Showing papers in "Theoretical Chemistry Accounts in 1966"

Molecular orbital calculations on copper chloride complexes

Abstract: The following paper describes a molecular orbital calculation of the Wolfsberg-Helmholz type. However, in contrast to the usual empirical determination of the H ii integrals, these integrals are here computed from the wave functions of the free ions. The calculation is performed on Cu in a tetrahedral, square planar and octahedral environment of Cl, the results being subsequently compared.

Approximate analytical orbital functions for second and third-row transition metals

Abstract: Approximate analytical functions fors, p, d, andf orbitals of the second and third-row transition metals have been constructed from the Herman-Skillman Hartree-Fock-Slater numerical wave functions. They consist of orthonormal linear combinations of Slater-type orbital functions, and depend on a weighted least-squares criterion to judge the accuracy of the fit. The option of using a single-μ (one orbital function per extremum) or double-μ representation for the outermost maximum of a given function is included. Whereas the latter basis set is more flexible, convergence problems in the fitting method as well as an inherent arbitrariness in choosing the fitting criterion result in derived parameters which are not uniquely determined by the least-squares criterion alone. By relaxing the orthogonality requirement for orbitals of the samel and differentn it is shown that the accuracy of the fit can be significantly improved. The importance of a proper choice for the fitting criterion is discussed.

Theoretical study of the electronic properties of the purine and pyrimidine components of the nucleic acids

Abstract: A semi-empirical self-consistent field calculation has been carried out for the purine and pyrimidine bases of the nucleic acids after an appropriate optimization of the integral values using reference compounds. Special emphasis has been laid upon results concerned with the ionization potentials, electron affinities, spectral transitions, triplet states, dipole moments and basicities of the compounds. They enable in particular a detailed interpretation of both the individual ultra-violet absorption spectra of the bases and of their interrelations.

The electronic spectra of the anion radicals of substituted benzenes

Abstract: The nitrosobenzene, benzonitrile, nitrobenzene, phthalonitrile, isophthalonitrile, terephthalonitrile and pyromellitonitrile anions were prepared by the alkali metal reduction method. Their electronic absorption spectra were measured in parallel with the ESR spectra. Furthermore, the electronic structures of these anion radicals were studied theoretically by combining the open shell SCF procedure with configuration interaction calculation. The observed transition energies and relative intensities were well explained by the present theoretical studies.

Pariser and Parr type calculations on saturated hydrocarbons I

Abstract: A Pariser and Parr-type approximation is proposed for saturated paraffinic hydrocarbons and is applied to methane, ethane and propane.

Low spin ferric hemoglobin complexes

Abstract: A caloulation has been made of the energy eigenfunotions and eigenvalues of low spin ferric ion in complexes with a strong cubic crystal field including the effects of tetragonal and rhombic distortions and of spin-orbit coupling among the ground state components and with excited states. Using the resultant, spin-orbit coupled eigenfunotions as a basis set, the magnetic susceptibility, the components of magnetic field energy, and the lattice and valence contributions to an electric field gradient at the iron nucleus were all calculated as a function of rhombic, tetragonal, and spin-orbit coupling strength used as parameters: R, u and δ. All of the calculated results agree reasonable well with experiment for the values of parameters R=1000 cm−1, u=2000 cm−1 and the free ion value (δ=420 cm−1. These values of parameters were selected for the excellent fit they gave of the calculated values of g x, gy and g z compared with the experimental ones obtained from single crystal electron spin resonance of ferrihemoglobin azide. With them, a value of 2.29 Bohr magnetons was calculated for the effective magnetic moment compared to the experimental value of 2.35. The total field gradient calculated under the same conditions, predicts a nuclear quadrupole moment Q in the range of. 107 –127. Barns, which is smaller than the range predicted from the high spin ferric ion results. Reasons for this discrepancy are discussed.

Das stereochemische Strukturmodell, ein mathematisches Modell zur gruppentheoretischen Behandlung der dynamischen Stereochemie

Abstract: Das in der vorliegenden Abhandlung entwickelte „Stereochemische Strukturmodell“ ist ein theoretischer Ansatz, in dem nAherungsweise gultige Analogien zwischen den EnergieeigenzustAnden von stereoisomeren Molekulen im Sinne einer mathematisch strengen Behandlung idealisiert sind. Diese Analogien sind beschreibbar durch eine Gruppe von umkehrbar eindeutigen Abbildungen der EigenzustAnde, Abbildungen, die nAherungsweise geometrisch-physikalisch interpretierbar sind. Die Gruppenstruktur liefert uns demnach mit einer physikalisch sinnvollen Klassifikation der EigenzustAnde die dem PhAnomen der Stereoisomerie angepa\ten Quantenzahlen. Da die Zustandssummen von Stereoisomeren bezuglich dieser Gruppe ein einfach uberschaubares Transformationsverhalten zeigen, ist dieser mathematische Ansatz als Modelltheorie fur Probleme der dynamischen Stereochemie geeignet. Es werden Gleichungen abgeleitet, mit denen die Konzentrationen der Produkte von stereoselektiven Reaktionen ohne gro\en Aufwand sowohl fur den Fall thermodynamischen Gleichgewichts als auch fur den Fall kinetisch gesteuerter Reaktionen berechnet werden konnen. Eine dieser Gleichungen fur einen speziellen Reaktionstyp deckt sich mit einer halbempirisch gefundenen Beziehung, die in einer fruheren Arbeit [9] innerhalb der Me\genauigkeit experimentell bestAtigt wurde. Die vorliegende Arbeit beschrAnkt sich auf rein theoretische Fragen, wie die Definition des Modells, die KlArung seiner gruppentheoretischen Struktur, die Diskussion seiner unmittelbaren algebraischen Konsequenzen und die Ableitung von Formeln fur die KonzentrationsverhAltnisse der Produkte bei einigen Reaktionstypen. Auf weitere theoretische Problemstellungen im Rahmen des Modells wird hingewiesen.

Der Einfluß der kationischen Umgebung auf die Lichtabsorption des zweiwertigen Nickels in oxidischen Wirtsgittern

Abstract: Es werden Absorptionskurven und Werte fur die Ligandenfeldstarke Δ hauptsachlich des Ni2+-Ions in verschiedenen oxidischen Wirtsgittern mitgeteilt. Die Grose von Δ variiert in charakteristischer Weise mit der Art des Wirtsgitters und seiner chemischen Zusammensetzung. Hierbei sind neben den Ni2+-O2−-.Abstanden Polarisationseinflusse und die kationische Koordination der O2−-Ionen von masgebendem Einflus. Besteht die uber gemeinsame O2−-Liganden benachbarte kationische Umgebung des Ni2+-Ions aus hochgeladenen Ionen, so resultieren fur Kristallstrukturen, in denen eine Anionenpolarisation begunstigt ist, im allgemeinen stark erniedrigte Δ-Werte (Perowskitgitter). Sind uberdies π-Bindungen moglich, so induzieren cuprokonfigurierte Kationen wesentlich kleinere Ni2+-Δ-Werte als Kationen mit unbesetzter nd-Schale (Perowskitgitter). Im Falle von Kristallgittern, in denen aus Symmetriegrunden eine Anionenpolarisation nur eine untergeordnete Rolle spielt, findet man eher entgegengesetzte Effekte.

Pariser-parr-pople calculations on different DNA constituents

Abstract: A semiempirical SCF LCAO MO-CI calculation has been performed for the nucleotide bases. According to the results we obtain a better agreement with the experimental singlet excitation energies, if we use the SCF eigenvectors of the Pople-type matrix in the CI calculation instead of the eigenvectors obtained after the first iteration step. On the basis of some parameter variation a set of integrals has been determined which yields as largest deviation between the experimental and theoretical excitation energies for the first two intensive excitations of the five bases (A, T, G, C and U) 1 eV. The possibilities of further improvements are discussed. In the second part of the calculations the oscillator strength values (f) of the G-C base pair and of the GpG, GpC, CpG and CpC dinucleotides have been determined using the first 16 singlet excited configurations of these composite systems for the CI calculation. The comparison of the results obtained with the appropriatef values of the constituent single bases shows a hypochromicity of the first absorption band system in the case of all the four dinucleotides and a slight hyperchromicity in the case of the G-C base pair.

Integral approximations for molecular orbital theory

Abstract: The electron repulsion integrals arising in LCAO-MO theory are approximated by replacement of the product of two orbitals on different centers by linear combinations of one-center products. The approximation differs from those previously proposed in that the coefficients of the various terms are determined by requiring agreement for certain integrals, and in the emphasis of the role of symmetry in selecting the one-center products. For two-center integrals, the new approximation is significantly better than older approximate methods. Reasons are given for expecting this improvement to extend also to multi-center integrals.

Self-consistent perturbation theory for conjugated molecules: I. General Theory and Method of Calculation

Abstract: The theory of one-electron perturbations is developed in the context of the self-consistent LCAO treatment of molecular wavefunctions. A numerical method of calculating changes in the charge and bond order matrix due to one-electron perturbations is described. The method is used to compute self-consistent atom-atom polarizabilities for a number of conjugated hydrocarbons. The results are applied to spin-density calculations and to a discussion of chemical reactivity.ZesummenfassungIm Rahmen des SCF-LCAO-Verfahrens wird eine Einteilchen-Störung behandelt and ein numerisches Verfahren zur Berechnung der Änderung der Matrix der Bindungsordnung angegeben. In der vorliegenden Arbeit wird diese Methode benützt, um Atom-Atom-Polarisier-barkeiten und Spindichten für eine Anzahl von Kohlenwasserstoffen zu berechnen sowie ihre chemische Reaktivität zu diskutieren.ResumeLa théorie des perturbations monoélectroniques est developpée au contexte du procédé LCAO autocohérent de fonctions d'ondes moléculaires. On décrit une méthode numérique pour le calcul de changements dans la matrice de charge et d'ordre de liaison dûs aux perturbations monoélectroniques. Suivant cette méthode sont calculées les polarisabilités atome-atome autocohérentes pour un nombre d'hydrocarbures conjugués. A l'aide des résultats on calcule des densités de spin, et discute la réactivités chimiques.

Electronic structures and spectra of adenine and thymine

Abstract: The near and vacuum ultraviolet spectra of adenine and thymine were measured with the evaporated film method and in solution. The absorption spectrum of the adenine anion in basic solution was also measured. The assignment of the absorption bands was made by comparing the observed transition energies and intensities with the theoretical results obtained by the LCAO-SCF-CI-method. The results indicated that the longest wavelength band of adenine at 269 mμ is a π→π * transition band. This was supported by the fact that a new band due to the n→π * transition was found at 278 mμ for the adenine anion. Furthermore, it was shown that the thymine molecule may interact strongly with the solvent molecules and that the electronic structure of thymine in solution may be different from that of the free molecule.

Two-configuration, self-consistent field theory

Abstract: The Hartree-Fock pseudoeigenvalues equations have been derived for configuration interaction in the case of two atomic configurations ns2np N and npN+2. A special case of investigation is the interaction between the nearly-degenerate configurations 1s22s22p N and 1s22pN+2. The LCAO form of the equations has also been established.

Extended Hückel method charge distributions and the chemical shift

Abstract: The extended Huckel theory (EHT) is used to calculate the charge distributions in alkanes, halogenated alkanes, and molecules containing methyl and vinyl groups. The charge densities are correlated with 13C, 31P, 19F, 35Cl and 1H, chemical shifts. The correlations are discussed in terms of the α and Β used in the extended Huckel method, and are compared to the results of the self-consistent group orbital and bond electronegativity (SGOBE) method.

Pariser-Parr-Pople-Rechnungen an Molekülen mit Aminogruppen

Abstract: Die Ergebnisse von PPP-Rechnungen fur Cyanine, Merocyanine und Aminochinone stehen in guter Ubereinstimmung mit den experimentellen Elektronen- und Schwingungsspektren. Das Spektrum des 2,5-Diaminobenzochinons-1,4 mus als unter Substituenteneinflus langwellig verschobenes p-Benzochinonspektrum gedeutet werden. Auch die Ladungsverteilung dieses Diaminochinons unterscheidet sich nur sehr wenig von derjenigen des Chinons. Diese Ergebnisse stutzen die Zuordnung der IR-Spektren der Aminochinone von Draber und Wallenfels und zeigen, das in den Aminochinonen der Chinoncharakter so ausgepragt ist, das es unnotig erscheint, sie als Quadrupol-merocyanine zu formulieren.

Transition probabilities in the CuCl42- complex

Abstract: Transition probabilities in the CuCl 4 2− -complex have been calculated using the L.C.A.O. functions obtained in the preceding paper by P. Ros and G. C. A. Schuit. For all transitions the calculated absolute oscillator strenghts turn out to be a factor 2 too large with respect to the experimental values. The calculated transition probabilities are mainly determined by the amount of mixing of the ligand functions in the molecular orbitals. If this mixing is assumed somewhat less the agreement is still better.

The electronic spectra of acenaphthylene and fluoranthene

Abstract: Acenaphthylene (I) and fluoranthene (II) in their electronic ground state are often regarded as consisting of two weakly interacting subsystems (naphthylene/ethylene or naphthalene/ benzene), which conserve much of their individuality. This seems to suggest that the lower electronically excited states of I and II can also be linked in a simple fashion to the low energy L b , L a , B b , and B a states of naphthalene and of benzene in II and of naphthalene in I, according to either Simpson's “Independent System Approach” or the“ Molecules in Molecules” scheme proposed by Longuet-Higgins and Murrell. Contrary to expectation this approach fails even for the lowest excited states of I and II, because of the extensive mixing of high energy locally excited and charge transfer configurations. These involve molecular orbitals of the subsystems naphthalene/ethylene or naphthalene/benzene, which do not contribute to their low energy L b , L a , B b or B a states.

Open shell SCF MO CI calculation of spin density distributions

Abstract: Self Consistent Field Molecular Orbitals (SCF-MO's) computed using the method described by Roothaan [3] were used as basis functions in a calculation of the spin density distributions of a number of (open shell) π electron anions and radicals. Electronic correlation was introduced into the ground state wavefunction by allowing it to interact with all singly excited configurations of the type 1/√6 {2 ¦ a x ¯n ¦ − ¦ a ¯x n ¦ − ¦ ¯a x n ¦ }. The results agree well with experiment, and the method was applied without difficulty to some open shell heterocyclic anions.

The relative importance of delocalization terms in proton spin-spin coupling constants

Abstract: An expression for the atom-atom poiarizability between the1s orbitals of geminal or vicinal protons has been derived by perturbation theory using thePople-Santry Theory ofσ electron systems. The coupling constants calculated on this basis agree well with these obtained from full molecular orbital calculations. Expressions have been obtained for the variation of vicinal coupling constants with dihedral angle in ethane and the HCC bond angle and C-C bond length in ethylene, and these agree well with corresponding expressions obtained using the VB method.

On the solution of the quantum mechanical two-electron problem by direct calculation of the natural orbitals

Abstract: The wave function of the ground state of the H2 molecule is calculated directly in its natural expansion form, the approximate natural orbitals (NO) being expressed as linear combinations of Slater type functions centered at the midpoint of the molecule. One obtains as total energy (in the equilibrium distance) − 1.168 a.u. (exact − 1.174 a.u.) which seems to be the best one-center result for H2 known so far; 96% of the binding energy is accounted for. The accuracy of this approach is limited due to the rather slow convergency of the onecenter expansion of the orbitals. The Hartree-Fock energy calculated with the same basis (−1.129 a.u.) is about as much in error with respect to the exact HF energy (− 1.136 a.u.) as the energy of the NO expansion with respect to the experimental one.

Quantum mechanical calculations on barriers to internal rotation: I. Self-Consistent-Field wave functions and theoretical potential energy curves for the hydrazine molecule, in the gaussian approximation

Abstract: The barrier to internal rotation in hydrazine has been studied by the non empirical SCF-LCAO method, in the gaussian approximation. Calculations have been performed for values 0°, 60°, 120°, 180° and 94° (equilibrium conformation) of the dihedral angle, with all other bond angles and bond lengths fixed. The gaussian basis set used consisted of 9s + 3p for nitrogen and 3s for hydrogen. The calculated total molecular energy for the equilibrium conformation, −111.030 a.u., is 0.865 a.u. higher than the experimental value. The theoretical dihedral angle 94° is in good agreement with experimental indications of 90–95°. The computed rotation barriers are 11.5 kcal/mole for the cis position and 4.7 kcal/mole for the trans.RésuméLa barrière de rotation interne de l'hydrazine a été étudiée par la méthode LCAO-SCF dans l'approximation des orbitales gaussiennes. Les calculs ont été effectués pour des valeurs de l'angle dièdre de 0°, 60°, 120°, 180° et 94° (valeur correspondant à l'équilibre), tout en gardant constants les autres angles et les longueurs des liaisons. On obtient pour la position d'équilibre une énergie moléculaire totale de −111.030 u.a., l'écart avec l'énergie expérimentale étant de 0.865 u.a. La valeur obtenue pour l'angle dièdre, 94°, est en bon accord avec les indications expérimentales de 90–95°. Les barrières de rotation théoriques sont de 11.5 kcal/mole pour la position cis et de 4.7 kcal/mole pour la position trans.ZusammenfassungDie Rotationsbarriere von Hydrazin wurde mit Gaußfunktionen nach einer nicht-empirischen SCF-LCAO-Methode studiert. Rechnungen wurden für die Werte 0°, 60°, 120°, 180°, 94° (Gleichgewichtslage) des Diederwinkels durchgeführt, wobei alle übrigen Bindungswinkel und -längen festgehalten wurden. Der Basissatz von Gaußfunktionen bestand aus 9s- und 3p Funktionen für Stickstoff und 3s-Funktionen für Wasserstoff. Die berechnete Gesamtenergie der Gleichgewichtskonformation, −111,030 at. E. liegt um 0,865 at. E. höher als der experimentelle Wert. Der theoretische Diederwinkel von 94° stimmt gut mit den experimentellen Daten von 90–95° überein. Die berechneten Rotationsbarrieren sind 11,5 kcal/mol für die cis- und 4,7 kcal/mol für die trans-Lage.

Calculations on the electronic spectra of anilino, phenoxyl and benzyl radicals

Abstract: The electronic spectra of benzyl, anilino and phenoxyl have been calculated, using two well known SCF-MO methods. Good agreement is found with experiment in all instances. However, the calculations still cannot explain the red shift produced by the addition of an extra electron to benzyl.

Etude des spectres électroniques de quelques p. quinones polycycliques par la méthode de Pariser-Parr-Pople

Abstract: Le calcul des transitions de typeπ →π* entre l'etat fondamental et les etats singulets monoexcites des p. benzo, naphto 1 – 4, anthra 9 – 10 et anthra 1 – 4 quinones a ete effectue par la methode de Pariser-Parr-Pople, avec interaction de toutes les configurations monoexcitees. Les transitionsn →π* ont ete egalement etudiees par une methode empirique. Les resultats obtenus permettent de rendre compte des spectres experimentaux.