An infrared spectroscopic study of NH4Br —ammonium bromide to 55 GPa

We have measured the infrared spectrum of NH4Br—ammonium bromide to 55 GPa at 300 K. The hydrogen bond in this system is initially nearly linear, and the magnitude of volumetric compression over this pressure range is between 45% and 55%: thus, dramatic compression of the N–H…Br distances is expected. However, we observe pressure shifts of the internal modes of the ammonium ion that are small, non-linear and change in sign under compression. For example, the ν3ν3-asymmetric stretching band of the NH4 group near 3145  cm−1 undergoes an initial decrease with pressure to ∼5 GPa, followed by near-constancy to 20 GPa, and increases in frequency at higher pressures: it has essentially an identical frequency at 30 GPa as it does at 2.5 GPa. The pressure dependences of the internal modes indicate that if strong hydrogen bonding is present in the system, or is induced by pressure, then there must be competing and compensating effects that produce near-constancy or pressure-induced increases of the internal ammonium ion stretching frequencies as well as decreases in the internal bending frequencies. Our results indicate that three effects likely prohibit the pressure-induced onset of strong hydrogen bonding within the system: (1) a change in the N–H…Br bond angle at high pressures, (2) overbonding on the bromine ion by the surrounding hydrogen atoms, and (3) inter-ammonium ion hydrogen–hydrogen repulsion which is enhanced with increasing pressure and decreasing volume. Systematics of the pressure dependences of the N–H stretching frequencies of the ammonium halides indicate that the last effect, H–H repulsion, dominates in these systems. Thus, it is the abundance of hydrogen within the NH4Br lattice that produces the primary effect on the pressure shift of the N–H stretching vibrations, rather than increased hydrogen bonding.