1721
ESROF NO2 IN FROZEN SOLUTIONS of a solvent-fixed frame is based on sound practical reasons, but it is an arbitrary choice of one reference component and becomes more unrealistic from a theo-
retical viewpoint as concentrations increase. The significant coefficients are L,,O coefficients for a Darken reference frame.
Electron Spin Resonance of Nitrogen Dioxide in Frozen Solutions' by B. H. J. Bielski, J. J. Freeman, and J. M. Gebicki Chemistry Department, Brookhaven National Laboratory, Upton, N e w York
11973 (Received November 6 , 1967)
The electron spin resonance of nitrogen dioxide trapped in frozen nitromethane, chloroform, carbon tetrachloride, and water were studied over a wide range of temperatures. In all solvents a triplet spectrum observed near - 100" split into many lines when the temperature was raised. Simultaneously, the hyperfine splitting constant due to interaction of the unpaired electron with the KI4magnetic moment decreased from 55 to about 12 G. The multiline spectra were interpreted in terms of an interaction between NO2 and Nz04.
Introduction A number OF attempts have been made to study the esr spectrum of the paramagnetic nitrogen dioxide and several accounts have appeared of the observation of triplet absorpt!ion spectra, characteristic of the interaction of the unpaired electron and IY14 magnetic moments.2-4 The isotropic component of the hyperfine splitting constant was found to be 54.6-57.8 g for KO2 trapped jn inert gas matrices at 4°K. Extension of these studies to solutions of SOz did not produce consistent results. Holm, et U Z . , ~ measured a line width of 155 G for NO2 in pure dinitrogen tetroxide at room temperature, which increased on dilution with carbon tetrachloride to 227 G. In carbon disulfide, Bird, et u Z . , ~ recorded a fully resolved NO2 triplet with a splitting constant of 107 G. This was about twice as large as the splitting observed for S O 2 trapped in inert matrices or in ice. Further attempts to repeat and extend these studies to solutions of NO2in carbon tetrachloride, chloroform, nitromethane, and dinitrogen tetroxide yielded only broad singlet^.^ On cooling, all signals disappeared because of rapid dimerization of the NOz. This paper reports results of esr studies of frozen solutions of N102 in water, nitromethane, chloroform, and carbon tetrachloride. In order to overcome the difficulties associated with dimerization on cooling, modified techniques mere used to introduce NO2 into the solvents. .[n one method, dilute solutions of nitromethane were frozen at boiling nitrogen temperature and irradiated with ultraviolet light. During photolysis, methyl and NO2 free radicals were released and trapped in the m a t r k 6 The methyl radicals were then removed by annealing so that the behavior of NO2alone
could be studied. I n a second method, mixing of cooled solvent and solid N204also resulted in the trapping of KO,. Except at very low temperatures, similar esr spectra were obtained with solutions prepared by either technique. Experimental Section Analytical grade chloroform, carbon tetrachloride, and nitromethane were distilled over a 30-plate column. Water and heavy water (99.7% deuterated) were purified by triple distillation from acid dichromate and alkaline permanganate, while deuterated chloroform (Merck, Sharp and Dohm, Ltd.) was used directly. The concentration of nitromethane in these solvents was varied between 0.05 and 1.0 M . A small sample of the solution was sealed in a 2-mm i.d. quartz tube and irradiated at boiling nitrogen temperature with light from a GE BH-6 high-pressure mercury arc lamp for periods between 0.5 and 2 hr. I n initial experiments the light was filtered through a Corning No. 7-59 uv filter (transmission peak at 3600 A), which passed uv (1) Research performed under the auspices of the U. S. Atomic Energy Commission. Part of this work was presented at the Sixth Australian Spectroscopy Conference, Brisbane, 1967. (2) (a) J. G. Castle, Jr., and R. Behringer, Phys. Rev., 80, 114 (1950); (b) C. K. Jen, S. N. Foner, E. L. Cochran, and V. A. Bowers, ibid., 112, 1169 (1958); (c) J. B. Farmer, D. A. Hutchinson, and C. A. McDowell, Fifth International Symposium on Free Radicals, Uppsala, 1961, 44-1. (3) G. R. Bird, J. C. Baird, and R. B. Williams, J. Chem. Phys., 28, 738 (1958). (4) P. W. Atkins, N. Keen, and M. C. R. Symons, J. Chem. SOC.,2873 (1962). ( 5 ) C. H. Holm, W. H. Thurston, H. M. McConnell, and N. Davidson, Bull. A m e r . P h y s . Soc., Ser. 17,397 (1956). (6) B. H. J. Bielski and R. B. Timmons, J . Phys. Chem., 68, 347 (1964). Volume 72, Number 6 M a y 1968
1722 light in the wavelength region corresponding to the onset of the first strong absorption band of nitromethane. Since identical esr spectra were obtained with solutions irradiated with unfiltered light, the use of the filter was omitted in later studies. In some experiments frozen dinitrogen tetroxide and cold solvent were mixed a t a low temperature. A portion of the resulting slurry was then transferred to a cooled quartz tube and sealed for esr studies. Yo attempt was made to exclude air or moisture from the system, but the operations were performed as rapidly as possible. The esr measurements were carried out in a Varian spectrometer Model V4500 equipped with a 12-in. magnet, 100-Kc field modulation, and a variable-temperature accessory. The first derivative of the absorption spectrum was recorded. The g values were measured by direct comparison with the singlet of 1,l-diphenyl-2-picrylhydraxyl (DPPH), g = 2.0036.
Results Water, heavy water, carbon tetrachloride, chloroform, deuterated chlororm, and nitromethane were used as solvents. Both the photolyzed frozen solutions of nitromethane and the cold mixtures of dinitrogen tetroxide and solvent gave esr spectra. Except at low temperatures, the method used to introduce the paramagnetic species into the matrix did not affect the appearance of the spectrum. At - B O 0 , samples prepared with N204 showed only a broad singlet absorption. I n all solvents, at - 180"the spectra of photolyzed nitromethane solutions consisted of a poorly resolved set of asymmetric lines. By analogy with an earlier study of NOz in solid nitromethane and in carbon tetrachloride,16 the spectra were identified as superimposed absorptions of the methyl and NOz free radicals. On warming, the signal due to the methyl radicals disappeared between -140 and -120" and a characteristic NOz triplet remained (Figure 1A). The average splitting of the triplet in all matrices studied was about 55 G. I n all solvents, the triplet split further on warming, the number of absorption lines and their intensities varying with temperature and the nature of the matrix. Although the best resolved hyperfine spectra were observed close to the melting point of each matrix, the rapid disappearance of the signal at that point caused difficulties in recording. All signal was lost when the matrix was allowed to melt. The best resolved spectra were recorded with KO, trapped in nitromethane and in chloroform. The sequence of spectra recorded in nitromethane at different temperatures is shown in Figure 1. At -GO0 the triplet recorded at - 110" split into nine symmetricql lines separated by 5.3 G Figure (1B). These were further split on warming to give a 15-line spectrum Figure (1C). The Journal of Physical Chemistry
B. H. J. BIELSKI,J. J. FREEMAN, AND J. M. GEBICKI
VUYII
U
2OG
H
Figure 1. Effect of matrix temperature on the esr spectrum of NO2 trapped in nitromethane: A-C, change on steady warming; D, spectrum after recooling from -34".
On recooling to -110", instead of the original triplet, a broad unresolved spectrum was recorded (Figure 1D). In chloroform the esr absorption of NO2underwent similar changes, except that the intermediate nine-line spectrum was not observed. Also, because of the differences in the melting points of these solvents, the final Table I: Features of the Electron Spin Resonance Spectra of NO2 in Frozen Solutions
Solvent
CHaN02
CHCla CDCls
KO
Sample iiumMelting temp, ber of point of solvent. OC "C peaks
0.0
DzO CCla
-94 -60 -34
3 9 15
ca. 5 7 . 5
-102 -68
3 15
CU.
-102 -32
3 7
-80 -42
3 27
-29.0
-63.5
Splitting constant, G
-22.8
g
value
5 . 3 i 0.3 12.2 i 0 . 2 5.3 i0.3
2.0035
56.0 1 2 . 6 i: 0 . 3 5.6 i0 . 3
2.0058
ca. 57.35 6 . 8 rt 0 . 6
2.0086
CU. 55.0
2.0038
Table I1 : Relative Line Intensity Distributions in the Electron Spin Resonance Spectra of NO2 in CHCla a t - 68' and CHa?S02a t -34' Solvent
CHCla CHaNOz
--------
Line number------------
1 15
2 14
3 13
4 12
5 11
6 10
1.5 2.3
4.4 5.1
6.7 7.1
1.2 1.5
3.6 4.6
4.4 4.4
7
8
9
1.1 7 . 6 1.0 7 . 7
1723
ESROF NOz IN FROZEN SOLUTIONS
- 42'C
G H
10
Figure 3. Electron spin resonance spectrum of NO2 in carbon tetrachloride.
Figure 2. Electron spin resonance spectrum of NO2 in water.
15-line spectrum was recorded at about -70". The slight differences in the line intensities and splitting constants observed in the two solvents were within the experimental error (see Tables I and 11). The substitution of deuterated chloroform for chloroform did not affect the spectrum. The esr signals of NOz in water and in heavy water were identical. The spectra were difficult to record because the signal disappeared rapidly even a t - 25". Figure 2 shows a typical noisy seven-line spectrum recorded in ice a t -30". In carbon tetrachloride the original triplet split at -48" into six groups of lines, each consisting of several unresolved components. At least 27 lines were observed a t -42"(Figure 3). Table I summarizes the main features of the esr spectra of NOz, in the various solvents used.
Discussion The esr triplets observed with annealed photolyzed solutions of nitromethane suggest that the absorption was due to the interaction of magnetic moments of an unpaired electron and a single N14nucleus. The identification of the molecular species containing the nitrogen atom is aided by an earlier study of the esr spectra of photolyzed pure nitromethane and nitromethane dissolved in carbon tetrachloride.6 Bielski and Timmons presented in their investigation evidence that the triplet observed at, low temperatures after the removal of methyl radicals is due to the presence of NOz in solid matrix. Further, although resolution of the triplet is far from satisfactory (Figure lA), the observed isotropic hyperfine constant of about 56 G agrees with values measured for NOz in other systems.' I n contrast with observations made by Atkins, et aLJ4with NOz trapped in ice at 77°K) no anisotropy was seen at low temperatures in the solvents used in this study. Poor resolution of the triplets may partly account for this apparent disagreement,
The possibility of a contribution to the esr signals by methyl free radicals, once the matrix was kept for several minutes near -loo", can be dismissed. The lifetime of this radical is known to be quite short in frozen solutions and the esr spectra were similar, whether the NOz was produced by photolysis of nitromethane or by the equilibrium dissociation of NzO+ Since prolonged photolysis of nitromethane might result in the formation of some nitric oxide, which has a paramagnetic z m / 2state, the esr spectra of deaerated carbon tetrachloride and water saturated with NO (up to 3 X M ) were investigated between -180" and room temperature. Only weak, broad singlets yere observed at low temperatures, unlike any of the spectra obtained with photolyzed solutions of nitromethane. In all systems studied, the triplets characteristic of NOz trapped at low temperature showed additional hyperfine splitting when the matrix was allowed to warm up. Such splitting could arise from coupling of spin of the unpaired electron to rotational moments of the KO,,from radical-matrix or from radical-radical interactions. Although spin-rotational coupling has been observed with NOz in the gas phase,zait is not possible to postulate the existence of rotational states in the various matrices investigated which could account for the observed esr absorption. The possibility of interaction between the electron and solvent nuclei was ruled out by experiments in which substitution of deuterated analogs of chloroform and water failed to affect the esr spectra. Contribution of lattice site effects is probably negligible, except for solutions in CCl, where the hyperfine interaction may have an anisotropic component. Both glassy and polycrystalline matrices were investigated and the spectra of trapped NOz did not depend to any observable degree on the orientation of solvent molecules. The absence of radical-radical interaction was indicated by two observations: the independence of the esr absorption of the concentration of trapped NOz, (7) See P. W. Atkins, N. Keen, and M.C. R. Symons, ref 3, for a summary of earlier results.
Volume 72, Number 6 May 1968
1724 and the nature of the multiline spectra. If the matrix prevented dimerization of KO2 while allowing interaction of the unpaired electron with two nitrogen nuclei, in the absence of anisotropy the spectrum should consist of five lines with relative intensities 1:2:3:2: 1. No such spectra were seen. The only other interactions which might account for the observed absorption are between NO2 and other stable molecules also trapped in the matrix. In photolyzed solutions, ethane, nitromethane, and dinitrogen tetroxide molecules may be present in considerable quantities. Only Nz04 has to be considered in solutions prepared by mixing this compound with the cold solvent. Since the nature of the multiline spectra was independent of the method used to introduce NO2 into the matrix, interaction with Nz04 is the one most likely to account for the observed spectra. It should follow that in all solvents studied the paramagnetic species responsible for the esr absorption should be the same. ,4 more detailed examination of the spectra supports this conclusion. The best resolved and most symmetrical spectra were obtained in nitromethane and in chloroform about 5" below the melting point of the solution (Figure IC). Data listed in Tables I and I1 show that these 15-line spectra differed only slightly in the relative line intensity distributions. The three most prominent lines (numbers 3 , 8, and 13) were in the ratio 1: 1: 1 and they can be assigned to interaction with one N14 nucleus. Comparison of the splitting constant for this coupling (aN = 12.4 G) with the value measured for NOz triplets a t low temperatures (a" = about 55 G) indicates that a considerable delocalization of the unpaired electron took place on warming. In addition to this narrowing, each line of the triplet split into a quintet, whose lines were separated, on the average, by 5.4 G. The line intensities within the quintets were slightly different in the two solvents: in nitromethane the ratios were 1.0:2.3 : 3.9 : 2.3 : 1.0, and in chloroform the ratios were 1.O :3.3:5.7 :3.4: 1.O. This analysis suggests that near the melting point of these two matrices the unpaired electron interacts with three nitrogen nuclei of which two are equivalent. Figure 4 shows a 15-line spectrum together with a line reconstruction based on this suggestion. One nitrogen atom is in the NOz molecule on which the single electron is largely localized and the two equivalent nitrogen nuclei belong t o a NZO4molecule. This suggests that even in the frozen solutions the NO2 has sufficient translational freedom to dimerize to some extent. The average line intensities for the quintets are not in the ideal ratio of 1: 2 : 3 :2 : 1, but the discrepancy is probably not significant. While this explanation appears to account for the 15line spectra observed in nitromethane and chloroform, it also affords an understanding of the intermediate 9line spectrum in nitromethane (Figure 1B). If it The Journal of Physical Chemistry
B. H. J. BIELSKI, J. J. FREEMAN, AND J. M. GEBICKI
Figure 4. A 15-line esr spectrum of NO$. The line reconstruction assumes interaction with one N14 nucleus, producing a splitting of 12.2 G, and two equivalent N14 nuclei, producing a splitting of 5.3 G.
is assumed that a t lower temperatures some of the closely spaced lines of the 15-line spectra cannot be resolved, the intensities of these lines can be added to produce a 9-line spectrum, as shown in Table 111. The resultant intensities compare well with values obtained from the experimental spectrum. Table I11 : Relative Line Intensities in Nitromethane -Temp = -eo0--(Total no. of lines, 0) Relative Line intensity
1 2 3
4 5 6
7 8 9
1.0 3.1 3.8 4.0 4.0 3.6 3.1 2.0 1.0
_____Temp
-340----(Total no. of lines, 15) Relative Line Intensity
1 2 3+4 5 $6 7 + 8 + 9 10 11 12 13
+ +
14 15
1.0 2.2 3.8 4.0
4.3 4.0 3.8 2.2 1.0
I n water and in heavy water great difficulty was encountered with the resolution of spectra of NOz. Although below - 80" these systems behaved similarly to solutions of NOz in organic solvents, there was rapid loss of signal at higher temperatures. It was, therefore, necessary to warm the aqueous samples rapidly to between - 50 and 40°, where the triplets split further. Loss of signal was observed in spectra recorded with fresh samples in the downfield and in the upfield directions, suggesting a true time-dependent decay of intensity. The best resolved spectra consisted of seven lines with an average intensity distribution of 1.6 :3.1 : 3.4:3.5:2.6:1.6:1.0 and a splitting constant of 6.8 G. The asymmetry of the spectrum probably arises from
ESROF NOz I N FROZEN SOLUTIONS
1725
a decrease in the concentration of paramagnetic molecules during the period of recording. It is likely that this spectrum corresponds to the nine-line spectrum in nitromethane, with the two outermost absorption lines obscured by background noise. The multiline esr spectrum of NOz in carbon tetrachloride could not be interpreted in terms of N02-N204 interaction. I n this solvent the N14 triplet split into six groups of poorly resolved lines a t -48'. Attempts to obtain better resolution at higher temperatures were not successful. The most probable stabilizing force in the N02-Nz04 complex is an electrostatic interaction between the 0 atoms of the NOz and the N atoms of the X204molecule. Two configurations allow the closest approach of these atoms and are, therefore, the most likely: in one, the plane of the NOz bisects the N-N bond of Kz04, so that each of the 0 atoms of the dioxide lies an equal distance from the N atoms of Nz04 (A). I n the
A
B
other configuration, the N-N bond lies in the plane of the NO2 molecule (B). I n calculating the potential energy of the systems, it was amumed that in any given
molecule each 0 atom carried one half of the net charge of the N atom and the magnitudes of the charges were assumed to be different in the two interacting species. No values were assigned to the net charges. The distance between 0 atoms in NOz is 2.202 8. This value is based on an N-0 distance of 1.188 8 and bond angle of 134°4'.8 Using the value of 1.75 8 for the N-N bond length in NzOd9it can be shown that the parallel configuration (B) has lower potential energy. Experimental estimations of this energy have not been made, but apparently the interaction stabilization is not sufficient for the complex to survive the melting of the matrix. The existence of the proposed complex would help to explain several observed phenomena. Figure 1 shows that once the complex was formed, then on recooling only a broad, weak singlet absorption was recorded instead of the usual triplet. This singlet probably corresponds to an envelope of unresolved 15 absorption lines. It is also apparent that an NOz molecule can complex with only one molecule of Nz04 and thus the esr spectrum should be independent of the concentration of Nz04 and of the nature of the matrix. It should, however, depend strongly on temperature, because freedom of NO2 to dimerize is a prerequisite of complex formation. This was found to be the case. The absence of any residual triplet spectrum from uncomplexed NOz a t higher temperatures suggests that on warming virtually every monomer molecule either dimerizes or complexes with a dimer already present in the matrix. (8) P. Gray and A. D. Yoffe, Chem. Rev., 55,1069 (1965). (9)M.Green and J. W. Linnett, Trans. Faraday Soc., 57, 10 (1961).
Volume 72, Number 6 M a y 1968
