J . Phys. Chem. 1986, 90, 1677-1683
1677
Kinetics of Complexation of NH,CIO,, AgCIO,, and TICIO, with the Macrocycle 18C6 in Dimethylformamide Sergio Petrucci,*+Raymond J. Adamic,* and Edward M. Eyring' Department of Chemistry, Polytechnic University, Farmingdale. New York 1 1 735, and Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12 (Received: August 28, 1985; In Final Form: November 26, 1985)
The kinetics of complexation of NH4CI04,AgClO,, and T1CIO4with the crown ether 18C6, in the solvent dimethylformamide (DMF) over the temperature range 5-40 OC, have been studied by the pulse and resonator ultrasonic relaxation techniques in the frequency range 0.5-400 MHz. Effects attributable to bond directivity (NH4+)and cation polarizability (Ag' and TI') on the kinetics and mechanism of complexation with the 18C6 macrocycle were found. Data are compared with previous results for KC104 reacting with 18C6 in DMF where the Eigen-Winkler scheme M'-C MC' 2 (MC)+valid for large formation constants K z was found to account for the data. M+ denotes the metal ion, C denotes the crown ether, and M'-C, MC', and (MC)' are three different forms of the crown ether-metal complex. The ultrasonic relaxation spectra for the cations studied in the present work can be interpreted by the sum of two Debye processes. In the case of NH4+the forward rate constants and activation parameters of Scheme I do not differ appreciably from those of K' reacting with 18C6 in DMF. The thermodynamic parameters are more affected, implying different dissociation rate parameters for NH4'. For the case of Ag', dramatic differences appear in the ultrasonic spectra when AgC104 or AgNO, are reacted with 18C6 in DMF. Competition between 18C6 and NO3- is suggested by evidence of complexation of Ag' with NO3- in DMF. Further the rate constants and maximum sound absorption per wavelength for the fast process involving Ag' follow the requirements of Scheme I whereas those for the slow process defy interpretation by Scheme I. TIC104 has a behavior similar to AgC10,. Rationalization of these deviations is sought by applying the Eigen-Tamm mechanism and its details for relatively small Kz's to the complexation of Ag' and TI' with 18C6 in DMF.
Introduction The kinetics of complexation of alkali metal cations such as Li', Na', and K' with the macrocycle 18C6 in DMF1sZand with the macrocycles dihexano-l8C6 (DH18C6) and dibenzo- 18C6 (DB18C6) in DMF3 have been studied recently by ultrasonic relaxation techniques. The general Eigen-Winkler scheme4
Scheme I M'
+C
kl k-I
k
k2
M'-C
k-2
MC'
2(MC)' k-3
[with M' denoting the solvated metal cation, C the free macrocycle, and M+-C, MC', (MC)' three complexed forms of the metal-macrocycle complex] accounted for the data. In fact, for large formation constants K , >> 1 Scheme I reduces to Scheme I1 which suffices to interpret the data for K' complexing with
Scheme I1 k
k2
M+-C
k.2
MC'
& (MC)' k-3
18C6 but not for Na' and Li', perhaps because of their smaller Kz values.* It was also found that, for Na' and K', the rate-determining step of the complexation is the rearrangement of the ligand in forming the complex (MC)', the encapsulated form of the metal-macrocycle It seemed reasonable to extend this investigation to ions that, because of their geometry or flexibility of their electron clouds compared to Na' and K', would perhaps offer new clues to the details of the complexation mechanism with macrocycles. The conformations of macrocycles because of their structure are sensitive to the bonding geometry of a cation or to its effective binding radius. To this end the cations NH4', Ag', and TI' were selected for this study. The first is known to have a tetrahedral geometry and partial ionic character of all the N-H bonds with the ionic charge being distributed over all four of them.5 The N-H interatomic distance
'
Polytechnic University. *University of Utah.
0022-3654/86/2090-1677$01.50/0
is5 1.03 8, but the ionic radius of NH4' is5 rNH4+ = 1.43 8, which is intermediate between that of K', rK+= 1.33 A, and that of Rb', rRb+ = 1.48 A. Hence it should be possible to compare rate constants and activation parameters of NH4' with the corresponding quantities available for K+ in D M F to see whether the geometrical potential bonding directivity of NH4+plays any role in complexing with 18C6 in DMF. Ag' and TI' have electronic structures symbolized6,' as [Kr]4dio and [ X e ] 6 ~ * 4 f ' ~ 5 with d ' ~ [Kr] and [Xe] denoting the two preceding rare gas structures (in the periodic table). Both the cations Ag+ and TI' are polarizable and "soft", and although their ionic radiis.9 are rAg+= 1.15 A and rTl+= 1.49 A, they behave in terms of affinity with 18C6 in water more as smaller cations do. In fact, the logarithm of the complex ion formation constant for Ag' is9 log K = 1.50 compared to log K = 0.80 and log K = 2.03 for Na' and K', re~pectively.~ Also the log of the formation constant for Tl', log K = 2.27, is comparable to that of K', log K = 2.03, and larger than that of Rb'? log K = 1.56, even though the ionic radius of Rb', rRb+= 1.48 A, is much closer to that of TI' than that of K+.8 It was therefore interesting to extend these thermodynamic considerations to a kinetic mechanistic study using ultrasonic (1) K. Maynard, D. E. Irish, E. M. Eyring, and S. Petrucci, J . Phys. Chem., 88, 729 (1984). (2) C. Chen, W. Wallace, E. M. Eyring, and S . Petrucci, J . Phys. Chem., 88, 2541 (1984). (3) W. Wallace, C. Chen, E. M. Eyring, and S. Petrucci, J . Phys. Chem., 89, 1357 (1985). (4) E. Eigen and R. Winkler in The Neurosciences: Second Study Program, F. 0. Schmitt, Ed., Rockfeller University, New York, 1970. (5) F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 2nd ed, Interscience, New York, p 334. (6) T. L. Cottrell, The Strength of Chemical Bonds, Butterworths, London,
1958. ( 7 ) H. B. Gray, Electrons and Chemical Bonding, W. A. Benjamin, New York, 1965. (8) J. D. Lamb, R. M. Izatt, J. J. Christensen, and D. J. Eatough in Progress in Macrocyclic Chemistry, Vol. 2 , R. M. Izatt and J. J. Christensen, Eds., Wiley, 1981, Chapter 3. (9) R. M. Izatt, R. E. Terry, B. L. Haymore, L. D. Hansen, N. K . Dalley, A. G. Avondet, and J. J. Christensen, J . Am. Chem. Soc., 98, 7620 (1976).
0 1986 American Chemical Society
1678 The Journal of Physical Chemistry, Vol. 90, No. 8. 1986
Petrucci et ai.
TABLE I: Ultrasonic Relaxation Parameters, Sound Velocities at the Various Temperatures. and Concentrations for the Svstems Investigated B X IO”, u X CNH4X. C18C6, t, o c anion mol dm-, mol dm-; F I x IO5 fi, MHz f i l l X 10’ f i i ? MHz cm-] s2 cm s-l 254 cia,0.16 0.093 100 25 70 5.5 29.5 I ,378’ 0.20 I80 25 290 3.5 31 1.483h 25 c10,0.20 25 (210,0.32 0.30 280 25 330 4.5 33 1.49Ih 0.41 0.40 380 -_ 1< 420 5.0 25 c10,34 1.518’ 40
32 15 I,
O C
25 25 25 25 25
c10,-
c10,c10,-
0.41 0.41 0.41 cAgX,
CiSCbr
anion
mol dm-,
CI0,-
0.042 0.09 1 0.15 0.185 0.26
mol dm-’ 0.039 0.092 0.16 0.20 0.26
c10,c10,-
(210,-
c10,-
0.40 0.40 0.40
45 30 20
450 500 380
6.5 5.5 4.5
fi, MHz
kIl X I O 5
fil, MHz
80 140 250 370 450
50 45 40 50 60
I90 3 20 420 470 570
2.3 3.5 4.0 4.5 5.5 2.8 2.0
450 400 350 X 10’
35 15 5
CI0,-
c104c10,-
0.15 0.154 0.147
0.16 0.154 0.147
240 350 330
60 30 20
400 390 300
25 25 25
NO,NO,NO;-
0.043 0.15 0.27
0.043 0.155 0.26
175 500 850
50 70 80
65 130 200
25f
CIO,’
0.098’
0.102J
251
360’
cTIX,
t,
O C
25 25 25 25 25
801
mol dm-3
mol dm-’
c10,-
0.09 1 0.17 0.24 0.32 0.39
0.094 0.17 0.24 0.33 0.40
IO5 135 220 270 350 450
0.27 0.25 0.27 0.34
CD-
c10,-
35 15
CIO;
5
CIO,
0.27 0.25 0.27
25
NO,-
0.36
clod-
5 IO 12 1.251
c18C6,
anion
c10,c10,-
5.0
fiI
X
fi. MHz
1111 X
IO5
fil,
MHz
32 35 32
1.427c 1.483‘ 1.567‘
E X IO’?, cm-’ s2 19 31 33 32 36
u X lo-’,
31.5 29 28
1 .424d
29 32 32
1.460 1.457 1.458
cm
I .462d 1.45Id 1.47ld 1.47@ I .494d I .53dd
31’
B x IO]? cm-’ s2 28.5 30 30 32 35
SKI
1.455d
1.460’ u
x 10-5, cm s-l
25
125
25 30 35 30
210 250 280 330
2.0 2.5 3.0 4.0 3.5
350 290 320
31 25 19
310 220 180
4.0 2.8 2.7
33 29 30
1.497e 1 .524e
550
I30
630
8.0
42
1.445
1.450 1.446 1.466 1.440
1.440 1.419e
OThis is the only run with R # 1 to check upon the validity of the assumed 1 : l interaction between cation and macrocycle. ’ r 2 = 0.85, u = (1.462 + 0.125C,8c6) X IO5 ern s-I, = 1.654 X l o 5 - (0.00551) X IO5? (“C) cm SKI,r2 = 0.99. d A t c = 0.15 M, u = 1.550 X 10’- (0.0037) X 10St ( “ C ) cm s-l, r 2 = 0.99. e A t c = 0.25 M, u = 1.546 X I O 5 - 0.0035 X IO5[ ‘C (r2 = 0.98). ’This run refers to t h e used macrocycle DB18C6. absorption techniques as discussed below. The results obtained with the three electrolytes NH,CIO,, AgC104, and TICIO, will be presented separately.
Experimental Part The pulse ultrasonic equipment has been described e1se~here.I~ The only relevant changes are in the data capturing which has been automated by the use of a digital micrometer (Mitutoyo Series 164 sensitive to a displacement of j~0.00005in.) and an associated electronic counter, interface, and miniprinter. The distances corresponding to 1 dB step increments of the standard signal are now automatically recorded. The data are then reported on a Hewlett-Packard 85A desk computer and subjected to linear regression, statistical analysis, and automatic plotting of dB vs. distances. In the case of the ultrasonic resonator an electronic thermometer with surface thermistor sensors records the temperature of the three available cells within f O . O 1 OC thus matching the precision of the thermostat (but avoiding any systematic error due to possible differences in temperature between the cells and the thermostatic bath). The materials AgCIO,, A g N 0 3 (Aldrich), and TIC104 (K and K) were dried at 60-70 O C under vacuum overnight. Purification of 18C6 and dimethylformamide and preparation and handling ( I O ) S. Petrucci, J . 6‘hJ.s. Chem., 71, 1174 (1967); S. Petrucci and M. Battistini, J . Phys. Chem., 71, 1181 (1967); S. Onishi, H. Farber, and S . Petrucci, J . Phys. Chem., 84, 2922 (1981); M. Delsignore, H . Maaser. and S. Petrucci. J . Phys. Chem.. 88. 2405 (1984); ref 2.
of solutions were as reported previously.’,*
Results and Discussion NH4CI04 + 18C6. Figure 1 shows a representative plot of the excess sound absorption per wavelength vs. the frequencyf for NH4CI04 18C6 in molar ratio R N 1 in the solvent D M F at 25 “C. The excess sound absorption is given by fi = (CY- By)u/J where a is the sound absorption coefficient expressed in N p cm-I, B = ( C X / ~ ) + , ~ , ~namely ,, the background sound absorption coefficient divided by the squared ultrasonic frequency for values of the latter much larger than the relaxation frequenciesf, and Ai under study. The u denotes the sound velocity. The solid line in Figure 1 has been calculated as the sum of two Debye single relaxation processes according to the function
+
where pi and pII are the maximum excess sound absorption coefficients per wavelength of the two relaxation processes centered at the frequencies fI and f,,. Table I reports the relaxation parameters wl, h l l ,fl, f,,, the quantities B, and the sound velocities u for all the NH4ClO4+ 18C6 systems investigated in DMF. From Table I it is clear that the relaxation frequencies at a given temperature are concentration independent and one can deduce that both the fills and pll’s vary linearly with the concentration. These results can be explained in terms of first-order or pseudo-first-order processes as done previously for KC104 + 18C6 in D M F using the Eigen-Winkler Scheme I1 which, as noted above, is the particular form of the
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1679
Complexation of NH,-, Ag-, and TlClO, with 18C6
TABLE 11: Kinetic and Thermodynamic Parameters Comparing the Complexation of NH4C104 with That of KCIOl Reacting with the Macrocycle 18C6 in DMF NH4C104 + 18C6 KCIO, + 18C6 k2 (298), s-' 1.6 X lo8 1.8 X lo8 AS**,cal/(K mol) -4.1 -7.7 AH**, kcal/mol 5.0 3.9 AH, + AH3, kcal/mol -0.9 -3.66
500-
500-
I
no 400
-
a 300-
d@,/d(c X k,(298), SKI A F 3 , cal/(K mol) AH*,, kcal/ mol AH,, kcal/mol AH,, kcal/mol
200-
'.
100I' _--I
0.5
0 0.1
2.0
1.0
5.0
10
20
50
100
' - _ _ dpI,/d(c X 200
500
I(MHZ1-
500
I,
-
-
one obtains r2 = 0.98, I = -17.54, S = -1842 from which AH, + AH, = -3.66 kcal/mol. Similarly given',2 1 d In ( p I I T / u 2 )= -AH3 (7) R
280x10-5 26MHz
pII - m X 1 o - 5
400-
P a 300
200
-
---
_ _ C C
1
IlMHI)-
Figure 1. Representative plots of the excess sound absorption per wavelength 1.1 vs. f for NH4C104+ 18C6 in DMF in molar ratio R = 1.
more general Scheme I for large overall complexation constants KB. Scheme I1 leads to the interpretation of the two relaxation times q (= (2aA)-') and T~~(= (2aJl)-l) according to the two relations
TC' = k-2
+ k2 = k-2(1 + K2)
i=
k2
(2)
for K2 = k 2 / k - 2>> 1 and (3) for K3 and K2 >> 1. Then since k2 = (kT/h)eM*2/Re-AH*2/RT, a plot of In ( T < I / T ) vs. 1/T will give In ( k / h ) + (ASt2/R)as the intercept and - A H t 2 / R as the slope. From the data of Table I for NH4C104+ 18C6 in D M F one gets by linear regression of In ( T < ' / T ) vs. (1/T) an intercept I = 21.713, slope S = -2531, and r2 = 0.93 from which one calculates A S 2 = -4.1 cal/(mol K) and AHt2 = 5.0 kcal/mol. Similarly, given k3 = ( k T / h)e'*3/Re-M*3/RT, from a plot of In ( q { I / T ) vs. l / T o n e obtains by linear regression r2 = 0.94, I = 14.948, and S = -1002 from which one calculates hSt3= -17.5 cal/(mol K) and AH*, = 2.0 kcal/mol . Scheme I1 leads also to the two following expressions for pl and wl,'~2
FI = 7r (AVShZ -28, R T (1
H 'I1
=
K2C
+ K2)(1 + K 2 +
K2K3)
H (AVSJ2 =---
2p,
RT
( l + K2)K2K3 C= (1 K , ~ 2 ~ 3 ) ' a (AvSII)2 K22K3C 1 = - -a (AVS1J2 --c -28, R T [K2(1 + K3)I2 2P9 R T K3
c K2K3 (4)
(AvSll)2
2P, RT
from linear regression of the quantity In ( p l 1 T / u 2vs. ) (l/T), one obtains r2 = 0.91, I = -17.825, S = -1727 from which AH3 = -3.43 kcal/mol and consequently AH2 = -3.66 - (3.43) = -0.2 kcal/mol. Table I1 contains all the kinetic and thermodynamic parameters extracted for the system NH4C104 18C6 in DMF. These results are compared to the corresponding ones for KC104 18C6. From Table I1 one notices similarities between the kinetic rate constants for the forward steps of Scheme 11, the only significant differences appearing between the two k3 values. Table I1 shows, however, larger differences in the thermodynamic parameters and in the slopes dpl/dc and dplI/dc, reflecting differences in the stability of the complexes, once they are formed and in the reverse rate constants of Scheme 11. The above seems to suggest that there is no striking effect on the forward rates of complexation attributable to the orientational requirement of 18C6 complexing NH4+. In fact, k , for NH4+ appears to be three times larger than for Kf reacting with the same ligand in DMF. The similarity between the k i s and the k3's can be rationalized by recalling that the rate-determining step of the overall Scheme I (or 11) for the alkali ions in D M F was found to be the rate of ligand rearrangement to form the final species (MC)'. Evidently differences in the energy barrier to the orienting of 18C6 around NH4+ compared to K+ are not significant when contrasted with the general energy barrier to wrapping 18C6 around a cation. If any overall difference exists between N H 4 + and K+ due to orientation and/or cation radius this seems to increase k3 for NH4+ with respect to Kf. AgClO, + J8C6 in DMF. Figure 2 shows a representative plot of the ultrasonic spectrum of AgC10, + 18C6 in D M F in the form of )I vs. f. The solid line corresponds as in the case of NH4CI04 + 18C6 in D M F to the sum of two relaxation processes. Table I reports the relaxation parameters p,, pll,h,fil, B and the sound 18C6 in D M F systems investivelocity u for all the AgClO, gated. Before proceeding further we should recall that in an earlier study' of KClO, + 18C6 and KSCN + 18C6 in D M F the same ultrasonic spectra were obtained in the two cases within experimental error. The polarizability of Ag' led us to suspect that other anions more nucleophilic than C104- could affect the ultrasonic results by competing with 18C6 for the first coordination sphere sites around Ag'. To check on this point three runs at various 18C6 in molar ratio R N 1 were concentration of AgNO, performed in DMF. The results are reported in Table I and a representative plot in Figure 3. Dramatic differences from
+
100-
0
18.7 9.1 X lo6 -15.3 3.4 -5.4 4.5 5.7
with 0,= (pp2)-' denoting the adiabatic compressibility. Then ) ( l / T ) given2,' by linear regression of the quantity In ( p I T / u 2 vs. 1 d In ( p 1 T / u 2 )= -(AH2 + AH,) (6) R
W 4 W 4 Oa?M+lBC8 0 3 0 M In DMF. I 25. C
--
9.4 3.2 x 107 -17.5 2.0 -3.4 -0.2 11.0
+ +
+
+
(5)
+
1680
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Petrucci et al. OAgNO3 +lac6 in DMF;
AgC104 0 . 1 5 M i l S C 6 D 15M
'O0I
in DMF. 1
I 5GOt
-
25' C
rl= 250~10-~ I, = ~ O M H Z
800 I
'
I
400t
O
=
N
I
300 t
200-
'03P
,
:I
._,__-
I
0 2
---
-/--AI
C 5
10
20
I
50
10
20
I
50
L - L X . 130
200
530
t(wtiz)---
Figure 2. Representative plot of
p
vs.ffor AgCIO,
DMF.
+ 18C6 ( R
N
I ) in
A ~ N 0O 15aM*18C6 ~ 0 155M ~
i
10
DMF, I
- 25'C
6001
1
pI=500x10-5
, 5001 400, 1
x 3 0 0 ~
-
600-
In
I I = 70MHz
E
130x10-~
Ill - 1 0 M H z
Y
~i32~io-17cm-1~2
3. '400
-
U= 1 4 5 1 ~ 1 0 ~ S "~ m
200r
@'
k -. 2oo/00
*-
0
500
-
I 0.2 0.2 C,M
I 0.1 0.1
lIMHzl--
Figure 3. Representative plot of
p
vs.ffor AgNO,
DMF.
+ 18C6 ( R
N
1) in
AgC10, + 18C6 are observable, both in the values of pI and p I I and in the relaxation frequencies fi and fil. Parts A and B of Figure 4 show pI and pullas a function of c for the two systems AgC10, + 18C6 and AgNO, + 18C6 showing the strong anion dependence of both pI and pII. Figure 5 confirms our suspicion that the anion NO,- competes for the first coordination sites of Ag'. The ultrasonic spectrum shows a single Debye relaxation which is absent for AgC10, in DMF, hence it is ascribable to an interaction of Ag+ with NO3-. This shows the possible importance of the anion in the mechanism of complexation of a cation with a macrocycle, an aspect which has been pointed out in the past" and that has been found to be of paramount importance in media of low p e r m i t t i ~ i t y . ' ~ . ' ~ Figure 4, A and B, the Table I show further that for AgCIO, + 18C6 in D M F the pi's change linearly with c and thefi's are independent of c whereas the pll's do not show a linearity with c and thefi,'s increase with c. This was previously noted2 also for NaC10, 18C6 and LiC104 + 18C6 and was tentatively attributed to the smallness of K, with respect to that of KCI04 + 18C6. In other words, when K , becomes relatively small Scheme I1 is no longer a valid representation of the process, but rather Scheme I ought to be used. The latter is synonymous with the well-established EigenTammi, mechanism used to interpret the complexation of ions by inorganic ligands. We shall follow then the Eigen-Tamm guidelines in trying to interpret the present data for AgC10, + 18C6.
+
( I I ) N . S. Poonia and A. V. Bajaj, Chem. Reu., 79, 389 (1979). (12) H. Farber and S. Petrucci, J . Phys. Chem., 85, 1396 (1981). (13) H. Richman, Y . Harada, E. M. Eyring, and S. Petrucci, J . Phys. Chem., 89, 2373 (1985). (14) M. Eigen and K. Tamm, Z . E[ektrochem., 66, 93, 107 (1962).
0.I3 0.3
Figure 4. p1 vs. c for AgC10, + 18C6 and for AgNO, + 18C6 in DMF at t = 25 'C. pl, vs. c for AgCIO, + 18C6 and for AgNO, + 18C6 in DMF at t = 25 " C . AgN03 0 2 1 8 M ~n DMF
I
1-25'c
U - 1459x1O5crn SS1
'i
300-
> 2001
I(MHz1-
Figure 5. Representative plot of
p
vs.ffor AgNO, in DMF; t = 25 ' C .
Following Eigen and Tamrnl4 for the fast observable process of Scheme I and the general condition k l , k-, > k2, k-, > k,, k-, one writes
with 8 = 2ac and K 1 = k , / k - , . u is the overall degree of dissociation of the complex, or K , = (1 - u)/u2c, neglecting activity coefficients. Then if 8 > K-l TI-'
= k-2
+ k2
as it appears in the present case that
T,-'
(9)
is concentration inde-
Complexation of NH4-, Ag-, and TlC104 with 18C6
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1681
TABLE 111: Collected Kinetic and Thermodynamic Parameters for the Complexation of AgCIO4 and TICI04 with the Macrocycle 18C6 in DMF k
k
M+ + c+M+...c&Mc++Mc)+ k-2 AgC104
+ 18C6
TlClO4
2.1 x 107 2.3 X IO8 0.10 -1.6 5.6 1.8 x 106
k2, SKI k-2, s-'
K2 cal/(K mol) AH*_2.kcal/mol k-,,
k k-3
s-I
+ 18C6
1.8 x 108 9.8 X IO6 18.4 -10.1 4.91 2.1 x 106
0.2
0
Figure 6. q,-'vs. c1I2 for AgCIO,
= k-2
kT + k2 = -eMS'2/Re-AH*2/RT( 1 + K2) h
+
18C6 in DMF at t = 25 "C.
TICIO, 0 . 3 2 M 1 l a C B 0 33M ~n DMF. I 2 5 ' C
-
pendent. (Parenthetically, one notices that the same result, namely a first-order process and T i 1 = k-2, is achieved if the condition k2 < k-2 would hold.) From eq 9 one writes 7I-I
os
04
@ I m d dm-3)1'2-
(10)
which gives1q2by plotting In ( s - ' / T ) vs. 1 / T intercept = In slope =
d In
(T
I
I
pi
5
pII
-
h -
.o
.?
135~10-~
3
11 - 2 5 M H z
200
-
30
125~10-~
-2.61Hz
lit
a
-
;
B - 2 8 . 5 ~ 1 0 - 1 7 s m - 1 s2
U - 1 . 4 Y ) x 1 0 s c m S1
*-_. with K2 = k 2 / k - 2 = (MC)/(M-C) and d In ( M ~ T / U=-' ) AH2 K 2 - 1 d(l/T) R K2+1
___---
(15)
- _- - - _ _ _
+
For AgC104 18C6 in DMF, a plot of In ( q - ' / T ) vs. 1 / T gives r2 = 0.99, I = 22.968, S = -2766 from which AS*-2 = -1.58 cal/(mol K ) and
Further a plot of In ( h 1 T / u 2 )vs. 1 / T gives I = -25.256, S = 315.9, namely AH2 K2 - 1 315.9 = - R K2+1 Equation 10 with cal/(mol K) gives
8.93
T~-'
X
= 2.5 X lo8 and with AS*-2 = -1.58
+
= e-AH*-2/RT(1 K 2 )
{exp[ - ( 2 7 6 6 -
)
K2 375.9 / T K2 - 1
>> 1 , a
= 1/K,'/2c'/2 and 0 = 2ac = ~ C ' I ~ / K , ' / ~ .
(18)
Equations 16-18 can be combined giving the transcendental equation
8.93 x 10-5 =
Also if K z Therefore
11
(1
+ K2)
which can be solved for K , by trial and errorI5 until the left part is obtained. The solution is K 2 = 0.10 from which AH2, AH*-2, ( 1 5 ) C. Chen and S. Petrucci, J . Phys. Chem., 86, 2601 (1982)
+
Figure 6 shows the data for 7 f 1 vs. c1I2for AgC10, 18C6 in DMF. Linear regression gives r2 = 0.99, I = 1.82 X lo6,S = 6.3 X lo7, namely k-, = 1.8 X lo6, and given K2 = 0.10, k3K1/K,1/2 = 3.15 X 10' and finally (KIK3/K,1/2)= 175. Further progress is negated by lack of knowledge about the numerical value of K,. T l c l o , 18C6 in DMF. The T1C104 + 18C6 system, despite the larger radius of TI+ with respect to Ag+, follows similar lines of behavior to those of the system AgC104 + 18C6 in DMF. Figure 7 shows representative plots of the quantity vs. f for TIC104 + 18C6 in DMF. The solid line is the the sum of two
+
1682 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Petrucci et al.
, n TIN03 0.36M118C6 0.34M in DMF , I= 25°C
zool .. ---
100~
e
0
0.5
-
-1
,
, ,
0
/
-,e
2
5
10
50
20
100
200
0
02
1.0
0.5
2.0
50
10
20
50
100
200
500
fiMHz)---
Figure 10.
fi
v s . f f o r AgC10,
+ DB18C6 in DMF; t = 25 OC
500
I(Mdr)-
Figure 8. Excess sound absorption per wavelength b vs. frequencyffor 0.36 M TINO, + 0.34 M 18C6 in D M F at 25 "C.
Similarly, application of eq 14 gives I = -21.036, S = -842.4, and AH2 K2 - 1 -842.4 = - R Kz+1
Maxmum Excess Sound absorption coefficient per wavelenght ,ul and ,unfor
Equations 21 and 22 together with
TlC104+18C6in DMF; 1 - 2 5 0 ~
kT 1.89 X 108 = -eAS'-dRe-AH'-dRT( h
/
400-
1
or 4.89 x 10-3 = e-AH'-?/XT (1 expression 4.89 x 10-3 = (.XP[
-(
1580 +
1
+ K2)
(23)
+ K 2 ) lead to the transcendental
K2
K2-l
which gives, by trial and error analysis,15 K 2 = 18.4 and k-2 E 1 X IO'. Then all the kinetic and thermodynamic parameters collected in Table 111 can be calculated. Application of the Eigen-Tamm theory14 and eq 19 or 20 and linear regression analysis give r2 = 0.85, I = 2.1 X IO6, S = 3.52 X lo7 or k_, = 2.1 X lo6 s-l, ( k 3 K , / K , 1 / 2 )= 9.6 X lo5, and KlK,/K,'12 = 0.46. Table I11 collects the calculated kinetic and thermodynamic parameters for TIC104 18C6 in DMF.
+
0
0.1
0.2
0.3
c X 1 o 3 (mol cm-3)
Figure 9. Excess sound absorption coefficient per wavelength TIC104 18C6 in DMF; t = 25 "C.
+
Conclusions 0.4
vs. c for
Debye relaxation processes. Table I reports all the relaxation parameters and the sound velocities for the systems investigated involving TI'. Figure 8 reports a plot of w vs.ffor 0.36 M T1N03 + 0.34 M 18C6 in D M F at 25 OC. The solid line is the interpretation of the spectrum by two Debye relaxation processes. As reported in Table I, huge differences with respect to the relaxation parameters of T1C104 18C6 at comparable concentrations appear. This shows again, as in the case of Ag', the influence of the nature of the anion in altering the mechanism of cation-macrocycle complexation. Figure 9 reports M~ and pullvs. c for TIC104 + 18C6 in DMF. Again w1 appears to vary linearly with c whereas jql is not linear with c. Application of the Eigen-Tamm theoryI4 and eq 2 gives 3 = 0.99, I = 18.666, S = -1580, whence AS*-*= -10.1 cal/(mol K) and
+
AH'-2 R
-1 580 = - --
K2 AH, 1+K,
R
(21)
It appears that the Eigen-Tamm theoryI4 or the equivalent Eigen-Winkler theory4 can account for the main features of the complexation of both Ag' and TI' with a macrocycle such as 18C6 in DMF. Comparison between Ag' and TI' (Table 111) shows that the main difference resides in the fast observable kinetic step, namely in the rate constants of desolvation and solvation of the cations. In particular, the rate of desolvation of TI', k2 = 1.8 X lo8 s-', appears identical with the one for K+ in DMF, very close to the and about one order of magone for NH4', k2 = 1.6 X los SKI, nitude larger than the one for Ag', k2 = 2 X lo7 s-', in DMF. The above is in line with ionic radii considerations. On the other hand, the rate constant of dissociation of the encapsulated species (MC)', k-3rappears to be close for Ag+ and TI' despite their differences in ionic radii ( r A g += 1.15 A, rTl+= 1.49 A). We also wished to know whether the rate-determining step for the complexation of a given cation such as Ag' was the final rearrangement of the ligand or cation desolvation. To this end we have run AgC104 + dibenzo-18C6 at R = 1 in D M F (Figure 10). The results are thatf, is a factor of two slower andfll a factor of three slower than the corresponding values for AgC104 18C6, a situation qualitatively similar to the alkali metal cations
+
1683
J . Phys. Chem. 1986, 90, 1683-1688 reacting with these macrocycles in DMF.2,3 This confirms also for Anf that the rate-determinating stet, of comDlexation is the ligand rearrangement to form (MC)+.
Acknowledgment. The authors are grateful to the National
Science Foundation for generous support through Grants CHE8108467 and CHE8513266. Registry No.
18C6, 17455-1 3-9; NH4C104, 7790-98-9; AgC104,
7783-93-9; TICIO,, 13453-40-2.
Electron-, X-ray-, and Ion-Stimulated Decomposition of Nitrate Salts Subhodaya Aduru, Salvatore Contarini, and J. Wayne Rabalais* Department of Chemistry, University of Houston, University Park, Houston, Texas 77004 (Received: September 26, 198.5; In Final Form: November 13, 198.5)
Decomposition of lithium, sodium, and ammonium nitrate stimulated by 500-eV electrons, 1254-eV X-rays, and 4-keV Ar' ions is investigated by X-ray photoelectron spectroscopy (XPS). The core level XPS spectra show that a steady-state composition is attiined at a total irradiation dose of 10'' particles/cm* in which the damaged layer contains predominantly M 2 0 and M,O, with smaller amounts of MNO, and MN02, where M = Li and Na. Irradiation of ",NO3 produces no changes detectable by XPS, suggesting that the decomposition products are all volatile. While the relative atomic concentrations of the constituent elements within the damaged layer change in a similar manner for the three modes of radiation, i.e. there is an increase in alkali metal, decrease in nitrogen, and only minor changes in oxygen concentration, the specific products and relative abundances of these products differ. Consideration of these results in terms of the mechanisms of energy deposition into the atomic lattice indicates that the energy density deposited by all three sources exceeds that required for decomposition and that the excess energy determines the extent of reaction.
Introduction Exposure of crystalline oxyanions to energetic radiation sources induces decomposition and results in the formation of different compounds within the radiation damaged layer. Such radiation-induced chemical changes have been studied by exposure of oxyanions to X-ray,"-* e l e c t r ~ nand , ~ ion beams.1w13 The techniques used in identifying the decomposition products have included chemical separation from acqueous solution,'%*Raman spectro~copy,~ X-ray photoelectron spectroscopy (XPS),4-13and ESR.'4!'5 The technique of irradiation within a UHV chamber followed by in situ XPS analysis has been particularly valuable in identifying reaction products through use of core level chemical shifts. Despite these studies, it is not possible to predict the radiolysis products of these complex salts because the mechanism of decomposition is not understood. The radiation damage studies on these crystalline oxyanions have typically employed only one type of radiation source. In our efforts13to obtain a better understanding of the radiation-induced (1) G. E. Boyd, E. W. Graham, and Q.V. Larson, J . Phys. Chem., 66,300 (1962). (2) G. E. Boyd and Q.V. Larson, J . Phys. Chem., 68,2627 (1964). (3) J. B. Bates and J. C. Pigg, J . Chem. Phys., 62, 4227 (1975); 65, 3901 (1976). (4) R. Prins. J . Chem. Phvs.. 61. 2580 (19741. (5) A. E. P o k y and P.M.'A.' Sherwood; J . Chem. Soc., Faraday Trans. 2, 70, 1240 (1974). (6) R. G. Copperthwaite, S. Afr. J . Chem., 36, 125 (1983); J . Chem. Soc. Chem. Commun., 320 (1980). (7) R. G. Copperthwaite and J. Lloyd, J . Chem. Soc., Dalron Trans., 1117 (19'7). (8) R. G. Copperthwaite and M. Steinberg, Solid State Commun., 28,915 (1978). (9) T. Sasaki, R. S. Williams, J. S. wong and D. A. Shirley, J . Chem. Phys., 68, 2718 (1978); 69, 4374 (1978); 71, 4601 (1979). (10) A. B. Christie, I. Sutherland, J. Lee, and J. M. Walls, Vacuum, 31, 513 (1981); Anal. Proc., 20, 480 (1983). (1 1) G. J. Coyle, T. Tsang, I . Adler, N. Ben-Zvi, and L. Yin, J . Electron Spectrosc., 24, 221 (1981). (12) G. J. Coyle, T. Tsang, I. Adler, and L. Yin, Surf. Sci., 112, 197 (1981). (13) S. Contarini and J . W. Rabalais, J . Elecfron Spectrosc., 35, 191 (1985). (14) D. Suryanarayana and J. Sobhanadri, J . Magn. Reson., 16, 274 (1974). (15) T. Sasaki, Phys. Status Solidi A , 34, 339 (1976).
0022-3654/86/2090-1683$01 .50/0
decomposition mechanism of oxyanions, we have used X-ray, electron, and ion beams to stimulate decomposition of L i N 0 3 , N a N 0 3 , and ",NO3. The damaged layer has been studied by in situ XPS analysis. The effects of these radiation sources can be particularly enlightening because X-rays and electrons transfer energy to the atoniic lattice through electronic excitations, whereas ions transfer both nuclear translational energy as well as electronic energy. The 1254-eV X-rays, 500-eV electrons, and 4-keV Ar' ions all penetrate deeper than the XPS sampling depth so that only the irradiated layer is being studied. Similarities and differences in the decomposition products induced by these radiation sources can provide clues to the energy channels that lead to their formation. Among the oxyanion radiolysis studies that have been analyzed by in situ XPS include electron irradiation9 of lithium nitrate, sulfate, chromate, tungstate, halates, and perhalates, X-ray exposure4-* of alkali metal chlorates, perchlorates, nitrates, and sulfates, and ion b ~ m b a r d m e n t ' of ~ ' metal ~ sulfates and carbonates. Sasaki et aL9 were able to identify L i 2 0 and L i N 0 2 decomposition products from electron irradiated LiNO,, while Copperthwaite6 identified NO;, NO-, N-, and N3- species in X-ray exposed alkali metal nitrates. The latter study did not report the metal or oxygen XPS levels and neither study reported the final atomic concentrations within the damaged layer. The nitrates were chosen for this investigation with three radiation sources because nitrogen can assume several oxidation states which exhibit substantial XPS chemical shifts and the previous work with single radiation s o u r ~ e sindicates ~-~ that specific products can be identified within the damaged layers. Radiolysis of such crystalline oxyanions is of general importance (i) as a means of preparing surfaces with unique properties, i.e. thin films of specific species on substrates, (ii) because of decomposition induced by routine XPS studies of materials, and (iii) for a fundamental understanding of the mechanism and results of energy deposition in insulating materials by the three radiation sources.
Experimental Methods The measurements were carried out on a Perkin-Elmer PHI Model 550 ESCA/SAM system using Mg Ka X-rays at 1253.6 eV as the excitation source. Analytical grade samples in the form of fine powders were pressed into disks of 1 cm diameter and
0 1986 American Chemical Society
