Gamma resonance spectroscopic study of compounds containing the π-allylirontricarbonyl group

The gamma resonance (GR) spectra of a series of -allylirontricarbonyl compounds and -cyclopentadienyldicarbonyl complexes of iron have been recorded. The relationship between the GR spectra of these compounds and the electronic structure of the iron atom in the complexes has been examined. It has been shown that tetrahydrofuran, benzene, and acetonitrile do not react with -allylirontricarbonyl chloride, whereas dimethylformamide decomposes it with the formation of high-spin complexes of iron. The formation of the [-C₃H₅Fe(CO₃)]^– anion in the reduction of [C₃H₅Fe(CO₃)]I by naphthalene-sodium in benzene has been confirmed.     

A Theoretical study of the interaction of ethylene with transition metal complexes

In order to be able to describe the ethylene molecule bonded to an active site of a Titanium- or Nickel fluoride crystal, we have used the Hartree-Fock-Slater model, implemented by a Discrete Variational Method, as introduced by Ellis and Painter. The study of the ethylene molecule at a crystal surface then gives a clear, easily interpreted picture of the electronic structure. The-back donation from metal to olefin is found to be extremely important, both in the Ti- and in the Ni-complex. This back donation is caused by a strong interaction of ad orbital of the central ion with a ^* as well as the ^* molecular orbital of ethylene. As a result of these interactions, the C-C bond of ethylene is weakened considerably.A comparison is made between the Ti-ethylene and the Ni-ethylene systems.

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Zusammenfassung Die Elektronenstruktur von Äthylen, das an ein aktives Zentrum eines Titan- bzw. Nickelfluoridkristalls gebunden ist, ist mittels der Hartree-Fock-Slater-Theorie kombiniert mit der sog. Discrete Variational Method theoretisch behandelt worden. Es zeigt sich, daß die-Backdonation sowohl im Nickel- als auch im Titankomplex außerordentlich wichtig ist; sie entsteht durch eine starke Wechselwirkung eines Metall-d-Orbitals sowohl mit einem ^* als auch einem ^*-Orbital des Äthylens. Dies führt zu einer erheblichen Schwächung der Doppelbindung.Außerdem werden die beiden Systeme (Titan und Nickel) miteinander verglichen.

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Alfred P. Sloan Research Fellow     

π -Type and σ -type cation- π complexes of atomic cations

MP2/6-31+G* calculations were performed on the cation- complexes of ethylene, cyclobutadiene and benzene with a number of atomic cations. It was found that except B⁺ all the atomic cations form -type cation- complexes with ethylene. On the other hand, with cyclobutadiene Li⁺, N⁺, Na⁺, P⁺ and K⁺ form -type complexes, whereas H⁺, F⁺, and Cl⁺ form covalent -type complexes. With benzene Li⁺, B⁺, Na⁺, Al⁺, and K⁺ form -type complexes whereas H⁺, F⁺, and Cl⁺ form -type complexes. It was concluded that the driving force to form the -type complex is chemical bonding, and that for metal cations to form -type complexes is non-covalent interaction.     

A 2pπ, 3dπ valence bond wave function for the 1B1u state of ethylene

A 2p, 3d valence bond wave function for the lowest ¹ B _1u state of the -system of ethylene is variationally optimized with respect to the atomic orbital exponents using a non-empirical -approximation. The resulting energy compares favourably with previous calculations and leads to a satisfactory value for the lowest ¹ A _1g-¹ B _1u transition energy. The optimized exponent for the 2p orbital is close to the Slater value whereas the exponent for the 3d orbital is found to be nearly hydrogenic. The implications of this result are discussed in some detail.

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Zusammenfassung Für den tiefsten ¹ B _1u -Zustand des -Elektronensystems des Äthylens wird nach der Variations-methode eine 2p, 3d-Valenzbindungs-Wellenfunktion bezüglich der Orbitalkoeffizienten optimiert, wobei eine nicht-empirische -Approximation zugrunde gelegt wird. Die berechnete Energie stimmt gut mit den Ergebnissen vorausgegangener Rechnungen überein. Für den tiefsten ¹ A _1g ¹ B _1u -Übergang ergibt sich ein befriedigender Energiewert. Der optimierte Orbitalkoeffizient für das 2p-Orbital stimmt gut mit dem nach der Slaterschen Regel bestimmten Koeffizient überein, während man für den 3d-Orbitalkoeffizienten einen Wert findet, der dem des Wasserstoffs ähnlich ist.

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Résumé Une founction d'onde V.B.(2p , 3d ) est optimée pour l'état ¹ B _1u d'aethylen. L'energie calculé aussi que l'energie de transition la plus basse ¹ A _1g -¹ B _1u est en bon accord avec des valeurs connues.

     

The activation of the C-H bond in acetylene by second row transition metal atoms

Summary The C-H activation reaction of acetylene by second row transition metal atoms has been studied including electron correlation of all valence electrons. Binding energies have been computed for both -coordinated complexes and C-H insertion products. It is found that for most atoms the -coordinated complexes are thermodynamically favoured, just as in the case for the corresponding ethylene reaction. The barrier height for the C-H insertion increases from acetylene to ethylene and to methane. This is in line with the experimental finding that there should be an inverse relation between C-H bond strengths and the difficulty to activate these bonds. To explain the detailed differences between the C-H activation of acetylene and ethylene, the interaction with two, rather than one, - and *-orbitals for acetylene is of key importance. The barrier height for the acetylene reaction increases significantly between niobium and molybdenum going to the right in the periodic table, just as for all oxidative addition reactions previously studied. The origin of this increase is that noibium has one empty 4d-orbital but for molybdenum all 4d-orbitals are occupied. Rhodium has the lowest barrier for C-H activation for all systems studied.     

Spectroscopic study of organosilic on derivatives of thiophene

It is shown that the basicity of substituted thiophenes depends on the electronic effects of the substituents. The basicity of organosilicon derivatives of thiophene is determined not only by the -I effect but also by the effect of d-p interaction in the Si-ring bond. The dependence of the degree of d-p interaction on the nature of the three other substituents attached to the silicon atom is examined.See [1] for communication II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No.8, pp.1057–1061, August, 1972.     

Orbit-orbit interaction between iron(III) kramers doublet and porphyrin π-orbitals studied by magnetic circular dichroism of low-spin ferriheme compounds

To explain the temperature dependence (as 1/T) of the magnetic circular dichroism (MCD) observed in the —^* bands of low-spin ferriheme compounds, we present the theoretical treatment based on the exchange-type —d interaction. The four-fold degeneracy of both B and Q excited states (3d-iron(III) Kramers doublet x porphyrin orbitalE()-doublet) is to be partially lifted due to intraconfiguration interaction. The transitions between the ground and excited Kramers doublets give rise to MCD of two types. Instead of temperature independentA-type effect usually observed in both B and Q bands of metal porphyrin MCD spectra, there will be twoA-type effects of the same sign and two temperature dependentC-type effects of opposite signs. The values of energy gaps between orbit-orbit split sublevels are predicted to be generally different for the visible Q- and the near UV B-band. The previous theoretical models [3, 4] are discussed and some experimental evidence in favour of our interpretation are presented.     

Quantum-chemical study of Ni complexes active in reactions of oligomerization of olefins and polymerization of allenes

Conclusions Based on EHM calculations of complexes of ethylene, allene, and dimethylallene with -allylnickel bromide, it was shown that the activity of the corresponding monomers in the oligomerization reaction is determined by the elevated charge density on the terminal C atom of the -allyl ligand in the trans position to the halogen. Addition of alienes to the C atom of -allyl takes place on the central C atom in allene, characterized by a reduced charge density, the activity of the complexes in substitution of the halogen increases in the order Cl < Br< I, and the activity of the monomers increases in the order ethylene < allene < 1,1-dimethylallene.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1821–1823, August, 1986.     

Calculations of electronic spectra of Fe and Co porphyrins and their anionic forms

Energies and intensities of electronic transitions of a number of Co and Fe porphyrins and their anionic forms have been calculated by the INDO/S-CI method in the Zerner parametrization. The results from the theoretical analysis are consistent with the basic relationships observed in the change of electronic spectra within the limits of isoelectronic series of compounds characterized by identical total number of -electrons and identical filling of the pair of d orbitals. It has been established that in the spectra of compounds of an isoelectronic series corresponding to the neutral state of the porphyrin ligand, the allowed electronic transitions are described by superpositions of two ^* configurations (la_1u 4e_g) ) and (3a ^2u 4e_g ) , two configurations of intermolecular charge transfer 3 (d 2b_1u) and (d 3b_2u), and two types of doubly excited configurations (la_1u, d 4e_g, 4e_g) and (3a _2u, d 4e_g, 4e_g). Data have been obtained on the energies and orbital nature of the even electronic states that are responsible for rapid exchange of excitation energy of the porphyrin molecules with the transition metals.Leningrad. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 2, pp. 144–151, March–April, 1991. Original article submitted September 28, 1989.     

Solvatochromism of Heteroaromatic Compounds: XIII. Methyl(aroyloxymethyl)fluorosilanes and (Aroyloxymethyl)trifluorosilanes

The UV spectra of heptane solutions of methyl(aroyloxymethyl)fluorosilanes and (aroyloxymethyl)trifluorosilanes of the general formula 4-RC₆H₄COOCH₂Si(Me)_{3 - 4n} F _n (n = 1-3; R = H, F, Cl, Me, MeO, O2N) were studied. The intramolecular coordination bond Oarr;Si causes a bathochromic shift of the second -* transition, because the energy gap between the ground electronic and Franck-Condon excited states decreases owing to additional stabilization of the latter. The spectral shifts correspond to stabilization of the five-coordinate state of the silicon atom in (aroyloxymethyl)trifluorosilanes by 5-6 kcal mol^{- 1}.     

Spectroscopic study of compounds possessing a bond of an element of group IVb to hydrogen

Conclusions In the silanes (C₂H₅)X₃SiH and X₃SiH, the substituants X of which form d-p bonds with the silicon atom, the effect of d-p conjugation depends on the inductive and mesomeric effects of X, which change the charge on the silicon atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 342–347, February, 1972.The authors would like to express their gratitude to G. A. Domrachev for discussing certain questions in this article.     

Orientational preference of two ethylene ligands bound to a nickel atom

CASSCF and CCI calculations have been performed to analyze the bonding in Ni(C₂H₄)₂. Three different relative orientations of the two olefins have been studied. It is found that a structure with D_2d symmetry, where the C-C bonds in the two olefins make a 90 degree angle to each other, gives the lowest energy. A D_2h form, with the two C-C bonds and Ni in the same plane, is 10.3 kcal/ mol higher in energy. The reason for the preference of the D_2d form is analyzed in terms of valence bond theory, and is found to be due to a d ⁸ structure with two simultaneous d bonds. A C _2v form, for which the two nickel olefin bonds make a 103 degree angle to each other and the C-C bonds are parallel to each other, is 32 kcal/mol higher in energy than the D_2d form. The low binding energy of the C _2v form is due to a poor interaction with inefficient sd hybridization.     

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.     

Porphyrins XXVIII. Extended Hückel calculations on metal phthalocyanines and tetrazaporphins

Extended Hückel (EH) calculations on Mg, Zn, Cu, Ni, Fe, Mn, and VO complexes of phthalocyanine and tetrazaporphin are reported and the results compared to similar calculations on porphyrins. The smaller ring size of phthalocyanine gives rise to a larger ligand field. The bridge nitrogen atoms give rise to n - ^* transitions, which are probably in the region of the Soret band. In Ni, Co, Fe, and Mn there is strong mixing of the bridge N2p and metal b _2g (d _xy ), which should affect the ligand field. Extra absorption bands observed in the near uv of NiPc and CoPc are attributed to d ^* transitions. A general symmetrized EH program is reported that speeds calculations on large systems.

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Zusammenfassung Für die Komplexverbindungen von Mg, Zn, Cu, Ni, Fe, Mn und Vo mit Phtalocyanin und Tetrazaporphin werden erweiterte Hückelrechnungen (EH) durchgeführt und die Ergebnisse mit ähnlichen Berechnungen an Porphyrinen verglichen. Die geringere Ringgröße des Phtalocyanins gibt Anlaß zu einem größeren Ligandenfeld. Das Brücken-Stickstoffatom gibt Anlaß zu n- ^*-Übergängen, die wahrscheinlich im Gebiet der Soret-Bande liegen. Beim Ni, Co, Fe und Mn tritt eine starke Mischung der Orbitale des Brückenstickstoff-Atoms N2p und b _2g (d _xy ) auf, die das Ligandenfeld beeinflussen sollte. Die zusätzlichen Absorptionsbanden, die im nahen UV bei NiPc und CoPc beobachtet werden, werden d ^*-Übergängen zugeordnet. Ein allgemeines, symmetrisiertes EH-Programm wird mitgeteilt, das die Berechnung großer Systeme beschleunigt.

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Paper XXVII. M. Gouterman, F. P. Schwarz, P. D. Smith, and D. Dolphin, J. chem. Physics, in press (1973).     

Theoretical Study of the Products of Acetylene Addition to HgCl2

The ab initio MP2 method is used with the LANL2DZ basis to calculate the mercury chloride ,-complex with two acetylene molecules (1) and various isomeric forms of mercury di()-vinyl chloride -complexes (2): cis-cis (2A), cis-trans (2B), and trans-trans (2C). The ,-complex is the most stable form of all those considered; the difference between 1 and 2A is 24.9 kcal/mole. A relation between the total energies (kcal/mole) for isomeric forms 2 is established to be 2A (0) < 2B (0.98) < 2C (1.58). Complex 1 is shown to be transformed into 2A via the intermediate formation of 3, which is a hybrid form of the complex (,-complex of mercury chloride with two acetylene molecules). The structures of the transition states for the transformations of 1 into 3 (structure 4) and of 3 into 2A (structure 5) and the corresponding transition activation energies are determined. The interaction of 2A, 2B, and 2C with the Cl^- anion as a model nucleophile is considered. It is shown that the resulting anions (6A, 6B, 6C) have a planar structure with the relative stability increasing in the series 6A<6B<6C.     

Spectroscopic study of organosilicon derivatives of thiophene

Substituents in disubstituted thiophenes have an additive effect on the chemical shifts of the ring hydrogen atoms. The electronic effects of organosilicon substituents are transmitted via inductive and conjugation (d-p interaction) mechanisms. The effect of d-p interaction in the Si-ring bond is absent for Si(OC₂H₅) and SiF₃ substituents.See [1] for communication III.Translated from Khimiya Geterotsiklicheskikh Soedinenli, No. 11, pp. 1483–1488, November, 1972.     

Study of relative acidity of some silazanes by infrared spectroscopy method

Conclusions The relative acidity of the hydrogen atom of the NH group in a number of silazanes was determined. The electron-acceptor character of the silyl substituents is caused by the existence of d—p interaction at the Si-N bond.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 644–646, March, 1973.     

Solvatochromism of Heteroaromatic Compounds: XVI. Theoretical Study of the Effect of Nonspecific Solvation on the Rotational Isomerism and Spectral Characteristics of 2-[2,2-Dicyano-1-(methylthio)vinyl]-5-methylpyrrole

The effect of nonspecific solvation of the relative stability of rotational isomers of 2-[2,2-dicyano-1-(methylthio)vinyl]-5-methylpyrrole was studied theoretically (HF/6-31G^*) in terms of the Onsager model. In the free form this compound exists as an ensemble of four nonplanar C, N, S rotamers, the largest energy gap between which being no more than 2 kcal/mol. The effects of media whose generalized parameters correspond to chloroform and 1,2-dichlroethane reduce this gap to 0.7 and 0.5 kcal/mol. The vibration spectra of the rotamers and characteristics of the ^* electronic transitions with intramolecular charge redistribution were calculated. Prospects of experimental studies of the solvatochromism and conformational isomerism of the title compound were discussed.     

Strain,orbital interaction,and conformation of propella[34]prismane and its precursor diolefin

Factors that influence the intramolecular [2+2] photochemical ring closure reaction of multiply bridgedsyn-tricyclo[4.2.0.0^2.5]octa-3,7-diene (2) leading to propella[3₄]prismane (1) are analyzed. The unfavorable, large increase in strain (89 kcal mol^–1) calculated for this reaction is overcome by the favorable ordering of frontier molecular orbitals, which is caused primarily by bridging of the juxtaposed double bonds with trimethylene chains in2. This conclusion is based on detailed analysis of the frontier molecular orbitals (FMO) of cycloocta-1,5-diene (6), cyclodeca-1,6-diene (7), and tricyclo-[4.2.0.0^2.5]octa-3, 7-diene (3), bridged with alkylene groups of various lengths, in terms of Paddon-Row's theory of through-bond interaction by the use of AM1 semiempirical MO method. In addition to the -orbital interaction through an even number of C-C bonds, the double-bond-double-bond distance is recognized to be an important factor for the ordering of FMOs. It is suggested that reduction of this distance in6 by only a few tenths of an Ångstrom will change the FMO ordering and allow the intramolecular [2+2] cycloaddition to proceed. Potential energy calculations by MM2 indicate that the trimethylene bridges in1 and2 are so flexible that they appear flat on the NMR time scale.     

A theoretical determination of the electronic spectrum of Methylenecyclopropene

The vertical electronic spectrum of methylenecyclopropene, the prototype of the nonalternant hydrocarbons known as fulvenes, has been studied using multiconfigurational second-order perturbation theory. The calculations comprise three valence states and the 3s, 3p, and 3d members of the Rydberg series converging to the first ionization limit. Vertical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation from the highest occupied orbital to a ^* orbital, while the other two are ^* excitations. The third transition gives rise to the most intense feature in the electronic spectrum. The results are rationalized within the scheme of two interacting double bonds. Comparisons are made between this and the previous theoretical calculations of the electronic spectra of related systems and also between the available experimental data of methylenecyclopropene in solution.