CA 3- IR+NMR Spectroscopy Flashcards
(34 cards)
True or False: Molecule’s component parts oscillate in different vibrational modes
True
IR Spectroscopy
How does IR spectroscopy work?
- The IR spectrometer exposes a sample to IR radiation (region- wavelength 2.5-40 um)
- Different bonds (vibrations) in a molecule absorb IR at different frequencies/wavelengths
- Absorbance/Transmittance is measured as a function of frequency/wavelength
IR Spectroscopy
Absorbance/Transmittance is measured as a function of what?
frequency/wavelength
IR Spectroscopy
What do vibrational frequencies depend on?
- Masses of atoms
- Bond stiffness (strength)- bond length
IR Spectroscopy
What is IR absorption accompanied with during vibration?
change in dipole moment
IR Spectroscopy
What does the intensity of bands depend on?
the magnitude of the dipole moment associated with the bond interacting with IR radiation
IR Spectroscopy
What are the options for peak shape? (Bands descriptions)
- Sharp/narrow (V shape)
- Broad band (U shape)
IR Spectroscopy
What are the options for relative intensity?
- strong (s)
- medium (m)
- weak (w)
IR Spectroscopy
What percent of y-axis is covered for a strong band?
60% of the y-axis or more
IR spectroscopy
What percent of y-axis is covered for a medium band?
40-50%
IR spectroscopy
What percent of y-axis is covered for a weak band?
around 20% of the y-axis, or less
NMR Spectroscopy
How does NMR spectroscopy work?
- Nuclei with magnetic properties, such as 1H, 13C, etc. interact with radiofrequency (Rf) radiations to produce signal
- Depending on their electronic environment, the same kind of nucleus can have different resonance frequencies (chemical shifts)
ex. H-NMR chemical shifts depend on what else is attached to the carbon bonded to the signal-producing proton
NMR Spectroscopy
What produces the signal in NMR spectroscopy?
Nuclei with magnetic properties interact with radiofrequency radiations, which produces the signal
NMR Spectroscopy
Can the same kind of nucleus have different resonance frequencies (chemical shifts)? Why?
Yes, due to differences in electronic environment
NMR Spectroscopy
What does chemical shift tell us?
the electronic environment around the proton(s) that are producing the signal
NMR Spectroscopy
What is signal intensity and what does it tell us?
- The area under each peak/integration
- Tells us the relative ratios of the different kinds of protons
NMR Spectroscopy
What does a signal’s multiplicity (peak spliting) tell us?
Tells us the number of neighboring protons to the one producing the signal (n+1 rule)
NMR Spectroscopy
What does the number of signals tell us?
How many kinds of inequivalent protons/sets of protons there are in a molecule
NMR Spectroscopy
What is important about equivalent protons?
- equivalent protons in a molecule have the same electronic/chemical environment
- Chemically equivalent protons resonate at the same frequency
NMR Spectroscopy
What happens between non-equivalent protons (protons in different electronic/chemical environments)?
Coupling or peak-splitting
What are IR and NMR spectroscopy valuale tools for?
- structure elucidation
- characterization of organic compounds
What does an IR spectrum provide information about?
Unique bonds/functional groups
Why does an IR spectroscopy provide infomration about unique bonds/functional groups?
because the bands (peaks) on the spectrum arise from vibrational motions of specific chemical bonds which have characteristic absorption frequencies
What spectrum is characteristic of a carboxylic acid?
- a strong, broad absorption band at 2800-3000 cm (O-H bond)
- a strong, narrow (sharp) band at 1715-1720 (C=O bond)
- a couple of strong, narrow bands in the region of 1000-1300 (C-O bond)
