Infrared spectra have commonly been collected using transmission methods. ATR has displaced transmission in many applications. However, the infrared spectrometer in transmission mode is an excellent method for analyzing gas. For example, FT-IR spectrometer systems, manufactured by Thermo Fisher Scientific, are now aboard U.S. aircraft carriers and are used in testing breathing oxygen. Gas phase IR spectra are also commonly used in teaching labs, such as the analysis of HCl vapor to determine the bond length.

A much less known, but fascinating, educational application of gas phase transmission infrared spectroscopy involves the spectrum of ammonia. Gas phase molecules are free to rotate and vibrate, so gas spectra typically reveal a symmetrical distribution of closely spaced peaks (due to combined vibration-rotation transitions) around a central peak (the pure vibrational component). There are many complexities and exceptions, of which ammonia is an especially interesting case.

The three N-H bonds in ammonia are identical, so there should be only one vibrational peak. However, as seen in the figure below, the gas phase spectrum consists of a large doublet surrounded by rotational bands. The doublet is caused by the inversion of ammonia (pyramid to planar to inverted pyramid). The full theory is developed in an application note available from Thermo; only a brief introduction is given here.

Figure 1: Gas phase spectrum of ammonia showing the doubled-Q peaks

The energy levels of the pyramid and the inverted pyramid combine through linear combinations, resulting in the ground state of ammonia now having two levels, as do all the other levels. Each linear combination is described by a wavefunction which is either odd (o) or even (e), so a short hand of this would be “0e” for the v=0, even, level. The four possible transitions are now 0e to 1e, 0e to 1o, 0o to 1e and 0o to 1o. Mathematically, the e-e and o-o transitions result in zero probability (see the full paper for the reason), so the only allowed transitions are 0e to 1o and 0o to 1e – hence the two observed peaks in ammonia.

A 5 cm gas cell filled with low pressure ammonia gives an excellent spectrum on the Nicolet™ IR100 spectrometer. The spectrum occurs in the region near 950 cm-1 which is easily detected. The coupling of a simple measurement with an accessible (but non-trivial) theoretical description makes this an excellent physical chemistry experiment.