Structure of HCOOK hydrated melts

Abstract X-ray diffraction and ATR-IR spectroscopic measurements have been carried out for HCOOK hydrated melts, (HCOOK) x (H 2 O) 1− x with x =0.3, 0.35 and 0.40, in order to obtain structural information on both the ion–water and ion–ion interactions in extremely concentrated aqueous solutions. The observed X-ray intermolecular interference terms were analyzed through the least-squares fitting procedure. The present value of the nearest neighbor K + ⋯H 2 O distance, r K + ⋯H 2 O =2.77(1) A, for the 40 mol% HCOOK solution is in good agreement with that observed in various aqueous solutions. On the other hand, the coordination number, n K + ⋯H 2 O , was revealed to be 2.6(1), which is much smaller than the value, n K + ⋯H 2 O ∼6, reported for more dilute aqueous solutions. Structural parameters for the contact ion pair, HCOO − ⋯K + , were satisfactorily determined to be r O(HCOO − )⋯K + =2.80(1) A, n O(HCOO − )⋯K + =1.3(1) and ∠C–O⋯K + =100(1)°, respectively. Hydration numbers, such as n K + ⋯H 2 O and n HCOO − ⋯H 2 O were found to be strongly concentration dependent from the ATR-IR spectra for uncoupled O–D stretching vibrational bands of HDO molecules in the melts.