2963
AN ESRSTUDY OF THE N204-N02SYSTEM
An Electron Spin Resonance Study of the Dinitrogen
Tetroxide-Nitrogen Dioxide System by David W. James and Robert C. Marshall Department of Chemistry, University of Queensland, Brisbane, Australia
(Received February 28, 1.968)
Dinitrogen tetroxide has been examined in the liquid and solid states between 77 and 300°K. The nature of the equilibrium N2O4+ 2502 has been studied in the liquid state, and the concentrations of NO2 have been determined. The equilibrium constant decreased from 43.54 k 1.5 mol/l. a t 296°K to 0.11 0.01 mol/l. a t 247"K, giving AH = 15.6 & 0.5 kcal/mol for the dissociation. When the liquid was quenched to 77"K, an anisotropic set of triplets was observed with g values gx = 2.0065, g, = 1.9960, g. = 2.0029, and gav = 2.0018 and hyperfine-splitting constants (in 0)A , = 50.7, A , = 50.1, A , = 67.0, and Aiso = 56.0. When the sample was warmed to 180°K, the triplets decreased in intensity and became isotropic with a splitting of 57 G. On recooling, the original anisotropic triplet was recovered. Spectral changes are discussed in terms of solid-state interactions.
*
Introduction The electron spin resonance spectrum of liquid dinitrogen tetroxide (X204)at room temperature consists of a single, broad unstructured band with a half-width of approximately 150 G and a g value of about 2.0.' This has been attributed to nitrogen dioxide (NO2) formed by the dissociation of N204. The unpaired electron on X02 is most probably in an orbital of a1 symmetry partially localized on the nitrogen atom;' hence the esr signal should be a triplet due to the electron-nuclear-spin interaction. Failure to resolve this band at temperatures in the region of 273°K has been attributed to a rate of dimerization and dissociation which is sufficient to broaden the lines beyond the point of resolution.' The resolved spectrum of NO2 molecules has been obtained by trapping NO2 radicals in various matrices, usually by irradiation of the matrix containing alkali nitrates. The resolved spectrum has been shown to consist of an anisotropic with the g values and splitting constants shown in Table I. Atkins and Symons2 were unable to obtain the spectrum of trapped NO2 in an K204 matrix, but Schaafsma and Kommandeur4 reported a well-developed triplet at the liquid nitrogen temperature. The equilibrium between dinitrogen tetroxide and nitrogen dioxide has been extensively studied in the gas phase6 and the kinetics of the reaction are reliably documented. In the liquid state, however, the available data show considerable variation. Magnetic susceptibility measurements were used by Sone6 and Whittaker' to determine the concentration of NOz. Steese and Whittake? examined the visible absorption spectrum of liquid N204, which shows a highly structured band due to NOz. By examining three of the subsidiary peaks at 6130, 5920, and 5760 A, the equilibrium constant and AH for the dissociation
were calculated. The dissociation constant at 293" K varied from 47.26 X to 224.7 X mol/l., depending on which peak was examined. The AH was calculated t o be 19.5 kcal/mol. The present study reports the esr spectrum of the N204-?JOaequilibrium mixture in the liquid and solid state between 77 and 293°K. The nature of the trapped species is examined and the equilibrium constants are derived.
Experimental Section The dinitrogen tetroxide used in this investigation was a Matheson chemical (99%) which was dried by passage through phosphorus pentoxide columns. It was handled in an all-glass vacuum system using DowCorning high-vacuum silicone grease, which showed the greatest resistance to corrosive attack. An esr tube the internal diameter of which had been calibrated was dried and degassed under vacuum and then was flushed several times with N204 and reevacuated. Kz04was then condensed into the tube so that the liquid column was at least 1.5 in. in length. With the X204condensed in liquid nitrogen, the tube was sealed off from the vacuum system. The esr spectra were recorded using a Varian Associates V-4500 epr spectrometer with a 100-kcps mod(1) P. W. Atkins, N. Keen, and M. C. R. Symons, J. Chem. Soc., 2873 (1962). (2) P. W. Atkins and M. C.R . Symons, "The Structure of Inorganic Radicals," Elsevier Publishing Co., Amsterdam, The Netherlands, 1967. (3) H. Zeldes and R. Livingston, J. Chem. Phys., 35,563 (1961). (4) T. Schaafsma and J. Kommandeur, ibid., 42,438 (1965). (5) M. Bodenstein, Z. Physilc. Chem. (Leipzig), 100, 68 (1922); L. Harris and K. L. Churrey, J.Chem. Phys., 47, 1703 (1967). (6) T . Sone, Sei. Rept. Tohoku Imp. Univ., 11,139 (1922). (7) A. G. Whittaker, J. Chem. Phys., 24,780 (1956). (8) C. M. Steese and A. G . Whittaker, ibid., 24,776 (1956).
Volume P.?,Number 8 August 1968
w.JAMES AND ROBERT c. M A R S H A L L
2964
DAVID
Table I: Esr Results for Nitrogen Dioxide at Low Temperature Temp,
Medium
Ref
Nz04
a
NaO4
a
Ice
1 3
NaNOz a
--.
B values 81
OK
77 77 (irradiated)
77 77
BY
2.0065 2.0048 2.0066 2.0057
1.9960 1 ,9996 1,9920 1.9910
85
2.0029 2 0034 2.0022 2.0015
7--
Bav
B*
2,0018 2,0016 2,0003 1.9994
-5.8 -2.7 -6.28 -5.27
Hyperfine interaction, G--BY B,
-6.4 -5.3 -7.04 -7.95
12.3 8.0 13.33 13.22
Aiso
56.0 59.0 56.88 54.71
Present study.
ulation and a V-2390 variable-temperature attachment. Spectra at 77°K were obtained by immersing the sample in liquid nitrogen in the cavity. The cavity temperature, as indicated by the dial reading on the variable-temperature attachment, changed over 3 months. The cavity was, therefore, calibrated periodically by replacing the esr sample by a standardized thermocouple and plotting the error in indicated temperature against the dial reading. I n order to have a known volume of sample in the field, the esr tube was allowed to extend through the cavity and this gave a 1-cm length of the tube in the field. The signals were standardized against a 0.1% pitch in KC1 sample which was run using dual-cavity operation. Where possible, spectra were run using an attenuation of 25 db in order to ensure the accuracy of band area and to prevent saturation of the sample. Areas of absorption curves were determined by double integration of the firstderivative curve. Because the standard sample was examined at 296"K, the integrated areas at other temperatures were corrected for the Curie dependence of susceptibility. Comparison of the areas with those obtained from a standard sample gave the number of absorbing paramagnetic molecules.
Results Between 293 and 243°K a single esr signal was obtained which had a half-width of 150 G and a g value of about 2.0. This signal showed a marked temperature dependence, decreasing in intensity as the temperaTable 11: Equilibrium Data for the Reaction Nz04 $ 2x02 -Weight Temp, "C
Sone
22.8 13.3 2.6 -7.1 -16.8 -26.5
0.87 0.45 0.19 0.07 0.00
% ' NOzPresent study
0.083 0.045 0.028 0.018 0.010 0.004
--K, Steese and Whittaker=
50.00 18.89 5.76 1.43
rnol/l.-----
Present studyb
43.54 rt 1 . 5 13.40 f 0 . 5 5.13 i 0 . 2 2.22 i 0 . 0 7 0.64 f 0.02 0.11 =I= 0.01
a Data are the average values obtained a t 613 mp, where agreement is best. bError limits indicate the precision of the measurements rather than the accuracy. The inherent uncertainty in the standard should be understood (see the text). ~
The Journal of Physical Chemistry
-.-.I 50 gauss
H
Figure 1. Electron spin resonance spectrum of NO9 a t 77°K.
ture was decreased. The band areas were determined at different temperatures and on different samples, and, after correction for Curie dependence, these allowed the spin concentration and hence the concentration of NO2 to be calculated at each temperature. By extrapolating the density data of Mittasch, et U Z . , ~ it was possible to calculate the concentration of Nz04, and hence the equilibrium constant was accessible. The data are collected in Table 11, which gives earlier values for both for the equilibrium constant and for the wei$ht per cent of NO2 for comparison. The data permitted the calculation of the enthalpy change on dissociation and this was found to be 15.6 * 0.5 kcal/mol. When the sample was cooled below 243°K the signal abruptly disappeared, and as the sample was warmed the signal did not reappear until the temperature was raised to 263°K. This behavior was in contrast to that observed when the sample was not allowed to cool below 243°K when the variation of the signal on heating and cooling cycles was completely reversible and reproducible. This behavior, which indicates that either supercooling of the liquid sample has occurred or a solid-state phase change had occurred at 243°K) will be discussed later. I n order t o investigate the low-temperature behavior of NOz, the radicals were trapped in the Nz04 matrix by quenching a liquid sample from room temperature in liquid nitrogen, the sample being maintained at 77°K for the esr examination. In this way a well-developed anisotropic set of triplets, as shown in Figure 1, was obtained. It was assumed that the NOz molecules were randomly orientated, and the spectrum was treated by Kneubuhl's method,1° as described by Atkins and (9) A. Mittasch, E.Kuss, and H. Schleuter, 2.Anorg. Allgem. Chem., 159,l (1927). (10) K. K. Kneubuhl, J. Chem. Phys., 33,1074(1960).
AN ESRSTUDY OF
THE
N2O4-NO2SYSTEM
Symonsa2 When the derived spectral parameters were used as initial parameters in a computer program which simulated the spectrum, the agreement was very poor. B y adjustment of the parameters, a better fit was obtained using a Gaussian curve, but a complete refinement of the data was not possible. The derived parameters are presented in Table I, which contains data for KO2 in other matrices for comparison. The computer simulations of the curve resulted in significant changes from the parameters obtained by Kneubuhl's method. When the temperature was raised from 77°K the spectrum broadened and became more difficult to observe, until at about 180°K it could not be discriminated from the background. When the sample was recooled the signal was recovered unchanged. I n order to increase the radical concentration, the sample was irradiated at 77°K for 4.5 hr using a high-pressure mercury lamp. This resulted in an increase in signal level by a factor of 20; there was a change in derived constants as shown in Table I. When the irradiated sample was warmed the spectrum changed as previously, until at 170°K the signal had broadened into an isotropic triplet with a hyperfine splitting of 57 G. This triplet was observed unchanged in form but with decreasing intensity to 210"K, and as before the original signal was regained on cooling to 77°K. The signal intensity observed at 77°K was about 60 times as great as that observed at 190°K. When the sample was warmed to 237°K no signal was observable, and when this sample was recooled to 77°K there was no signal, indicating that radical annihilation had occurred at the higher temperature.
Discussion (i) Liquid State. The spectrum obtained in this study is closely similar to that found in other studies of N204 dissolved in organic solvents and there seems no doubt that it is due to NOz species. There is uncertainty as to the state of aggregation of the sample below 261.8"K, where Yz04freezes. l 1 If solidification took place at this temperature, the dissociation reaction is independent of whether the material is liquid or solid, which seems unlikely. It has been reportedI2 that N204may be readily supercooled by 30", and, in view of the lack of agitation and the small bore of the esr tube, it is considered that measurements between 262 and 238°K were carried out on a supercooled liquid. Determination of the freezing point was carried out using a sample of several milliliters volume, and a freezing point of 261°K was obtained. In addition no evidence for a phase transition at 238°K could be obtained by calorimetry, although this should be verified by the use of a differential scanning calorimeter. The equilibrium data presented in this paper show marked differences from those previously reported. The enthalpy change of 15.6 =t0.5 kcal/mol may be compared with the earlier value of 19.5 kcal/mol.*
2965 Although the esr technique has some inherent uncertainties, they will not be significant in this determination, which has an uncertainty of about 3%- The determinations of Steese and Whittaker utilized the structure near the low-energy tail of the ultraviolet absorption of SOz. Such measurements are of uncertain accuracy, and this is reflected in the variation of values obtained at different wavelengths by these workers. In the estimation of the equilibrium constant, use is made of a standard sample of 0.1% pitch in KC1 which has 3 X 1016 spins/cm (rt25%). The uncertainty in spin concentration appears large and is reflected in a corresponding uncertainty in the concentration of NOz and in the equilibrium constant. However, since the spin concentration in the standard sample is fixed, the equilibrium data are internally consistent and certainly represent the best available results. There arc considerable differences between previously published data for the equilibrium constant and the concentrations of NO2 and those reported here. The NOz concentrations reported by Sone6 were obtained from magnetic susceptibility measurement. The variation in the susceptibility over the range of measurements was A x = 0.32 and the uncertainty in the measurement was *O. 11. Hence the experimental uncertainty could account for 30% of the measured change. This could lead to an error in X02 concentration of more than 100% at low concentration. The equilibrium constants previously reported are derived from measurements on the low-energy tail of the ultraviolet absorption band of NOz. As indicated above these values arc of low reliability. It is evident that although the standard samples used for calibration have large uncertainty, the esr technique is capable, in systems such as this, of yielding data of greater reliability and accuracy than other techniques. The equilibrium data for the dissociation of Xz04 in the liquid phase presented here are thus to be preferred to those previously reported. (ii) Solid State. The behavior of trapped NO2 molecules at low temperature gives some insight into the nature of the low-temperature solid phase. When NzO4 is quenched it forms a glass and so the trapped NO2 molecules will not be orientated by the crystal field. It is this random orientation of NO2 molecules which allows Kneubuhl's method'* to be applied. The spectrum of the quenched sample resembles previous workers' results very closely, and the differences may be attributed to the effect of the different matrix. There were small variations noted in peak positions and peak intensities when the same sample was examined on separate occasions. This may be due to some measure of preferred orientation in the matrix or to inter(11) C. C . Addison, Angew. Chem., 6,193 (1960). (12) G . M . Begun and W. H. Fletcher, J . Mol. Spectrosc., 4 , 388 (1960).
Volume 7t,Number 8 August 1968
2966
CECILM. CRISSAND EUGENE Lu~saa
molecular interactions due to different distributions of spin-free species. The remarkable decrease in intensity of the signal as the sample is warmed is not well understood. The marked tendency of the N204molecule to form adducts may result in the unpaired electron of the NOz molecule becoming delocalized over one or more of the matrix molecules. Such an electron exchange would result in the broadening and apparent weakening of the signal. The conversion from an anisotropic to an isotropic triplet is more easily rationalized. As the temperature of the matrix rises, the trapped NO2 molecules become free to rotate and so all orientations are averaged; this indicates that the trapping sites are quite large. It has recently been shownIa that the rotation of the NO2 species is not an isotropic rotation but rather has rotation about one axis only (the 0-0 axis). This has little effect on the volume requirement for the rotation. However, the reduction in temperature yields the original spectrum unchanged, and so this free rotation is not accompanied by rapid diffusion with consequent annihilation. Such dimerization does not
occur until the temperature is raised to 238°K. Little is known of the solid-state changes with temperature, and so it is not possible to decide whether the dimerization of the trapped NO2 occurs because of lattice expansion or because of a phase transformation. The spectrum obtained after the sample had been irradiated with uv light shows considerable variation from the unirradiated quenched sample. This is reflected in the different parameters derived (Table I). Since the irradiation took place in the solid state, it is likely that most of the radicals were generated near the surface. The considerable increase in intensity indicated that the surface concentration of radicals was high and thus interaction between radicals would be likely. The presence of such interactions invalidates the use of Kneubuhl’s method, as the orientation of radicals could no longer be considered random.
Acknowledgment. The authors thank Dr. J. O’Donnell and Mr. M. J. Bowden for useful discussions during the progress of this work. (13) T. J. Schaafsma and J. Kommandeur, Mol. Phys., in press.
Thermodynamic Properties of Nonaqueous Solutions. IV.
Free
Energies and Entropies of Solvation of Some Alkali Metal Halides in N,N-Dimethylformamidel by Cecil M. Criss2 Department
of
Chemistry, University of Miami, Coral Gables, Florida 38124
and Eugene Luksha Department of Chemistry, University of Vermont, Burlington, Vermont OS4Oi
(Received March 4 , 1968)
Solubilities of LiF, YaF, NaCl, KCl, CsC1, CsBr, AgRr, and AgI in N,N-dimethylformamide have been measured and the free energies of solution and free energies of formation of the respective electrolytes in N,Ndimethylformamide have been calculated. Plots of the free energies and entropies of solvation of the alkali metal and halide ions vs. l/(ri 6) show a linear relationship, as predicted by the modified Born equation, when 6 = 0.85 A for cations and 1.00 A for anions. The entropies of solvation have been used to evaluate the absolute partial molal entropies of individual ions in DMF.
+
I. Introduction a recent series of
thermodynamic properties of several electrolytes in N-methylformamide 182.4) and N,N-di(NMF) (dielectric constant E methylformamide (DMF) ( e = 36.7) have been reported. In continuation of obtaining thermodynamic The Journal of Physical Chemistry
papers,8-5
data for these solvent systems, the solubilities of several electrolytes in D M F have been determined and used t o (1) This paper was taken in part from the work submitted by E. Luksha to the Graduate School of the University of Vermont in partial fulfillment of the requirement for the Degree of Doctor of Philosophy, (2) TO whom correspondence should be directed.
