It's interest idea but I think it's difficult to make it real. Due to IR spectra of conformers can be little distinguishable and if you have more than 2 conformers the obtained results will have a large error.

Maybe this way:

If you have all gas-phase conformers and their vibration frequencies you may calculate mole fractions of conformers using Free Energy. And you can apply the obtained values to scale and sum IR spectra for further correlating with the experiment.

Hey Mikhail, thanks for the intput. I'm definitely aware of the large errors, especially in my case where I am trying to "fit" theoretical spectra of peptides. I just want to get a general rough idea of what things are.

At the moment what I am doing is manually applying weights to specific spectra to get a general idea of what the distribution is, but I can only do this adequately for 2-3 conformers at once. I'd like to feed in up to 10 conformer spectra and have a computer automagically apply weights for me (perhaps this is wishful thinking).

For example, I have a Boltzmann weighted average of 8 conformers (top) vs a 60:20:20 weighted average of 3 conformers. You can see that both make reasonable sense, but my manual fit seems to qualitatively match better. Either way I'm not a specialist in spectroscopy so I literally have no Idea what I am doing.

Mr. Batoon,

According to the data provided, first of all you need a chemometric analysis, in order to determine are your patterns coincide or not, furthermore, quantitatively. Please pay attention to an attachment, showing two patterns (the examples are of polarized IR spectra in solid-state), and the chemometrics applied.

Your patterns are significantly different, which can be determined even without chemometric analysis (qualitatively). So that you cannot claim that you have coincidence between the set of optimized molecular geometries, and the real state in gas-phase.

In other words you have achieved an averege picture, but this average picture of conformations as shown in your posting is no representative to the real pattern.

The quantum chemistry is very sensible towards any change of the molecular conformations, and in this context you need to obtain a set of conformations which usually are far from equilibrium, in order to reproduce, exactly, the experimental patterns. For a flexible molecules the operation at global minimum of the energy usually fails from the real energy picture. And this is valid for gas- and condense phase. The correlation between MS data about free energies and quantum chemical ones at global minimum can deviate about 30 % when the molecules are flexible.

For condense phase analysis you can pay attention to few of our papers (refs 1-4) where we have correlated quantum chemistry to crystallographic structures. In all those cases the global minimum corresponds to extended geometry of the aliphatic chains (Refs. 1,2,4). But experimentally even we have cases when in a single unit cell there is possible stabilization of two conformers of one and the same molecules (for example ref. 1, fig. 1). And, in parallel, those conformers are different comparing with the theoretical one at the global minimum. Moreover there is possible to obtain two non-equivalent pairs of molecules even within the frame of aromatic substances (for example, ref. 2).

Or you should continue your computations of possible geometries and conformational search, but at higher criteria, in order to obtain theoretical patterns coinciding with experimental ones, according to the demonstrated quantitative chemometric criteria (attachment).

When you have achieved this picture, then again by chemometrics you can determine percentage contribution. Bur currently you have a significant deviation of the profiles of both patterns.

[Ref. 1] Bis(tyrammonium) sulfate dihydrate: Crystal structure, solid-state IR-spectroscopic and theoretical characterization, J. Mol. Struct. 888, 2008, 138-144; B. Koleva, T. Kolev, R. Seidel, M. Spiteller, H. Mayer-Figge, W. Sheldrick

[Ref. 2] Anyles of 4-benzoylpyridine: Crystal structure and spectroscopic properties, Dyes Pigm. 82, 2009, 95-101, T. Kolev, T. Tsanev, S. Kotov, H. Mayer-Figge, M. Spiteller, W.S. Sheldrick, B. Koleva

[Ref. 3] Struct. Chem. 2008, 19, 147-154, Are there preferable conformations of the tryptammonium cation in the solid state? Crystal structure and solid-state linear polarized IR-spectroscopic study of tryptammonium hydrogentartarate, B. Koleva , T. Kolev, H. Mayer-Figge, R. Seidel, M. Spiteller, W. Sheldrick

[Ref. 4] J. Food Measur. Character. 2014, 8, 15-28, Simultaneous quantitation of naturally occurring insecticides, acaricides, and piscicides in rapeseed oil by UV-MALDI mass spectrometry, B. Ivanova, M. Spiteller