Ch. 11: Spectroscopy Flashcards
Infrared (IR) Spectroscopy
Measures absorption of infrared light, which causes molecular vibration (stretching, bending, twisting, and folding)
Plotting IR spectra
Percent transmittance vs wavenumber (1/λ). The normal range is 4000 to 400 cm-1
Fingerprint region of IR spectra plot
Between 1500 and 400 cm-1. Contains a number of peaks that can be used by experts to identify a compound
How does a bond 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.
O-H peak
Broad peak around 3300 cm-1. Molecules w O-H include alcohols, water, and carboxylic acids; the carboxylic acid O-H peak will be shifted around 3000 cm-1
N-H peak
Sharp peak around 3300 cm-1. Molecules w N-H include some amines, imines, and amides
C=O peak
Sharp peak around 1750 cm-1. Molecules w C=O include aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides
Ultraviolet Spectroscopy
Measures absorption of ultraviolet light, which causes movement of electrons between molecular orbitals
UV spectra plotting
Percent transmission or absorbance vs wavelength
How does a molecule appear on a UV spectrum?
To appear on a UV spectrum, a molecule must have a small enough energy diff between its highest occupied molecular orbital and its lowest unoccupied molecular orbital to permit an electron to move from one orbital to the other. Smaller the diff between the HOMO and LUMO the longer the wavelengths a molecule can absorb
Conjugation
Occurs in molecules w unhybridized p orbitals. Conjugation shifts the absorption spectrum to higher max wavelengths (lower frequencies)
Nuclear Magnetic Resonance Spectroscopy
Measure the alignment of nuclear spin w an applied magnetic field, which depends on the magnetic environment of the nucleus itself. Useful for determinging the structure (connectivity) of a compoud, including functional groups. Nuclei may be in the lower energy alpha state or higher energy beta state; radiofrequency pulses push the nucleus from the alpha state to the beta state and these frequencies can be measured
MRI
Magnetic resonance imaging Medical app of NMR spectroscopy
NMR spectra plotting
Frequency vs absorption of energy. They are standardized by using chemical shift, measured in parts per million (ppm) of a spectrophotometer frequency
NMR spectra calibration
Tetramethylsilane (TMS) is used to calibrate NMR spectra. It has a chemical shift of 0 ppm
Chemical shifts
Higher chemical shifts are located to the left (downfield); lower chemical shifts are located to the right (upfield)
Most common NMR
Proton (1H) NMR
Each unique group of protons…
Has its own peak
Integration
The integration of this peak (area under the curve) 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.
Spin-Spin Coupling (splitting)
When hydrogens are on adjacent atoms, they interfere w each other’s magnetic environment, causing spin-spin coupling. A proton’s (or group of protons’) peak is split into n+1 subpeaks, where n is the number of protons that are 3 bonds away from the proton of interest
Splitting patterns
Include doublets, triplets, and multiplets
Where do protons on diff hybridized carbons fall?
Sp3: 0-3 ppm
Sp2: 4.6-6.0 ppm
Sp: 2.0-3.0 ppm
Where do protons on diff compounds’ hydrogens fall?
Aldehydic hydrogens: 9-10 ppm
Carboxylic acid hydrogens: 10.5-12 ppm
Aromatic hydrogens: 6.0-8.5 ppm
