# Are the vibrational modes of CO2 active at room temperature?

I was doing an experiment whereby I had to measure the specific heat ratios of certain gases. Carbon dioxide came out to be around 1.3, and checking with the accepted values this is close. My question is, the only way I can justify this answer is if $$\ce{CO2}$$ had 7 active degrees of freedom at room temperature. This implies that the molecule is vibrating. I am not a chemist and I thought that molecules only store energy in this degree of freedom at high temperatures.

• Your results implies just rotational degree of freedom. Why you see it as to require activated vibrations? 7/5 is 1.3 – Alchimista Mar 14 '19 at 8:54
• Its 1.4, the tables in my textbook have the specific heat of C02 listed has 1.28, which is closer to 9/7. – Vishal Jain Mar 14 '19 at 10:02
• I see. I was astray by you saying " the only way I can justify this answer is if C02 had 7 active degrees of freedom at room temp". – Alchimista Mar 14 '19 at 10:07

Yes, the vibrational modes are present at any temperature, including absolute zero where the lowest vibrational energy levels only are populated; the zero-point levels. The molecule is linear ( OCO ) so it has $$3N-5=4$$ vibrational modes for $$N=3$$ atoms. The symmetric stretch corresponds to both CO bonds stretching in phase, and asymmetric stretch to one CO stretching while the other compresses (this has highest frequency), and there are two degenerate bends where the OCO angle changes (lowest frequency). If the z-axis is along the OCO bonds, bends are in the zx and in the zy planes. Wikipedia has lots of diagrams showing these vibrational normal modes.