ACKNOWLEDGEMENTS
A ll participants of the research and educational projects as w ell as the authors o f the selected publications express their thanks to IBM for providing the basic hardware and software tools within the framework o f the IBM Academic Initiative Programme in Hungary.
A ll data processing work for the educational activities and all or m ost work in the various fields of research were carried out on the IBM 3090 170J mainframe computer operated by the IBM Competence and Development Center. Budapest.
AULA Kiadó kft. Budapesti Közgazdaságtudományi Egyetem A kiadáséit felel: dr. Halász Géza, az Aula Kiadó igazgatója.
Készült a BKE házi sokszorosító üzemében Nyomdavezető: Ifkovits Józsefné
Ferenc Bogár1, Gyuia Tasi2, István Pálinkó3'* and Béla Török3
[Department o f Theoretical Physics, 2Applied Chemistry Department, 3Department o f Organic Chemistry, J ó zse f A ttila University, Szeged, H -6720 Hungary
Introduction
Cinnam ic acids and its derivatives are important compounds in several aspects. They are key com pounds in the shikim ic acid pathway of secondary metabolism [ 1 ], and they offer the possibility o f studying hydrogen bonding and aromatic packing in the liquid and solid phases. Their formation through condensation reactions are also o f interest, since usually geom etric isomers form and the isom eric ratio may depend on several not fu lly explored factors. Finally, experim ental and theoretical investigations concerning their geom etric isom erization reactions may resolve som e controversies related to the thermodynamical stability of the isomers.
A particularly useful candidate for these sort of studies is the isom er pair o f a-phenyl cinnam ic acid. An a-phenyl cinnam ic acid m olecule contains one phenyl group attached to each pillar atom o f the double bond. Since another bulky group, the carboxyl group is also bonded to one o f the olefinic carbon atoms, (when one phenyl and the carboxyl groups are on the same side the configuration is Z when they are on different sides: it is E) the m olecule becom es very crowded and the optimal spatial arrangement cannot be predicted just by view ing e.g . a stick-and-ball model of the compound.
The final goal o f the project is to attack and possibly solve each problem outlined in the first section o f the paragraph. Firstly, a relatively sim ple system was studied experim entally as w ell as theoretically. This was the isom erization reaction in the liquid phase. Experiments revealed that an almost 5 0 -5 0 % isom eric distribution could be achieved either starting from the pure Z or the E isomer. This observation clearly shows that the thermodynamic stability o f the isomers are nearly identical. This contradicts to earlier finding [2], i.e. Zimmerman et al. reported 4 for the E /Z ratio, which is the sign o f significant difference in thermodynamic stability. In order to provide additional inform ation, theoretical calculations on the optimal conform ation and stability o f the m olecules seem ed desirable. These calculations may give support to one o f the experim ental findings and at the same tim e may point to experim ental artifacts in one o f the experim ental studies.
Puli Geometric Optim ization of a-Phenyi Cinnamic Acid Isomers
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Methods
Since semiempirical and ab initio methods are suitable for studying the properties of isolated m olecules, they seem ed to be good choices for m odelling m olecules in dilute solvents, especially if the solvent is inert ni the sense that van der W aals interactions are negligible.
Initial optim ization was started by semiempirical calculations. A M I, MNDO as w ell as PM3 m ethods w ere applied. Full geom etric optim ization w as done by the BFGS algorithm [3].
The output o f the semiempirical calculations served as the input for ab initio com putations. Initially, a small basis set was used (STO 3G) but calculations with other larger sets are also planned.
Results
Semiempirical calculations revealed that the thermodynamic properties o f the m olecules are very close to each other. The standard enthalpies o f form ation by the AM I method are as follow s: DHe= -2 3.48 kcal/m ol, DHZ= -2 3 .3 3 kcal/m ol. The PM3 method gave very similar results (dHe= -232.6 kcal/m ol, DHZ= -2 3 .3 7 kcal/m ol). w hile the MNDO method provided w ith higher standard heats o f form ation (dHe= -2 1 2 1 kcal/m ol, dHz= -2 6 2 5 kcal/m ol). It is to be noticed that the stabilities o f the isom ers are practically equal by.
each m ethod. These results clearly coincide w ith our experim ental observation i.e. in equilibrium the mixture contains equal amounts o f E and Z isom ers.
It is also uncovered that the Z isomer is more polar than the E (pg= 1.88 and p z= 2.53) which predicts that this isomer may be more abundant in protic solvent.
As far as the optim ized geom etries are concerned, the plain o f the phenyl groups are perpendicular to each other for the E isom er, w hile they are alm ost parallel in the Z. The carboxyl group is near perpendicular to the phenyl group attached to the sam e olefinic carbon atom in both isom ers. These features are to be seen in Figures 1 and 2
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Figure 1 The AMS optimized structure of Figure 2 The AMI optimized structure of
the E isomer the Z isomer
A s mentioned above ab initio calculations w ere also performed on a STO 3-G basis. Full geom etric optim ization without geom etric constraints could be performed only for the Z isom er. The procedure did not converge for the E isom er. The ab initio optim ized structure is displayed in Figure 3.
Figure 3 The ab initio opti-m izec structure o f the Z iso-m er
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Further work
After having the db initio optim ized structure of the E isom er, the pathway o f the isom erization w ill be studied and the saddle point geom etry w ill be determ ined. Then the semiempirical as w ell as the ab initio optim ized geom etries o f the hydrogen bonded dimers w ill also be calculated to model structures in more concentrated solutions and as a first step to obtain a deeper insight into the crystalline form o f these m olecules.
Acknowledgem ent
This work has been supported by the IBM Academic Initiative Program and the National Science Foundation o f Hungary through grant F 4 2 9 7 /1 9 9 2 .
References
[1] T orssell, KJB.G., Natural product Chemistry, John W iley & Sons Lim ited, 1989, Chichester, N ew York, Brisbane, Toronto, Singapore.
[2] Zimmerman, H., Ahramijan, L., J. Am. Chem. Soc. 81 (1959) 2 0 8 6 .
[3] Broyden, G.. J. Inst. Math. A ppl 6 (1970) 76; Fletcher, R ., Comput. J. 13 (1970) 317; Goldfarb, D ., Math. Comp. 24 (1970) 23; Shanno, DJF. Math. Comp. 24 (1970) 6 4 7 .
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