Ammonia polyatomic molecule vibrations

Vibrational spectra are accompanied by rotational transitions. Figure 1.3 shows the rotational fine structure observed for the gaseous ammonia molecule. In most polyatomic molecules, however, such a rotational fine structure is not observed... 

Vibrational spectra are accompanied by rotational transitions. Rgure 1-4 shows the rotational fine structure observed for the gaseous ammonia molecule. In most polyatomic molecules, however, such a rotational fine structure is not observed because the rotational levels are closely spaced as a result of relatively large moments of inertia. Vibrational spectra obtained in solution do not exhibit rotational fine structure, since molecular collisions occur before a rotation is completed and the levels of the individual molecules are perturbed differently. Since Raman spectra are often obtained in liquid state, they do not exhibit rotational fine structure. 

Accurate ab initio theoretical calculations of harmonic and anharmonic force fields of molecules as large as benzene have recently been performed. By comparing the experimentally determined molecular constants with the calculated ones, the reliability of theoretical calculations can be assessed. All the molecular constants of COF2 in Table 1 can be evaluated from the theoretical anharmonic force field. For molecules such as ammonia the computed fundamental wavenumbers are within 3 cm or better of the experiment. For anharmonic constants and centrifugal distortion constants, which are much smaller than the fundamental frequencies, the deviations are on average around 15%. The experimental determination of anharmonic constants x j that appear in the equation for the vibrational energy of a polyatomic molecule with the harmonic frequencies cop and without degenerate vibrations ... 

---