Infrared, Mass Spectrometry, and NMR: Key Concepts for Organic Analysis
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47 Terms
What does IR spectroscopy measure?
Absorption of infrared light causing bond vibrations
What determines IR wavenumber?
Bond strength and atomic mass
How does bond strength affect IR frequency?
Stronger bonds absorb at higher wavenumbers
How does atomic mass affect IR frequency?
Heavier atoms absorb at lower wavenumbers
Effect of hydrogen bonding on IR peaks?
Broadens peak and lowers wavenumber
Effect of resonance on carbonyl IR?
Lowers C=O stretching frequency
Where does O-H (alcohol) appear in IR?
3200-3600 cm^-1 (broad)
Where does O-H (carboxylic acid) appear?
2500-3300 cm^-1 (very broad)
Where does N-H appear?
Around 3300 cm^-1 (sharp)
Where does C=O appear?
Around 1700 cm^-1 (strong)
Where does C=C appear?
1600-1680 cm^-1
Where does C≡C or C≡N appear?
2100-2260 cm^-1
Where does sp3 C-H appear?
2850-2960 cm^-1
Where does sp2 C-H appear?
3000-3100 cm^-1
Where does sp C-H appear?
Around 3300 cm^-1 (sharp)
What determines IR peak intensity?
Change in dipole moment
Why is C=O peak strong?
Large dipole change
Why are some peaks weak or absent?
Little or no dipole change
What is Electron Ionization (EI)?
Hard ionization using high-energy electrons to form radical cations
What is formed in EI?
Molecular ion (M+•)
Why does EI cause fragmentation?
Molecular ion is unstable
What is FAB?
Fast atom bombardment, softer ionization method
What is MALDI?
Matrix-assisted laser desorption ionization, very soft ionization
Which ionization gives least fragmentation?
MALDI
What is the base peak?
Most intense peak (100%) representing most stable fragment
What determines fragment stability?
Carbocation stability and resonance
Order of carbocation stability?
Tertiary > secondary > primary
Which fragments are especially stable?
Allylic and benzylic
What is alpha cleavage?
Breaking bond next to functional group
Common neutral losses in MS?
H2O (18), CO (28)
What is NMR spectroscopy?
Absorption of radio waves by nuclei in magnetic field
What does chemical shift indicate?
Electronic environment of protons
What are homotopic protons?
Identical protons, same signal
What are enantiotopic protons?
Mirror-related, same in achiral environment
What are diastereotopic protons?
Non-equivalent, different signals
Where do alkyl protons appear?
0-2 ppm
Where do allylic/benzylic protons appear?
2-3 ppm
Where do protons near electronegative atoms appear?
3-4 ppm
Where do alkene protons appear?
5-6 ppm
Where do aromatic protons appear?
6-8 ppm
Where do aldehyde protons appear?
9-10 ppm
Where do carboxylic acid protons appear?
10-12 ppm (broad)
What causes downfield shift?
Deshielding (electronegative atoms, pi systems)
What causes upfield shift?
Shielding (electron density)
What is the n+1 rule?
Number of neighboring protons + 1 determines splitting
What does integration show?
Number of protons
What is a coupling constant (J)?
Distance between split peaks indicating interaction
