Question

1. Explain (in your own chemically accurate words) why and how you can use IR spectroscopy to measure bonding parameters for a polar, diatomic molecule. You will need to address
   • why the ro-vibrational spectrum is in the IR region of the electromagnetic spectrum (this may include a discussion of vibrational and rotational motion and the selection rules associated with them)
   • the origin of the P, Q and R branch (including a figure to indicate the origin of each set of peaks and where they will appear in the spectrum you will obtain)
   • an explanation of how the P,Q, and R branches can be used to calculate bond length and Morse Potential Parameters.
   • Define- quantum number, selection rule, rigid rotor, moment of inertia, harmonic oscillator, dissociation energy

This is for physical chemistry lab (Rovibrational Spectrum of HCl). Any help would be very appreciative!

Answer 1

1. The symmetrical diatomic molecules like H₂, O₂ and N₂ with zero permanent dipole moment and also do not interact with the lights electric field, So do not absorb IR radiation, although their vibrational frequencies are in the IR region. Molecules like HCl, and HBr with a permanent dipole, due to an unequal sharing of the electrons, dipole oscillates as the bond distance between the atoms oscillates. When frequency of electric field of the IR radiation approaches the frequency of the oscillating bond dipole and the two oscillate at the same frequency and phase, the chemical bond can absorb the IR photon.
2. IR radiation absorbed by molecules converted into molecular vibrational energy, either bending, or stretching. Therefore different types of bonds, and functional groups absorb IR radiation. The far-IR region which is neighboring to the microwave(MW) region, used for rotational spectroscopy.
3. The IR spectrum with vibrational transitions with same rotational quantum number (ΔJ = 0) in ground state and excited states is called the Q-branch. High frequency side of Q-branch the rotational transitions energy is added to the the vibrational transition energy of, called as R-branch (ΔJ = +1). P-branch is nearly a mirror image of the R-branch.