to be submitted for publication. (6) Alfred P. Sloan Fellow.
Department of Chemistry Northwestern University Evanston, Illinois 60207
fi
George P. Lozos Brian M.
Received September 12. 1973
Solvent Effects on the Fluorescence Lifetime of 2-Aminopyridine
u +..-
~AN*~AAI
+
Figure 1. Esr of DTBN at 77°K: (A) type I spectrum on silicaalumina (--) and that of frozen toluene solution (- - - ) ; (B) type I I spectrum on silica-alumina; (C) AI& molecular com-
plex (ref 5).
surface species should act as a strong Bronsted acid and be able to protonate a nitroxide.2b DTBNH+ has A N = 47.62c G and no metal splitting, but no such spectrum with that large a splitting was observed either as a predominant or minority species upon addition of nitroxide t o catalyst activated a t this temperature. However, under these conditions the nitroxide underwent considerable decomposition, far more than was found to occur a t higher T i . A large excess had to be added before the site I spectrum could be observed. Removal of nitroxide by heating and pumping revealed only a weak site I1 spectrum. Since the protonated nitroxide formed from HzO:AlC13 is much less stable than is the A1C13 complex, it may be inferred t h a t similar behavior occurs on a silica-alumina surface; a Bronsted site is formed by moderate T,, but decomposes the nitroxide whereas coordinately unsaturated surface aluminum itself forms a stable complex with the radical. We have used DTBN to examine surface sites on silicaalumina and to compare their properties to those of welldefined molecular species. In particular, we have observed a coordinatively unsaturated surface aluminum whose properties as a Lewis acid resemble those of AIC13 and which is present even with T.1 = 125". More detailed in-
Sir: The absence of luminescence frofn electronically excited states in pyridine has remained a problem of continuing interest, In previous reports from this laboratory we demonstrated significant solvent effects on the fluorescence of aminopyridines.l.2 It was also shown that the lowest excited singlet states of these molecules were more basic than the ground state, which is usually observed in heterocyclic molecules. Aromatic amines exhibit increased acidity upon excitation to S1*.3 We have selected 2-aminopyridine (2-AMP) as a model compound, which approximates the excited state behavior of pyridine, but exhibits fluorescence so that environmental changes could be followed. Our choice was prompted by the gas-phase spectral , ~ the 2980-A system in analysis by Hollas, et ~ l . that 2-AMP is essentially a pyridine A,A* transition perturbed by the amino group. Lamotte and L o u s t a ~ n e a u on , ~ the other hand, reported that the first excited electronic state in 2-aminopyridine shows n , r * character. With the aim of trying to better understand radiationless processes in pyridine we have measured the fluorescence lifetimes of 2-AMP in different solvents, in order to establish, if possible, (1) the constancy of the triplet yield, (2) the constancy of the natural radiative lifetime of the lowest excited singlet, and (3) whether or not radiationless processes occur from the singlet, triplet, or in both manifolds. The fluorescence lifetime for vacuum degassed solutions were measured using the time correlated single photon counting technique and are tabulated in Table I, for five different solvents, in the order of increasing dielectric conThe Journal of Physical Chemistry, Yo/. 78, No. 2, 1974
