MCAT Spectroscopy Flashcards
How does Infrared spectroscopy work?
Absorption of IR light causes molecular vibrations (twisting, bending, stretching, and folding).
Different groups absorb different wavelengths of IR light causing a change in the dipole moment.
It’s plotted wavenumber v. transmittance
What’s the fingerprint region of IR spectroscopy?
Fingerprint region spans from 1500 to 400 where the total region spans from 4000 to 400.
What is the wavelength absorption for the different groups in IR spectroscopy
Alcohols- 3600 - 3200
Carboxylic acids- 3600-2500
Amines- 3500- 3350
Alkynes- 2160-2260
Nitriles- 2260-2220
Describe UV spectroscopy
Spectroscopy that measures the absorption of UV light which causes electrons to move between molecule orbitals ( LUMO and HUMO).
graphed absorbance v. wavelength
The smaller the energy difference between the longer wavelength is absorbed.
Conjugation shifts to longer wavelength absorption ( lower frequency).
Nuclear magnetic resonance (NMR)
Measures the aligned spin of a nucleus and external magnetic field.
Plotted frequency v. absorbance of energy
Measure chemical shift (measured in ppm).
Higher chemical shift = located to the left (downstream). Lower chemical shift = located upstream (right).
Proton (‘H) NMR
Alignment of protons to an external magnetic field.
Integration
Proton NMR
It’s the area under the curve and is proportional to the number of protons.
Deshielding v. Shielding
Deshielding happens when the proton is near EWGs causing electron density to be pulled away exposing it more to the magnetic field. This causes a shift downstream (to the left).
Shielding happens when protons are near EWG causing a shift upstream (to the right).
Splitting
When there is nearby protons, they interfere with each other’s magnetic field causing spin-spin coupling ( splitting).
We find the amount of splitting via n+1, where n = number of protons three bonds away from proton of interest. Can be doublets, triplets, and multiplets.
