tailieunhanh - Nuclear Magnetic Resonance 2

NMR in solids, like solution-state, relies on the behavior of nuclear spin energy levels in a magnetic field. However, the interactions that affect NMR spectra act differently. In liquids, molecules reorient and diffuse quickly, leading to narrow isotropic resonances. In solids, the fixed orientation of individual crystallites leads to a range of resonance frequencies for anisotropic interactions. | a W VILLANOVA UNIVERSITY Nuclear Magnetic Resonance 2 Lecture Date February 13th 2008 NMR Experiments NMR experiments fall into some basic categories - Basic pulse methods Single pulse Selective pulse or selective decoupling Solvent suppression - 2D and multi-dimensional experiments unravel complex spectra by separation of overlapping signals control of mixing between signals to obtain more data - Multiple resonance heteronuclear techniques Are often 2D or nD sequences - Diffusion dynamics and relaxation experiments 1 Common Solution-state NMR Experiments for Organic Structural Analysis Experiment Acronym Information Provided GASPE DEPT Gated-spin echo Distortionless editing by polarization transfer 13C multiplicity C CH CH2 CH3 COSY correlated spectroscopy 1H-1H covalent bonding 2-4 bonds HMQC heteronuclear multiple quantum coherence 1H-13C covalent bonding 1 bond HMBC heteronuclear multiple bond correlation 1H-13C covalent bonding 2-4 bonds NOE difference NOESY ROESY nuclear Overhauser effect spectroscopy 1H-1H proximity in space 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 - Chemical shift - Quadrupolar coupling - Dipolar coupling Or are designed to measure a bulk effect - Relaxation - Diffusion - Chemical exchange or dynamics 2 Basic Pulse Sequences A single pulse and acquire An Example of 1D NMR Top - 1H spectrum Middle - Selective pulse Bottom - homonuclear decoupling