a Spectroscopic and Structural Correlation of Salts of the 1,1,2,3,3--PENTACYANOPROPENIDE Anion.

Four salts of the 1,1,2,3,3-pentacyanopropenide (PCP) mono-anion are synthesized and studied spectroscopically and crystallographically. The crystal structures of pyridinium pentacyanopropenide (PyPCP), rubidium pentacyanopropenide (RbPCP), and potassium pentacyanopropenide (KPCP), are reported. The pyridinium salt forms in the space group P2_12_12 with the pentacyanopropenide anion lying on the 2-fold c-axis. The pyridinium cation also nominally lies on the 2-fold axis, but disorder required moving all atoms of the cation to general crystallographic positions. The rubidium and potassium structures are isomorphic, but different than the previously reported structure of the cesium salt. Although all three alkali lattices have the anion atoms on general crystallographic positions in the P2_1/c space group, the cesium structure has the anions forming sheets whereas the rubidium and potassium structures do not. The rubidium and potassium lattices significantly distort the anion from the calculated C _ {2v}>=ometry, particularly along the propene backbone. UV-visible diffuse reflectance spectra and low temperature (4 K) emission spectra for microcrystalline powders of the four salts are recorded and discussed in terms of the known structures. Extended Huckel theory is used to predict first transition energies. For the pyridinium salt, the disorder in the pyridinium ion is discussed in terms of a dynamic system, and is modeled by a double-minimum potential. Vibrational (Raman and FTIR) spectra are recorded for the different PCP salts in the 100-2500 cm ^{-1} region. The results are used to extend and clarify a previous vibrational analysis of the ion based on spectra recorded from the cesium salt. Assignments are made in terms of the parent C _{2v } point group. In addition, a novel combination of Self-Consistent-Field Hartree-Fock (SCF-HF) calculations and the Gordy relation is used to model the perturbation of the ionic lattice on the normal mode frequencies of the PCP anion.