Degrees of freedom and triatomic gas

[anigeo]

i wonder how can a non linear triatomic gas molecule have 6 degrees of freedom.As per me three of those will come from translational motion,two from rotational and two from vibrational modes.so it should be 7.
could you please tell me where am i going wrong.
note:the triatomic gas molecule has the arrangement of an equilateral triangle.

 

[sodaboy7]

there will be 3 from translation
3 from rotation about each atom
and 1 from from rotation of 3 atoms about center of equilateral triangle

 

[nasu]

A triatomic molecule has 9 degrees of freedom, in general.
There are 3 translational modes, 3 rotational and 3 vibrational.
The "6 degrees of freedom" triatomic molecule neglects the vibration modes, which usually are not active at room temperature.

Same as for diatomic gas (6 degrees of freedom) the vibration mode does not contribute to heat capacity at room temperature so the gas has only 5 active modes.

 

[anigeo]

thanx nasu.
i think i got it

 

[klimatos]

A triatomic molecule has 9 degrees of freedom, in general.
There are 3 translational modes, 3 rotational and 3 vibrational.
The "6 degrees of freedom" triatomic molecule neglects the vibration modes, which usually are not active at room temperature.

In general, yes, but not always. The vibrating mode is often coupled with a flexing mode (KE of libration), and many authorities prefer to consider this coupling as a single mode with a KE of kT rather than two modes with 1/2 kT each.

CS_2, for instance, has 10 observed degrees of freedom.