Vibration-Rotation Spectrum of HClA classic among molecular spectra, the infrared absorption spectrum of HCl can be analyzed to gain information about both rotation and vibration of the molecule. The absorption lines shown involve transitions from the ground to first excited vibrational state of HCl, but also involve changes in the rotational state. The rotational angular momentum changes by 1 during such transitions. If you had a transition from j=0 in the ground vibrational state to j=0 in the first excited state, it would produce a line at the vibrational transition energy. As observed, you get a closely spaced series of lines going upward and downward from that vibrational level difference. The splitting of the lines shows the difference in rotational inertia of the two chlorine isotopes Cl-35(75.5%) and Cl-37(24.5%). From the spectrum above, you can examine details about the following:
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Index Molecular spectra concepts Reference Tipler & Llewellyn Sec. 9-4 | ||||||
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Bond Force Constant for HCl
Despite the approximations, this value is quite close to the value given in the table.
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Index Molecular spectra concepts Reference Tipler & Llewellyn Sec. 9-4 | ||||
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Bond Length of HCl
Substitution of numerical values leads to an estimate of the bond length r: This compares reasonably with the value r=.127 nm obtained from pure rotational spectra.
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Index Molecular spectra concepts | ||||
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