Linear molecule - degrees of freedom

These are rotations of the molecule as a whole, if it were assumed to be rigid. The atoms have no external rotational degrees of freedom, linear molecules 2, non-linear molecules 3. 

In contrast to diatomic molecules vibrating in a single mode only, polyatomic molecules can vibrate in many modes. A molecule containing N atoms has 3N 6 vibrational degrees of freedom (linear molecules 3N — 5), which correspond to the number of possible fundamental vibrations (normal modes). As an example. Figure 2.2 shows the various vibration modes feasible for a 3-atomic side group. Here, N >3 due to the polymer rest (not shown). 

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See also in sourсe #XX -- [ ]

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