J. A. JACKSON AND H. TAUBE
1844
Chemical Shifts in the Nuclear Magnetic Resonance Absorption for Oxygen-17 in Oxy Ions'
by J. A. Jackson and H. Taube University of California, Los Alams Scientific Laboratory, Los Alamos, New Mexico (Received November 16, 1064)
Chemical shifts in the oxygen-17 n.m.r. absorptions for the bridging and terminal oxygens of Crz07-2 are reported. The broadenings of these signals, and of that for the solvent caused by adding acid, are interpreted as arising from the operation of the Crz07-z-HCr04equilibrium catalyzed by Hf. The specific rate kz multiplying the function (H+)(Cr~07-~) is calculated as 1.3 X lo4m-l sec.-l in 3.1 m NazCrz07 a t 15 f 1". The n.m.r. absorption of MnO4- is shifted by Cof2, suggesting appreciable complex formation; it is broadened by n/In04-', and this broadening can be accounted for by electron exchange between Mn04and MnOrV2. A solution of SOz in water shows a single peak, at the position expected for the averaging of the SOz and solvent peeks. Exchange of oxygen between SO2 and HzO sec. Shifts for a number of oxy ions as solvent is rapid, the half-time being less than not previously reported are included in this paper.
Introduction
Experimental
Chemical shifts in the nuclear magnetic resonance (n.m.r.) absorption for oxygen-17 in oxy ions have been reported by Figgis, Kidd, and Nyholm.2 Early in our work exploring n.m.r. effects for 0 1 7 in solutions, we made a survey of chemical shifts for oxy ions. Where we have measurements on the same ions, namely for MII04-, Cr04-', Se04-2, M004-', C104-, C103-, and our values agree with theirs, within the limits of experimental error, which for our measurements we assess as 0.1 part in lo4 (10 p.p.m.). We have observations for a number of oxy ions not reported on by Figgis, et al., which we deem worthy of publication even though refined measurement with samples enriched in 017may reveal effects which were hidden from us. At the very least, our data can help to guide the initial efforts of others planning research in this field. In particular, we wish to report our results for solutions of NazCrz07. We have observed the peak for the bridging oxygen displayed separately from that of the terminal (nonbridging) oxygen, and have studied the effect of acid in broadening these signals, and the signal for the oxygen contained in the solvent.
The spectra were taken a t 6 , 7, or 8 Mc. on a Varian wide-line n.m.r. spectrometer as described previously. Unless otherwise stated, the samples were in HzO of normal abundance in 01' (0.04%) or in DzO enriched to about 0.1% O17. All shifts (AH/HO)are reported as parts per lo4, a paramagnetic shift being given a negative sign. The temperature in all the experiments was 20 f lo.
The Journal of Physical Chemistry
Results
+
The System CrzO~-2 HzO. In Figure 1 is displayed the n.m.r. spectrum in the dispersion mode for a solution approximately 2 m in NazCrz07made up in water ea. 1.4% in 0". We observe three peaks lying a t +0.07, -3.38 f 0.05, and -10.90 0.05 relative to a sample of pure water enriched in O17. The intense absorption a t the left is evidently that of the
*
(1) Work done under the auspices of the U. S. Atomic Energy Commission. (2) B. N. Figgis, R. G. Kidd, and R. S. Nyholm, Proc. Roy. SOC. (London), A269, 469 (1962). (3) J. A. Jackson, J. F. Lemons, and H. Taube, J . Chem. Phys., 38, 836 (1963);32,553 (1960); W.B.Lewis, J. A. Jackson, J. F. Lemons, and H. Taube, ibid., 34, 694 (1962).
CHEMICAL SHIFTS IN THE N.M.R.
i
n
ABSORPTION FOR OXYGEN-17 IN OXY IONS
I
OXYGEN-I7 NMR SPECTRUM NazCrzO7 IN H20
IIHnO
S~LVENT
1845
OXYGEN-I7 N M R SPECTRUM Na2Cr201 IN HO ,
1
OXYGEN
I
SOLVENT
BRIDGING OXYGEN
I
NONBRIDGING OXYGEN
I I/
BRIDGING OXYGEN
-I
d
l
1 ’ 1
-5
1
1
I
’/
! I I I I l 1 1 -10
AH/HO (PARTS / IO4) Figure 1. Oxygen-I7 n.m.r. spectrum of NazCrz07(-2 m ) in water (enriched t o -1.4% in O17). Derivative of dispersion.
solvent, that of intermediate intensity a t the right we attribute to the terminal oxygens of CrzO7-2 (our shift is close to that reported by Figgis, et a1.,2 but outside the limits of error placed by them on their results), and that of lowest,intensity we attribute to the bridging oxygen. This assignment is consistent with the intensity of the peak a t the right compared to that at -3.38 (the ratio lies between 5 and 7) and with the behavior of this peak when acid is added, as will be discussed more fully later. Figure 2a shows a trace in the absorption mode. Figure 2b shows an absorption trace a t higher gain in which we observed a peak of still lower intensity a t -9.0 which is probably caused by HCr04- present at equilibrium with Cr207-2(note: Cr04-2 lies a t - 8.3). Equilibrium quotients applicable to the very concentrated solutions we found it necessary to use have not been reported. Those appliabl le^-^ to lower ionic strength place the equilibrium concentration of HCr04- a t -0.3 and this is probably accurate to better than one order of magnitude. The peak width for FICr04- is expected to be particularly sensitive to acid; this, and the low intensity, may explain why we did not observe it in all samples. The width of the absorption peak for solvent water in the presence of Crz07-2is essentially identical with that which we observe for pure water and thus the operation of the equilibrium process Crz0T-2
+ HzO = 2HCr04-
(1) is not contributing to broadening of the n.m.r. signals. The rate of hydration of Crz07-2 has been measured by oxygen exchange studies: and by a relaxation technique,s and values for the specific rate for the firstset.-' and 2.7 X order process of 3.3 X sec. respectively, have been reported. These
0
f l l l l l l l l l l l l l l l l l l f 1 1 -5 -10 AH/Ho (PARTS / 18)
Figure 2a. Oxygen-17 n.m.r. spectrum of NazCr207(-2 m ) in water (1.4% 0 1 7 ) . Derivative of absorption.
values are not in conflict. Some of the oxygen exchange studies were done in 3 M solution, and the calculation of the specific rate quoted involved the use of equilibrium quotients6 which are not known for such concentrated solutions. The relaxation technique was applied to solutions of ionic strength 0.1 M . Even using the larger of the specific rates, no measurable contribution to line broadening by the process 7cl(Crz0T-2) is to be expected. The line widths are insensitive to the introduction of NazCr04 a t low concentrations, but are sensitive to the introduction of HC104. In Figure 3, we show the dispersion mode for the solution used for the trace of Figure 1 but with 0.06 M HCIOl added. The marked broadening of the water and terminal oxygen peaks is evident, and the peak for the bridging oxygen has been so much broadened as not to appear. In Figure 4 we show data on the line width of the three peaks as a function of concentration of acid in a solution 3.1 m in NazCrzO,. The line broadening is consistent with a linear variation in (H+) but the data are too sparse and imprecise to demonstrate that the variation really depends exactly on the first power of W+). The broadening is greatest for the bridging oxygen and least for the solvent. The broadening for the solvent, though small, is unmistakable, and shows that the solvent is part of the exchange pool which is causing (4) W. M. Latimer, “Oxidation Potentials,” Prentice-Hall, Inc., New York, N. Y., 1962. (5) At p = 0.1, G. Schwareenbach and J. Meier, J. Inoro. NucZ. Chem., 8, 302 (1957). (6) At p = 3.0, Y . Sasaki, Acta Chem. S c a d . , 16, 719 (1962). (7) H. Baldwin, private communication. (8) J. H. Swinehart and G. W. Castellan, I n o r g . C h m . , 3, 278 (1964) *
Volume 69,Number 6 June 1066
J. A. JACKSON AND H.TAUBE
1846
NON ERlDGlNO OXYGEN
BRIOPING
"&O
L
\
I
I
Figure 2b. Derivative of absorption a t higher gain. I
NMR SPECTRUM
II
SOLVENT OXYGEN
1
I
0
r
l
r
l
r
!
l
l
AH/lj,
l
l
-5
r
!
l
l
l
l
l
l
I
l
-10
i
l
(PARTS / IO4)
0.01 002 0.03 0.04 0.05 0.06 0.01 0.08
MOLALITY-HCI04
Figure 3. Oxygen-17 in 2 m NazCrzO, with 0.06 M HCIOl added. Derivative of dispersion.
Figure 4. Line width of 0 1 7 resonance in aqueous NazCr~07as a function of acidity.
the broadening. We shall show that the effects are consistent with the operation of equilibrium 1, the hydration of Crz07-2 now not being spontaneous, but catalyzed by H +. If we let the rate of the process kz(H+)(Cr~07-~) as given in m-l set.-' of water, oxygen be given by R, then the rates of the three exchange processes are: bridging to nonbridging = 3/dZ,nonbridging to water = 3/&,and water to bridging = I/&, with, of course, forward and reverse rates the eame in each case. The derivation of two of the relations will be given to illustrate how they come about. The reaction of HzO with Crz07-2 produces HCr030b- and HCr030,-, where subscripts w and b identify solvent and bridging oxygen. When HCrOaOb- and HCrOaO,recombine to form Crz07-2,solvent oxygen is restored to solvent in '/a of the events, becomes bridging oxygen in '/*of them, and becomes nonbridging in "* of them. To relate these rates to the line broadening, which from the shapes of Figure 1, we take a t 0.06 m H + to be 7.5 X lo2, 2.3 X lo2, and 48 C.P.S. for bridging, terminal, and solvent oxygen, respectively, we must take account of the abundances of the oxygen species. 1/Tz and For our treatment we assume that 6H/H
-
The Journal of Physical Chemistry
"0
thus, in an approximate manner, from the broadening observed for each of the species of oxygen, we can calculate the rate R . It should be noted that the total fluxes for the bridging, terminal, and solvent positions are 7/gR, 1.5R, and 7/&, and that the concentrations of the three kinds of oxygen in g.-atoms/1000 g. of HzO are 3.1, 18.6, and 55.5, respectively. Thus we have the relations
7/aR = 3.1 X 7.5 X lo2 X G / 2 1.5R = 18.6 X 2.3 X lo2 X &/2
7/gR = 55.5 X 48 X &/2 where the factor d / 2 is to convert width between points of maximum slope to widths at half-height. These yield for R the values of 2.3 X lo3, 2.5 X loa, and 2.6 X lo3 m-l sec.-l. The three self-consistent values of R prove that the line widths are due to uncertainty broadening. Taking the mean value of R to be 2.5 X loam-l sec.-l, the specific rate for the hydration of C r ~ 0 7 -by ~ the path k(H+)(Cr207-2)is 1.3 X lo4 7n-l sec.-l. The specific rate would be approximately 30% greater on a molar basis. Using either scale, it is clear that the rate at which the Crz07-2-
CHEMICAL SHIFTS I N
THE
N.M.R.ABSORPTION FOR OXYGEN-17 I N
HCr04- equilibrium is established is profoundly affected by acidity, in accord with the qualitative observation of Schwarzenbachand Meier.5 No appreciable shifts in the positions of the H2O and terminal oxygen peaks are observed even at 0.06 M (H+). The peak for the bridging oxygen, however, seems to have shifted slightly toward that for the terminal oxygens and appears a t -3.5 when (H+) = 0.06 M . In another series of experiments a t higher acid, the concentration of Na2Cr207was varied, keeping the ionic strength constant by using Na2S208 to replace Na2Cr207. The results of these experiments are summarized in Table I. Table I: Broadening in N.m.r. Absorption of OL7in HzO Caused by Acid Dichromate (NaPCr30,)
(HCIO,) = 2 M
(NazCrzOd
Line width
1.0 1.1 1.2
0.00
0.10 0.22 f 0.02 0.35 f 0.1 0.75 f 0.1
1.3" a
+ (Nai3sOa) = 0.8 M ;
Expt. no.
0.16 0.36 0.80
Solution 1.3 is 0.96 m NazCr207and 2.4 m HClOa.
The line broadening is a t least approximately linear in (H+). It is furthermore approximately of the magnitude expected from the operation of the mechanism discussed more fully above. If the difference in ionic strength are neglected, the broadening observed in experiment 1.3 is expected to be 12 times that for the experiment of highest acidity shown in Figure 1. The actual ratio of the broadenings is a factor of 11 f 3. The peak observed in the data shown in Table I is by no means that resulting from the coalescence of the three component peaks. The water peak has shifted to -0.30 for the solution used in experiment 1.3; if the rate of exchange were so rapid that the peaks would coalesce, the resulting peak would be expected a t ca. -1.1, that is assuming that the nature of C r 2 0 ~ -is~ not altered by adding 2 M HC104to the solution. Despite the fact that substitution in Cr(II1) is slow, chromate complexes of Cr (111) form rapidlyg because bond rupture can take place a t the Cr(V1) rather than the Cr(II1) center. However, no shift was observed in the absorption peak of the terminal oxygen in CrzO T - ~when 0.6 M Cr(NO& was added to a 3.5 M solution of NazCr207. This result may merely mean that although formation of the Crz07-2 complexes is rapid compared to those of most Cr(II1) complexes,
OXY
IONS
1847
the process is still too slow to make the bound and unbound Cr207-2 equivalent with respect to n.m.r. absorption. Some Observations with Mn04- in Water. Solutions of MnO4- were prepared by dissolving NaMn04 of normal isotopic composition in water enriched in 0 1 7 to the level of 1.8%. No attempt was made to measure the rate of exchange, but it was found to be substantially complete in 5 days a t room temperature, in 0.5 M HC104 or 0.2 M NaOH. In each solution Mn02 was also present because the solid sodium permanganate had undergone some decomposition on storage and there is the possibility that the oxide increased the rate of exchange (the solutions were filtered before the n.m.r. spectra were taken). Oxygen-17 in Mn04- was found a t -12.3 f 0.1 parts/104 with respect to pure H2OI7. The line width for Mn04- in acid solution is about 0.45 part/104 and the line shape gave evidence of saturation even at relatively low radiofrequency power levels. The effect of C O + on ~ the n.m.r. absorptions for MnO4- was investigated, for the interest this has in comparison with C104- which has the same charge and geometry. In 0.36 M MnOd-, a concentration of 0.43 M Co(C104)~ caused a shift of -0.48 f 0.05 part/104 from the normal position of -12.3. This is much greater than the shift of 017in C104- (less than 0.2 part/104 from the normal c104- position of -2.8 with respect to pure H2OI7)under the same conditions. Significant conversion of Rho4- to a complex is required to produce a shift of the magnitude recorded, but these data provide no firm basis for calculating an equilibrium constant for the association. An estimate can be made if it is assumed that the molal shift for each MnOr- oxygen is the same as that for a water oxygen. Then for each species, the molal shift a t constant ( C O + ~is) proportional to the bound oxygen compared to the total. For the solvent, this ratio for our solution is approximately (6 X 0.4)/52 (the coordina~ taken as 6), and the shift for tion number of C O + is the solvent was measured as -7.3 f 0.2 parts/104. Thus the ratio bound to total for ?tln04- can be calculated as (6 X 0.4)/52 X (0.48)/7.3 = 0.003 and the equilibrium quotient for the association constant as 0.003 X 4/0.4 = 0.03. The factor four enters because for each permanganate oxygen directly bound to C O + ~four , are contained in the complex. Unfortunately, there are no comparative data which give us a means of judging how good the assumption (9) E. L. King and J. A. Neptune, J. Am. Chem. Soc., 77, 3186 (1955).
VoZunte 69,Number 6 June 1966
1848
we introduced is, but it seems likely that it is good to a t least one order of magnitude. The 0'' n.m.r. absorption in Mn04- for the solution in 0.2 M NaOH showed much less tendency to saturate than in acid solution. When the alkalinity was increased to 2.5 M and enough NaI was added to generate ca. 0.01 M Mn04-2, the relaxation time increased as evidenced by the fact that the absorption showed a greater tendency to saturate. When enough NaI was added to generate 0.08 M %fn04-2, the absorption peak was broadened by 0.5 part/104 to about 1.0 part/104. This effect we can with some confidence ascribe to electron exchange between Mn04- and the paramagnetic species Mn04-2. From the observed broadening and the concentrations (0.7 M Mn04- and 0.08 M Mn04-2) we calculate for the specific rate of the exchange process the value 7 X lo3 M-l sec.-l which can be compared to 3 X lo3 M-I see.-' a t 21.0' and p = 0.16 as obtained by isotopic tracer methods and 4.5 X lo3 M-l see.-' at 15.0' and p = 1.3 as interpolated from the results of n.m.r. measurementslO using MnS5. Allowing for the differences in ionic strength, our value is consistent with those which have been published. We are unable to explain the change in the n.m.r. absorption characteristics of MnOe- caused by the f i s t increment in M D O ~ - ~In . general terms, the behavior observed means that a potent agent for relaxing the nuclear spins was generated in the slightly alkaline solution and was destroyed by the first portion of sodium iodide which was added. Even a t 0. I M Mn04-2, no shift in the peak position for Mn04- is observed. This presumably means that the absorption peak for Mn04-2 experiences a paramagnetic shift so large that the frequency corresponding t o the 1\InO~--~!h104-~peak separation is large compared to the rate of exchange. The SO2-H20 System. Observations with SO2 in water show that oxygen exchange between solute and solvent is so rapid that the two peaks coalesce. In one experiment, enough SO2 was added so that we were dealing with a two-phase system. Peaks at -5.2 and -0.83 were observed, the former ascribable to liquid so2 and the latter to a coalescence of the water peak with that Of This peak is quite 'lose to that expected for the averaged peak, assuming that the species so2 in water has the SfJJM? shift as it has in the liquid (i-e.,its chemical nature is unaltered by water)and taking into accountthe tive " l n t s of so2 and HzO oxygen in the aqueous phase. There appear to be no solubility data for our conditions 150 but an estimateof the solubility can be made from data covering the temperature range The Journal of Physical Chemistry
J. A. JACKSON AND H. TAUBE
20 to 6Oo.l2 If we assume that solubility is proportional to pressure, correct for the change in activity of SO2 caused by the dissolution of H2013in liquid SO2 assuming Raoult's law, and extrapolate the data to our temperature, we calculate that the aqueous phase contains 5.2 g.-atoms of HzO oxygen for each g.-atom of SO2 oxygen. With this proportion of the two kinds of oxygen, the averaged peak would be expected at -0.84 (-0.83 observed). This agreement indicates that no large fraction of the SO2 is converted to a form such as H2SO3, a conclusion consistent with that based on Raman work.14 Sulfurous acid would be expected to have shifts fairly close to those recorded for Na2SOa and NaHS03 (vide infra). Since the SOT HzO peaks coalesce, a lower limit on the rate of oxygen exchange between SO2 and HzO can be calculated. If the rate of exchange is defined by the equation R = k(SOn), the value of k must exceed loa sec., a value somewhat outside the limits on the rate of hydration set by von Biinau and Eigen. l5 The solubility of H2O in SO2 is approximately 0.04 g.-atom of H20 oxygen for each g.-atom of SO2oxygen. If exchange were rapid in the SO2 phase, a shift from -5.2 to -5.0 would be expected, but no appreciable shift is observed. It is of course entirely in order that exchange be much slower when SO2 is solvent than when HzOis solvent. The chemical shift we observed for SOa-2 was -2.35, and for 5 M NaHS03,16it was -1.67. In the latter solution, HS03- and S205-2 are probably in labile equilibrium. When 0.5 M HC1 is added, the water peak is shifted to -0.30 f 0.05. The position of the S(1V) peak is left unaltered, probably because of compensating effects-on the one hand, formation of SO2 with a large paramagnetic shift, and on the other, admixture with water oxygen. Both the H 2 0 and "HSOn-" peaks are broadened by the addition of acid. The dynamic processes occurring in this system can probably be studied by the 017n.m.r. technique, but oxygen a t a higher level of enrichment than we had at hand when these experiments were done will be needed to unravel the complex effects. (10) 0. E. Mewrs and J. C. Sheppard, J. Am. Chem. SOC.,83,4739 (1961); see also A. D. Brett and W. M.Yen, aid.,83, 4516 (1961). (11) J. C, Sheppard and A. C. Wahl, aid., 79, 1020 (1957). (12) W.L.Beusohlein and L. 0. Simensen, ibid., 62,610 (1940). (13) K.Wickert, 2.anorg. allgem. C h . ,239,89 (1938). (14) H.Nisi, Japan. J . Phys., 6,1 (1930); P.Fadda, NWO Cimento, 9, 168 (1932); w. J. Nijveld and H. Gerdine, Nature, 137, 1080 (1936). (15) G. von Bunau and M. Eigen, 2. physik. C h . (Frankfurt), 7, 108 (1956). (16) SZOS-~is probably the dominant form in these solutions: H. Simon and H. Kriegsmann, Ber., 89,2442 (1956); H.Simon and K. Waldman, z. ~ ~ O T Qallgem. . Chem., 283,359 (1956); 281,135(1956).
CHEMICAL SHIFTS IN
THE
N.M.R.ABSORPTION FOR OXYGEN-17 IN
Miscellaneous Oxy Ions. For a number of oxy ions, absorption only at the solvent position was observed. For a number of solutions the absorption was broader than for pure water, but we are not certain whether this was caused by the increased viscosity of the salt solutions or by a more chemical effect. Absorption only a t the solvent position was observed for the following solutions: 4.5 M H3P04, 3 M NaHJ?04, 2 M Nas(P03)B (broad), 1.5 M HAS04 1.5 M NaH2AsO4, 6 M NaAl02, 1.7 M Na2SiO3, 2.5 M NaC102, and 6 M NaC10. For none of these, with the possible exception of the aluminate and arsenate, is exchange of oxygen between the anion and solvent so rapid that the solvent and oxygen peaks coalesce; even if this were the case for these two species we would conclude that the chemical shift for oxygen in the oxy ion is not significantly different from that for water, because no shift
+
OXY
IONS
1849
(within *0.05 part in lo4) in the water peak is observed. For the other solutions, either the oxy ion peak is so broadened, presumably by a quadrupole interaction, as to be undetectible, or the peaks coincide with those for water. A simple experiment with the solution of phosphoric acid served to demonstrate that in this case the latter situation obtains. When about 0.2 M Co(C104)2 is present, the water peak is broadened and shifted to -2.9; another peak, quite broad, can be discerned a t - 1.7, and this is undoubtedly the peak for %PO*, apparently also shifted and broadened by Co +z. When a similar experiment was done with the arsenate solutions, a single broad peak at -3.5 was observed.
Acknowledgments. We wish to thank Dr. B. B. McInteer and Mr. R. M. Potter of this laboratory for providing the supply of enriched oxygen-17.
Volume 69,Number 6 June 1966
