Magnetic circular dichroism of the first spin-forbidden transition in 2[Cr(en)3Cl3] . KCl . 6H2O

The longitudinal Zeeman effect of the ² E ←⁴ A ₂ transition of the Cr⁺³ ion in single crystals of 2[Cr(en)₃Cl₃] . KCl . 6H₂O has been measured using circularly polarized light as a function of magnetic field strength between 0 and 160 kgauss at ～80 K. The g factor for the ground state was determined to be g _‖(⁴ A ₂) = 2·01±0·05, in agreement with the E.S.R.-determined value. The ratio between the excited state g values and that of the ground state was determined; using the value g _‖(⁴ A ₂) = 1·99, the g factors g _‖(2) = 2·61±0·04 and g _‖() = 1·47±0·04 were obtained.     

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

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:

     

Ab initio potential surfaces for the à and states of HCN+

Ab initio configuration interaction calculations for the à ²Σ⁺ and states of HCN⁺ are presented. Minima occur at r _CH = 2·03 a ₀, r _CN = 2·25 a ₀ (à ²Σ⁺) and r _CH = 2·75 a₀, r _CN = 2·26 a ₀ (). The potential surface for the state has a local maximum as the hydrogen atom is pulled away from CN. The barrier height is calculated to be 0·27 eV.     

E.S.R. and ENDOR studies of X-irradiated 1-phenyl-3-thiosemicarbazide single crystals

By means of E.S.R. and ENDOR spectroscopy, a radical identified as C₅H₆NNHCSNH₂ has been studied in X-irradiated single crystals of 1-phenyl-3-thiosemicarbazide, C₆H₅NHNHCSNH₂. The unpaired electron interacts with one nitrogen and three hydrogen nuclei. The nitrogen interaction, studied with E.S.R., has the principal values A ₁=76·2±8 MHz; A ₂=10·0±8 MHz; A ₃=0·0±8 MHz, and is characteristic of a π-type radical with 50 per cent of the spin density localized in a nitrogen 2p orbital. The three hydrogen interactions, studied with ENDOR, have the principal values

These interactions are of the CH α-type and arise from hydrogen atoms on the phenyl ring. The remaining spin density is distributed on the phenyl ring with virtually the same amount in each of the three different carbon 2pπ orbitals.     

Rotational band contour analysis in the 2760 Å system of p-chlorofluorobenzene

The origin band of the 2760 å system of p-chlorofluorobenzene has been shown to be a type B band of a prolate asymmetric top. The electronic assignment of the system is therefore ¹ B ₂-¹ A ₁.

The excited state rotational constants are:

compared with the estimated ground state constants:

The rotational origin of the band is at 36275·1 ± 0·2 cm^-1.     

On the kinetics of a two-level system with a diffusion-modulated interaction

For a two-level system corresponding to a particle of spin ½ in a random field in the Z direction, the relaxation function

has been estimated, the magnitude Ω(t) being the sum of the isotropic interactions of the particle in question with particles j executing diffusional motion. Specifically Ω(t)=ω₀ + Σ ω(Mj, rj), where ω₀ = constant, M_j is a random time-independent parameter, ω(M, r) decreases with r faster than r ^-3 and r _j = r _j (t) is a diffusional, random function. From the expression for <σ⁺(t)>, we establish general features of the relaxation phenomenon for diffusional processes, and calculate the relaxation rate 1/T ₂ and relaxation shift Δω to be 1/T ₂-iΔω = 4π CAv_M λ _M , where C is the concentration of particles and λ _M is the scattering length for an equation of the Schrödinger type with an imaginary potential -iω(M, r) instead of U/?, and diffusion coefficient instead of ?/2m. We also found that for the case of ‘external’ relaxation, the Redfield approach proved valid only under the simultaneous restrictions of low concentration and weak interaction.     

Vibrational frequency correlation and vibrational dephasing in aqueous nitrate solutions

The vibrational frequency correlation functions

of the v ₁(A′₁) mode of NO₃ ^- ions in aqueous NH₄NO₃ solutions are calculated directly from isotropic Raman profiles, assuming that the process ω(t) is gaussian. The calculated correlation functions are oscillatory decaying functions with a time period of about 0·35 ps. A model based on the generalized Langevin theory is proposed and a theoretical correlation function, obtained by first-order truncation of Mori's continued fraction representation, is found to reproduce both the observed vibrational frequency correlation functions and the vibrational correlation functions. The observed short-time oscillation with a frequency of about 95 cm^-1 is attributed to an inter-molecular librational motion between a NO₃ ^- ion and surrounding water molecules and cations.     

The gas-phase electron paramagnetic resonance spectrum of vibrationally excited SO radicals

Gas-phase E.P.R. spectra from the first six vibrational levels in X ³Σ^- SO have been detected and analysed using a Hund's case (b) formalism. The spectra can be fitted using the following molecular constants:

The first and second derivatives of with respect to the normalized internuclear distance ξ were calculated from these constants. A simple analytic formalism is described which enables the variation of with internuclear distance to be separated into contributions from the matrix elements and from the change in energy spacing between interacting states. Both effects are important in the derivatives of , but it is suggested that in SO most of the variation of with rotational and vibrational state is due to the increase in the second-order part of the parameter as the separation between the ground and the dominant perturbing state decreases. This explanation implies that the expression proposed recently by Veseth and Lofthus for the rotational variation of is not accurate, and enables the conflict between the results of their analysis and ab initio calculations to be resolved.     

Thermodynamic parameters for gaseous sulphate ion

Electrostatic energies and Madelung parameters for M ₂SO₄ (M = Li, Na, K, Rb and Cs) and for MSO₄ (M = Ca) are reported. An assessment of the charge distribution within the sulphate ion (q ₀ = -0·80) is made and values:

and

are assigned to the enthalpies of formation and solvation of the gaseous sulphate ion. The total lattice potential energies are found to be

A value of -1267 kJ mol^-1 is assigned for the oxide ion affinity of sulphur trioxide.