The Journal of Physical Chemistry, Vol. 82, No. 15, 1978 1731
Photoionization and Radiolysis of Fluorobromomethanes
TABLE 111: Effects of Repulsive Imaging on the Capacitance and Repulsive Pressure for Reduced Surface Potentials e$ L / k T = e @R / k T = 1 [salt],
M
d,A
crLim/crL
Pim/P
s
m
10-3
50 100 200 300
0.882
20
100
0.932 0.959 0.963
1.00 1.00 1.00 1.00
50
1.02
15
200 300
0.913 .0.939 0.945 0.945
1.02 1.02 1.02
50 75 100
0.938 0.939 0.939
1.03 1.03 1.03
10-2
100
10-1
60 60 40
80 60 60
60 10
80 80 80
M 1:l salt solution ( K - ~ = 30 A) and for a 10-1 M solution (u-l = 10 A) we find
lo-' switch in P switch in u L
M 276 A 297 A
lo-'
M 92 A 99 A
The anomalous results seen in Table I1 occur for separations near those shown above. A delicate "flip" in P and uL can occur at large separations. The flip occurs at different values of d when imaging is included. The sign on uLirn/uL will be negative if uL has flipped and uLimhas not (or vice versa). Table I1 shows no peculiar results for a M solution because the table does not include large enough values of d.
Table I11 describes the influence of repulsive imaging, such as occurs if the "electrode" is replaced by a material of dielectric constant EH less than the dielectric constant of water ( E H < e). In the limit eH 1.5), and 1197 cm-l ( A = 0.99). Irradiation of this sample with the full light of an AH-4 mercury arc lamp for 45 min produced new absorptions at 1099 ( A = 0.15)
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978 1735
Photoionization and Radiolysis of Fluorobromomethanes
TABLE 111: Product Absorptions (em-' ) and Intensities (Absorbance Units) in Different Experiments with Ar/CF,Br, = 400/1 Samples, and after Filtered Hg Arc Photolysis with Several Wavelength (nm) Regions argon discharge photoionization 500-1000 290-1000 220-1000 H+beam assignmenta initial absorption 0.11 CF,Br,+ 0.00 0.07 0.00 406 0.07 0.08 CF,Br,+ 428 0.05 0.05 0.00 0.00 514 0.18 0.01 0.00 0.00 0.29 P+)X 554 0.21 0.16 0.00 0.00 0.00 (CFzBr, - 1 611 68 5 182 868-873 988 1001-1004 1099 1103 1121 1168 1180 1188 1197 1219 1222 1234 1244 1309 1330 1364 1481
a
0.11 0.05 0.04 0.09 0.13 0.12 >1.3 0.56 0.09 0.06 0.05 0.83 0.26 0.58 0.12 0.39 0.27 0.04 0.07 0.39 0.21
Parentheses denote tentative assignment.
0.08 0.05 0.03
0.00
0.00
0.05 0.03
0.05 0.02
0.10
0.00
0.00
0.13
0.12
0.08
0.10 0.00
0.00 0.00
0.00 0.00
0.56 0.13 0.01 0.05 0.04 0.21
0.47 0.12
0.42 0.14
0.00 0.12 0.02 0.27 0.04 0.39 0.21
0.00
0.00
0.04
0.05
0.00
0.00
0.22
0.25 0.00 0.09 0.00 0.08b 0.03 0.06 0.20 0.09
0.00 0.10 0.00 0.08b 0.04 0.06 0.27 0.13
?
0.04 0.17 0.06 0.24 0.12
CF,Br CFBr, CF,Br,+ CFBr,+ (CF,Br*-) (CF,Br-) CF, C, F, Br,
0.00 0.85 0.13 0.25 0.14 0.20 >1.3 0.72 0.35
?
C, F4 P+)X CF,Br (CF,Br-) CF, CF,Br,+ (P'F
0.18 0.62 0.55 0.05 0.26 0.21 0.09
CFBr, ' CZF, CF,Br+ CF,Br+
Small residual parent band at 1243 an-'.
Y"
'
t
D'
P-
Y
CF2Br2
CF,Br
Figure 3. Infrared spectra of the photoionizatlon and radiolysis products of CF,Br trapped in solid argon at 15 K. Trace a was recorded from an Ar/CF,Br = 400/1 sample exposed to argon resonance light from a 2-mm orifice discharge tube during deposition for 18 h: the inset scans a' and a" were recorded after high-pressure Hg arc photolysis for 30 min with 290-1000- and 220-1000-nm filters, respectively. Trace b was
recorded from a similar sample deposited for 12 h with simultaneous proton radiolysis: scan b' followed photolysis by 220-1000-nm BH-6 light for 60 min.
and 1219 cm-l ( A = 0.10), and reduced the CF2absorptions and 685-, 1137-, and 1197-cm-l bands by 12%. Exposure to 290-1000-nm BH-6 light for 30 min destroyed the 1099and 1219-cm-l bands, reduced the CF2absorptions slightly, and left the 685-, 1137-, and 1197-cm-l bands unchanged. CF3Br. Six argon discharge studies were done with Ar/CFsBr = 400/1 samples. The spectrum from a 2-mrn orifice tube experiment is shown in Figure 3a. New bands were observed at 1666.5,1484-1481,1368-1364,1293,1255, 918-914,662, and 469 cm-l. The sample was subjected to
30-min periods of 500-1000-, 290-1000-, and 220-1000-nm BH-6 photolysis. The first irradiation did not affect the spectrum; however, the two ultraviolet irradiations changed the above bands as shown in the traces displaced above the original scan in Figure 3a. The bands labeled P+ and P- were destroyed, the 1484-1481- and 1368-1364-cm-l bands were decreased by 45%) and the 1666.5-cm-l band was reduced by 25%. The data from an open tube argon photoionization experiment with an Ar/CF3Br = 400/1 sample are listed
1736
The Journal of Physical Chemistry, Vol. 82, No. 75, 1978
F. T. Prochaska and L. Andrews
TABLE IV: Product Absorptions (cm-' ) and Intensities (Absorbance Units) for Open Tube Argon Resonance Photoionization and Radiolysis of Ar/CF,Br = 4 0 0 / 1 Samples, and after Filtered Hg Arc Photolysis with Several Wavelength (nm) Regions argon discharge photoionization
H+beam
absorption
initial
340-1000
290-1000
220-1000
469 631 662 685 692 712 819 914 918 948 959 1103 1137 1145 1222 1252 1255 1274 1293 1369 1398 1484 1517 1666.5
0.56 0.06 0.20 0.16 0.11 0.08
0.33 0.06 0.20 0.16 0.08 0.07 0.15 0.34 0.48 0.04 0.09 0.25 0.70
0.02 0.07
0.00
0.00 0.11 0.00
0.16
0.16
0.46 0.12 0.02 0.21
0.00
0.00
0.00
0.025 0.22 0.00 0.02 0.04
0.025 0.25 0.00
0.03 0.16 0.02 0.04 0.09
0.10 0.32 0.38 0.04 0.11 0.28
0.70
0.20
0.10 0.9 0.9 0.56 0.50 0.58 0.07 0.25 0.03 0.17
0.07 0.9 0.6 0.64 0.32 0.62 0.06 0.26 0.015 0.17
0.00 0.04
0.00
0.00
0.22 0.70 0.22 0.07 0.9
0.18 0.70 0.26 0.06 0.9
0.00
0.00
0.00
0.35
0.21 0.60 0.28 0.12 0.82 0.70 >1.3 0.39 0.24
0.00
0.00
0.00
0.22 0.00 0.15
0.12
0.06
0.00 0.11
0.00
0.78
1.3
0.00
000
0.52
0.12
assignmen tu CF,Br+ CF, (CF,Br- ) CF,Br (anion ?) (anion ?) CF,Br, (CF,Br-) (CF,Br-) c, F y Br, (anion ?) CF, CF,Br CF,Br, CF, CF, CF,Br+ CF, CF,Br+ CF,Br+ (cF,B~+)~ CFzBrt (CF,Br + ) b CF,'
Position and relative intensities of these small bands in several experiments a Parentheses denote tentative assignment. suggest assignment t o perturbed CF,Br+ trapped near an anionic species, and hence readily photodestroyed.
in Table IV. Compared to the 2-mm orifice tube experiment of Figure 3a, all band intensities were increased in the open tube experiment by approximately one-third, except the 918-914- and 662-cm-' absorptions which were reduced by one-quarter. This sample was photolyzed by 340-1000-nm BH-6 light for 30 min; the 1293-, 1255, and 469-cm-l P+ bands decreased by 40%, while the 14841481-, 19%1364-, 918-914-, and 662-cm-' bands increased slightly in intensity. As seen in Table IV, the 290-1000and 220-1000-nm ultraviolet irradiations of this sample had the same effect as that discussed for the sample of Figure 3a. Another argon discharge experiment was done with CF3Br using a LiF filter in front of the open discharge tube. All of the major product bands were observed with reduced intensities; the absorbances of the product bands, measured after 8 and 18 h of photoionization through the LiF window, and then after an additional 4-h discharge and deposition without the LiF filter, are listed in Table V. Note the observation of the 1666.5-, 1484-, 1368-, and 1293-cm-l bands after 18 h of photoionization through the LiF window, and the large yields of the 914-918- and 662-cm-l absorptions produced during this period. After an additional 4 h of photoionization without the LiF filter, all the major product bands grew significantly, except for the 914-918- and 662-cm-' absorptions. One open tube argon photoionization experiment was performed with an Ar/CF3Br = 1000/1 sample; this spectrum is shown in Figure 4b. The same product bands were observed with intensities reduced 50-90 % relative to the 40011 open tube study of Table IV. This concentration study suggests all new product bands are single carbon species. A (3-13 experiment was performed with an Ar/CF3Br = 400/1 sample of 90% C-13 enriched CF3Br using the open discharge tube, and the spectrum is shown in Figure 4a. The C-13 isotopic shifts can be clearly seen by comparison with the C-12 material in the same spectrum, and with the natural isotopic spectrum of Figure 4b. The
TABLE V: Product Absorptions (cm-l) and Intensities (Absorbance Units) in an Open Tube Discharge Experiment with an Ar/CF,Br = 4 0 0 / 1 Sample, after 8 and 1 8 h of Argon Resonance Photoionization through a 1-mm Thick LiF Window, and after an Additional 4 h of Photoionization without the LiF Filter
absorption 469 662 685 692 712 914 918 959 1137 1252 1255 1274 1293 1369 1484 1666.5 a
after after after 4 h more 8h 18 h (without (with LiF) (with LiF) LiF) 0.015 0.20
0.00 0.04 0.02 0.18 0.16 0.035 0.04 0.27 0.03
0.01
0.015 0.03 0.005 0.018
0.03 0.28 0.015 0.06 0.03 0.23 0.24 0.045 0.05 0.45
0.04
0.025 0.02 0.05 0.015 0.025
0.14 0.29 0.055 0.055 0.025 0.22 0.25 0.04 0.39 0.85 0.26 0.57 0.13 0.21 0.055 0.080
assignmenta CF,Brt (CF,Br-) CF,Br (anion ?) (anion ?) (CF,Br- ) (CF,Br-) (anion ?) CF,Br CF, CF,Br+ CF, CF,Br+ CF,Br+ CF,Br+ CF, +
Parentheses denote tentative assignment.
I3CF3+,I3CF2Br+,and P+ absorptions in the 1200-1700cm-I region exhibited large C-13 shifts: the new (2-13 absorptions and intensities are listed in Table VI. Note the clear resolution of pure carbon isotopic components without an intermediate feature, which indicates that single carbon species were produced and trapped. The 13CF3Brsample was photolyzed with 340-1000-, 290-1000-, and 220-1000-nm light, and the spectrum following the final photolysis is shown in the inset trace in Figure 4a. Table VI lists the product band absorbances after each of the filtered photolysis periods. A proton radiolysis experiment was done with an Ar/ CF,Br = 400/ 1sample; the spectrum is shown in Figure
The Journal of Physical Chemistry, Vol. 82,No. 15, 1978 1737
Photoionization and Radiolysis of Fluorobromomethanes
220 - loo0 nrn
"CF;
(a) 13CF3Br
"C5*
H P
''Cp*
(b)
"CF,Br
I
,
1660
I
1 1600
ICI
I
"1500
I
I 1440
WAVENUMBERS (ern-') Figure 4. Infrared spectra of the argon resonance photoionization products of 90% '%-enriched CF,Br in solid argon at 15 K using the 10-mm open tube. Trace a was for Ar/CF,Br = 400/1, 90% %-enriched CF,Br, inset scan follows photolysis sequence ending with 30-min 220-1000-nm photolysis. Trace b shows a Ar/CF3Br = 1000/1 sample for comparison.
TABLE VI: Product Absorptions (cm-') and Intensities (Absorbance Units) for Open Tube Argon Resonance Photoionization of a 90% "C-Enriched 13CF,Br Sample. Ar/CF,Br = 400/1. and after Filtered Photolysis absorption
initial
340-1000
290-1 000
220-1000
468 629 652-650 662 672 675 685 695 792 903 918 953 1026 1109 1136 1191 1219 1222 1233 1252 1259 1274 1293 1332 1361 1369 1444 1470 1485 1600.5 1643 1666.5
0.66 0.07 0.40 0.05 0.10 0.11 0.025 0.15 0.11 0.45 0.05 0.16 a05 1.5 0.18 0.10 0.75 1.5 0.95 0.13 0.64 0.16 0.05 0.58 0.05 0.07 0.23 0.025 0.03 0.20 0.05 0.02
0.53 0.07 0.43 0.06 0.12 0.10 0.025 0.14 0.18 0.49 0.06 0.14 0.03 1.5 0.20 0.09 0.75 1.25 1.20 0.13 0.59 0.21 0.04 0.67 0.04 0.07 0.26 0.02 0.02 0.23 0.05 0.02
0.13 0.08 0.12 0.02 0.11 0.00 0.025 0.11 0.23 0.17 0.02 0.02 0.00 1.5 0.19 0.08 0.75 0.20 1.30 0.13 0.26 0.25 0.01 0.58 0.00 0.07 0.21 0.00 0.02 0.18 0.035 0.02
0.00 0.10 0.00 0.00 0.12 0.00 0.025 0.11 0.29 0.00 0.00 0.02 0.00 1.5 0.18 0.10 0.75 0.00 2 0.13 0.17 0.28 0.00 0.34 0.00 0.04 0.11 0.00 0.01 0.14 0.03 0.015
Parentheses denote tentative identifications.
I,
assignmen tu 13CF3Br+
WF, ("CF,Br') (laCF3Br-) I3CF,Br ?
WF,Br ? t iesidual P l3CXF, Br, ?
(13CF3Br-) (I2CF3Br-) ? ?
WF,Br Y!F,Br I3CF, 13CF, I3CF,Br
+P +
WF, WF,
13CF3Br++ WF,
'TF,
WF,Br+ 13CF,Br+ (13CF2Br+)b 'TF,Br+ 13CF,Br+ ( 13CF2Br+)b 'WF,Br+ 13CF,+ 13CF3+ 12CF,+
Same as Table IV.
3b, and the product band absorbances are listed in Table IV. The radiolysis technique again produced increased yields of radicals and stable products. As seen in Figure 3b', the 220-1000-nm output of the BH-6 lamp destroyed the P+ and P- absorptions, reduced the 1484- and 1368cm-l bands 25%, and decreased the 1666.5-cm-l absorption by 15%. Deposition of an Ar/CF,Br = 400/1 sample with Na atoms for 20 h produced a weak NaBr band at 278 cm-l,
CF, radical14 at 1251 cm-l ( A = 1.5), and new features at 668 ( A = 0.15), 943 ( A = 0.231, and 1130 cm-l ( A = 0.25). Medium pressure mercury arc photolysis for 35 min reduced these bands to A = 0.10,0.15, and 0.10, respectively, and produced intense new bands at 918 ( A = 0.33) and 662 cm-' ( A = 0.26). A 30-min photolysis with 290-1000-nm BH-6 light reduced these new bands to A = 0.12 and 0.09, respectively, and further decreased the 668-, 943-, and 1130-cm-l features. A final 20-min exposure to 220-
1738
The Journal of Physical Chemistry, Vol. 82,No. 15, 1978
1000-nm BH-6 light destroyed these bands except for the one at 943 cm-l (A = 0.06). An identical sodium experiment was performed with C-13 enriched CF3Br. In addition to bands due to NaBr and 13CF3,new absorptions were observed at 669 ( A = 0.231, 925 (A = 0.14), and 1101 cm-l ( A = 0.13). Medium pressure mercury arc photolysis for 45 min reduced these bands to A = 0.13,0.07, and 0.06, respectively, and produced intense new absorptions at 652 (A = 0.28) and 903 cm-l ( A = 0.30). A 30-min photolysis with 290-1000-nm BH-6 light reduced these two new bands to A = 0.25 and 0.27, respectively, and further decreased the 669-, 925-, and 1101-cm-l features to A = O.09,0.05, and 0.05, respectively.
Discussion The new product absorptions were separated into four distinct groups according to mercury arc photolysis behavior and appearance in sodium-fluorobromomethane experiments. Absorptions produced by sodium reactions which were not affected by photolysis are due to free radicals: new absorptions which were reduced by highintensity 220-1000-nm radiation will be assigned to the daughter cations; bands produced in large yield in the radiolysis experiments and destroyed by photolysis wavelengths longer than 290 nm will be attributed to parent cations; and absorptions generated in smaller yield by the proton beam, destroyed by wavelengths longer than 290 nm, or produced upon photolysis of sodium-fluorobromomethane samples will be assigned to molecular anions. Free Radicals. The CF,, CC13,CBr,, CF2C1,and CFClz free radicals have been observed in previous photoionization and proton beam radiolysis s t u d i e ~ ; ~ -these ~ J ~ Jfree ~ radicals were unaffected by intense high-pressure mercury arc photolysis. The present reaction of Na atoms with CFBr, produced NaBr and new bands at 1136 and 782 cm-'; these new bands are assigned here to the C-F and antisymmetric C-Br stretching vibrations, respectively, of the CFBr2 free radical. The weaker 1127-cm-l feature observed in Na reactions with CFBr3 is believed to be a matrix site effect on the 1136-cm-' band. Reaction of Li or Na atoms with CFzBrzyielded LiBr or NaBr, CFz, and new bands a t 1197,1137, and 685 cm-l; these new bands are assigned here to the two C-F stretching modes, and C-Br stretching vibration, respectively, of CFzBr. Comparison of the positions and relative intensities of the two C-F stretching modes of this radical with those of CF2Cl (for which C-13 shifts have been measured5), suggests the assignment of the lower, stronger absorption a t 1137 cm-l to the antisymmetric stretching mode, v5, of CF2Br,and the assignment of the higher band at 1197 cm-l to the symmetric stretching mode, vl. In the 13CF3Br photoionization experiment, the antisymmetric C-F stretching frequency of l3CFZBrwas observed to shift to 1109 cm-l, while the C-Br stretching frequency shifted to 672 cm-'. The symmetric C-F stretching mode of 13CFzBr could not be observed due to interference by parent 13CF3Brat 1166 cm-l. The CFBrz and CF2Br free radicals gave strong infrared absorptions in the present photoionization and radiolysis experiments, and these absorptions did not change in intensity upon high-pressure mercury arc photolysis. Daughter Cations. The CC13+cation absorbing at 1037 cm-l, the CFClZ+species at 1353 and 1142 cm-', and the CF2C1+ion at 1515 and 1415 cm-l have been discussed previously;z-6p8the fundamental stretching frequencies of each are substantially higher than those of the corresponding neutral radical, and the absorptions of each cation were found to decrease 35-6070 upon ultraviolet
F. T. Prochaska and L. Andrews
BH-6 mercury arc photolysis for periods of 1-6 Intense absorptions a t 1311 and 991 cm-l in the present CFBr, experiments exhibited similar photolysis behavior; they were reduced by approximately 25% upon exposure to 290-1000-nm and then 220-1000-nm BH-6 light for a total period of 1h. These bands are assigned here to the daughter cation CFBr2+. The 1311-cm-l absorption, which lies above the 1136-cm-l band of neutral CFBr2,is assigned to the C-F stretching mode of CFBr2+; the weaker 1322-cm-l feature is believed to be a matrix site effect, as similar splittings were observed for the C--F stretching mode of CFClZ+at 1353 and 1363 cm-Ia4 The 991-cm-l absorption, appearing above the 782-cm-l band of the radical, is assigned to the antisymmetric C-Br stretching mode of CFBrz+. In the CFzBr2 photoionization and radiolysis experiments (Figure 2), CFBr2+was produced, absorbing a t 1309 and 988 cm-'; the 2-3 wavenumber displacement of the bands to lower frequencies in these experiments is probably due to a minor difference in matrix environments. The ion CFBrz+ has thus been produced from two different precursors, CFRr, and CFzBr2. A previous paper described the evidence supporting photoionization as the mechanism for production of new species in these discharge experiment^.^ Argon resonance lines at 1048 and 1067 A (11.83 and 11.62 eV) comprise the major discharge radiation, with approximately 10% of the light a t higher energy (13-15-eV range).17 The ionization potential of CFBr, has been reported as 10.67 eV;ls by analogy with results found in photoionization studies on CFC13,19yz0 the CFBr3+parent cation is probably of limited stability, and the appearance potential of CFBr2+from CFBr, is at this energy or slightly higher. The suggested mechanism for formation of CFBr2+in the present CFBr, discharge experiments is reaction 1,using argon resonance radiation. CFBr, hv (11.6 eV) CFBr2+ + Br e- (1) In CF2Br2experiments, CFBr2+may be formed from direct photoionization of CF2Brzor, more likely, by photoionization of the CFBr2 radical. The strong site-split absorptions centered at 1364 and 1481 cm-l, with splittings a t 1369 and 1484 cm-l, in the CF2Br2experiments, and the 1369- and 1484-cm'l absorptions in the CF3Br studies, exhibited a 50% reduction when subjected to 290-1000-nm and then 220-1000-nm BH-6 radiation for successive 30-min periods; these bands appear above the two C-Fz modes of neutral CF2Brat 1197 and 1137 cm-l. The 1369- and 1484-cm-l absorptions are assigned here to isolated CFzBr'; these bands have been observed in CHFzBr photoionization studiesz1 which produce CFzBr radical that can be photoionized. The slightly displaced bands centered at 1364 and 1481 cm-l in CF2Br2experiments are assigned to CFzBr+with a Br atom in the same matrix cage but not bonded to the carbon cation center. In photoionization studies on the CF2ClBr molecule,22these same two bands were observed slightly shifted to 1360 and 1477 cm-', which are attributed to CFzBr+perturbed by a nearby C1 atom not bonded to the carbon cation center, The cation CF2Br+has thus been produced from four different precursors, and Figure 5a-d compares the infrared spectrum of the photoionization products from these four precursors in the region 1300-1500 cm-l. Broad absorptions centered at 1412 and 1512 cm-l in the CF2ClBr experiment (Figure 5d) are attributed to a perturbed CF2C1+species. Photolysis of this sample increased the CFzBrf absorptions, as shown in Figure 5e, which will be discussed below. The position and relative intensity of the two CFzBr+ absorptions suggest assignment of the weaker 1484-cm-l
+
-
+
Photoionization and Radiolysis of Fluorobromomethanes
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978 1739
resonance radiation transmitted by LiF, and its subsequent photoionization by the same light. Since the appearance potential of CF2Br+from CF3Br is probably 14-15 eV,20 its direct production would require higher energy light than can be transmitted by the LiF filter. Thus, the sharp bands in CF3Br and CHF2Br experiments are assigned to the isolated CF2Br+cation. The appearance potential of CF2Br+from CF2Br2has not been reported; however, it is probably less than the 12.1-eV appearance potential of CF2C1+from CF2C12,19'20 and CF2C1+is a major product in CF2Cl2discharge experiments. The matrix environment can red shift the absorption spectrum by approximately 1eV25making this photoionization of CF2Brzby the argon resonance lines possible, and providing a cage to retard the escape of the leaving Br atom, reaction 2. This could cause a vibrational CF2Br2 hu (11.8 eV) (CF2Br+)Br+ e(2) perturbation on the CF2Br+species and the 3-5 wavenumber shift in the observed bands for this precursor. In the CF,ClBr experiments a similar mechanism is suggested for the formation of CF2Br+,where in this case a C1 atom is trapped in the same matrix cage further shifting the vibrational frequencies of CF2Br+to 1360 and 1478 cm-l. The presence of Br atoms near CF2C1+in these experiments is indicated by the broad contour of the C F 2 W absorptions, and the 3 wavenumber shifts from the sharp 1415- and 1515-cm-' absorptions assigned to isolated CF2C1+.5 Photolysis of this CFzCIBr sample with 500-1000and 340-1000-nm BH-6 radiation produced no changes in these absorptions. However, after photolysis for 30 min with 290-1000-nm BH-6 light, as shown in Figure 5e, the 1412- and 1512-~m-~ bands were destroyed, and the 1360and 1478-cm-l bands increased from A = 0.23 to 0.48, and from A = 0.11 to 0.32, respectively. It is postulated that photolysis caused rearrangement of the (CF2C1+ Br) ensemble trapped together in one matrix cage to the (CFzBr+ C1) ensemble. Further irradiation of this sample with 220-1000-nm BH-6 light for 30 min caused the 1360- and 1478-cm-' bands to decrease to A = 0.13 and A = 0.07, respectively; this is a normal diminishment in the CF2Br+absorption intensities upon exposure to 220-nm cutoff radiation. The above photolytic behavior provides strong evidence that the molecular ions formed in these matrix experiments are frequently not completely matrix isolated, but they often have neutral atoms in the same matrix cage. The radicals CFBr, and CF2Br are probably nonplanar; ionization is expected to produce a planar species with extensive P bonding. Hence, a marked increase in C-F fundamental stretching frequencies is expected on going from the radicals to the cations. This increase is about 200 cm-' for CFBrz+,300 cm-l for CFzBr+,and 400 cm-l for CF3+. The 1666.5-cm-' band in the present CF3Br photoionization and radiolysis experiments has also been observed in photoionization studies on CF3C1, CF31, and CHF3, and has been assigned to the u3 mode of the CF3+ cation.26 The large carbon-13 shift of this band to 1600.5 cm-l, with the appearance of an additional small product band at 1643 cm-l in the carbon-13 spectrum (Figure 4a), has also been observed in photoionization studies on 13CHF3,and has been explained by Fermi resonance between v3 and the (vl + u4) combination band of the 13CF3+ species.26 Appearance potentials for the chlorofluoromethanes20 show that dissociation of CX3+to CX2+and X requires approximately 6 eV, which is beyond the mercury arc range of energy. The gradual decrease in size of daughter cation absorptions upon ultraviolet photolysis is probably due to
+
I
I , 1520
/
I 1460
/
,
I
,
,
1400
I 1340
WAVENUMBERS kin-')
Figure 5. Infrared spectra of CF,Br+ in the 1300-1500-cm-' region produced from different precursors: (a) CF,Br, (b) CHF,Br, (c) CF,Br,, (d) CF,CIBr, and (e) CF,CIBr after 30 min of 290-1000-nm photolysis.
band to the symmetric C-F2 mode of CF2Br+ and the stronger 1369-cm-l band to the antisymmetric stretch of this species, since CF2 has a similar position-intensity relationship between the two C-F, modes.14 The 90% carbon-13 enriched CF3Br study produced both carbon isotopic absorptions without an intermediate mixed component, which indicates a single carbon atom vibration; the large carbon-13 shifts are ih accord with carbonfluorine vibrations. The shift of the 1369-cm-l band to 1332 cm-l is in good agreement with the shift calculated for the antisymmetric C-F, stretch of a planar symmetrical CF2Br+species, using an estimated F-C-F angle of 107' (taken from the isoelectronic planar CFzS species23)and the antisymmetric C-F2 stretching symmetrized G matrix element. The shift of the 1484-cm-l band to 1444 cm-l exceeds that expected for the symmetric C-F2 mode, which indicates mode mixing with the unobserved C-Br stretch in CF2Br+; analogous behavior has been reported for CFOSZ3 and CF,C1+.5 The ion CF2Br+has been produced from four different precursors: CF3Br,CHF,Br, CF,Br2, and CF2C1Br (Figure 5). The first two sources gave the same major bands at 1369 and 1484 cm-l; CF2Br2gave strong site-split absorptions at 1364 and 1481 cm-l, with resolved side bands a t 1369 and 1484 cm-l; CFzCIBr yielded strong bands at 1360 and 1478 cm-l, with side bands at 1364 and 1484 cm-l, and weaker broadened CF2C1+absorptions centered at 1412 and 1512 cm-l. A common mechanism of formation of CF2Br+is indicated for the former pair of parents, which is different for the latter two sources. In the CHF2Br case, the radical CF2Br is produced by photolysis,21and it can be photoionized by the same radiationsz4The LiF filter experiment with CF3Br indicates a similar mechanism, involving first the photoproduction of CF,Br with argon
-
+
+
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The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
neutralization of the daughter cations by electrons photodetached from bromide ions, the major electron trap in these matrix samples, since the onset of this process comes just to higher energy than the electron affinity of Br a t 27 100 ~ m - ~ . ~ ~ Parent Cations. In the previous matrix photoionization study of CCl,, evidence was presented for an asymmetric carbon tetrachloride parent cation represented as (CC12-C12)+,which was destroyed by 500-1000-nm phot o l y s i ~ .Radiolysis ~ and photoionization experiments on CFC13produced four bands assigned to the CFC13+parent cation, which was produced in larger yield in the proton radiolysis and Ar-10% Xe discharge experiments and totally destroyed by exposure to 420-1000-nm BH-6 light for 30 min. Carbon-13 isotopic substitution studies confirmed the vibrational assignment^.^ In the CFBr, photoionization and radiolysis experiments, bands observed at 1167-1160, 853, 423-417, 405-399, and 316 cm-l were in much larger yield in the radiolysis and Ar-10% Xe discharge experiments (Table I). These bands were slightly reduced by exposure for 30 min to 660-1000-nm BH-6 light, and were gradually destroyed by successive 30-min exposure to 500-1000-, 290-1000-, and 220-1000-nm light (Table 11). The broad 1167-1160-~m-~ absorption lies above the C-F stretching mode of CFBr3 a t 1061 cm-l, and is here assigned to the C-F stretching mode of the parent ion CFBr3+. The 853-cm-l band, occurring above the 745-cm-l CFBr, absorption, is assigned to the antisymmetric C-Br, stretching mode of this parent cation. The 417-, 399-, and 316-cm-I bands consistently tracked with 1167-1160- and 853-cm-l absorptions in different experiments and on photolysis, and are reasonable frequencies for this parent ion; the higher bands are near the symmetric C-Br3 stretching mode of CFBr, at 398 cm-l, and the 316-cm-' absorption is just above the u5 mode of CFBr3 at 306 cm-1.28 The photoelectron spectrum of CFBr, has been observed;18 however, no mass-selected photoion study has been reported. By analogy with photoionization experiments on CFCl,, where very little CFC13+parent ion was detected in the gas phase,20 the CFBr3+ parent ion is probably of limited stability with respect to dissociation to CFBr2+and Br. As evidenced by its diminishment with 660-1000-nm BH-6 light, this species can easily photodissociate to eliminate a bromine atom, but because it is more difficult for bromine atoms to diffuse away through the matrix than for chlorine atoms, this parent ion was more resistant to destruction by mercury arc photolysis than the CFC13+parent ion., The growth of CBr3+at 875 cm-l upon ultraviolet photolysis of CFBr, samples (Figure 1, Tables I and 11)indicates that fluorine atom elimination is involved when the CFBr3+species is destroyed by high energy light. Similar evidence for fluorine atom elimination was observed upon photodissociation of CF2C12' by ultraviolet light.5 In the CF2C12 studies, bands assigned to the parent cation CF2C12+showed appropriate carbon-13 shifts, were stable to 500-1000-nm light, but were reduced significantly by 290-nm cutoff radiation, and destroyed by 220-1000-nm p h o t ~ l y s i s .In ~ the present CF2Br2experiments, a group of bands a t 1244,873468,428, and 406 cm-l were stable to 500-1000-nm light but destroyed by 290-1000-nm radiation (Table 111). Appearance of the 1244-cm-l band above the two C-F2 stretching modes of neutral CF2Br2 at 1142 and 1082 cm-l suggests assignment to CF2Br2+. The other C-F2 stretching mode of CF2Br2+is possibly obscured by these intense neutral parent absorptions. The 873-868-cm-l doublet is assigned to the antisymmetric
F. T. Prochaska and L. Andrews
C-Br2 stretch of CF2Br2+which appears at 819 cm-l for CF2Br2. The weaker 428- and 406-cm-' bands are tentatively assigned to fluorine deformation modes of CF2Br2+. The most photosensitive bands in CF2Br2studies appeared at 1234,1188, and 514 cm-l, which were destroyed by 500-1000-nm BH-6 photolysis. These bands were produced in greater yield by radiolysis (Table 111)which gives more complete precursor fragmentation. These observations suggest the tentative assignment to CF2Br2+ trapped with an adjacent reactive species such as a Br atom, which could be formed by aggregation of CF2Br2+ and Br during sample condensation. The 1234- and 1188-cm-l bands are reasonable frequencies for the two C-F2 modes of such a species, and the 514-cm-l band can be attributed to a fluorine deformation mode. Intense product bands were observed in CF3Br experiments at 1293 and 1255 cm-', above the antisymmetric and symmetric C-F3 stretching modes of the neutral parent molecule at 1200 and 1076 cm-l, respectively. These product bands shifted in the C-13 CF3Br experiment to 1259 and 1222 cm-l, respectively; the 1293-cm-l C-13 counterpart appears on top of the 1258-cm-l band of 12CF4, which accounts for the residual absorption at 1258 cm-l after photolysis. As seen in Tables IV and VI, these two product bands were greatly reduced by 340- and 290-nm cutoff light, and then totally destroyed by 220-1000-nm radiation. The photolysis behavior and their appearance above the neutral CF,Br bands suggest assignment of these new absorptions to the CF3Br+parent cation. The large carbon-13 shift of the 1293- and 1255-cm-' bands of 34 and 33 wavenumbers, respectively, correlates with the 34 wavenumber C-13 shift of the antisymmetric C-F3 parent mode as compared to the 26 wavenumber shift for the symmetric C-F3 mode. The proximity of the 1293- and 1255-cm-l bands and their (3-13 shifts preclude their assignment to the two C-F stretching modes of CF3Br+;a more likely vibrational assignment for these two bands, which tracked together to all experiments, is to split components of the v4 antisymmetric C-F stretching mode of CF3Br+. It is presumed that the parent ion retains C3" symmetry although ionization removes an electron from one of the degenerate nonbonding p orbitals on bromine, and the possibility of Jahn-Teller distortion in the antisymmetric C-F stretching mode or matrix site asymmetry could lead to splitting in the degenerate u4 mode of CF3Br+. In this regard, the 1299-cm-l antisymmetric C-F3 stretching absorption of CF3C1+ was reduced to a completely resolved 1302-1296-cm-l doublet with one-third of the original 1299-cm-' intensity upon 340-1000-nm photolysis, The symmetric C-F3 stretching mode of CF3Br+ is perhaps hidden under the intense 1200-cm-l CF3Br parent band. The strong 469-cm-l absorption consistently tracked with the 1293- and 1255-cm-l bands, and it shifted to 468 cm-I upon carbon-13 substitution. A reasonable assignment is to the F-C-F bending mode of the parent cation; the isotopic frequency shift is in accord with the shift of the F-C-F bending mode of neutral CF3Brfrom 550 to 548 cm-l upon C-13 substitution. From photoelectron spectra, ionization potentials of CFBr3, CF2Br2,and CF,Br are 10.7, 11.2, and 12.0 eV, r e s p e c t i ~ e l y .It~ is ~ ~proposed ~~ that the parent cations are formed by direct photoionization of the parent molecules by the argon resonance lines during sample condensation. In addition, the argon matrix can red shift the photoionization spectrum by about 1eV,26which places argon resonance radiation in a more intense portion of the photoion spectrum. The observation of CF3Br+ in the
Photoionization and Radiolysis of Fluorobromomethanes
CF3Br experiment using a LiF filter (Table V) confirms this method of parent cation generation. The parent cations CF,Br2f and CF3Br+were found here to photolyze with near ultraviolet light. As evidenced by the small growth in CF3+and CF2Brt absorptions upon photolysis of CF3Br" by 340-1000-nm light (Tables IV and VI), these parent cations probably photodissociate to give halogen atom elimination and the daughter ion. Appearance potentials for the chlorofluoromethaneszoindicate that halogen atom elimination from the parent cations should require energies appropriate to the ultraviolet region of the spectrum. Anions. The final category of new product species was favored in the argon photoionization experiments, and completely destroyed by 500-1000- or 290-1000-nm photolysis. In addition, sodium-fluorobromomethane matrix samples subjected to medium-pressure AH-4 mercury arc photolysis produced many of these new bands formed in argon photoionization experiments. Photolysis of sodium samples is an accepted procedure for generating matrix-isolated anions: and it thus appears that these new bands are due to anionic species. In CFC13experiments, new bands at 1056,776,486, and 440 cm-l exhibited similar photolysis behavior and were assigned to a negatively charged species; the data could not provide a definitive identification, but CFC1,- was ~uggested.~ In the present CFBr, studies, absorptions at 1148 and 644 cm-l were destroyed by 500-1000-nm BH-6 light, while bands at 1029-1019,626, and 465 cm-l were significantly diminished by 500-nm cutoff radiation, and then destroyed by 290-1000-nm BH-6 light (Table 11). Full AH-4 photolysis of a sodium-CFBr, sample produced the 1148- and 644-cm-' bands which were then destroyed by a 15-min exposure to 290-1000-nm BH-6 light. This indicates that the 1148- and 644-cm-' absorptions are due to a negatively charged species probably formed through electron capture by the CFBr, radical or CFBr, parent. The photosensitive nature of these absorptions and their appearance near the expected absorption positions for CFBr suggest the tentative CFBrBr- assignment. This species is probably a carbene-bromide complex. The 1029-1019-, 626-, and 465-cm-' bands, which were not observed in photolyzed sodium-CFBr, samples, are tentatively assigned to the CFBr3- parent anion, analogous to the CFC13- parent anion.4 The 1029-1019-~m-~ site-split doublet represents a reasonable frequency for the C-F stretching mode of CFBr3-, while the 626-cm-' band could be the antisymmetric C-Br, stretching frequency. The 465-cm-l absorption is tentatively assigned to a deformation mode of this species. The CF2C12photoionization experiments produced three new bands at 1029,626, and 564 cm-l which were destroyed by 290-1000-nm light, and generated upon AH-4 photolysis of sodium-CF2C12 samples. In the present CF,Br2 photoionization experiments, absorptions were observed at 1004-1001 and 554 cm-l; a parent band at 622-628 cm-I obscured the 625-cm-I region of the spectrum. Upon exposure to 500-1000-nm BH-6 light for 30 min, these bands diminished by 20% and 290-1000-nm radiation destroyed these bands in 30 min. Absorptions showing similar photolysis behavior were observed at 1013,622, and 564 cm-l in the CF2C1Br photoionization study.22 The observation of three analogous groups of bands from three CFzX, precursors suggests the assignment to the parent anion in each case. The bands in the area of 1000 cm-l lie in the expected region for a C-F stretching mode of this species, and the lower wavenumber bands in each case are reasonable frequencies for a F-C-F bending mode and a
The Journal of Physical Chemistty, Vol. 82, No. 15, 1978 1741
deformation mode, respectively, of the parent anion. Strong sharp bands in the CF2Clzphotoionization and radiolysis studies a t 1220 and 1101 cm-l were reduced to 25% of their original intensity by 340-1000-nm light, and then destroyed by 290-nm cutoff radiation, uncovering the weaker, very sharp bands of CF, at 1222 and 1103 cm-l. The C-13 shift of the 1220- and 1101-cm-l bands to 1191 and 1072 cm-l, respectively, confirmed their assignment to C-F2 vibrations. In the CF2Br2photoionization and radiolysis experiments, strong, sharp bands were reproducibly observed at 1219 and 1099 cm-l and were destroyed by 500-1000-nm light. Deposition of Na atoms with a sample of CF2Brzproduced large amounts of the CFzBr radical at 1197 ( A = 0.99), 1137 ( A > L3), and 685 cm-' ( A = 0.26), and also CF2 at 1222 and 1103 cm-l. After photolysis with the AH-4 lamp for 45 min, the 1219- and 1099-cm-l absorptions grew in with absorbance units of 0.10 and 0.15, respectively, while the CF2Br absorptions were reduced by 12% , and the CF2 bands were left unchanged. Irradiation of this sample with 290-1000-nm light destroyed the 1219- and 1099-cm-l bands, and only reduced the CF2 absorptions by 20%. This observation indicates that the 1219-, 1099-cm-l absorber in CF2Br2 studies is an anionic species formed by electron capture, which is different from the 1220-, 1101-cm-l absorber in CF2C12experiments. Possibilities include the parent and radical electron-capture products (CF2Br)Br-and CF2Br-. In the CF2ClBr photoionization studies, strong, sharp bands were observed at 1219 ( A = 0.42) and 1099 cm-' ( A = 0.60), which were destroyed by 420-1000-nm photolysis;22these bands are identical with those produced from CF2Br2. These data support the CF2Br- possibility. In addition the photodestruction of the 1219- and 1099-cm-l absorptions by 500-1000-nm light is indicative of a relatively photosensitive species, which favors the CF2Bridentification over (CF,Br)Br-. The 1219- and 1099-cm-' bands are tentatively assigned to CF2Br-. This species can be viewed as CF, weakly bound through the empty p orbital to a bromide anion, since the two C-F stretching modes observed here are very close in frequency to those of CF2. The 916- and 662-cm-' absorptions in CF,Br studies were strongest in argon discharge experiments, destroyed by 290-nm cutoff radiation, and generated upon AH-4 photolysis of Na-CF3Br samples; these bands thus must be assigned to an anion. Bands with similar behavior but showing small, distinct halogen shifts have been observed in discharge and Na experiments with CF&l and CF,I; the band at 916 cm-l in CF3Br studies shifts to 933 cm-l in CF3C1studies and to 892 cm-l in CFJ experiments.26 The C-13 shift observed in CF3Br experiments, 916 to 903 cm-l, and the lack of an appreciable heavy halogen effect, suggest a predominantly C-F stretching mode. The 662-cm-l band shifts to 652 cm-l upon C-13 substitution which is reasonable for a C-F, deformation mode. These absorptions are assigned to the CF3Br- radical anion which has been identified in recent ESR studies.30 The fluorobromomethane molecules and radicals undoubtedly have high electron capture cross sections, and associative electron capture by the radical or parent molecule is the likely mechanism for the formation of molecular anions in these experiments. The electrons can, of course, come from mercury arc excitation of sodium or photoionization of the precursor molecule. The method of destruction of these molecular anions upon photolysis probably involves electron detachment or halide ion elimination. It is interesting that most of the fluorobromomethane molecular anions are more photolytically
7742
The Journal of Physical Chemistry, Voi. 82, No. 15, 1978
unstable than their chlorine-substituted counterpart^.^^^ Conclusions The molecules CFBr3,CF2Br2,CF3Br,and C-13 enriched CF3Br have been studied by matrix photoionization, radiolysis, and mercury arc photolysis techniques. The infrared spectra show stable products, free radical intermediates, and parent and daughter ions. The identification of the CFBrz and CFzBr free radicals were confirmed by Li and Na metal atom reactions. At least three different charged products were identified for each precursor based on their photolysis behavior. The daughter cations CFBr2+, CFzBrf, and CF3+have substantially higher vibrational stretching frequencies than the corresponding radicals. These absorptions were decreased by 220-1000-nm light, as the daughter cations were probably neutralized by photodetached electrons in the matrix. The parent cations were totally destroyed by mercury arc irradiation; these species are expected to undergo halogen atom elimination upon ultraviolet photolysis. Medium pressure mercury arc irradiation of sodium-fluorobromomethane samples generated many of the bands attributed to molecular anions in the photoionization experiments; associative electron capture by the radical or parent molecule is the likely method by which molecular anions are formed in these experiments. Acknowledgment. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We thank Mr. D. Bielefeldt and Dr. H. Willner for preparing the 13CF3Brsample. References and Notes (1) R. 0. Allen, J. M. Grzybowski, and L. Andrews, J. Phys. Chem., 79, 898 (1975).
Tordo et al. (2) L. Andrews, J. M. Grzybowskl, and R. 0. Allen, J. Phys. Chem., 79, 904 (1975). (3) F. T. Prochaska and L. Andrews, J . Chem. Phys., 67, 1091 (1977). (4) F. T. Prochaska and L. Andrews, J . Chem. Phys., in press. (5) F. T. Prochaska and L. Andrews, J. Chem. Phys., in press. (6) M. E. Jacox and D. E. Milligan, J . Chem. Phys., 54, 3935 (1971). (7) B. S. Auk and L. Andrews, J. Chem. Phys., 63, 1411 (1975). (8) L. Andrews, C. A. Wlght, F. T. Prochaska, S.A. McDonald, and B. S.Auk, J . Mol. Spectrosc., in press. (9) M. E. Jacox and D. E. Milligan, Chem. Phys., 16, 195 (1976). (10) M. E. Jacox and D. E. Milligan, Chem. Phys., 16, 381 (1976). (11) F. T. Prochaskaand L. Andrews, Paper TH9, Symposium on Molecular Spectroscopy,Ohio State University, Columbus, Ohio, June 14, 1977. (12) L. Andrews, J. Chem. Phys., 48, 972 (1968). (13) D. W. Smith and L. Andrews, J. Chem. Phys., 55, 5295 (1971). (14) D. E. Milligan and M. E. Jacox, J. Chem. Phys., 48, 2265 (1968). (15) D. E. Milligan, M. E. Jacox, J. H. McAuley, and C. E. Smith, J . Mol. Spectrosc., 45, 377 (1973). (16) L. Andrews and T. G. Carver, J. Chem. Phys., 49, 896 (1968). (17) L. Andrews, D. E. Tevauk, and R. R. Smardzewski, Appl. Spectrosc,, 32, 157 (1978). (18) F. T. Chau and C. A. McDowell, J. Electron Spectrosc., 6,357 (1975). (19) J. M. Ajello, W. T. Huntress, Jr., and P. Rayermann, J. Chem. Phys., 64, 4746 (1976). (20) H. W. Jochims, W. Lohr, and H. Baumgartel, Ber. Bunsenges. Phys. Chem., 80, 130 (1976). (21) C. A. Wight and L. Andrews, unpublished results. (22) F. T. Prochaska and L. Andrews, unpublished results. (23) A. Haas, H. Wlllner, H. Burger, and G. Pawelke, Spectrochim. Acta, submitted for publication. (24) The IP of CF,Br should be below the 9.2-eV value for CF3. T. A. Water, C. Lifshitz, W. A. Chupka, and J. Berkowitz, J. Chem. Phys., 51, 3531 (1969). (25) A. W n k e n , B. Raz, and J. Jortner, J. Chem. Phys.,58, 1178 (1973). (26) F. T. Prochaska and L. Andrews, J . Am. Chem. Soc., 100, 2102 (1978). (27) R. S.Berry and C. W. Reimann, J. Chem. Phys., 38, 1540 (1963). (28) A. G. Melster, S.E. Rosser, and F. F. Cleveland, J. Chem. Phys., 18, 346 (1950). (29) J. Doucet, R. Gilbert, P. Sauvageau, and C. Sandoriy, J. Chem. Phys., 62. 366 (1975). (30) A. Hasegawa, M. Shiotani, and F. Williams, Faraday Discuss. Chem. Soc., 63, 157 (1977).
Phosphorus-Substituted Nitroxides. 3.4 Hyperconjugative Ability of Carbon-Phosphorus Bonds in Five-Membered Ring Nitroxides Paul Tordo," Marc Boyer, Alain Frledmann, Odette Santero, and Louis Pujol Laboratoires de Chimie Organique Physique et de Chimie Structurale Thlorique, Universitl de Provence, Rue Henri Poincar6, 13397 Marseille Cedex 4, France (ReceivedJanuary 1 I , 1978)
5,5-Dimethyl-l-pyrroline 1-oxideis used to trap a series of phosphorus-centered radicals .PLn. In the resulting nitroxides a cos2 0, dependence is exhibited by the hyperfine phosphorus splitting. The hyperconjugative parameters B, have been estimated for the different carbon-phosphorus bonds and are shown to be linearly correlated with the Pgscharacter in the bond.
Introduction Aryl or tertiary alkyl nitroso compounds are the most effective scavengers in the spinrtrapping technique.1,2 We recently reported3i4that phosphorus-substituted nitroxides are formed spontaneously when these nitroso compounds are mixed with different organophosphorus compounds. The nitroxides arise either by an electron transfer mechanism (cf. arylnitroso compounds3) or as a consequence of the homolytic reactivity of nitric oxide (cf. photolabile 2-methyl-2-nitro~opropane~). As a consequence, the use of nitroso compounds as radical scavengers could lead to errors of interpretation when applied in the field of homolytic organophosphorus 0022-3654/78/2082-1742$01 .OO/O
reactions: On the other hand nitrones do not undergo any spontaneous homolytic process in the presence of organophosphorus compounds. Moreover their use as scavengers would lead to nitroxides bearing a P-phosphorus atom whose hyperfine splitting could give information about the nature of the trapped radical. We report here the results obtained when 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) is used to trap phosphorus-centered radicals.
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0 1978 American Chemical Society
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