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Advanced Organic Chemistry — Spectroscopy

NMR, IR, MS, and UV-Vis for structure determination

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nmr_nadia 23 terms Feb 26, 2026
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Terms 23

1
¹H NMR
Proton nuclear magnetic resonance; detects environments of hydrogen atoms in molecule
2
Chemical Shift (δ)
Position of NMR signal in ppm; determined by electron density around proton; deshielded = higher δ
3
TMS (Tetramethylsilane)
Reference standard for NMR; δ = 0 ppm; highly shielded protons
4
Spin-Spin Coupling (J)
Splitting of NMR signal by neighboring nonequivalent protons; n+1 rule
5
N+1 Rule
Signal split into n+1 peaks by n equivalent adjacent protons; doublet = 1 neighbor
6
Coupling Constant (J)
Separation between split peaks in Hz; indicates dihedral angle and distance between coupled nuclei
7
Integration
Area under NMR peak proportional to number of equivalent protons
8
¹³C NMR
Carbon NMR; one peak per unique carbon; no coupling; gives carbon skeleton information
9
DEPT
Distortionless Enhancement by Polarization Transfer; distinguishes CH, CH₂, CH₃ vs quaternary C
10
2D NMR (COSY)
Correlation Spectroscopy; shows which protons are coupled (adjacent); cross-peaks indicate neighbors
11
HMBC/HSQC
HSQC: one-bond C-H correlations; HMBC: two-to-three-bond C-H; connect ¹H and ¹³C spectra
12
IR Spectroscopy
Measures molecular vibrations; identifies functional groups by characteristic absorption frequencies
13
IR — O-H stretch
~3200–3600 cm⁻¹ (broad); alcohol or carboxylic acid; broad due to hydrogen bonding
14
IR — Carbonyl (C=O)
~1700–1800 cm⁻¹; strong absorption; frequency distinguishes ketone, ester, amide, acid
15
IR — N-H stretch
~3300–3500 cm⁻¹; primary amine shows two peaks; secondary shows one
16
Mass Spectrometry
Fragments molecules by ionization; measures m/z ratio; molecular ion (M⁺) gives molecular weight
17
Base Peak
Most abundant fragment in mass spectrum; most stable carbocation or radical
18
Molecular Ion (M⁺)
Peak at actual molecular mass; confirms MW; M+1 and M+2 peaks reveal isotope patterns
19
McLafferty Rearrangement
Mass spec fragmentation of carbonyl compounds with γ-hydrogen; 6-membered transition state
20
UV-Vis Spectroscopy
Measures absorption of UV/visible light; detects conjugated π systems and chromophores
21
Chromophore
Structural feature absorbing UV/Vis light; conjugated systems, aromatic rings, C=O
22
λmax
Wavelength of maximum absorption; increases with conjugation length (bathochromic shift)
23
Beer-Lambert Law
A = εlc; absorbance = molar absorptivity × path length × concentration