FT-NMR Sample Preparation for Organic Molecules

Flashcards covering key vocabulary, definitions, and procedures for preparing samples for FT-NMR analysis, including solvent types, reference standards, and sample preparation steps for liquids and solids.

FT-NMR

Fourier Transform Nuclear Magnetic Resonance, an analytical technique used for determining the structure of organic molecules.

Ideal FT-NMR Sample Composition

A properly prepared sample contains 5-10 mg of compound, 0.7 - 1.0 mL of a deuterated solvent, and 0.1% v/v Tetramethylsilane (TMS) as an internal reference.

Deuterated Solvents

Solvents used for FT-NMR samples where hydrogen atoms are replaced with deuterium, such as CDCl3, D2O, C6D6, acetone (C3D6O), acetonitrile (CD3CN), and tetrahydrofuran (C4D8O).

Primary Reason for Deuterated Solvents (Signal Obscurity)

To prevent the strong proton signal from a protonated solvent from obscuring the weaker signal from the low concentration (10-2 molar) organic molecule.

Secondary Reason for Deuterated Solvents (Instrument Lock)

To provide a deuterium resonance signal that the FT-NMR instrument can 'lock' onto, ensuring a consistent magnetic field and compensating for drifts.

Residual Nondeuterated Solvent Peaks

Proton signals observed in NMR spectra due to the incomplete deuteration (typically 97-99%) of NMR solvents.

Tetramethylsilane (TMS)

The internal reference normally used in proton NMR, with its proton resonance assigned as 0 ppm, serving as a reference point for other organic proton resonances.

Recommended TMS Concentration

An extremely small amount, 0.1% v/v, is necessary in an NMR sample; higher concentrations can negatively affect the NMR spectrum.

Importance of Sample Purity in FT-NMR

Samples must be free of any particulates and prepared with proper amounts of sample and solvent to ensure accurate data from the precision FT-NMR instrument.

Liquid Sample Preparation (Sample Amount)

Place 5-10 mg of liquid sample (drawn by capillary action into a Pasteur pipette) into an 8-inch NMR tube.

Solid Sample Preparation (Sample Amount)

Weigh 5-10 mg of solid sample into a clean glass vial and dissolve it thoroughly in the NMR solvent before transferring to the NMR tube.

NMR Tube Fill Level

The NMR sample (liquid or dissolved solid) should fill the NMR tube to the 4.5 cm mark on the side of the NMR tube rack.

Chemical Shift of Residual Protons in CDCl3

7.27 δ ppm from TMS.

Chemical Shift of Residual Protons in D2O

4.80 δ ppm from TMS.

Chemical Shift of Residual Protons in C6D6 (Benzene)

7.23 δ ppm from TMS.

Chemical Shift of Residual Protons in C3D6O (Acetone)

2.05 δ ppm from TMS.

Chemical Shift of Residual Protons in CD3CN (Acetonitrile)

1.94 δ ppm from TMS.