The temperature dependence of Mössbauer quadrupole splitting values: a quantum chemical analysis

The two key parameters extracted from Mössbauer spectroscopy, isomer shift and quadrupole splitting, have well-known temperature dependencies. While the behavior of the values following a temperature change has long been known, its microscopic origins are less clear. For quantum chemical calculations - formally representing the situation at 0 K - significant discrepancies with the experiment can arise, especially at elevated temperatures. Herein, we introduce an ensemble-based approach for capturing the temperature dependence of the quadrupole splitting. Our method is exemplified with [Fe(TPP)(2-MeHIm)], an iron(II) high spin system. We rationalise the temperature dependence by changes in the shape of the charge distribution due to vibrational distortion. By using a normal mode fitting approach, we isolated collective nuclear movements associated with the change in the quadrupole splitting.