Flashcards in Organic Chemistry Chapter 11: Spectroscopy Deck (26)
Infared spectroscopy measures
absorption of infared light, which causes molecular vibration (stretching, bending, twisting and folding)
IR spectra are generally plotted as:
percent transmittance vs. wavenumber (1/wavelength)
The normal range of spectrum is:
4000 to 400 cm-1
What is the fingerprint region?
between 1500 and 400 cm-1. It contains a number of peaks that can be used by experts to identify a compound.
What needs to happen 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.
A broad peak around 3300 cm-1
The caboxylic acid O-H peak will be around 3000 cm-1
a sharp peak around 3300 cm-1
sharp peak around 1750 cm-1
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 on a UV spectrum, a molecule must have a small enough energy difference between:
highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) to permit an electron to move from one orbital to another.
The smaller the difference between HOMO and LUMO --
The longer the wavelengths a molecule can absorb.
Conjugation occurs in:
molecules with unhybridized p-orbitals. Conjugation shifts the absorption spectrum to higher maximum wavelengths (lower frequencies)
Nuclear magnetic resonance spectroscopy
measures alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself. It is useful for identifying the structure of a compound.
What are the two states of nuclei?
lower energy alpha-state or higher energy beta-state. radiofrequency pylses push the nucleus from the alpha-state to the beta-state and these frequencies can be measured.
NMR is generally plotted as
frequency versus absorption of energy. Standardized using chemical shift, measured in parts per million of spectrophotometer frequency.
NMR spectra are calibrated using
tetramethylsilane (TMS), which has a chemical shift of 0 ppm
Integration area under curve
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.
Spin-spin coupling (splitting)
when hydrogens are on adjacent atoms, they interfere with each other's magnetic environment - peak is split into n+ 1 subpeaks, where n is the number of protons that are three bonds away from the proton of interest.
Protons of on sp3 hybridized carbons appear
usually in the 0 to 3 ppm range
sp2 hybridized protons appear
4.6 to 6.0 ppm range
sp hybridized protons appear
2.0 to 3.0 ppm range
aldehydic hydrogens appear
9 to 10 ppm range
CA hydrogens appear
10.5 and 12 ppm range