University:
The University Of Western Ontario
Course:
2223B | Organic Chemistry of Biological Molecules
Academic year:

2023
Views:

491

Pages:

1
Author:

Cole M.

Infrared & Ultraviolet/Visible Spectroscopy Introduction to Infrared Spectroscopy: Wavenumber (v with a tilde over it) is: 𝑣 = - - 1 ! → inverse to wavelength c = 𝜆v (wavelength x frequency) In an IR Graph, the left half (<1500 cm-1) is known as the diagnostic region, meaning it can be indicative of certain functional groups 2100 cm-1 shows the presence of a triple bond Fingerprint region is unique to each molecule, but is much harder to decipher Only vibrations that produce a change in the dipole moment are observed as signals on an IR spectrum Bonds as Springs: For frequency of a spring, use the equation to the right → µ is the reduced mass (use if there are two masses) Double bonds need to double the k value (approximately) IR Spectra for Hydrocarbons: 3300 cm-1 shows up for sp hybridized Carbons, 3100 cm-1 shows up for sp2 hybridized carbons, and 2900 cm-1 for sp3 hybridized carbons Signal Characteristics - Intensity: About 1700 cm-1 is a carbonyl bond stretch (if the signal is strong) For symmetrical compounds, the dipole moments cancel out with each other and therefore do not often show on the spectra Signal Characteristics - Shape: OH groups typically show broad peaks If there is steric hindrance for hydrogen bonding, then there won’t really be broad signals 1° Amines will have one peak at about 3300 cm-1 while 2° amines will have two peaks, an extra at 3400 cm-1 (same thing can happen with other molecules, just need the option of symmetric and asymmetric stretches) UV/Vis Spectroscopy: E = hv = hc/λ Increased conjugation means increased absorbed wavelength of light