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 NH₄⁺ 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 $T_d$ , and the plausibility of this is discussed. A comparison of the frequencies with those of other ammonium salts indicates moderate hydrogen bonding in NH₄CN, 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⁻ $\pi$ orbitals is considered; evidence favoring the former is presented, but the data do not permit a decision.