James J. Worman, Ronald L. Atkins,' and David A. Nelson2 Soulh Dakota State Unwersity Brookings, South Dakota 57007
I
Com~arisonof the Electronic Transitions of Unconjugated Thiones and lmines An exercise in the application of symmetry selection
It is the purpose of this paper to show how symmetry selection rules can be used to determine whether certain of the electronic transitions exhibited by the title compounds are allowed or forbidden by symmetry. Our treatment assumes an elementary knowledge of symmetry operations, point groups, symmetry species, and character tables as given in other sources (1-6). Recent investigations i m the imine chromophurei have established the exlstenre and t v ~ of e certain electronir transitions (7-10). Using the symmkiry selection rules, we should be able to obtain agreement with the experimental data shown in Table 1. In ~ i & e 1 is illustrated the orientation of the imine chromophore about the Cartesian coordinate system. The molecular orbital picture in Figure 2 assumes sp2 hybridization based on experimental data which shows the C=N-R bond angle to he approximately 119' (11,12). The hasic rules of symmetry allow us to assign this chromophore to the C, point group. If we now operate on each orbital of the chromophore separately with I and ax, (the symmetry operations of the C, point group) we obtain the species representation for the orhitals as shown in the C, character table (Table 2). Only two different species are obtained, and they are designated A' and A". At this point we must introduce a uuantitv known as the dipole moment vector, M. This qnantfty is reiated to the av: erage charge dis~lacementdurine an electronic transition. ~ i k any e v&tor, s c a n he resolved rnto three components, one along each of three mutually perpendicular axes. These component vectors are now associated with the Cartesian coordinate system of the point group C,, and each of the components Mx, My, and Mz are operated on by the symmetry operations of the group. An operation either leaves the direction of the component vector unchanged, resulting in a +1 character, or reverses the direction of the vector. (-1 character). Each component vector can then be assigned to the pruper symmetry species, and thece are indicated in the C, character table. The symmetry selection rule with which our discussion is concerned states that for a transition to be allowed, the product of the symmetries of the ground and excited states must be of the same species as a t least one component of the dipole moment vector. If this is not the case, the transition is said to he forbidden hv. svmmetrv. . Returning now to a descriptinn of the electronic transition of the imine chromophnres, let us look at the lowest energy n r *transition. The gn)und state can be represented as o, 2, and this beromes cuuivalent to an A'soecies. The n m* state can be represented as a,,2, n.9, r . a 2 , u . * which u* transition can then be becomes an A" species. The n = A" and since represented as A" -A'. The product of &+, M , transforms as this species, the transition is said to be symmetry allowed. Experimental data show an n r * transition for the imine chromophore occurring a t 245 nm with a f of 200 (8).This is
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Paper 1 in the series appeared in J. CHEM. EDUC., 47. 709 (1970).
'Present Address: United StatesNaval Research Labs, China Lake,
California. Present Address: University of Wyoming, Laramie, Wyoming 82070. 502 1
Jwrnal of Chemical Education
Figure 1. Orientation of G?e imine chrornophare in a Cartesian cowdinale system.
Figure 2. Partial mo1ec"lar orbital representation of the imine chmmophae.
Table I.Absorption Maxlma and Molar Absarptkltles for Compounds Containing the Slmple UnconjugatedAzomethlne Chromophore n
-
r' Transition
Cyclahexane compound
A,-*
emb
u
Ethanol Amd
emb
-
r '
-Heplane Ama
enurb
Transition TMPc AmB emxb
Kisopropyli-
245
140
232
200
179
8900
183
...
buiylamine N-cyclohexyildene-butyia-
245,
...
238
230
180
9500
185
...
mine
'b nanometers. a Liters/male om.
"Trimemyl phosphate "Reference ( lo).
Table 2. Character Table lor the C. Polnt Group
Table 3. Character Table For the c2,
A, A2
6, B2
I
Cz(d
+ + +
+ +-
+
-
%=I
++-
ClrPoint Group % ' =I
+-
+
MZ
4
4
greater than the correspmding t for the same transition in the thiocarhonyl chromophore which is forhidden hy symmetry. All other electronic tramitions of the imine chromophore are also allowed by symmetry since there is at least one dipole moment vector assigned to each species in the character table. The thiocarbonyl chromophore, like that of the carhonyl, belongs to Cz, point group. One pair of non-bonding electrons is best represented in a 3p orbital, the other in a non-honding 3s orbital or in other words there is no hybridization about the sulfur atom in the thiocarhonyl chromophore. Using an approach similar to that for the imine chromophore and now using the C2,, character table (Table 3), we arrive a t the following clasification of the electronic transitions of the thiocarhonyl chromophore which fit the observed data; 500 nm (c = 5) n a* (A2 A1) symmetry forbidden; 232 nm ( 6 = 5000) a a* (A, -AT) svmmetrv allowed (13). 1 n conclusion-it is &&ble to-understand why certain electronic transitions are allowed or forbidden using symmetry considerations. It should be emphasized that a simplified
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picture was given so that the actual use of symmetry in understanding electronic transitions could be demonstrated. This representation, however, is an oversimplification, primarily because we have considered the geometry of the ground and excited states to be the same. This is usuallv not true. because the orbitals in excited states reorient themselves in order to minimize such forces as d i ~ o l e - d i ~ ointeractions. le Such considerations are as yet not fully understood, and much work in this area is continuine" to further elucidate the theow of electronic transitions. Another neglected area in this discussion is the spin allowed or spin forbidden states. I t is obvious that all the possible transitions can either proceed t o singlet or triplet state. Of these, the triplet is spin forbidden and would make a transition already forbidden by symmetry even less likely to occur. We feel the above discussion presents information which is absolutely necessary for anyone attempting to understand ultraviolet absorption spectroscopy as related to organic structural analysis. Literature Cited (1) Jaffe. J. J., and Orchin, M.. "Theory and A~piicationsof Ultraviolet SpeNoampy? John Wiley and Son&Inc, New York, 1962. (2) Drag,. R. S.,"Physical Methods in Inorganic Chemi~try,"Reinhold Publishing Corporation, New York, 1965. (3) Cotton. F. Aikrt, "Chemid Applications of Group Theory? John Wiley and Sons. Ine.. Near York, 1971. 141 Jaffc. H.H.. and Orchin. M.. "Svmmetn. in Chemistrv." John Wilev and Sons, Inc.. N& ~ o r k1965. . (51 Zeldin. M., J. CHEM. EDUC., 43,17 (19661. (61 Orchin, M., and Jaffe, H. H., J. CHEM. EDUC., 47,216,372,510 (1970). (7) U n m d e , H. E., (Edirorl, "Olganic E l ~ ~ nSipcb d Data: lntePsienee Publishers, I&New ., York, 1960. (8) Bonnett, R., David, Ne J., Hamlin, J.. and Smith, P., Chom. ondlnd., 1936 (1963). (9) Bannett, R., J. Chrm. Sor., 2313 (1965). (10) Nelson, D. A,, and Worman,J. J.. Tetrahedron Lett., 5 . W (1966). (111 Se~tzy,K.V. L. N.,andCurl.R. F., J. Chom.Phys.. 41.77 (19641. (12) Sfsdd,P., J e w , W., Woman, J.,snd Jaeohaon,R. A , J.Chem Soe., PerkinII,W6 (19761. (13) Mayer,R,andFabi. J.,S@ctmhimihihi Ado, 20,299(1961):Niehols.P,M.ShiTh~ia. South Dakota Sfate University. June 1970. (14) Worman, J. J., Pml, G.L., end Jensen, W. P., J. CHEM. EUUC., 41.709 (1970).
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Volume 55, Number 8, August 1978 / 503
