J. J. W o r m a n , G. 1. Poolli and W. P. J e n s e n South Dakota State University Brookings, South Dakota 57006
Comparison of Electronic Spectra
I
Carbonyls, thiocarbonyls, a n d azomethines
T h e large number of papers appearing in the recent literature dealing with photochemistry is a good indication of the current interest in photochemical processes. I n order to appreciate the possible uses of photochemical data a chemist must first be able to interpret electronic absorption data. The purpose of this paper is to demonstrate the use of visible and ultraviolet spectroscopy in distinguishing the three chromophores mentioned in the title of this article. The data examined demonstrate the n + a* transition and interpretations are based on simplified but commonly accepted ideas. With the recent studies on the azomethines ( I , 2 ) and the extensive but less complete studies on the carbonyls and thiocarbonyls (3)) it is possible to compare the differences in electronic absorption of the n a* transitions of three different heteroatoms pi-bonded to carbon. The molar absorptivities and the absorption maxima for the n + a* transition in both a polar and nonpolar solvent for compounds containing these simple chromophores are reported in the table.
carbonyls 490-510 mp. This then provides an excellent method for determining which of the three chromophores is present in a simple compound. In addition to the different regions of absorption the azomethines possess molar absorptivities ten times greater than those for the carbonyls or thiocarbonyls. Although infrared spectroscopy can be used to distinguish between the azomethine (approximately 1670 cm-l) and the carbonyl (approximately 1720 cm-I) chromophores (7), the position and intensity of the thiocarbonyl stretching frequency has not been established. On the basis of visible and ultraviolet data alone it is possible, by the values of X max and molar absorptivity, to distinguish between the three simple unconjugated chromophores. Discussion
-+
Molar Absorptivitives a n d Absorption Maxima for Compounds Containing Simple Unconjuged Carbonyl, Thiocarbonyl, a n d Azomethine Chromophores Cyclohexane Compound
k maxa Zmaxf
Acetoneb Methyl ethyl ketoneb N-isopropylidenemethylamined N-isopropylidenecyclohexyl&mined Thiodipropylketonee Thiocam~hor~ a
In millimicrons.
* Reference (4). In hexane.
f
~thanol Xmaxa t max
29P 277" 244
1P 2OC 160
272 270 231
18 16 161
247 503 493
196 10 10
235 492 481
180 10 10
Reference ( 2 ) . Reference (6). Liters/mole cm.
The distinguishing feature of the reported absorption maxima is that they undergo a blue shift for each compound in going from a nonpolar to a polar solvent. This is characteristic of an n -+ a* transition and is only one of several criteria used in characterizing this transition (6). I t is possible from the available data, the table and references therein, to group the chromophores into three distinct areas of absorption: azomethines 230-260 mp; carbonyls 270-300 mp; thioPresented in part a t the 157th National Meeting of the American Chemical Society, Minneapolis, Minnesota, April 15, 1969, Division of Chemical Education, Paper No. 19. Present Address: Department of Chemistry, University of Minnesota, Minneapolis, Minnesota.
The fact that thiocarbo-nyls absorb a t a lower energy than the carbonyls or azomethines in the same solvent is reasonable since in the case of the thiocarbonyl the excitation involves 31, electrons of sulfur while in the carbonyl and azomethine the excitation involves 2p electrons of oxygen and nitrogen, respectively. This energy difference is attributed to the fact that 2p electrons show a higher ionization potential than do 3p electrons. On the basis of electronegativity, one would expect the carbonyl to absorb at a higher energy than the azomethine, and since this is not observed, some other factor must be dominant. The azomethine is best represented by sp2 hybridization since the experimental data has shown the C=N-R angle to be 117" (8). This places the nonbonding electrons in an sp2 orbital. If we assume that the oxygen of the carbonyl is not hybridized, one pair of nonbonding electrons is best represented in a p orbital. The greater s character of the nonbonding electrons of the azomethine place them closer to the nucleus, and they would therefore require a greater energy for excitation. The dominant factor for the energy difference between the absorption a* transition of the carbonyl and maxima for the n azomethine chromophores would seem to be the ground state hybridization. This is only true provided there is not a corresponding lowering in energy of the a* level of the azomethine. Since the a* orbitals in both the carbonyl and azomethine are formed from two 2ptype atomic orbitals, one would expect a smaller relative energy change in this level than in the nonbonding ground state where the hybridization of the two chromophores differs significantly. This suggests that the relative difference in energy between the nonbonding -+
Volume 47, Number 10, October 1 970
/
709
and ?r* levels of the azomethine and the carbonyl is due to a lowering in energy of the nonbonding ground state of the former as a result of its hybridization. The hybridization of the specific chromophores in the excited state may be of importance in rationalizing the electronic absorption data, but the lack of experimental data concerning the geometry of the excited states precludes any speculation on this matter a t present. A reasonable explanation for the observation that the molar absorptivities of the azomethines are ten times that of the carbonyls or thiocarbonyls is found in a consideration of the symmetry of the transitions. Using the concept of local symmetry the carbonyls and thiocarbonyls in the ground state belong to the point group Czv The n -+ a * transition in this point group is described as 'Az + 'A1and is forbidden by symmetry. The resulting molar absorptivity is observed because of vibrational interactions and is predicted to be small. The data in the table substantiate this, showing the molar absorptivities of the carbonyls and thiocarbonyls to be approximately 10. The azomethines on the other hand belong to the point group Cs. The n + a * transition in this point group is described as ' A K +' A f and is allowed by symmetry. This should result in a molar absorptivity greater than that of the carbonyls or thiocarbonyls and as stated earlier this is observed experimentally. An analysis of the symmetry of the n + a * transitions of the azomethines has been reported (9). For a similar analysis of the symmetry of the n + a * transitions of carbonyls (and this should also apply for the thiocarbonyls) see Jaff6 (10). The appearance of an additional transition (n a*) -+
710
/
lournal of Chemical Education
for the thiocarbonyl a t 230 mp is again the result of a lower energy of excitation of 3 p relative to 2 p electrons. The carbonyl and azomethine would have similar absorptions a t higher energies, and this is a t least evident in the case of acetone in which the n + a* transition has been reported to occur a t 186 mp (2). This article has discussed a convenient way in which the three title chromophores can be distinguished by electronic absorption spectroscopy. I t is hoped that the reader will apply these techniques to higher energy transitions as well as to more com~lexmolecules. The information presented should also provide for a better understanding of the photochemistry of simple carbonyls (12), azomethines (I$), and thiocarbonyls (14). Literature Cited BONNETT,R., DAVID,N. J., HAMLIN, J., A N D SMITH,P., Chem. Ind. 1836 (1963). NELSON, D. A., AND WORMAN, J. J., Tetrahedron Lett., 507 (1966). MASON, S. F.,Quart. Rev., XV, 287 (1961). GILLAM,A. E., AND STERN, E. S., "Electronic Absorption Spectroscopy," Edward Arnold (publishers) Ltd., London, 1957, p. 56. FABIAN. J.. AND MAYER.R.. 8vectrochim. Acta.. 20.. 299 (1964). . . S I D ~ A N J.' ~ w., Chem. R&., '58;689 (1958). CONLEY, R. T., "Infrared Spectroscopy," Allyn and Bacon, Inc., Boston. 1966. DD. 135: 157. (8) S A S T ~ Y , K.v.-L.N.:AND CURL,R. F., J . Chem. Phys., 41,77 (1964). J. J., AND ATKINS,R. L., J . Cola.-Wyo. Acad. (9) NELSON,D. A., WORMAN, Sci., 5, 42 (1966). H. H.,AND ORCHIN,M., "Theory and Applications of Ultra(10) JAFF*. violet Spectroscopy," John Wiley & Sons, Inc., New York, 1962, p. 105. (11) JANSSEN, M. J., Rec. Trau. Chim., 79,454 (1960). J. G., AND PITTS,J. N., "Photochemistry," John Wiley & (12) CALVERT, Sons, Inc., New York, 1966,p. 539. (13) NELSON, D. A,, ATKINS,R. L., and CLIFTON,G. L.,Chem. Commun., 399 (1968). Y., FUKUYAMA, M., A N D TSUCHIHASHI. G., J . Amer. (14) OHNO,A.,OHNISHI, Chem. Soc., 90, 7038 (1968). - ~-
~-
