The far-infrared and Raman spectra have been analyzed for the following compounds: dicesium neptunium hexachloride Cs2NpCl6, ditetramethyl ammonium neptunium hexachloride [(CH3)4N]2NpCl6, and ditetraethyl ammonium neptunium hexachloride [(C2H5)4N]2NpCl6. Infrared studies on polycrystalline samples were carried out at room temperature and 98°K. Single-crystal Raman spectra were recorded at room temperature. Far-infrared absorption peaks were found as follows: Cs2NpCl6−265, 117, and 58 cm−1; [(CH3)4N]2NpCl6−259, 116, and 65 cm−1; and [(C2H5)4N]2NpCl6−259, 117, and 59 cm−1. Observed Raman spectra were found as follows: Cs2NpCl6−310 and 128 cm−1; [(C2H5)4N]2NpCl6−301 and 123 cm−1. An interpretation of the observed spectra was made. The interaction force constants necessary to describe the observed spectrum are the Np–Cl stretch, K; the Cl–Np–Cl bend, H; the nonbonded Cl–Cl stretch, F; the interaction, k, between two Np–Cl stretches on opposite sides of the Np atom; and an angle-angle interaction, h, between angles at right angles to each other which share a common bond. The spectrum of the complex NpCl6−2 in each salt can be described by a given set of octahedral force constants, calculated using the Wilson FG matrix technique and a Urey-Bradley potential. The Cs2NpCl6 spectra are described by force constants K=1.31 mdyn/Å, H=0.053 mdyn/Å, F=0.061 mdyn/Å, k=0.40 mdynÅ, and h=0.015 mdyn/Å. The [(CH3)4N]2NpCl6 and [(C2H5)4N]2NpCl6 salts can be described by the force constants K=1.271 mdyn/Å, H=0.053 mdyn/Å, F=0.05 mdyn/Å, k=0.38 mdyn/Å, and h=0.015 mdyn/Å. The spectrum calculated using these parameters agrees to within 1% of the observed spectrum.

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