Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
9 Cards in this Set
- Front
- Back
|
Infrared (IR) spectroscopy |
*measures absorption of infrared light, which causes molecular vibration (stretching, bending, twisting, and folding) *to appear on an IR spectrum, vibration of a bond must change the bond dipole moment. Certain bonds have characteristic absorption frequencies, which allow us to infer the presence (or absence) of a particular functional group *generally plotted as percent transmittance vs. wavenumber (1/λ) *normall range of a spectrum is 4000 to 400 cm-1 *fingerprint region is between 1500 and 400cm-1 (it contains a number of peaks that can be used by experts to identify a compound) |
|
|
Characteristic absorption frequencies in IR spectroscopy |
1) The O-H peak is a broad peak around 3300cm-1. (the carboxylic acid O-H peak will be shifted around 3000 cm-1) 2) The N-H peak is a sharp peak around 3300cm-1 3) The C=O peak is a sharp peak around 1750cm-1 |
|
|
Ultraviolet (UV) spectroscopy |
*measures absorption of ultraviolet light, which causes movement of electrons between molecular orbitals *to appear on a UV spectrum, a molecule must have a small enough energy difference between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) to permit an electron to move from one orbital to the other *The smaller the difference between HOMO and LUMO, the longer the wavelengths a molecule can absorb *conjugation occurs in molecules with unhybridizes p-orbitals. Conjugation shifts the absorption spectrum to higher maximum wavelengths (lower frequencies) *UV spectra are generally plotted as percent transmittance or absorbance vs. wavelength |
|
|
Nuclear magnetic resonance (NMR) spectroscopy |
*measures alignments of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself. It is useful for determining the structure (connectivity) of a compound, including functional groups *nuclei may be in the lower-energy α-state or higher-energy β-state; radio frequency pulses push the nucleus from the α-state to the β-state, and these frequencies can be measured *magnetic resonance imaging (MRI) is a medical application of NMR spectroscopy *generally plotted as frequency vs. absorption of energy. They are standardized by using chemical (shift), measured in parts per million (ppm) of spectrophotometer frequency *NMR spectra are calibrated using tetramethylsilane (TMS), which has a chemical shift of 0 ppm *Higher chemical shifts are located to the left (downfield); lower chemical shifts are located to the right (upfield) *proton (1H) NMR is most common |
|
|
Proton (1H) NMR |
*Each unique group of protons has its own peak *The integration (Area under the curve) of this peak is proportional to the number of protons contained under the peak *deshielding of protons occurs when electron-withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by the magnetic field. Deshielding moves a peak further downfield *When hydrogens are on adjacent atoms, they interfere with each other's magnetic environment, causing spin-spin coupling (Splitting). A proton's (or group of protons') peak is split into n+1 sub peaks, where n is the number of protons that are three bonds away from the proton of interest *splitting patterns include doublets, triplets, and multiplets *protons on sp3-hybridized carbons are usually 0-3 ppm (but higher if electron-withdrawing groups are present). Protons on sp2-hybridized carbons are usually 4.6-6.0 ppm. Protons on sp-hybridized carbons are usually in the 2.0-3.0 *aldehydic hydrogens appear 9-10ppm *carboxylic acid hydrogens appear 10.5-12ppm *aromatic hydrogens appear 6-8.5ppm |
|
|
Integration in NMR |
The integration (area under the curve) of this peak is proportional to the number of protons contained under the peak |
|
|
Deshielding |
Deshielding of protons occurs when electron-withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by the magnetic field. Deshielding moves a peak further downfield |
|
|
spin-spin coupling (splitting) |
When hydrogens are on adjacent atoms, they interfere with each other's magnetic environment, causing spin-spin coupling (splitting). A proton's (or group of protons') peak is split into n+1 sub peaks, where n is the number of protons that are three bonds away from the proton of interest |
|
|
Characteristic NMR peak ranges |
*protons on sp3-hybridized carbons are usually 0-3 ppm (but higher if electron-withdrawing groups are present). Protons on sp2-hybridized carbons are usually 4.6-6.0 ppm. Protons on sp-hybridized carbons are usually in the 2.0-3.0 *aldehydic hydrogens appear 9-10ppm *carboxylic acid hydrogens appear 10.5-12ppm *aromatic hydrogens appear 6-8.5ppm |
