Dissertation Abstract

Pressure and Temperature Dependence of NMR Chemical Shifts of Simple Ions in Solution

Publication Number:  AAT9016015
Author:  Sarkar, Subhendra Nath
School:  Washington State University
Date:  1989
Pages:  159
Subject:  Chemistry

The NMR frequencies of simple ions (Li$\sp+$, F$\sp-$, Sc$\sp{+3}$ etc.) in various solvents have been found to shift with increasing pressure and temperature. The pressure induced shifts are in opposite direction to the temperature induced shifts. The frequencies of ions with unfilled d orbitals decrease with increasing pressure and increase with increasing temperature. Shifts in the opposite direction are observed in ions with filled s, p or d orbitals.

The magnitude of these shifts varies from 1 ppb/MPa to 170 ppb/MPa for pressure induced shifts, and from about 1 ppb/$\sp\circ$C to 1800 ppb/$\sp\circ$C for temperature induced effects. Most of the shifts are linear with pressure and temperature.

The pressure as well as temperature effects for closed shell ions are found to be proportional to the ionic volume. Since the polarizability of the ions is directly related to the volume, it seems that under pressure, more polarizable ions show a greater frequency change. These effects can be explained by changes in the chemical shielding $\sigma$, particularly by the changes in its negative component $\sigma\sb{\rm p}$ with the assumption that the positive and diamagnetic component $\sigma\sb{\rm d}$ does not change appreciably with applied pressure or temperature.

An overlap model of the chemical shift assigns the frequency shifts to changes in overlaps of atomic orbitals and assumes $\Delta$, the average excitation energy, to be constant. This model is known to agree fairly well with the observed $\sigma$ in alkali halide metal ions by an extended Huckel program. The change of overlap with temperature and pressure has been estimated and ${{{\rm d}\sigma}\over {\rm dP}}$ and ${{{\rm d}\sigma}\over {\rm dT}}$ calculated.

The distance between the metal ion and the first sphere solvent molecules is altered by the temperature and pressure and this, in turn, is related to the force constant and anharmonicity of the potential between the ion and solvent molecules.

For the trends of Sc$\sp{+3}$, Mn$\sp{+1}$ and Co$\sp{+3}$, which have unfilled d orbitals and behave in an opposite manner to closed shell ions in the frequency shift experiments, it can be concluded that the increase in charge overlap is more than compensated by the increase in $\Delta$ with pressure which causes a reduction in the magnitude of $\sigma\sb{\rm p}$. Hence for these ions, the frequency decreases with pressure. The temperature behavior remains consistently opposite due to the decrease in $\Delta$.

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