page_title: Rotational–vibrational spectroscopy
section_title: Inversion in ammonia
hierarchical_section_title: Rotational–vibrational spectroscopy / Symmetric top molecules / Inversion in ammonia
caption_attribution_description: English: Simulated spectrum of symmetric bending vibration in ammonia
context_page_description: Rotational–vibrational spectroscopy is a branch of molecular spectroscopy concerned with infrared and Raman spectra of molecules in the gas phase. Transitions involving changes in both vibrational and rotational states can be abbreviated as rovibrational transitions. When such transitions emit or absorb photons, the frequency is proportional to the difference in energy levels and can be detected by certain kinds of spectroscopy. Since changes in rotational energy levels are typically much smaller than changes in vibrational energy levels, changes in rotational state are said to give fine structure to the vibrational spectrum. For a given vibrational transition, the same theoretical treatment as for pure rotational spectroscopy gives the rotational quantum numbers, energy levels, and selection rules. In linear and spherical top molecules, rotational lines are found as simple progressions at both higher and lower frequencies relative to the pure vibration frequency. In symmetric top molecules the transitions are classified as parallel when the dipole moment change is parallel to the principal axis of rotation, and perpendicular when the change is perpendicular to that axis.
context_section_description: For ammonia, NH₃, the symmetric bending vibration is observed as two branches near 930 cm⁻¹ and 965 cm⁻¹. This so-called inversion doubling arises because the symmetric bending vibration is actually a large-amplitude motion known as inversion, in which the nitrogen atom passes through the plane of the three hydrogen atoms, similar to the inversion of an umbrella. The potential energy curve for such a vibration has a double minimum for the two pyramidal geometries, so that the vibrational energy levels occur in pairs which correspond to combinations of the vibrational states in the two potential minima. The two v = 1 states combine to form a symmetric state (1⁺) at 932.5 cm⁻¹ above the ground (0⁺) state and an antisymmetric state (1⁻) at 968.3 cm⁻¹.
  The vibrational ground state (v = 0) is also doubled although the energy difference is much smaller, and the transition between the two levels can be measured directly in the microwave region, at ca. 24 Ghz (0.8 cm⁻¹). This transition is historically significant and was used in the ammonia MASER, the fore-runner of the LASER.
caption: Spectrum of central region of the symmetric bending vibration in ammonia simulated with Spectralcalc,[6] illustrating inversion doubling.
exif: {}