New aspects of the semiempirical theory of27Al-chemical shift and its application to glasses

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Abstract
In the first part of the paper the semiempirical theory of nuclear shielding is extended to the case of the 27Al-chemical shift. The central point is a more precise interpretation of the formula for the dependence of the bond polarity on the electronegativity of the bond partners. The theory gives only a general guideline for the change of the chemical shift with substitution in the first coordination sphere. Using the form of the parabola obtained for the dependence of the chemical shift on the electronegativity of the bond partner it is possible to give an explanation for the large span of shifts in the case of neighbours with high electronegativity like oxygen. In the second part of the paper the recently developed bond polarization theory is applied to explain the influence of substituents in the second coordination sphere on the 27Alchemical shift. This theory establishes a linear correlation between the bond polarization energy and the chemical shift. The bond polarization energy can be calculated once the charges on the next neighbour atoms and their geometrical arrangement is known. From this correlation detailed interpretations can be derived for the influence of bond angles and substituents in the second sphere on the chemical shift.