NMR Lecture 3 Oct.26, PSC 311 Zhenming (Jimmy) Du … · 2017. 12. 23. · 1.The absolute value...
Transcript of NMR Lecture 3 Oct.26, PSC 311 Zhenming (Jimmy) Du … · 2017. 12. 23. · 1.The absolute value...
NMR Lecture 3Oct.26, PSC 311
Zhenming (Jimmy) [email protected]; NSC 134
Q1: What is 2D NMR?
Q2: Why 2D NMR?
Q3: Which 2D NMR?
1. Resolves crowed peaks
2. 2nd Dimension information and correlation
3. Cross‐peak information (phase, intensity…)Etc .NOESY intensity for structure determination
Homonuclear Heteronuclear
COSYDQF‐COSYTOCSYNOESYROESY
INADEQUATE
HETCORHSQCHMQCHMBC
Pro Con
SimpleEasy for set upForgiveness to pulse length error
Least information for H‐H 2DMainly used for smallmolecule assignment
Warnings:Strong spin lock power (25 μs)Sample heating
Spin‐lockLower power level in ROESY
Absolute value or phase sensitive HETCOR?1.The absolute value HETCOR is easier to set up and process.
2.The phase sensitive HETCOR gives you narrower lines. You also have the choice to see the CH‐groups only or to see the CH‐and CH3 resonances positive and the CH2 resonances negative.
3.The final work‐up of the phase sensitive HETCOR requires phasing in both dimensions, which can be rather time‐consuming and difficult, but is do‐able. You need to be aware of peaks coming from incomplete cancellations, and have to neglect them in your phasing effort.
HETCOR Advantages:
1.The set‐up is shorter.
2.To cancel artifacts, less scans are acquired, so overall HETCOR requires less time to run.
3.1H‐1H couplings are removed, while they are present in the normal HMQC. However, you can run a homonucleardecoupled version of the HMQC experiment to also remove those couplings.
4.Variations of the HETCOR can show only methines, or methines and methyls positive and methylenes negative. HMQC does not have these variations.
HMQC Advantages:
1.The sensitivity is 2 to 4 times greater than HETCOR for small molecules, 4 times for large molecules.
2.The 1H dimension is the "high resolution" dimension; while in HETCOR this is the 13C dimension. Typically, 1H chemical shifts span only ~10ppm, while 13C chemical shifts span ~200ppm and are normally well resolved anyway. It is therefore advantageous for the 1H dimension to have high resolution.