NOTES than solvent radical anions are the dominant species in solutions.
2659
Acknowledgments. This work was supported in part by the National Science Foundation.
NOTES
Direct Detection of the Hexaaquocobalt(I1) Ion in Aqueous Solutions by Proton Magnetic Resonance Spectroscopy
by N. A. Matwiyoff and P. E. Darley Chemistry Department, Pennsylvania State University, University Park, Pennsylvania 16801 (Received January 1.9,1968)
Nuclear magnetic resonance studies of the contact chemical shifts1g2and enhanced relaxation rates3-* induced in water molecules by paramagnetic ions have provided much valuable information concerning the hydration of cations.9 Of these studies the ones which yield the most direct information about the hydration number of the cation are those which result in a distinction between the nmr signals of the water molecules within and those outside the first coordination sphere of the cation. Such a distinction is not easily obtained for aqueous solutions of paramagnetic cations owing to: the large water-exchange rates, the broad signals of the bonded water molecules, and the fact that the chemical shift between bonded and free water is not always large compared to the signal widths. Connick and Fiat,lO using "0 nmr spectroscopy, observed a separate first-solvation-sphere signal for Ni(I1) in aqueous solutions. Also separate solvation sphere proton signals have been observed for solutions of Co(C104)2 in methano1,'l N,N-dimethylformamide,12 and a~etonitri1e.l~We wish to report here that at temperatures below -38", it is possible to distinguish the pmr signal of water molecules within the primary coordination sphere of Co(I1) in concentrated aqueous C0(C104)~solutions. From the relative intensities of the bonded and free water signals, a primary hydration number of 6, within experimental error, is obtained for Co(I1). Furthermore, we have been able to show that the primary hydration number is invariant with respect to solution composition in the temperature range -63 to +670.14 A 100-MHz pmr spectrum of an aqueous Co(C104)2 solution at -60" is represented in Figure 1. From the
dependence of the relative signal intensities upon the solution composition, the low-field resonance is assigned to water within, and the high-field resonance to water outside, the first hydration sphere of the Co(I1) ion. The primary hydration numbers calculated from the relative signal intensities are listed in Table I, along with the signal line widths and relative chemical shifts, The proton chemical shifts of Co(OH2)2f, Avm, with respect to those of bulk water are plotted us. the reciprocal temperature in Figure 2. I n the temperature range, 4.51 2 103/T 2 4.78, the shifts are a linear function of the reciprocal temperature and conform to the Bloembergen e q u a t i ~ n ' ~ Avrn - = - - - A 2n S(S VI
h 3
+ 1)gP
71kT
(1)
where A / h is the proton-electron hyperfine coupling constant in hertz, V I is the resonance frequency, p is the Bohr magneton, y~ is the proton magnetogyric ratio, and S is the electron spin of Co(I1). At temperatures 103/T 2 4.42, the shifts deviate from the linear relation, owing to the onset of chemical exchange4 which is also reflected in the line-width data in Table I. (1) 2. Luz and R. G. Shulman, J . Chem. Phys., 43, 3750 (1965). (2) B.B.Wayland and W. L. Rice, Inorg. Chem., 5, 54 (1966). (3) R.A.Bernheim, T. H. Brown, H. S. Gutowsky, and D. E. Woessner, J . Chem. Phys., 30,950 (1958). (4) T.J. Swift and R. E. Connick, ibid., 37, 307 (1962). (5) T. J. Swift and T. A. Stephenson, Inorg. Chem., 5, 1100 (1966). (6) N. Bloembergen and L. 0. Morgan, J . Chem. Phys., 34, 842 (1961). (7) L. 0.Morgan and A. W. Nolle, ibid., 31, 365 (1959). (8) R.Hausser and C. Laukien, 2.Phys., 153, 394 (1969). (9) For a general discussion of the nmr of paramagnetic molecules, see D. R. Eaton and W. D. Phillips, Advan. Magnetic Res,, 1, 103 (1965). (10) R. E.Connick and D. N. Fiat, J . Chem. Phys., 44,4103 (1966). (11) Z.Luz and 9. Meiboom, ibid., 40, 1058 (1964). (12) N. A. Matwiyoff, Inorg. Chem., 5, 788 (1966). (13) N. A. Matwiyoff and 5. V. Hooker, ibid., 6, 1127 (1967). (14) I n the temperature range -63 to Oo, aqueous solutions of Co(ClO& of the compositions used in this study are viscous but flow freely and are free of solids. No time dependence of these properties, or the nmr spectra, was noted over a 4-hr period. (15) N. Bloembergen, J . Chem. Phys., 27, 695 (1957). Volume 71,Number 7 July 1968
NOTES
2660
Tabte I : Solvation Numbers, Line Widths, and Relative Chemical Shifts for the Ion Co( OH&2+ Derived from the 100-MHz Spectra of Aqueous Co( C10& Solutions --Solution [CO(II)l
oomposition, mol--IH~OI
1.oo
18.0
1.oo
16.0 23.1
1
.oo
Temp, OC
Solvation no. (10.3), n
-63.7 -60.0 -55.8 -51.5 -47.0 -42.8 -38.0 -63.7 -38.0
5.8 5.7 5.8 5.9 5.9 5.8 6.1 5.9 5.9
A Yo
widthsQ-Avf
Chemical shiftb
1450 1430 1440 1600 1900 2550 4300 1490 4100
400 390 430 540 800 1200 2000 410 1900
10,000 9,850 9 ,650 9,500 9,150 8 ,700 7,530 10,050 7,430
-----Line
a Full width (in hertz, &l.O'%) of the line a t half the maximum height; Avo refers to the coordinated water signal and A v f to the free water signal, * Chemical shift (in hertz, &l,O'%) of the coordinated water signal downfield with respect to free water.
i
/o,ooo
9,000-
$8,m-
?E Figure 1. Proton magnetic resonance spectrum (100 MHz) of a 3.2 m solution of Co(ClO& in water a t -60". The magnetic field increases from left to right.
-
71000
6,000-
-
5,000
Also included in Figure 2 are chemical shifts calculated from data obtained at high temperatures in the rapid chemical exchange limit (where a single exchangeaveraged proton signal is distinguished) using the equation4
Av
= PAv,
(2)
where P is the fraction of the total amount of water coordinated to Co(I1) and Av is the difference in chemical shift between the exchange-averaged pmr signal of the Co(C104)ssolution and that for a solution of Ng(C104)2,16117each measured relative to the internal standard 3-(trimethylsilyl)-l-propanesulfonic acid sodium salt. The P values were calculated using a primary hydration number of 6 for Co(I1). The shift measurements in the rapid-exchange limit were made under the following conditions (Figure 2 ) and all the shift data were adjusted to 100 MHz: 60 MHz for a 0.400 m solution, 100 MHz for a 0.400 m solution, 100 MHz for a 3.02 m solution, 60 MHz for a 0.309 m solution, and 60 MHz for a 1.020 m solution. Because the shift data obtained in the rapid- and slow-exchange limits fall along the same straight line, we can conclude that the primary hydration number of Co(I1) is 6 and that it is independent of the temperature and solution composition. However, in view of the The Journal of Physical Chemistry
2.0
Z#
2.8
3.2
3.6
103/r ,OK-!
4.0
4.4
4.8
The proton chemical shifts, Avm, of Co(OH,),++ 10a/T: 0, proton chemical shifts of Co(OHg),2+ a t 100 MHz in the slow-exchange limit; e, 60 MHz for a 1.020 m Figure 2.
us.
+,
solution in the rapid-exchange limit; 60 MHz for a 0.309 0, 100 MHz for a 3.02 m solution; A, 100 MHz for a 0.400 m solution; I , 60 MHz for a 0.400 m solution.
wz solution;
error limits for the shift measurements, we cannot exclude the formation of small amounts of species such as, Co(OHz)s2+ or Co(OH2)&104+, in these solutions. The coupling constant, A / h , calculated using the slope of the line in Figure 2 is (3.7 jl 0.1) X lo6 Hz, in acceptable agreement with the value 3.9 X lo5, obtained by Luz and Shulmanl from measurements a t 20 and 100". Similar conclusions about the temperature independence of the coordination number of Co(I1) in aqueous solutions of Co(C104)2 have been reported by Chmelnick and Fiat,18 who studied the (16) The primary hydration number of Mg(I1) in aqueous solutions of Mg(Cl0a)a is 6.17 (17) N. A. Matwiyoff and H. Taube, J . Amer. Chem. Soc., in press. (18) A. M. Chmelnick and D. Fiat, J . Chem. Phys., 47, 3986 (1967); in this paper, reference is made to unpublished work by D. Fiat, Z. Luz, and B. L. Silver, who observed the 1 7 0 resonance for solvent molecules within the first coordination sphere of Co(I1) in methanolwater mixtures.
NOTES
2661
water 170resonance over a wide temperature range in the rapid chemical exchange limit. Although the coordination number of Co(I1) was not determined in that study, the water 170chemical shifts and line widths were found to be consistent with a constant coordination number for Co(I1) over the temperature range from -10 to +183'. Although the temperature dependence of the proton line widths in Table I is characteristic of that for a system undergoing chemical exchange, we have not attempted a detailed analysis because of our uncertainty about the magnitude and temperature dependence of the interionic electron-nuclear dipolar interactions. Such interactions must be large in these concentrated solutions.19 We can, however, estimate the electron-spin relaxation time of C O ( O H ~ ) ~T ~~from +, , the line-width data at low temperatures ( 5-55.8') where the chemical exchange of water is very slow. The Solomon20 and Bloembergen6 equations for the longitudinal and , nuclei transverse relaxation rates, 1/2'21,f and l / T l ~ of bonded to paramagnetic ions reduce to the following, under the conditions obtained in this study
1 TIM
1 Tm
nAv, = - - - -
3
s(s
2 2 2
+
r6
7s
+ is(& + l ) ( $ - a
(3)
where r is the Co-H distance. The first term in eq 3 gives the contribution of the electron-nuclear dipolar relaxation to the line width and the second term that for isotropic spin exchange, which, in this case, accounts for less than 1% of the line width. Using a value of 2.1 8 for the Go-H distance and the pmr line width of C O ( O H Z ) ~at~ + -63.7", we obtain 7s = 1.6 X 10-13 sec. The value is substantially less than that calculated by Bloembergen and Morgan,6 5 X 10-13 sec, who used proton TI data obtained a t 25', and it is also smaller than that estimated from 1 7 0 T2 data, 1.7 X 10-l2 see a t -10'.ls The low value obtained in this study may be due to electron-spin exchange contributions to T~ in the concentrated solutions employed.21
Experimental Section Cobalt (11) perchlorate hexahydrate was prepared by treating CoC12 (Fisher) with a large excess of 70% HC104 (Baker) at 80" until a C1- ion test of the resulting solution with aqueous Agn'O3 was negative. The solid recovered after the HClO, solution had been concentrated by distillation was recrystallized from distilled water several times and was spectrophotometrically analyzed for Co(I1) as the COc14'- ion in 13 m HCl. The nmr spectra were obtained with the Varian A-60A and HA-100 spectrometers, the latter being operatled in the HR mode. The measurements
were made and the systems were calibrated in the manner described previously. (19) The "free''-water line widths in concentrated diamagnetic solutions of Mg(C104)z in water a t -70' are approximately 50 HI, nearly a factor of 10 less than those for the corresponding CO(c104)2 solutions.17 (20) I. Solomon, Phgs. Rev., 99, 559 (1955). (21) The calculated 7s is a sensitive function of r . In the determination of r , we assumed a dipolar coordination of HzO to Co(I1) in which all four atoms are coplanar. Angular coordination, in which Co(I1) is bonded t o a lone pair of oxygen electrons, results in smaller T (1.93 and T~ (9.5 X 10-14) values.
w)
Proton Magnetic Resonance Spectra of DL- and LL-I'henylalanylvalines
by Vito J. Morlino and R. Bruce Martin Chemistry Department, Unicersity of Virginia, Charlottesville, Virginia 22901 (Received November 27, 1967)
I n a recent publication we reported on the chemicalshift nonequivalence of the methylene hydrogens observed in the pmr spectra of a series of dipeptide Laminoacylglycines and some tripeptides. A striking feature of this study was the downfield shift of one of the nonequivalent methylene hydrogen resonances of the glycyl residue in L-phenylalanylglycine upon ionization of an ammonium hydrogen. The observation of this downfield shift for only one of the nonequivalent methylene hydrogens suggested that a study of the diastereomeric LL- and DL-phenylalanylvahes would be informative. Results for the LL dipeptide are reported in the previous paper,l but because of its limited solubility in neutral solutions, no results were reported for the dipolar ion form of ~-phenylalanyl-~-valine. We have recently obtained a lOO-h/Ic nmr spectrometer, and the results obtained with the aid of a computer of average transients for the dipolar ion form of D-phenylalanyl-L-valine are recorded in Table I, along with the values obtained for the cationic and anionic forms. Though obtained with the use of external tetramethylsilane (TWIS) as a lock signal, the values reported in Table I are converted to parts per million downfield from sodium (trimethylsily1)-1-propanesulfonate(DSS) as an internal standard in order to be directly comparable with the values reported for: the LL dipeptide. Values for the methyl resonances recorded in Table I are averages for the doublet produced by coupling of about 7 cps with a-CB. Table I shows for the DL dipeptide a downfield shift for resonance of the a-CH of the valyl residue upon ionization of an ammonium hydrogen. Thus the unusual downfield shift of the high-field glycyl hy(1) V. J. Morlino and R. B. ,Martin, J . Amer. Chem. SOC.,89, 3107
(1967). Volume 78, Number 7 July 1988
