Vibrational Properties of Cu(Pz)₂(ClO₄)₂:  Evidence for Enhanced Low-Temperature Hydrogen Bonding in Square<i>S</i>=¹/₂Molecular Antiferromagnets

Abstract

We report the variable temperature vibrational properties of the two-dimensional S = 1 / 2 quantum Heisenberg antiferromagnet Cu(Pz) 2 (ClO 4 ) 2 . With decreasing temperature, softening is observed in the ν 4 (ClO 4 ) mode. In addition, the resonance frequencies of several pyrazine ring-related modes are temperature independent. This unusual behavior is attributed to enhanced hydrogen bonding interactions between the ClO 4 - counterion and the pyrazine rings in a neighboring layer, in line with the recently observed low-temperature red-shifts of numerous vibrational modes in the one-dimensional analogue, copper pyrazine dinitrate (Cu(Pz)(NO 3 ) 2 ). The proposed change in hydrogen bonding and the low-temperature symmetry breaking, made manifest by the concomitant doublet structure of several pyrazine-related modes, is supported by an analysis of the structure.