This cheat sheet covers the main tools chemists use to identify unknown organic compounds: IR spectroscopy, NMR spectroscopy, mass spectrometry, and UV-Vis spectroscopy. Students need these methods to connect molecular structure with experimental evidence. It is designed as a quick reference for recognizing functional groups, interpreting spectra, and combining clues from multiple instruments.
IR spectroscopy identifies bonds by their absorption frequencies, while NMR shows the chemical environments of hydrogen or carbon atoms. Mass spectrometry gives molecular mass and fragmentation clues, and UV-Vis measures electronic transitions in conjugated systems. The most important strategy is to use each spectrum for the information it gives best, then check that all evidence supports one structure.
Key Facts
- IR absorption wavenumber is related to wavelength by , where is usually measured in .
- Broad stretching usually appears near , while carboxylic acid can be very broad near .
- A strong carbonyl absorption usually appears near , making it one of the most useful IR signals.
- In proton NMR, chemical shift is calculated by and is reported in parts per million, or .
- The rule predicts that a proton signal with equivalent neighboring protons is split into peaks.
- NMR integration gives the relative number of protons producing a signal, so an integration ratio of represents relative proton counts.
- In mass spectrometry, the molecular ion peak gives the molecular mass when the molecule remains intact after losing one electron.
- In UV-Vis spectroscopy, absorbance follows Beer-Lambert law, , where is absorbance, is molar absorptivity, is path length, and is concentration.
Vocabulary
- Wavenumber
- Wavenumber is the number of wave cycles per centimeter and is commonly used as the x-axis unit in IR spectra.
- Chemical Shift
- Chemical shift is the position of an NMR signal relative to a reference standard, measured in .
- Integration
- Integration is the area under an NMR signal and shows the relative number of nuclei producing that signal.
- Molecular Ion
- The molecular ion is the charged form of the entire molecule in mass spectrometry, often written as .
- Base Peak
- The base peak is the tallest peak in a mass spectrum and is assigned a relative intensity of .
- Chromophore
- A chromophore is a part of a molecule that absorbs UV or visible light because of electronic transitions.
Common Mistakes to Avoid
- Treating every IR peak as equally important is wrong because the diagnostic region and strong functional group absorptions usually give the clearest structural clues.
- Confusing NMR integration with peak height is wrong because integration depends on signal area, not how tall the peak appears on the spectrum.
- Applying the rule to non-equivalent neighboring protons is wrong because splitting only comes from nearby equivalent protons under typical introductory conditions.
- Assuming the base peak is always the molecular ion is wrong because the base peak is simply the most abundant fragment, while may be weak or absent.
- Using one spectrum alone to prove a full structure is risky because IR, NMR, mass spectrometry, and UV-Vis each provide different types of evidence.
Practice Questions
- 1 An IR spectrum shows a strong absorption at and no broad peak above . What functional group is strongly suggested?
- 2 A proton NMR signal has equivalent neighboring protons. How many peaks should appear according to the rule?
- 3 A UV-Vis sample has , , and . Calculate using .
- 4 Why is it useful to combine IR, NMR, and mass spectrometry instead of relying on only one spectrum when identifying an unknown compound?
Understanding Spectroscopy (IR, NMR, Mass Spec)
Each method begins with an interaction between energy and matter. In IR, bonds vibrate by stretching or bending. A bond absorbs infrared energy only when its vibration changes the molecule's dipole.
This is why some bonds give strong peaks while symmetrical bonds can be weak or absent. Peak position depends mainly on bond strength and the masses of the bonded atoms. Stronger bonds vibrate at higher wavenumbers.
Hydrogen bonding changes the shape of an O-H signal because molecules pull on one another. The region below about fifteen hundred inverse centimetres contains many coupled vibrations. It is called the fingerprint region because the full pattern can help distinguish similar compounds, even when single peaks are hard to assign.
NMR depends on tiny magnetic properties of certain nuclei. In a strong magnetic field, hydrogen nuclei can absorb radiofrequency energy. Electrons around each hydrogen partly shield it from the field.
A hydrogen near oxygen, nitrogen, a halogen, or a double bond is usually less shielded, so its signal appears further downfield. Signals from chemically equivalent hydrogens merge into one environment. Hydrogens that can be exchanged, such as those in O-H or N-H groups, may appear broad and may not show normal splitting.
Their position can move when the solvent, concentration, temperature, or amount of water changes. For this reason, students should treat these signals as supporting evidence rather than exact proof.
Splitting reveals which hydrogen groups are close enough to influence each other through bonds. The spacing between lines in a multiplet is called the coupling constant. Coupled signals share the same spacing, which helps connect parts of a structure.
The simple neighbour rule works best for first-year examples with one set of equivalent neighbours. Real spectra can be more complicated when neighbouring hydrogens are not equivalent or when signals overlap. Integration measures peak area, not peak height.
A tall narrow peak can represent fewer hydrogens than a broad peak with greater area. Carbon thirteen NMR gives a separate count of carbon environments, though ordinary carbon spectra do not give reliable carbon ratios because different carbons respond differently.
Mass spectrometry first turns molecules into ions, then separates those ions by mass-to-charge ratio. Fragment peaks are not random damage. Common fragments form when breaking a bond leaves a relatively stable positive ion, such as one next to a carbonyl or within a branched carbon chain.
Isotope patterns provide another useful check. Chlorine commonly gives a pair of peaks two mass units apart with a larger lower-mass peak. Bromine gives a similar pair with nearly equal heights.
When solving an unknown, start by listing firm observations rather than naming a compound immediately. Use molecular mass to limit possible formulas, use isotope evidence to spot certain elements, use IR to rule functional groups in or out, then test whether NMR environments, ratios, and neighbour patterns fit one structure. A correct answer must explain every major signal, not only the convenient ones.