# Lecture 22: NMR Spectroscopy II: 13C NMR and Structure Determination
## Organic Chemistry I
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the key features of 13C NMR spectroscopy and how it differs from 1H NMR
2. Predict the number of signals in a 13C NMR spectrum using symmetry analysis
3. Interpret 13C chemical shift values to identify carbon environments
4. Explain the principles of broadband decoupling and DEPT experiments
5. Integrate data from 1H NMR, 13C NMR, IR, and MS to determine the structure of an unknown compound
6. Solve multi-spectral structure determination problems
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## Lecture Content
### I. Basics of 13C NMR
**13C** is an NMR-active nucleus with spin I = 1/2, but it presents significant experimental challenges compared to 1H. The 13C isotope has a natural abundance of only 1.1%, while the dominant isotope 12C (98.9%) is NMR-inactive with I = 0. Combined with a lower gyromagnetic ratio, this makes 13C NMR approximately 6,000 times less sensitive than 1H NMR. To compensate, 13C NMR experiments typically require more sample, longer acquisition times, and extensive signal averaging over many scans.
Despite these challenges, 13C NMR provides invaluable structural information. A 13C spectrum shows one peak for each unique carbon environment in the molecule, with the number of signals equaling the number of chemically nonequivalent carbons. Molecular symmetry reduces the signal count, as equivalent carbons produce a single signal. The chemical shift range extends from 0 to 220 ppm, far wider than the 0-12 ppm range of 1H NMR. This broader dispersion means that signals from different carbon environments are less likely to overlap, giving 13C NMR excellent resolution for distinguishing carbon types. TMS serves as the reference at 0 ppm.
### II. 13C Chemical Shifts
The same factors that govern 1H chemical shifts (electronegativity, hybridization, and magnetic anisotropy) also determine 13C chemical shifts, but the ranges are broader and more diagnostic. Alkyl carbons (sp3, not bonded to electronegative atoms) appear between 0 and 50 ppm, with CH3 groups typically at 8-25 ppm, CH2 at 15-40 ppm, CH at 20-50 ppm, and quaternary C at 25-50 ppm. Carbons bonded to electronegative atoms shift downfield: C-O in alcohols and ethers appears at 50-80 ppm, C-N at 30-65 ppm, and C-Cl at 25-50 ppm.
Alkene and aromatic carbons (sp2) resonate between 100 and 150 ppm. Carbonyl carbons (C=O) are the most deshielded common carbon type, appearing farthest downfield: carboxylic acids, esters, and amides at 160-185 ppm, and aldehydes and ketones at 190-220 ppm. A signal in the 190-220 ppm region is strong evidence for a ketone or aldehyde carbonyl.
### III. Broadband Decoupling
In a proton-coupled 13C spectrum, each carbon signal would be split by attached and nearby protons, producing complex multiplets that reduce signal intensity and complicate interpretation. **Broadband (proton) decoupling** eliminates this problem by irradiating all 1H nuclei simultaneously during acquisition. This removes all C-H coupling, producing a simplified spectrum in which each unique carbon gives a single peak (a singlet). Broadband-decoupled spectra are the standard way 13C NMR data are recorded.
The tradeoff is that information about the number of hydrogens attached to each carbon is lost. Additionally, integration in 13C NMR is not reliable for counting carbons because different carbon types have different relaxation times and experience varying nuclear Overhauser effects. Peak height gives a qualitative sense of relative abundance but cannot be trusted for quantitative analysis.
### IV. DEPT Experiments
**DEPT** (Distortionless Enhancement by Polarization Transfer) experiments recover the information about attached hydrogens that broadband decoupling removes. Three DEPT subspectra can be acquired, each providing distinct information.
**DEPT-135** is the most commonly used and most informative single experiment. In a DEPT-135 spectrum, CH3 and CH carbons produce positive (upward) peaks, CH2 carbons produce negative (downward) peaks, and quaternary carbons (bearing no hydrogen) produce no signal at all. **DEPT-90** shows only CH carbons, allowing them to be distinguished from CH3. **DEPT-45** displays all protonated carbons (CH3, CH2, and CH) as positive peaks, with quaternary carbons again absent.
By comparing the broadband-decoupled spectrum with the DEPT-135 spectrum, every carbon can be categorized. Peaks present in the broadband spectrum but absent in DEPT-135 are quaternary carbons. Positive peaks in DEPT-135 are either CH or CH3 (distinguished by DEPT-90). Negative peaks in DEPT-135 are CH2.
A three-panel comparison of 13C NMR data for 2-butanone (CH3-CO-CH2-CH3). Panel A: The broadband-decoupled 13C spectrum showing four singlet peaks at approximately 209 ppm (C=O), 37 ppm (CH2), 29 ppm (CH3 next to C=O), and 8 ppm (CH3). Each peak is labeled with its assignment. Panel B: The DEPT-135 spectrum of the same compound. The C=O peak at 209 ppm is absent (quaternary). The CH2 at 37 ppm points downward (negative). The two CH3 peaks at 29 and 8 ppm point upward (positive). Panel C: A summary table showing: Carbon type | Broadband | DEPT-135 | DEPT-90 -- CH3: present, up, present; CH2: present, down, absent; CH: present, up, present; C (quaternary): present, absent, absent. A caption reads: "DEPT experiments reveal the number of hydrogens attached to each carbon, complementing the broadband-decoupled spectrum."
### V. Solving Structures with 13C NMR
The number of 13C signals provides immediate information about molecular symmetry. Fewer signals than the total number of carbons in the molecular formula indicates the presence of symmetry elements that make some carbons equivalent. Para-xylene (C8H10), for example, shows only 3 signals instead of 8 because of its high symmetry.
Matching signals to structural features follows the chemical shift ranges described above. A signal above 160 ppm indicates a carbonyl or an aromatic carbon bonded to oxygen or nitrogen. A signal at 190-220 ppm specifically identifies an aldehyde or ketone. Signals between 100 and 160 ppm correspond to alkene or aromatic carbons. Signals at 50-80 ppm indicate carbons bonded to oxygen or nitrogen. Signals at 0-50 ppm correspond to sp3 alkyl carbons. DEPT data then adds the CH, CH2, CH3, or quaternary assignment to each signal, creating a detailed picture of the carbon framework.
### VI. Integrated Structure Determination Strategy
No single spectroscopic method is sufficient to determine a complete molecular structure. The combination of molecular formula (from HRMS), IR spectroscopy, 1H NMR, and 13C NMR provides a powerful, complementary toolkit.
The recommended step-by-step strategy begins with determining the **molecular formula and degree of unsaturation** from mass spectrometry or combustion analysis. Each degree of unsaturation represents one ring or one double bond; a triple bond equals two degrees; a benzene ring equals four. Next, examine the **IR spectrum** to identify which functional groups are present or absent: is there a broad O-H stretch, a strong C=O, an N-H, or a triple bond absorption? Then analyze the **1H NMR** to determine the number of proton environments, their chemical shifts, integration ratios, and splitting patterns. Assemble structural fragments from the NMR data (for example, a triplet and quartet with 3:2 integration identify an ethyl group). Examine the **13C NMR and DEPT** to count unique carbons, determine their chemical shift environments, and assign the number of attached hydrogens. Finally, assemble the fragments into a complete structure consistent with all data, and verify that the proposed structure predicts every observed spectral feature.
A worked example of multi-spectral structure determination. An unknown compound has molecular formula C4H8O2 (degree of unsaturation = 1). Panel A: The IR spectrum shows a strong broad O-H stretch (~2500-3300 cm-1) and a strong C=O stretch at ~1710 cm-1, annotated as "carboxylic acid." Panel B: The 1H NMR spectrum shows a singlet at ~11.5 ppm (1H, broad, COOH), a triplet at ~2.3 ppm (2H), a sextet at ~1.6 ppm (2H), and a triplet at ~0.9 ppm (3H). Integration ratio 1:2:2:3. Splitting patterns are labeled. Panel C: The 13C NMR (broadband decoupled) shows four peaks at approximately 180 ppm (C=O), 36 ppm, 18 ppm, and 14 ppm. DEPT-135 below shows the 180 ppm peak absent (quaternary C=O), the 36 ppm and 18 ppm peaks negative (CH2), and the 14 ppm peak positive (CH3). Panel D: The structure of butanoic acid (CH3CH2CH2COOH) is drawn, with arrows connecting each spectral feature to its corresponding structural fragment. A caption reads: "Combining molecular formula, IR, 1H NMR, and 13C NMR data enables unambiguous structure determination."
### VII. Common Structure Determination Patterns to Recognize
Several structural fragments produce instantly recognizable spectral signatures. An **ethyl group** appears as a triplet (3H) and quartet (2H) in 1H NMR, with two sp3 carbons (one CH3 and one CH2) in DEPT. An **isopropyl group** produces a doublet (6H) and septet (1H), with two equivalent CH3 carbons and one CH. A **tert-butyl group** gives a singlet (9H) in 1H NMR, with one quaternary carbon and one CH3 signal (representing three equivalent methyls) in 13C. A **monosubstituted benzene ring** shows signals at 6.5-8.5 ppm in 1H NMR, produces m/z = 77 (C6H5+) in mass spectrometry, and has signals at 125-140 ppm in 13C NMR. A **methoxy group** (OCH3) appears as a singlet near 3.3-3.8 ppm (3H) in 1H NMR with a peak at 50-58 ppm in 13C. An **aldehyde** shows a signal at 9-10 ppm (1H) in 1H NMR, a peak at 195-205 ppm (CH by DEPT) in 13C, two weak C-H absorptions at 2720 and 2820 cm-1 in IR, and a strong C=O absorption.
Practicing structure determination with multiple spectra simultaneously is the single most effective way to master spectroscopy.
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