Photochemistry of Alkyl Iodides tive. However the reaction sequence 13-16 is consistent with all of the data. Proofs of reactions 13-16 will have to await the observation of "202 and NHzO by direct means.
Acknowledgment. This work was supported by the Atmospheric Sciences Section of the National Science Foundation through Grant No. GA-42856 and the National Aeronautics and Space Administration through Grant No. NGL-39-009-003 for which we are grateful. References and Notes (1) N. Cohen and J. Heicklen, "Comprehensive Chemical Kinetics", C. H. Bamford and C. F. H. Tipper, Ed., Elsevier, Amsterdam, 1972, p 1. (2) K. J. Olszynaand J. Heicklen, Adv. Chem. Ser., No. 113, 191 (1972). (3) H. Gesser, J. Am. Chem. SOC.,77, 2626 (1955).
437 (4) F. W. Dalby, Can. J. Phys., 36, 1336 (1958). (5) C. H. Barnford, Trans. Faraday SOC.,35, 566 (1939). ( 6 ) A. Serewicz and W. A. Noyes, Jr., J. Phys. Chem., 63, 843 (1959). (7) R. Srinivasan, J. Phys. Chem., 84, 679 (1960). (8) S. Gordon, W. Muiac, and P. Nangia, J. Phys. Chem., 75,2087 (1971). (9) M. Gehring, K. Hoyermann, H. Schacke and J. Wolfrum, Symp. (lnt.) Combust., [Proc.], 14, 1972, 99 (1973). (IO) G. Hancock, W. Lange, M. Lenzi, and K. H. Welge, Chem. Phys. Left. 33, 168 (1975). (1 1) R. Simonaitis and J. Helckien, J. Phys. Chem., 80, I (1976). (12) R. Simonaltis, R. I. Greenberg, and J. Heickien, lnt. J. Chem. Kinet., 4, 497 (1972). (13) D. Garvin and R. F. Hampson, Natlonal Bureau of Standards Report NBSlR 74-430 (1974). (14) H. A. Wiebe, A. Villa, T. M. Hellman, and J. Heicklen, J. Am. Chem. SOC..95, 7 (1973). (15) J. D. Saizman and E. J. Bair, J. Chem. Phys., 41, 3654 (1964). (16) R. A. Back and T. Yokata, lnt. J. Chem. Kinet., 5 , 1039 (1973). (17) L. Stockburger, ill, B. K. T. Sle, and J. Heicklen, Center for Air Environment Studies Report No. 407-75, Penn State University, 1975, Sci. Total Environ.. in press.
Photochemistry of Alkyl Iodides T. Donohue and J. R. Wiesenfeld' Department of Chemistry, Corneli University, lthaca, New York 14853 (Received June 9, 1975) Publication costs assisted by the Office of Naval Research
The chemical kinetics of ground state (5p5 2P3/2) and electronically excited (5p5 2P1/2)iodine atoms following the flash photolysis of RI (R = H, CnHZn+l,and CnFln+l)has been monitored using time-resolved attenuation of atomic resonance radiation. The deactivation efficiencies of I(2P1/2) by RI have been determined and are discussed in terms of formation of a short-lived collision complex. In addition, the role of the photolytically generated R group in the chemistry of I(2P1/2) and I(2P3/2)has also been investigated and rates of a number of recombination and deactivation processes are estimated using modeling techniques. Of special significance to development of the iodine photochemical laser are the rate constants for deactivation of I(2P1/2)by alkyl radicals and hydrogen atoms.
1. Introduction There have been numerous studies on the photochemistry of alkyl iodides.1-6 The earliest of these were concerned with the determination of the energy content and reactivity of the dissociated alkyl radical which receives considerable excess vibrational and translational energy in the initial p h o t 0 1 y s i s . l ~The ~ ~ ~electronic state of the corresponding iodine atom remained a matter of conjecture until the iodine photodissociation laser was reported.7 The observation of laser action following the photolysis of some of the simpler alkyl iodides6i7 indicates that a substantial amount of electronically excited iodine, I(52P1/2), is formed in the photodissociative process. As the iodine laser is currently being considered as a candidate for laser fusion experiments due to its potential for high output power? interest has been renewed in the role of radicals in the photochemistry of alkyl iodides. There are several important chemical and physical processes in the kinetics of the iodine photodissociation laser. The extent of population inversion is related to the initial photolytic branching ratio, a, where =
[I(52p1/2)lo/[I(52p3/2)l~
This parameter has recently been investigated in some detaiLg The inversion can be depleted in a number of different processes. Most important is physical quenching of I(Ci2P1/2) (hereafter, I*) by the source gas to produce ground state iodine atoms, I I*
+ RI
-+ I
RI
(1)
which is presumably accompanied by some internal excitation of the RI. Also, the following reactions involving R, I*, and I must be considered: I* R
+ RI
--
-+
+ R (+MI
I2
+R
R2 (+M)
R + R (+MI -,other hydrocarbons R + I* - R +
I
+ I* (+M) RI (+M) R + I (+M) RI (+M) R + RI R2 + I, I*
R
-+
(2) (34 (3b) (4)
(5) (6)
(7a,b)
Note that reaction 7b could actually increase the inversion The Journal of Physical Chemistry, Voi. 80, No. 5, 1976
T. Donohue and J. R. Wiesenfeld
438
following photolysis; indeed, this effect has been invoked to account for an apparent “late-time gain” observed in some laser experiments.loJ1 There are a number of other reactions possible, particularly if vibrationally and translationally “hot” radicals are included, but they are not considered, as a large excess of inert buffer gas was used in these experiments to ensure thermal equilibration of the radicals.
2. Experimental Section The apparatus used in these experiments has been described p r e v i o ~ s l y . ~Briefly, J~ alkyl iodides (at 1-50 fi pressure) in 20 Torr of argon buffer gas were photolyzed by a 100-5 Kr flash with absorption occurring in the region 240-280 nm. The resulting I and I* products were monitored using time-resolved absorption spectroscopy. Here, a microwave powered electrodeless discharge lamp, containing a small amount of 12 in Ar, was used to produce an atomic iodine emission spectrum. Appropriate resonance transitions [178.3 nm (62P3/2 5’P3/2) and 206.2 nm (@]P3/2 52P1/2)]were selected using a vacuum monochromator. Transient absorption signals were digitized and the signal-to-noise ratio enhanced by addition of successive runs in a signal. averager. The volume of the photolysis vessel was swept out between flashes to prevent accumulation of photolysis products. A modified form of Beer’s law was used to relate the observed absorption signal to the concentration of the atomic state being monitored, where
-
TABLE I k , + k,, lo-” om3 molecule-’ s--’
Selected lit. values
Source
Collision no.
This work
_ l _ ”. _”. l CH,I 2.6 + 0.6a 5.7 i 0.6 29 5 1.9 i 0.26 6.1 t 0.3 30 5 C,H,I n-C,H,I 2.0 t 0.26 8.0 i 0.7 270 6.3 1.9 335 i-C3H71 2.0 i 0.26 n-C,H,I 2.9 i 0.26 9.3 i 0.3 225
*
12.0 i 2.4 185 11.1T 0.7 20 5 >5.2d > [I*] indicate negligible contribution from processes 4, 5, and 7b, which would not display overall first-order kinetics. However, the branched iodides (isopropyl, sec-butyl, and tert-butyl), along with HI, display a pronounced curvature in their In [I*] vs. time plots (Figure 2), indicating the importance of other processes. Therefore, a modeling technique (discussed below) was necessary to estimate the deactivation rates given by (1) and (2) for these four compounds. Table I lists the deactivation rates determined here. While several perfluoro compounds were studied, their deactivation rates were too slow to be observed at the low RI pressures used in these experiments.14 The correlation between deactivation rates and alkyl group size is interesting. The rates increase with radical size, in an almost linear fashion, so that the collision number (number of collisions required for deactivation) remains almost constant, except for HI, which gives a much lower collision efficiency. There does, however, appear to be a tendency for the collision efficiency to increase slightly as the alkyl group becomes larger. These effects can qualitatively be explained in terms of a short-lived collision complex, I-RIt, where the extent to which this entity can be formed is determined by the magnitude of the attractive forces between RT and 1.l6-ls Alternatively, the larger number of hydrogen atoms or multitude of vibrational modes in The JQurnal of Physical Chemistry, Vol. 80,No. 5, 1976
T i m e (mS)
Deactivation of I* in the presence of increasing partial pressures of CHd: pnr = 20 Torr: pcHs1 = 1.4, 7.9, 21.2, 44.1 fi (top to bottom). Traces are shifted by 1.O In unit for clarity. Figure 1.
0
‘
n
I
I
I
I
I
I
I
Time (mS)
Deactivation of I * in the presence of 9 . 9 /A of sec-butyl iodide and 20 Torr of Ar. Dashed line represents calculated fit (shifted downward by 0.3 In unit for clarity) to the data, using rate constants iven in Tables I and II. Initial concentrations used: [ l ” ] =~ 8.9 mfi; Flgure 2.
110 = 78 mfi; [RIo = [I”]o + [ll0= 87 mfi.
the heavier radicals might also be invoked to account for the observed enhancement of deactivation by these molecules.
439
Photochemistry of Alkyl Iodides
The extent of chemical reaction 2 with respect to physical quenching (1) in the deactivation of I* by alkyl iodides has been vigorously debated.3J9920 The importance of these processes in laser kinetics is clear. While (1) produces a ground state iodine atom, (2) results in an iodine molecule which has a large cross section for quenching of I*, and a radical, which can further participate in reactions 3-7. Experiments performed by Meyer and othersz0 using CH3I indicate that the rate of process 2 could be quite large compared to that of (1). On the other hand, there is extensive evidence to indicate that deactivation is completely dominated by (l), a t least for the normal, perhydro species.3J9,21p22As has been previously suggested,2l one would expect the reactive channel, (2), to have equal probability for CH31 and CD31. Since the measured deactivation rate is several orders of magnitude smaller for CD31,21the dominant channel leading to deactivation of I* by CH3I must be quenching. An upper bound for kz/(kl k z ) of 20% may be set for all of the normal, perhydro alkyl iodides examined here.23 Unfortunately, a more detailed comparison between these compounds is precluded due to limitations imposed by the signal-to-noise ratio for the ground state line, as well as by limited accuracy of the numerical analysis. However, our results extend earlier work on CH31 to higher members in the homologous series and confirm that quenching of I* by R I proceeds with a much higher efficiency than reaction. B. Kinetics of Processes Involving Free Radicals. While free radicals cannot be directly detected using the methods discussed here, their role may be deduced by observation of the temporal profiles of I* and I following photolysis. The coupled, nonlinear differential kinetic equations describing reactions 1-7 may be integrated numerically by the standard Runge-Kutta method in order to obtain estimates of the kinetic parameters associated with processes 4-7. The relative importance of these processes following photolysis allows the iodides discussed here to be conveniently divided into three groups: primary iodides, branched iodides, and HI. Primary Iodides. The kinetics for removal of I* following the photolysis of primary iodides (Figure 1) are sensibly first order with respect to reactant concentrations. The removal of I is also a first-order process, since it is governed by diffusion through Ar to the walls, where heterogeneous recombination takes place.lZ These facts imply that processes 4-7 are slow compared to (1) (2), particularly if (3) is fast. The rate of the radical recombination step (3a) has been measured for CH3 by a variety of methodsz4 and has been found to be rapid, occurring in every 5-10 collisions. Reaction 3b is a disproportionation step, where R R + R H R(-H); it is slow compared to (3a) for primary alkyls ( k 3 b / k s a < 0.2).25As the observed kinetic profiles would permit detection of a 2% nonlinearity in the I* decay rate, a limit of 5 5 X 10-l2 cm3 molecule-l s-l can be set on K 4 125 for the primary species. This parameter has recently been estimatedz2(for R = CH3) by modeling of laser amplifier gain measurements to be 5.5 x 10-12 cm3 molecule-1 s-l, in agreement with the limits suggested here. Deviations from first order would be difficult to observe in the case of I atom decay, especially since processes 4, 6, and 7a could combine in such a fashion as to yield no perceivable effect. However, since processes 4 5 and 6 are less important for primary radicals (due to rapid radicalradical recombination) than the iorresponding ones involving branched radicals (see below), it would be expected
+
+
+
+
+
+
that the rates for reactions 4, 6, and 7a would be similarly unimportant. Branched Iodides. In contrast to the primary compounds, the branched iodides all display curvature in both the plots of In [1*]/[1*]0 vs. t (Figure 2) and In [I]/[I]o vs. t (Figure 3). As k l k z is similar for all of the hydrogenic iodides, this curvature suggests that the rates of processes 4 5 and 6 are much larger for the branched alkyls than for the primary isomers. Clearly, the enhanced contribution to I* and I removal by processes 4 5 and 6 must be due to higher concentrations of alkyl radicals at times on the order of the kinetic observations (ca. 5-10 ms). This is reasonable in view of the relatively slow recombination of the branched radicals with respect to the unbranched speciesz6 (Table 11). By fixing the measured values of h l and k 3 , as well as the known branching ratio of the initial photolysis (a = 0.10), it is possible to obtain estimates of the rate constants for processes 4 5 and 6 as well as overall photolysis yields. Process 2 is deemed to be of negligible importance. The value of does not affect k4+5 or ke so long as &I > 1, would result in very low concentrations of I. However, lasers based on the use of inefficient photolytic sources or electric discharge initiationz8 might depend on significant depletion of I by (6) before laser threshold is achieved. HI. HI displays curvature in its plots of In [I*]/[I*]o vs. t in a manner similar to those of the branched iodides (Figure 2), but in this case, it is clear that this deviation is caused by process 4 and not 5 as the latter is not probable; the HI+“complex” could exist for dnly about one vibrational period, leaving little possibility for collisional stabilization. This argument also applies to processes 3 and 6. Modeling of I* decay data demonstrates that the quenching process 4 occurs with a surprisingly large probability, with
+
+
+
+
+
+
+
+
N
The Journal of Physical Chemistry, Vol. 60,No. 5, 1976
T. Donohue and J. R. Wiesenfeld
440
TABLE 11: Radical Reaction
Rates
and Photolysis Yields
cm3 molecule" s-'
k,
-
k,
k, + k ,
k,
i-C,H, 9a 5 s-C,H, 2b I t-C,H, la 5 a Reference 26. b See text.
Photolysis yield, % p e r flash 3.0 3.5
9 10 5
4.0
Figure 4 demonstrates the r i s e in [I] for varying HI concentrations. The rate of increase is clearly a function of [HI], and results from processes 4 and 7a. Modeling of the concentration profile of the I atoms, using the value determined above for k4, yields a rate constant, kVa, of 2.0 f 1.0 x 10-11 cm3 molecule-l s-l. Cadman et al.,30 in calculations on some earlier experimental data of Sullivan,31 obtained 3.3 X cm3 molecule-l s-l. As had been suggested earlier,32 no evidence of reaction 7b could be found in our results.
4. Conclusions Rate constants have been determined for a variety of processes involving alkyl iodides and related photofragments. Deactivation rate constants for I* by RI measured here appear -2-4 times faster than those reported earlier. No systematic cause for these discrepancies (Le., impurities, such as 0 2 , in the samples used were found to be negligible) have been found. Alternatively, a systematic error in the determination of y for the transition (62P3/2 52P1/2) at 206.2 nm could result in ~ C H being ~ I as much as 1.75 times too large if y were 1.0 instead of our measured value, 0.57. However, the rates for HI and H212 determined in the same apparatus agree, within error limits, with recent literature value^.^^,^^ Processes involving alkyl radicals have been found to be rapid, and of potential significance to practical laser systems. Of course, the estimates of the rate constants associated with these processes depend upon the absolute values of the radical-radical recombination rates, and so can be no more accurate than these parameters.26
-
T i m e (mS) Figure 3. Decay of I in the presence of 3.3 p of sec-butyl iodide and 20 Torr of Ar. Dashed line represents calculated fit (shifted down by 0.2 In unit) to the data, using rate constants given in Table II. Initial concentrations used: [I*l0= 3.0 mb; [Il0 = 26 mp; [R]o = 29 mw. 10 ms) is due to diffusion by I to the walls The long-time decay ( t (kd,ff = 45 s-l).
>
Acknowledgment. We are indebted to the Office of Naval Research for their generous support of this research.
References and Notes (1)J. R. Majer and J. P. Simons, Adv. Photochem., 2, 137 (1964),and references clted therein.
(2)M. R. Levy and J. P. Sirnons, J. Chem. SOC., Faraday Trans. 2, 71, 561 (1975),and references cited therein. (3)D. Husain and R. J. Donovan, Adv. Photochem., 8, 1 (1971),and references cited therein.
(4)S. J. Riley and K. R. Wilson, Faraday Discuss. Chem. SOC., 53, 132 (1972). ( 5 ) G. M. Harris and J. E. Willard, J. Am. Chem. SOC.,76, 4678 (1954). (6) J. V. V. Kasper, J. H. Parker, and G. C. Pimentel, J. Chem. Phys., 43,
1827 (1965). (7)J. V. V. Kasper and G. C. Plrnentel, Appl. Phys. Lett., 5 , 231 (1964). (8)K. Hohla and K. L. Kornpa, Appl. Phys. Lett., 22, 77 (1973). (9)T. Donohue and J. R. Wiesenfeld, J. Chem. Phys., 63, 3130 (1975).
(IO) T. L. Andreeva, V. I. Malyshev, A. I. Maslov, I. I. Sobel'man, and V. N. I
I
0
I
I
I
I
2
I
I
I
3
T i m e (rnS)
Flgure 4. Initial rise of I in the presence of increasing partial pressures of HI. pnr = 20 Torr; h2= 90 w; PHI = 1.0, 2.3,4.2, 16.8 I.L (bottom to top). Data have been smoothed by hand and traces are not normalized to one another. H2 (kH2= 1.2 X om3 molecule-' s-l, for I* H2 I H2) is used to control the rate at I; this feature can aid in data interpretation. which I *
-
+
-+
a rate constant of -5 x cm3 molecule-l s-l. All of the energy released in this quenching process (21.7 kcal mol-l) must go into translation; the hydrogen atom is produced extremely "hot" following such a quenching collision, as i t acquires over 99% of the available kinetic energy. Process 4 has also been observed in the H atom quenching of c1*(32P1/2) atoms, which proceeds at an even faster rate,29 -7 X 10-l" cm3 molecule-l s-1, about the gas kinetic value. The Journal of Physical Chemistry, Vol. 80, No. 5, 1976
Sorokin, JETP Lett., 10, 271 (1969). (11)K. Hohla and K. L. Kompa, Chem. Phys. Lett., 14, 445 (1972);Z. Naturforsch. A, 27, 938 (1972). (12)T. Donohue and J. R. Wiesenfeld, Chem. Phys. Left., 33, 176 (1975). (13)R. J. Donovan, D. Husain, and L. J. Kirsch, Trans. Faraday SOC., 66, 2551 (1970). (14)D. Husain and J. R. Wiesenfeld, Trans. Faraday SOC.,63, 1349 (1967).
(15) A more detailed treatment of collision complex dynamics can be found in S. J. Rlley and D. R. Herschbach, J. Chem. Phys., 58, 27 (1973),and references cited therein. (16) P. J. Robinson and K. A. Holbrook, "Unimolecular Reactions", WileyInterscience, New York, N.Y., 1972. (17)W. Forst, "Theory of Unimolecular Reactions", Academic Press, New York, N.Y., 1973. (18)The theory of unimolecular decay is now well understood: see ref 16 and 17. However, the effect of electronically excited ieveis in energy partioning among the modes of a complex has now just begun to be treated. See D. W. Setser, MTP Int. Rev. Sci., Phys. Chem., Ser. One, 9,l(1972);J. C. Tully, J. Chem. Phys., 62, 1893 (1975). (19)S.Adltya and J. E. Willard, J. Chem. Phys., 44, 418 (1966). (20)R. T. Meyer, J. Chem. Phys., 46, 4146 (1967);J. Phys. Chem., 72, 1563 (1966);D. M. Haaland and R. T. Meyer, lnt. J. Chem. Kinet., 6, 297
(1974). (21)R. J. Donovan and C. Fotakis, J. Chem. Phys., 61,2159 (1974). (22)T. D. Padrick, private communication. (23)A recent studyZ2using CH31 gives 2% for this ratio.
' ~
Chlorophyll-Poiy(viny1pyridine) complexes (24) F. C. James and J. P. Simons, int. J. Chem. Kinet., 6, 887 (1974), and references clted thereln. (25) E. Whittle, MTf lnt. Rev. Sci., fhys. Chem., Ser. One, 0, 75 (1972). (26) D.M. Golden, 0. N. Spokes, and S. W. Benson, Angew. Chem., 12, 534 (1973). (27) S. V. Kuznetsova and A. I. Maslov, Sov. J. Quantum Electron. (Engi. Trans.), 3, 468 (1974).
441 (28) L. D. Pleasance and L. A. Weaver, Appl. fhys. Lett., 27, 407 (1975). (29) I?. J. Donovan, D. Husain, A. M. Bass, W. Braun, and D. D. Davls, J. Chem. fhys., 50,4115 (1969). (30) P. Cadman, J. C. Polanyi, and I. W. M. Smith, J. Chim. Phys., 64, 111 (1967). (31) J. H. Sullivan, J. Chern. Phys., 36, 1925 (1962). (32) R. J. Donovan and D. Husain, Trans. faraday Soc., 62, 1050 (1966).
Chlorophyll-Poly(viny1pyridine) Complexes. V. Energy Transfer from Chlorophyll to Bacteriochlorophyll1 G. R. Seely Charles F. Kettering Research Laboratory, Yellow Springs, Ohio 45387 (Received June 20, 1975) Publication costs assisted by the Charles F. Kettering Laboratory
The transfer of singlet excitation energy from chlorophyll a to bacteriochlorophyll on poly(4-vinylpyridine) in nitromethane solution has been measured by quenching and sensitization of fluorescence. The directly excited fluorescence of both chlorophyll a and bacteriochlorophyll is strongly quenched by transfer to weakly interacting, weakly fluorescent pairs of pigment molecules. The dominant quencher is apparently a bacteriochlorophyll-chlorophyll a pair. The fluorescence of bacteriochlorophyll is correlated with the presumed density of quenching pairs, but the quenching of chlorophyll a fluorescence has also a component due to transfer to bacteriochlorophyll. The quenching of chlorophyll a fluorescence is described mathematically by Forster’s equation for transfer from a donor to randomly distributed acceptors, provided that the effective critical transfer distance Ro is proportional to the sixth root of the donor/acceptor ratio. In this way energy trapping can be described, when there is extensive transfer among randomly distributed donor molecules. The equations pertaining to the model system can be extrapolated to pigment systems typical of photosynthetic units, and maximum sizes of the units can be estimated, which depend on pairwise transfer parameters and attainable pigment concentrations. The estimated sizes approximate those of the presumed natural ones.
Introduction Chlorophyll or its derivatives bound to the randomly coiled macromolecule poly(4-vinylpyridine) in nitromethane solution constitutes a three-dimensional aggregate of variable pigment density, useful as a model for studying energy transfer and trapping processes like those of the photosynthetic unit of green plant^.^-^ In a previous paper we demonstrated energy transfer in this system by depolarization of fluorescence, and have related the probability of it to the configurational properties of the p01ymer.~The other “classical” way of demonstrating energy transfer is by sensitization of the fluorescence of one kind of molecule through excitation of another.6 While depolarization of fluorescence detects whether energy transfer occurs at all, sensitization of fluorescence detects only energy transferred from donor to acceptor, and therefore can be studied a t pigment concentrations at which many transfers among donor molecules occur. Sensitization of fluorescence is therefore an appropriate means by which to study energy transfer in dense pigment aggregates, including the photosynthetic unit itself. We have measured energy transfer from chlorophyll b to chlorophyll a, and from chlorophyll a to bacteriochlorophyll, in mixed pigment aggregates on poly(4-vinylpyridine). Because of unexpected complications in the former
system, we confine ourselves here to the latter, more straightforward system. The essential question to be examined is how multiple transfer among donor molecules (chl) affects the probability of transfer to acceptor (bchl), as a function of the ratio of acceptor to donor, /3 = (bchl)/(chl), and of the pigment density in the polymeric aggregate, expressed as the ratio of bound chlorophyll a or bacteriochlorophyll to polymer pyridine units. It will be found possible to include the effect of multiple transfer within the conceptual framework of energy trapping developed by Duysens7 and Forster.8 The pigments of the present system resemble those of the green sulfur bacteria, where transfer occurs from an antenna array of chlorobium chlorophyll to a core array and reaction center of bacteriochlorophyll.
Experimental Section Materials. Chlorophyll a was extracted from spinach and purified chromatographically. Bacteriochlorophyll was extracted from species of photosynthetic bacteria with acetone and chromatographed twice on sugar with toluene +0.5% 1-propanol eluent. The poly(4-vinylpyridine) used was a fraction of molecular weight 320 000 (fraction A3 of ref 3). The purification of nitromethane has been de~cribed.~ The Journal of Physical Chemistry, Vol. 80, No. 5, 1976
