Measures molecular vibrations which can be seen as bond stretching, bending or combinations of different vibrational modes.
Intramolecular Vibrations and Rotations
Infrared light range has a wavelength range of: 700 nm to 1 mm
Useful absorptions occur at wavelengths between 2500 to 25,000 nm.
Wavenumber: like the frequency but in units of cm-1.
Wavenumber = 1/λ
When light with wavenumbers between 4000 to 400 cm-1 is absorbed, the molecules enter an excited vibrational states.
Four types can occur: Symmetric bend, asymmetric bend, symmetric stretch, asymmetric stretch
Fingerprint Region: wavenumbers in range of 1500 to 400 cm-1 where complex vibration patters occur. Patterns are associated with the motion of the molecule as a whole.
For the absorption to be recorded, the vibration must result in a change in the bond dipole moment.
Molecules with same electronegativity (O2 & Br2) are not useful, neither are symmetric molecules like C2H2.
Can be used to identify the functional groups. Most important peaks to know are:
Hydroxyl (O-H): absorbs wide peak at 3300 cm-1 for alcohols and 3000 cm-1 for carboxylic acids.
Carbonyl (C double bonded to O): sharp peak at 1700 cm-1
N-H Bonds: have a sharp peak at 3300 cm-1
Bond between any atom and hydrogen always has a high absorption frequency
As more bonds are added between carbon atoms, the absorption frequency increases
All frequencies in the fingerprint region are out of scope.
Transmittance: amount of light that passes through the sample and reaches the detector
IR spectra is plotted as Transmittance vs wavenumber
UV spectra are obtained by passing ultraviolet light through a sample that is usually dissolved in an inert, non-absorbing solvent, and absorbance is caused by the electron transitions between orbitals.
Most important information gathered: wavelength of the maximum absorbance
Can tell us the extent of conjugation within the system
As conjugation increases, the energy lowers and thus the wavelength increases.
UV spectroscopy works because molecules with pi-bonds or nonbonding electrons can be excited by UV light to high energy orbitals
Molecules with a lower gap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) are more easily excited and can thus absorb longer wavelengths.
Conjugated molecules are molecules with unhybridized p-orbitals
These can be excited by UV light
Conjugation causes a shift in the absorption spectrum that results in higher maximum wavelengths.
Nuclear Magnetic Resonance Spectroscopy
NMR: the most important spectroscopic technique on the MCAT
Certain atomic nuclei have magnetic moments that are oriented at random, but when placed in a magnetic field, the moments of these nuclei tend to align with the field or opposite to the field.
Nuclei with magnetic moments that are aligned with the field are in the alpha state
From the alpha state, radiofrequency pulses can be radiated onto the nuclei to excite the lower energy nuclei into the Beta-state.
Absorption of radiation leads to excitation at different frequencies
MRI: a non-invasive diagnostic tool that uses proton NMR
NMR Spectrum: plot of frequency vs absorption of energy
Standardized method uses a chemical shift (d) with units of ppm of spectrometer frequency.
This is plotted on the x-axis and increases to the left (downfield)
Tetramethylsilane (TMS) is used as a calibration standard to mark the location of 0 ppm
Can be conducted on any atom which possesses a nuclear spin (odd mass number or odd atomic number or both)
Proton NMR (1H-NMR)
Most hydrogens come into resonance from 0 to 10 ppm downfield from TMS
Protons that are chemically equivalent will have the same magnetic environment and will thus correspond to the same peak.
Height of each peak is proportional to the number of protons.
Deshielding: the more a protons electron density is pulled away (by more electronegative elements), the less it can shield itself from the applied magnetic field.
Spin-Spin Coupling (splitting): when two protons are in close proximity but are not magnetically identical.
Results in a doublet being formed: two peaks of identical intensity, equally spaced around the true chemical shift of a proton.
n+1 Rule: if a proton has n protons that are three bonds away, it will split into n+1 peaks
Do not include protons attached to oxygen or nitrogen
Magnitude of splitting is called the coupling constant.
Chemical Shift ranges:
Deshielded aldehyde: 9-10 ppm
Carboxylic acid:10.5-12 ppm
Hydrogen on Aromatic Ring: 6.0-8.5 ppm
Sp3Hybridized carbons: 0-3 ppm
Sp2Hybridized carbons: 4.6-6.0 ppm
Sp Hybridized carbons: 2.0-3.0 ppm
When electronegative groups are present, they pull electron density away from the proton and further deshield it.