finding the resonance frequency of substances is apparently a VERY VERY difficult
It depends on molecule structure. More complex substance, more oscillation axis, more resonant frequencies. So, most substances have many resonant frequencies, not one, and it is more common to talk about "vibrational spectrum", instead of "resonant frequency". So, use "vibrational spectrum of ..." to find what you want.
It depends on molecule structure. More complex substance, more oscillation axis, more resonant frequencies. So, most substances have many resonant frequencies, not one, and it is more common to talk about "vibrational spectrum", instead of "resonant frequency". So, use "vibrational spectrum of ..." to find what you want.