Dcr., 1062
nIliECTED
SOLUTE-SOLVEST
ISTER.4CTIOXS I S
BENZENE SOLUTIONS
2653
DIRECTED SOLUTE-SOLVENT INTERACTIOSS I N RESZENE SOLUTTONS‘
Downloaded by NANYANG TECHNOLOGICAL UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: December 1, 1962 | doi: 10.1021/j100818a070
BY W. G. SCHKEIDER Division of Pure Chemistry, National Research Council, Ottawa, Canada Received May
8, 1968
By means of proton magnetic resonance techniques it has been possible to demonstrate directed molecular interactions between polar solute molecules and benzene solvent molecules. The pronounced sensitivity of the proton resonance method for this purpose is due to the large magnetic anisotropy of the benzene solvent, which greatly magnifies the solute proton shifts arising from non-randomization in the solution. For polar alkyl-X and vinyl-X solutes the interaction with benzene is interpreted in terms of a dipole-induced dipole interaction, which leads to preferred mutual orientations of the solute and solvent molecules. The magnitude of the interaction appears to depend on the magnitude of the molecular dipole moment of the solute, as well as its molecular volume; molecular shape of the solute molecule is not a determining factor. With phenyl-X solutes in aromatic solvents the interaction is more extensive and cannot be adequately accounted for in terms of a simple dipole model.
The molecula,r force field of benzene, and of other words, the resonance is shifted to lower aparomatic molecules generally, has a pronounced plied field. On the other hand protons which find directional character. The r-electron system in themselves in positions immediately above or below such molecules represents a relatively exposed re- the plane of the aromatic ring will have their resogion of electronic charge directed normal to the nances shifted to higher applied field. This is bemolecular plane. Accordingly, when a second cause here the component of the secondary field molecular species interacts with benzene, the in- is opposed to that of the applied field and hence to teraction will be primarily through the a-electrons, bring such protons into resonance we have to apply and in the resulting complex the interacting mole- a field H greater than Ho. We have here considered cules will tend to have preferred mutual orienta- a fixed orientation of the aromatic ring with retions. It turns out that nuclear magnetic reson- spect to the external field. If the external field is ance techniques provide us with a method of ex- applied parallel to the plane of the ring there will traordinary sensitivity for studying systems of this be no induced ring current and no resonance shifts. kind. The reason for this is the large anisotropy So even after averaging over all directions we will in the magnetic susceptibility of aromatic molecules, still observe a net effect as indicated. also commonly referred to as the “ring current A simple example of the kind of behavior diseffect.”2 I n order to illustrate how the ring cur- cussed here is provided by the proton resonance of rent effect influences nuclear magnetic resonance CHCh contained in a dilute solution of benzene. measurements we consider a very simple model, This is illustrated in Fig. 2. When CHCla is disshown in Fig. 1. In this model we regard the aro- solved in cyclohexane, there will be no specific inmatic ring as a, simple circular conducting loop. teraction with the solvent, other than relatively X magnetic field Ho applied normal to this loop weak van der Waals and dipole interactions. For induces a circular current, which generates a the present purpose we may regard the cyclohexane secondary magnetic field opposed in direction to solvent as an inert medium and accordingly we that of the applied field. We can approximate this arbitrarily assign the chloroform resonance in this very crudely by a dipole placed a t the center of the case the value zero. I n ether, which is an n-type ring. This dipole has magnetic lines of force as donor, the chloroform hydrogen interacts by formillustrated by the dotted lines. Suppose now we ing hydrogen bonds with the ether oxygen, giving are measuring the proton resonance of the hydro- rise t o a shift of the chloroform proton resonance to gen atoms in benzene, only one of which is shown lower applied field.a This low-field shift is quite here. The component of the secondary field a t this characteristic of all hydrogen atoms involved in proton is in the same direction as the applied field, hydrogen bonding with n-type donors. In benzene i.e., it enhances. the a,pplied field Ho. Therefore we observe the chloroform proton resonance shifted to bring this proton into resonance we will require very much to higher field. We interpret this to an external field H which is less than Ho, or, in mean that the chloroform is here also forming a (1) Presented a t t h e Prof. Hildebrand 80th Birthday Symposium a t Berkeley, Calif., September 11th a n d 12, 1961. (2) (a) L. Pauling, J . Chem. Phys., 4, 673 (1936): (b) J. .1. Pople. ibid., 24, 1 1 1 1 (19%).
(3) The proton resonance of the CHClzin ether, as well a8 i n benzene, was measured relative to the proton resonance of cyclohexane, a small amount of which was added t o t h e solutions to serve as a n internal reference.
2654
W. G.SCHNEIDER
i
Ho
i
for the acetonitrile resonance leads us to conclude there is a preferred mutual interaction between benzene and acetonitrile. On the basis of previous work4 on solvent effects in proton resonance measurements the observed shift in acetonitrile in the present experiment5 may be regarded as made up of the following separate contributions G(benzene soln.)
I I
- G(neopentane soln.) 6a
I \
Figure 1. Downloaded by NANYANG TECHNOLOGICAL UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: December 1, 1962 | doi: 10.1021/j100818a070
Vol. 66
weak hydrogen bond, the aromatic solvent functioning as a n-electron donor. Accordingly the hydro-
gen of the CHCls is directed normal to the molecular plane of the aromatic solvent molecules and, because of the ring current effect, its resonance will be shifted very much to higher field. If the ring current effect did not exist we might have expected this hydrogen-bonding interaction t o result in a small shift of the CHCla resonance to lower field. Another example involving a different type of interaction is illustrated by CH&N when dissolved in benzene (Fig. 3). If we dissolve a small amount of acetonitrile in neopentane we find the proton resonance of the acetonitrile to appear about 50 c./a. to the low field side of the proton resonance of the neopentane solvent. Again it may be assumed that neopentane is an inert solvent, and there will be no specific interaction of the acetonitrile with neopentane other than the relatively weak van der Waals interactions. If now we dissolve a small amount of acetonitrile in benzene and at the same time we add a small amount of neopentane, to serve as an internal reference signal, we now find the acetonitrile resonance on the highfield side of the neopentane reference. In other words in the benzene solution the acetonitrile resonance has been displaced to high field by 56.8 c./s. (at 60 Mc./s.). The most probable explanation of this is one which involves an induced dipole interaction with the benzene. Acetonitrile has a large dipole moment, the negative end of which is concentrated mainly in the N atom. This will be repelled by the n-electron distribution of the benzene and will tend to lie off the ring. Since benzene has a large polarizability in the plane of the ring, a dipole is induced as shown in Fig. 3, and the resulting attraction will tend to locate the CHa group of the acetonitrile over the aromatic ring. In this orientation the ring current effect will then cause a pronounced high-field shift of the acetonitrile proton resonance. Now if there were no specific interaction between benzene and acetonitrile involving preferred mutual orientations of these molecules, Le., if we had complete randomization of this system, both the acetonitrile and the reference neopentane molecules would experience the same environment due to the solvent benzene. To a first approximation both should be affected in the same way by the magnetic anisotropy of the benzene solvent. The observed high-field shift
= A8 =
+ + 8a
63
60
Here 6, is the shift due to the different magnetic environment of the neopentane and benzene solutions, ie., due to the magnetic anisotropy effects of the benzene solvent. The contribution 6, is the shift arising from different van der Waals interaction of the solute with neopentane and with benzene. (Since the solutions employed were dilute, the solute concentration being 5 mole To, solute-solute interactions are negligible.) The term BE is the shift due to the “reaction field” e f f e ~ t . ~Finally 6, is the shift due to specific molecular interaction or complex formation in the all benzene solution. The terms 6,, 83, and arise from small perturbations of the electronic charge distribution of the solute molecule due to the electric fields of neighboring molecules. They are generally negative, that is, the proton resonance is shifted toward lower field. The term 6, has a purely magnetic origin, and for aromatic solvents it is positive in sign. In the systems being considered here the terms 6, and 6 3 are relatively small and may be neglected for our present purpose. If 6, = 0, implying complete molecular randomization, then in the present experiment 6, should be zero. This is because, as indicated above, both the acetonitrile and neopentane solutes in benzene should experience the same magnetic environment. If 6, # 0, that is, if there is a specific interaction between acetonitrile and benzene causing preferred mutual orientations, then 6, # 0. Hence the quantity actually measured is A6 = 6, 6,, and since for acetonitrile in benzene this is a large positive quantity, 8, >> 6,. In other words small proton chemical shifts of solute molecules resulting from specific complex formation with benzene molecules are effectively “amplified” because of the large magnetic anisotropy of the solvent molecules. At this point there is a further question to be examined. Acetonitrile is a linear molecule and benzene is disk-shaped. Is it possible that the apparent non-randomization, indicated by the proton resonance shifts, arises not so much from molecular complex formation, but from molecular shape effects? To investigate this question we have carried out similar experiments with a number of hydrocarbon solutes, three of which are rod-like, and tu70 are planar. The results are shown in Table I. The observed values of A6 are not significantly different from the estimated experimental error of the measurements, and may be
+
(4) A. D. Buokingham, T. Schaefer, and W. G . Schneider, J . Cham. Phys., 33, 1227 (1960). (5) See alao R. J. Abraham, c=c
