J . Phys. Chem. 1988, 92, 2521-2525
2521
Interaction of Activated Magnesium Oxide Surfaces with Spin Traps Kenneth J. Klabunde* and Ileana Nievest Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: June 26, 1987; In Final Form: November 17, 1987)
Thermal activation of high surface polycrystalline MgO at 200-1 100 "C produces room temperature surface reactivity with subsequently adsorbed spin traps such as N-cy-diphenylnitrone (PNP) and 2-nitroso-2-methylpropane (NOMP). ESR measurements indicate the MgO surface to be initially diamagnetic. The added spin traps are also initially diamagnetic. However, upon interaction high concentrations of paramagnetic species are formed. Two primary reactions appear to be (1) electron transfer from electron-rich defect sites on MgO to PNP and NOMP to form P N P - and NOMP-. In the case of PNP the anion radical is stable and strongly adsorbed. In the case of NOMP-, decomposition to tert-butyl radical and presumably 0-NO- occurs, and the alkyl radical is trapped by NOMP to form the stable di-tert-butyl nitroxide. (2) Surface Mg-oxy radicals are trapped by NOMP to form &NOM€" which subsequently also decomposes to tert-butyl radical and is then also trapped by NOMP. Activation temperature for the MgO profoundly affects the course of these radical-forming processes. Thus, addition of spin traps to such active oxide surfaces serves as a method of distinguishing different types of surface reactivity and features. Partially dehydroxylated surfaces are unusually reactive, suggesting the importance of OH groups next to exposed cations as important sites.
Results and Discussion
Introduction Ionic, insulator oxides can be prepared in high surface area forms resulting in a relatively high proportion of surface/bulk metal oxide moieties. Such microcrystalline particles can possess a wide variety of defect sites including structural (kinks, steps, cleavage planes, cation, and anion vacancies), and electronic (electron excess/deficient centers).'-3 In addition, surface impurities, especially adsorbed H 2 0 , O H , and H+, can have important effects. Such defects play an important role in the rich surface chemistry of metal oxide^.^-^ However, progress toward learning what defects are responsible for specific surface reactions has been very slow. The complexity of the surface structure has hampered real progress. One serious problem is that it has been difficult to distinguish diamagnetic from paramagnetic defects since ESR detection of some paramagnetic sites has been very difficult due to relaxation times.6 One important reaction that is indicative of the presence of diagmagnetic defect sites is electron transfer. Thus, if MgO is thermally activated in vacuo (to clear the surface of adsorbed H 2 0 , COz,and some O H groups), cooled to room temperature, and an electron-demanding molecule allowed to adsorb, electron transfer to form the adsorbed anion radical readily takes place. High concentrations of anion radicals, even monolayers, can be formed in this way.' Tanabe has referred to such surface electron donation sites as reducing sites,* and it has been proposed that electron-rich substructures, perhaps cation vacancies, are resp~nsible.~~~ In order to learn more about these electron-rich substructures, as well as try to differentiate diamagnetic and paramagnetic sites, we have carried out a study of adsorbed spin traps on the surface of MgO. Spin traps are molecules that are able to scavenge free radicals to form stable paramagnetic adducts.e11 They are also capable of accepting free electrons. Thus, they have the potential of allowing simultaneous detection of both diamagnetic and paramagnetic electron-rich sites. Two common spin traps have been employed as shown below:
7I ?-
I
CH,--C-N=O
CgHs-C=N
I
- 2 - methylpropane
-C8H5
N - a - dipenylnit rone (PN P)
CH3 2-nitroso
It
(NOMP)
On leave from University of Puerto Rico, Humacao University College, Humacao, Puerto Rico. 0022-3654/88/2092-2521$01.50/0
After thermal activation of a MgO sample (see Experimental Section) and cooling to room temperature, a spin trap molecule was allowed to adsorb in one of two ways: (1) by dissolving the spin trap in pure, degassed toluene and adding this solution to the MgO, and removing the toluene in vacuo, or (2) in the case of N O M P by evaporation of the spin trap onto the MgO sample at room temperature. After a prescribed time any N O M P that did not adsorb was pumped away. Since ESR was our most important tool, MgO blank samples (no spin trap) were examined frequently. Figure 1A shows the ESR spectrum for a blank MgO sample (500 "C activation temperature). Very weak absorptions were detected and these are due to small amounts of MnZ+impurity which served as convenient internal standards. After 800 OC activation a singlet at g = 1.9797 appeared for the first time and it became more intense as the activation temperature was increased (Figure 1B,C). This finding will be discussed later. PNP. Upon adsorption of P N P a strong, broad signal (Figure 2) was generated with a g value of 2.0039 and AN = 16 G. Hyperfine splitting due to a-H and H atoms on the aromatic rings could not be resolved. The spectrum did not change with time. If a surface radical oxide species were trapped to yield I it can be reasonably assumed that AN should be similar to PNP trapped alkoxy radicals.
(1) Sondor, E.; Sibley, W. A. In Poinr Defecrs in Solids; Crawford, J. H., Slifken, L. M., Eds.; Plenum: New York, 1972; p 201. (2) Henderson, B.; Wertz, J. E. Defects in Alkaline Earth Oxides; Halstead: New York, 1977. (3) Klabunde, K. J.; Hoq, M. F.; Mousa, F. In Preparatiue Chemistry Using Supported Reagents; Laszlo, P., Ed.; Academic: New York, 1987; p 35. (4) (a) Che, M.. Bond, G. C., Eds. "Adsorption and Catalysis on Oxide Surfaces", in Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1985; Vol. 21. (b) Che, M.; Tench, A. J. Adu. Catal. 1982, 31, 77. (5) Klabunde, K. J.; Matsuhashi, H. J . Am. Chem. SOC.1987, 209, 1111. (6) Driscoll, D.; Martir, W.; Wang, J.; Lunsford, J. J . Am. Chem. SOC. 1985, 107, 58. (7) Morris, R. M.; Klabunde, K. J. Inorg. Chem. 1983, 22, 682. (8) Tanabe, K. Solid Acids and Bases; Academic: New York, 1970. (9) Evans, C. A. Aldrichim. Acta 1979, 12, 23. (10) Zubarev, V. E.; Bekevskii, V. V.; Bugaenko, J. Russ. Chem. Reu. 1979, 48, 729. (11) Oleshko, V. P.; Bychkova, T. V.; Golubev, V. B.; Lunina, E. V.; Nekarosov, L. I. Rum. J . Phys. Chem. 1978, 52, 605.
0 1988 American Chemical Society
Klabunde and Nieves
2522 The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 --i 50 G
j 1)
(C)
I l l
___I__
Figure 1. ESR spectrum of thermally activated MgO (A) representative of activation temperatures of 200-800 O C , (B) 900 O C , (C) 1100 OC. All recorded a t room temperature.
(1) 10
G
(2) 26
G
11k1;
(3) 19 G,
Figure 3. ESR of NOMP adsorbed on thermally activated MgO (600 "C) at room temperature.
In the literature three relevant species have been characterized by ESR, as shown be lo^.'^,'^ Note the large variations in A,. CH3 0*
I I CH3-C-N--OR I Figure 2. ESR of N-a-diphenylnitrone (PNP) adsorbed on thermally activated MgO (600 "C) at room temperature; g = 2.0039, AN = 16 G .
In the 'OR trapped case AN values of 10.5-10.7 G have been reported,I2 so structure I is probably not the paramagnetic species produced. A second possibility would be that P N P accepted an electron from the MgO surface to form a radical anion PNP'as shown below:
I1
Structure I1 should exhibit a large AN value since much of the unpaired spin density should reside on nitrogen and carbon.I3 In order to make an authentic sample of PNP'- we added potassium metal to a THF solution of P N P and recorded an ESR spectrum. Multiple hyperfine splitting was observed superimposed on a triplet with AN = 12.3 G and g = 2.004. Although this value is not in very good agreement with that observed for 11, it is substantially higher than the RO-PNP' species.'2 Also the g value is in excellent agreement with that found for PNP-. We therefore believe that PNP'- is the major product from this adsorption process and that it is stable at room temperature and strongly adsorbed. NOMP. When N O M P was allowed to adsorb on activated MgO (600 O C activation), three different radical species were found. Initially, t h e following s p e c t r a l patterns were observed simultaneously. A triplet was observed (Figure 3) with AN = 10.4 G and g = 2.0056. A second triplet was superimposed with AN = 26 G and g = 2.0048. Both of these species decreased with time while a third triplet increased AN = 19 G, g = 2.0048 that was stable with time.
(12) Bluhm, A. L.; Weinstein, J. J . Org. Chem. 1972, 37, 1748.
(13) In structure I1 the 0'-species might also be expected to be observed by ESR. However, no electron-transfer processes on MgO (or other oxides) have allowed the observation of this species. Two possible explanations are (1) (O-?*-) dipolar interactions cause severe broadening, and (2) the 0species IS so reactive that it does on to react further with adsorbed molecules. This latter explanation has been favored by Che and co-workers: Che. M.: Dyrck, K.: Louis, C. J . Phys. Chem. 1985, 89, 4531.
CH3
0-
I . ! CH3-C-N I
CH3
CH3
A N = 27-30 G g - 2.0054
A N = 11.2-12.20 g = 2 0059
I11
IV
CH3O* CH3 CH3-C-
I I
l
l I
N-C-CH3
CH3
CH3
AN
12-16 G g = 2.00586
V
We assume that AN and g values would not change dramatically upon adsorption, which proved to be a valid assumption in earlier ~ 0 r k . lTherefore, ~ ~ ~ ~ we think it is reasonable to explain our spectra using structures 111-V. The radical anion IV is known to be unstable toward decomposition to tert-butyl radical and NO-.I5 Likewise I11 is known to be unstable and has been shown to also decompose to tert-butyl radical and RN02.14 In our system if tert-butyl radicals were produced they could be trapped by physisorbed N O M P to yield V. Thus, we believe two kinds of surface sites are available to NOMP, an oxy radical site (electron difficient anion, A) and an electron-transfer site (electron-rich defect reducing site, B). Both sites react to form adsorbed I11 and IV in comparable amounts, and both decompose eventually yielding V. Our proposed sequence on the surface is shown below where analogous structures VI, VII, and VI11 are shown."
(A)
(B)
MQO A N = 26 G A N = 10.4G g = 2.0048 g = 2 0056
(14) (a) Libertini, L. J.; Griffith, 0. H. J. Chem. Phys. 1970, 53, 1359. (b) Berliner, L. J. Spin Labelling. Theory and Applications: Academic: New York, 1976: p 565. (15) Sosonkin, I. M.; Belevskii, V. N.; Strogoy, G. N.; Domarev, A. N.; Yarkov, S. P. J. Org. Chem. USSR (Engl. Transl.) 1982, 18, 1313. (16) Morris, R. M.; Klabunde, K. J. J. Am. Chem. SOC.1983,105,2633. (17) Note that PNY- showed a larger AN compared with PNY-OR, while NOMY- shows a smaller A N . This is apparently due to the basic structural differences, the nitrone anion radical (PNY-) placing more unpaired density on N, while the nitroso anion radical (NOMY- behaves oppositely.
Interaction of Activated MgO Surfaces with Spin Traps
1
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2523
C.
50 0
Figure 4. ESR of NOMP adsorbed on thermally activated MgO (500 " C )at room temperature initially using (A) 1.O mg of NOMP dimer/100 mg of MgO, (B) 2.0 mg of NOMP dimer/100 mg of MgO; ( C ) 4.0 mg of NOMP dimer/100 mg of MgO. W e do not observe spectra characteristic of adsorbed N0.I8 Either N O is lost to the vacuum after decomposition of I11 and IV, or cannot be observed due to the overlap with Fe3+impurity or for other unknown reasons. W e also do not observe the tertbutyl radical by ESR, indicating that when released it is trapped rapidly by physisorbed N O M P present. Variation of Initial NOMP Concentration. Studies varying N O M P initial concentration at a given activation temperature (500 "C) of MgO were undertaken. It is known that in solution and under the influence of light N O M P decomposes to N O and tert-butyl radical." In the presence of excess of NOMP the radical adds to another N O M P molecule forming a stable di-tert-butyl nitroxy radical (V). Upon NOMP adsorption on MgO the radical decomposition is initiated by the catalyst surface. Figure 4 shows the effect of different initial N O M P concentration upon its adsorption on MgO. At low concentration (1.0 mg of N O M P dimer/100 mg of MgO) an anisotropic signal at g = 2.003 and with line width (AW) of 3.0 G predominates (Figure 4A). In this case there was no excess of N O M P molecules and the signal observed should not be attributed to the di-tert-butyl nitroxide radical. It has been reported that, when N O was added to MgO a signal at g = 2.0047, AW = 6.0 G was obtained and it was attributed to N022-since the NO reacted with the excess oxide ions around a magnesium ion vacancy.Is These data are not comparable with our experimental results. Another possibility that we considered was the observation of VI centers at this low N O M P concentration range. In a previous report,I9 a VI signal appeared at g = 2.0030, but the signal was very sharp (AW = 0.7 G) and upon treatment with O2 it disappeared and converted to a new signal with g = 2.0039 and AW = 4 G. The concentration variation experiments were done using MgO activated at 500 or 600 O C . At such activation temperatures the VI sharp signal might be hidden in the blank samples due to the poor signal to noise ratio obtained (see Figure IA). In our case, even though the g values are comparable, a sharp signal was not observed, but oxidation of the surface by N O (or NO,) may have taken place after N O M P decomposition to t-Bu and N O (or NOz) in a manner similar to the behavior of VI centers in the presence of O2 Therefore, the observed signal may be due to these oxidized surface sites. A signal similar to ours (at low NOMP concentration) has been reported for lithium-doped MgO at 500 OC that was attributed to a Fe3+ impurity: which did not allow the observation of signals with g values around 2.001-2.005 since it was a very strong and (18) Lunsford, J. H.J . Chem. Phys. 1967, 46, 4347. (19) Boudart, M.; Delbouille, A.; Devouane, E. G.; Indovina, V.;Walters, V. J . Am. Chem. SOC.1972, 94, 6622.
Figure 5. ESR of NOMP adsorbed on thermally activated MgO (300 "C)at room temperature. broad band. At those g values a very broad signal, different from that observed at low N O M P concentration, can be observed in the blank samples at higher activation temperatures, (900-1 100 "C, see Figure IC). As stated before, due to the poor signal to noise ratio of the blank samples at 500 or 600, it is difficult to establish the presence of any signal in that position. Therefore, if we assume that the broad signal observed at higher activation temperature is that of Fe3+impurity, then the one we observe with low N O M P concentration should not be attributed to it. For these reasons we are inclined to attribute the signal observed in Figure 4A to oxidized VI surface sites (by N O or NOz). At intermediate initial N O M P concentration (2.0 and 3.0 mg of NOMP dimer/100 mg of MgO) the spectrum obtained is shown in Figure 4B. The superposition of the signals corresponding to the species 111, IV, and V discussed previously is observed. This coverage range of NOMP is sufficient to form the various trapped radicals mentioned before, each differing in nature. The spectra obtained are typical of slow-moving species which have wellseparated outer hyperfine extremes with overlapped central regions. Such spectra are characteristic of strongly adsorbed species. The high initial NOMP concentration range (4 mg of N O M P dimer/100 mg of MgO and over) allowed the tert-butyl radical to form and be trapped by NOMP to yield V. These di-tert-butyl nitroxy radicals predominate over this range of N O M P coverage. The spectra are characteristic of highly mobile species, which indicates weak adsorption. Variation in Activation Temperature. Since the NOMP system promised to be of some use in comparing different types of surface sites generated on activated MgO, we next carried out a study where MgO activation temperature was varied. In all cases N O M P was added at room temperature. Activation a t 200, 300,and 400 O C . These lower activation temperatures resulted in a certain characteristic MgO that interacted at 25 OC (room temperature) with N O M P to yield predominately VI11 (AN= 15 G). Apparently intermediates VI and VII, if formed, rapidly converted to VIII. Furthermore, the spectra indicate that VI11 is not strongly bound and exhibits an isotropic behavior (cf. Figure 5). Since thermal activation at 200-400 OC does not remove much of the adsorbed H 2 0 nor chemically remove O H groups, it is likely that many active sites continue to be occupied by H 2 0 / O H , and therefore strong adsorption of VI11 may be inhibited. However, it also indicates that many surface sites are capable of decomposing NOMP, perhaps even those bearing O H groups. Activation a t 500 and 600 O C . Very often 500 or 600 OC are optimum temperatures for thermal activation of MgO. Electron-transfer processes as well as catalytic R H + D2 RD H D processes are most effective over MgO activated at these temperature^.^-^-^^ It is apparent that a variety of active sites
-
+
(20) Hoq, M. F.; Klabunde, K. J. J . Am. Chem. SOC.1986, 108, 2114.
2524
Klabunde and Nieves
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
TABLE I activation
Figure 6. ESR of NOMP adsorbed on thermally activated MgO (700 " C ) at room temperature.
50 G
Figure 7. ESR of NOMP adsorbed on thermally activated MgO (1 100 " C ) at room temperature.
are exposed by this treatment due to more complete removal of H 2 0 and OH groups. With these activation temperatures structures VI-VI11 were observed as discussed previously (Figure 3). The apparent longer life of these species on these MgO samples may reflect a stronger adsorption mode; therefore, the spectral line shape suggests more anisotropic character, which is an indication of slow tumbling species on this surface environment. Activation a t 700, 800, 900, 1000 "C. Higher temperature activtion caused a change in the spectrum from anisotropic described previously to the one shown in Figure 6, which indicates more weakly adsorbed VI11 species characterized by a more isotropic spectrum. The AN value for VI11 is 15.5 G. Activation a t 1100 "C. Figure 7 shows a spectrum which also is due to VIII. However, a strong singlet at g = 1.9797 appeared for the first time. This band was also present in the blank MgO sample activated at 1100 "C (Figure 1C) and it was a very weak signal at 800 "C which became more intense as the activation temperature was increased. We noted that the MgO sintered badly at this activation temperature, which was not the case at other temperatures. It is known that 1100 "C is a special temperature, and MgO possesses activity as a hydrogenation catalyst when activated in this way.21 Thus, there must be a unique site developed and this ESR band may be an important clue as to what it is. However, note that this band is unaffected by NOMP, and this indicates that it is probably a chemically unreactive site. It is possible this band is due to an impurity metal ion, probably Cr3+ whose signal is enhanced in some way by the very high temperature treatment. Such temperature-dependent behavior has been reported previously for [Li+O-] centers.6 Concentration of NOMP Radical Species. Upon addition of larger and larger amounts of NOMP to MgO samples, the amount of VI11 formed progressively increased and finally remained constant even in the presence of a large excess of NOMP. We have measured the number of spins/gram and have calculated the surface coverage as follows: Surface area MgO = 140 m2/g;7,22 area occupied by one surface MgO moiety= 8.86 A2;ratio of total (21) Tanaka, Y.; Hattori, H.; Tanabe, K. J . Am. Chem. SOC.1976, 98, 4652. (22) Klabunde, K. J.; Kaba, R.; Morris, R. M. Inorg. Chem. 1978, 17, 2684.
temp, "C
spins
200 400 500
27 x 1019 5.5 x 1019
0.3
x
MgO moieties/spin
surface MgO moieties/spin
5000 55 270
530 6 30
1019
MgO/surface MgO moieties = 9.4; area occupied by one molecule of VI11 assuming it is spinning on the N - 0 axis = 50 A2, (R = t-Bu). Measurement of the number of spins of VIII/g of MgO for several samples showed the values listed in Table I. Accordingly, a monolayer of VI11 would require about 5-6 surface MgO moieties. Thus, very high concentrations of VI11 are generated, especially by 400 and 500 "C activation temperatures (close to monolayer coverages). It is indeed interesting that the intermediate activation temperature, 400 "C, yielded the highest concentration of surface spins. A possible rationale is that a partially dehydroxylated surface provides increased activity for NOMP decomposition. For example, if OH groups existed next to exposed (dehydroxylated) cations, a very reactive site may be available that could lead initially to HO-NO and tert-butyl radical (which would be rapidly trapped by another NOMP molecule) .23 Summary and Conclusions. Spin traps react vigorously at room temperature with thermally activated MgO. High concentrations of trapped, radical species are formed and absorb on the MgO surface. Three reaction modes have been identified: (1) attack of surface Mg-oxy radicals on the spin trap, (2) electron transfer to the spin trap, and (3) decomposition of the spin trap via reactions (1) and (2) to yield an alkyl radical that is subsequently trapped. Under some activation conditions, surface OH groups also appear to be important. Variation in thermal activation temperature has a profound effect on the extent and mode of interaction of MgO with the spin trap. Our observations are that at lower activation temperatures process (3) is dominant. Indeed, 400 "C activation yielded a surface that was covered with a monolayer of VIII, a very high concentration. At 400 "C partial dehydroxylation is likely, and very reactive sites result possibly due to the presence of exposed cations and neighboring O H groups: H
H
O-Mg2+& I Mg2+02-Mg2+
400 ' C
-H'
H O-Mg2'bMg2+02-M$+
-nZ 100 * C o r higher
1
N-0-H
0- M+ ;O-
0- M t t b
Mg2+02-Mg2+
Mg2+O2-M;+
+ (CH3 )&'
At higher activation temperatures the concentration of OH would diminish, and Mg-oxy radical species (Mg2+-0-) may become more important. This idea is supported by the observation that at lower activation temperatures only VI11 is observed and is a rapidly tumbling species and exhibits an isotropic spectrum with AN = 15.9 G. With activation temperatures of 500 and 600 "C, VI, VII, and VI11 are observable, and VI11 appears to be anisotropic compared to low (200-400 "C) and high (700-1000 "C) range of temperatures studied, showing an AN value of 19.0 G. Stronger adsorption of VI11 at these two temperatures may be an explanation. Higher activation temperatures (700,800,900, 1000 "C) allowed observations of strongly adsorbed VI1 and VI11 species shown by more anisotropic spectra compared to lower activation temperatures (200, 300,400 "C). Similar results were observed at the highest activation temperatures (1 100 "C). Sintering of MgO also occurred at this temperature and a new (23) We are indebted to a reviewer for this suggestion.
Interaction of Activated MgO Surfaces with Spin Traps A
(1)
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2525
E
(1)
Figure 8. Reactor used for MgO activation (A) and further addition of dissolved P N P in toluene and (B) for further addition of solid NOMP dimer.
ESR absorption was observed in the blank MgO, probably due to an enhanced impurity metal ion.
Experimental Section Materials. Magnesium hydroxide was purchased from ROC/RIC (99.99+% pure). The procedures for washing and drying have been previously described.16 N-a-Diphenylnitrone was purchased from Alfa and 2-methyl-2-nitrosopropane was purchased from Aldrich. Both were used without further purification. The spin standard, DPPH, was from Aldrich Chemical Co. Toluene used to dissolve N-a-diphenylnitrone was dried over calcium hydride and freeze-thaw degassed prior to use. Thermal Activation. One hundred milligrams of Mg(OH)2 was added to a U-shaped 8-mm-diameter quartz reactor (I, see Figure 8), connected to two high-vacuum stopcocks (4 mm). The reactor was maintained under constant nitrogen gas flow (300 mL/min) for a 3-h activation period at the desired activation temperature (200, 300,400, 500, 600, 700, 800,900, 1000, 1100 "C). After activation was over the system was evacuated and cooled to room temperature. Addition of N-a-Diphenylnitrone (PNP). In a separate chamber (chamber 11, see Figure 8A) connected to the quartz reactor and to a high-vacuum stopcock, 5% (by weight) of P N P was dissolved in 3 mL of toluene (that was previously freezepumpthaw degassed). The solution was then covered (to avoid interaction with light) and held under vacuum until the activation period was over; it was then transferred to the U-shaped quartz reactor, allowed to pour onto the oxide, and left to equilibrate for 5 min. The excess of solution was decanted to chamber 11. It was cooled with liquid nitrogen and the solvent was cryogenically pumped back to this chamber. After the solvent removal, the treated catalyst was transferred to the ESR tube (111, Figure SA), which was sealed off with a flame and spectrally analyzed. Addition of a 2-Methyl-2-nitrosopropane (NOMP). Thermal activation was done as described in the previous section. Previous to thermal activation the solid dimer of the spin trap 2-methyl2-nitrosopropane (NOMP, 1-1 6 mg for the concentration variation experiments and 7-8 mg for the activation temperature variation experiments) was added under N,(g) to an additional chamber (IV, see Figure 8B) inserting glass wool at the middle of the tube (V). During the 3 h, the chamber was covered with aluminum foil to avoid any light, since N O M P is sensitive to photolytic decomposition. In addition N O M P is kept cold (0 "C) during
-1
(3)
Figure 9. Apparatus used for the preparation of spin trap anion radical in THF.
this period since it has a high vapor pressure. After the activation period was over, N2(g) was evacuated and MgO was cooled to room temperature. About 50 km pressure was observed due to NOMP. The catalyst was transferred to chamber IV and placed on the glass wool (at V). The reactor was flame separated from chamber IV. The system was completely evacuated through the stopcock (VI) connected to the ESR tube (VII). N O M P was then heated at 5 5 "C for 2 h avoiding light during this period. Then the excess NOMP was evacuated and the MgO transferred to the ESR tube, sealed off with flame, and covered from light until it was spectrally analyzed. Anion Radical Preparation. A known amount (3 mg/lO.O mL) of spin trap (PNP or NOMP) and a small preweighed piece of K metal were added to chamber 2 (see Figure 9). Exactly 10.0 mL of T H F (previously dried under Na/benzophenone mixture) were added to chamber 1. The system was placed under vacuum and after the solvent was freeze-pump-thaw degassed it was cryogenically distilled from (1) to (2). Bulb 1 was separated from bulb 2 with a flame and the PNP solution in the presence of K was stirred, analyzing ESR samples (3) at different stages. Electron Spin Resonance Studies. All ESR samples were analyzed by using the X-band of an IBM-Bruker 200D spectrometer. The spin standard 2,2-diphenyl-l-picrylhydrazyl (DPPH; g = 2.0036) was used to determine the number of spins/gram of unknown samples by area comparison and the g values of the samples where Cr3+impurity was not observable. This Cr3+ impurity was used as an internal standard (g = 1.9797) in the cases where it has a strong observable signal.
Acknowledgment. The support of the Army Research Office is acknowledged with gratitude. Partial support of the National Science Foundation is also appreciated. Registry No. PNP, 1137-96-8; N O M P , 917-95-3; MgO, 1309-48-4.
