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X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects
Published on: June 8, 2016
Preparation, characterization, X-ray crystal structure, and energetics of cesium 5-cyano-1,2,3,4-tetrazolate:
H P Arp1, A Decken, J Passmore
1Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6E2.
Abstract:
Cesium 5-cyano-1,2,3,4-tetrazolate, Cs[NCCNNNN] (Cs1), was prepared in 100% yield by a 3 + 2 cycloaddition reaction of CsN3 and (CN)2 in SO2. Cs1 forms monoclinic crystals (a = 8.297(2) A, b = 11.040(3) A, c = 6.983(2) A, beta = 120.31(2) degrees, space group C2/c, Z = 4, R1 = 0.048, wR2 = 0.120 for 1217 independent reflections). Cs1 is best described as a three-dimensional array of cations and anions connected by weak Cs(+)-N delta- contacts. The cations and anions each form a diamond-type lattice (tetrahedral arrangement of ions) with the counterions lying in hexagonal channels running parallel to the c-axis. The anions in the channels form stacks with the CN groups pointing in opposite directions in adjacent layers. The calculated (RB3PW91/6-311 + G*) geometry of 1 is in agreement with the X-ray crystal structure, and the calculated vibrational spectrum is in good agreement with the observed FT-IR and FT-Raman spectra. The lattice enthalpies and heats of formation of M1 (M = Cs, K) as well as the enthalpy of formation from MN3(s) and (CN)2(g) were estimated. The 13C and 14N NMR spectra of Cs1 are also reported. The ionization potential (450.7 kJ mol) and electron affinity (427.4 kJ/mol) of 1 were calculated. Attempts to oxidize 1 with AsF5 led to the formation of Cs1.xAsF5 (x approximately 2). The 7 pi dianion [NCCNNNN].2- is calculated to be a stable entity in the gas phase, but Cs(2)1 is estimated to be unstable with respect to dissociation to 2 CsCN and 3/2 N2 (delta Hdiss = -132.4 kJ/mol). The preparation of the potassium salt of 1 and the corresponding thermodynamic quantities are reported.
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