Nuclear Magnetic Resonance 2

Nuclear Magnetic Resonance 2 Lecture Date: February 13th, 2008 NMR Experiments  NMR experiments fall into some basic categories: – Basic pulse methods  Singlepulse  Selectivepulse or selective decoupling  Solvent suppression – 2D and multi-dimensionalexperiments  unravel complexspectra by separationof overlappingsignals, controlof “mixing”between signals (to obtainmoredata) – Multipleresonance (heteronuclear techniques)  Are often 2D or nD sequences – Diffusion,dynamics and relaxationexperiments 1 Common Solution-state NMR Experiments for Organic Structural Analysis Experiment GASPE DEPT COSY HMQC HMBC NOE difference, NOESY, ROESY Acronym Gated-spin echo Distortionless editing by polarization transfer correlated spectroscopy heteronuclear multiple quantum coherence heteronuclear multiple bond correlation nuclear Overhauser effect spectroscopy Information Provided 13C multiplicity (C, CH, CH2, CH3) 1H-1H covalent bonding, 2-4 bonds 1H-13C covalent bonding, 1 bond 1H-13C covalent bonding, 2-4 bonds 1H-1H proximity in space, 1.8-4.5 A Pulse Sequences  Modern NMR involves flexible spectrometers that can implement pulse sequences, which are designed to extract and simplify relevant information for the spectroscopist  Designed to harness a property or properties of the nuclear spin Hamiltonians – J-coupling – Chemicalshift – Quadrupolar coupling – Dipolar coupling  Or, are designed to measure a bulk effect – Relaxation – Diffusion – Chemicalexchange or dynamics 2 Basic Pulse Sequences  Asingle pulse and acquire An Example of 1D NMR Top – 1H spectrum Middle– Selectivepulse Bottom– homonuclear decoupling 3 Multi-dimensional NMR  The general scheme of 2D and multi-dimensionalNMR: Can includeNOE or J-couplingmixing Preparation Evolution (t1) Mixing (tm) Detection (t2) ExperimentTime  2D NMR data has two frequency dimensions: FT(t1) FT(t2) A Simple 2D NMR Spectrum 1 Cross peak (“correlation”) 2 3 F1 (ppm) 4 Diagonal Peak 5 5 4 3 2 1 F2 (ppm) 4 An Example of 2D NMR – the COSY Experiment Correlationsare observed between J-coupled protons! (Exampleis a sample of sucrose in D2O) Applications of NMR  Structural analysis  Quantitativeanalysis  Stereochemical and conformational analysis  Solid-stateanalysis 5 ... - tailieumienphi.vn