IR Spectroscopy and Mass Spectrometry: Flashcards
Revise IR absorptions of functional groups, the fingerprint region, molecular ions, fragments and isotope patterns.
IR Spectroscopy and Mass Spectrometry: Flashcards
- QuestionWhat does an IR spectrum show?Answer
Which bonds (functional groups) are present: each bond absorbs IR at a characteristic wavenumber.
- QuestionWhat is a wavenumber?Answer
The number of waves per centimetre, 1/λ, in cm⁻¹. Higher wavenumber = higher energy.
- QuestionWhere does a C=O bond absorb in IR, and what does it look like?Answer
1680–1750 cm⁻¹, strong and sharp.
- QuestionHow do the O–H absorptions of alcohols and carboxylic acids differ?Answer
Alcohol: broad, 3200–3550 cm⁻¹. Acid: very broad, 2500–3300 cm⁻¹.
- QuestionWhat is the fingerprint region?Answer
Below about 1500 cm⁻¹: complex, unique to each compound, used to match a known spectrum.
- QuestionIn a mass spectrum, what is the molecular ion?Answer
M⁺, the peak at the highest m/z (ignoring small isotope peaks); its m/z equals the molar mass.
- QuestionWhat is the base peak?Answer
The tallest peak (100 %), from the most stable fragment.
- QuestionGive the ions for fragments at m/z 15, 29 and 43.Answer
15: CH₃⁺. 29: C₂H₅⁺ or CHO⁺. 43: C₃H₇⁺ or CH₃CO⁺.
- QuestionWhat M : M+2 ratio shows one Cl atom? One Br atom?Answer
Cl: 3 : 1. Br: 1 : 1.
- QuestionWhy do N₂ and O₂ not absorb IR?Answer
Their vibration does not change the dipole moment of the molecule.
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