N+1 rule: n equivalent neighbors = n+1 peaks (splitting pattern)
NMR Splitting Patterns
A proton with n equivalent neighboring H's splits into n+1 peaks. Singlet(1), doublet(2), triplet(3), quartet(4), quintet(5). Integration tells number of H's. Coupling constant J measures peak separation.
⚗️ Spectroscopy — NMR & IR
M+ = molecular ion peak = molecular weight of compound
Mass Spectrometry Basics
M+ peak = molecular weight. M+1 peak: 13C isotope contribution (~1.1% per carbon). M+2 peak: indicates Cl (M:M+2 = 3:1) or Br (M:M+2 = 1:1). Base peak = most abundant fragment.
⚗️ Spectroscopy — NMR & IR
DEPT NMR: CH3 and CH point up, CH2 points down, C = absent
DEPT NMR
DEPT (Distortionless Enhancement by Polarization Transfer) distinguishes C types: CH3 and CH give positive peaks, CH2 gives negative peaks, quaternary C gives no signal. Used with 13C NMR.
⚗️ Spectroscopy
¹H NMR integration: ratio of peak areas = ratio of number of protons
Using NMR Integration
The area under each NMR peak is proportional to the number of protons responsible for that signal. Integration is reported as a ratio. Example: ethanol (CH₃CH₂OH) shows peaks with ratio 3:2:1 (methyl:methylene:OH). To determine actual number of protons, use molecular formula. Integration + splitting pattern + chemical shift together identify each proton environment. Integration does NOT give absolute numbers — only ratios.
⚗️ Spectroscopy
¹³C NMR: one peak per unique carbon environment — no splitting, no integration
Carbon-13 NMR
¹³C NMR gives one peak per chemically distinct carbon (due to low natural abundance of ¹³C, C-C coupling is negligible). Key differences from ¹H NMR: NO integration (peak heights are not proportional). NO splitting from adjacent carbons (decoupled in routine spectra). Chemical shifts: 0–50 ppm (alkyl), 50–90 ppm (C-O or C-X), 100–150 ppm (alkene/aromatic), 160–220 ppm (carbonyl). Count peaks = count unique carbon environments. Equivalent carbons give ONE peak.
0–50 ppm
Alkyl carbons (sp³)
50–90 ppm
C-O, C-X (alpha to electronegative atom)
100–150 ppm
Alkene or aromatic carbons (sp²)
160–220 ppm
Carbonyl carbons (C=O)
⚗️ Spectroscopy
Chemical shift anisotropy: aromatic and alkene protons deshielded by ring current
Anisotropic Effects in NMR
Not all deshielding is due to electronegativity — ring currents in aromatic compounds create a magnetic field that deshields protons on the periphery → aromatic H appear at 6.5–8.5 ppm (far downfield for C-H). Aldehyde H also deshielded by C=O anisotropy (9–10 ppm). Alkyne H are shielded by the cylindrical pi system → appear upfield (~2–3 ppm) despite being on sp carbon. These are tested on every spectroscopy exam.
⚗️ Spectroscopy
Mass spectrum fragmentation: alpha cleavage occurs next to heteroatom or C=O
Mass Spectrometry Fragmentation Patterns
Common fragmentation patterns: Alpha cleavage: bond adjacent to carbonyl, heteroatom, or radical breaks. Methyl loss: M-15 (loss of CH₃). Loss of water: M-18 (alcohols). Loss of HCl: M-36 (chloro compounds). McLafferty rearrangement: gamma-H transfer → loss of alkene (M - alkene). Base peak = most stable carbocation fragment. Work backwards: M⁺ minus fragment masses = identify structural pieces. Odd molecular ion → nitrogen present (nitrogen rule).
M-15
Loss of CH₃ (methyl)
M-18
Loss of H₂O (alcohol, carboxylic acid)
M-29
Loss of CHO or C₂H₅
M-31
Loss of OCH₃ (methoxy)
Odd M⁺
Contains odd number of nitrogen atoms
McLafferty
Gamma-H transfer → M minus alkene
⚗️ Spectroscopy
UV-Vis: pi → pi* (conjugated systems, ~200–400 nm) and n → pi* (~300–400 nm)
UV-Visible Spectroscopy
UV-Vis measures electronic transitions. pi → pi* transitions: conjugated dienes (~217 nm), extended conjugation shifts to longer wavelength (bathochromic shift). n → pi* transitions: lone pair → pi* (carbonyl ~280 nm, low intensity). More conjugation = longer wavelength absorption = color. Chromophore = light-absorbing group (C=C, C=O, aromatic). Auxochrome = group that shifts absorption (NH₂, OH). Beer-Lambert law: A = εlc. Used to determine conjugation and aromatic systems.
⚗️ Spectroscopy
Solving a structure: DoU first, then MS for MW, then IR, then NMR
Systematic Structure Determination Strategy
Step 1: Calculate DoU from molecular formula (rings + pi bonds). Step 2: Mass spectrum — get molecular weight (M⁺) and fragmentation clues. Step 3: IR — identify functional groups (carbonyl ~1700, OH broad 3300, NH 3100–3500). Step 4: ¹H NMR — count environments (peaks), count H per environment (integration), identify neighbors (splitting), identify type (chemical shift). Step 5: ¹³C NMR or DEPT — confirm carbon types. Step 6: assemble structure consistent with ALL data.
Step 1
DoU = (2C+2+N-H-X)/2 — rings and pi bonds
Step 2
MS: M⁺ = MW, fragmentation clues
Step 3
IR: functional groups (OH, C=O, NH, C≡N)
Step 4
¹H NMR: environments, ratio, neighbors, shift
Step 5
¹³C/DEPT: carbon environments and types
Step 6
Assemble structure consistent with all data
⚗️ Spectroscopy
Equivalent protons give ONE NMR signal — check symmetry before counting peaks
Chemical Equivalence in NMR
Chemically equivalent protons give ONE NMR signal. Two H atoms are equivalent if they can be interchanged by a rotation axis or mirror plane of symmetry. Examples: CH₄ — 1 NMR signal (4 equivalent H). CH₃Cl — 1 signal. CH₂Cl₂ — 1 signal. CH₃CH₃ — 1 signal. ClCH₂CH₂Cl — 1 signal (the two CH₂ are equivalent). Diastereotopic H (adjacent to chiral center, in different environments) are NOT equivalent — give different signals.
⚗️ Spectroscopy
Long-range coupling: allylic (4J) and W-arrangement coupling are small but observable
Long-Range NMR Coupling
Normal vicinal coupling (³J) is between protons on adjacent carbons (~6-8 Hz for free rotation). Long-range coupling (⁴J and beyond): allylic coupling through C=C (~0-3 Hz, small). W-arrangement: 4 sigma bonds in a specific geometry allows 4-bond coupling. Aromatic coupling: ortho (³J ~8 Hz), meta (⁴J ~2 Hz), para (⁵J ~0-1 Hz). Geminal coupling (²J on sp³): between protons on same carbon (~10-16 Hz). Reported as J-value in Hz.
🎓 Common Exam Questions
Q: What are the key NMR chemical shift ranges for common proton environments?
A: A proton with n equivalent neighboring H atoms splits into n+1 peaks. Singlet (1 peak): no neighboring H. Doublet (2): one neighbor. Triplet (3): two neighbors. Quartet (4): three neighbors. This is first-order splitting. The coupling constant J (in Hz) measures the separation between split peaks and is the same for coupled protons. Integration tells you the number of protons.
Q: What are the key IR absorption peaks to memorize?
Q: How do you interpret the molecular ion (M⁺) peak in mass spectrometry?
A: M⁺ = molecular ion = molecular weight of compound. M+1 peak: ~1.1% per carbon (due to ¹³C natural abundance — counts carbons). M+2 peak: Cl gives M:M+2 = 3:1. Br gives M:M+2 = 1:1. Base peak = most abundant fragment (most stable carbocation). Loss of 15 = CH₃. Loss of 29 = CHO or C₂H₅. Loss of 31 = OCH₃.
Q: What does DEPT NMR show and how do you read it?
A: DEPT (Distortionless Enhancement by Polarization Transfer) distinguishes carbon types in ¹³C NMR. CH₃ and CH give positive (upward) peaks. CH₂ gives negative (downward) peaks. Quaternary C (no attached H) gives NO signal — absent from DEPT. Used alongside regular ¹³C NMR to assign peaks. DEPT-135 is most commonly used.
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