Question

MCAT Tutorial: IR Spectroscopy

Answer 1

\({\bf{Basic~Mechanism:}}\) when molecules are excited with energy from the IR region of the EM spectrum they are induced to have different vibrational states. Molecular bonds are somewhat like springs that can stretch/compress to varying degrees.

Two types of vibrational patterns: stretches (bond length changes) and bends (bond angle changes)
> subtypes of stretches: symmetryc and asymmetric
> subtypes of bends: rocking, scissoring, wagging, twisting





IR radiation will only be absorbed if the dipole of the molecule is changed. Symmetric stretches are not IR active.

\({\bf{Units:}}\) usually in wavenumbers (cm^-1)

to convert from wavenumbers to SI units: note that cm is in the "denominator" of the unit so we would multiply by 100cm/m to convert to m^-1, then since

c/wavelength = frequency
1/wavelength = frequency/c

so simply multiplying by the speed of light will convert from 1/wavelength to frequency units (Hz)

Answer 2

\({\bf{IR-Absorption~Spectra:}}\) every compound produces a characteristic pattern of peaks based on the number of bonds, types of bonds, and the atoms involved in its bonds

General Trends for peaks:
- Intensity (size) of peaks: determined by polarity, asymmetry, and concentration
- Stretching vibrational modes have higher frequencies (requires more energy to excite that state)
- triple bond modes have higher frequencies than double bonds, which are higher than single bonds
- functional groups with electron with-drawing groups and conjugated bonds have lower frequencies
- hydrogen bonding makes peaks wider
- each major functional group has a characteristic region on the IR spectra, there will be some overlap but some major ones you should be looking for are -OH's, aromatics, C=O double bonds, and ethers

Answer 3

let's use this problem for example (from the Mastering Organic Chemistry blog, check them out btw, they're fantastic) https://www.masterorganicchemistry.com/2016/11/29/ir-spectroscopy-some-simple-practice-problems/

Answer 4

Using index of hydrogen deficiency (will get to this in a later tutorial) we get 1 which means we should be looking for either a double bond or a ring. Let's try looking for C=O which gives a strong peak around the 1680-1750 region, we don't really have one here, so we probably have a ring of sorts

Answer 5

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT

I'm a little pressed for time but will add problems upon request if anyone needs a bit of guidance, I'm a little rusty myself (which is why I'm trying to teach myself with these tutorials haha)