4822
J . Phys. Chem. 1985, 89, 4822-4824
Kinetlcs of Complexatlon of the Cesium Ion with Large Crown Ethers in Acetone and in Methanol Solutions Bruce 0. Strasser, Mojtaba Shamsipur, and Alexander I. Popov* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: January 25, 1985)
The kinetics of complexationof the Cs+ ion with the ligands dibenzo-21-crown-7(DB21C7) and dibenzo-24-crown-8(DB24C8) were studied in acetone and in methanol solutions by cesium-133 NMR. In all systems, the predominant mechanism of exchange between the solvated and complexed Cs+ ion sites is the bimolecular process. The kinetic parameters for the exchange have been determined. For Cs+.DB21C7 in acetone at 220 K, E , = 8.7 (0.5) kcal-mol-', AH* = 8.1 (0.5) kcalmol-l, AS* = -2.7 (2.5) eu, AG* = 8.7 (0.2) kcalmol-I, and k = 9.7 (3.0) X lo3 M-l s-l. For Cs'sDB21C7 in methanol at 220 K, E , = 6.6 (0.3) kcalmol-I, AH* = 6.0 (0.3) kcal.mol-l, AS* = -10.5 (1.5) eu, AG* = 8.3 (0.1) kcal-mol-', and k = 2.7 (0.4) X lo4 M-' s-'. For Cs+.DB24C8 in acetone at 220 K, E , = 7.3 (0.3) kcalmol-I, AH* = 6.7 (0.3) kcal-mol-', AS' = -5.0 (1.5) eu, AG* = 7.8 (0.1) kcalmol-', and k = 7.4 (1.0) X lo4 M-l s-I. For Cs'qDB24C8 in methanol at 220 K, E, = 3.5 (0.5) kcal.mol-', AH* = 2.9 (0.5) kcalmol-I, AS' = -23.2 (2.5) eu, AG' = 8.0 (0.2) kcal-mol-', and k = 5.6 (2.0) X lo4 M-' s-l. With both crown ethers, the free energy barrier for the exchange process appears to be independent of solvent.
Introduction We have commented previously' that studies of the kinetics and mechanisms of macrocyclic complexation reactions are rather sparse as compared with the thermodynamic data. In particular, little is known about the influence of solvent properties, or of the counterions, on kinetic processes. Apart from the valuable chemical information one can obtain from such studies, they may also provide a better insight into the mechanism of ion transport through biological membranes. Recently we reported',2 a correlation between the solvating abilities of solvents, as given by the Gutmann donor number^,^ and the free energy barrier for the decomplexation step for those systems in which the dissociative exchange mechanism is predominant. However, in those systems in which the bimolecular exchange mechanism predominates, the free energy barrier for the decomplexation step is essentially solvent independent. The bimolecular exchange mechanism has also been observed in several solvents to be predominant for the complexation reaction of the potassium ion with crown ether 18-crown-6 (18C6). The free energy barrier for the decomplexation step is also essentially solvent-inde~endent.~ Complexation kinetics of the cesium ion with 18C6 were previously studied in propylene carbonate5and in pyridine6 solutions. In both cases the dissociative mechanism was assumed to be dominant. We report a study of the kinetics of complexation of the Cs' ion with the crown ethers, dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) in acetone (AC) and in methanol (MeOH) solutions. Experimental Part Cesium thiocyanate (Pflatz and Bauer) was dried under vacuum at 60 "C for at least 2 days. Methanol (Fisher) was refluxed over sodium methoxide for 1 day. Acetone (Fisher) was refluxed over calcium sulfate for 2 days. The solvents were fractionally distilled, and the middle 60% of the distillates was kept and stored over activated molecular sieves. Water content was less than 100 ppm, as determined by gas chromatography. Purification of 18C6 has been described previously.' Cesium-133 NMR measurements were carried out on a Bruker WH-180 spectrometer at a field of 42.27 kG and a frequency of 23.62 MHz. Cesium-I33 chemical shifts are corrected for bulk (1) Strasser, B. 0.; Hallenga, K.; Popov, A. I . J . Am. Chem. SOC.1985, 107, 789. (2) Strasser, B. 0.; Popov, A. I., in press. (3) Gutmann, V.; Wychera, E. Inorg. Nucl. Chem. Letr. 1966, 2, 247. (4) Schmidt, E.; Popov, A. I . J . A m . Chem. SOC.1983, 105, 1873. (5) Mei, E.; Popov, A. I.; Dye, J. L. J . Phys. Chem. 1977, 81, 1677. (6) Mei, E.; Dye, J. L.; Popov, A. I. J . A m . Chem. SOC.1977. 99, 5308.
0022-3654185 12089-4822$01.50/0
TABLE I: Mean Lifetimes as a Function of Temperature for CsSCN-Crown in Acetone Solutions" T,c OC 1047,s crown PCs+b DB24C8 0.384 -17.8 (* 1) 0.417 (0.013)d -34.2 0.384 1.11 (0.04) -50.0 3.51 (0.08) 0.384 -60.4 8.84 (0.17) 0.384 -72.5 21.3 (1.0) 0.384 -17.7 0.536 (0.012) 0.608 -28.8 0.608 1.09 (0.06) -39.2 1.81 (0.04) 0.608 -55.4 0.608 7.05 (0.22) -72.6 24.8 (1.6) 0.608 DB21C7 0.385 -17.7 1.49 (0.04) -29.6 0.385 5.81 (0.26) -39.0 9.61 (0.22) 0.385 -49.9 0.385 30.0 (1.0) -60.5 76.3 (4.4) 0.385 -17.7 3.34 (0.10) 0.663 -28.5 5.70 (0.20) 0.663 -39.1 14.4 (0.6) 0.663 -49.9 0.663 28.9 (0.7) -60.4 66.2 (3.2) 0.663 -72.5 261.0 (41.0) 0.663
"0.02 M in salt. *Concentration of free Cs' ion. Estimated standard deviation.
Cfl
O C .
magnetic susceptibility and are referenced to infinitely dilute aqueous cesium chloride solution. Determinations of line widths and of kinetic data have been described previously.' Results The cesium ion may exchange between the solvated and crown complexed sites by two mechanisms as proposed by Shchori et al.' These are the bimolecular exchange mechanism (I) and the dissociative pathway (11):
*CS+
+
+ CS+.L CS+ + L
k
k-2
*CS+.L CS+.L
+ CS+
(1) (11)
where L refers to the crown ether. The general expression for the mean lifetime of the cesium ion in terms of these two mechanisms is
(7) Shchori, E.; Jagur-Grodzinski,J.; Luz, Z . ; Shporer, M. J . Am. Chem. SOC.1971, 93, 7133.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4823
Cs+ Complexation with Crown Ethers
TABLE II: Kinetic Parameters for the Complexation of Cs' with Several Crown Ethers in AC and in MeOH Solutions at 220 K 1O4kIC hS*b AGIO E,' AH*# solvent crown AC AC MeOH MeOH a
kcal.mol-'. beu.
DB21C7 DB24C8 DB21 C7 DB24C8
8.7 7.3 6.6 3.5
(0.5)d (0.3) (0.3) (0.5)
8.1 6.7 6.0 2.9
(0.5) (0.3) (0.3) (0.5)
-2.7 -5.0 -10.5 -23.2
8.7 7.8 8.3 8.0
(2.5) (1.5) (1.5) (2.5)
0.94 (0.3) 7.4 (1.0) 2.7 (0.4) 5.6 (2.0)
(0.2) (0.1) (0.1) (0.2)
CM-' s-I. dStandard deviation estimate. 5000a'
-
h
T
t
40000.
:1
t
c
I
E 2.50
t
30000.
-
7
I
k
4
4-
0
Y
+
20000.
h
U
i
v
;
v
10000.
t
-;
0.
I
I
0
50
100
150
200
Figure 2. Plots of I/[r(Cs+),,,] vs. l/(Cs+), for Cs'crown complexes at 200 K: (A) Cs'eDB21C7 in acetone; (B) Cs+.DB21C7 in methanol; (C) Cs+-DB24C8 in acetone; (D) Cs'eDB24C8 in methanol (206 K).
correct size for the cavity of DB21C7 but is smaller than the cavity of DB24C8, thus the Cs+.DB21C7 complex is somewhat more stable than C S + . D B ~ ~ C ~ . ~ It was previously observed that, in acetone and methanol solutions of CsSCN, the 133Cschemical shift shows a slight concentration dependence' which could be due to the formation of contact ion pairs. However, neither the above measurements nor a literature search indicates to what extent such association (if any) occurs. In fact, at room temperature, and in dilute solutions, alkali salts are not appreciably associated in methanol solutions.IO Since the salt concentration in our solutions was 0.02 M and since the measurements were carried out at low temperatures which increase the dielectric constant of the medium (for MeOH, D = 48.5 at -50 OCll vs. 32.7 at 25 OCI2) we can conclude that at least in MeOH solutions ion pair formation is negligible. If we compare the complexation kinetics of the alkali metal ions with crown ethers a trend emerges. As one goes through the series Na', K+, Cs+ the predominant exchange mechanism varies from that of either the dissociative or bimolecular process for Na+ ion to primarily the bimolecular process for the Cs+ ion. This trend may be due to ion pairing or to the decrease in charge density as one goes to the larger cation. Such a decrease in charge density will minimize the charge-charge repulsion in the bimolecular exchange process and, thus, allow it to predominate. If we consider the kinetic data reported previously for Cs+ ion complexation with 18C6 in pyridine solutions6 and with dicyclohexyl- 18-crown-6 in propylene carbonate solutionsS and assume that in both cases the predominant exchange mechanism is the bimolecular one, free energy barriers for AG*18C6,w and for AG*Dclec6,pcof 9.30 and 9.21 kcal-mol-I are obtained. These are the magnitudes one would expect considering the nature of the (8) Shamsipur, M.; Rounaghi, G.; Popov, A. I. J . Solution Chem. 1980, 9. 701. (9) DeWitte, W. J.; Shoening, R. C.; Popov, A. I. Inorg. Nucl. Chem. Lett. 1916. -, 12. -, 251. (10) Janz,G. J.; Tomkins, R. P. T. 'Nonaqueous Electrolytes Handbook"; Academic Press: New York, 1972; Vol. 1. (11) Sears, P. G.; Holmes, R. R.; Dawson, L. R. J . Elecrrochem. SOC. 1955, 102, 145. (12) Riddick, J. A,; Bunger, W. B. "Organic Solvents";Wiley-Interscience: New York, 1970. ~
~
4824
J . Phys. Chem. 1985,89, 4824-4828
bimolecular exchange mechanism. In the transition state of this process, two positive ions must approach one another. It is reasonable that the ions with the higher charge density will have the higher free energy barrier. For the Na+ ion complexation with 10.5 18C6 by the bimolecular exchange process, AG'Na+ kcal-mol-I. For K+ complexation with 18C6, AGIK+ 10.0 kcal-mol-I, and if cesium ion complexation with 18C6 proceeds via this mechanism, AGlcs+ 9.3 kcal-mol-I. Thus, the observed trend is what one would predict based on charge density arguments.
-
In summary, it is important to investigate not only the rates of exchange involved in the complexation kinetics of crown ethers but also the mechanism of exchange. Such information is invaluable in determining the kinetic processes of these complexes.
--
Acknowledgment. The authors gratefully acknowledge the support of this work by the National Science Foundation research grant CHE-85 15474. Registry No. 7440-46-2.
DB21C7, 14098-41-0; DB24C8, 14174-09-5; Cs,
Thermal Decomposition of Energetic Materials. 7. High-Rate FTIR Studies and the Structure of 1, I ,I ,3,6,8,8,8-0ctanitro3,6-dlazaoctane Y. Oyumi, T. B. Brill,* and A. L. Rheingold Department of Chemistry, University of Delaware, Newark, Delaware 1971 6 (Received: March 18, 1985)
The thermal decomposition of a gem-trinitroalkylnitramine,1,l,I ,3,6,8,8,8-octanitro-3,6-diazaoctane,C,H8(NNOz)z[C(N02)3]2 (ONDO), was investigated by FTIR spectroscopy. ONDO decomposes in the 426-435 K range without melting. Rapid pyrolysis (20-170 K s-l) at subatmospheric pressure produces NOz as the main product along with lesser amounts of HONO, CO,, CO, HCN, NO, N20, and HNCO. The results of slower heating (10 K S-I) of ONDO under 1-atm or higher pressure are unique. Decomposition initially occurs followed momentarily by the transition to deflagration. During deflagration a sharp decrease in [NO,] and an increase in [CO,] take place, suggesting that NOz is the principal oxidant of the molecular backbone. The crystal structure of ONDO was determined: monoclinic, P2,/n, a = 5.9716 (8), b = 12.0170 (15), c = 11.8407 (14) A, @ = 97.1 14 (IO)', Z = 2, D(ca1cd) = 1.88 g ~ m - R~ = , 0.035. The NOz groups of the C(NOJ3 fragment adopt an extreme conformation owing, apparently, to the intramolecular electrostatic influence of the neighboring nitramine group.
Introduction The characteristics of high-rate thermal decomposition of energetic materials, particularly in relation to their molecular structure, are of intense current interest to A study of several cyclic gem-dinitroalkylnitramines has been completed r e ~ e n t l y . Related ~ to these compounds are acyclic nitroalkylnitramines such as 1,1,1,3,6,8,8,8-octanitro-3,6-diazaoctane (ONDO) whose molecular formula is C,Hs(NNOz)2[C(N0,)3]z. ONDO also contains gem-trinitroalkyl groups which may or may not thermalize differently from the gem-dinitro group. The thermolysis of O N D O was studied by rapid-scan FTIR spectroscopy using a cell that permits real time, in situ analysis of the gas product^.^ The results are particularly intriguing because O N D O is the first compound we have encountered that shows the transition from decomposition to deflagration in the signature of the gas products. The crystal of O N D O was determined by X-ray diffraction. The dihedral planes in the C(N02)3 fragment have the greatest distortion yet observed in a gem-trinitroalkyl fragment. Experimental Section A sample of O N D O was generously provided by Drs. Milton B. Frankel and Sun 0. K. Myong (Rocketdyne, Canoga Park, CA) and was recrystallized from ethanol. Infrared spectroscopic characterizations were conducted on a Nicolet 60SX FTIR (1) Brill, T. B.; Karpwicz, R. J.; Haller, T. M.; Rheingold, A. L. J . Phys. Chem. 1984,88,4138. (2) Oyumi, Y . ;Brill, T. B.; Rheingold, A. L.; Lowe-Ma, C. J. Phys. Chem. 1985,89, 2309.
( 3 ) Oyumi, Y . ;Brill, T. B. Combust. Flume, in press. (4) Oyumi, Y . ;Brill, T. B. Combust. Flame, in press. ( 5 ) Oyumi, Y . ;Brill, T. B. Combust. Flume, in press. ( 6 ) Oyumi, Y . ;Brill, T. B. J . Phys. Chem., in press. (7) Cronin, J. F.; Brill, T. B., submitted for publication in J . Phys. Chem. (8) Oyumi, Y . ;Brill, T. B., submitted for publication in Combust. Flame. (9) Oyumi, Y . ;Brill, T. B., submitted for publication in Combust. Flame.
0022-3654/85/2089-4824$01 .50/0
spectrometer employing an MCT-B detector. Variable-temperature spectra were obtained on polycrystalline samples very thinly spread between two NaCl plates by using procedures described Thirty-two accumulated interferograms were averaged at 2-cm-' resolution. A home-built calibrated cell containing an electrically heated nichrome filament was used for the variable-pressure, high-rate pyrolysis studies. The details of this cell and the procedures employed have been fully de~cribed.~ The essential features for this work include the following: Ar atmosphere, 2 mg of polycrystalline sample, 10 scans s-l with two spectra per file, 4-cm-l resolution. The heating rates and final filament temperatures (Tf) are given in the text. For a typical fast heating run, the filament reached Tfat the rate of about 10 K s-' and remained constant at Tf for the duration of the experiment. Each gas product was quantified by integrating the intensity of a characteristic band and ratioing this to its known absolute i n t e n ~ i t y .A ~ small amount of H N C O (
