Showing papers on "Tetrahedral molecular geometry published in 1976"

On the determination of the intermolecular potential between a tetrahedral molecule and an atom or a linear or a tetrahedral molecule - application to CH4 molecule

Abstract: A brief summary of the general theory of the long-range potential energy between two molecules, previously developed by Buckingham, is first presented. Then the first detailed expressions of this potential energy are given for the three following molecular pairs : tetrahedral molecule-atom, tetrahedral molecule-linear molecule and tetrahedral molecule-tetrahedral molecule. These expressions are applied to the calculation of the spectral moments of the far infra-red pressure induced absorption band, the second virial coefficient and the intermolecular mean square torque. This allows a discussion of the magnitude of the various terms contributing to this potential energy and leads to a determination, for CH4, of the octupole and hexadecapole moments, and of the hyperpolarizability:

Determination of molecular multipole moments and potential function parameters of non-polar molecules from far infra-red spectra

Abstract: The quadrupole, Θ, and hexadecapole, Φ, moments of N2 and O2, and the octupole, Ω, and hexadecapole moments of CF4 and SiH4 are obtained from far infra-red collision-induced absorption in the gas phase at low densities where bimolecular collisions predominate. An accurate theory for evaluating Θ and Φ for non-polar diatomic molecules is presented here ; the theory for evaluating Ω and Φ for tetrahedral molecules has been presented previously. A reliable value of a multipole moment obtained by a direct method which does not depend on the intermolecular potential can be used to obtain an accurate value of the molecular diameter, σ. Thus using the quadrupole moment of N2 obtained by induced birefringence and the far infra-red value of Θ2/σ5, we obtain σ and with this the value of Φ.

Pressure-induced absorption in nonpolar gases containing tetrahedral molecules

Abstract: The theory of pressure-induced absorption of far infrared radiation by gases is extended to include the contribution of the dipole moment induced in a molecule by the field gradient due to its neighbours. This dipole is nonzero when the molecule lacks a centre of inversion, as in a tetrahedron. In the collision of two tetrahedra, the dipole induced in molecule 2 by the electric field of the octopole moment Ω1 of the partner leads to transitions in which ΔJ(1) = 0, ± 1, ±2, ±3, and ΔJ(2) = 0. The dipole induced by the field gradient of Ω1 leads to ΔJ(1) = 0, ±1, ±2, ±3, and ΔJ(2) = 0, ±1, ±2, ±3, and therefore gives a required increase in absorption at higher frequencies. The field-gradient contribution vanishes in a collision involving a tetrahedral and a spherical molecule. General expressions are given for the field-gradient contributions to the integrated intensity and to the −2 spectral moment.

Kinetic secondary deuterium isotope effects for substituted benzaldehyde cyanohydrin formation

Abstract: ..cap alpha.. secondary deuterium isotope effects have been measured for the observed rate constants for addition of cyanide ion to a series of substituted benzaldehydes in aqueous solution at 25/sup 0/C. Under the experimental conditions employed, these reactions did not proceed to completion, and the observed isotope effects were corrected to account for the influence of the reverse reaction employing measured equilibrium constants for cyanohydrin formation and previously determined secondary deuterium isotope effect for the equilibrium constant for 4-methoxybenzaldehyde cyanohydrin formation. In the four cases studied, values of k/sub D//k/sub H/ varied from 1.15 to 1.20, only slightly lower than the calculated maximal value for complete formation of the anionic tetrahedral species which is the immediate product of the rate-determining step, 1.21. A trend in isotope effect as a function of substrate reactivity could not be definitively established. The results suffice to establish that addition of cyanide to benzaldehydes proceeds via transition states in which rehybridization of carbonyl carbon to the tetrahedral geometry is nearly complete.

Theory of pure rotational Raman transitions in the vibronic ground state of a tetrahedral molecule

Abstract: It is well known that the polarizability tensor of a rigid tetrahedral rotor is isotropic. Hence, pure rotational Raman transitions in the vibronic ground state of a tetrahedral molecule are forbidden. However, the distortion of the molecule by centrifugal forces produces an effective anisotropic polarizability tensor, which depends on the components of the total angular momentum, and results in a pure rotational Raman spectrum. The purpose of this communication is to give explicit expressions for the polarizability tensor operator and the scattering intensity of the pure rotational Raman transitions. The feasibility of observing such transitions and an estimate of the relative intensity are given for the case of methane. The results presented here have been derived partially by considering the matrix elements of the linear normal coordinate terms in the polarizability tensor, between perturbed wavefunctions that were obtained by mixing the ground state with the degenerate excited vibrational states. The existence of similar ''forbidden'' pure rotational transitions in the far infrared region were established recently both on theoretical and experimental grounds.

Actinide metallocarbaborane complex: synthesis and X-ray structure determination of the bis[η5-(3)-1,2-dicarbollyl]dichlorouranium(IV) dianion

Abstract: The first actinide metallocarbaborane complex, [U(C2B9H11)2Cl2]2–, has been obtained by reaction of the 1,2-dicarbollide ion with UCl4 in tetrahydrofuran (THF), and a crystal structure determination of its lithium salt has shown the complex anion to have a distorted tetrahedral geometry with pentahapto-bonded dicarbollide ligands.

Mössbauer studies of a synthetic analog for the active site in reduced rubredoxin

Abstract: The electronic structure of the complex anion tetrakisthiophenolate Fe(II) (Fe(SPh)4) has been studied by Mossbauer spectroscopy in order to demonstrate its validity as a synthetic analog of the active site in reduced Rubredoxin. The orbital degeneracy is completely removed due to the distorted tetrahedral geometry. From the temperature dependence of the quadrupole splitting the separation of the two low lying orbital states – those originating from the tetrahedral E states — is estimated at 1 150 ± 100 cm-1. Magnetically perturbed spectra at 1.5 K and 4.2 K have been parametrized with a spin Hamiltonian formalism. Spin Hamiltonian parameters have been calculated by second order perturbation theory from a ligand field model including fine and hyperfine structure coupling. The optimized spin Hamiltonian parameters are D = 10,87 K, E = 2,43 K, gx = 2,12, gy = 2,19, gz = 2,01 for the electronic spin Hamiltonian and Ax = 1,16 mm/s, Ay = 0,63 mm/s, Az = 2,09 mm/s, ΔEQ = 3,24 mm/s, η = 0,67 for the nuclear spin Hamiltonian. The close agreement of these values with reported values for Rdred constitutes strong evidence that Fe(SPh)4 is a satisfactory synthetic analog for the active center in Rdred.