I understand that a diatomic molecule has 3 translational and 2 rotational degrees of freedom. But since there is only 1 vibrational mode associated with a diatomic molecule and 1 vibrational mode is associated with 2 degrees of freedom, shouldn't the total degrees of freedom be f=7? I've seen it given as f=6 in many sources. Similarly, for a linear triatomic molecule, since there are 4 vibrational modes, the total degrees of freedom, f=13 right? Please explain.
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$\begingroup$ You might refer to: en.m.wikipedia.org/wiki/… $\endgroup$– Darth VaderCommented Jan 9 at 11:13
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$\begingroup$ This seems to agree with the fact that at high temperatures for diatomic molecules, f=7 and for triatomic linear molecules, f=13. So that's correct right? In other sources different answers have been given. $\endgroup$– Srijan DasCommented Jan 9 at 11:16
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$\begingroup$ Each vibrational mode has kinetic and potential energy associated with it. So each vibrational mode contributes two degrees of freedom. You can verify it in the link that you posted, for linear triatomic molecule, no. of vibrational modes = (3N-5) and vibrational degrees of freedom = 2(3N-5). Please correct me if I'm wrong. $\endgroup$– Srijan DasCommented Jan 9 at 11:50
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1$\begingroup$ You're right in your analysis. The confusion between the terms stems from the different definitions in physics and chemistry. This answer clears that up. $\endgroup$– Darth VaderCommented Jan 9 at 12:39
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1$\begingroup$ A diatomic molecule has one vibrational mode, so 2 vibrational degrees of freedom (dof's). Thus: 3 trans. + 2 rot. + 2 vibr. = 7 dof's. A (linear) triatomic molecule that cannot bend has two vibrational modes, so 4 vibrational dof's: 3 transl. + 2 rot. + 4 vibr. = 9 dofs. A nonlinear triatomic molecule has three vibrational dof's, and can rotate in three dimensions, so: 3 trans. + 3 rot.+ 6 vibr. = 12 dof's. I don't think 13 is right, and wikipedia agrees with me. $\endgroup$– marchCommented Jan 9 at 17:24
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