L39 - NMR And MRI - Advance Interpretation
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How can you measure coupling?
• measure a, measure b • measure spliiting for a, how far a ppm is for b • divide a by b, multiply by the Hz per ppm
What is coupling constant?
Protons couple to each other, with certain intensity (vary from 0 to 18 Hz, usually ~ 7)
What happens when a molecule contains a proton which is coupled to 2 different protons with different coupling constants?
Different splitting pattern
What happens to the coupling constant when there’s a freely rotating alkane?
There’s am observed average across the different coupling constants
What are coupling constants like in alkenes?
• differ depending on cist or trans • trans - quite large (12-18Hz) • cis - smaller (8-11Hz)
How does symmetry in an aromatic systems affect NMR?
• 2 Ha protons = 2 Hb protons • chemically equivalent, same chem shift, same coupling • different couping pathway, JHH not the same
What is coupling like in simple aromatic rings?
• size of coupling depends on orientation of protons to each other • coupled by 2C (ortho) = JHH 8-10Hz • coupled by 3C (meta) = JHH 1-2.5Hz • coupled by 4C (para)= JHH 0.25-0.50Hz (not observed) • meta coupling (3C) aka W coupling
What is 13C-NMR like compared to H-NMR?
• had 1.1%natural abundance • 1/2 nucleus • 400 times less sensitive than H nucleus • dont usually see 13C-13C coupling • chem shift 0-220ppm • chem shift measured with respect to tetramethylsilane • simalar factors affect chem shift • long relaxation times = no integrations • broadband, proton decoupled = single lines • number of peaks = number o types of C atom
What is broadband decoupled like?
• one bond C-H coupling retained • signal for particular carbon determined bt number of attached protons in accord with n+1 rule
What is 13C chemical shift like?
• electronegativity of groups attached to C atom • Hybridisation of C
What is fully proton decoupled 13C NMR like?
• rapidly collected • appear as single peaks irrespective to num of protons connected to C • same height unless originate from 2 chemically and structurally equivalent protons
What is distortionless enhancement of polarization tranfer (DEPT)?
• used to give similar info to off-resonance decoupled spectra • sequence of pulses with various delays, used to create DEPT • dif peaks respond in different fashions, based on num of protons attaches
What are the 2 DEPT experiments used in Bath?
• DEPT-135 • DEPT-90
What is DEPT-135 like?
• CH and CH3 = positive peaks • CH2 = negative peaks • Cq = invisible
What is DEPT-90 like?
• CH = yields peaks • Cq, CH2 and CH3 = invisible peaks