600
J . Phys. Chem. 1985, 89, 600-606
dissociated oxygen will also increase hydrocarbon ~ x i d a t i o n ~ * ~ ~ ~ ~effects ' @ of the individual reactant and product molecules. Careful thermal etching studies with each individual reactant and hetaccelerating carbon deposition in fuel-excess conditions. It is also erogeneous reaction product gas showed that no individual gas possible that the radical species in the gas-phase independently produced more than faceting and minor amounts of pitting on enhances the coking rate. For example, the methylene radical the platinum surface. In contrast, during catalytic etching may simply decompose leaving carbon and hydrogen or may platinum particles, or platinum-containing carbon layers, formed induce surface polymerization or even polycyclization of adsorbed on the surface. Furthermore, it was found that the nature of the ethylene to produce higher hydrocarbons. It is clear that these same mechanisms may operate in region catalytic etching was a strong function of surface temperature 11 as well. However, in that excess oxygen region all surface and ethylene to oxygen ratio in the gas phase. Indeed, a phase carbon will quickly be burned away. diagram of the type of catalytic etching was developed showing Region V. Catalytic etching in this region produces a thin five regions on the plane defined by surface temperature and (relative to region IV) graphitic layer. The apparent structure oxygen to ethylene ratio and three types of catalytic etching. A of this layer, however, is a function of the pretreatment temnovel model was developed which explains all the regions of the phase diagram. In brief, the catalytic etching of platinum during perature. Tentatively, the carbon deposition in this region is attributed ethylene oxidation was attributed to the formation of methylene to the incomplete oxidation of hydrocarbons. The layer is thinner radicals above about 770 K in the above boundary layer above than in region IV, probably because there are no platinum particles the catalyst. This model is consistent with all the observations formed and also possibly because there are no radical species made in this study and the salient features of the model have precedents in the literature. However, the work performed here produced above 970 K. The change in structure of the surface layer as a function of is not sufficient to prove this model and further study, already underway, is required. pretreatment probably results from the difference in the initial platinum surface structure. Acknowledgment. The authors thank the Department of Summary. It has been shown that the catalytic etching of Chemical Engineering at The Pennsylvania State University and platinum during ethylene oxidation is not the sum of the etching the donors of the Petroleum Research Fund, administered by the American Chemical Society, for supporting this research. (67) Lang, B.; Joyner, R. W.; Somorjai, G . A. Surf. Sci. 1972, 30,440. Registry No. Pt, 7440-06-4;C2H4, 74-85-1. (68) Carberry, J. J. Kinei. Katal. 1977, 18, 562.
Sodbm-23 Nuclear Magnetic Resonance Study of Sodium Bromide in Methylamine Solutions That Contain Macrocyclic Polyethers Richard C. Phillips,+Sadegh Khazaeli,* and James L. Dye* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: July 23, 1984)
Sodium-23 NMR spectra of NaBr in methylamine with and without various amounts of 18-crown-6 or 15-crown-5(C) were obtained as functions of temperature. The data were best described by a two-site model in which both the solvated and the complexed sodium cation are completely ion-paired according to Na+,Br- + C
k k-i
Na+C,Br-
For C = 15-crown-5, variation of the chemical shifts with temperature and mole ratio gave (KL)298 = 5.4 f 0.2, AHlo = -2.2 & 0.1 kcal mol-', and PSIo= -3.9 f 0.4 cal mol-' K-'. The exchange rate with this complexant was too fast to measure by the NMR technique. For complexation by 18-crown-6, (K1)298= 220 f 80, MIo= -5 f 3 kcal mol-', and AS1' = -5 f 3 cal mol-' K-I. The dependence of transverse relaxation times, T,, with mole ratio and temperature below -215 K yielded (k1)298= (1.2 f 0.6) X lo8 M-I s-' with AHl* = 1.1 f 0.8 kcal mol-' and ASl* = -18 f 3 cal mol-' K-I.
Introduction The solubility of sodium metal in amine and ether solvents is greatly enhanced by the addition of cryptands or crown The presence of excess metal gives equimolar amounts of the complexed cation, Na+C, and the sodium anion, Na-, in solution. Even when the complexant is present in excess, so that all of the metal dissolves, the concentration of solvated electrons as second counterions is very small. The =Na N M R spectra of sodium metal in methylamine solutions in the presence of either 18-crown-6 (18C6)e7 or 15-crown-5 (15C5)4*7consist of broad Na+-crown Department of Chemistry, Youngstown State University, Youngstown, OH 44555. At Michigan State University on a Faculty Improvement Leave, 1978-1979.
*Presentaddress: Department of Chemistry, Southern Illinois University, Edwardsville, IL 62026.
0022-3654/85/2089-0600$01.50/0
ether and narrow Na- bands at low temperatures which coalesce to a single band at higher temperatures. In order to study the exchange mechanism between Na+C (C = crown ether) and Na-, as described in a companion paper, it was first necessary to estimate the complexation constants and (if possible) the rate constants for the complexation reaction ( 1 ) Dye, J. L.; DeBacker, M. G.; Nicely, V. A. J . Am. Chem. SOC.1970, 92, 5226. (2) Lok, M. T.; Tehan, E. J.; Dye, J. L. J . Phys. Chem. 1972, 76, 2975. (3) Dye, J. L. In "Progress in Macrocyclic Chemistry"; Izatt, R. M., Christensen, J. J., Eds.; Why-Interscience: New York, 1979; Vol. 1, Chapter 2. (4) Andrews, C. W. M.S. Thesis, Michigan State University, 1975. (5) Dye, J. L.; Andrews, C. W.; Ceraso, J. M. J . Phys. Chem. 1975, 79, 3076. (6) Smith, P. Ph.D. Dissertation, Michigan State University, 1978. (7) Phillips, R. C.; Khazaeli, S.; Dye, J. L., following paper in this issue.
0 1985 American Chemical Society
Sodium Bromide in Methylamine Solutions
+
Na+ C Na+C (1) The complexation constants of solvated Na+ with 15C5, 18C6, and various disubstituted 18C6 complexants have been measured in a variety of solvents:-ls and some exchange rate constants have also been determined.I6+l7 Andrews4 measured chemical shifts and transverse relaxation rates ( l / T z ) for N a I in methylamine with and without added 18C6 or 1925, but the data were too sparse to permit determination of rate and equilibrium constants. At the metal concentrations used in the exchange study,7 0.1-0.25 M, ion pairing is probably nearly complete. For example, previous with Cs+, 18C6, and the anions I-, SCN-, and BPh4- yielded ion association constants that predict > l/TZA. The relaxation rates for Ro = 13.0 (not shown in Figure 4) were larger than the values of l / T z B based upon the computer fit of all the data. Since the volume percentage of 15C5 in that solution was 20%-30%, these larger relaxation rates may be attributable to the higher viscosity of the solution. Model for Both 18C6 and 15C5 Solutions. A model that is compatible with all of our data is a two-site model in which Na+ and Na+C are essentially completely ion-paired Na+,Br(site A)
34
38
42
46
50
54
1 0 5 (KI)
Figure 3. Sodium-23 chemical shifts vs. reciprocal temperature for me-
thylamine solutions that contain NaBr and 15C5. The solid lines are calculated on the basis of fast chemical exchange. The probabilitiesp s are the calculated values at 20 O C , and Ro is given in parentheses. shifts at low temperatures is difficult. NaBr with 15-Crown-5 in Methylamine. Chemical shifts and relaxation rates for 16 NaBr solutions containing 15C5 were measured over a wide temperature range (179-298 K). The initial NaBr concentrations ranged from 0.0378 to 0.0946 M, and Ro was varied from 0.254 to 13.0. The chemical shift as a function of temperature for some of these solutions is shown in Figure 3. The solid lines represent the computer fit of the data for fast
kl +Ce Na+C,Brk - ~ (site B)
(4)
where C is 18C6 or 15C5. Solvent has not been explicitly included in the representations of the ion pairs in eq 4; however, the ion pairs are probably mainly solvent separated, at least in site A.1s319 For fast exchange the chemical shifts and relaxation rates are given by 6 = P A ~ A+ P B ~ B
(5)
and
/ T2
= PA/
T2A
+ PB/
T2B
(6)
(31) For 15C5 solutions it was necessary to reduce the initial Naf concentration at larger Ro values to avoid changes in the solvent properties because of high concentrations of 15C5. Therefore, identifying a particular solution bypBat a particular temperature (20 "C)is more informative than identifying it with Ro.The pBvalues at 20 OC are the computer-calculated values from the data analysis.
604
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
where the subscripts designate sites A and B. The complexation constant is given by Kl =
[Na+C,Br-]
[Na+,Br-] [C]
= (eaI0/R)(e-Ml"/R3
(7b)
which leads to the following expression for the chemical shift
where 6 and [Na+Ioare experimental chemical shifts and the initial NaBr molar concentrations, respectively. A fit of this equation , AS1' to the data with adjustment of the parameters BB,M I oand is preferable to the determination of K I at each temperature from the temperfollowed by the evaluation of M l 0and SIo ature dependence of K1. Only three parameters are required, and eq 8 can accommodate variability in [Na+Ioand in T from sample to sample. The experimental chemical shifts were used together with the for solutions two-site model to determine bg, AHlo, and Sl0 containing either 18C6 or 15C5 (see Table I). The chemical shift of the Na+,Br- peak, was assumed to be the same as that for solutions containing only NaBr. For 18C6 solutions the complexation constant is so large at low temperatures that only data in the +25 to -25 "C temperature range could be used. Even in this range, the decrease in the chemical shift with decreasing temperature is not large so that uncertainties in the various parameters obtained from the data are relatively large. For solutions with RoC 1, chemical shifts at temperatures below -25 OC could not be used because the fast-exchange approximation is not valid. For 15C5 solutions the complexation constants were low enough and the exchange rates large enough to permit use of all of the data. Because the uncertainties in M I o and A S l 0 for the 18C6 systems are large, there is no significant difference of these parameters for 18C6 solutions compared with those obtained when 15C5 was used. However, because of the correlation between MIo and ASl", the values of Kl at a particular temperature have far less uncertainty and the complexation constants with 18C6 are nearly 2 orders of magnitude larger than those with 15C5. The parameters obtained from the chemical shift data, the experimental relaxation rates for solutions containing only NaBr, and the two-site model were then used to calculate the relaxation rates as functions of Ro and temperature for the fast-exchange region. Table I shows the parameters obtained from the computer fit of the experimental relaxation rates. For 18C6 solutions, because of the effect of exchange, only the data from +25 to -25 "C were used to determine the various parameters. The parameters calculated for 15C5 solutions fit the experimental relaxation rates better than is the case for 18C6 solutions because of the extended fast-exchange region and, consequently, the larger array of data which could be used (194 points compared with 57 for the 18C6 solutions). Relaxation Mechanism for Na'C. The activation energies for relaxation of the Na+C species are considerably larger than expected on the basis of solvent reorientation only. They are about the magnitude expected for rotational diffusion of the Na+C (or Na+C,Br-) complex ion. Another possible relaxation mechanism is the reorientation of the crown ether within the Na'C complex ion.32 The latter mechanism appears to be less likely than the rotational diffusion mechanism because the activation enthalpy for complexation is only 1.1 kcal mol-' (see Table 11). As complexation probably represents an extreme conformational change of the crown ether, additional conformational changes within the complex ion would be expected to have even smaller activation ( 3 2 ) Shizuka, H.; Takada, K.; Morita, L. J . Phys. Chem. 1980, 84, 994
Phillips et al. energies. The relaxation activation energies reported by Andrews4 for NaI solutions that contain 18C6 or 15C5 are similar to ones we have found for solutions that contain NaBr (Table I). The relaxation rate at a particular temperature for 18C6 solutions is approximately 10 times that for 15C 5 solutions at the same temperature (except for 18C6 solutions at lower temperatures and Ro values where the experimental band is an exchangebroadened Naf,Br- band). Line widths of 18C6 solutions for which Ro> 1 are nearly 4000 Hz at -80 "C. Solid-state X-ray data for Na'(l8C6)SCN- indicate that the structure of this salt is a pentagonal bipyramid with an intimate coordination of a solvent molecule and a greater Na+-ether oxygen distance axially above the pentagonal plane than in the plane.33 If a similar structure is present in methylamine solutions which contain the Naf-18C6 species, a large electrical field gradient at Na' is expected. This would result in substantial quadrupolar broadening. By contrast, 15'25 is expected to form a nearly planar Naf-l 5C5 complex with nearly equal distances between ether oxygen atoms and with solvent molecules occupying the axial positions. This would lead to less asymmetry than is found in the Na+-18C6 species and hence narrower lines. Kinetic Parameters for 18C6 Solutions. For a two-site exchange mechanism (eq 1) the rates of leaving sites A and B are
and ~ / T B=
k-1
(10)
where kl and kTlare the complexation and decomplexation rate constants and [C] is the equilibrium molar concentration of the crown ether. The correlation times T~ and rBrepresent the residence times of Na+ in sites A and B, pBis the fraction of Na+ in site B (complexed), and the other terms have already been defined. As the relaxation rates for 18C6 solutions at low temperatures with & C 1 are compatible with an exchange-broadened Na+,Br- band, these rates may be used to determine the rate constants for the reaction. Shchori and c o - w ~ r k e r s ~used ~ , ' ~the Bloch-McConnell equations to derive the following expression for l / for~ a two-site ~ model in which the chemical shift between sites A and B is negligible compared with the relaxation rates
in which 1/T2 is the experimental relaxation rate of the exchange-broadened band for site A and ( 1/T2) is the average relaxation rate for the fast exchange extrapolated to lower temperatures where exchange is slow. Although the chemical shift difference between the magnetic enviroments A and B, bAB, is relatively large in our case, 16.2 ppm (AwAB = 1600 s-'), the relaxation rates for 18C6 solutions at Ro C 1 in the slow-exchangeregion were used with eq 11 to determine 1 / ~ * .The values of 1 / ~ *obtained from eq 11, together with the appropriate chemical shifts and relaxation rates previously obtained, were then used with the complete Bloch-McConnell equations to simulate N M R spectra in the slow-exchange region. The simulated spectra consist of a broad, relatively intense band and a very broad band which only appears as an asymmetry in the more intense band. Relaxation rates determined from the more intense band of the simulated spectra as well as those calculated from eq 11 agree with the experimental values reasonably well for solutions in which RoC 0.7 (see, for example, Figure 2). For Ro > 0.7, the effects of chemical exchange on the relaxation rates are minimal except at the very lowest temperatures, and in this region the observed rates tend to be somewhat lower than those calculated, probably because of deviations from Lorentzian line shapes. ( 3 3 ) Duritz, J. D.; Dobler, M.; Seiler, P.; Phizackerley, R. P. Acta Crystallogr., Sect. B 1974, B30, 2133.
Sodium Bromide in Methylamine Solutions
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985 605
TABLE II: Kinetic Parameters for Comolexation of Sodium Ions bv Crown Ethers'
NaBr NaFF NaBPH4 NaCNS NaCNS NaCNS NaCNS NaCNS
18C6 MeDBC DBC DCC (IB) DBC DBC NDBC AmDBC
MeNHz THF DMF MeOH MeOH DMF DMF DMF
1 X lo*
4.3 x 105
1.1 f 0.8
-18 f 3
5.9
-13
12.5 6.5 x 107 2.6 X 10' 3.2 X lo8 6 X lo7 2.3 x 107
1.2 x 107
1.5 x 104 5.2 X lo4 1.4 X lo4 1 x 105 2.0 x 105
9.9 2.7
1.9 x 105
7.1
-6.2 -28
-14
13.5
8.3 11.7 12.6
12.5 -1 1
13.1
22 7.8 19 19 17 22
'The kinetic parameters for methylamine solutions were determined from the present studies. All other values were taken or calculated from data given in ref 17, Table 111. bKey: MeDBC = dimethyldibenzo-18-crown-6,DBC = dibenzo-18-crown-6,DCC (IB) = dicyclohexano-18-crown-6 isomer B, NDBC = dinitrodibenzo-18-crown-6, AmDBC = diaminodibenzo-18-crown-6.CFluorenylsodium salt. d A t 25 OC. By use of the low-temperature relaxation rate data for Ro < 0.7, the exchange parameters AHI*and A S l * were estimated (Table 11). The activation enthalpy is only about 1 kcal mol-', not much larger than that expected for simple solvent exchange of Na+. Shchori and co-workers17have measured and compiled activation energies for the decomplexation of several disubstituted 18C6 species from Na+. The activation energies for decomplexation of dibenzo-18-crown-6 and substituted dibenzo-18-crown-6 complexes are all approximately 12 kcal mol-' and are independent of solvent. The decomplexation activation energy for dicyclohexano-18-crown-6 is 8.3 kcal mol-'. These authors have attributed the lower activation energy for dicyclohexano-18-crown-6 compared to the dibenzo-18-crown-6 reagents to the greater flexibility of dicyclohexano-18-crown-6molecule. The activation enthalpy for decomplexation from 18C6 obtained in the present study is 5.9 kcal mol-'. Our data are in accord with their explanation as 18C6 is expected to be even more flexible than dicyclohexano- 18-crown-6. The solvents used in their studies generally have larger dielectric constants than methylamine. In our system the complex ion is completely ion-paired and possibly the complexed Na+ is also intimately in contact with the solvent. Therefore, the decomplexation of Na+C may be more complicated than simple distortion of 18C6 to allow Na+ solvation. The activation entropy for complexation, ASl*,is -18 f 3 cal mol-' K-I, which with ASIo = -5 f 3 cal mol-' K-' gives an activation entropy for decomplexation of -13 f 6 cal molT1K-' compared with values of +20 and +7.8 cal mol-l K-' for dibenzo-18-crown-6 and dicyclohexano- 18-crown-6, respectively. The transition state for 18C6 appears to be more ordered relative to the complex than is the case with substituted 18C6 complexants. Utility of 18C6 and 15C5for Dissolving Sodium Metal. A major purpose of the present study was to determine the kinetic and thermodynamic properties of complexation of Na+ with two crown ethers so that these properties could be used to determine the electron transfer mechanism and exchange rate for methylamine solutions which also contain sodium metal. Essentially completely ion-paired Na+,Br- and Na+C,Br- species are the dominant species in the solutions used in the present study. As ion pairing may significantly affect the magnitude of the chemical shifts and relaxation rates, and thus the parameters generated from these measurements, it is uncertain whether the parameters necessary for electron exchange studies can be directly determined from studies with salts. In methylamine solutions that contain a crown ether and sodium metal the anion is primarily the sodium anion, Na-. The ion-pairing constant for Na- with Na+18C6 may be significantly different from that of Br-. We have measured the 23NaN M R spectra of methylamine solutions which contain sodium metal and 18C6.7 The relaxation rates of the Na+C species at lower temperatures in the metal solutions are significantly smaller than those found for NaBr-18C6 solutions in this work. This shows that Br- has a larger effect on the relaxation rate of Na+C than does Na-. However, the important comparison is between Na+,Br- and Na+C,Br- on the one hand and Na+,Naand Na+C,Na- on the other hand so that the effect of differences in ion-pair formation constants on the complexation constant will tend to cancel. In addition, the dominant relaxation mechanism for Na+C in the two solutions may be different.'
The equilibria that appear to be compatible with the data of the present study are
+ Br- Na+,BrNa+ + C & Na+C Na+C + Br- & Na+C,BrNa+
(12) (13) (14)
where the notation of Khazaeli, Popov, and Dye18*19 is used for the equilibrium constants. The equilibria 12-14 lead to the following expression for the equilibrium constant of eq 4: These author^'^^'^ have found that K 1 = Kc, that is, the complexation between Na+ and the crown ethers is independent of whether ion pairing occurs as long as KA K I P . If KA and K I P are also equal in metal-18C6 solutions, the complexation constant K 1 for these solutions may be similar to the same constant for salt-18C6 solutions even if K A and K I Pfor metal solutions differ from these constants for salt solutions. At -20 "C, the typical temperature used for dissolving Na metal in methylamine solutions, the complexation constants K , and (presumably) Kc are 920 and 10 M-' for 18C6 and 15C5 solutions, respectively. This suggests that 15C5 would not be a particularly effective complexant for increasing the solubility of sodium metal in methylamine. This is in accord with our observation that sodium is not very soluble in methylamine in the presence of 15C5.'
Summary and Conclusions 23Na chemical shifts and relaxation rates for methylamine solutions containing NaBr and either 18C6 or 15C5 were measured. They are compatible with a simple, two-site model for the complexation of the crown ether with Na+(solv) in which both the complexed and uncomplexed Na+ are ion-paired with Br-. The exchange between the two sites is, in general, fast on the N M R time scale. However, the exchange rate for 18C6 solutions is slow enough at low temperatures to be measured. Where the slowexchange approximation is valid, the spectrum consists of an exchange-broadened Na+,Br- band and a very broad Na+C,Brband that is unobservable. The chemical shifts were used to determine AHl", Aslo, and dB for both kinds of solutions. AH,' and ASloare negative for both kinds of solutions as is typical for the complexation of metal ions by crown ethers in other The complexation constants at 25 O C are 220 and 5.4 M-I for 18C6 and 15C5 solutions, respectively. The complexation constant K I is not large enough to provide complete complexation at all temperatures for solutions that contain either 18C6 or 15C5. For solutions with 18C6, complete complexation (PB2 0.99) does not occur at 20 OC even at the largest value of Ro (R,= 2.96); however, the complexation is complete enough that the chemical shifts and relaxation rates are (34) Christensen, J. J.; Eatough, D. L.; Izatt, R. J. Chem. Reu. 1974, 74, 351.
606
J . Phys. Chem. 1985, 89, 606-612
essentially equal to the limiting values and 1/ T 2 for ~ Ro L 1.50 even at the highest temperatures studied. At lower temperatures complexation is essentially complete at all values of Ro. Complexation of Na+(solv) with 15C5 is not complete at any temperature or R , value used; however, p B is 0.96 at the lowest temperature and largest Rovalue ( R , = 13.0). For 18C6 the exchange rate is slow at the lowest temperatures for Ro < 0.7. For R , in the range from 0.7 to 1.0 at lower temperatures, the exchange rate is intermediate. The low-temperature data for which Ro < 0.7 were used to determine values of AHl * and AS1* of 1.1 kcal mol-' and -1 8 cal mol-' K-I, respectively. The activation enthalpy is similar to that expected for simple solvent exchange and considerably smaller than the values reported by other authors for complxation of Na+(solv) by the more rigid, substituted 18-crown-6 complexes in other solvent^.'^ As 18C6 is more flexible than the substituted 18C6 species, it may more easily complex Na+(solv) and require a smaller activation enthalpy.
The activation entropies for complexation, AS1*,and decomplexation, A X l * , are -18 and -13 cal mol-' K-l, respectively. Activation entropies determined from previous studies with substituted crown ethers are negative for complexation and positive for de~omp1exation.l~Possibly the greater flexibility of 18C6 allows for a more ordered transition state. The activation energies for relaxation of Na+C for 18C6 and 15C5 are not significantly different and are considerably larger than expected for a solvent reorientation relaxation mechanism but are of the right magnitudes for relaxation by rotational diffusion of the Na+C (or Na+C,Br-) species. Acknowledgment. Partial financial support for this research was provided by N S F Grant No. DMR 79-21979. Youngstown State University partially supported R.C.P. through its Faculty Improvement Leave Program. Registry No. Na, 7440-23-5;NaBr, 7647-15-6; 18-crown-6, 1745513-9; 15-crown-5, 33100-27-5.
Sodium-23 Nuclear Magnetic Resonance Study of Sodium Metal in Methylamine Solutions That Contain 18-Crown-6. Kinetics of Sodium Cation-Sodide Ion Exchange Richard C. Phillips,+ Sadegh Khazaeli,t and James L. Dye* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: July 23, 1984)
Sodium-23NMR spectra of Na+-l8-crown-6,Na- (Na+C,Na-) solutions in methylamine were studied as functions of temperature and mole ratio of crown ether to sodium. A broad band of Na+C and a narrow band of Na- were observed at low temperatures, but coalesced into a single band at -6 to +5 OC depending upon the ratio of dissolved sodium to free crown ether and exhibited exchange narrowing as the temperature was increased further. Although the data could be quantitatively fit to the simple mechanism Na+ + C Na+ + Na-
k-1
Na'
k
& Na- + Na+
the ionic species probably exist largely as ion pairs at the concentrations used. A modified mechanism that fit the data was k
Na+,Na- + C & Na+C,Nak-1
Na+,Na- + Na+C,Na-
& Na+,Na- + Na-,CNa+
in which the first equilibrium involves cation complexation but no electron exchange while the second involves exchange of Na+C with Na-. Neither cation-anion exchange within the ion pairs Na+,Na- and Na+C,Na- nor cross exchange between two Na+C,Na- species contributed significantly to the exchange rate. Exchange broadening of the Na- band was used to determine the apparent rate constant k 2 / K 1= (1.0 i 0.2) X lo5 s-' at 25 OC with AH*,,, = 19 & 1 kcal mol-' and AStapp = 27 & 3 cal mol-' K-I. By using the value of K 1 obtained for NaBr solutions in methylamine, we calculated approximate values of k2 = (2.4 f 0.8) X lo7 M-I s-', AHz* = 14 & 4 kcal mol-I, and ASz* = 22 & 6 cal mol-' K-I. Several unsuccessful attempts were made to obtain similar data with sodium and 15-crown-5. Although broad Na+C and narrow Na- bands were observed at low temperatures, rapid decomposition prevented study of the exchange kinetics in this case.
Introduction Previous 23Nastudies have been made with sodium metal solutions that contain either cryptand [2.2.2], (C222),'v2 18-crown-6, (18C6),293or 15-crown-5, (15C5).3 The crown ether data were obtained in methylamine solutions whereas three different solvents including methylamine were used for the cryptand studies. The Department of Chemistry, Youngstown State University, Youngstown, OH 44555. At Michigan State University on a Faculty Improvement Leave, -1978-1 - . - - 979. - . ..
f Present address: Department of Chemistry, Southern Illinois University, Edwardsville, IL 62026.
0022-3654/85/2089-0606.%01.50/0
low-temperature NMR spectrum for each of these systems consists of a broad band associated with the Na+-ligand complex (Na+C) and a narrow band associated with Na-. Edwards and co-workers have observed the Na- band in solutions prepared from Na metal and either hexamethylphosphoric triamide4 or N,N-diethylacetamideSin the absence of any complexing agent. In all cases the (1) Ceraso, J. M.; Dye, J. L. J . Chem. Phys. 1974, 61, 1585. (2) Dye, J. L.; Andrews, C. W.; Ceraso, J. M. J . Phys. Chem. 1975, 79, 3076. ( 3 ) Andrews, C. W. M.S. Thesis, Michigan State University, 1975. (4) Edwards, P. P.; Guy, S.C.; Holton, D. M.; McFarlane W. J . Chem. SOC.,Chem. Commun. 1981, 1185.
0 1985 American Chemical Society
