4594
the exchange rates and k,, of reaction 6 are under investigation. This kind of rather fast exchange of HOI among different metal ions may have an important role in similar reactions in biological systems.
NOTES
charge-transfer effects with HFB, if any, would become apparent in mixtures of this species with X and RI. I n this note we report the results of such measurements.
Experimental Section Details of the experimental methods have been previAcknowledgment. We gratefully acknowledge the ously described.2 The benzene and hexafluorobenzene support of this research by the U. S. Atomic Energy were the same materials used in the previous study; reCommission under Contract AT(30-1)-3753. agent grade p-xylene (11atheson Coleman and Bell) and research grade mesitylene (Phillips Petroleum) were distilled from over sodium just before use. Gas chromatographic analysis showed these latter materials to have a purity of 99.98 and 99.95%, respectively. Dielectric Constant and Refractive Index Refractive indices at 40" were measured at seven of W e a k Complexes in Solution. II1& wavelengths for each mixture and the data were fitted t o a three-term Cauchy dispersion formula by the by M. E. Baur,* C. 11.Knobler, method of least squares. The values of the constants n w jb, and c obtained by this procedure are listed in D. A. Horsma, and P. Perez Table I. The dispersion relations fit each set of data Departmelzt of Chemistrg,Ib Ulziversitu of California, with a standard deviation no greater than 0.0002, the Los Angeles, California 90024 (Receiued J u n e 29, 1970) estimated uncertainty of an individual refractive index measurement; the standard deviations listed for n, have also been obtained from the analysis. I n a previous communication,2 we reported the reFor the p-xylene and mesitylene mixtures, dielectric sults of measurements of the molar polarization ( P ) constants were measured for the pure components and and molar refraction ( R )of benzene (B) and hexafluorothree mixtures at 40" No new data were obtained for benzene (HFB) liquid mixtures over the entire compothe B-HFB system at 40", However, in our previous sition range at 25'. One of the goals of that work was paper2 Fye reported dielectric measurements on this systo obtain evidence bearing on the nature of the molectem a t 10" intervals from 25 to 65". Values of the ular complex believed t o be present in that ~ y s t e m . ~ molar polarization for B-HFB reported here have been The P of the mixtures studied was found to be additive obtained from these data by interpolation, to within experimental error; R showed a negative deviThe molar volume of hexafluorobenzene was taken ation from additivity well in excess of that typically from the data of Counsell, et ~ l .and , ~ the volumes of the found in mixtures of nonpolar species not exhibiting hydrocarbons from the compilation by Timrnermans,lo specific complexing effects. From these observations, These data for the pure Components were combined with we concluded that a significant concentration of B-HFB measurements of the excess volumell to obtain the complex is present in the mixtures. The observed admolar volumes of the mixtures as given in Tables I and ditivity in P is, however, incompatible with any impor11. tant contribution of charge-transfer effects4 to the stability of the complex, for the maximum dipole moment * To whom correspondence should be addressed. associated with this complex could not be more than (I) (a) Supported in part by the National Institutes of Health, about 0.1 D on the basis of our data. An alternative Public Health Service under Grant No. GM 11125. (b) Contribution No. 2645. possibility is that the formation of the complex in the (2) M. E. Baur, D. A . Horsma, C. R.1, Knobler, and P. Perez, J , B-HFB system is to be attributed t o electrostatic, inPhgs. Chem., 73, 641 (1969). duction and dispersion interactions. Recent theoreti(3) D. V. Fenby, I. A. McLure, and R. L. Scott, ibid., 70, 602 cal studies6.6 have indicated the importance of such in(1966). (4) G. Briegleb, "Charge-Transfer Complexes," Springer-Verlag, teractions even in the benzene-halogen complexes, usu1962, Section 111. ally considered the archetype of charge-transfer species. ( 5 ) M. W. Hanna, J . Amer. Chem. Soc., 90, 285 (1968). Since our data could not be considered altogether con(6) J. C. Lippert, >I. W .Hanna, and P. J. Trotter, ibid., 91, 4035 clusive, it seemed appropriate to extend the optical and (1969). dielectric measurements to liquid mixtures of HFB with (7) W. A. Duncan and F. L. Swinton, Trans. Faraday Soc., 62, 1082 (1966); J. C. A. Boeyens and F. H. Herbstein, J . Phys. Chem., 69, the nonpolar methyl-substituted species p-xylene (X) 2153 (1965). and mesitylene (M), with which it is miscible in all pro(8) M. Kroll, J . A m e r . Chem. SOC., 90, 1097 (1968). portions a t somewhat elevated temperatures and with (9) J. F. Counsell, J . H. S. Green, J. L. Hales, and J. F. Martin, Trans. Faraday Soc., 61, 212 (1965). which i t forms 1-1 solid solution^.^ The donor strength (10) J. Timmermans, "Physico-Chemical Constants of Pure Organic for charge-transfer complex formation in the aromatic Compounds," Elsevier Publishing Co., New York, N. Y., 1965. hydrocarbons is known to increase in the sequence (11) 'CV. A . Duncan, J. P. Sheridan, and F. L. Swinton, Trans. F a r e B-X-WJ and it might therefore be expected that day Soc., 62, 1090 (1966). I
The Journal of Physical Chemistry, Vol. 74, hro. 86,1970
Table I: Molar Refraction at Infinite Wavelength b X 10-6
(A
-2)
c
x lo-= (A-9
t (om*/mol)
CeHe-CeFa, 40.10"
I , 4678 4 0.0012 1.4337 f 0.0014 1.4086 1 0 , 0 0 0 5 1.4091 1 0 , 0 0 0 9 1.3946 2C 0.0013 1.3774 z!= 0.0011 1.3713 f 0.0016 1.35462CO0.0O09
0 IO000 0.1728 0,3683 0.3794 0.4753 0 6466 0.7622 1.0000 I
6.32 7.26 4.83 3.62 5.44 5.65 4.02 3.89
3.1 0,7 2.6 3.8
91.09 96.41 101.93 102.46 104,85 109.33 112.28 118,32
25.30f0.06 25.09 25,18 25.33 f 0.05 25.11 f 0.07 25.17 i 0.07 25.47 25.76 i 0.06
0.9 2.8 3.1 3.2 2.6 0.9 3.1
125.85 124.89 123 83 122.60 120.06 120103 118.33
34.60=k00.06 33.37f0.03 31 97 30.44&Oo.l0 27.61f0.09 27 39 25.942C0.11
1,7 2.0 3.7 5.3 0.9 1.5 3.0 3.9
144,68 136.66 134.94 131 80 131.12 127.99 125,02 118.31
39.36 f 0.06 36,31&0,03 35.45 3 3 . 8 7 f 0.11 32,92 31.14 Ilt 0 . 0 7 2 9 . 6 3 r t 0.11 25.99 f 0 . 0 9
1,o 0.1 1.8 1.3
CsHio-CeH~,4O.1l0
0 IO000 0.1392 0.2858 0,4444 0.7736 0.7761 1,0000
1.4621 rt 0.0010 1,4472 f 0,0005 1.4298 =t 0.0014 1.4112 4 0.0015 1.3771 It 0.0014 1.3738 f 0.0006 1 3574 & 0.0018 I
7.70
5,65 4.58 3.97 3.26 4.72 2.35
I
I
I
C~HI~-CCF~, 40.19' 0.0000 0.2069 0,2779 0.4083 0,4371 0,5709 0.7020 1.0000
1.4677 & 0.0008 1.4442 f 0.0004 1,4385 0,0010 1.4275 =t 0.0015 1.4163 f 0.0005 1.4018 f 0,0010 1.3901 f 0.0016 1.3582 i 0.0014
7.02 5.79 4.15 2.37 5.67 4.89 3.23 1.74
Results
Table I1 : Molar Polarization
CeH6-CeF6, 40. Oo ir Xcsrs
0.0000 0.2522 0 3817 0.4466 0.4850 0,6167 0.7419 0.8687 1.0000 I
f
2.275 2.187 2,152 2.130 2.127 2.091 2.066 2.052 2.029
(cm'/mol)
89.41 96.82 100.33 102.02 103.01 106.36 109.46 112.60 115.79
pM400C
(omg/mol)
26.72 27.52 27 90 27.98 28.22 28.44 28.76 29.32 29 68 I
I
CsHio-CaFs, 40100 0.0000 0.2392 0.5047 0.7560 1 0000 I
2.2402 2.1816 2.1169 2.0542 1,9922
125.72 124.09 122.06 120.15 118.31
36.78 25.06 33.11 31.24 29.41
C ~ H I ~ - C40.0" ~F~, 0.0000 0.2702 0.4730 0.8304 1.0000
2.2580 2.1959 2.1452 2.0431 1.9922
I
141.67 135.04 131.26 122.15 118.31
41 -86 38.48 35.98 31.51 29.41
Values of R", the molar refraction a t infinite wavelength, are given in Table I for each of the three systems studied. They have been calculated from the infinite wavelength refractive index, n,, and the molar volume P. The dielectric constants and molar polarizations for these systems are likewise given in Table 11. From these data it is easily seen that for both the X-HFB and AI-HFB mixtures, as for the R-HFB mixtures, P is additive to within experimental error over the entire concentration range, but as for the B-HFB mixtures, a simble negative deviation in R is found for both of the new systems. Plots of P and R vs. concentration for the new systems are identical in general aspect with Figure 1 of ref 2 and are not included here. These observations are best analyzed by considering the values for the quantity A defined in eq 13 of ref 2, the differential increment between P and R. As noted there, this quantity if interpreted in terms of the contribution of a complex t o the low-frequency electric properties of a solution is given in lowest order by
where K , is the mole fraction equilibrium constant for The Journal
of
Physical Chemistry, Vol. 74, No. 26, 1970
SOTES
4596 formation of the complex and po and ag*are the dipole moment and (incremental) atomic polarizability of the complex, respectively. For HFB mole fraction equal to 1/2, our results yield A(B-HFB) = 0.35, A(XHFB) = 0,33 and A(M-HFB) = 0,38 at 40". The experimental uncertainty in these values is + 0.05, Hence little significance can be ascribed to this small observed variation, and to within experimental error we conclude that A is constant throughout the series. Other tests for trends in the data for the three systems could be given, but would be less sensitive to changes than comparison of A values, The pronounced departure from additivity in R provides positive evidence for the presence of a complex in both X-HFB and 11-HFB mixtures, However, the analysis2 of the maximum F~ consistent with the data previously obtained for B-HFB mixtures also holds in an approximate way here. The dipole moments of the X-HFB and 34-HFB complexes accordingly cannot be greater than 0.1-0.2 D. We cannot in practice make reliable direct estimates of K , and po separately from our data. Nevertheless, it is clear that either both these quantities remain sensibly constant through the series of mixtures studied or that an increase in one with ring methylation is largely compensated by a decrease in the other. Some further considerations can be adduced. If one makes the physically plausible assumption that the entropy of formation of the complex is about the same throughout the series investigated here, then K , is a measure of the heat of formation of the complex. If charge-transfer effects were to play a significant role in stabilizing the complex, then for given geometry K , would be expected t o increase with methylation of the benzene ring. However, such an increase in chargetransfer character would also entail an increase in p c , 4 behavior inconsistent with what is observed. This reinforces our conclusion that charge-transfer effects are of negligible importance for the formation of complexes in the systems studied here. The role of electrostatic, induction, and dispersion interactions in stabilizing complexes formed by methylated benzenes with the halogens and TCNE has recently been subjected t o theoretical analysis,5'6 and it was concluded that such interactions make contributions to the energy of formation of these complexes comparable in magnitude to that of charge transfer, The case of the B-TCNE complex is most relevant for comparison with the systems studied here; TCKE is a T electron system, like HFB, and is expected to have electrostatic and dispersion interaction parameters not significantly different from the latter. It should be noted that the principal contribution to stabilization of the B-TCNE complex from other than charge transfer appears t o come from the electrostatic quadrupole-quadrupole interactions6 The magnitude of this interaction in the B-HFB complex should be comparable to that in the B-TCICE complex. Of parThe Journal of Physical Chemistry, Vol. 74, .Yo. 16,1970
ticular interest is the result6 that the electrostatic, induction, and dispersion interactions contribute approximately -9 kcal/mol t o the heat of formation of the B-TCNE and X-TCNE complexes (the precise figure depending on the angle of orientation of TCNE in the complex) but only 0.11 and 0.14 D, respectively, to the dipole moments of these complexes. Thus, there exists an example of a system for which the computed electrostatic, induction, and dispersion interactions give a significant stabilization of the complex without producing a significant dipole moment. In view of the probable similarity in interaction parameters between the TCXE complexes and the complexes formed by benzene and its methylated homologs with HFB, it appears reasonable, therefore, to assert that the latter complexes are stabilieed nearly exclusively by such noncharge-transfer interactions, Detailed theoretical calculations for the HFB complexes would be of help in making this conclusion precise and are being carried out.12 We may also remark that the calculations on the B-TCSE and XTCKE systemse indicate a net decrease in magnitude of the heat of formation of the complex in passing from B to X in consequence of the greater exchange repulsion energy of the complex formed from the latter species, A similar moderate decrease in heat of formation in the H F B complexes in passing from B to X, together with a small increase in the dipole moment of the complex, would be consistent with our data. Xo very strong conclusion on this point should be drawn, however, as it is evident that steric factors may play a more significant role in the case of the H F B complexes than in that of the TCKE complexes. Finally, it should be noted that we previously found A(B-HFB) to be 0.55 at 2 5 O O 2 If we assume that the decrease in this quantity to 0.35 at 40" reported here is entirely due to the temperature dependence of K,, we obtain an estimate for the heat of formation of the BH F B complex of - 5 kcal/mol, This value is in reasonable agreement with an estimate obtained from measurements on B-HFB mixtures in the gas phase,13 (12) M , W. Hanna, private communication. (13) E. M . Dantzler and C. ,VI. Knobler, J . Phys. Chem.. 73, 1602 (1969).
Field-Induced Ion Dissociation and Spontaneous Ion Decomposition in Field Ionization Mass Spectrometry by James C. Tou Chemical Physics Research Laboratory, The Dow Chemical Company, Midland, Michigan 48640 (Received J u l y 10, 1070)
The fundamental mechanistic studies of field ionization mass spectrometry1,z and the great usefulness in
