Organic Spectroscopy & Structure Elucidation
Complete M.Sc. Organic Chemistry masterclass covering UV-Vis Woodward-Fieser Rules, FT-IR Carbonyl Frequencies, 1H-NMR Anisotropy & Karplus Equation, 13C-NMR DEPT Decoupling, and Mass Spectrometric McLafferty Rearrangements for CSIR NET & GATE Chemical Science.
- UV-Visible Spectroscopy: Electronic Transitions & Woodward-Fieser Rules
- FT-IR Spectroscopy: Molecular Vibrations & Carbonyl Diagnostic Trends
- 1H-NMR Diamagnetic Anisotropy & The Karplus Dihedral Curve (SVG)
- Mass Spectrometric McLafferty Rearrangement & DEPT Subspectral Matrix (SVG)
- 13C-NMR Multiplet Editing (DEPT-135/90) & 2D-NMR Correlations (COSY, HSQC, HMBC)
- Mass Spectrometry: Isotope Clusters, α-Cleavage & Fragmentation Rules
- High-Yield CSIR NET & GATE Chemical Science Solved Spectral Problems
1. UV-Visible Spectroscopy: Electronic Transitions & Woodward-Fieser Rules
UV-Visible spectroscopy measures the absorption of electromagnetic radiation (200–800 nm) causing electronic transitions between bonding/non-bonding orbitals (σ, π, n) and antibonding orbitals (π*, σ*). Conjugation dramatically lowers the energy gap (ΔE = hν) between the HOMO and LUMO, inducing a bathochromic shift (red shift, λmax increases) accompanied by a hyperchromic effect (εmax increases).
| Electronic Transition | Typical Wavelength (λmax) | Molar Absorptivity (εmax) | Selection Rule & Characteristic Chromophore |
|---|---|---|---|
| σ → σ* | < 150 nm (Vacuum UV) | Moderate | Symmetry allowed; saturated C−C, C−H bonds (alkanes). |
| n → σ* | 160 – 200 nm | 100 – 1,000 | Symmetry allowed; lone-pair heteroatoms (alcohols, amines, halides). |
| π → π* | 170 – 260 nm (Conjugated > 210 nm) | 1,000 – 40,000 (Very Strong) | Symmetry allowed; isolated alkenes (175 nm), conjugated dienes, enones, arenes. |
| n → π* | 270 – 330 nm | 10 – 100 (Weak, Forbidden) | Laporte/Symmetry forbidden (low intensity R-band); carbonyls (∼280 nm), nitro groups. |
- Base Value:
• Heteroannular (transoid / acyclic) diene: 214 nm
• Homoannular (cisoid in one ring) diene: 253 nm - Incremental Additions:
• Extended conjugation (per conjugated double bond): +30 nm
• Alkyl substituent or ring residue directly on diene: +5 nm each
• Exocyclic double bond (projecting outside any ring): +5 nm each
• Polar auxochromes: −OAc (+0 nm), −O-Alkyl (+6 nm), −S-Alkyl (+30 nm), −Cl/−Br (+5 nm), −NR2 (+60 nm).
2. FT-IR Spectroscopy: Molecular Vibrations & Carbonyl Diagnostic Trends
Infrared spectroscopy measures transitions between vibrational energy levels. According to Hooke’s Law for a harmonic oscillator, the vibrational stretching wavenumber (ν̄) is directly proportional to the square root of the bond force constant (k) and inversely proportional to the reduced mass (μ):
ν̄ = [1 / (2πc)] × √(k / μ) where μ = (m1m2) / (m1 + m2)
| Carbonyl Functional Class | Stretching Frequency (νC=O, cm−1) | Electronic / Structural Origin of Shift |
|---|---|---|
| Acid Chlorides (RCOCl) | 1815 – 1785 | Dominant −I inductive effect of Cl pulls electron density, strengthening C=O bond force constant. |
| Acid Anhydrides [(RCO)2O] | 1820 & 1760 (Twin doublet) | Fermi resonance / symmetric and asymmetric vibrational dipole coupling between twin carbonyls. |
| Esters (RCOOR’) | 1750 – 1735 | Electronegative ester oxygen (−I) outcompetes resonance donation, elevating frequency relative to ketones. |
| Aldehydes (RCHO) | 1740 – 1720 (plus Fermi doublet at 2820 & 2720 cm−1) | Characteristic C−H aldehydic Fermi doublet confirms aldehyde identity. |
| Acyclic Saturated Ketones (RCOR) | 1715 (Standard reference baseline) | Unperturbed baseline for unstrained, non-conjugated aliphatic carbonyls. |
| α,β-Unsaturated Enones & Aromatic Ketones | 1685 – 1665 (Decreases by 30–40 cm−1) | Resonance delocalization gives single-bond character to C=O (O−C=C−C+), lowering force constant k. |
| Amides (RCONH2, RCONHR’) | 1690 – 1650 (Amide I Band) | Strong resonance donation from nitrogen lone pair (O−−C=N+) drastically weakens C=O force constant. |
| Cyclic Ketones (Ring Strain Effect) | Cyclohexanone: 1715 Cyclopentanone: 1745 (+30) Cyclobutanone: 1780 (+65) Cyclopropanone: 1850 (+135) | Decreasing C−C(=O)−C bond angle increases s-character of the exocyclic C=O double bond, strengthening it! |
3. 1H-NMR Diamagnetic Anisotropy & The Karplus Dihedral Curve
In NMR spectroscopy, induced local magnetic fields generated by circulating π-electrons either oppose or reinforce the applied external field (B0), generating pronounced spatial regions of shielding and deshielding.
4. Mass Spectrometric McLafferty Rearrangement & DEPT Subspectral Matrix
Modern organic analysis relies on tandem fragmentation patterns in Mass Spectrometry combined with multiplicity editing in 13C-NMR spectroscopy.
5. 13C-NMR Multiplet Editing (DEPT-135/90) & 2D-NMR Correlations
Broadband 1H-decoupled 13C-NMR spectra collapse all carbon multiplets into single lines, losing all information regarding the number of attached protons. The DEPT pulse sequence utilizes polarization transfer from sensitive 1H nuclei to 13C nuclei to unambiguously differentiate carbon multiplicities:
- Broadband Decoupled 13C Spectrum: Displays all carbons present in the molecule (Cquat, CH, CH2, CH3).
- DEPT-90 Spectrum: Uses a 90° pulse on the 1H channel; ONLY methine (−CH= / −CH<) carbons appear as positive signals. All CH3, CH2, and quaternary carbons are completely suppressed.
- DEPT-135 Spectrum: Uses a 135° pulse on the 1H channel; CH3 and CH carbons point UP (positive phase), whereas CH2 carbons point DOWN (inverted negative phase). Quaternary carbons remain completely absent.
- Quaternary Carbon Identification: Peaks present in the broadband 13C spectrum but missing in the DEPT-135 spectrum represent quaternary carbons (e.g., carbonyls, CAr ipso carbons, tetrasubstituted alkenes).
- 1H–1H COSY (Correlation Spectroscopy): Homonuclear 2D experiment. Cross-peaks (off-diagonal signals) reveal scalar spin-spin coupling between mutually coupled protons (typically across 2 or 3 bonds, 2J and 3J), establishing direct proton-proton connectivity networks.
- NOESY (Nuclear Overhauser Effect Spectroscopy): Homonuclear 2D experiment detecting cross-relaxation through space. Cross-peaks appear between protons that are physically close in space (≤ 5 Å), regardless of whether they are coupled through bonds. Invaluable for establishing cis/trans geometries, axial/equatorial positions in steroids, and polypeptide tertiary folds.
- HSQC / HMQC (Heteronuclear Single Quantum Coherence): Correlates each proton directly with the carbon atom to which it is covalently bonded via one-bond coupling (1JC−H). Each peak represents a distinct C−H pair.
- HMBC (Heteronuclear Multiple Bond Correlation): Long-range heteronuclear correlation detecting two-bond (2JC−H) and three-bond (3JC−H) couplings. Connects protons to quaternary carbons (such as carbonyls, ester carbons, and quaternary aromatic centers) across heteroatoms, locking together disconnected structural fragments.
6. Mass Spectrometry: Isotope Clusters, α-Cleavage & Fragmentation Rules
In electron ionization mass spectrometry (EI-MS, 70 eV), energetic electrons knock out an electron from the analyte to form an odd-electron radical cation (the molecular ion, M+•). The molecular ion undergoes unimolecular decompositions governed by carbocation stability and the thermodynamic extrusion of stable neutral molecules (CO, H2O, C2H4, HCN).
| Diagnostic MS Feature | Isotopic / Fragmentation Signature | Structural Implication |
|---|---|---|
| Nitrogen Rule | Odd nominal MW ⇒ Odd number of Nitrogens (1, 3, 5 N atoms). Even nominal MW ⇒ Zero or Even number of Nitrogens. | Applies to all organic compounds containing C, H, N, O, S, and halogens. |
| Chlorine Isotope Cluster | M : M+2 = 3 : 1 ratio (due to 35Cl [75.8%] and 37Cl [24.2%]). | Confirms the presence of 1 Chlorine atom. (2 Cl atoms show M : M+2 : M+4 = 9 : 6 : 1). |
| Bromine Isotope Cluster | M : M+2 ≈ 1 : 1 ratio (twin peaks of equal intensity; 79Br [50.7%] & 81Br [49.3%]). | Unambiguous hallmark of 1 Bromine atom. (2 Br atoms show 1 : 2 : 1 triplet). |
| Tropylium Ion Formation | Intense base peak at m/z = 91 (C7H7+), which further loses neutral acetylene (26 Da) to give m/z = 65 (C5H5+). | Diagnostic for alkylbenzenes bearing a benzylic methylene (−CH2−Ph, e.g., toluene, ethylbenzene). |
| α-Cleavage (Radical-Induced) | Cleavage of the bond adjacent to a heteroatom (O, N, S) or carbonyl group, driven by oxonium or iminium ion formation: R−CH2−O+•−R’ → CH2=O+−R’ (m/z 31 for primary alcohols) + R•. | Favors loss of the largest alkyl radical to maximize resonance stability. |
7. High-Yield CSIR NET & GATE Chemical Science Solved Spectral Problems
Standard CSIR NET 4-Mark Problem: An organic compound with molecular formula C10H12O2 exhibits the following spectral data:
• IR: 1735 cm−1 (strong), 1240 cm−1 (strong), 1600 cm−1, 1500 cm−1.
• 1H-NMR: δ 2.05 (s, 3H), 2.90 (t, J = 7.0 Hz, 2H), 4.25 (t, J = 7.0 Hz, 2H), 7.20–7.35 (m, 5H) ppm.
• 13C-NMR: δ 20.9 (CH3), 35.1 (CH2), 64.8 (CH2), 126.5 (CH), 128.5 (CH), 128.9 (CH), 138.1 (Cquat), 171.0 (Cquat) ppm.
Deduce the complete structure and assign all spectral signals.
- Index of Hydrogen Deficiency (IHD / DBE):
DBE = C + 1 − H/2 = 10 + 1 − (12/2) = 11 − 6 = 5 degrees of unsaturation.
The 5H multiplet at δ 7.20–7.35 ppm + IR bands at 1600/1500 cm−1 confirms a monosubstituted benzene ring (C6H5−, 4 DBEs).
Remaining DBE = 5 − 4 = 1 DBE. - IR Analysis:
Strong peak at 1735 cm−1 indicates an aliphatic ester carbonyl (−COOR). This accounts for the remaining 1 DBE and both oxygen atoms. - 1H-NMR Analysis:
• δ 2.05 (s, 3H): Isolated methyl group adjacent to a carbonyl: CH3−C(=O)− (Acetate group).
• δ 2.90 (t, J = 7.0 Hz, 2H) and δ 4.25 (t, J = 7.0 Hz, 2H): An isolated −CH2−CH2− ethylene bridge.
The heavily downfield triplet at δ 4.25 ppm is directly attached to the electronegative ester oxygen: −CH2−O−C(=O)−CH3.
The triplet at δ 2.90 ppm is attached to the phenyl ring: Ph−CH2−CH2−. - Final Assembled Structure:
PhCH2CH2OCOCH3 (2-Phenylethyl acetate / Phenethyl acetate).
Verification with 13C: Exactly 8 signals (6 aromatic carbons with 2 pairs degenerate + 2 aliphatic CH2 + 1 CH3 + 1 ester C=O at δ 171.0 ppm).
Frequently Asked GATE Question: In the electron-impact mass spectrum of 2-hexanone [CH3COCH2CH2CH2CH3], what is the m/z value and structure of the base peak resulting from the McLafferty rearrangement?
- Molecular ion: CH3−C(=O+•)−CH2(α)−CH2(β)−CH2(γ)−CH3 (MW = 100).
- Hydrogen abstraction occurs from the γ-carbon via a six-membered cyclic transition state:
• Oxygen radical abstracts a γ-hydrogen atom.
• Homolytic cleavage of the Cα−Cβ bond occurs. - Eliminated neutral molecule: Propene [CH2=CH−CH3, MW = 42 Da].
- Observed radical cation fragment: Acetone enol radical cation [CH3−C(OH)=CH2]+•.
- Fragment mass: m/z = 100 − 42 = 58 (Base Peak).
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