Unit 13: Mass Spectrometry, IR, and NMR Spectroscopy
How chemists know what they made: a fragmented molecular ion, a functional group's fingerprint, and a proton's neighbors counted from its own peak pattern.
9 topics · 33 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Mass Spectrometry and the Molecular Ion
- How it worksHow does knocking one electron out of a molecule generate the molecular ion that mass spec actually detects?
- Worked exampleFind the molecular weight of a compound whose molecular ion peak appears at m/z 86 in its mass spectrum.
- Common mistakeWhy isn't the molecular ion peak always the tallest peak in a mass spectrum?
Your turn1 of 2
A compound's molecular ion peak appears at m/z 221 in its mass spectrum, formed by knocking one electron out of the neutral molecule to give a single positively charged radical cation. What is the compound's molecular weight?Fragmentation Patterns in Mass Spectrometry
- How it worksHow does a molecular ion break apart preferentially at its weakest bonds to form stable cationic fragments?
- Worked exampleFind the mass of the fragment lost when a molecular ion at m/z 100 produces a base peak at m/z 57.
- Common mistakeWhy doesn't every bond in a molecule break with equal probability once fragmentation begins?
- Going furtherWhat happens to a fragmentation pattern when the molecular ion can lose a particularly stable fragment like a methyl radical?
Your turn1 of 2
A molecular ion appears at m/z 198 and its base peak, the major fragment, appears at m/z 58. What is the mass of the neutral fragment lost in forming that base peak?IR Spectroscopy and Characteristic Absorptions
- How it worksHow does a bond's vibration absorb infrared light only at the specific frequency matching its own stretching motion?
- Worked exampleFind the functional group responsible for a strong broad absorption near 3300 wavenumbers alongside a sharp peak near 1710.
- Common mistakeWhy doesn't every peak in an IR spectrum correspond to a separate functional group in the molecule?
- Going furtherWhat happens to a carbonyl's stretching frequency when it moves from an open ketone into a strained four-membered ring?
Your turn1 of 2
An IR spectrum shows a strong, broad absorption near 3300 wavenumbers along with a sharp, strong peak near 1710 wavenumbers. What functional group is responsible for both together?1H NMR Chemical Shift
- How it worksHow does nearby electron density shield a proton from the magnetic field and shift its signal upfield?
- Worked exampleFind which proton signal, near 2.1 ppm or near 9.7 ppm, belongs to an aldehyde hydrogen rather than a methyl beside a ketone.
- Common mistakeWhy doesn't a proton's chemical shift depend on how many other identical protons happen to sit on the same molecule?
Your turn1 of 2
Which proton signal, near 2.1 ppm or near 9.7 ppm, belongs to an aldehyde hydrogen rather than a methyl group beside a ketone?1H NMR Integration
- How it worksHow does the number of protons producing a signal directly set the integrated area underneath that peak?
- Worked exampleFind the ratio of protons behind three NMR signals that integrate to 3, 2, and 1 in a molecule with six total protons.
- Going furtherWhat happens to an integration ratio when a molecule has a plane of symmetry that makes two sets of protons equivalent?
Your turn1 of 2
An NMR integration trace rises by 38 units for one signal out of a total rise of 118 units summed across the whole spectrum. If the compound's molecular formula shows 19 total protons, how many of those protons does this one signal represent?Spin-Spin Splitting and the n+1 Rule
- How it worksHow does each neighboring proton's spin, up or down, add a tiny magnetic nudge that splits a signal into a doublet?
- How it worksHow does stacking up the up/down combinations from n neighboring protons build exactly n+1 peaks in a set ratio?
- Worked exampleFind the splitting pattern and peak ratio expected for a proton with three equivalent neighboring protons.
- Common mistakeWhy doesn't a proton ever split its own signal, only the protons sitting next to it?
- Going furtherWhat happens to a splitting pattern when a proton has two different kinds of neighbors instead of just one?
Your turn1 of 2
A proton has 5 equivalent neighboring protons on an adjacent carbon. According to the n+1 rule, into how many peaks does this proton's signal split?Complex Splitting Patterns
- How it worksHow do two different coupling constants combine to split a single proton's signal into a doublet of doublets?
- Worked exampleFind the number of peaks expected for a proton coupled to two neighboring protons with different coupling constants.
- Going furtherWhat happens to a splitting pattern when the coupling constants to two different neighbors happen to be equal?
Your turn1 of 2
A proton is coupled to 2 neighboring proton(s) with one coupling constant and 2 neighboring proton(s) with a different coupling constant. Assuming the two coupling constants are different enough that no peaks overlap, how many total peaks appear in this proton's signal?13C NMR Spectroscopy
- How it worksHow does proton decoupling erase the coupling between carbon and its attached hydrogens during the experiment?
- Worked exampleFind the number of distinct 13C signals expected for the symmetric molecule para-xylene.
- Common mistakeWhy doesn't the height of a 13C NMR peak reliably tell you how many carbons are producing that one signal?
- Ties togetherHow does combining the carbon count from 13C NMR with the proton count from 1H NMR narrow a structure down further?
Your turn1 of 2
Para-xylene (1,4-dimethylbenzene) is a highly symmetric molecule. How many distinct 13C NMR signals does it show?Combining Spectra to Solve a Structure
- How it worksHow does combining a formula from mass spec, functional groups from IR, and connectivity from NMR triangulate one structure?
- Worked exampleFind the structure of a compound with formula C4H8O2, an IR peak at 1740, and 1H NMR signals at 4.1, 2.0, and 1.3 ppm.
- Worked exampleFind the structure of a compound with molecular ion at m/z 72, no IR carbonyl peak, and one 1H NMR signal at 0.9 ppm.
- Ties togetherHow does this structure-solving process pull together every spectroscopic tool from this entire unit into one workflow?
Your turn1 of 2
A compound has molecular formula C4H8O2, one degree of unsaturation, an IR peak at 1740 wavenumbers, and 1H NMR signals at 4.1, 2.0, and 1.3 ppm. What is the structure?