Electronic Structure and Vibrational Modes of Cobalt Oxide

Transkript

1 Electronic Structure and Vibrational Modes of Cobalt Oxide Clusters CoO n (n=1-4) and their Mono-anions Ellie L. Uzunova 1, Georgi St. Nikolov 1, *, Hans Mikosch 2 1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, BULGARIA 2 Institute of Solid State Chemistry and Technical Electrochemistry, Vienna University of Technology, Vienna 1060, AUSTRIA Stationary points on the potential energy surfaces (PES) of cobalt oxide clusters were studied by the density functional theory. A number of local minimums were detected on the doublet, quartet and sextet PES of CoO n (n=1-4) and the singlet, triplet and quintet PES of the corresponding anions. The normal vibrations of all optimized structures were calculated and interpreted in terms of point group theory. The global minimums and the low-lying local minimums were also examined by the coupled cluster method with single and double substitutions and perturbational estimate of triple excitations, CCSD(T). The ground state of CoO is a quartet ( 4 )state in agreement with previous assignments, while the ground state of CoO - is a quintet ( 5 ) state. The CoO 2 oxides are more stable than the Co(O 2 ). peroxides The neutral CoO 2 oxide in its ground 6 A 1 state is quasilinear; the mono-anion in its ground 5 g state is linear. Oxoperoxides OCo(O 2 ) are more stable than CoO 3 oxides and OCoOO superoxides. Diperoxides are the stable structures for uncharged CoO 4. In general, the mono-anions are more stable than the neutral clusters; however, electron attachment to peroxide destabilizes the O-O bond. The ground states of CoO n (n=2-4) and the ground states of their anions [CoO n ] - are high-spin states sextets for the neutral species and quintets for the anions. The thermodynamic stability of different structures was examined for possible fragmentation paths. The CoO n clusters and their mono-anions dissociate preferably with the release of dioxygen molecule. Geometry optimization and harmonic frequency calculations were performed by the B1LYP method, which includes local and non-local terms as implemented in the Gaussian 98 package. The Becke one-parameter functional was derived from the B3LYP functional, the three parameters being substituted by one. The standard G* basis set was employed. Potential energy surface scans were performed for the three-atomic CoO 2 incrementally at steps R Co-O = 0.05 Å and OCoO = 5.

2 6 A 1 b) C 2v D h C 2v R (Co-O), Å OCoO, deg Figure a) Potential energy surface of CoO 2 in its 6 A 1 state. The local minimums and the dissociation products are denoted. The ground state is quasilinear, OcoO = b) Contour diagram of CoO 2 in its 6 A 1 state as a function of the internal coordinates R CoO and OCoO. Energy values are scaled (E tot +1533) 10 2 Hartree. The paper is submitted to J Phys Chem B in November 2001

3 DFT Study of Zeolite FAU Structural Fragments Double Six-Member Rings of Oxygen-bridged Silicon and Aluminum Atoms Ellie L. Uzunova 1, Georgi St. Nikolov 1, *, Hans Mikosch 2 1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, BULGARIA 2 Institute of Solid State Chemistry and Technical Electrochemistry, Vienna University of Technology, Vienna 1060, AUSTRIA The electronic structure of M x H 12 Si 12-x Al x O 18 structural fragments with Si and Al atoms located at tetrahedral sites, together with oxygen-coordinated chargecompensating cations M=Na, K, were studied by density-functional theory. Calculations were performed with the B3LYP exchange-correlation functional, which includes both local and non-local terms, as implemented in the Gaussian 98 package. The 6-31G(d) basis set was employed; for the fragments, containing hydroxyl groups, polarization functions at the hydrogen atoms were included via the 6-31G(d,p) basis. Extraframerwork cations were calculated with the 6-311G(d) basis. The influence of Si, Al ordering, the type and positions of the charge compensating cations on the framework stability were evaluated. Frequency calculations were performed for the protonic forms of the D 6 R and the OH stretching vibrations and they agree with the experimental values for FAU zeolites. A relation between the TO(1)T angle, where O1 denotes the bridging oxygen atom, connecting the two six-member rings and the position of the extraframework cations was established. Frequency calculations were performed for all the electroneutral species at their optimized geometry. The ring-opening vibrations of the cluster models were correlated to the FAU framework modes via the higher symmetry group D 6h.

4 Bericht über Rechnungen am Rechner fpt während des Jahres 2001 Dr. Ferenc Billes Lehrstuhl für Physikalische Chemie Technische und Wirtschaftswissenschaftliche Universität Budapest und Institut für technische Elektrochemie und Festkörperchemie Technische Universität Wien A. Das Ziel ist die Benutzung der quantenchemischen Rechnungen zur Zuordnung der Schwingungsspektren vorwiegend organischer und einiger weniger anorganischer Verbindungen. Diese Berechnungen wurden vor allem mit DFT Methoden durchgeführt. Für größere Moleküle wurden Hartree-Fock-Verfahren oder semiempirische Methoden angewendet. Dabei wurden als Basissätze 6-311G** oder 6-31G*, manchmal LANL2DZ, der Problemstellung entsprechend angewendet. Zur Zuordnung (Interpretation) der Schwingungsspektren wurden die Ergebnisse der quantenchemischen Rechnungen anschließend mittels Normalkoordinatenanalyse und Kraftkonstantenoptimierung an die experimentellen Spektren angepaßt. Folgende Rechnungen wurden dazu durchgeführt: 1. Wir beschäftigen uns seit vielen Jahren systematisch mit der Zuordnung der Schwingungsspektren der Diazinderivate. Dieses Jahr haben wir die Schwingungsfrequenzen der Chlor- und Methylderivate von Pyrazin berechnet, und zwar die 2-Chloro- und 2,5- Dichloro-, 2-Methyl-, 2,3-Dimethyl-, 2,5-Dimethyl- und 2,6-Dimethylpirazine, alle mit Becke3P86/6-311G**. Die Ergebnisse der auf den quantenchemischen Rechnungen beruhenden Normalkoordinatenanalyse zeigen, daß hier die quantenchemisch berechneten Frequenzen nahe den experimentellen liegen. 2. Derivate des N-Methyl-Phenothiazin bilden eine neue Klasse der organischen Verbindungen; die 3-substituierten Verbindungen der Grundverbindung, ihre Sulfoxide und Sulfone sind interessant als potentielle Medikamente. Die Rechnungen für die Aufklärung ihrer Struktur und die Zuordnung ihrer Schwingungsspektren haben wir dieses Jahr begonnen. Diese Moleküle sind wesentlich größer als die Pyrazinderivate, darum berechneten wir diese mit kleinerem Basissatz (Becke3P86/6-31G*). Die Bearbeitung der Ergebnisse ist im Gange. 3. Wir setzten die Rechnungen an anorganischen Chalkogeniden mit Hartree-Fock Verfahren (LANL2DZ Basis) fort. Die Schwingungsspektren von Arsenselenid,

5 Germaniumselenid und Germaniumtellurid Clustern wurden berechnet (Modellverbindungen). Diese Substanzen sind in der Optoelektronik und in der Optik als optische Gläser anwendbar: Im infraroten Bereich sind sie als Laserlichtleiter anwendbar, spezielle Spiegel werden auch aus diesem Material gefertigt. Die gute Übereinstimmung mit den experimentellen Werten der mit dieser Methode berechneten Frequenzen ist überraschend. B. Modellrechnungen an großen Molekülen mit semiempirischen Methoden wurden fortgeführt. 1. Kalixarene sind wichtige Verbindungen als chelatkomplex-bildende Substanzen. Mit langen Nebenketten an den aliphatischen Methylengruppen dienen sie als Modelle der Ionenkanäle von Zellen, z.b. Tetraundecylresorzinaren. Wir berechneten das all-trans Konformer mit AM1; die Optimierung führte zur richtigen Zahl von Frequenzen. Auch die anschließende Einführung eines Na + Ions in den Kanal ergab richtige Ergebnisse. Weitere zu untersuchende Probleme sind die Änderung des Potentials als Funktion der Position des Na + Ions in der Molekülachse und die Frage des Platzbedarfs im Kanal bei Einführung eines Wassermoleküls zusätzlich zum Na + Ion. 2. Hydroxiharnstoff ist ein sehr wichtiges Heilmittel; es dient als Hilfsmaterial neben anderen Medikamenten in der Krebs- und AIDS-Heilkunde; es ist ein wichtiges Medikament gegen einer Art der Blutarmut. Die Strukturaufklärung des Substanzen (starke Assoziation wegen vieler möglicher Wasserstoffbrücken) hilft in der Aufklärung seines Wirkungsmechanismus. Die Modellrechnungen an Hydroxiharnstoffpolymeren wurden semiempirisch (AM1 und PM3) durchgeführt. Die Ergebnisse zeigen deutlich, daß mit der steigenden Anzahl der Moleküle sich das auf dieser Grundlage berechnete Infrarot-Spektrum den experimentellen Ergebnissen annähert. Doch hier muß noch die Anzahl der Moleküle im Modell weiter erhöht werden. Literatur: 1. Billes, F., Mohammed-Ziegler, I.: Ab initio geometry and vibrational spectroscopic study of 25,25,27,28-tetrahydrocalix[4]arene Supramolecular Chemistry (im Druck) 2. Billes, F., Mohammed-Ziegler, I., Mikosch, H., Holmgren, A.: Vibrational spectroscopic and conformational analysis of pinosylvin. J. Phys. Chem. (in Begutachtung)

6 Electronic structure and vibrational spectra of 1,3-propanediale and 2,4-pentanedione and their tautomeric enol forms Vasil Borisov Delchev Department of Physical and Theoretical Chemistry, University of Plovdiv, Bulgaria 1,3-propanediale and 2,4-pentanedione compounds show interesting properties: They can exist as two stable conformers - so-called enol- and diketo-form. The enol- form is stabilized by an intramolecular hydrogen bond, which makes it possible to form a pseudoaromatic structure. Our theoretical investigation was connected with details of the keto-enol- tautomerism, i.e. the equilibrium between these forms for both compounds. First we optimised the isolated structures and calculated the vibrational spectra of the enol- and the diketo-form for both compounds. The next step was related with the task to find the exact transition state of the keto-enoltransformation - its structure and its vibration spectra. We used the options QST3 and FREQ=RAMAN of the GAUSSIAN 98 package. The transition state is defined by one imaginary frequency in a vibrational spectrum, describing the path of the reaction. Knowing the energy of the transition state we estimated the energy barriers of the forward and the reverse reaction and calculated the equilibrium constant of the keto-enol equilibrium for both compounds. It should be noted that we have studied all reactions for isolated molecules. The solvent effect was not taken into account. Therefore, our proposed mechanism for the reactions would only be able to describe the path of a keto-enol-equilibrium in the gas phase. We have studied also eight planar enol conformers for each compound a structure and a vibration spectrum in order to find the most stable one and to investigate the mechanisms of the conversions between a pair of conformers. The results show that low energy barriers have conversions connected with rotations around single bonds and accordingly the rotations (conversions) connected with rotations around a double bond show high-energy barriers. Keto-enol- mechanisms for barbituric acid were also studied. We have performed DFT level calculations using D95** basis set of GAUSSIAN 98. The aim was to find the most stable form of the compound and to estimate the energy barriers of the different keto-enol- reactions.

7 That was again connected with determining of the transition states of the reactions - their structures and vibrational spectra. We have published and prepared for publication the following papers and reports during the period January December 2001: 1. Delchev V.B. and Hans Mikosch Ab initio Study of the Rotamers and Rotations of Propane- 1,3-dial by means of DFT and SCF Calculations, Monatshefte für Chemie, 2001, 132:2, Delchev V.B., H. Mikosch and G. St. Nikolov Keto-Enol Equilibrium of Pentane-2,4-dione Studied by Ab initio Methods, Monatshefte für Chemie, 2001, 132:3, Submitted: 1. Delchev V.B., Ab initio study of rotamers of the enol form of 2,4-pentanedione, Chemistry and industry - Bulgaria. 2. Delchev V.B., Ab initio study of the attachment of CN - to malonaldehyde and some correlations between characteristic frequencies, Chemistry and industry Bulgaria 3. Delchev V.B., AM1 study of the influence of halogen substitutes over the energy of HOMO and LUMO of symmetric and asymmetric malonyldihalogenides, Chemistry and industry Bulgaria. 4. Delchev V.B. and Hans Mikosch, Ab initio Study of the Rotamers and Rotations of Pentane- 2,4-dione by means of DFM and SCF, Monatshefte für Chemie. 5. Delchev V.B. and Hans Mikosch, Ab initio Study of the Keto-enol Tautomerism of Barbituric Acid by means of DFT, Monatshefte für Chemie. Reports and posters: 1. Delchev V.B. and G. St. Nikolov, Ab initio study of rotamers of the enol form of 2,4- pentanedione, IVth National Chemistry Conference, Sofia, , p Delchev V.B., Ab initio study of the attachment of CN - to malonaldehyde and some correlations between characteristic frequencies, IVth National Chemistry Conference, Sofia, , p.82