Computer-Assisted QuantitativeInfrared Conformational Analysis of a#-Unsaturated Ketones P. Tlsnes and M. Perry Laboratoire de SynthGse et Physicochimie Organique, ERA 686 du CNRS, Universite Paul Sabatier, 118, route de Narbonne 31062 Toulouse Cedex, France Infrared spectroscopy has proved to be a very useful tool in conformational analysis, especially for the study of rapid equilibria for which other techniques like NMR give an average information. Nevertheless. for such an anolication. . each conformer must display different vibrational proper: ties. Amone the manv examvles renorted in literature. the sriu. ;s-transronformationalequilibriumof nJ-unsaturated ketones studied by Haves and Timmons ( 1 J was rerained for the commeicial &ailability or the easy synthesis of these products.
.
s - trans
hence
The molar fraction y of the other conformer is similarly obtained by comparison with another single-conformation reference molecule (eqn. (4)).Thevalidity of this approximation will be estimated afterwards by the sum of the molar fractions which must be nearest to one (eqn. (5)).
s - cis
The uc.ustretching band is disrinct for each ronformer, and the analysis consists in 1 ) the attribution of each band to the corresponding conformer by comparison to model singleconformation molecules, 2) the decomposition of the resulting curve for the determination of the integrated absorption of hoth component bands. This decomposition is achieved with a n iterative program adapted for this purpose' with an Apple I1 microcomputer, the shape of the infrared bands beina avvroximated to a Lorentzian distribution. .. The quantitative approach proposed to the students is more especially focused on the two following points: 1) the determination from the soectra of the a and b constants of the I.orcnrr rquntion (eqn ,111 and their w e for rhecalculatlonof Ihe inregrated mdar absorplim H i c q n ( 2 , ) .
where A is the absorbance, T the transmittance, v the wavenumber, YM the wavenumber for the maximal absorbance, and a and b constants.
The Experlrnent The studied a,8-unsaturated ketones are, on the one hand, reference compounds with a single conformation, the 2methyl-1-phenyl-1-penten-3-one (s-trans, RI = Ph, Rz = Me, R3 = Et), I, and the henzylidene pinacolone (s-cis, R1= Ph, Rz = H, R3 = t-Bu), 11, and on the other hand compounds displaying s-cis ==s-trans conformational equilibrium, the benzylidene acetone (RI = Ph, Rz = H, Rg= Me), 111, and the chalcone (R,= Ph.. Rv- = H., R?" = Ph). .. IV. These -. products are commercially available, except I which is easily synthesized (2) by a crotonization reaction between diethyl ketone and henzaldehyde. The configuration of the ethylenic douhle bond is in all the cases the most stable E one. Spectra are recorded with 0.05 A1 solutions in rarbon tetrachloride in 0.5-mm sodium rhloride cells usina an Arrulah 10 Beckman spectrometer. Assignment of the Bands to the Corresponding Conformers
where C is the concentration of the solution in mol.1-', 1 the cell length in cm, and S w is the area of the band 2) the criticism of the approximations used for the application of the values of the integrated absorptions to the conformational analysis. This approximation consists in assuming that the integrated molar absorption R of a given band is unchanged for single or multiconformation molecules. The molar fraction x of a conformer is thus obtained by the ratio of the area of its absorption band S, to the area of the absorption band S, of a conformer of the same nature in a single-conformationreference molecule recorded at the same concentration (eqn. (3))
' This program is available on request from the authors.
Molecular models demonsrrate that benzylidene pinacolone, 11, is sterically less crowded in the s-cis than in the strans conformation. For compound I conformation is more difficult t o deride on the basis of models ( I , Y), hut the same wavelenmh (.f i.) reoorted for the ur-n stretchine band of 2.3dimetha cyclohex-2-en-1-one, wgere the s-trans conformation is fixed. is an exnerimental evidence for the vresence of a single s-trans conformation. In hoth conformers a restricted rotation around the bond between the carbonyl and the methylene group is required for the minimization of the steric interactions. If the analysis of the steric interactions does not argue definitively for a s-trans single conformation, the electrostatic interactions, repulsive for the s-cis covformer and attractive for the s-trans, support the explanation of the observed unique s-trans form. &
Volume 62
Number 10 October 1985
903
Average Experimental Values 8-cis
Mrans
Compound
Y
S
I
1676
20.73 f 0.20
s P
+
6-I&
h
A
o
H
g-
7
%
swans
% kcis
rnn
St
I8W
T% ,,-I
s+
% s-bans
+ % s-cis
ISM)
,S
n
1750
Woo
7850
'em-*,
H.
"+HCH3
H
CH3
h
S
0s-
+
The students also have to interpret the lower value of the wavenumber for the s-trans conformer on the basis of a more important conjugation in this conformer (4). The double bond character of the C-to-0 bond is thus reduced, with respect to the s-cis conformer leading t o a lower wavenumber value (5). Such an interpretationis supported bythe uc=o values (6) in cyclic a-8 unsaturated ketones, such as 2,3-dimethylcyclohex-2-en-l-one (s-trans position fixed) and 2-ethylidene cyclohexanone (s-cis position fixed). Quantitative Analysis The measurement of the area of the band is necessary to determine the intepraud molar absor~tion.The inteeration of the Lorentz equation (eqn. (1)) gives the area ( e q k (6)). and the a and b constants are determined from eqn. (1) as follows, AM being the absorbance corresponding to the v~ value and Auln the band width a t half height All2 of the absorption band.
50
Spectrum of benrylidene acetone.
(12)) is needed in eqn. (10) and allows, by means of eqn. (13), the determination of the T ordinate, which corresponds to the measurement of the Aullz value on the transmission record.
Hence
The resolution of the eqns. (7) and (8)gives b
= '~Av~/z
(9)
and
Hence a u S=U-=-A~AY~/~ b
2
(11)
Therefore, the area of a Lorentz curve can be approximated to the area of a triangle provided that a =I2 multiplicative coefficient is used. However, for measuring the Avll~value, the absorption curve A = f(v) is needed, and the routine infrared spectrometers, as those used by students, give the only transmission record T = f(u). This difficulty may be overcome by measuring the transmission TM corresponding to the V M abcissa. The corresponding absorption AM (eqn.
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Journal of Chemical Education
For the compounds I and I1 with a single conformation, the measurement of a and b leads in both casesto the area and to the integrated molar absorptions values. For the compounds I11 and IV for which a conformational equilibrium exists (it gives the partial overlapping of the bands as illustrated in the figure for III), the measured (a, b, VM),.,~,and (a, b, UM),.~,,values are introduced as initial parameters in the iterative program. These parameters are optimized by comparison of the experimental curve previously entered in the computer with the calculated one, which is the sum of the two Lorentz functions. The agreement is measured with a statistical correlation factor. Results
The average values resulting from many experiments are listed in the table. The most important errors result from the estimation of the initial values of Avllz, which may be difficult to measure when the overlapping of the bands is large, implying a greater number of iterations or even a divergence of the calculation.
The approximation concerning the equality of the integrated molar absorptions for the same type of conformer seems roughly justified for the benzylidene acetone, 111, the sum of the percent values being very near 100%. This compound and the two reference compounds I and I1 all are alkyl styryl ketones and the approximation seems reasonable. On the contraw for the chalcone.. IV.. which is a ohenvl stvrvl ketone displaying a crossed conjugation, the structural analo g with ~ the reference comnounds is not obvious. This discrepancy can be seen in the ieduction of the wavenumhers of both s-trans (Au = 28 em-') and s-cis (Au = 18 cm-') conformers. Hence, the percent values leading to an absurd 124% sum have no meaning in this case.2 However, this example has the advantage oiinitiating criticism of approximations and delimiting the domain of generalization of a method. For example, the application to other alkyl styryl
. . ."
" iPr) leads tocoherent % s-trans/% ketones (R3 = E t and R s-cis values, which are, respectively, 59%/43% and 45%/52%. Literature Cited (1) Hayes,W.P.,andTimmons.C.J.,Spoctroehim.Actn,A,24.323 (1968). (21 Vorlander, D., and Hobohm, H., Ann., 294,296 (18971. (3) Kronenbetg,M. E.,andHavinga, E..Rec. Tmu. Chim.,S4,17(1965). (4) Braude, E. A,, and Waight, E. S.. in "Progreaa in SLereoehemistry," (Editor: Klyne, W I , ButterworthsScientific Publications, Landon, 1958, p. 73. (51 Siivenfein, R. M., and Beaslor. G.C., "Spectrometric ldcntiflcation of Organic Compunds,l'2nded.,J.Wiley,NewYork, 1967.p.67. ( 6 ) a) Erskine, R. L., and Waight, E. S., J. Chom. Soc., 3425 (1960); bl Noaek, K., and Jones, R.N., con.J. Cham.,39,2201and 2205 (1961).
- -
Disreoardina the amount of s-trans conformer for IV.. its .Dresenca thls
is undeniable and. taking into account the strong interaclions In form, it is oener assigned as a nonptanar form.
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