5057
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J . Phys. Chem. 1985,89, 5057-5061 stant for the addition of I8F to C2H2that was 2% smaller than that used in Table VI.) The values are displayed in Table VI with errors calculated only for the precision relative to this standard. As indicated above, the precision of the relative rate constant measurements is very much better than the accuracy of any of the individual absolute rate constants involved in the comparisons. The largest discrepancy between the values in Table VI and those given in Table V is that for k l o ,the reaction with HI. An internal consistency check can also be made for the various relative rate constants measured by the moderated 18F recoil technique. The ratio of reactivity for CHI vs. H2 has been measured in this laboratory as 2.43 f 0.12 with (CH3)2Hgas the comparison reactant,222.64 f 0.16 with CH2C1CH=CHz as the The HI/C2H2 base, and 2.9 f 0.3 in HI-scavenged a~ety1ene.l~ measurement was made at 283 K,I4 while the other two were performed at 287 K. Root et al. measured the same CH4/H2pair vs. CF3CF=CF2 and found a ratio of 3.1 1 f 0.14 at 283 K and 2.28 f 0.10 at 300 K.' The indications are strong that the ratio of these particular reaction rate constants is relatively sensitive to temperature and that the individual rate constants may vary even more widely with temperature than their ratio. In addition, the C2H2/HI measurements from this laboratory a decade agoI4 were evaluated from measurements at moderator ratios as low as 10 to 1. The current techniques rely on much higher moderator ratios of 50 to 400 because of concern for the contributions from (22) McKeown, F. P.; Iyer, R. S.; Rowland, F. S.J . Phys. Chem. 1983, 97, 3972.
energetic atom reactions at the lower moderator ratios. The mutual error limits for this set of experiments are large enough that there is no necessary inconsistency within this group of relative rate constants. An extensive reexamination of the HI-scavenged C2H2system is probably necessary to improve the precision of agreement, and determine whether significant discrepancies remain. The only apparent substantial difference in the relative reactivity ratios is that found for I8Fbetween HI and H2, which has the value 4.2 for the competition here and 2.8 in the HI-scavenged C2H2.14 Again, the differences in moderator and temperature conditions may provide an explanation for the discrepancy. Another consideration is that these very fast reactions with molecules such as H I may not exhibit normal Arrhenius behavior in thermal reaction systems. Measurements of the rate constants vs. temperature for C1+ H2have been reported to deviate widely from the Arrhenius form~lation?~ and indicate that uncertainties may still exist when the fluorine atom rate constants have been intercompared with high precision. Acknowledgment. This research has been supported by Department of Energy Contract DE-AT03-76ER-70126. Registry No. F, 14762-94-8; lSF, 13981-56-1; C1CH2CH=CH2, 107-05-1; HZ, 1333-74-0; ClCHZCHCH2F*,96706-04-6; CICH2CHFCH2', 96706-05-7; C12, 7782-50-5; HI, 10034-85-2; C H E C H , 74-86-2; CH4, 74-82-8. (23) Mei, C. C.; Moore, C. B. J . Chem. Phys. 1979, 70, 1759.
Oxygen-17 and Proton Nuclear Magnetic Resonance Studies on Acetic Acid Exchange Processes of the Chloride, Nitrate, and Acetate of Nickel( I I ) in Acetic Acid' Akiharu Hioki; Shigenobu Funahashi,* and Motoharu Tanaka Laboratory of Analytical Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya 464, Japan (Received: February 22, 1985)
The exchange rates of acetic acid coordinating to nickel(I1) chloride, nickel(I1) nitrate, and nickel(I1) acetate in neat acetic acid and acetic acid/dichloromethane-dzmixtures were measured by the oxygen-17 and proton NMR line-broadening methods. The activation parameters for the acetic acid exchange on these nickel(I1) salts were independent of the concentration of acetic acid (HOAc) in the mixed solvents. The first-order rate constants at 25 OC and the activation parameters are k = (5.5 0.2) X lo5 s-l, AH*= 41 i 2 kJ mol-', and AS* = 3 7 J mol-' K-I for NiC12, k = (3 1) X lo5 s-I, AH* = 37 i 5 kJ mol-', and AS* = -18 20 J mol-' K-' for Ni(N03)2,and k = (3 1) X lo5 s-l, AH* = 50 5 kJ mol-', and M* = 28 20 J mol-' K-' for Ni(OAc)2. Solvent exchange was proposed to proceed via a dissociative-interchange mechanism.
* *
*
Introduction Solvent exchange processes on metal ions are the fundamental phenomena of metal ions in solution and studies on such processes are very important for elucidating the reaction mechanisms of metal complex formation." Recently we have studied the kinetics of complex formation of nickel(I1) chloride, nickel(I1) nitrate, and nickel(I1) acetate with 1-(2-pyridylazo)-2-naphtholin acetic acid (HOAC).~This study prompted us to attempt measurements of the exchange rates of acetic acid coordinating to nickel(I1). Acetic acid is an amphiprotic solvent with a low dielectric constant. All electrolytes exist as nondissociating species in this solvent. Since the pioneering work of Swift and Connick,6 fast solvent exchange on paramagnetic ions has been studied extensively by the N M R m e t h ~ d . ~Oxygen-17 ,~ N M R studies on the exchange of nonaqueous solvent are few7-" as compared to many studies Present address: The National Chemical Laboratory for Industry, Yatabe, Ibaraki 305, Japan.
*
*
*
on water exchange on metal ion^.^,'^ Since the oxygen-17 nucleus has a wide range of chemical shifts, it has some advantages in comparison with the proton. Making use of oxygen-17 we can (1) Metal Complexes in Acetic Acid. 7. For part 6, see ref 5. (2) Wilkins, R. G. "The Study of Kinetics and Mechanism of Reactions of Transition Metal Comalexes": Allvn and Bacon: Boston. 1974. (3) Martell, A. E. "Cwrdination Ckemistry"; American Chemical Society: Washington, DC, 1978; Vol. 2. (4) Burgess, J. "Metal Ions in Solution"; Ellis Horwood: New York, 1978. (5) Hioki, A.; Funahashi, S.;Tanaka, M. Bull. Chem. SOC.Jpn. 1984.57, 1255. (6) Swift, T. J.; Connick, R. E. J . Chem. Phys. 1962, 37, 307. (7) Stengle, T. R.; Langford, C. H. Coord. Chem. Reu. 1967, 2, 349. (8) Moore, P.; Sachinidis, J.; Willey, G. R. J . Chem. SOC.,Dalton Trans. 1984, 1323. (9) Babiec, J. S.; Langford, C. H.; Stengle, T. R. Inorg. Chem. 1966, 5 , 1362. (10) Fiat, D.; Luz, Z.; Silver, B. L. J . Chem. Phys. 1968, 49, 1376. (11) Poupko, R.; Luz, 2. J . Chem. Phys. 1972, 57, 3311. (12) Hunt, J. P. Coord. Chem. Reu. 1971, 7, 1.
0022-3654/85/2089-5057$01 SO10 0 1985 American Chemical Society
5058 The Journal of Physical Chemistry, Vol. 89, No. 23, 1985
Hioki et al.
TABLE I: Comwsition of SamDle Solutions
solution A0 Alc A2a BO Blc B2n B3a
co Clc C2a DO
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Dlc D2n D3a D4a
Ni(I1) salt reference solution NiC12 Ni(OAc),
[Ni(II)]/m
CHOAcu/m
[CD,Cl,l/m
0
16.65
0
3.37 x 10-2 2.14 X lo-,
16.65 16.65
0 0
4 6 (CH,) 4 (OH)
reference solution NiC1, Ni(N03)2 Ni(OAc),
0
1.65
10.4
5.83 x 10-3 3.86 X IO-' 2.89 X
1.65 1.65 1.65
10.4 10.4 10.4
reference solution NiC1, Ni(OAc)* reference solution NiCI, Ni(NW2 Ni( OAc), Ni(OAc),!
0
16.52
0
1.72 X IO-, 1.45 X lo-, 0
16.52 16.52 3.31
0 0 9.16
1.09 X 1.15 X lo-, 3.68 x 10-3 6.80 x 10-3
3.37 3.37 3.31 3.31
9.16 9.16 9.16 9.16
PM
nb
4 6 6 (CH,) 4 (OH)
HOAc,
"0,
wt %
atom %
0
100
8.16 x 10-3 7.77 X 5.17 x 10-3 0
100 100
1.43 X 1.41 X
9.9 9.9 9.9 9.9
1.06 X 7.05 x 10-3 0
100
5.13
4 6
2.10 x 10-3 2.63 x 10-3 0
IO0 100 20.4
5.13 5.13 5.13
4 6 6 6
6.60 x 1.05 X 3.30 x 6.11 X
20.4 20.4 20.4 20.4
5.13 5.13 5.13 5.13
10-3 lo-, 10-3
lom3
OCHOAcis the total concentration of HOAc. bSee text.
monitor relatively fast processes and observe the chemical exchange region at higher temperatures. Moreover since an oxygen atom in a solvent molecule acts mostly as a ligating atom, oxygen- 17 is convenient as a probe nucleus for solvent exchange. Though the natural abundance of oxygen-17 is extremely low (0.037%), the preparation of oxygen-17 enriched acetic acid enabled us to observe I7O N M R readily. In this paper we describe the results of our investigation on the rate of exchange of acetic acid between the bulk acetic acid and the coordination sphere of a cation in NiCl,, Ni(N03),, or Ni(OAc), by the I7O FT-NMR line-broadening technique. For confirming the frequency effect under conditions where chemical shift contributes to observed line width, we measured I7O N M R a t 8.16, 12.15, and 54.21 MHz. Moreover, the behavior of the methyl protons is compared with that of the hydroxyl proton and the results of 1 7 0 N M R . Experimental Section Reagents. The purification of acetic acid (HOAc),13 dichloromethane-d, (CD2C12),14and tetramethylsilane (SiMe4)l 4 was described previously. Oxygen-17 enriched water (I7O, 20.7 atom %; I8O, 38.7 atom %) was purchased from CEA France. The mean molecular weight is 18.994. The purified acetic anhydride was mixed with an equivalent amount of enriched water. The mixture was kept at room temperature for 2 weeks and then a t 85 OC for about 10 days. The water content was confirmed to be less than mol dm-3 by Karl-Fischer titration. The concentration of this oxygen-17 enriched acetic acid is 16.52 m. The percentage of oxygen-17 is 5.13 atom % of all oxygen atoms. DzO, methanol-d4 (CD30D), and dimethyl-d, sulfoxide ((CD3),SO) were obtained from Merck and Aldrich. The preparation ~] of hexakis(solvent)nickel(II) nitrate ( [ N ~ ( H O A C )(NO3)2), tetrakis(solvent)nickel(II) chloride (N~CI,(HOAC)~), and tetrakis(solvent)nickel(II) acetate (Ni(OAc),(HOAc),) was described el~ewhere.~ These nickel(I1) species in acetic acid have octahedral structures, and chloride ions in NiC1, and acetate ions in Ni(OAc), exist in the inner sphere of the nickel(I1) ion5 All operations for the preparation of N M R sample solutions were carried out on a standard vacuum line. A reference solution without the nickel(I1) ion for N M R measurement was prepared by distillation. An N M R sample solution containing the nickel(I1) ion was prepared in the following manner. A known amount of acetic acid solution of the nickel(I1) salt was placed into a 5-mm (13) Sawada, K.; Ohtaki, H.; Tanaka, M. J. Inorg. Nucl. Chem. 1972, 34, 625. (14) Hioki, A.; Funahashi, S.; Tanaka, M. Inorg. Chem. 1983, 22, 749.
0.d. N M R tube. The acetic acid in the N M R tube was removed by distillation on a vacuum line. A reference solution already prepared was added to the NMR tube by distillation and the tube was weighed and sealed. The composition of the sample solutions used for the N M R experiments is summarized in Table I. NMR Measurements. Solvent-nucleus 'H NMR spectra were recorded at 60 MHz on a high-resolution N M R spectrometer (JNM-C-60H, JEOL Ltd.) equipped with a variable-temperature controller (JNM-VT-3C, JEOL Ltd.). The external lock (D20) was operated. Care was taken to keep the rf level low enough to avoid saturation. The temperature was measured by a substitution technique using a thermister (SPD-02-1OA, Takara Thermister Co.) or a potentiometer (P-IB, Yokokawa Electric Co.) with a copper-constantan thermocouple. About 10 min was required for temperature equilibration of the sample solutions. The uncertainty of the temperature was estimated to be f0.3 'C. SiMe, (ca. 0.5 wt %) was used as an internal standard for the chemical shift measurement. Variable-temperature Fourier-transform 170NMR spectra were obtained with JEOL JNM-FX60, JNM-FX90QE, and JNM(3x400 instruments operating at 8.16, 12.15, and 54.21 MHz, respectively, with an internal deuterium lock (DzO, CD,OD, or (CD,),SO). Pulse widths of 20, 23, and 60 p and pulse intervals of 0.2, 0.2, and 0.15 s were used at 8.16, 12.15, and 54.21 MHz, respectively. Typically, the free induction decay spectrum was accumulated over 2000 to 12000 pulses. A 5-mm 0.d. NMR sample tube was immersed in a 10-mm 0.d. NMR tube containing a lock solvent. Since samples are heated by 'H decoupling and are surrounded by a lock solvent, temperature equilibrium of sample solutions was achieved after 30 min. Sample temperature was measured by using a thermistor (SPD-02-1 OA, SPD-01- 10A or D111-1031, Takara Thermister Co.) or a copper-constantan thermocouple put in a 5-mm 0.d. N M R tube with acetic acid, methanol, or ethylene glycol instead of the sample. The uncertainty in the temperature was estimated to be f0.5 'C. Basic Equations for Treatment of NMR Data. The following presentation of the N M R data is based on theories and experimental analyses extensively discussed in the l i t e r a t ~ r e . ~ $ ~The .'~-'~ solvent NMR line broadening ( TzPPM)-l due to the paramagnetic ion is expressed as ( TzPphl)-l = *(Avo, - Avwlv)PM-',where AvOM and AvsOlv(Hz) are the half-height widths of the solvent N M R spectra in the presence and absence, respectively, of the para(15) Funahashi, s.; Nishimoto, T.; Hioki, A.; Tanaka, M. Inorg. Chem. 1981, 20, 2648. (16) Luz, 2.; Meiboorn, S. J . Chem. Phys. 1964, 40, 1058. (17) Granot, J.; Fiat, D. J . Magn. Reson. 1974, 1 5 , 540.
The Journal of Physical C h e m i s t r y , Vol. 89, No. 23, 1985 5059
170and IH N M R Study of Solvent Exchange magnetic ion. PMis given as PM=fpM', where f is a factor (vide infra) and PM'is the mole fraction of solvent molecules bound to the paramagnetic ion. In an acetic acid molecule bound to nickel(I1) ion, one oxygen atom is directly bonded to the nickel(I1) ion and the other oxygen atom does not coordinate. The contribution of the latter to the line width and the chemical shift should be negligible. On the other hand, the two oxygen atoms of an acetic acid molecule in the bulk are equivalent. Therefore For IH N M R f is unity. for I7O N M R f i s accepted to be ( TZpPM)-I is derived by Swift and Connick6J7 as
- -1 - -1 TZPPM
TM
T2M-2 (TM-'
+ (TMTZM)-' + AuM2 1 + T ~ M - '+) ~A W M ~ + T20
(1)
where iM and TZMare the mean lifetime and the transverse relaxation time of the observed nucleus of solvent in the first coordination sphere of the paramagnetic ion, A w M is the difference of the resonance frequency between the observed nucleus of solvent in the inner sphere and that of free solvent, and TzO-l is the relaxation term due to the interaction in the outer sphere of the paramagnetic ion. The solvent exchange rate constant k is equal to TM-' = (kBT/h) exp(-A"/RT AS*/R). The temperature is given by A w M = -C,/T.18,22 TZM is assumed dependence of bM to have a simple temperature dependence given by TZM-I= (CM/T)eXp(EM/RT) and T20is also expressed as T2d1= (Co/T) exp(E0/RT).l8 For T2M-2>> AuM2,eq 1 reduces to
I
I
2t 3
4 I / T xi03
Figure 1. Temperature dependence of the line width for oxygen-17 of HOAc in the absence of metal ions: (0)sample CO; (A)sample DO.
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+
--1 T2PpM
1 TM
+
+-1 T2M
T20
I
I
(2)
When TZM-Iis small enough compared with the other relaxation terms, eq 1 becomes
--1 T2PpM
1 (TMAOM~)-'
+ TM
1 + -T20
(3)
The chemical shift of the solvent nucleus due to the paramagnetic ion is expressed as Aw,b,dPM-' = 27T(vobsd - V ~ , I ~ ) P M - ~ . The value of AWObsdPM-l depends on A w M , TM, TZM, as given by6 -AuM
-Awobsd - M '
(1
+ TM/T2M)2 + ( 7 M A w M l 2
(4)
For TZM-l
