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Example: general case of internal rotation in polyatomic molecules

The conventional approximation with one term potential

V(f)=V0/2[1+cos(nf)]

is inapplicable to the general case of internal rotation (IR) with a complicated potential. The case of torsional potential for relatively simple methyl n-propyl ether is shown on the picture. The potential was obtained from quantum-chemical calculation and is fully unsymmetrical. The most general shape of the periodic function including cosine and sine terms should approximate it. The results are presented in the figure. The direct summation over calculated torsional levels using such potential gives reliable values of IR contribution to the thermal functions.

 

Potential function of internal rotation and torsional energy levels for methyl n-propyl ether (CH3 – O – CH2CH2CH3)
Potential function of internal rotation and torsional energy levels for methyl n-propyl ether (CH3 – O – CH2CH2CH3).

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