Homonuclear diatomic molecules are an important special case. Clearly, the symmetry of the problem dictates that both atoms will have identical coefficients in a molecular orbital. We will consider the special case of the linear combination of atomic orbitals for two identical orbitals as a way to understand the more complex general case. We will examine the Aufbau principle applied to diatomic molecules as an example of molecular orbital theory to be developed in more detail in coming chapters.

Heteronuclear diatomic molecules

Heteronuclear diatomic molecules are asymmetric, which means that they have dipoles. Thus, heteronuclear diatomic molecules have pure rotational and infrared vibrational spectra. Actually, their infrared spectra are the result of a combination of vibrational and rotational spectra so that they are called rovibational spectra as discussed in the next section. We will examine by treatment of the electronic structure of heteronuclear diatomic molecules using the LCAO-MO approach.

Analysis of rovibrational spectra

In this section we consider the observed infrared rovibrational spectra of heteronuclear diatomic molecules. We will consider the analysis of rovibrational spectra by linear regression. The analysis will include an understanding of the additional effects of anharmonicity and centripedal distortion of the molecule during rotation. These effects results in a deviation of the rotational line spacing from perfect 2B (where B is the rotational constant). We note that homonuclear diatomic molecules have no infrared spectra since they have no dipole moments. However, homonuclear diatomics do have both vibrational and rotational Raman spectra. We will consider Raman spectroscopy in a future chapter.