Spectroscopy and Structure Determination: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry II Topic 8, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
Calculated from the molecular formula, this count fixes the total number of rings and pi bonds before any spectrum is examined.
Infrared spectroscopy
Detects bond vibrations, so it identifies which functional groups are present without describing the carbon skeleton around them.
Carbonyl stretch
A strong sharp absorption near 1700 wavenumbers whose exact position separates acid derivatives, with conjugation lowering it and ring strain raising it.
Hydroxyl stretch
A broad absorption above 3200 wavenumbers, broadened by hydrogen bonding. In a carboxylic acid it broadens further and overlaps the carbon-hydrogen region.
Nitrile and alkyne stretch
A weak sharp absorption near 2200 wavenumbers, one of the few signals in an otherwise empty region of the spectrum.
Fingerprint region
The crowded low-wavenumber region that is unique to a compound. It is useful for matching against a reference but rarely for assigning individual bonds.
Mass spectrometry
Measures mass-to-charge ratios of ions produced from a sample, giving molecular mass and, from the fragments, structural clues.
Molecular ion peak
The signal from the intact ionized molecule, giving the molecular mass. It is weak or absent when the molecule fragments very easily.
Nitrogen rule
An odd molecular mass implies an odd number of nitrogen atoms, which is a fast first check on a proposed formula.
Isotope pattern
Chlorine and bromine give characteristic companion peaks two mass units apart, so their presence and count can be read directly.
Alpha cleavage
Fragmentation next to a heteroatom, producing a stabilized cation that reveals the substituents around that atom.
McLafferty rearrangement
A carbonyl compound with a suitably placed hydrogen loses a neutral alkene through a cyclic transition state, giving a diagnostic even-mass fragment.
Proton nuclear magnetic resonance
Reports the number of distinct hydrogen environments, how many hydrogens are in each, and which environments are adjacent.
Chemical shift
The position of a signal relative to a reference, set by how much electron density shields the nucleus. Nearby electronegative atoms move signals downfield.
Integration
The relative area of each signal, giving the ratio of hydrogens in each environment rather than their absolute number.
Spin-spin splitting
Neighbouring hydrogens split a signal into a predictable pattern, so the number of lines reveals how many hydrogens sit on adjacent carbons.
Coupling constant
The spacing between split lines, measured in hertz and independent of instrument strength. Matching values confirm which signals are coupled to each other.
Equivalent hydrogens
Hydrogens related by symmetry give a single signal. Recognizing symmetry is usually the fastest way to narrow a set of candidate structures.
Exchangeable proton
A hydrogen on oxygen or nitrogen that appears broad and unsplit and disappears when the sample is shaken with heavy water.
Aromatic ring current
Circulating pi electrons deshield ring hydrogens, pushing their signals far downfield and providing a reliable marker for aromaticity.
Carbon nuclear magnetic resonance
Counts distinct carbon environments. Signals are usually shown without splitting from hydrogens, so each carbon environment appears as one line.
Distortionless enhancement experiment
Edits a carbon spectrum so that signals point up or down according to how many hydrogens each carbon carries, distinguishing carbon types directly.
Two-dimensional correlation spectrum
Plots couplings as cross peaks, so connectivity between environments can be traced through the skeleton rather than inferred.
Nuclear Overhauser effect
An intensity change between nuclei that are close in space rather than through bonds, which is how stereochemistry and conformation are assigned.
Combined structure determination
The working method: get the formula from mass data, functional groups from infrared, environments and counts from resonance, then assemble one consistent structure.