INFRARED SPECTROSCOPIC STUDY OF POLYCRYSTALLINE NH4CN

The infrared absorption spectrum of polycrystalline NH4CN is reported at temperatures between 125° and 4°K. At 77°K the main features occur near 3165, 3030, 2854, 2094, 1850, 1438, and 215 cm-1. A few of these features show two or more components at lower temperatures. The NU4+ torsional fundamental does not appear in the spectrum, but a value of 410 cm-1 is inferred for it from other observed frequencies. The spectrum does not show the complexity allowed by factor-group analyses based on the x-ray symmetry of the crystal. The results at 77°K are understandable on the basis of an effective NH4 + site symmetry of Td, and the plausibility of this is discussed. A comparison of the frequencies with those of other ammonium salts indicates moderate hydrogen bonding in NK4CN, comparable to that in NH4Cl and NH4N3. The possibility of hydrogen bonding of NH4+ to either the four nearest C or N atoms of adjacent CN-ions or to CN~ π orbitals is considered; evidence favoring the former is presented, but the data do not permit a decision.