1548
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
Gergely et al.
Effect of -NH2 H 2 0 Proton Exchange on the Paramagnetic Relaxation of Water Protons. 1. Nuclear Magnetic Resonance Study of the Copper(I1)-Glycine System IstvTn Nagypsl, Etelka Farkas, Ferenc Debreczeni, and Arthur Gergely * Institute of Inorganic and Analytical Chemistry, Lajos Kossuth University, 40 10 Debrecen, Hungary (Received September 2, 1976; Revised Manuscript Received January 2, 1978)
Fast proton- and ligand-exchange processes have studied in the copper(I1)-glycine system by measuring the transverse relaxation time of the water protons. When the experiments were carried out over wide pH and concentration ranges, it was found that at >0.001 M copper(I1)concentration the experimental data could not be explained by considering the effects of TB and T z B alone. To explain this deviation, it was assumed that at higher copper(I1) concentration the rate of the ligand exchange becomes comparable with the rate of -NH2 e H20 proton exchange in the diamagnetic environment. With the above assumption the results could be evaluated within experimental error. It was possible to obtain information on the T2B values, the ligand-exchange rate constants, and the -NH2 e H20proton-exchange rate constants. The advantages and disadvantages, and the possibilities and limitations of this method (suitable for the simultaneous study of proton exchange between dia- and paramagnetic environments and within a diamagnetic environment) are briefly outlined.
TABLE I: Cu(glycine), and Excess Glycine Introduction Concentrations of t h e Samples Studied In earlier work1 we determined the rate constants of Cu(glycine), proton-exchange reactions in the dia- and paramagnetic M Glycine excess = TG - 2Tcu M environments in aqueous solutions of some copper(I1)amino acid systems. The proton exchange in these systems 0.0010 0.150 0.0020 0.150 takes place via the ligands, and thus a correlation could 0.0030 0.150 be found between the structures of the amino acids, the 0.0050 0.150 thermodynamic stabilities of the complexes and the 0.0101 0.150 proton-exchange rate constants. The relation between the 0.0202 0.150 0.100 0.050 line width and the free ligand concentration was derived 0.0303 0.1 50 from kinetic and probability theory considerations. The 0.0404 0.150 0.100 0.050 0.0631 0.124 0.0863 0.0488 result agreed with the relation derived from the B l o ~ h ~ - ~ equations. The measurements showed the line width to It was assumed that a new method could be developed be linearly proportional to the free-ligand concentration to study the -NH2 * H20 proton exchange if the exin the concentration range used ( TL< 0.03 M, Tcu < 0,001 perimental results were interpreted in terms of the above M) with the exception of the copper(I1)-glycine system. factors. Thus, the aim of the present work was to clarify The saturation curvelike change in the copper(I1)the possibilities and limitations of this new approach. In glycine system was interpreted by taking into account the order to achieve this, the copper(I1)-glycine system was effect of TZB,in accordance with the earlier finding of studied. Pearson and Lanier.2 This means that, as a consequence of fast ligand exchange, T B (the average lifetime of the Experimental Section protons in the paramagnetic environment) becomes The C~(g1ycine)~ and excess glycine concentrations of comparable with T2B(the relaxation time of the protons the solutions studied are listed in Table I. in the paramagnetic environment) at a relatively low In order to investigate the line width (relaxation time) free-ligand concentration, and the line width is controlled vs. pH correlation, 20-cm3 samples containing 1.0 M KC1 only by T2Ba t higher ligand concentration. were titrated against 1.485 M KOH solution. 0.5-cm3 For the rate constant of the process samples corresponding to the individual titration points k were taken in standard NMR tubes. After recording the CuGG + G * - A CuGG* t G (1) spectrum, these samples were carefully returned to the titrated solution. (The change of the total concentrations a value of 5 X lo7 M-l s-l was given. in the samples during the titration remained below 2% .) T o determine T 2 B we also carried out measurements a t higher copper(I1) concentrations (Tcu> 0.01 M). It was The NMR spectra were recorded a t 28 f 1 “C on a JEOL JNM MH-100 instrument. observed that an increase in the copper(I1) concentration The p H and concentration distribution of the species causes a continuous trend in T2Band k2. This indicated in the samples were calculated with a computer program that the paramagnetic relaxation of the protons was inreported earlier.* Values of pKNHz= 9.60 ( I = 1.0 (KC1, fluenced by other factors in addition to T B and TzB. 25 OC),9and K3 = 1.8 for the processlo CuG, + G- CuGg The high value of the exchange rate constant led to the possibility that a t higher copper(I1) concentration the rate were used to calculate the species distribution. of ligand exchange becomes comparable with the rate of Experimental Results and Their proton exchange between -NH2 and H2O. This would Qualitative Interpretation mean that the relaxation time for water protons is inThe experimental results are shown in Figures 1-5. fluenced by the -NH2 + H 2 0 proton exchange as well. Figure 6 shows the line width as a function of the cop(The “Aw” relaxation mechanism does not play a role in per(I1) concentration at different pH values in samples the case of copper(I1) c ~ m p l e x e s . ~ ~ ~ ) 0022-3654/78/2082-1548$01 .OO/O
0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
NMR Study of the Copper(I1)-Glycine System
HZ
80
*ool
%?
-
Hz
60 -
40 100 20
1549
-
0
4
8
10
9
pH
Figure 1. pH dependence of the NMR line width of the water protons in the copper(I1)-glycine system at different compositions: (1) T,, = o.001; (2)r,, = 0.002;(3) r,, = 0.003; (4) r,, = 0.005; (5) T," = 0,0101 M. (1-5) Glycine excess = 0.15 M.
AY/z
9 10 pH 8 Figure 3. pH dependence of the NMR line width of the water protons in the copper(I1)-glycine system at different compositions. T,, = 0.0202 M in all cases: (6) glycine excess = 0.15; (10) glycine excess = 0.10; (1 1) glycine excess = 0.05 M.
0
8
Hz
HZ
200 -
200 -
100-
100 -
L
I
I
a
9
10
1 pH
Figure 2. pH dependence of the NMR line width of the water protons in the copper(I1)-glycine system at different compositions: (5) T,, = 0.0101; (6)Tcu = 0.0202;(7)Tcu = 0.0303; (8) T,, = 0.0404;(9) T,, = 0.0631 M. (5-8) Glycine excess = 0.15 M; (9) glycine excess = 0.124 M.
containing a 0.15 M glycine excess. The partial mole percentages of the species HG*, G-, CuG2, and CuG3- are given in Figure 7 as a function of the p H at the highest 1igand:metal ratio. From the shapes of the curves in Figures 1-7, the following more important conclusions can be drawn: (1) It can be seen from Figures 1 and 2 that the A v l j z values tend toward a copper(I1) concentration-dependent limit with increasing pH &e., increasing free ligand concentration). These saturation curvelike functions show that ?B is a free-ligand concentration-dependent quantity.
L
I
I
8
9
10
pH
Figure 4. pH dependence of the NMR line width of the water protons in the copper(I1)-glycine system at different compositions. T,, = 0.0404 M in all cases: (8) glycine excess = 0.15; (12) glycine excess = 0.10; (13) glycine excess = 0.05 M.
A t higher ligand concentrations rB C TZB, Le., the line width limit is determined by T 2 B and naturally by the copper(I1) concentration. (2) It can be seen from Figure 6 that at the same pH (i.e., practically the same free-ligand concentration) the line width depends on the copper(I1) concentration in accordance with a saturation curve also. This change cannot be interpreted by the effect of TZB, nor by the formation of CuG3-, because the partial mole fraction of CuG3- does not change along the curves. The probable explanation of this deviation from linearity is that a t higher copper(I1) concentration the average lifetime of the free ligand in the diamagnetic environment
1550
Gergely et al.
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
OIo
All2
80
Hz
\
/'
/
\
\
/ \
200-
/
\
60
\
/
I
40
100-
20
I
I
I
8
9
10
8 pH
Figure 5. pH dependence of the NMR line width of the water protons in the copper(I1)-glycine system at different compositions. ,T = 0.0631 M in all cases: (9) glycine excess = 0.124; (14) glycine excess = 0.0863; (15) glycine excess = 0.0488 M.
9
10
pH
Figure 7. Partial mole percentages of the proton and copper(I1) complexes as functions of pH. Tcu = 0.001 M. Glycine excess = 0:15 M: (1) HG"; (2) G-; (3) CuG,; (4) CuG,-.
Scheme I
n
At12 HZ
250
4
200
150
100
0.01
0.02
0.03
0.04
Tcu (MI Figure 6. Copper(I1) concentration dependence of the NMR line width of the water protons in the copper(I1)-glycine system at different pH values. Glycine excess = 0.15 M: (1) pH 8.5; (2) pH 9.0;(3) pH 9.5; (4) pH 10.0,
is comparable with the average lifetime of protons bound to nitrogen, and thus the line-width limit is controlled by H 2 0 proton exchange. the rate of the -NH2 (3) It can be seen from Figures 3-5 that the line-width limit also depends on the total glycine concentration. This indicates the presence of an -NH2 + HzO proton exchange which depends on the total glycine concentration too. In the interpretation of the results, it has to be taken into account that the complexes CuGz and CuG3- contain water molecules in the axial position. The effect of the protons of these molecules is not significant in the line
broadening, as their relaxation time is some orders of magnitude higher than that of the -NHz protons. The molar line-width increasing coefficient of the water molecules in CuG2 has been determined in samples a t rt 2 (no ligand exchange) and found to be 80. The molar line-width increasing coefficient of the water molecule in CuG; cannot be determined experimentally, as CuG3- is formed only at a high free-ligand concentration where the effect of ligand-exchange determines the line width. Therefore, a value of 40 (half that for CuG2)was assumed for the molar line-width increasing coefficient of the water molecule remaining in the coordination sphere of the CUG~-. On the instrument available the line width of pure water was found to be 3.0 Hz.ll Thus, the line width assigned to the proton- and ligand-exchange processes can be obtained via the equation A v , , ~= A ~ l , -2 3.0 ~ -~80[CuG?] ~ ~ 40[CuG3-] (2) From Figures 1-5 and the above equation it can be seen that the correction of the expermentally measured line width lies below 10% in general. Mathematical Considerations The relaxation and proton-exchange processes occurring in the system studied can be illustrated as shown in Scheme I. The circles in this scheme represent the protons in different chemical environments, and the arrows the rates of the proton exchange between them. The processes transforming the effect of the paramagnetic environment
NMR Study of the Copper(I1)-Glycine System
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1551
to the HzO protons can easily be seen in this scheme. The proton-exchange processes can be divided into two parts. The index r represents the protons which are relaxed but have not been in the H 2 0 form since their relaxation (i.e., undetected) and the index u represents the protons which have not been relaxed since their last occurrence in the HzO form. In brief, r and u denote relaxed and unrelaxed protons. For an interpretation of the experimental data the rate u,9 has to be known. This rate is in close connection with the relaxation time (TJ, Le., with the corrected line width: 9
1 ur9 u, =A v I , ~= -= nT2 2 n [ H 2 0 ] 348.7
reactions lead to proton exchange in the diamagnetic environment: -OOCCH,NH, t HOH
k- 4
i.e. G - t H,O
G-
(4)
and the exchange of protons in CuG3- and G- through the process k,
CuG, e CuG, t G -
(5)
k-3
in the concentration range studied. This latter process means the exchange of protons between CuG3- and CuGz as well. According to Scheme I, this means that the following relations are valid:
u,' Ur3
+ u U 1 = vr2 + u,' = 2hz[CuG2] [ G I = A (6) + U U 3 = Ur4 + U U 4 = 1 / 2 ( u , 5 + u, 5 ) = 1 / 2 ( U r 6 + uU6) = 2k3[CuG3] = B
(7)
The probability of relaxation during the residence time of the protons in the CuGz and CuG3- forms is questionable. Let us denote the probability of relaxation in CuG2 by w2 and that in CuG3- by w3. With these, the rates of the leaving of the protons from the CuG, and CuG3- forms can be expressed as follows: u,'
+ ur6 = ur2 + ur5 + (u,* + u u 5 ) . w 2 + 24.5 = + + + UU6).W3
(8)
(9) w2 and TB@) and T2B(,)relating to CuGz can be correlated as follows:
u,3
ur6
u,4
(u,4
w2 = J ;e-t/7~(Z)e-t/TzB(Z)
dt/Jye-t/TzB(2)dt =
+
rB(2)/(7B(2) T ~ , B ( ~ ) ) From eq 4 and 5
T B ( ~ can )
= ~ [ C U G/(A ~]
be given as
t HG" + H G ~ t H,O t G -
ur7
+ vu7 = u,' + u,' 2k5[G-] [HG']
By similar considerations, w3 can be expressed as wg = Y / ( 3 B+ Y ) (13) where Y = rate of relaxation in CuG3-; i.e., Y = 6[CUG3-]/ T ~ B ' ~ ) . (2) For the derivation of v,9 and Aullz depending on it, the rates of the proton-exchange processes between G- and HG* and between HG* and HzO have to be known. With u1trasonid2 and spin-echo13 methods, it was earlier found that in the pH range studied by us the following
= 2k4[G-] + + 2k5 *[G-] [HG']
=D
(16)
k5 and h5* in eq 16 denote the proton exchange between -NH2 and -NH3+ with and without water participation. The rate constants h4 and h5 include the water concentration. The factor 2 expresses the fact that the exchange of one proton means that two protons go to the -NH3+ form from -NH2 and vica versa. (3) The exchange of protons between -NH3+ and H 2 0 takes place through processes 14 and 15, but the direct -NH2 -NH3+ proton exchange does not play a role in this, i.e.
ul0
=
ur9
+ u U 9 = k 4 [ G - ] + k5[G-][HG']
= C (17)
Apart from the aboves, the following equations (based on equilibrium conditions) can be used for the derivation of v,9: u,'/u,'
=
u, 5/u,
= u, 3/u,
u,2/u,
=
(19) =
u,6/u,
+ VI()) = = u,' + + + =
u,7/(uu7 u,7
(18)
U,6/UU6
(20) (21)
u,7/u,'
u,8/u,'
=
u,g/u,g
u,9
u,8
4 3
u,2
+ u,6 + 4
7
(22) (23)
From eq 6-9, 12, 13, and 16-23, u,9 can be expressed as follows:
+ 2B)
(11) Let us denote the rate of relaxation of CuG, by X ,i.e., X = ~[CUG~]/T~ and B (substitute ~) it into eq 10: ~2 = X / ( A + 2B + X) (12) TB")
+ H,O
(15)
Chang and Grunwald14have recently found that, besides reaction 15 there is direct proton exchange between -NHz and -NH3+ without water participation. Taking into account the above, the proton exchange between -NHz and -NH3+ could be given by the following equation:
u,'
(10)
(14) k
For the derivation of v,9 on the basis of the diagram, we have assumed the following relation: (1)The exchange of protons in CuGz and G- takes place through the process
+ G-
OH-
-OOCCH,NH, t HOH t HNH,CH,COO' -OOCCH,NH,+ t HOH t H,NCH,COO-
i.e.
k
k
4HG" t k-4
(3)
CuGG t G*- A CuGG*
'"4 OOCCH,NH,+ t OH-
CD
+C+D W
(24)
where
+ Y)+ (A + B ) X Y + 2B2(X + Y ) W = (A + 2B + X ) (3B + Y ) - 4B2
2 = 3AB(X
The chemical meaning of the rather complicated eq 24 is relatively simple. The first term of the product is the rate of the leaving of relaxed protons from the coupled paramagnetic environments. It is easy to realize, that if the effect of one of these is negligible compared to the other, then the relaxation rate of the protons is simply a
1552
Gergely et al.
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
TABLE 11: Results of the Least-Squares Fit of Eq 24 Setting the Value of One Parameter Equal to Zero Assumption
k, = O
T,B(,)= 0
k, = 0 ( T 2 R ( 3 )= 0)
( ZAy2/n)’”
32.0
32.0
5.7
k, = 0 19.0
k, = 0 7.6
TABLE 111: Results of the Five-ParameterFit of Eq 24 and the Derived Rate Constants Together with the Earlier Reported Values Present work k , , M-l s - * T , ~ ( z s) , k 3 ,s - ’
1.0 x
lo8
Ref 10 1.5 X
3.4 x
1.4 X
Accepted
lo7
lo’ 6 X 10’f 1.4 X 1.4 X k , , s-l 4.2 X lo6 k , , M-l s-’ 4.2 X 10’ a See ref 2, based on water protons. See ref 2, based on -CH,-protons. f s e e note 39 in ref IO.
8.2 x
lo6 a
I x lo-’
2.6 X 4x
lo6
6X
T , B ( ~ )S,
saturation curve originating from the comparison of the rate of chemical exchange and relaxation:
(Le. B , Y
-
0)
(Le. A , X 0). The second term of eq 24 consists of a comparison of the rate of relaxation in the paramagnetic and the rate of proton exchange within the diamagnetic environment. If proton exchange within the diamagnetic environment is much faster than relaxation, Le., the value of the second term is nearly one, the v,9 is determined only by the relaxation rate. If the two rates are comparable, then u,9 is determined by their ratio. The numerator and denominator of the second term comprise a fraction which contains the proton exchanges within the diamagnetic environment. If we assume that there is no direct proton exchange between the -NH2 and -NH3+ i.e. k6 = 0, then the expression can be simplified to
C D / ( C + D ) =,/3hq[G-]
+ ’//3ks[G-]
[HG’]
(27)
If, however, h,[G-][HG*] is very large compared to the others, then the simplification leads to
C D / ( C + D )= k,[G-] -t k5[G-] [HG’]
(28)
The measured null2and pH data pairs have been fitted by the equation derived from eq 24 by substituting it into eq 3. The full lines in Figures 1-5 represent the theoretical functions defined by the “best” set of constants calculated. For the curve-fitting procedure, the earlier result of Scheinblatt and Gutowsky15 has been accepted, namely that the sum of the rate constants h b and hb* is 3.8 X lo8 M-l s-l, i.e., D = 2h4[G-] + 7.6 X 1Oa[G-][HG*]. It has been found, moreover, that the k3 rate constant could not be calculated exactly from the data, as the maximum concentration of CuG3- was only 15% of the total copper(I1) concentration (see Figure 7). Thus a value of 6 X lo7 for h3 determined by Beattie et a1.l’ from ultrasonic measurements has been accepted, and has been kept constant in the curve fitting procedure. Discussion The fitting of eq 24 to the experimental points gave a value of 4.8 Hz for (CAy2/n)1/2.The measured line widths
lo5 1.6 X lo6 3.4 x l o 6 e 7.6 X 10‘ See ref 12. ‘See ref 14. e See ref 13.
8X
‘
were between 10 and 280 Hz, and thus this fitting can be accepted. Systematic deviations from the experimental data can be seen only in curves 3, 5 , 6, and 9. This may be due to the experimental error, but the following reasons are also possible: (1)The proton-relaxation time in this pH range may be influenced by some other proton-exchange process which was not taken into account. (2) A value K3 = 1.8 was used to calculate the CuG3concentration. This value was determined by Beattie et a1.l’ under somewhat different conditions from those used in this work. (3) We have only assumed a value for the molar linewidth increasing coefficient of the axially bound water molecule in CuG3-. (4) The CuG3- + G- CuG; + G- proton exchange may take place in the system. Taking this process into account the fitting of seven parameters is necessary. We have tried to fit the data by including this process too, but (EAy2/ n)lJ2decreased by only 0.8 Hz. Thus, the above process was considered to be unimportant compared to the others. (5) The temperature of the samples was maintained constant only to fl “C. With regard to the above sources of error, the average deviation of 4.8 Hz was considered good. For the appropriate fitting of the experimental data, all parameters are necessary. This is illustrated in Table 11, where the (EAy2/n)1/2data are given for those cases when the value of one parameter was taken as zero, and only the remaining parameters were calculated. From these tabulated data it can be seen that if the proton-exchange processes represented by h4 and h5 are not taken into account, then the fit is much worse. This indirectly proves the influence of proton-exchange processes on the relaxation time (line width). A relatively good fit can be achieved if h3 and 1/T2B(3) (Le., B and Y)are set equal to zero. As regards the four constants calculated with the assumption h3 = 0, only T2B(2)changes by 20% compared with its “best” value, the others remaining practically unaffected. This means that only T2B(3) (and to a smaller extent T2B(2) is uncertain, the other parameters can be regarded as exact. The inclusion of h, in the calculation is justified by chemical evidence rather than for computational reasons. The calculated values of the constants are listed in Table 111. The corresponding constants determined by other authors for the same system are also given. In an interpretation of the results it has to be considered that Beattie et a1.l0 measured the line broadening of the -CH2- protons, and therefore their experimental conditions were basically different. This could explain in part
NMR Study of the Copper(I1)-Glycine System
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1553
the almost one order of magnitude deviation in rate constant kz. Another explanation could be that, in principle, the G- HG* proton exchange must be taken into account even if the T z values relating to -CH2- are measured. It seems, moreover, that Beattie et al. have not taken into account that the process CuG3- CuGz Grepresents the transfer of four -CHz- protons from CuG; to the CuG2 environment. The difference in T2B(2)(this value is only slightly sensitive to the experimental conditions) reflects that it relates to the -NHz protons in our case, and to the -CH2protons in the work of Beattie et al.IO It must be taken into account, however, that, as a consequence of the form of the equation representing the line broadening, the product k2TzB(’)can be determined much more exactly than their values separately. Considering the basically different experimental method and conditions, the proton-exchange rate constants k4 and k 5 agree well with the recently reported data of Chang and Gr~nwa1d.l~ Pearson and Lanier2 measured the line broadening of the H 2 0 and -CH2- protons as well. They did not take into account, however, that copper(I1) was present mainly as CuG3- under their experimental conditions.
+
Conclusion Our results are in complete agreement with the critical comments of Beattie et concerning paramagnetic line broadening as a method for locating the binding site of the ligands, as our results prove that the fast exchange condition is valid only over limited pH and concentration ranges. The most important conclusion of this work, however, is that in this and similar systems the relaxation of the water protons cannot be described in the usual manner, by considering the T~ and T2Bvalues only. The relaxation time is influenced by proton-exchange processes within the diamagnetic environment too. As we have seen, this gives a possibility for the simultaneous study of the exchange processes between the dia- and paramagnetic environments as well as within the diamagnetic environment. The method used to examine fast reactions can be supplemented by using this approach, and a more complete picture of the exchange processes occurring in these systems can be obtained, especially with the combination of other methods available for the study of fast exchange processes. The advantage of the method is that it is experimentally very simple; because of the very high proton concentration of water, the measurements can be carried out on the
simplest commercial NMR instrument. It is in part advantageous and in part disadvantangeous that the method gives a rather complicated picture of the system studied. Resolution of the different effects is accompanied by significant mathematical and computational difficulties. At the same time, the relatively complicated picture means that the results given by different methods can be mutually cross-checked. It is necessary to make further investigations to discover the possibilities and limitations of this method. First of all that range of metal ions and ligands has to be defined for the complexes of which this approach can be used. It would be expedient to combine this method with the relaxation study of the nonlabile protons of the ligands. The basic equation has to be derived starting from the Bloch equations, taking into account proton-exchange processes within the diamagnetic environment. Work to answer the questions outlined above is continuing in our laboratory, and will be dealt with in a following paper.
Acknowledgment. We are indebted to Professor R. E. Connick, Department of Chemistry, University of California, for his helpful critical comments and his suggestion to improve the explanation of the results. We thank Dr. E. Brucher for helpful discussions, and Dr. P. JBkel for his help in solving the computational problems. References and Notes (1) I.Nagypll, E. Farkas, and A. Gergely, J . Inorg. Nucl. Cbem., 37, 2145 (1975). (2) R. G. Pearson and R. D. Lanier, J. Am. Chem. Soc., 86, 765 (1964). (3) H. M. McConnel, J . Chem. Pbys., 28, 430 (1958). (4) T. J. Swift and R. E. Connick, J . Cbem. Pbys., 37, 307 (1962). (5) N. Bloembergenand L. 0. Morgan, J. Chem. Pbys., 34, 842 (1961). (6) Z. Luz and S. Meiboom, J . Chem. Pbys., 40, 2686 (1964). (7) D. E O’Reilly and C. 6. Pooie, J . Pbys. Cbem., 67, 1762 (1963). (8) I.Nagypll, Acta Cbim. Acad. Sci. Hung., 82, 29 (1974). I A. Gergely, I. Nagypll, and E. Farkas, Magy. Kem. Foly., 80, 25 (1974). J. K. Beattie, D. J. Fenson, and H. C. Freeman, J. Am. Cbem. Soc., 98. 500 - - - (1976). - -, The resolution of the instrument is ca. 1.0 Hz. The measured line width therefore shows that saturation occurs in this sample. The extent of saturation, however, is proportional to the product of T, and T,. In the samples studied, therefore, where T2was some orders of magnitude smaller than in pure water, the saturation had no effect on the line width. D. Grimshaw, P. J. Heywood, and E. Wyn-Jones, J. Cbem. SOC., Faraday Trans. 2 , 756 (1973). D. D. Eley, A. S. Fawcett, and M. J. Hey, J. Chem. Soc., faraday Trans. 7 , 399 (1973). K. C. Chang and E. Grunwald, J . Pbys. Cbem., 80 1422 (1976). M. Scheinblatt and H. S.Gutowsky, J. Am. Cbem. Soc., 86, 4814 (1964). - 1
\
