Carbon-13 DEPT NMR: Determining Substitution Patterns

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.

DEPT (Distortionless Enhancement by Polarization Transfer) NMR determines how many hydrogens are attached to each carbon through three spectra: (1) Broadband decoupled shows all carbons, (2) DEPT-90 shows only carbons with one hydrogen, (3) DEPT-135 shows carbons with one/three hydrogens as positive peaks and carbons with two hydrogens as negative (inverted) peaks. Structural elucidation combines multiple data types: chemical formula, degrees of unsaturation, carbon NMR peak positions using correlation tables, peak counts indicating equivalent carbons, and DEPT NMR for hydrogen counts. The process involves identifying puzzle pieces (functional groups, equivalent carbons, hydrogen counts) and assembling them into a consistent structure that matches all observations.

This video demonstrates how to solve an unknown organic compound (C5H9N) by integrating multiple spectroscopic techniques: degrees of unsaturation calculations reveal two degrees of unsaturation (ruling out aromatic rings), mass spectrometry shows a molecular ion at m/z 83 with a base peak at m/z 82 indicating an acidic proton, IR spectroscopy reveals a terminal alkyne (2114 cm⁻¹) and NH2 group (3295 cm⁻¹), DEPT carbon NMR identifies three CH2 groups and a quaternary carbon, and proton NMR confirms the structure with integration and splitting patterns showing a primary amine and terminal alkyne; the final structure is CH2CH2CH2C≡CH-NH2.

DEPT NMR distinguishes CH₃, CH, and CH₂ groups while quaternary carbons remain invisible. Combining IR (functional group identification), proton NMR (hydrogen connectivity), carbon NMR (carbon environments), and DEPT (hydrogen counts) provides complete structural information. Symmetric molecules show reduced signal counts. This integrated approach enables confident structure assignment from spectral data alone.

Solving complex organic structure problems requires integrating multiple spectroscopic techniques: IR identifies functional groups via characteristic absorptions (carbonyls at 1700-1800 cm⁻¹, alkenes at 1650-1750 cm⁻¹, terminal alkynes at 2100-2200 cm⁻¹, alcohols/broad absorptions at 3200-3600 cm⁻¹); mass spectrometry determines molecular formula and reveals fragmentation patterns; NMR provides connectivity information. The systematic approach involves: (1) IR analysis to identify functional groups; (2) mass spec to determine molecular weight and isotope patterns; (3) applying the nitrogen rule and isotope effects; (4) using fragmentation patterns to deduce structure; (5) confirming with NMR data. This integrated methodology enables confident structure determination even for complex molecules.

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.

HSQC (Heteronuclear Single Quantum Coherence) and HMBC (Heteronuclear Multiple Bond Correlation) are two-dimensional NMR spectroscopy techniques used to determine molecular structures by correlating hydrogen and carbon atoms through direct and indirect bonds, respectively; HSQC shows direct one-bond correlations between 1H and 13C nuclei, while HMBC reveals long-range correlations (typically 2-3 bonds) that help establish connectivity across quaternary carbons and complex molecular frameworks.

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.

Two-dimensional heteronuclear NMR correlation experiments, such as HSQC and HMQC, enable the correlation of chemical shifts between different types of nuclei (typically carbon-13 and proton) by transferring magnetization through one-bond couplings, allowing researchers to assign molecular structures by correlating the chemical shifts of coupled nuclei in a two-dimensional spectrum; these experiments are most effective when observing the high-gamma nucleus (proton) in inverse mode, though they can also detect long-range couplings by adjusting the delay times, with HMBC being particularly useful for establishing connectivity through multiple bonds despite its more complex intensity modulation.

This lecture explains how complex NMR pulse sequences work by introducing key concepts including the rotating frame, spin echo refocusing, and J-coupling modulation. The APT (Attached Proton Test) experiment uses a 90-degree pulse followed by a 180-degree pulse after waiting 1/J time, with proton decoupling applied mid-sequence to distinguish quaternary carbons (negative peaks), methylene groups (negative peaks), and methine/methyl groups (positive peaks). The DEPT (Distortionless Enhancement by Polarization Transfer) experiment transfers polarization from abundant protons to less abundant carbons, with variants (DEPT-45, DEPT-90, DEPT-135) producing different spectral patterns that can be combined for spectral editing. Critical factors include coupling constant values (J1CH ~125 Hz for sp3, ~160 Hz for sp2, ~250 Hz for alkynes, 208 Hz for chloroform), which affect interpretation accuracy.

This section explains APT (Attached Proton Test) NMR spectroscopy. APT distinguishes carbon types: methyl (CH3) and methine (CH) appear as positive peaks, methylene (CH2) appears as negative peaks, and quaternary carbons (C) appear as positive peaks. The section demonstrates how to interpret APT spectra to identify carbon types. The example shows how to distinguish between CH3, CH2, and CH carbons based on peak assignments.

The APT experiment is a 2D NMR spectroscopy technique used to assign carbon-hydrogen multiplicities in 13C NMR spectra. It provides information about all types of carbons within a single experiment. The technique works based on the number of hydrogens bound to a carbon atom: CH and CH3 groups have spin vectors that evolve differently after the initial pulse compared to C and CH2 groups. In the APT spectrum, a delay time T1 of 1/J is applied, resulting in positive signals for carbon and CH2 groups, while CH and CH3 groups appear with negative signals. A delay of 0.5/J produces a spectrum showing only quaternary carbons.

APT instrumentation operates in ultra-high vacuum (10^-8 Pascals) with a sample stage capable of three-dimensional movement. The counter electrode is a hollow copper foil (10-60 microns diameter) that concentrates electric field lines, reducing the field factor. Two pulsing methods control field evaporation: high-voltage pulsing (0.5-5 nanoseconds duration) maximizes ion evaporation at peak voltage, while laser pulsing (tens of nanoseconds to hundreds of femtoseconds) uses focused laser beams to increase tip temperature and assist evaporation. The detector consists of Multi-Channel Plates (MCP) in chevron configuration and a delay-line detector for position-sensitive detection.

The attached proton test uses spin echo experiments with proton decoupling to identify carbon types based on their coupling environment. By setting the delay time to specific fractions of the J-coupling constant (typically 1/2J or 1J), different carbon types can be distinguished: (1) At 1/2J delay, only quartet carbons (no attached protons) are visible; (2) At 1J delay, CH and CH3 carbons show negative signals while CH2 carbons show positive signals. This technique helps identify whether a carbon is attached to 0, 1, 2, or 3 protons, which is essential for structural elucidation in NMR spectroscopy.