Effects Of Molecular Symmetry Research Articles

The crystal and molecular structures of the compounds [(η5-C5H5)Fe(CO)2]2(CH2)n, where 0 n = 3 and 4

The crystal stuctures of [(η5-C5H5)Fe(CO)2]2(CH2)n, n = 3 and 4, have been determined.[(η5-C5H5)Fe(CO)2]2(CH2)3: a = 21.20, b = 10.39, c = 7.88Å, β = 101.6°, U = 1699Å3, C2/c, Z = 4, R = 0.059, 1036 observed data.[(η5-C5H5)Fe(CO)2]2(CH2)4: a = 7.63, b = 10.54, c = 21.87Å, β = 96.4°, U= 1748Å3, P21/c, Z = 4, R = 0.051, 1418 observed data.In each compound the iron atoms are joined by simple chains of sigma bonded CH2 groups. Bond lengths are similar in both: mean Fe-CO 1.75, C-O 1.15, FeC(cp) 2.11, FeCH2 2.08, (cp)CC(cp) 1.41, CH2CH2 1.55Å. The (CH2)3 compound retains a 2-fold axis of symmetry in the crystal. The (CH2)4 compound has no imposed symmetry, but closely approximates centrosymmetry. The effects of molecular symmetry on the IR spectrum between 2100 and 1900 cm-1 are discussed. The13C and1H (270 MHz) NMR spectra in solution are shown to be consistent with the structures found crystallographically.

ChemInform Abstract: THE ELECTRONIC STRUCTURES OF CU(II) COMPLEXES AS DETERMINED BY MEANS OF THE CONFIGURATIONAL INTERACTION METHOD

The electronic structures of Cu(II) complexes with D4h effective molecular symmetry have been investigated by means of the configurational interaction method. The locally-excited configurations of the central metal ion and the charge-transfer configurations corresponding to an electron transfer from the ligand system to the central metal ion have been taken into account. A correspondence with the crystal-field treatment has been obtained using the partitioning technique of solving the secular equation. The energies of the charge-transfer configurations, the resonance integrals, and the electrostatic contribution, Dsion, were determined so as to fit the observed d-d spectra.

The Electronic Structures of Cu(II) Complexes as Determined by Means of the Configurational Interaction Method

Abstract The electronic structures of Cu(II) complexes with D4h effective molecular symmetry have been investigated by means of the configurational interaction method. The locally-excited configurations of the central metal ion and the charge-transfer configurations corresponding to an electron transfer from the ligand system to the central metal ion have been taken into account. A correspondence with the crystal-field treatment has been obtained using the partitioning technique of solving the secular equation. The energies of the charge-transfer configurations, the resonance integrals, and the electrostatic contribution, Dsion, were determined so as to fit the observed d-d spectra.

The electronic spectra and magnetic circular dichroism of the uranyl ion

The MCD and absorption spectra of (Bu4N)UO2(NO3)3 have been measured over the range 18 000—33 000 cm-1 in a rigid polymer matrix at ∼10 K. A large number of sharp peaks are observed including structure in the high energy region previously referred to as the ‘structureless continuum’. It is shown that group theory places some rather strict limitations on the electronic states and the possible transitions between them, and we conclude that the most probable orbital ordering is e 1g < a 1g < a 2u < e 1u and that the low energy bands arise from transitions to the triplet states 3 E 1g and 3 E 2g . The higher energy band structure we assign as due to transitions to 3 E 1u . We show that second order spin-orbit interaction is necessary to account for the unsymmetric splitting of the triplets. Furthermore it is demonstrated that the symmetry D ∞h will not account for the intensity of absorption and MCD of the observed transitions and that the effective molecular symmetry is D 3.

Excited State Symmetry Assignment Through Polarized Two-Photon Absorption Studies of Fluids

We consider the effect of molecular symmetry on the two-photon absorption constants of a randomly oriented sample of molecules. It is shown that a complete polarization study of an allowed two-photon transition of a fluid generally yields enough information to permit an unequivocal identification of the symmetry species of the excited state. Ambiguities occur in only a few cases. Thus, oriented molecule studies for symmetry assignment will be largely unnecessary in two-photon molecular spectroscopy. Symmetry assignment rules are given for all of the 32 crystallographic point groups and for the two groups of linear molecules. It is shown that certain transitions, which are allowed for two photons of different energies, are forbidden for two photons of the same energy. The application of this formalism to Raman scattering problems is discussed.

Dodekaeder und Archimedisches Antiprisma als Molekülstrukturen bei Oktacyanokomplexen des Molybdän(IV) und Wolfram(IV)

Infrared spectra (300—4000 cm-1) have been measured on the octacyanide complexes K4Mo (CN)8·2H2O and K4W (CN)8·2 H2O in aqueous solution, in the solid dihydrates, and in the solid anhydrous compounds. Comparison with relevent Raman data, particularly in the 2100 cm-1 region, provides conclusive evidence for the effective molecular symmetry of the M (CN)8 4- group in the different surroundings. In aqueous solution, the ions [Mo (CN)8]4- and [W (CN) 8] 4- assume the symmetry of an Archimedean antiprism, D4d . In the dihydrate crystals, the groups Mo (CN)8 4- and W (CN)8 4- approach the full D2d symmetry of a dodecahedron with triangular faces, possibly slightly distorted by packing effects. In the anhydrous compounds, the symmetry of these groups is considerably lower, very likely C8. Reasonable assignments for all normal frequencies in the 2100 cm-1 region are proposed, both in D4d and D2d symmetry.

Two-Photon Capture Cross Sections of Pyrene and Benzpyrene from SCF–MO Calculations

The two-photon capture cross sections for pyrene and 3,4-benzpyrene are calculated using SCF–CI pi-electronic wavefunctions and the theory of Goeppert-Mayer. The cross section is shown to be dependent on the product of the photon flux and a constant, δ, which may be calculated from suitable matrix elements and the line shape function. For randomly oriented molecules and polarized light the calculated values of δ for pyrene and 3,4-benzpyrene are, respectively, 260×10—51 and 1500×10—51 cm4·sec atom—1 photon—1. These are to be compared with the experimental values of, respectively, 1×10—51 and 50×10—51 cm4·sec atom—1·photon—1. The effect of molecular symmetry on the two-photon absorption of polarized light is illustrated for molecules of D2h symmetry. For a two-photon transition to a B1g state, the maximum probability is shown to lie approximately 45° between the x and y axes of the molecule which are the axes of maximum probability for one-photon absorption.

Ultrasonic Absorption in Liquids

The Debye-Sears method for measuring ultrasonic absorption is used for various organic liquids at room temperature and at frequencies of 3 to 15 mc/sec. The effect of molecular symmetry on absorption is studied for the pairs benzene, toluene; paraxylene, orthoxylene; and cyclohexane, methyl cyclohexane. The effect of double versus single bond is given for tetrachlorethane versus tetrachloroethylene. The effect of viscosity on absorption is shown to be almost negligible for solutions of polystyrene in either toluene or benzene. Finally, a possible explanation along relaxational lines is suggested.