Understanding DEPT NMR: Identify Carbon Types via 13C Spectroscopy

2D NMR spectroscopy extends traditional 1D NMR by providing two frequency dimensions (ppm axes) instead of one, enabling chemists to analyze complex molecules that produce overlapping signals in 1D spectra. The three primary 2D NMR techniques are: COSY (Correlation Spectroscopy) which shows proton-proton coupling relationships through cross-peaks symmetric about the diagonal; HSQC (Heteronuclear Single Quantum Coherence) which correlates directly bonded carbon-hydrogen pairs; and HMBC (Heteronuclear Multiple Bond Coherence) which reveals long-range carbon-hydrogen relationships across two to three bonds. Reading 2D NMR spectra follows a spreadsheet-like approach where peaks are located on one axis and traced across to find corresponding peaks on the perpendicular axis, with intensity represented as depth or contour lines rather than vertical height.

This section introduces three essential 2D NMR techniques. HSQC (Heteronuclear Single Quantum Coherence) detects 1-bond correlations between hydrogen and carbon, showing which hydrogens are directly bonded to which carbons. HMBC (Heteronuclear Multiple Bond Correlation) detects 2-3 bond correlations, allowing connection of molecular fragments not directly bonded. COZY (COrrelation spectroscopy) detects correlations between hydrogens, showing which hydrogens are adjacent in the structure. The instructor explains how cross-peak intensity varies with bond count: 2-3 bonds produce strong peaks, 4-5 bonds produce weak peaks.

Three common heteronuclear correlation experiments are HSQC (Heteronuclear Single Quantum Coherence), HMQC (Heteronuclear Multiple Quantum Coherence), and HMBC (Heteronuclear Multiple Bond Correlation). HSQC uses one-bond heteronuclear coupling (typically 150 Hz for carbon-proton) to correlate directly bonded nuclei. HMQC uses multiple quantum pathways during evolution, providing identical correlation information but being more robust against experimental imperfections. HMBC detects long-range correlations (2-3 bonds away) using 0-10 Hz coupling constants, suppressing one-bond correlations while retaining remote couplings. These experiments can be applied to various nucleus pairs including carbon-proton, nitrogen-proton, and fluorine-carbon.

This section covers three fundamental 2D NMR correlation experiments. HSQC correlates abundant spins (protons) with dilute spins (carbon-13, nitrogen-15) through polarization transfer and 180-degree pulse during T1 period, with variations in decoupling dimensions. HMBC captures long-range correlations (2-3 bonds) using accept and reject filters to suppress one-bond correlations. INADEQUATE (Insensitive Nuclei Double Quantum Transfer Experiment) addresses the unique challenge of correlating two dilute spins like carbon-13 to carbon-13. The main challenge is the 1% natural abundance of carbon-13, making simultaneous observation of two carbons extremely rare (1 in 10,000 probability). INADEQUATE uses a double quantum filter to suppress parent signals from individual carbons while preserving correlations between coupled carbons. Double quantum filtering is sufficient because observing three carbons simultaneously is negligible (1 in a million).

Three major heteronuclear correlation techniques serve different structural analysis purposes. HETCOR detects one-bond proton-carbon couplings using two channels with decoupling during acquisition, but suffers from low sensitivity due to carbon-13 detection. HMQC employs inverse polarization transfer, moving magnetization from sensitive protons to insensitive carbons, then detecting via proton channel for higher sensitivity. HMBC specializes in long-range connectivity (2-4 bonds) by first removing direct couplings, then evolving and detecting long-range interactions. Each technique addresses specific structural analysis needs in organic chemistry.

Carbon-13 NMR spectroscopy employs three key methods for structural elucidation: 13C CPD (Composite Programmed Decoupling) provides quick results showing all carbon atoms; 13C APT (Attached-Proton-Test) distinguishes primary, secondary, tertiary, and quaternary carbons through peak polarity (primary and tertiary carbons appear downfield while secondary and quaternary appear upfield); and 13C DEPT (Distortionless Enhancement by Polarization Transfer) variants (DEPT-45, DEPT-90, DEPT-135) reveal primary, secondary, and tertiary carbons but cannot detect quaternary carbons. For comprehensive structural analysis, chemists should combine these methods: use CPD first to identify total carbon count, then apply APT to distinguish carbon types, and finally use DEPT to confirm specific group identifications.

DEPT (Distortionless Enhancement by Polarization Transfer) is a specialized carbon-13 NMR technique that provides information about the number of hydrogen atoms attached to each carbon. At 45° excitation angle, only CH₃ and CH groups are visible; at 90°, only CH groups are visible; at 135°, CH₃ groups appear above the baseline (positive polarity) while CH₂ groups appear below the baseline (negative polarity). This allows chemists to distinguish between different types of carbons (methyl, methylene, methine, quaternary) based on their polarization patterns in the DEPT spectrum.

Carbon-13 NMR spectroscopy uses APT (Attached Proton Test), DEPT (Distortionless Enhancement by Polarization Transfer), and INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) techniques to determine molecular structure by distinguishing between CH3, CH2, and CH carbon environments through characteristic signal patterns and polarization transfer mechanisms.

This video explains how to interpret carbon-13 NMR spectra using different techniques: broadband decoupling shows all carbons as singlets, DEPT-45 displays all carbons (CH3, CH2, CH, quaternary) as positive peaks, DEPT-90 shows only CH carbons as positive peaks, and DEPT-135 displays CH3 and CH as positive peaks while CH2 appears as negative peaks; APT provides similar information with CH3 and CH as positive peaks, CH2 as negative peaks, and quaternary carbons as positive peaks, allowing chemists to determine the number of carbons and their hydrogenation states in organic molecules.

The DEPT (Distortionless Enhancement by Polarization Transfer) NMR technique classifies carbon atoms by their number of attached hydrogen atoms. In DEPT-90, only tertiary carbons (CH) appear as positive peaks. In DEPT-135, primary (CH3) and tertiary (CH) carbons appear as positive peaks, while secondary carbons (CH2) appear as negative peaks. Quaternary carbons (C with no hydrogens) do not appear in any DEPT experiment. To determine carbon types: (1) Normal DEPT gives total carbons, (2) DEPT-90 gives tertiary carbons, (3) DEPT-135 negative peaks give secondary carbons, (4) Primary carbons are calculated by subtracting tertiary and secondary from total, (5) Quaternary carbons are found by subtracting all other types from total. This systematic approach allows complete characterization of the carbon skeleton.

This lecture demonstrates how to determine molecular structures using 1H NMR, 13C NMR, and DEPT spectroscopy by analyzing chemical shifts, integration values, and coupling patterns to identify functional groups and connectivity, as illustrated through multiple worked examples of organic compounds.

This segment demonstrates complete molecular structure determination using NMR spectroscopy. Proton NMR multiplicity follows the n+1 rule: triplets indicate two neighboring protons, sextets indicate five. Integration ratios reveal proton counts. For C7H14O showing only three proton peaks, symmetry is indicated. Carbon-13 NMR shows carbonyl carbons at ~212 ppm. DEPT-135 distinguishes carbon types: positive signals for CH3 and CH, negative for CH2, and no signal for quaternary carbons. DEPT-90 shows only CH carbons. For C7H14O, only four carbon peaks indicate symmetry. DEPT-135 shows two negative signals (two CH2 groups) and one positive signal (CH3), confirming the propyl group structure. The compound is identified as 3,7-dimethyloctan-4-one. This combined spectroscopic approach provides comprehensive molecular architecture information.

Structure determination combines multiple spectroscopic techniques: (1) Mass spectrometry provides molecular weight and identifies halogens/nitrogens, (2) C-13 NMR identifies carbon types and environments, (3) Proton NMR identifies hydrogen environments and functional groups. For C5H11Br: IHD = 0, C-13 NMR shows a peak at 51.55 ppm (CH attached to Br), DEPT confirms it's a CH group. The correct structure is 1-bromobutane. For C4H8O: IHD = 1, peak at 208 ppm indicates carbonyl, DEPT shows no hydrogens (ketone), structure is 2-butanone.

The number of NMR signals equals the number of chemically distinct atoms. For bicyclic compounds, symmetry reduces signal count: highly symmetric isomers show fewer signals. In 1H NMR, methyl groups attached to aromatic rings appear at 2.3-2.7 ppm as singlets, methoxy protons above 3.5 ppm as singlets, and aromatic protons at 6.5-8.5 ppm. Electron-donating groups cause upfield shift of ortho protons, while electron-withdrawing groups cause downfield shift. Combined NMR data (1H and 13C) enables complete structure elucidation by matching chemical shifts to functional groups and symmetry patterns.

This extended case study demonstrates solving complex structural problems by integrating multiple NMR techniques. For C4H10O with DU=0: calculate hydrogen count from integration and formula; use DEPT spectra to identify CH, CH2, and CH3 groups; apply symmetry considerations to explain fewer signals than total carbons; match chemical shifts to functional groups; assemble fragments into complete structure. For C5H10O with DU=1: analyze proton NMR for functional group identification (carboxylic acid, aromatic protons); use carbon NMR for carbonyl and aromatic carbon identification; apply symmetry considerations to explain signal multiplicity; combine all evidence to propose and verify structure. The key insight is that no single technique provides complete information—successful structure elucidation requires synthesizing data from proton NMR, carbon NMR, and DEPT experiments.