NOTES
736 mechanism appears to govern the interaction of short alkyl chains with ribonuclease at denaturant concentration of 1 M or less, which is independent of the nature of the polar head group. This statement is supported by the internal consistency of the values in Table I. Table I : Transition Temperature Changes for Ribonuclease Produced by Alkyl Chains from 1 M Solutions of Various Denaturants Alkyl chain
----ROHb
CHaCHaCHzCHaCH2CHzCHaCHtCH2CHz-
-1.6 -3.1 -7.1 -13.1
ATrn(R)a---
RNHaCl
R I N B ~ /RCOONa ~~
0 -1.2 -3.1 -6.5
0 -1.1 -3.5 -6.5
+0.6
-1.3 -3.2
...
Polar group contribution subtracted as described in text. Data of Schrier, Ingwall, and Scheraga.1 Data of von Hippel and Wong.2 b
0
It has been suggested‘ that the chain-length effect on the transition temperature arises from the increasing ability of the longer alkyl chains to form hydrophobic bonds with nonpolar groups which are exposed to the solvent as the protein unfolds. The apparent special interaction of the charge with the carbon (and associated solvent molecules) nearest it may be viewed in the light of the assumed mechanism. In an examination of the free energies of transfer of amino acids from water to various nonpolar solvents, Tanfords has pointed out that alkyl groups near charges are likely to be less hydrophobic than those adjoining other nonpolar groups. On this empirical basis, it might be expected that the presence of a charge near one of the sites for hydrophobic interaction would tend to reduce the strength of the bond formed. The results presented above suggest that this reduction occurs mainly for the alkyl group immediately adjacent to the charge. The interaction of the remainder of the chain with the protein does not appear to be modified by the presence of the charge. Acknowledgment. This work was supported in part by Public Health Service Research Grant GM-11762 from the Institute of General Medical Sciences. (9) C. Tanford, J . Am. Chem. Soc., 84, 4240 (1962).
A Nuclear Magnetic Resonance Investigation of Paramagnetic Relaxation in Viscous Solutions
by Lawrence S. Frankel’ Department of Chemistry, University of Massachusetts, Amheret, Massachusetts 01 00.3 (Received June 19, 1067)
The effect of paramagnetic cations on the transverserelaxation time (T2M)of rapidly exchanging solvent The Journal of Physical Chemietry
mo1ecules2J has been the subject of considerable interest.4 Of principle concern here is the relationship between the correlation time and the viscosity of the bulk solution. A large or small viscosity dependence is contingent upon the physical process which serves as the correlation time. Proton frequency6 and temperature dependencee-8 have shown that for the hexa-hydrated complexes of Ni2+, Co2+and Cu2+,T2Mis governed solely by a magnetic dipole-dipole interaction between the paramagnetic electrons and the proton nuclear spin. To a good approximation, the interaction can be expressed as699 1/T2p = P/T2M = PC[77o
+ 137o/(1 +
(1) where T2p is the experimentally observed relaxation time;lO P is the probability of a proton being in the primary coordination sphere of the metal complex; 7c is the dipolar correlation time; us is the electron precessional frequency; and C contains physical and (M = Co, Ni) at numerical constants. For M(H20)62+, a resonant frequency of 60 Mc/sec, the high frequency 7 . ~ while ) for C U ( H ~ O ) ~ ~ + , condition (1 >> ~ ~ ~ prevails, only a small effect is due to this term.6 The dipolar correlation time can be governed by either a diffusion , is the case for C U ( H ~ O ) ~ or~by +,~ process ( r 0 = T ~ ) as the electron relaxation time (7c = TIJ, as is the case for M(H20)62+.6The bases for these assignments of T~ are the activation energies obtained from variable temperature studies (E+,d= 5 kcal, whileET,le= 1 kcal) and approximate calculations of values of T~ needed to account for experimental relaxation times. With the aid of these previous investigations, the relation between the viscosity of the solution and T~ has been explored for the purpose of developing a third method of assigning T~ to a physical process and to further understand the relaxation process. The viscosity of the solutions was varied by the addition of glycerin obtained from Matheson Coleman and Bell and labeled 99+%. The viscosity of the solvent mixtures at 40” has been previously reportedall (5s27O2)]
(1) Correspondence and request for reprints should be sent to Physical Chemistry Laboratory, Research Laboratory, Edgewood Arsenal, Md. 21010. (2) H.M. McConnell, J . Chem. Phys., 28, 430 (1958). (3) T . J. Swift and R. E. Connick, ibid., 37, 307 (1962). (4) A paper which can serve as a key to earlier literature is Z. Lua and S. Meiboom, ibid., 40, 2686 (1964). (5) L. 0.Morgan and A. W. Nolle, ibid., 31, 365 (1959). (6) N. Bloembergen and L. 0. Morgan, ibid., 34, 842 (1962). (7) R. A. Bernheim, T . H . Brown, H. 8. Gutowsky, and D. E. Woessner, ibid., 30, 950 (1959). (8) P. F.Cox and L. 0. Morgan, J . Am. Chem. SOC.,81,6409 (1969). (9) I. Solomon, Phys. Rev., 99, 559 (1957). (10) A small correction for the relaxation time in the absence of the transition metal is to be subtracted from Tap. To correct accurately for this, the relaxation time of all solvent mixtures was determined immediately prior to the addition of paramagnetic solute. (11) .“Handbook of Chemistry and Physics,” Chemical Rubber Publishing Co., Cleveland, Ohio.
NOTES
737 "01
I
, 1.6
I
1.8
2.0 22 PXl02
2.4
2.6
2.*
Figure 1. Plot of observed contact shift us. the probability of a proton being in the primary coordination sphere in water-glycerin solutions; [Co2+] is constant at 0.15 M . The straight line corresponds to complete preferential solvation by water; I, experimental points.
Hexaaquo metal perchlorate salts obtained from G. Frederick Smith were used to minimize contact ionpair effects. All measurements were made on a Varian Associates A-60 spectrometer at a probe temperature of 401". The relaxation times are obtained by measuring the full width ( A Y ) at half height ( T A Y = 7'2p-l). For water-glycerin solutions, there is a rapid exchange between the water protons and the three hydroxy protons on glycerin12 which must be taken into account in the calculation of P . In the past, one could not change the viscosity by changing the composition of the solvent without the danger of changing the composition of the complex ion.13 A question which is then to be considered is over what range of solvent composition does the primary coordination sphere of the metal contain essentially all water and no glycerin. An approximate answer can be obtained by studying the observed contact shift (Aw) in mixed s01vents.l~ Under rapid exchange conditions, A W ' ~is given byS AW = -PAOM
where the contact shift, AwM, should be independent of viscosity and relaxation effects. If no glycerin is coordinating, a linear relation between Aw and P is anticipated and is drawn as a straight line in Figure 1. The data in this figure are for CO(H~O)S~+. The last experimental point corresponding to a viscosity of 9.40 cp and a mole fraction water of 0.685 shows clear deviation from thiEi line and was taken as the limit with respect to solvent composition in this investigation. More obvious deviation from linearity in Figure 1 is observed at lower mole fractions of water. Previous investigations of other alcohols have shown that water strongly preferentially coordinates to a variety of transition metals aiver this mole fraction range.'6r17 A plot of the reciprocal of the transverse-relaxation time ( T A V P - I ) vs. viscosity is given in Figure 2. It is
Figure 2. Reciprocal of the proton transverse-relaxation time for C O ~Ni*+, + ~ and Cu2+ us. viscosity in water-glycerin solutions.
evident that Ni(H20)62+ and CO(HZO)~~+ show only small effects, while C U ( H Z O ) ~shows ~ + a large effect. For the former complexes with r, = TI,, only a small viscosity dependence would be expected, since upon going from solution to solid, Tle usually shows only small variation.l* For C U ( H ~ O ) ~where ~ + , r,, has been assigned to rotational diffusion of the entire hydrated species, a large effect is anticipated. It is clear that the process which serves as rc may be strongly viscosity dependent, and therefore a viscosity study may be helpful as a method of assigning a physical process to r,. However, it is also possible that Tie itself may be controlled by or have contributions from a diffusion process and, therefore, show some viscosity dependence.6,'9 From a model of a sphere turning in a viscous fluid, i.e., cU(H~0)6~+ rC = 4aqaa/3lcT
(2) where q is the viscosity of the solution and a is the radius of the sphere, which is approximately equal to the metal proton internuclear distance. The other symbols have their usual meaning. A test of this relation can be made by plotting r, vs. q ; a straight line (12) At 40°, a single resonance line is observed for all these protons, (13) R. L.Conger, J . Chem. Phys., 21, 937 (1953). (14) L. S. Frankel, T. R. Stengle, and C. H. Langford, in press. (15) The data for Am have been corrected for paramagnetic susceptibility shifts by the method outlined by D. F. Evans, J. Chem. SOC., 2003 (1959). (16) R.F. Pasternack and R. A. Plane, Inorg. Chem., 4, 1171 (1965), and other references cited therein. (17) N. Matwiyoff (private communications) has studied a variety of solvent systems which contain more than one hydroxy group and water and concluded that coordination by the nonaqueous component cannot be neglected at moderate mole fraction of nonaqueous component. (18) N. Bloembergen, J . Chem. Phys., 27, 572 (1957). (19) R.Wilson and D. Kivelson, ibid., 44, 154 (1966);P.W.Atkins and D. Kivelson, ibid., 44, 169 (1966); A. Carrington and G. R. Luckhurst, Mol. Phys., 8 , 125 (1965). Volume 72, Number 2 February 1068
738
NOTES
Figure 3. Plot of dipolar correlation time vs. viscosity for Cu(H,0)2+ in water-glycerin mixtures.
whose slope can be used to calculate a should result. ~ ~water-glycerin + solutions Values of r0 for C U ( H ~ O )in were calculated from eq 1 by the following procedure. Values of us corrected to 60 Mc/sec and T~ at 40" in pure water were obtained from previous investigations.6p8 The value of P was calculated by assuming a coordination number of 6. A value of the constant C was obtained and subsequently used to calculate r0 in water glycerin solutions. This procedure was adopted to avoid calculating C from first principles, since it contains a RW6(R = ion-proton internuclear distance) dependence and R is not accurately known. The resulting excelIent linear plot is given in Figure 3. A least-squares treatment of the slope gives a value of a = 2.3 A, which is in as good agreement, as can be expected, with previously suggested values of R of 2.520 and 2.8 A.6 A macroscopic solvent continuum model (eq 2) appears to do a surprisingly good job of accounting for the data, in view of obvious difficulties which result if a microscopic picture is considered.21122 Acknowledgment. This investigation was supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant No. 212-65. I wish to thank Dr. T. R. Stengle and Dr. C. H. Langford for many helpful discussions and the reading of the manuscript. (20) N.Bloembergen, Physica, 16, 95 (1960). (21) S. Broersma, J . Chem. Phys., 24, 659 (1955). (22) L. S. Frenkel, T. IC. Stengle, and C. H. Langford, Chem. Commun., 393 (1965).
Pyrolysis and Energetics of Halocarbons i n Cryochemical Studies of Divalent Carbon1*
by W. J.
and H. A. McGee, Jr.
School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Ueorgia $0888 (Received July 7, 1967)
Difluorocarbene is unusually unreactive in the gas phase, presumably due to its singlet electronic ground The Journal of Physical Chemistry
state,2 the electronegativity of the fluorine atomslaand the favorable contributions of resonance structures.4 Although it has not been observed, diiodocarbene may be additionally unreactive due to steric shielding of the carbon by the enormous iodine atoms. Consequently, as part of a continuing study of the preparation and properties of unusual reagents (not trapped free radicals) by cryochemical procedures, it has seemed reasonable to explore the stability and reactivity of CF2 and CI2 at very low temperatures. In related earlier experiments, CCl2 was ostensibly prepared as a stable yellow solid melting at - 114" and boiling at -20" by means of a liquid air quench of the effluent from the low-pressure pyrolysis of CC14 in a graphite tube at 1300°,5 but this product substance was later shown to be an equimolar mixture of C2Cl2 and CIZ.~Studies with the related molecule, SiFz, which has a half-life of 150 sec in the gas phase at 0.1 torr and 25", have been more fruitful.' The primary analytical tool in this study was the Bendix time-of-flight mass spectrometer and cryogenically cooled inlet systems which usually permits the identification of species regardless of their stability even at very low temperatures. A number of halocarbons were pyrolyzed either in a tubular monel furnace or on heated filaments of platinum and tungsten. Calculations indicated that the furnace pyrolyses should have attained equilibrium. An essentially collision-free path between the pyrolysis and the ion source was necessary, and this was accomplished in the usual way by the use of low pressures, close spacing, and differential pumping. The filaments and the furnace outlet were positioned "8 and in., respectively, from the ionizing electron beam. Furnace temperatures were limited to 500°, but the filaments were usable up to temperatures at which they collapsed. Ionization occurred with tungsten filaments either on the hot metal surface or from the acceleration of the surrounding electron cloud by the potential between opposite ends of the filament, but these ions were readily biased out of the pyrolysis products before (1) (a) The authors gratefully acknowledge support of this research byNASA Grant NsG-123-61; (b) abstracted from a thesis presented by W. J. M. in partial fulfillment of his requirements for the Ph.D. degree in Chemical Engineering, March 1965; (c) W.J. M. exprebsea appreciation to NSF for a Cooperative Graduate Fellowship during 2 years of graduate study. (2) F. X. Powell and D. R. Lide, Jr., J . Chem. Phys., 45, 1067 (1966). (3) J. P. Simons and A. 5. Yarwood, Nature, 192, 943 (1961). (4) J. Hine, "Divalent Carbon," The Ronald Press Co., New York, N. Y., 1964,p 40. (5) M. Schmeisser and H. Schroter, Angew. Chem., 7 2 , 349 (1960). (6) M. Schmeisser, H. Bchriiter, H. Schilder, J. Massonne, and F. Rosskopf, Ber., 95, 1648 (1962). (7) P. L. Timms, R. A. Kent, T. C. Ehlert, and J. L. Margrave, J . Am. Chem. SOC.,87, 2824 (1966). (8) H. A. McGee, Jr., T. J. Malone, and W. J. Martin, Rev. Sci. Instr., 37, 561 (1966).
