4132
NOTES
Reaction Rates of Alkyl and Peroxy Radicals with Copper Ion-Pulse Radiolysis Studies
by A. RlacLachlan Contribution N o . 1379 from the Central Research Department, Experimental Station, E. I . U u Pont de Nemours and Company, Wilmington, .Delaware 19898 (Received June 19, 1967)
The pulse radiolysis technique' has been applied to observe directly the rates of interaction of substituted alkyl and peroxy radicals with CUI and CUI' ions. Some effect's of radical structure and ligands were also studied. Pulse radiolysis of solutions involves subjecting the sample to a 5-psec beam pulse of electrons (energy 5-8 MeV) which produces during that time a concentration of free radicals dependent on the G(so1vent). If the solution is deoxygenated, only alkyl-type radicals are produced, and correspondingly, if the solution is saturated with oxygen before pulsing, high yields of peroxy radicals are obtained.2 In this work, radicals were generated from three solvent systems: ethanol (EtOH) , 98% ethanol-2% acetic acid (98-2 E-A), and glacial acetic acid (HOAc). The efficiencies and rates of interaction of Cu'I and CUI ions with the radicals produced were examined. Table I lists the interaction efficiencies for CuClz and c ~ ( 0 A . c ) ~CuC12 . in EtOH and in 98-2 E-A Mlpulse, in agreement with the captures 2 X known radical yield in ethanoL3
Table I: Capture Efficiencies of Alkyl Radicals by Cu" Ions (2.00 X M) Solvent
Mole of CuII reduoed/pulse
CUClZ
EtOH
2.3 x 10-4
CUCll
98-2 E-A 98-2 E-A HOAc HOAc
2 . 9 x 10-4 5 . 2 X 10-6 4.5 x 10-6 1.0 x 10-4
98-2 E-A
1.6
Compound
Cu(0Ac)z Cu(0Ac)z C ~ ( 0 A c ) z4- 1 M NH~OAC C ~ ( 0 A c ) z4- 1 M XIJ~OAC
x
10-4
in pure ethanol because of the immediate precipitation of the CUI produced. However, in 98-2 E-A no precipitation problem occurred; surprisingly only 5 X 10-5 mole of CU" was reduced per pulse. From kinetic arguments, Kochi3 has concluded that CU"(OAC)~dimer is unreactive toward alkyl radicals. The spectrum of CU(OAC)~ in 98-2 E-A clearly indicates the presence of dimer. Table I shows that the reduction yield of CU(OAC)~ is also low in pure HOAc (4.4X mole/ pulse), another solvent where dimer is presents4 This result could also be due to a low yield of substituted alkyl radicals or to the low reactivity of the CHzCOOH radical as a reducing agent. To distinguish these possibilities, Cu(0Ac)z in HOAc containing 1 M NH40Ac was irradiated. Spectral evidence indicated a significant diminution in the dimer concentration, and the reduction yield increased to 1 X mole/pulse. Addition of 1 M XH40Ac to 98-2 E-A also dissociated CU(OAC)~, and as the last entry in Table I shows, there was a corresponding increase in reduction yield. These results agree with K o ~ h i . I~n addition, we find the . CH2COOH radical to be a good reducing agent with a rate constant greater than lo9 l./mole sec. CUI ions were not reoxidized by substituted alkyl radicals to Cu" in any of the solvents used; thus, this is a relatively slow reaction if it occurs at all. Peroxy radicals were studied by presaturation of the solutions with oxygen. Since it has already been shown that the substituted alkyl radicals react at a diffusioncontrolled rate, it is necessary to use low concentrations of Cu" ( 2 X M )to have peroxy radical formation compete with the alkyl radical reduction reaction. Transient observation and end product spectral analysis revealed no detectable reaction of a-ethanol peroxy radicals with Cu" chloride or acetate. Cuprous ion was investigated by first reducing a CuCL ( 2 X M ) solution in ethanol by electron irradiation of a deoxygenated solution. This sample was then saturated with oxygen and pulse radiolyzed. Transient analysis showed a rcoxidation by the a-ethanol peroxy radical that followed the beam pulse duration. Thus the rate constant is at least 109 l./mole sec and may be diffusion controlled. It was not possible to run these experiments with cuprous acetate because of the rapid rate of reoxidation by dissolved oxygen. In future work, the effects of ligands on the rates of alkyl and peroxy radical reactions with cupric and cuprous ions will be studied.
-
~
Direct observation a t 3400 A showed that reduction of CU" in 98-2 E-A foIIows the beam pulse shape even down to 2 X. M CuCL. Consequently, the rate constant is greater than lo9l./mole sec and is probably diffusion controlled. Cu(0Ac)z could not be examined The Journal of Physical Chemistry
(1) R. L. McCarthy and A. 1187 (1960).
MacLachlan, Trans. Faraday SOC.,5 6 ,
(2) A. hlaclachlan, J . Am. Chem. SOC.,87, 960 (1965). (3) J. K. Kochi and R. V. Subramanian, ibid., 87, 4855 (1965). (4) J. K. Kochi and K. V. Subramanian, Znorg. Chem., 4, 1527 (1965).
NOTES
4133
Acknowledgments. The author is indebted to J. R. Merrill for helpful discussions and to R. T. Edwards, Jr., and R. B. Uhlig for assistance in the experimental work.
8 = (w
Determining Nuclear Spin-Lattice Relaxation Times
p
=
V(.)
ScientificLaboratory, Ford Motor Company, Deurbwn, Michigan 481.81 (Received January 28,1967)
Perhaps the least complicated method of determining nuclear spin-lattice relaxation times (TI) in liquids is that of saturation-recovery. 1--3 This method consists of saturating a particular resonance line with a large HI field4 and following the subsequent recovery of the nuclear signal after HI has been reduced to a small, nonsaturating, value.5 It is presumed that the v mode will recover exponentially with a time constant TI. It is well known that residual saturation effects in the low HI field impose an effective upper limit on the TI values that can be determined by this method. The saturation-recovery method has been less critically examined in the range of short TI values. Experimentally, it has been observed in a number of laboratoriesaP6that there is large initial distortion in the recovery curve, due to a “transient” which seems to last about 200 msec. A typically distorted recovery curve is shown in Figure 1. The cause of this distortion has not been well understood: it has been attributed to both experimental and nuclear origins. The transient caused us considerable concern, for being uncertain of its true duration, we were unable t o gauge the accuracy of short TI values obtained from saturation-recovery measurements. It is hoped that the following discussion will prove valuable t o others who may face the same problem. Torreyz has obtained a general solution to the Bloch equations, including transient terms. We shall restrict our attention to the case where TI = Tz. This case is easy to treat mathematically, and it describes the relaxation behavior found in many simple liquids. When T1 = Tz, Torrey’s solution for the v mode becomes
+ 1 + p2 +
62
)cos (sr)
+boy*
(IC)
T = TI = Tz
(14
s = (1
l/THIT;
by J. E. Anderson and Robert Ullman
e-qvo
Ob)
The quantities ~ 0 ,vo, and mo are the initial values of u, v, and m,, respectively, for a particular value of 6. Equation 1 describes the radiofrequency absorption of a particular “spin isochromat” a distance (6) away from resonance. The experimental absorption signal is obtained from
On the Saturation-Recovery Method for
=
- Uo)/YH1
+
=
J”; 9 (6)v (6,
(2)
where g(6) is a weighting factor reflecting the Ho field inhomogeneities. Specifically, we will take g(6) = (a/.) [az a2]-’, where a = l/-yHITz* (Tz*is the apparent value of T2in the inhomogeneous field). We are interested in v ( r ) following the reduction of HI at t = 0. For this reason, it is convenient to let the symbol HI
+
-10
0
01
02
03
04
05
06
07
TIME IN SECONDS
Figure 1. Solid curve: saturation-recovery curve obtained from eq 3. The initial and final HI values are 5.9 and 0.030 mg, respectively; T I = Tz = 0.40 sec; Tz*= 0.04 sec. Dashed curve: exponential recovery. The two curves have identical asymptotic behavior.
(1) See, for example, E. R. Andrew, “Nuclear Magnetic Resonance,” Cambridge University Press, New York, N. Y., 1958,p 107 ff. (2) H. C. Torrey, Phys. Rev., 76, 1059 (1949). (3) A. L. Van Geet and D. N. Hume, Anal. Chem., 37, 983 (1965). (4) The notation used in this article has become conventional. A complete exposition is given in ref 2. (5) J. G. Powles and D. J. Neale, Proc. Phys. SOC.,77, 737 (1961), have used a variation of this saturation-recovery method which is preferable for long TI values. Following the reduction of the saturation HIfield, they shift the HOfield off resonance. The recovery of the magnetization is then monitored by periodic sweeps through resonance a t low HI levels. The absence of a continuously resonant HI greatly reduces residual saturation effects. It does not circumvent the difficulties with short TIvalues, however. (6) G. Weill (private communication, University of Strasbourg, France) has observed the transient on a Varian HA100 spectrometer.
Volume 71, Number 12 November 1967
