NOTES
sorption, and ion exclusion from small pores.’ This conclusion was based on his experimental results along with those of Dalton, et a1.,2 and Dugger, et aLa It i8 the purpose of this note to show that there has been some misinterpretation of the results of Dalton, et al., and that there is therefore no need to postulate physical sorption and ion exclusion for these systems. Tien found that the gel preference for cation is in the order Cs > Rb > K > Na > Li. This order, determined by a very accurate tracer method, is in agreement with the results for K, Na, and Li of Dugger, et al. (who did not work with Rb and Cs), taking into account their reported experimental error. Tien, however, bases much of his interpretation on a seemingly contradictory order deduced from pH measurements reported by Dalton, et al. There is actually no contradiction among the results reported in ref. 1-3. The pH values reported by Dalton, et al., were given to indicate that the amount of exchanged metal cation was orders of magnitude less than the amount of unexchanged metal cation in the pores. This gel and other commercial silica gels must be acid-treated before pH differences as small as those found by Dalton, et al., who did not acid-treat, can be used to deduce differences in the amount of exchange. To show that the gel which has not been acid-treated does not give the desired reproducible results, compare the equilibrium pH values obtained by Dalton, et al., with those of Daniel, et u Z . , ~ who used the same materials in very similar experiments and who measured pH values for the same reason. The difference between the two sets of experiments is that the former equilibrium was attained at 22” and the latter at go”, although in both cases the measurements were made at room temperature. The pH values are, respectively: Cs, 2.7, 2.4; K, 3.1, 2.8; Na, 2.6, 2.5; Li, 2.2, 2.7. The order is thus radically different. It does not seem likely that the temperature of equilibration is the important variable. Rather, there is an impurity problem: the necessity of acid-treating such gels to eliminate such surface reaction problems has been shown.96 There then remains no experimental result indicating that some experimental conditions can alter the order Cs > Rb > K > Na > Li found by Tien. Since Dugger, et al., determined affinities by H+ release from the surface, their results (and, by inference, those of Tien, who observed metal-metal exchange) can be interpreted in terms of ion exchange with surface H+, without invoking physical sorption. It seems diacult to visualize H + release from the surface connected with physical sorption. Furthermore, while Dalton, et al., did suggest that large, hydrated ions are excluded from the smallest pores, this effect has been shown not to
3197
exist? (Reference 7 had, however, not appeared when ref. 1 was submitted.) By considering a geometric effect found at any solution-solid interface it was shown that even hydrated AI3+of radius 5-7 much larger than any alkali metal ion, can enter the smallest pores in all the gels used by this group. There thus seems to be no need to postulate either physical sorption or ion exclusion from small pores in alkali metal cation-silica gel systems. Dugger, et al., in considering the silica gel reactions of 22 very different cations, found reasonable correlation with the tendency of the cation to hydrolyze. On the other hand, with ions as similar as the alkali metal cations only s m d differences in reactivity are expected and found. To make a correct prediction of affinity order, one would obviously need detailed knowledge concerning several factors; an “order of hydrolysis” is not enough. Rosseinskp showed that the matter is very complicated: cation hydration, the nature of the anion, and other factors must be considered.
fi.,
Acknowledgment. This work has been supported by A.E.C. Contract At (11-1)-1354. (1) H.T.Tien, J. Phga. C h . ,69,360 (1966). (2) R. W. Dalton, J. L. McClanahan,andR. W. Maatman, J . Colloid Sci., 17, 207 (1962). (3) D. L. Dugger, J. H. Stanton, B. N. Irby, B. L. McConnell, W. W. Cummings, and R. W. Maatman, J. Phga. C h . , 68, 757 (1964). (4) J. L. Daniel, J. N e t t e d e , and R. W. Maatman, J . MksisSippi A d . Sci., 8, 193 (1962). (6) S. Ahrland, I. Grenthe, and B. Noren, Ada C h . Scud., 14, 1059 (1960). (6) J. Stanton and R. W. Maatman, J. Colloid Sci., 18, 132 (1963). (7) B. L. McConnell, IC. C. Williams. J. L. Daniel. J. H. Stanton. B; N. Irby, D. L. Dugger, and R. W. Maatman, J . Phya. C h . ,68; 2941 (1964). (8) D.R.Rosseinsky, J . C h . SOC.,786 (1902).
Observations Concerning Directly and Nondirectly Bonded W-H Couplings with Respect to Symmetry Considerations by T. Vladimiroff Department of C h m k t w and Chemical Enginwing, Steoens Instilzlte of Technobm~,Hoboken, New Jmaey (Rm’ved April 6, 1966)
The Fermi’ contact contribution to n.m.r. spinspin coupling for nuclei N and N’ was first given by Ramsey2to be
NOTES
3198
*,FS
=
- (’/&)
(16rPm)
2 ~W~(En^- /Eo)~-’ ~ X c
(nla(pfN)sj[o) (1) YN is the gyromagnetic ratio of nucleus N, 10 )(In)) represents the ground (excited state) of the molecule ) a &function centered on with energy Eo(E,), 8 ( F k ~ is nucleus N, fix is the electron spin operator, and all the other symbols have their usual meaning. After making the “average energy” approximation2 some progress has been made by McConnella using a molecular orbital approach and by Karplus and Anderson‘ using a valence-bond wave function. Recently Pople and Santry6used eq. 1 in conjunction with a molecular orbital approximation. Exact calculations, however, are diilicult? and any general observations that can be made are helpful. Some preliminary results on the basis of symmetry considerationsare reported here. Consider the operator which reflects the electronic states of ethylene in the plane of symmetry which is perpendicular to the C-C axis of the molecule. All the states of this molecule may be characterized as symmetric, IS), or antisymmetric, / A ) , such that RIS) = IS) and RIA) = - [ A ) . Equation 1 may be written as a sum of contributions due to symmetric and antisymmetric states. In the light of this, the direct and indirect 13C-H coupling in ethylene can be written as (o18(pkN>gICln>
~’ICZ-HI
=
Kc(ES Skj
- EO)-’(o16(FEz)fik[S) x
+
(Sl8(?jH~)8jlo) Kc(E.4 Akj
- EO)-’ x
(0 18(%c3f3 IC 1 4 ( AI8(F,EIl) 8 F l s C r C p H ~ =z
KC(E.9 Skj
g,lo>
(2)
- EO)-1(01a(%C8)@IC(@x
+
(Sla(~;Et)&[0) KC(E‘4 Akj
(JIC~ =E 156.4 I ~ C.P.S. and J l ~ a ~=r -2.4 ~ l c.P.s.) imply that the symmetric and antisymmetric contributions are 77.0 and 79.4 c.P.s., respectively. The difference in magnitude and relative sign for the directly and nondirectly bonded I3C-H coupling can be interpreted as the result pf approximately equal contributions from the symmetric and antisymmetric states. Molecular symmetry requires that these contributions add in the case of the former and cancel in the case of the latter. Thus, a minimum of two excited states must be considered when eq. 1is used. This analysis also suggests that in the (‘average energy” approximation7 different A-values should be used in the case of direct and indirect couplings because the symmetric ground state used in such calculations could not be expected to take into account the symmetry properties of all the excited states. Couplings in other molecules with a high degree of symmetry are presently being investigated in this laboratory. Acknowledgment. The author wishes to thank Dr. E. R. Malinowski and Dr. T. J. Dougherty for helpful discussiom and acknowledges the support of the U. S. Army Research Office (Durham), Contract DA-31124-ARO(D)-90. (1) E.Fermi, 2.PhyaiR, 60, 320 (1930). (2) N. F. Ramsey, Phys. Reu., 91, 303 (1953). (3) H.M.McConnelI, J. Chem. PhyS., 24, 460 (1956). (4) M. Karplus and D. H. Anderson, ibid., 30, 6 (1959). (5) D.P.Santry and J. A. Pople, Mol. Phys., 8 , 1 (1964). (6) R.M.Lynden-Bell and N. Sheppard, Proc. Roy. SOC.(London), A269, 385 (1962). (7) The “average energy” approximation has also been discussed by M. Karplus, J . Chem. Phys., 33, 941 (1960),and A. D. McLachlan, ibid., 32, 1263 (1960).
- Eo)-’ x
( o l ~ ( % c * ) ~ k I(AlW,FII)@,lO) A) Since the value of a matrix element is invariant under any symmetry operation, it follows that
Ionization Potentials and Mass Spectra of Cy clopentadienylmolybdenum
Dicarbonyl Nitrosyl and
(018(%C*)skIS)= R ( o \ W m J ~ k l S= ) (Ol8(~EP>’BICls)
1,3-Cyclohexadieneiron Tricarbonyl’ (O[8(%c8)8k[A) = lI(Ol8(%cJga[A) = - ( O I ~ ( ~ E J ~ I C I A )
It becomes obvious that the terms in the expansion of and 8 F 1 ~ 8 ~ aare - ~identical 1 in magnitude but have different signs for the antisy-etric contributio~. Denoting the symmetric and antisymmetric contributions by S and A , eq. 2 may be written as
by Robed E. winter8and Robert
w. zser
SFIC,-H1
+A
8 F ~ ~ 2= - ~81
b F ~ c d l - ~=l 8
-A
The data of Lynden-Bell and Sheppards for ethylene The Journal of Physical Chemary
Department Of Chemistry, Kansas State UniVmaity, Manhattan, Kansas 66604 (Received April 13,1966)
Recently, we reported2 mass spectrometric studies of the ionization and dissociation in several transition metal carbonyls. From the measured ionization potentials it was suggested that the electron removed
