Electronic Structure, Extraframework Cation Distribution and Vibrational. Modes of Zeolite Fragments: DFT Study of Double-Six Member Rings Built of

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1 Electronic Structure, Extraframework Cation Distribution and Vibrational Modes of Zeolite Fragments: DFT Study of Double-Six Member Rings Built of xygen-bridged Silicon and Aluminum Atoms ans Mikosch, *, Ellie L. Uzunova, Georgi St. ikolov Institute of Chemical Technologies and Analytics, Vienna University of Technology, Vienna 0, AUSTRIA Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 3, BULGARIA Zeolites are aluminosilicates with a very open structure, formed by corner-sharing T 4 tetrahedra, where T=Si,Al. The orientation of adjacent tetrahedra is not fixed and the flexibility of the T--T angle permits the formation of cage-like polyhedra. Extraframework cations balance the negative framework charges, which arise from the substitution of tetrahedral Si atoms with lower-valence elements. These cations are positioned in the voids; they are loosely bound to the framework and are easily exchangeable. A convenient way to model the zeolite frameworks is to segregate the highly symmetric structure into smaller units, known as secondary building units (SBU). 3 The D R, or hexagonal prism, is a SBU in a large number of zeolite structures: CA, ERI, FAU, EMT, GME, LEV, FF, EAB, KFI, LTL, LT and also in structural intermediates EMT/FAU. Such units of general composition (M n+ ) /n Si A l and (M + ) Si Al 4, M = +, Li +, a +, K +, Ca + are the subject of the present study. 4,5 The electronic structure and relative stability of clusters with different Si,Al orderings were examined. The extraframework cations were allowed to take up different positions and the most favorable sites for a given Si/Al ratio were determined. They were positioned as found in FAU zeolites. The SI site lies in the center of the D R, SI' - above the T plane and pointing to a sodalite cage, and SIII' - near the fourring edges and located in the -member ring of the supercage, Figure. The calculated vibrational modes of the different structural fragments were correlated to the framework vibrations of FAU zeolites. 5 Further, the ring-opening vibrations were analyzed by introducing external symmetry coordinates, which define the radial synchronous atomic displacements in a direction parallel to the ring planes. Ring-openings and pore-size fluctuations are important because they control the access to active sites and are responsible for the permeability of zeolite frameworks to different reactants. The frequency shifts that arise from specific Si,Al orderings and extraframework cation positions, have been calculated.,

2 The cm - IR and Raman bands can be used as fingerprints for the detection of specific SBUs. SII SII' SI' SI SIII SIII' Figure. Schematic presentation of the FAU zeolite framework. Extraframework cation sites are denoted. xygen atoms are positioned near the centers of the lines connecting tetrahedral Si and Al atoms. Methods Geometry optimizations were performed by the B3LYP method as implemented in the Gaussian package. ydrogen atoms were used to terminate the clusters via Si- and Al- bonds. The standard -3G(d) basis set was employed for all atoms involved, with the exception of Ca + and K + for which the basis was extended to -3G(d). armonic frequency calculations were performed for all geometry optimized clusters. The harmonic vibrations were analyzed in order to determine the nature of the resulting stationary points. The points that correspond to minima on the potential energy surface were identified by the absence of negative eigenvalues in the diagonalized essian matrix, which give rise to imaginary vibrational modes.

3 Structure and Stability Related to Extraframework Cation Distribution D R of ratio Si/Al=5 were examined in detail with respect of possible stabilization of particular Si,Al orderings, that may violate Dempsey s rule. The following configurations were examined: 3 D 3d ; Si C s ; -Al--Si--Al- C h ; -Al--(Si-) 3 -Al- C v ; -Al--(Si-) -Al- Figure. D R of ratio Si/Al = (Si- only) and Ratio Si/Al=5, containing bridging hydroxyl groups to balance the negative framework charge. Both a + and Ca + form more stable clusters with two Al atoms placed in one T unit and stabilize Al(Si) Al linkages relative to Al(Si) 3 Al linkages instead of reaching maximum negative charge separation within the D R. This finding can be explained by comparing the a- distances in these clusters. In the C v fragment, Figure, the two a + are coordinated to four oxygen atoms each, at Å distances and the remaining oxygen atoms are at 3.3 Å (in the Si ring) and 3.3 Å (in the Si 4 Al ring) distances. The a-al internuclear distance is 3.0 Å. In the C h fragment, each a + is at.4 Å from the nearest four oxygen atoms and at 3. Å from the remaining two oxygen atoms in the Si 5 Al ring; the a-al distance in this case is 3.0 Å. The energy difference between the two fragments should be attributed to a more favorable position of the a + cations with respect to the centers, bearing the negative framework charge. The distance from the extraframework cations to the ring plane, defined by the six T atoms, increases with increasing ionic radius of the cations. In C v fragments, Ca + is found at 0. Å above the Si 4 Al ring, a + is at 0.5 Å from the Si 4 Al ring and at.05 Å from the Si ring. The SI sites inside the D R are 3

4 energetically slightly more favorable for Ca + ; the cation is in this case displaced from the D R center by 0.4 Å in direction of the Si 4 Al ring. Depending on extraframework cation type and position, the following scheme of stabilization of Al--(Si-)n-Al linkages was obtained, Figure 3. All cations favor maximum separation of Al atoms within the T rings. Concurrent occupation of extraframework cation sites in the center of the D R and above the T window is energetically unfavorable for Li + and a + cations, while this is achieved with K +, in agreement with experimental data. The Si,Al ordering with one Al atom per T ring and at the largest distance within the D R is the most stable one with Li + and K + as extraframework cations, while a + and Ca + favor the configuration with two Al atoms in one T ring. The small and medium size cations Li +, a + and Ca + approach the regions of high electron density in the D R, but Li + are unlikely to occupy neither the SI, nor SI site, when Al--(Si-) n -Al linkages (n=,) exist in the D R Al(Si) 3 Al Al(Si) Al AlSiAl Energy , artree K + at SIII' a + at SIII' K + at SI' Li + at SIII' a + at SI' C h C v C s Figure 3. Relative stability of Si,Al ordering at tetrahedral sites in D R The relative energy of the different clusters increases with decreasing the Al-Al distance. a + cations at SI sites stabilize Al--(Si-) -Al linkages rather than Al--(Si-) 3 - Al and Al--Si--Al linkages, which contradicts Dempsey s rule. Divalent charge- 4

5 compensating cations Ca + positioned above the T ring planes (SI' sites) were considered for clusters with a Si/Al = ratio. The Si,Al distribution was varied so as to obtain a fragment with maximum separation of the negative charges in the Si 4 Al rings (C symmetry) and to allow the presence of Al--Si--Al sequences in each T ring (C s symmetry). It should be noted that Al--Si--Al linkages exist in both isomers, but their number increases by two in the C s fragment. Summarizing the results on S R and D R with Ca + as charge compensating cations, we may conclude that Ca + allow the formation of Al--Si--Al linkages at ratios Si/Al<3. D R models with divalent Co and Cu at SI sites were selected in the studies of transition metal cations in zeolites. The interactions with framework oxygen atoms were found to be considerably weaker, as compared with the alkali and alkaline-earth cations. As a preliminary step in the modeling of guest-host interactions with zeolite frameworks, the formation of transition metal oxo-, peroxo- and superoxo- species has been studied in parallel with cluster models., Vibrational Analysis DFT studies of D R structural fragments (M n+ ) x/n Si -x Al x with x=0,, in which the negative charge, arising from Si,Al substitution, is compensated by Ca +, Li +, a +, K + and +, reveal that the shape and energy of the ring-opening vibrations depend on the type and positions of the extraframework cations and the Si,Al distribution. The vibrational spectra of FAU zeolites were interpreted on the basis of D R fragment models with descending symmetry (D h -> C s ). Maximum negative framework charge separation is attained in clusters with C h symmetry and Al--(Si-) 3 -Al sequence. With decreasing charge separation the symmetry of fragments is lowered to C v, with Al--(Si-) -Al linkages, and further to C s, with closely spaced Al atoms within one T ring. The ring-opening vibrations of D R with D 3d, C h, C v, and C s symmetry were identified in the cm - range and they were found to be correlated to the IR- and R-active FAU framework modes of the factor group h. The C h symmetry model with maximum separation of the Al atoms reproduces well the vibrations of ay, while the C v model describes the vibrational band broadening. The frequency shifts, arising from the specific Si,Al ordering in most of the examined clusters with Si/Al=5 ratio are within 0 cm -. Larger shifts were observed when cations take the two non-equivalent positions SI and SI. The lower symmetry clusters (C s ) display more significant band broadening in the IR and Raman spectra of zeolites. xygen atom 5

6 displacements contribute to all ring-opening modes. Ring-opening modes in the cm - range are found to be particularly sensitive to the type of T-atoms and their ordering. The totally symmetric ring-opening modes of the D 3d and C h fragments can contribute only to the Raman spectra of FAU zeolites, while for the C v - and C s fragments these modes should be active in both the IR and Raman spectra. REFERECES: () J. V. Smith, Chem. Rev.,, 4. () J. M. ewsam, In Solid State Chemistry: Compounds, A. K. Cheetham, P. Day; Eds., xford University Press:, vol., p. 34. (3) W. M. Meier, D..lson Atlas of Zeolite Structure Types, Butterworth, London:. (4) Gaussian, Revision A., Gaussian, Inc., Pittsburgh PA,. (5) E. Uzunova,. Mikosch, J. Phys. Chem. B, 004,,. (). Mikosch, E. Uzunova, G.ikolov J. Phys. Chem. B, 004,, 300. () E.Uzunova, G.ikolov,.Mikosch, ChemPhysChem 004, 5,. (). Mikosch, E. L. Uzunova, G. St. ikolov, In: 0-th Austin Symposium on Molecular Structure, The University of Texas at Austin, March, 004.

7 Ab initio study of the keto-enol tautomerism of guanine in the gas phase Vassil Delchev, ans Mikosch University of Plovdiv, Plovdiv, Bulgaria, Dept. Physical Chemistry; Technical University Vienna, Institute of Chemical Technologies and Analytics, Getreidemarkt, A-0 Wien Published in Scientific works of Scientists Union of Bulgaria Plovdiv, 004, IV, (in Bulgarian) ITRDUCTI Guanine is one of the DA and RA bases that is built by purine ring []. It exists in four stable tautomeric forms, depicted in Scheme A 3 3 B 3 C 3 D Scheme An evidence for the first three tautomers gave UV-spectroscopy [-5] with bands for each of them at 34, 33 and 33 cm -. The calculations have shown that the keto- and enol forms differ not much in energy [,]. ne interesting phenomenon in guanine is the mechanism of the tautomeric conversions. They have been studied for free guanine and cytosine at the MP level of theory [], as the estimated energy barriers have been found to be quite higher. ur idea was to check the influence of the computational method on the barriers height. We performed DFT calculations with the hybride functional B3LYP and -3+G(d) basis functions. RESULTS As expected [] the amino group in the four tautomers is situated out of the plane of the purine skeleton. In other words () is in sp 3 hybrid state. Several vibrations of the tautomers in the interval over 3000 cm - were correlated with the experimentally measured ones taken from ref. []. The best linear correlation we found between the

8 theoretical and experimental frequencies of tautomer A. In this case the angular coefficient is closest to the scale factor 0. [] for this kind of computations. The transition states of the tautomeric conversions were found as first order saddle points on the energy hypersurfaces. The energy barriers are shown in Table. Table. Energies of the transition states (a.u.) and energy barriers of the keto-enol conversions (kj mol - ) Conv. E E forvard E reverse E _ forvard E _ reverse B C A D A B C D The predicted energy barriers here are high enough to provoke tunneling. These barriers are as high as those predicted at the MP level []. References: [] J. D. Watson, F.. C. Crick, ature (London), 53,, 3. [] M. Mons, I. Dimicoli, F. Piuzzi, B. Tardivel, M. Elhanine, J. Phys. Chem. A, 00,, 50. [3] F. Piuzzi, M. Mons, I. Dimicoli, B. Tardivel, Q. Zhao, J. Chem. Phys., 00, 0, 05. [4] E. ir, K. Kleinermanns, L. Grace, M. S. de Vries, J. Phys. Chem. A, 00, 5, 5. [5] E. ir, C. Janzen, P. Imhof, K. Kleinermanns, M. S. de Vries, J. Chem. Phys., 00, 5, 5. [] A. Broo, A. olmen, J. Phys. Chem. A,,, 35. [] Y. Podolyan, L. Gorb, J. Leszczynski, I. J. Mol. Sci., 003, 4, 4. [] A. K. Chandra, M. T. guyen, T. Uchimaru, T. Zeegers-uyskens, J. Phys. Chem. A,, 3, 53. [] J. B. Foresman, Æ. Frisch, Exploring chemistry with electronic structure methods, nd ed., Gaussian Inc., Pittsburgh, p. 4.

9 Ab initio Study of the Keto-Enol Tautomerism of Barbituric Acid by means of DFT Vassil B. Delchev Department of Physical Chemistry, University of Plovdiv Paisij ilendarski, Plovdiv 4000, Bulgaria, Tzar Assen 4 Str., vdelchev@hotmail.com Published in Russ. J. Struct. Chem., 004, 45:4, 0- ITRDUCTI Barbituric acid is an important compound with a heterocyclic structure and possibility to exist in several tautomeric forms because of the mobility of the hydrogen atoms in its molecule. Some experimental investigations of that compound and its derivatives are available [,] mainly turned to its application in medicine and as dye in some branches of chemical industry. Ramondo et al have been studied [3] the effects of intermolecular hydrogen bonding on the molecular properties of barbituric acid at a high theoretical level (B3LYP). The theoretical spectra of all investigated monomers have been compared with their FT-IR ones in argon and nitrogen [3]. The main conclusion has been that B3LYP gives vibration frequencies differing not much from the experimental ones. In other words, B3LYP showed good accuracy describing the different motions in the molecules with comparatively low cost in computational time. The aim of this theoretical investigation was using the B3LYP DFT method and the D5** basis set to find the transition states of all tautomeric transformations and to estimate their energy barriers in isolated state. RESULTS According to the frequency caluclations all structures (Scheme ) were determined as local minima (do not have imaginary vibrations in the calculated spectra). Their energies showed that the tautomer A is the most stable one and the most unstable 3

10 tautomer is F one. The remain seven tautomers are arranging in the following row by decrease in their stability: G>E>B>D>>C>F E F A B C D G Scheme The estimated transition states of the conversions A B, B C, A E, E F and G have planar geometries. onplanar geometries have the transition states of the C D and A G. The lasts involve hydrogen transitions between carbon and oxygen atoms out of the molecular plane and the mechanisms are accomplished with a significant ring deformation. The transformation A G has the highest energy barrier, whereas the lowest one has the conversion B A. Almost symmetric is the energy curve of the conversion C D because of the close value of the energy of the two compounds taking part into it. Strongly asymmetric is the energy curve of the transformation. The high energy barrier established by us lead to the conclusion that the most of the conversion can not occur in the gas state. Probably in the gas phase only the tautomer A would be available. References [] Wurthner F., Yao S., eise B., Tschierske C., Chem. Commun., 00,, 0. [] Rezende M. C., Campodonico P., Abuin E. et al., Spectrochim. Acta. PT A-Mol. Bio. 00, 5:, 3. [3] Ramondo F., Pieretti A., Gontrani L., Bencivenni L., Chem. Phys., 00, :3, 3. 4

11 Electron and Geometry Structure of ydrogen-bonded Complexes of Guanine with one Molecule Methanol: a DFT Level Study Vassil B. Delchev a) and ans Mikosch b) a) University of Plovdiv, Department of Physical Chemistry, Tzar Assen Str. 4, Plovdiv 4000, Bulgaria, b) Technical University Vienna, Institute of Chemical Technologies and Analytics, Getreidemarkt, A-0 Wien Published in Monatshefte für Chemie, 004, 35:, 33-3 ITRDUCTI The nucleobases are DA and RA fragments that play an important biological role in the living world. They can constitute short, respectively strong, -bridges with other protic molecules. For example, with water molecules nucleobases form heavily hydrated systems [,]. According to several reported data from laser spectroscopy measurements [3,4] three tautomers of free guanine are abundant in the jet experiment: oxo-amino, oxo-amino and hydroxy-imino. The purpose of this paper is to throw light upon the geometry and electron structure of several stable -bonded complexes formed between the two oxo-amino guanine tautomers and the methanol molecule. This investigation would contribute to a better understanding of the nature of the interactions between methanol and nucleobases. The investigation was carried out with the B3LYP/-3+G* combination. RESULTS ) The structures of the optimized complexes are depicted in Fig.. It was demonstrated that the existence of an intermolecular -bond between the amino group and the methanol or water drastically lowers the pyramidal character of this group and the () atom turns almost sp hybridized; ) All complex formations are exothermic as only three of them have negative variations of the standard free energy. These are the processes leading to the formation of the most stable complexes with the shortest intermolecular -bonds; 5

12 Gua-- Gua-- Gua--3 Gua--4 Fig.. ptimized -bonded complexes between guanine forms and methanol 3) The values of the bonding energies of the supersystems guanine-methanol in the gas phase are very close to these of the guanine-water complexes. Therefore the complex formations between guanine and methanol, and water are competitive and can occur together. References [] Shishkin V, Sukhanov S, Gorb L, Leszczynski J (00) J Phys Chem Chem Phys 4: 535 [] Saenger W (4) Principles of ucleic Acid Structure. Springer-Verlag, ew York [3] ir E, Janzen Ch, Imhof P, Kleinermanns K, de Vries MS (00) J Phys Chem Chem Phys 4: 3 [4] ir E, Janzen Ch, Imhof P, Kleinermanns K, de Vries MS (00) J Chem Phys 5: 404

13 A DFT Study of Electron Structure, Geometry, and Keto-Enol Tautomerism of 3- xopropionyl alogenides Vassil B. Delchev University of Plovdiv, dept. Physical Chemistry, Tzar Assen 4 Str, Plovdiv 4000, Bulgaria Published in Monatshefte für Chemie, 004, 35:4, 3-34 ITRDUCTI Two most important representatives of the β-diketones group have been widely studied by theoretical and experimental methods [,]. These are malonaldehyde [3] and acetylacetone [4,5], which play an important role in organic synthesis. In this aspect, the composition with respect of enol and diketoform at certain conditions should be known. Two kinds of prototropic processes are observed in the tautomeric forms of malonaldehyde, acetylacetone, and their derivatives: ) a proton exchange between the two oxygen atoms in the enol form; and ) a transformation of the enol form into diketone and vice versa. The purpose of this paper was to investigate the kinetics of the keto-enol conversion of all malonylmonohalogenides in the gas phase passing through one transition state. The study involves a determination and characterization of the transition states and a tracing of the reaction paths by means of the B3LYP functional. X C... C C X =, F, Cl, Br, I RESULTS The computations contributed to an elucidation of the relationship stabilitystructure of five monosubstituted malonylhalogenides. The computations revealed the mechanism of the keto-enol conversion of the compounds in the gas phase passing through one transition state. The following general results were obtained: X C C C

14 The enol forms of malonylmonohalogenides are more stable than the cis- and transdiketones due to the stabilizing role of the intramolecular hydrogen bonds in their molecules. The strongest intramolecular hydrogen bond was found in the enol form of malonaldehyde. The halogen atom in the remaining enol tautomers provokes an elongation of this bond as in this way decreases its strength. Cis-diketoforms are unstable compared to trans-diketoforms. They do not correspond to minima because possess imaginary frequencies in their vibration spectra. An exception is the cis-diketoform of malonylmonochloride, which was assigned as an intermediate structure along the E-Cl trans-k-cl reaction pathway. Each cis-keto- / trans-keto - enol tautomeric isomerization passes through one nonplanar transition state. In other words all cis-keto-enol conversions occur with motion of the proton () out of the molecular plane. The high energy barriers of the keto-enol conversions indicate that in the gas phase they occur with difficulty. According to the IPCM calculations non-proton solvents with different polarity like chloroform and acetonitrile provoke an insignificant decrease in the energy barriers of the forward reactions versus the gas phase. References [] Buemi G, Gandolfo C () J Chem Soc Faraday Trans 5: 5 [] Andreassen AL, Bauer S () J Mol Struc : 3 [3] Foresman JB, Frisch (5-) Exploring chemistry with electronic structure methods, Second edition, Gaussian Inc. [4] Lowrey A, George C, D Antonio P, Karle J () J Am Chem Soc 3: 3 [5] Schaefer JP, Wheatley PJ () J Chem Soc A 5

15 Theoretical study of the binding sites in -hydroxy-4-methylcoumarin atasha Trendafilova (Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 3 Sofia, Bulgaria Günther Bauer (Institute of Chemical Technologies and Analytics, Technical University of Vienna, Vienna A-0, Austria) I. Introduction The binding sites of -hydroxy-4-methylcoumarin (mendiaxon) were studied on the basis of different interaction models, mend - -X + (X + = a +, Cu +, + ), mend. (), mend. () and mend. (Fig. ). F and B3LYP calculations were performed for the neutral mendiaxon (mend), its deprotonated form (mend - ), the sodium salt (menda) and the copper model complex (mendcu). Comparative structural and vibrational analysis of the species studied was performed at different levels of the theory. The vibrational shifts of modes sensitive to mend - -X + (X + = a +, Cu +, + ) interactions were evaluated. Geometry optimization and frequency calculations were performed for the hydrogen-bonded water complex, mend. at B3LYP level of the theory with - 3G(d) and -3++G(d,p) basis sets. The mend - 4 () and mend - 5 () hydrogen bondings were estimated on the basis of two models, mend. (), mend. ().. Methods To find the most reliable method for description of the experimental structural data for mendiaxon, first, full geometry optimization with F and DFT methods with - 3G(d) basis set was performed for its hydrogen-bonded water complex, as obtained from X-ray diffraction analysis. The reliably accurate description of the interactions in the systems studied required a treatment of electron correlation. ur previous studies on hydrogen bonded coumarin derivatives showed that density functional calculations with Becke s three-parameter hybrid method with the correlation functional of Lee, Yang and Parr (B3LYP) are reliable for studying system with hydrogen bonds.

16 Fig.. -bonded complex of -hydroxy-4-methylcoumarin (mendiaxon). To verify the reliability of B3LYP method with -3G(d) basis set we have performed geometry optimization and frequency calculations of mend. with larger basis set, -3++G(d,p), supplemented with polarization and diffuse functions for and diffuse functions for C and atoms: [3sp] for and [4s3pd] for C and. The reliability of B3LYP functional was further verified by comparison of the calculated and experimental first (vertical) ionization potential of mend. B3LYP/-3G(d) IP calc =. ev IP exp =.4 ev B3LYP/-3++G(d,p) IP calc =. ev B3LYP method with -3G(d) and -3++G(d,p) basis sets underestimated the experimental ionization potential of mend with 5.% and 3.4%, respectively. The larger basis set gave IP value, which was in better agreement with experiment. owever, comparison of the B3LYP/-3G(d) geometry of mendiaxon water complex with the

17 experimental one showed that the calculated geometry is sufficiently accurate. The average deviations of the calculated bond lengths and bond angles are: 0. % and 0.5 % at B3LYP/-3G(d), and.05 % and 0.5 % at B3LYP/-3++G(d,p), respectively. Therefore, B3LYP/-3G(d) level of the theory was used to obtain equilibrium geometries of mend, mend -, menda, mendcu, mend. (), mend. () and mend.. on-relativistic effective core potential, LanLDZ, was used for Cu ion. et atomic charges have been obtained using the natural population analysis (PA) of Weinhold and Mulliken scheme (MPA). The electronic structure and bonding features of the compounds studied were analysed using the natural bond orbital (B) analysis and the topological analysis of the electron density distribution, developed by Bader. Using the AIM theory we have located the bond critical points (i.e, points where the electron density, ρ b, is a minimum along the bond path and maximum in the other two directions). The Laplacian of the density, ρ b, as has been shown in the literature, identifies regions of the space wherein the electronic charge is locally depleted ( ρ b > 0) or concentrated ( ρ b < 0). The positive values of the Laplacian ( ρ b > 0) are associated with closed-shell interactions (ionic bonds, hydrogen bonds and van der Waals molecules), while the negative values ( ρ b < 0) characterize covalent bonds where the electron density concentrates in the internuclear region. The proton affinity of mend - was computed as the negative of the enthalpy change of the protonation reaction. The gas-phase basicity was estimated from the negative of the Gibbs energy change (- G), according to the equation: G = - T S, where the entropy contribution to G was obtained by T S = T[S(mend) - S(mend - ) - S( + )]. At K, the TS( + ) was. kcal mol -. Single point charge model was used to analyse the energy contributions to mend- a, mend-cu and mend- binding energies. Firstly, we have calculated the deformation energy of mend - as a difference between the energy of the deformed mend - in the complex and the energy of the optimised mend - structure. Secondly, the electrostatic interaction was estimated by the energy lowering of the deformed mendiaxon in the presence of a single point charge (Q + ) without allowing electronic relaxation. The point charge is placed at the mend - -a +, mend - -Cu + and mend distances. After that, we 3

18 allowed the electronic relaxation of mendiaxon and the energy thus obtained included both the electrostatic and the polarization terms. All computations were performed using GAUSSIA program, Revision A.. 3. Results The F calculated bond lengths were found in agreement with the experimental bond order in coumarin ring, the absolute deviations of C-C bonds being lower than those obtained at B3LYP. The B3LYP calculations overestimated the C-C bonds but they showed excellent coincidence with experiment for the double C = and the single C 3 - bonds. The predicted 4 () -bond length (mend - 4 ()) at B3LYP level was in a very good agreement with experiment. At the same time, both basis sets used showed longer distance than experiment (0.5 Å deviation with -3G(d) basis set and 0.45 Å with -3++G(d,p) basis set. A possible explanation of the calculated longer distance could be the B3LYP calculated larger - 4 valence angle ( ) in comparison with experiment (4 ). ur calculations showed stronger -bonding when the water molecule is a proton acceptor, mend 4 () (. kcal mol - ) in comparison with the -bonding in which the water is a proton donor, mend () (. kcal mol - ). Both -bondings produced similar bond length- and frequency changes in the coumarin ring. The structural and vibrational changes obtained in mend. () and mend. () model systems, were strengthened in mend.. The calculated gas-phase basicities of mend - predicted the deprotonated atom as the most probable for binding of positive charge atoms, a +, Cu + and +. In the order: menda, mendcu and mend the induced polarization of bonds increased, producing larger bond length changes. It was found that the different nature of mend - -X + interaction, especially, the strength of the electrostatic attraction, altered the C = properties, although the group is significantly remote from the position of the coumarin ring. The ν(c = ) vibration is very sensitive to the type of the mend - -X + interaction. Stronger induced polarization upon mend - -X + interaction led to shorter C = bond length and higher ν(c = ) frequency (ν(c=) mend - ν(c=) menda = 0 cm - ), whereas mend.. bonding 4

19 produced longer C = bond and ν(c = ) appeared at lower frequencies, by 50 cm -. Thus, the calculated higher ν(c = ) mode in menda than that one in mend. is in agreement with the experimental spectral behavior of ν(c=) in mendiaxon and its sodium salt. It was further found, that mendcu and mend. () compounds showed similar ν(c = ) and ν(c 3 - ) band positions. In the frame of the project the following publications appeared in 004:.. Trendafilova, G. Bauer and Tz. Mihaylov, DFT and AIM studies of intramolecular hydrogen bonds in dicoumarols, Chemical Physics, 30, 5-4 (004)... Trendafilova, I. Kostova, I. Manolov, G. Bauer, T. Mihaylov and I. Georgieva, Synthesis and spectroscopic study of new Lanthanum(III) complexes of 3,3 -benzylidenedi-4-hydroxycoumarin, Sinthesis and Reactivity in Inorganic and Metal-rganic Chemistry, (004). 3. I. Georgieva, T. Mihaylov, G. Bauer and. Trendafilova, Effect of the nature of mendiaxon - -X + interactions (X + = a +, Cu +, + ) and the hydrogen bonding on the ν(c=) behavior. Theoretical and spectroscopic study, Chemical Physics, 300, -3 (004). Acknowledgement The authors thank the Computer Center of the Technical University of Vienna and Ass.-Prof. ans Mikosch for the computational facilities and the services provided. Financial support from the Technical University of Vienna is greatly acknowledged. 5

20 Modelle für Strukturen und Schwingungsspektren von Aflatoxinen und ihrer ydrolyseprodukte Abschätzung der Reaktionsenergien Ferenc Billes,, ans Mikosch, Agnes M. Móricz 3, Ernö Tyihák 3 Lehrstuhl für Physikalische Chemie, Budapester Universität für Technologie und Wirtschaftswissenschaft, -5 Budapest, Budafoki út, Ungarn. fbilles@mail.bme.hu Institut für chemische Technologien und Analytik, Technische Universität Wien, A-0 Wien, Getreidemarkt. ans.mikosch@tuwien.ac.at 3 Institut für Pflanzenschutz der Ungarische Akademie der Wissenschaften, -55 Budapest, Pf., Ungarn. etyih@nki.hu Quantenchemische Rechnungen an vier Aflatoxinen und ihren ydrolyseprodukten wurden durchgeführt. Mit den Ergebnissen wurden Infrarotspektren simuliert und thermodynamische Kennwerte für die ydrolyse berechnet. Die Ergebnisse zeigen, daß die so berechneten thermodynamische Daten der Aflatoxine mit deren Toxizität in Zusammenhang gebracht werden können.. EILEITUG Die Aflatoxine sind Micotoxine, Produkte der Schimmelpilze Aspergillus flavus und Aspergillus parasiticus. Diese Toxine sind sehr verbreitete Verunreinigungen menschlicher und tierischer Lebensmittel. Die am häufigsten vorkommenden Aflatoxine werden mit B, B, G und G bezeichnet (s. Abb. ). Die Reihenfolge ihrer Toxizität ist B >G >B >G []. Diese Toxine werden in den rganen enzymatisch (Demethylase) abgebaut zu Methanol; in weiterer Folge bildet sich in den Zellen durch enzymatische xidation Formaldehyd. Die Aktivität der Demethylase wird über die Bildung von Formaldehyd bestimmt [,3]. Der Ziel dieser Arbeit war die ptimierung der Strukturen und die Simulierung der Infrarotspektren der vier Aflatoxine und ihrer Reaktionsprodukte mit ilfe quantenchemischer Methoden, sowie die Abschätzung thermodynamischer Größen, und mit ilfe dieser Ergebnisse die Berechnung der Reaktionsenergien ihrer ydrolyse. Und darüberhinausgehend: Kann ein Zusammenhang zwischen den thermodynamischen Daten und der Toxizität gefunden werden? Wir arbeiteten an diesem Projekt, als in Ungarn die Alfatoxine sehr populär wurden: Im erbst 004 importierte Ungarn Paprikagewürz (Pulver), und viele Lieferungen waren mit Aflatoxinen verunreinigt. Tageszeitungen und Zeitschriften waren voll mit Artikeln über dieses Ereignis und über Aflatoxine. Darum wurde unsere Arbeit besonders aktuell.. RECEVERFARE Die Rechnungen wurden mit dem Programmpaket Gaussian [4] durchgeführt. Die Dichtefunktional Methode (DFT) mit Becke3LYP Funktional und -3G* Basissätzen wurde benutzt. Als erste Stufe der Rechnung wurden die optimierte Molekülstruktur (r e ), die entsprechende Molekülenergie und die Ladungsverteilung erhalten. Die Ergebnisse der zweiten Rechenstufe waren die Schwingungskraftkonstanten, die Elektronenenergie, ihre Korrektur mit der Inneren Energie ( U), bzw. mit der Enthalpie ( ), und die Standardwerte einiger thermodynamischen Daten des isolierten Moleküls. Für die Simulierung der Schwingungsspektren wurden eigene Programme zur ormalkoordinatenanalyse verwendet. Als Ergebnis erhielten wir die Grundschwingungen

21 der Moleküle und die Zuordnung der Schwingungsarten, sowie die simulierten Infrarotspektren. 3. ERGEBISSE 3..ptimierte Molekülgeometrie Abb.. stellt die optimierten Molekülstrukturen der Aflatoxine dar. Die ydrolyse beeinflußt kaum die Gerüststrukturen. B B G G Abb.. Aflatoxin B und B enthalten vier koplanare Ringe (die in Abb.. rechtsstehenden). Sie unterscheiden sich voneinander durch den Ring links außen: die Doppelbindung fehlt in B. Der Ring rechts außen in den G Aflatoxinen ist statt eines Fünferrings ein Sechserring mit einem Ring Sauerstoffatom. Dieser Ring ist nicht koplanar mit den drei zentralen Ringen. G und G unterscheiden sich analog zu B und B. 3.. Schwingungsspektren Für die ormalkoordinatenanalyse wurden zuerst die internen (chemischen) Koordinaten definiert. Die in kartesischen Koordinaten gegebenen Kraftkonstanten werden in diese Koordinaten transformiert (F Matrix). Mit ilfe der kartesischen Koordinaten und der Definition der internen Koordinaten wird die inverse kinetische Energie Matrix (G Matrix) berechnet. Die Schwingungsfrequenzen (ν) werden nach der Gl. berechnet (E ist Einheitsmatrix): GF - 4π ν E =0 ()

22 Wegen der systematischen Fehler der quantenchemischen Rechnungen stimmen die mit Gl. berechneten Frequenzen nicht ganz überein mit den gemessenen. Die Anpassung der berechneten Frequenzen erfolgt durch Skalierung des F Matrizen. Ein sehr gutes Modell ist, dieselben Skalierungsfaktoren für chemisch ähnliche interne Koordinaten zu benutzen. Die Skalierungsfaktoren in diesem Fall, wo experimentelle Daten wegen der hohen Toxizität nicht zu Verfügung stehen, müssen einheitlich sein: Wir benutzten den Skalierungsfaktor 0,. eben den Frequenzen wurde auch eine Charakterisierung der Schwingungsarten durch die Verteilung der potentiellen Energie (PED) erhalten. Abbildung stellt die simulierten Infrarotspektren der Aflatoxine dar. Es wurden Lorentz-verteilte Peaks mit 5 cm - albwertsbreite angenommen und die quantenchemisch berechneten integrierten Intensitäten verwendet. Die mit B bezeichneten Absorptionspeaks sind charakteristisch für die B Aflatoxine, die mit G für die G Arten, die mit für die mit Index, und die mit bezeichneten für die mit Index. - Intensität / km mol 4 4 B B Simulierte Infrarotspektren der Aflatoxine B B 4 4 G G G Wellenzahl / cm - G Abb.. Aus den Ergebnissen der quantenchemischen Rechnungen können auch Spektren für experimentell nicht einfach zugängliche Substanzen, in diesem Fall für die nach Gl. hydrolisierten Aflatoxine, berechnet werden. Afl-C 3 + = Afl- + C 3 - ()

23 3.3. Thermodynamik der ydrolyse Tabelle. enthält die berechneten thermodynamischen Daten für die in Gl.. vorkommende Moleküle, sowie die berechneten Enthalpien und chemischen Potentiale der Reaktionen und die entsprechenden Gleichgewichtskonstanten. Tabelle. Molekül E e U m m µ S m r m r µ MJ mol - kj mol - J mol - K - kj mol - K AfB-C 3 -,3 3,0 4,00 5, 5,5, -3,00,004 AflB- -0, 5,3, 4, 54,55 AflB-C 3-5,4 04,0 0,50 3,05 5,4,5-3,40,003 AflB- -,5 4,4,30 5,4 55,4 AflG-C 3-3, 55, 5,5 53,345 5,4, -4,,00 AflG- -300,,44,0 5,55 55,0 AflG-C 3-33,0, 3,43 4, 53,4, -4,033,003 AflG- -30,0 44,444 44, 55, 54,54 Methanol -304, 44,4 4,04,3 3, Wasser -0,04 3, 5,4,,0 Es gibt nur kleine Unterschiede zwischen den Gleichgewichtskonstanten, aber alle Energieangaben (molekulare Innere Energie, molekulare Enthalpie, chemisches Potential) der Aflatoxine folgen der Toxizitätsreihe (s. Einleitung) mit steigenden Energiewerten. Daraus kann geschlossen werden, daß die Toxizität mehr bestimmt wird durch die Molekülstruktur als durch das chemische Gleichgewicht. LITERATUR. McLean, M., Dutton, M. F.: Pharm. Ther., 5, 3 (5).. Domngang, F., Emerole, G.: Biochem. Pharmacol., 3, 3 (). 3. Móricz, Á.,.-tta, K., tt, P., Tyihák, E.: J. Planar Chromatogr., 4 (003). 4. Gaussian, Revision A., M. J. Frisch, G. W. Trucks,. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K.. Kudin, M. C. Strain,. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. chterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. rtiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. anayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. ead-gordon, E. S. Replogle, and J. A. Pople, Gaussian, Inc., Pittsburgh PA,.