Nuclear Magnetic Resonance (NMR) Spectroscopy
NMR spectroscopy is an analytical technique used to determine the chemical molecular structure of a compound. NMR provides both quantitative and qualitative data on the composition of a sample. This technique can be used for quality control, for research, to identify an unknown, or to determine the content and purity of a sample.
A simplified description of NMR is that the sample, dissolved in a liquid, is placed into the instrument, which contains a magnetic field. A radio frequency pulse is then sent through the sample solution in order to orient the magnetic moments of the nuclei in the solution. As the magnetic moments relax, they exhibit a free induction decay. The free induction decay is Fourier transformed into an NMR spectrum. The NMR spectrum displays chemical shifts for the individual nuclei; and from these chemical shifts, the structure of the compound can be determined.
Impact Analytical currently has a Varian 400MHz NMR System spectrometer, equipped with a broadband Pulse-Field Gradient (PFG) probe for analysis of low frequency X nuclei (15N - 31P). Typical analyses include proton and carbon-13, as well as silicon-29 and fluorine-19. The NMR also has variable temperature (-25 to 130 C) capabilities, suitable for analyzing polymers and other materials at high temperature, or monitoring chemical reactions at low temperatures. Advanced one and two-dimensional experiments, such as APT, DEPT, COSY, HETCOR, and the use of PFG, are available and necessary tools for the deconvolution of complex mixtures and materials, especially for pharmaceutical characterization.
- 400MHz Varian 400 MHz Varian NMR System Spectrometer with Autosampler
- 5mm Broad Band ATB Probe with Pulse Field Gradient (PFG): 1H, 19F, 13C, 29Si, 31P, etc. Software: Linux Red Hat 4.0u3, VnmrJ 2.2C
- Temperature: -20°C-120°C GxP Compliant Instrument
APT (Attached Proton Test)
Differentiates between C, CH, CH2, and CH3.
DEPT (Distortionless Enhancement by Polarization Transfer)
Uses a polarization transfer from protons to another nucleus (e.g., 15N or 29Si), over one or more bonds, to increase signal strength. Can also distinguish between CH, CH2, and CH3 groups.
INEPT (Insensitive Nuclei Enhanced by Polarization Transfer)
Enhances sensitivity for nuclides with low natural abundance (13C, 29Si, 15N) by polarization transfer from the protons through X,H spin coupling.
1-D INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) Allows for the observation of small 13C,13C couplings over 2-3 bonds.
T1 (Spin-Lattice Relaxation Experiment)
Determines the relaxation time or time constant to reach the equilibrium of z-magnetization.
T2 (Spin-Spin Relaxation Experiment)
Determines the decay of the transverse magnetization and is related to line width.
2-D J-Resolved 1H NMR Spectroscopy
Separates chemical shift information from spin-spin coupling information by displaying them on different axes.
*2-D J-Resolved 13C NMR Spectroscopy
As above, displays chemical shift information on one axis, but other axis contains C,H coupling information.
H,H-COSY (COrrelation SpectroscopY)
A 2-D spectrum is produced which correlates the signals of a 1H spectrum. Cross peaks will occur if spin coupling is present. This experiment is helpful in determining chemical structure of an unknown. Other types of COSY experiments: Long-Range COSY, Phase-sensitive COSY, and Double Quantum Filtered COSY.
*HETCOR (HETeronuclear CORrelation Spectroscopy)
C,H correlation by polarization transfer produces cross peaks for and 1H and 13C nuclei which are connected by a C,H coupling over one bond.
HMQC (Heteronuclear Multiple Quantum Coherence)
An inverse H,X correlation experiment which suppresses unwanted signals through phase cycling.
TOCSY (TOtal Correlation SpectroscopY)
Can provide total correlation of all protons with each other. Mostly used for peptides and oligosaccharides.
*NOESY (Nuclear Overhauser Enhancement SpectroscopY)
This experiment produces correlation signals which are caused by dipolar cross-relaxation between nuclei.
*2-D INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) Provides all C,C connectivities, but very poor sensitivity. This experiment may not be successful with all samples.
*These experiments can be very long, and in some instances may require overnight analysis.
**All 2-D experiments require first running a standard 1H and/or 13C experiment.
- VnmrJ Liquids NMR User Guide. Applicable to Varian NMR superconducting spectrometer systems with Linux software and VnmrJ® software installed. Pub. No. 01-999313-00, Rev A 0106
- Autoswitchable, 4 Nucleus, and ATB NMR Probes Manual. Pub. No. 01-999121-00 Rev. J0505.
- S. Braun, H.O. Kalinowski, S. Berger, 100 and More Basic NMR Experiments, VCH Publishers, New York, NY