O-H broad IR peak question

[zoner]

I wan't sure why the middle comes down the most in the broad o-h IR peak. Is that simply implying that there are more concentration of O-H bonds that are in between H bonded and non H bonded?

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[jphwki82]

I wan't sure why the middle comes down the most in the broad o-h IR peak. Is that simply implying that there are more concentration of O-H bonds that are in between H bonded and non H bonded?

The graph is plotting % Transmittance (y-axis) vs Wavenumbers (v) or frequency (x-axis). It shouldn't have anything to do with concentration of a sample. These results paired with NMR could tell you how many -OH bonds there are, and where they would be placed in a given sample. However, IR is only going to tell you what functional groups are. %T is the ratio of radiant power transmitted by the sample you are testing, I, to the radiant power incident on the sample I sub 0. It will allow you to calculate absorbance, A, which is the log, base 10, of the reciprocal of the transmittance, T. A= log10 (1/T). They use transmittance as a graphical depiction because it has a range of 0 - 100% veruse Absorbance, which ranges from infinity to zero. Wavenumbers (1/wavelength) are directly proportional to frequency, and reflect the energy of IR absorption.

For the purposes of the MCAT, I would just worry about the above equations, and memorize the types of bending and stretching vibrations and know the major frequencies for certain functional groups. On the actual MCAT, you are likely to be asked maybe 2 or 3 questions regarding spectroscopy at the most. So I wouldn't worry to much about a more in depth understanding of IR other than it is used to ID functional groups, and how to use IR results to do so.

Key functional groups to note -

1) -OH = 3300ish (Broad)
2) -NH = 3300ish (Sharp)
3) -CHO = 2800ish
4) C=O = 1750
5) Aromatic C-H = 2900
6) Acids = 1750 (C=O) & 3300 broad (-OH)