Electron Capture Probabilities in105Ag

TheK-electron capture probabilities for the 1/2^? → 3/2^? and 1/2^? → 1/2⁺ transitions in the decay of¹⁰⁵Ag were measured for the first time using the sum coincidence method. The experimental P_K values were estimated to be 0.824 ± 0.042 and 0.851 ±0.046 for the allowed and first-forbidden beta transitions, respectively in agreement with the theory. The P_L experimental values to these two levels were also computed using the experimental P_L/P_K values reported by earlier authors. These results are also found to be consistent with the theoretical P_L values.     

NDDO MO Calculations

Siegbahn's potential model as extended by Ellison et al. is used with density matrix elements calculated by the NDDO/2 procedure, to correlate the K-shell binding energy shifts of C, N and O atoms in a few molecules containing only the first-row atoms. The correlation is not superior to that obtained with the CNDO/2 method when only the monopole term is retained in calculating the Madelung potential energy. However, the results are in excellent agreement with experiment when the two-parameters model including the dipole and quadrupole terms is used.     

Ring size effects on 1,2-shifts in carbocations

1,2-Hydride shift barriers in classical carbenium ions increase with decreasing ring size; this can be attributed to the increased strain of the H bridged structures in the small rings.     

Optical properties of mixtures of right- and left-handed cholesteric liquid crystals

A rigorous theory is developed of the optical properties of a mixture of right-handed and left-handed cholesteric liquid crystals, taking into account the effect of absorption. The dependence of the rotatory power and circular dichroism on pitch and on sample thickness is discussed.     

Liquid crystals of disc-like molecules

Thermotropic mesomorphism has been observed in pure compounds consisting of simple disc-like molecules, viz., benzene-hexa-n-alkanoates. Thermodynamic, optical and x-ray studies indicate that the mesophase is a highly ordered lamellar type of liquid crystal. Based on the x-ray data, a structure is proposed in which the discs are stacked one on top of the other in columns that constitute a hexagonal arrangement, but the spacing between the discs in each column is irregular. Thus the structure has translational periodicity in two dimensions and liquid-like disorder in the third.     

Thermodynamic analysis of enzyme catalysed reactions: new insights into the Michaelis–Menten equation

A simple thermodynamic analysis of the well-known Michaelis-Menten equation (MME) of enzyme catalysis is proposed that employs the chemical potential to follow the Gibbs free energy changes attending the formation of the enzyme-substrate complex and its turnover to the product. The main conclusion from the above analysis is that low values of the Michaelis constant K _M and high values of the turnover number k _cat are advantageous: this supports a simple algebraic analysis of the MME, although at variance with current thinking. Available data apparently support the above findings. It is argued that transition state stabilisation—rather than substrate distortion or proximity—is the key to enzyme catalysis.     

Sterically Hindered Pentacoordinated Phosphorus and Silicon Compounds

Abstract

The synthesis and X-ray structures of the first examples of five-coordinated acyclic derivatives of phosphorus and silicon incorporating the bulky substituents, the 1-naphthyl and t-butyl groups, are reported. Increases in bond distances induced by steric and electronic interactions are apparent for these compounds. The results are related to proposed intermediates which model these derivatives in nucleophilic substitution reactions suggesting the importance of steric control.