MATRIXINFRARED STUDIES OF OF RADICAL SYSTEMS
3577
Matrix Infrared Studies of OF Radical Systems' by Alfred Arkell Contribution N o . 1466, Texaco Research Center, Beacon, New Yorlc 19.508 (Received April 1 1 , 1969)
The photolysis of OFZ-NzOor OFZ-CO~ combinationsin a nitrogen matrix at 4°K has produced increased yields of OF radical compared to OFZwithout added N2O or COZ. The OF radical was also observed when Fz-NZO was photolyzed in a nitrogen or argon matrix at 4'K. This latter observation confirms a previously proposed mechanism and supports a lower-limit estimate of about 40 kcal/mol for the bond energy of OF. No radical was observed during the photolysis of Fz-CO~in nitrogen and alternate mechanisms involving C03are proposed. During the course of previous work2 in which the OF radical was produced by photolysis of OFZin N2 or Ar at 4"K, it was also observed that the addition of NzOto the matrix gave a pronounced increase in OF radical production. A similar but less pronounced increase in OF radical production was also observed when COz was added to the matrix. According to previous observations by Kaufman, et aLJ3it was proposed that C10 radical was produced by reaction of atomic chlorine with N 2 0 C1
+ NzO
--t
C10
F
+ N2O
--t
OF
+ N2
(1) Therefore, it was assumed that similar reactions of atomic fluorine with N20and C 0 2
+ Nz
F+COz+OF+CO
(2) (3)
could account for increased OF radical production. Subsequent work has now shown that reaction 2 does occur but that reaction 3 does not. It is of interest to note that Ogden and Turner4 have completed a similar N 2 0 reaction and, although they did study of the F not observe the OF radical directly, their results supported the conclusion that OF radical was an intermediate in the reaction. The present low-temperature matrix studies describe the effects of photolyzing OFzor Fzin matrices containing N20or COS.
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Experimental Section The low-temperature infrared cell, Beckman IR-9 spectrophotometer, and high-pressure mercury arc (General Electric BH6) used in this work have been described previously. Two component gas blends were prepared by standard manometric procedure using a matrix to reactant ratio (M:R) of 40. Three-component gas mixtures were produced by simultaneous deposition from two manifolds, and flow rates were adjusted to give the desired M:R. The N2 (Airco, prepurified), argon (Airco), OF2 and F2(Allied Chemical Co.), C02 and NzO (Matheson) were used without further purification. A summary of the runs to be discussed is given in Table I. Deposition rates were 0.12 and 0.06 mmol/min for runs 1-3 and 4-6, respectively.
In all runs a deposition time of about 30 min was required. Photolyses were carried oyt using a BH6 water filter (5-cm quartz cell, 2200-9000 A) combination.
Results
Photolysis of OF2 in an N2 Matrix. As shown in Table I1 and Figure 1 the photolysis of OFz in an Nz matrix appeared to be reaching a steady-state production of OF radical; that is, a balance between formation, recombination, and secondary photolysis, in the 120-130 absorbance ( X lo3) region. Diffusion (4-42-4°K) in this run showed complete loss of the OF radical absorptions, a large increase in OFz, and trace formation of OzF2 and 0 3 . In contrast to this, when N20 was added to the matrix (run 2), several changes occurred. The level of OF radical production was increased significantly and the relative intensities of the two OF radical absorptions at 1030 and 1026 cm-l were reversed. After 51 min photolysis, and during a liquid helium refill, an exotherm5 occurred which temporarily increased the temperature to about 18°K. An infrared spectrum after the refill showed that the OF radical absorbances had decreased. Continued photolysis up to the 101 min total indicated (Figure 1) that the OF radical was being produced a t about the same rate as it was prior to the exotherm. A trace of 02F also formed during photolysis. Diffusion (4-40-4°K) in this run gave as the major product 02F2 and a trace of 0 3 ; however, no increase in OF2was found. (1) This work was supported by the Research and Technology Division, AFSC, Edwards, Calif., under Contract AF 04(611)-9577. (2) A. Arkell, R. R. Reinhard, and L. P. Larson, J. Amer. Chem. Soe., 87, 1016 (1965). (3) F. Kaufman, N. J. Gerri, and D. A. Pascale, J.Chem. Phys., 24, 32 (1956). (4) J. 9. Ogden and J. J. Turner, J.Chem. Soc., A , 1483 (1967). The OF radical may not have been observed in the work by Ogden and Turner because of the presence of SiF4 in their fluorine sample. Silicon tetrafluoride has a strong absorption in the OF radical region. (5) Exotherms, or temperature spikes, ranging from 5 to 20°K increase in temperature were observed many times in this and other radical work. If the liquid H e container was near empty or in the process of being filled then the exotherm showed on the temperature recorder as a spike. However, if the liquid H e container had a sufficient reserve a t the time of the exotherm then no temperature spike was observed but a n increase in the liquid He loss rate occurred.
Volume 76, Number 11 November 1969
ALFRED ARKELL
3878 Table I : Summary of Runs. Photolysis of OFZor FZin Matrices Containing NzO or COZa t 4°K Run no.
Na
1 2 3 4 5 6
40 40 40
a
Ar
Components (mole ratio) OFa Fa
1 1 1 40
40 40
-
NaO
1 1 3 1
Table 11: The Effect of NzO and COZon the Photolysis of OF2 in
a
Photolysis" time, rnin
Table and/or figure
90 90 48 160 48
18-69 24-101 24-91 8-70 11-110 7-38
I, I1 I, I1 I, I1
90 90 90 48 48 48
1 1 1 1
used-Na0 or COa
2
+ HzOFilter (2200-9000 A).
Photolysis using BH6
Run no.
-#Mol OFa or Fz
COa
Composition
1
OFz
+ Nz
2
OF2
+ N2 + NzO
3
OFz
+ Nz + COZ
Photolysis time, min
18 36 53 69 24 51 51a 76 101 24 49 74 91
NZa t 4°K -Absorbance X--ol
r
1030 cm -1
44 67 77 83 44 61 57 74 84 32 44 56 62
1026 om-1
26 36 37 45 83 150 104 169 214 56 106 93 126
Ratio
Total
1030: 1026
70 103 114 128 127 211 161 243 298 88 150 149 188
1.7:l 1.9:l 2.1:l 1.8:l 1:1.9 1:2.5 1:1.8 1:2.3 1:2.5 1:1.8 1:2.4 1:1.7 1:2.0
Second infrared spectrum taken after liquid helium refill because of exotherm during refill (4-18-4°K).
In run 3, in which COz was added, the results were very similar to the system containing NzO. The 1026 cm-1 absorption was, on the average, twice as large as the 1030 em-' absorption and the total OF radical absorbance was again considerably higher than that found in the nonadditive system. No diffusion could be made in this run because of a shortage of liquid helium. Photolysis of F2 N 2 0 in N 2 or Ar at 4°K. The photolysis of F2 NzO Nz (1 : 3 :40) gave steady production of both OF radical and OF2. After 11 rnin of ultraviolet irradiation (Figure 2, curve B) the OF radical absorption at 1026 cm-l was about twice as large as the OF2 absorption at 822 cm-l. A high-resultion scan of the OF radical absorption showed that only one band was present at 1026 em-l. This was of interest because all previous work with OF2 had shown that two OF radical absorptions at 1026 and 1030 em-' were produced on photolysis. As shown in Figure 2 (curves C and D) total photolysis times of 44 rnin and 77 rnin showed formation of the split V I bands of OF2in the 920 cm-' region. There was also a continued increase in both OF and OFz absorptions. A second high-resolution scan of the OF radical absorption after 77 min photolysis still showed only a single absorption at 1026 cm-I. Diffusion (4-29-4°K) of this sample gave com-
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The Journal of Physical Chemistry
plete loss of OF radical (1026 em-', Figure 2, curve E), a small increase in OFZ,and formation of OZFz,as evidenced by the doublet near 620 cm-l and the bending vibration near 460 cm-I. The small absorption a t 900 em-l remains unidentified. The photolysis of Fz N20 Ar (1: 1:40) gave essentially the same results as in the nitrogen matrix except that the OF radical absorption was at 1028 em-' which is in agreement with previous work.2 As a consequence of the higher F2: NzO ratio (1: 1) in this run compared to the previous run (1 :3)) the diffusion operation gave both OF2 and OzFzas major products. Photolysis of F2 COzin N z at 4°K. The photolysis C02 N2 (1 : 1:40) gave no evidence of new of F2 band formation after photolysis times of 7 and 38 min. No additional photolysis was attempted because of the complete absence of new bands after 38 rnin of irradie tion.
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Discussion As shown in Figure 1the photolysis of OF2 in a matrix leads to the formation of OF radical with a gradual increase in its absorbance (x lo3)up to about 120 at which point it appears to have reached a steady-state concentration. It was therefore considered of interest to determine how the addition of NzO or COz to the
3879
MATRIXINFRARED STUDIES OF OF RADICAL SYSTEMS 320 A - OEPOSlTION F 2 t N 2 0 t N 2 B I I MIN PHOTOLYSIS
[ I :3:40'
-
MIN 0 - 7 7 MIN. E - DIFFUSION 14-39-4%!
300
C -44
260 240
-
7
5.
,
220
U
g- 200 + :: 180
__0
160 X
5
140
U
j! a
lZO 1 OL
80
,
I I
I
I
I
I
I
I
0 10
20
30
40 50 60 70 IRRADIATION TIME (MIN)
80
90
100
110
+ NzO + NZat 4°K.
Figure 2. Photolysis of FZ
Figure 1. Effect of NzO and COz on OF radical formation.
matrix would allow significantly greater concentrations of OF radical to accumulate. It can be assumed that OF radical may be lost by any combination of the equations shown below OF*-+O+F
F
+ OF +OF2
OF&O+F
0
+ OF +OzF
(4) (5)
(6) (7)
It should be noted that the loss of O F by decomposition as in (4) or (6) is particularly bad because it increases the concentration of 0 and F atoms in the matrix, both of which are capable of removing additional OF as in eq 5 and 7. Considering eq 4,the decomposition of OF* could occur if the radical is in a vibrationally excited state. In this case the NzO or COZmay be acting as a stabilizer by allowing the transfer of this excess vibrational energy from diatomic OF* to the multiply bonded triatomic additives NzO or COZ. Although stabilization by NZO and COZ cannot be rigorously excluded it seems quite probable that the two different environments provided by NzO and COzin the same cage with OF radical would have caused a perturbation of the OF radical absorption. However, this was not observed as evidenced by the fact that O F radical absorptions were a t the same frequencies in both NzO and COZruns.
If it is assumed that ( 5 ) is the most likely path by which O F radical is lost, that is by recombination, then anything which tends to remove atomic fluorine should allow a greater accumulation of OF radical. As shown by the F NzOreaction (run 5 , Figure 2), NzO not only effectively removes F by reaction but it produces another OF radical in the process. A comparison of the
+
OF2
+ NzO +2 0 F + Nz
(8)
absorbances in runs 1 and 2 (Figure 1) shows that about twice as much OF is being produced in run 2 (eq 8) compared to run 1. Additional evidence regarding the availability of fluorine atoms was found by comparing the products of diffusion from the two runs. In run 1the formation of OF2as the major product after diffusion showed that atomic F was readily available in the matrix. However, in run 2, in which atomic F was reacted (2) as it formed, the major product after the diffusion operation was 02F2 produced by combination of OF radicals. In view of the improved O F radical production from OFz when COz was present (run 3, Figure 1) and the absence of reaction between atomic F and COZ (run 6 ) it is necessary to consider alternate mechanisms for improved O F radical production in run 3. In recent work by Moll, et a1.,6 it was shown that the new species (6) N. G . Moll, D. 45,4469 (1966).
R.Clutter, and W. E. Thompson, J. Chem. Phys., Volume 79, Number 11 November 1969
E. G. HOHN,J. A. OLANDER, AND M. C. DAY
3880
COScould be produced by the photolysis (9) of OSin a COzmatrix. They proposed that (9)
COSwas produced by the reaction of atomic oxygen from with COz. In the present study it was suggested that a limiting factor to O F radical production may be photolysis of the radicals as shown in eq 6 to give oxygen and fluorine atoms. If the generated oxygen atoms’ react (eq 10) with C02 to give COS then OF radicals could be regenerated by the reaction (eq 11) 0 3
+ coz +co, F + COS+OF + COz 0
(10) (11)
of atomic F with COS. In this manner, CO:, would be acting as an oxygen transfer agent and would offset the photolytic loss of OF radical by allowing for the reformation of OF as shown in eq 10 and ll. If this proposed mechanism is valid, then the magnitude of the observed improvement (run 3, Figure 1) in the presence of COZ suggests that photolytic loss of OF (eq 6) contributes significantly to total OF losses. The absence of observed infrared absorptionse for COSin the COZrun was not considered as negative evidence for their presence in view of their transient existence in this system. An alternate function for COSmust also be considered because of the previous observatione that COa itself is subject to photolytic decomposition to give 0 atoms and
Ion-Solvent Interactions.
C02. The overall effect of improved OF radical production would be the same as in the previous mechanism; however, in this case it would be unnecessary to invoke the F COSreaction. The improved OF radical production would occur because the reversible reaction
+
hu
O
+ COa C,COS
(12) would increase the range of migration of 0 atoms within the matrix. Thus, in the absence of C02the generated 0 and F atoms from O F decomposition would migrate to isolated matrix sites. However, in the presence of GO, throughout the matrix, reaction 12 could increase the range of 0 atom migration and thereby increase the probability of an 0 F reaction to regenerate OF. OF Radical Bond Energy. The observation of OF radical formation during photolysis of F2-Nz0 mixtures confirms the presence of this intermediate in the forrnation of OFZ,as previously proposed by Ogden and Turnern4 It also supports their lowerlimit estimate of about 40 kcal/mol for the OF radical bond energy based on the energy requirements for reaction 2. Acknowledgments. The author wishes to thank Drs. P. H. Lewis and S. A. Francis for helpful discussions and Mr. G. H. Post for his help with the experimental portion of the work.
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(7) As suggested by a reviewer it should be noted that eq 10 assumes the dissipation of hot 0 atom energy in the formation of COa.
Infrared Studies of Solvation of the Sodium Ion1i2
by E. G. Hohn,8 J. A. Olander, and M. C. Day Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70805 (Received April 14, 1969)
The specific solvation of the sodium ion by tetrahydrofuran has been observed by infrared techniques. Unperturbed voo0bands occur at 1071 and 913 cm-1. In the presence of the sodium ion, new bands occur at -1053 and ~ 8 9 cm-l, 5 respectively, indicating complexation of the sodium ion. The band intensities are observed to be dependent on the ratio of ether: salt. This is used to approximate stability constants for the stepwise complexation of the sodium ion by tetrahydrofuran.
Introduction The importance of ion-solvent interactions on ionic conductance and extent of ion pairing was clearly pointed out by Gilkerson4 and has been extensively studied by several research groups.5-9 More recently there has been considerable interest in the effects of solvent interactions on the rates of anionic polymerization reaction^,^^-^^ but in Spite Of the large amount Of reThe Journal of Physical Chemistry
search in this area, there are yet numerous unanswered questions concerning the nature of the solvated species (I) Reprint requests should be sent to M. C. Day. (2) Presented in part at Southwestern Meeting of the American Chemical Society, Dec 1967. (3) Department of Chemistry and Chemical Engineering, University of Saskatchewan.Saskatoon, sask, (4) R. Gilkerson, J . Chern. Phys., 25,1199 (1956).
w.
