Electrical properties, isotope effects

Abstract Although the electronic structure and the electrical properties of molecules in first approximation are independent of isotope substitution, small differences do exist. These are usually due to the isotopic differences which occur on vibrational averaging. Vibrational amplitude effects are important when considering isotope effects on dipole moments, polarizability, NMR chemical shifts, molar volumes, and fine structure in electron spin resonance, all properties which must be averaged over vibrational motion. 

In this way one obtains the mean value of P averaged over vibrational motion in terms of mean displacements, mean square displacements, and so on. This approach has been long used to discuss isotope effects on electrical properties of molecules. 

Methods for determining permanent dipole moments and polarizabilities can be arbitrarily divided into two groups. The first is based on measuring bulk phase electrical properties of vapors, liquids, or solutions as functions of field strength, temperature, concentration, etc. following methods proposed by Debye and elaborated by Onsager. In the older Debye approach the isotope effects on the dielectric constant and thence the bulk polarization, AP, are plotted vs. reciprocal temperature and the isotope effect on the polarizability and permanent dipole moment recovered from the intercept and slope, respectively, using Equation 12.5. 

To explain this different fractionation behavior, Taube (1954) postulated different isotope effects between the isotopic properties of water in the hydration sphere of the cation and the remaining bulk water. The hydration sphere is highly ordered, whereas the outer layer is poorly ordered. The relative sizes of the two layers are dependent upon the magnitude of the electric field around the dissolved ions. The strength of the interaction between the dissolved ion and water molecules is also dependent upon the atomic mass of the atom to which the ion is bonded. 

Isotope effects can be divided in two main groups phenomena that are directly connected to the differences in the molecular mass (thermal motion, motion in gravitational, electric, and other fields) and those connected to different mass distributions within the molecule (isotope effects on molecular spectra, chemical equilibria, reaction rate, etc.). Isotope effects may also be classified according to the field in which they are observed physics, chemistry, biology, geology, spectroscopy, etc. Isotope effects play an important role in the variation of stable isotope compositions in nature. The differences in chemical and physical properties of the isotopes form the basis of their separation from each other and make possible the production of enriched isotopes for industrial, military, medical, and research purposes (see Chap. 51 in Vol. 5). 

Relativistic effects in core properties like electric field gradients can be extremely large [103]. As the isotope 197-Au is widely used is Mossbauer spectroscopy [104], an... 

Changing the electrical charge of an atom usually has a major effect on its chemical properties. The two electrically neutral carbon isotopes should have nearly identical chemical properties. 

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