Spectroscopy ** Flashcards
(25 cards)
IR spectroscopy
measure absorption of infrared light., which causes molecular vibration (stretching, twisting , and folding).
IR spectra are generally plotted as
percent transmittance versus wavenumber . (1/lambda). Te normal range of spectrum is 4000 to 400 cm^-1. Then fingerprint region is between 1500 and 400cm^-1.
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 particular functional groups
IR O-H peak is
a broad peak around 3300. Molecules with OH include alcohols, water, and carboxylic acids; the carboxylic acids OH peak will be sifted around 3000.
N-H IR peak is
sharp peak around 3300. Molecules with NH includes amines, imines, and amides
C=O peak is
sharp peak around 1750. This includes aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides.
UV spectroscopy measures
absorption of ultraviolet light, which causes movement of electrons between molecular orbitals.
UV spectra are
generally plotted as percent transmittance or absorbance vs wavelength.
To appear in 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
occurs in molecules with unhybridized p-orbitals. Conjugation shifts the absorption spectrum to higher max wavelength (lower frequencies)
Nuclear magnetic resonance (NMR) spectroscopy
measure alignment 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 alpha state or higher energy beta state; radio frequency pulses push the nucleus from the alpha state to the beta state, and these frequencies can be measured.
Magnetic resonance imaging is a
medical application of NMR spectroscopy.
NMR spectra are calibrated using
tetramethylsilane (TMS), which has a chemical shift of 0 ppm.
higher shifts are located
to the left (downfield); lower chemical shifts are located on the right (upfield)
Proton (H) NMR
most common. Each unque gropus of proton has its own peak.
Desheilding
Deshielding of protons occurs when electron withdrawing groups pull electron density away form the nucleus, allowing i to be more easily affected by a magnetic field. Deshielding moves peaks further downfield.
integration
The integration (area under the curve) of this peak is proportional to the number of protons contained under the peak.
Spin to spin coupling (splitting)
when hydrogens are on adjacent atoms, they interfere with each other’s magnetic environment. A proton/protons peak is split into n + 1 subpeaks, where n is the number of protons that are three bonds away from the proton of interest.
Splitting patterns
doublets, triplets, and multiplets
protons on sp3 hybridized carbons are usually in the
0 to 3 ppm range (but higher if electron if electron withdrawing groups are present). protons on sp2 hybridized carbons are usually in the 4.6 to 6.0 ppm range. Protons on sp-hybridized carbons are usually 2 to 3 ppm.
Aldehydic hydrogens tend to
appear between 9 and 10 ppm.
Carboxylic acids hydrogens tend to appear
between 10.5 and 12 ppm
