Some help understanding calculated vibrational frequncies

[gspeed]

I am looking at systems where amino acids are bound to divalent cations (Be2+, Mg2+ and Ca2+) through their carboxylate functional...in particular i am interested in the frequency of the CO symmetric and antisymmetric stretches in these systems...

I have performed MP2 and B3LYP calculations on the systems using the gaussian 03 program. When i have looked at the CO stretching frequencies,
I notice that the reduced mass in both sym CO vibration and asym CO vibrations are not consistent and vary slightly (for the same metal ion) and also vary dramaticaly depending on what ion is used...I find this strange as i thought that for C-O frequencies were just based on the reduced masses of C and O...can anyone help explain this

Cheers

 

[TeethWhitener]

I notice that the reduced mass in both sym CO vibration and asym CO vibrations are not consistent and vary slightly (for the same metal ion) and also vary dramaticaly depending on what ion is used...I find this strange as i thought that for C-O frequencies were just based on the reduced masses of C and O...can anyone help explain this

While our language tends to simplify vibrational motion in molecules to a subset of the most highly mobile atoms in a vibrational mode (e.g., CH stretch, or CH2 wag, bend, etc.), in reality, many of these vibrational modes in polyatomic molecules involve the concerted movements of several atoms at once. I can guarantee if you've bonded alkaline Earth metals to carboxylate ends of an amino acid, there are a lot of atoms moving for each normal mode, and the alkaline Earth's are going to be among them. So you'll definitely see differences in reduced masses for different alkaline Earth complexes.

The symmetric-antisymmetric stretch is the same idea: in a polyatomic molecule, the words "symmetric" and "antisymmetric" are approximations at best, and there will be differences in which atoms are involved in which normal mode vibrations.