2232
NOTES
w-I
l
\ \ 8 Urea 0 MMU 0 NNDEU
\
!
\ \
0
A NN!DMU "!DEW
0
1
Figure 1. Structural temperatures of urea-water solutions as a function of molality. 0 , HMT d a t a of hexamethylenetetramine-water solutions taken from ref 15.
the experimental one, showing a small breaking effect on the water structure, all its derivatives act as light structuring agents, the efficiency being in the order hlMU < NNDEU < NN'DMU 'v NN'DEU < TMU. This sequence is significative since it follows the increasing amount of hydrophobic substituents. It is interesting to note the inversion between the NK'DXU and the NNDEU that can be attributed to the fact that two ethyl groups bound to the same nitrogen atom may interact with each other and require less organized water to be solvated than two methyl groups bound to different nitrogen atoms. The effect of the ureas on the enthalpy of H-bond formation, computed through eq 2, has also been estimated and the results seem to agree with the structuring efficiency order previously given, the data reported below (Table I) are only indicative. Table I : D20-H20 Mixtures"
0 . 1 0 f 0.03 -0.05f0.02 -0.13&0.05
Urea MMU NNDEU NN'DMU NN'DEU TMU AHr'
= -2.33
-0.13&0.05 -0.13f0.05
-0.23kO.05 & 0.05 kcal/mol
In connection with the opposite effect on water structure of urea and of its derivatives, we like to remember that solubility in water of hydrocarbons and nonpolar solutes as well as critical micelle concentration of surThe Journal of Physical Chemistry
(18) D. B. Wethlaufer, S. K. Malik, L. Stoller, and R. L. Coffin, J . Amer. Chem. SOC.,86, 508 (1964). (19) W. Bruning and A. Holtzer, ibid., 83, 4865 (1961). (20) P. Mukerjee and A. Ray, J . Phys. Chem., 67, 190 (1963). (21) M. J. Schick, ibid., 68, 3585 (1964). (22) G. Barone, V. Crescenzi, A. M.Liquori, and F. Quadrifoglio, ibid., 71, 984 (1967). (23) D. R . Robinson and W. P. Jencks, J. Amer. Chem. floc., 87, 2462 (1965). (24) P. H. Von Hippel and K. I. Wong, J . BkZ. Chem., 240, 3905 (1965).
Nuclear Magnetic Resonance of Aqueous Solutions of Sodium Perrhenate
by R. A. Dwek, Z. Luz, and M. Shporer dAHr'/dm, (kcal/mol of OH)/ (mol of solute/ 1000 g of DzO)
Compd
factants is increased both by structure-breaker substances, such as urea or guanidinium and by structure-maker ones, such as methylurea or hexamethylenetetramine,22 so that both classes of compounds must weaken the hydrophobic effect, even through a different mechanism. Protein denaturation by urea cannot be ascribed only to its effect on water structure, and specific interactions of the amino groups with the peptide chain must be taken in account; it is significative on this respect to remember the effect of urea and its derivatives on the solutions of some molecules containing several peptide links. A salting-in effect is observed which decreases by increasing the number of substituents to the amino hydrogens and which parallels decreasing denaturating power.23 On the other hand, the increasing efficiency as denaturing agents of tetralkylammonium salts by increasing their structuring power shows the existence of a water structure effect on protein d e n a t ~ r a t i o n . ~ ~ Work is in progress in our laboratory to evaluate the relative weight of both effects on the conformat'ional stability of some proteins.
Isotope Department, Weizmann Institute of Science, Rehovot, Israel (Received December $0, 1969)
In the present note we report nmr measurements on aqueous solutions of NaRe04. The nmr spectrum of the two magnetic rhenium isotopes was recorded using the broad line method at a field of -14 kG. The following values for 1/T2 (in sec-I) were obtained for a 0.8 M solution at room temperature: lE5Re,4.5 X lo4; I*7Re, 4.25 X 104. These values are exceedingly large and may only arise from quadrupolar relaxation. Broadening due to chemical exchange can be ruled out since an unlikely large chemical shift between the exchanging rhenium species would have to be assumed; moreover, from the decrease of the Re line width with
NOTES
2233
temperature (measured between 25 and 93”) an activation energy of 3.4 kcal/mol has been calculated. This value is consistent with a rotational tumbling process but is too low for a chemical exchange reaction. Also the widths of the two Re isotopes are proportional to the square of their quadrupolar moments as expected if the relaxation is governed by quadrupolar interaction. On the other hand, the possibility that the relaxation is via interaction with paramagnetic impurities is excluded because they would affect the 170and ‘H resonances of Re04- and solvent water contrary to the experimental observations. From the Re nmr line width the Re quadrupolar interaction (QI) constant can be calculated using the equation’ 7
where cylindrical symmetry of the &I tensor is assumed. If we assume that the relaxation is due to rotational tumbling, T is the tumbling time of the complex and may be calculated from Debye’s relation: r = (47a3q/3kT) = 1.2 X lo-” sec (where the radius a is taken%,$as 2.3 A). From this value and the experimental result for l/Tz the &I constant of lS7Reis calculated to be (e2q&/h) 60 Mhz. This value is about one-third of the value found by Rogers and Rao in solid KRe04.4 The question now arises as to the origin of the Re &I in the Reo4- solution, i.e., whether it is due to ion pairing, solvation effects, or to an intrinsic distortion of the Re&- ion. To test these possibilities the following observations were made. (a) The Re nmr line width was found to be independent of the NaReO4 concentration within the range 0.25 M to 1.2 M . (b) Addition of up to 2 M NaCl to a 1 M aqueous solution of NaReOa had no effect on the observed Re nmr line width. (c) The line width was independent of pH in the range pH 1-13. (d) Na+ longitudinal relaxation times were measured at a field of 7 kG using the pulse method. The results for l/Tl in a 1 M solution of NaReO4 and NaCl were 27 sec-‘ and 20 sec-’, respectively. (e) The 170nrnr spectrum in a 1 M NaReO4 solution (containing -10 atom % l7O) consisted of the bulk water line and only one additional line shifted to low field by 579 ppm (cf. Figgis, et al.5). The widths of the peaks were: l/Tz(HzO) = 160 sec-l and 1/Tz(Rel704) = 210 sec-’, and the ratio of the integrated intensities of the two signals was consistent with having four oxygen atoms per NaRe04 molecule in the solution. These results indicate that ion pairing is not responsible for the &I,for if this were the case the broadening of the Re would depend on the concentration of the electrolytes, or if ion pairing were complete, even in the most dilute solution, the sodium relaxation time would
have been significantly affected too. (The difference in l/T1 of N a between the NaC1 and NaRe04 solutions is of the order commonly observed in aqueous solutions of sodium salts.)6 It seems quite certain that there is no permanent distortion from tetrahedral symmetry of the Reo4- ion in solution. This conclusion rests on ir, Raman, and uv and visible It is, however, possible that the &I arises from loosely bound solvent molecules having a very short lifetime in the solvation shell. These complexes may be thought of as “collision complexes” which cause an instantaneous distortion of the tetrahedron. If this is so, then the correlation time to be used in eq 1might well be shorter than lo-” sec and the result for e2Qp/h (which in this model is the root mean square value of the &I) correspondingly higher. A similar interpretation was previously offered for the quadrupolar relaxation of Mn in aqueous solution of ?(/In04-.12 There it was found that l/Tl(Mn) = 20 sec-’. If the relaxation mechanism of the central atoms in both X04- ions were analogous, one would expect the ratio of their relaxation rate to be as the square of the quadrupole interaction in compounds of similar structure. Examination of quadrupole resonance data shows this ratio to be around 100,la and in at least one case about 1000.14 (These values are considerably larger than the ratio of the square of the quadrupole moments, apparently due to the greater electronic polarization of Re relative to Mn.) The ratios of the l/TIJs is, however, -2000 which is significantly higher.
Acknowledgment. R. A. D. is indebted to the Royal Society for a travel grant to Israel and to the Weizmann Institute for hospitality during the summer of 1969.
(1) A. Abragam, “The Principles of Nuclear iMagnetism,” Oxford University Press, Oxford, 1960, Chapter VIII. (2) The value 2.3 A was taken as the sum of the Re-0 distance‘ (1.7 h) and the oxygen ionic radius (0.6 A). (3) J. C. Marrow, Acta Cryst., 13, 443 (1960). (4) M. T. Rogers and K. V. S. Rama Rao, J . Chem. Phys., 44, 1229 (1968). (5) B. N. Figgis, R. G. Kidd, and R. 5.Nyholm, Proc. Roy. SOC., Ser. A , 269, 468 (1962). (6) M. Eisenstadt and H. L. Friedman, J. Chem. Phys., 44, 1407 (1962). (7) L. A. Woodward and H. L. Roberts, Trans. Faraday SOL,52, 615 (1956); L.A. Woodward, (bid., 54, 1271 (1958). (8) H. H. Classen and A. J. Zielen, J. Chem. Phys., 22, 707 (1954). (9) R . H. Busey and 0. L. Keller, Jr., ibid., 41, 215 (1961). (10) E. J. Wells, A . D. Jordan, D. 5.Alderdice, and I . G. Ross, A w ~ . J. Chem., 20, 2315 (1967). (11) G. E. Boyd, J . Chem. Educ., 36, 3 (1959). (12) M. Broze and 2. Luz, J . Phys. Chem., 73, 1600 (1969). (13) A. N. Nesmeyanov, G. K. Semin, E. V. Bryuchova, T. A. Babushkina, K. N. Anisimov, N. E. Kolobova, and Yu. V. Makarov, Tetrahedron Lett., 37, 3987 (1968). (14) E. A. C. Lucken, “XIVth Colloque .4mpere, Ljubljana, 1966,” R. Blinc, Ed., North-Holland Publishing Company, Amsterdam, 1967, p 1121; S. Segel and R. Barnus, Phys.Rev.,107, 638 (1957). Volume 74,Number 10 May 14, 1070
