How to find vibrational frequency for specific bond? (DFT)

[Dan Zar]

Hello,
I performed an optimization and frequency calculation at the ub3lyp 6-311g level of theory. The compound has the formula C₃₉H₄₁PS₂. Using Molden, I am having a hard time identifying the frequency corresponding to the stretching of a phosphorus carbon double bond present in the molecule. I expect the frequency to be very low in intensity which makes my task harder. I am wondering if there is any way to obtain this information directly from the output file without having to click every frequency on Molden. Thank you very much.

 

[DrDu]

The calculation does not give you a value for the stretching of only this bond. Rather this stretching will contribute to several normal modes. Looking at the output of your calculation, you should see a list of the normal vibrations and the contribution of the individual atoms. Identify those normal modes where the C and P atom in question make a large contribution.