PACIFIC SOUTHWEST ASSOCIATION . O F CHEMISTRY TEACHERS 0
THE ULTRAVIOLET ABSORPTION SPECTRA OF AROMATIC COMPOUNDS ADSORBED ON SILICIC ACID' MELVIN ROBINZ University of California at Los Angeles, Los Angeles, California
S T ~ ~ I N color G changes occur when certain aromatic compounds, such as 4-nitroaniline, are adsorbed on activated silicic acid from inert solvents. The present study was undertaken with the dual purpose of explaining these color changes and trying to correlate them with known properties of the adsorbate and adsorbent. Accuracy and the fact that most of the "color changes" occur in the ultraviolet region of the spectrum demand that the "color" (spectrum) be recorded spectrophotometricdly. The plan was to record the spectrum of a solution of the compound in an inert solvent and the spectrum of that compound adsorbed on silicic acid from a solution in the same solvent, and then to try to correlate any changes in the spectra with the change of the adsorbate's environment on adsorption. These correlations can be expected to exist only when the electron being perturbed on adsorption is also the one excited in the electronic transition, a situation not often found to exist. It is not immediately obvious how one can obtain an ultraviolet transmission spectrum of a compound adsorbed on a solid which is as apparently opaque as silicic acid. The opacity is caused by the multiple refraction of the light at the numerous air-silicic acid interfaces. If the silicic acid were suspended in a medium which had the same refractive index as silicic acid, then the mixture would appear transparent. because there would be no refraction of light at the liquid-silicic acid interface (1). Cyclohexane has a refractive index (1.426) for the sodium-D line which is very close to that of silicic acid (1.420); it is transparent in the ultraviolet region to 220 mp, and it is PSACT EDITOR'S NOTE: The Student affiliates of the South-
ern California Section of the American chemied saeietv held their sixth annual regional convention on April 28, 1956;at the California Institute of Technology. Thia report is taken from ~ r ~ .o b i n ' s~rize-winningpaper presented at that meeting. The work was done under the supervision of Dr. Kenneth N. Trueblood as a senior problem in physical chemistry at U.C.L.A. Present address: Department of Chemistry, university of Washington, Seattle, Washington.
inert and nonpolar; consequently, it was chosen as the solvent in this work. The refractive index of a substance changes with the wave length of the incident light in a way which is characteristic of the substance. Hence, the matching of the refractive indexes of silicic acid and cyclohexane in the visible region of the spectrum does not guarantee a match in the ultraviolet region. The divergence of the refractive indexes in the ultraviolet region increases with decreasing wave length. At 235 mp the refractive indexes of the silicic acid and the cyclohexane are so diierent that the slurry becomes opaque and thus a lower limit is set on the wave length region available for investigation of this system. Slurries mere prepared by mixing 7.0 ml. of Eastman Spectro Grade cyclohexane and 2.50 g. of Mallinckrodt Reagent Grade silicic acid vbich had been heated a t 160°C. for at least 18 hours. After the mixture had been stirred magnetically to remove entrapped air bubbles, the resulting slurry had an absorbancy of 1.41 at 350 mp when viewed through a thickness of 0.3 cm. The absorhancy of slurries prepared in this manner was reproducible to *0.05 absorbancy units. The absorption cells used had a path length of 0.3 em. and could be disassembled for easy cleaning. The assembled cells were made leakproof with Nonaq Stopcock Lnbricant, a product of Fisher Scientific Company. The compounds investigated had either been recrystallized or distilled immediately before use. Spectra were recorded using a Cary Recording Spectrophotometer, Model 1lPMS. Because of the high molar absorptivities of most of the aromatic compounds, it was necessary to use solutions of low concentration in order to obtain absorbancies in the 1.0-1.5 range. Fortunately, even a t these low concentrations each of the solutes except benzene was more than 98 per cent adsorbed so that the slurry spectra did not have t o be corrected for unadsorbed solute' Appropriate were that the same number of absorbing molecules was in the light path in both slurry and solution spectra.
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VOLUME 33, NO. 10. OCTOBER, 1956
Spectra of benzene, fluorobenzene, phenol and Nmethylaniline in cyclohexane and adsorbed on silicic acid were investigated. These compounds were chosen to demonstrate both the results of the above method and the interpretation used in explanation of the results. The spectra are~shownin Figures 1-4. I n each case the solid line represents the spectrum of the solute in cyclohexane, the dotted line its spectrum when adsorbed on silicic acid a t the same concentration.
Figure 1.
Spectra of Benwne
INTERPRETATION OF RESULTS
It is first necessary to define two terms to be used frequently in what is to follow. The familiar singletsinglet 2600 A. absorption band in benzene involves the excitation of one of the mobile electrons in the a cloud, the excitation being to a vacant antibonding s * orbital. A transition in which a a electron is excited to a higher energy antibonding orbital, desig-
electrondonor atoms or groups which can participate in hydrogen bonds (3). The electro@c transition in benzene which occurs around 2600 A. is theoretically forbidden because of sixfold symmetry of the velectron cloud, but is partially allowed because of asymmetric vibrations of the ring carbon skeleton which introduces asymmetries in the r cloud. If a proton from an acidic silanol group of the silicic acid interacts with the s-electron cloud of benzene, then the cloud will be distorted, and
Figure 3.
Speche of Phenol
one might expect a stronger absorption intensity. Infrared evidence has been presented for interactions bonds (4). Graphical integration of the benzene spectrum, Figure 1, plotted as absorbance versus frequency (cm.-I), shows that the integrated absorption in-
Figure 4. SPF~... of N-Methyknilin.
nated a*, is known as a s - s * transition, that in benzene being a prime example. The n - a * transition involves the excitation of a nonbonding electron in an atomic orbit to the same a * antibonding molecular orbital (8). The adsorption-active sites on the surface of silicic acid are the acidic silanol groups, Si-0-H, which are capable of acting as proton donors or acceptors in hydrogen bonds. All compounds which have been found to be strongly adsorbed on silicic acid contain
tensity actually decreases on adsorption. Bayliss and Hulme (6) investigated the spectrum of benzene in water and in cyclohexane and found the same small shift to shorter wave lengths and broadening in water solution as found on silicic acid. They also found that the integrated absorption intensity remained unchanged on changing the solvent from cyclohexane to water. This fact discourages an interpretation based on the interaction of an acidic proton with the r electrons of benzene.
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I t seems more reasonable that the weak adsorption of benzene is caused by dispersion forces rather than the "directed hydrogen bond" considered above. These forces result from the continuously changing field produced by the motion of the electrons in the atoms. Since these forces are additive, an increase in the size of the adsorbate would result in an increased molar heat of adsorption if the adsorption were actually caused by forces of this kind. Beggerow and Harteck (6) report the molar heats of adsorption for benzene, toluene, ortho-xylene, para-xylene, and cyclohexane as 13.0,14.4, 16.6, 16.6, and 10.2 kcal./molerespectively, which shows that the adsorption is indeed caused by dispersion forces. Because dispersion forces fall off as the inverse sixth power of the distance, it is necessary that all of the atoms in the adsorbate molecule be in contact with the surface if maximum interaction is to be obtained. The higher adsorbability of molecules with double bonds is thus thought t o be the result of their planar structures which allow closer approach to the adsorbent surface, rather than the higher polarizability of the a electrons (7). All of the compounds investigated in the present work showed a broadening of the absorption bands on adsorption. For benzene the broadening is thought to be caused by the statistical distribution of the electronic energies in the molecule in the field of the polar adsorbent (8). The slight shift to shorter wave lengths on adsorption is unexplained. An identical shift has been reported for the spectrum of benzene adsorbed from the vapor phase onto transparent s~licagel (9). The close similarity between the spectra of fluorobenzene (Figure 2) and benzene strongly suggests that the transition in fluorobenzene is of the same kind as in benzene, i. e., r - r *. This is reasonable when one considers the high electronegativity of the fluorine atom. Because the substitution of a fluorine atom on the ring for a proton results in but a slight effect on the absorption intensity, it appears that the fluorine atom has very little effect on the a cloud, either by induction or resonance. Compounds containing the fluorine atom are known to form hydrogen bonds under a variety of conditions, and it is probable that the adsorption of fluorobenzene on silicic acid takes place through such a bond. This hydrogen bonding perturbs the nonbonding electrons on the fluorine atom. However, since the fluorine atom has only a very small effect on the electron system, the perturbation of the fluorine nonbonding elertrons would have little or no effect on the spectrum if the transition is indeed a - r *. As seen in Figure 2, the spectrum remains unchanged except for the broadening and slight shift t o shorter wave lengths which are also observed for benzene. The great similarity in the shifts in the spectra of benzene and fluorobenzene demonstrates that the fluorobenzene molecule probably lies parallel to the silicic acid surface as benzene has been said to do (10).
JOURNAL OF CHEMICAL EDUCATION
If the transitions in the halobenzenes were nr* with a nonbonding electron on the halogen atom being excited to the same a* orbital in each case, it
would be reasonable to expect the wave length maximum to increase in the order fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene, in view of the decreasing ionization potential of the substituent atoms in this order. The abso~ptionbands for these compounds in- iso-octane solutions are all centered around 2600 A., which indicates that the transitions are a - r*. This agrees with the a - a* assignment for fluorobenzene arrived at by comparison with the benzene spectrum. I t is not easy to say whether phenol acts as a protou donor or acceptor when adsorbed onto silicic acid, for silicic acid has a pK. of 10-11 (11) and phenol a pK. of 9.9. I n fact, Sporer (18) has suggested that the AH.a, of phenol is about 2.5 kcal./mole greater than that of anisole because the phenolic -OH group acts as both a proton donor and acceptor on adsorption. The spectra of phenol and anisole in cyclohexane are almost identical, while on silicic acid they are approximately identical at least in peak positions, both spectra being shifted to shorter wave lengths by about 3 mp; the relative intensities are not known. Because the spectra are so similar while the extent of interaction at the substituent group is so different, it is concluded that the electron being excited is one from the r cloud and not a nonbonding electron on the oxygen atom. No conclusion can be reached as to how the phenol molecule is situated on the silicic acid surface. N-Methylaniline offers an example of the n - r* transition. On adsorption, the nonbonding electrons on the nitrogen atom are stabilized in a hydrogen bond, thus lowering their energy in the electronic ground state. On excitation, the electron will occupy an antibonding a* molecular orbital where the electron does not have the benefit of hydrogen bond stabilization. This exclusive lowering of the ground state energy level results in a larger energy difference between the ground and excited states. The corresponding shift to shorter wave lengths of the N-methylaniline spectrum on adsorption is shown in Figure 4. This shift corresponds to a lowering of the ground state energy by about 5 kcal./mole, an energy of the proper magnitude for a hydrogen bond. When aniline is dissolved in aqueous hydrochloric acid the nonbonding electrons on the nitrogen atom become tightly bonded in a nitrogen-hydrogen bond and are thus unavailable for excitation. The spectrum of the anilinium ion is essentially that of benzene with an enhanced absorption produced by the -NH3+ group. Presumably a similar result can be expected for N-methylaniline. Even though the spectrum of N-methylaniline adsorbed on silicic acid is shifted towards that of benzene and is reduced in intensity, the fact that the spectrum is still far from that of benzene shows that salt formation on adsorption of the basic N-methylaniline on silicic acid does not occur.
VOLUME 33. NO. 10, OCTOBER, 1956 ACKNOWLEDGMENT
It is a pleasure to acknowledge the patience and encouragement given the author by Professor Kenneth N. Trueblood who suggested the problem and it means of solut,ion. LITERATURE CITED (1) TRUEBLOOD, K. N., PH.D. Dissertation, Cdifornia Institute of Technology, 1947, p. 156. M., Dimmi011.s Faraday Soe., 9, 14 (1950). (2) KASHA, R. K., "The Colloid Chemistry of Silica and Silicates," (3) ILER, Cornell University Press, Ithacs, 1955, p. 59. (4) MECKE,R., D i ~ e u ~ ~ i Fo anrsd a y Soe., 9, 161 (1950).
529 (5) BAYLIS~~, N.S., A N D L. HULME,Australian J. Chem., 6 , 257 (1953). (6) BEGGERO~, G., AND P. HARTECK, z. physik. C h m , 193,265 (1944). (7) DEBOER, J. H., "Advances in Colloid Science,'' Interscience Publishers, Inc., New York, Val. 111, 1950, p., 28. R. L.,in A. HOLLAENUER, "Radmtion Biol(8) SINSHEIMER, oev." "" , McGrsw-Hill Book Co.. , he.., New York. 1955. D. 169. (9) PAVLOVA, E. N., C m p t . rend. acad. sci. U.R.S.S., 49, 265 1194.51 - - --,. (10) ROBERT, L., Compl. rend., 234, 2066 (1952). 2. physik. Chent., A146, (11) HAHN,F. L., AND R. KLOCKMANN, 373 (1950). A. H., Ph.D. Dissertation, University of California (12) SPORER, at Los Angeles, 1956, p. 68.
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