The Varian Mercury-VX 400 MHz BB is available for challenging projects involving natural products, proteins, complex synthetic molecules, inorganic compounds and materials studies. The Mercury-VX 400 is equipped with z-pulsed gradients (PFG), 5mm broad band auto-swichable 1H/19F, (15N-31P) probe, variable temperature capabilities, waveform generators and can perform all of the latest 2-, 3-dimensional NMR experiments (COSY, TOCSY, NOESY, ROESY, HETCOR, DEPT and other advanced NMR pulse sequences). A Sun Ultra Sparc 5 host computer is available on the instrument.
This instrument is suitable for:
- direct observation of 1H, 13C, 19F, and 31P without need of hardware modification or tunning
- multinuclear NMR experiments
- extended duration variable temperature experiments in the range –80 to 130°C
- structure elucidation on small samples of complex molecules using 2-D experiments with gradients and indirect detection.
MERCURY-VX 400 MHz NMR at WORK
How can we use NMR system power to solve a complex chemical structural problem efficiently and quickly?
The key is to use 1-D and 2-D NMR techniques to map out the connectivity of the atoms as well as the 3-dimensional molecular structure and stereochemistry. Let’s look at a complex compound to illustrate the power of modern structure determination experiments, all of which are available at the push of a button with a Mercury-VX 400 MHz NMR spectrometer.
Taxol : antineoplastic Concentration:
anticancer drug 5 mg/0.6 ml in CDCl3
MW: 853.906 (MS)
C47H51 NO14 (number of constitutional isomers >> 1030 !!!)
Fundamental Structure Determination Techniques
To determine the fundamental structure, we explore:
- techniques to correlate carbon atoms to directly attached protons,
- techniques to assign proton-proton spin couplings
- techniques to assign long-range proton-carbon connectivity,
- techniques to determine the 3-dimensional structure by through-space proton-proton interactions.
Strategy and Experimental Plan
The more detailed strategy and experimental plan is as follows:
1. Determine the number of 1H’s, the 1H chemical shifts, the 1H-1H couplings
- 1-D 1H spectra
2. Determine the number of and types of 13 C’s (C, CH, CH2, CH3)
- 1-D 13 C spectra
3. Determine the direct and long-range 1H-13 C connectivities
- 2-D 1H-13C correlation experiments
4. Determine the 1H spin systems and coupling networks
- 1-D and 2-D 1H correlation experiments
5. Determine the 3- dimensional structure and stereochemistry
- 1-D and 2-D NOE experiments
1. 1-D 1H NMR
gives information about the number and chemical shifts of 1H and 1H-1H couplings
- Variation on 1-D experiment include 1-D NOE
- Selective decoupling
- Solvent saturation
- T1 determination
2. COSY- Proton-proton correlation experiment
Gives information about pairs of protons that are J-coupled. This usually indicates that the protons are on adjacent carbons, e.g., 3-bonds away (though protons further apart may in some cases be J-coupled)
- Variations on COSY
- DQF (Double Quantum Filtered)- COSY
Diagonal peaks are narrower
4-5 bond connectivities
- TOCSY- total proton correlation spectroscopy
- Gives correlations for all protons within a spin system
3. NOESY-proton-proton through space interactions via NOE
Gives information about pairs of protons that are close in space(< 5 A° apart)
ROESY-rotating frame NOE
- Compounds of molecular weigh t~1000-2000
- Exchange peaks are opposite sign from NOE peaks
4. HMQC- Heteronuclear Multiple Quantum Correlation Experiment
Gives information about strong proton-carbon J-couplings. A strong proton-carbon J-coupling indicates that the proton is directly bonded to the carbon. This experiment gives information that is identical to HETCOR, but because it is proton-detected, it is more sensitive than the standard HETCOR especially on indirect-detection probes.
5. HMBC- Heteronuclear Multiple Bond Correlation Experiment
Gives information about weak proton-carbon J-couplings. A weak proton-carbon J-coupling indicates that the proton is two, three, or four bonds away from the carbon. This experiment gives information about which protons are near to (but not directly bonded to) different carbons. This experiment (in conjuction with the HMQC) can give an enormous amount of information about molecular structure, since the long range proton-carbon correlations can include quaternary carbons, in addition to protonated carbons.
1. 1D Carbon experiment
Gives information about the numbers of magnetically non-equivalent carbon atoms.
2. DEPT experiment
Gives information about the number of protons bonded to each carbon.
3. HETCOR – proton –carbon correlation experiment
Gives information about strong proton-carbon J-couplings. A strong proton-carbon J-coupling indicates that the proton is directly bonded to the carbon. This experiment gives information identical to HMQC, but because it is carbon-detected, it is less sensitive than the HMQC experiment.