Model 3 - Changes in rotational frequency /energy at room temperature A gas-phas

Question

Model 3 - Changes in rotational frequency /energy at room temperature A gas-phase molecule can rotate (spin) as well as vibrate (stretch/contract), and nearly all gas- phase molecules are rotating at room temperature. In a pure sample of gas molecules, there are some molecules not rotating, some rotating with low frequency/speed/energy, some rotating at high frequency/speed/energy etc. When a diatomic molecule is not rotating it is said to be in the J -0 rotational energy level, when it is rotating it could be in J = 1, J-2, J-3 ete energy levels. There is a distribution across the many options for rotational energy. Once again, quantum mechanics only allows certain energies for each pure sample of molecules it does to It does not take as much energy to change the frequency (and energy) of rotation as it does to change the frequency (and energy) of vibration, so when an IR photonis absorbed by amolecule causing vibrational excitation, changes in rotational frequency (and energy) can occur at the same time. The oscillating electric field in incident IR radiation can interact with the electric charges on rotating molecules, causing the molecules to rotate with higher or lower energy than they had prior to their interaction with the IR radiation. This can complicate gas-phase spectra. causing vibrational excitaion, chanof wibration, so when Critical Thinking Questions 1. Pure samples of the following molecules are in the gas-phase at room temperature. Which ones will have IR spectra that are complicated due to simultaneous changes in vibrational and rotational energy? HCl(g), N2O (g), CO (g), CC14 (g) 2. Please give a general selection rule that can be used for predicting which molecules will have rotational peaks in their IR spectra

Explanation / Answer

1. HCl(g), N2O(g) and CO(g) will have complicated IR Spectra due to roto-vibrational coupling. CCl4(g) will have relatively less complex spectra as no rotational contribution will be there.

2. It will be harsh to say a rule, but it is mostly followed. A molecule with net zero dipole moment does not show any rotational spectra.A knowledge of space group may help. It is known that polyatomic species with higher symmetry does not exhibit rotational spectra. Why it cannot be said a rule is that CH4 though has the same symmetry as that of CCL4 yet shows ro-vibronic spectra as the symmetry is lost in the excited state which does not happen with CCl4.

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