FTNMR University of Massachusetts Boston a student 傅里叶变换核磁共振麻州大学波士顿个学生.pptVIP

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FTNMR University of Massachusetts Boston a student 傅里叶变换核磁共振麻州大学波士顿个学生.ppt

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FTNMR University of Massachusetts Boston a student 傅里叶变换核磁共振麻州大学波士顿个学生

FT-NMR Fundamentals Nuclear spin Spin quantum number – ? Nuclei with spin state ? are like little bar magnets and align with a B field. Can align with (++) or against (+-) B Small energy gap between + and – spin alignment (NMR insensitive/Boltzman dist) Can probe difference with RW (NMR insensitive/Boltzman dist) Small population difference between +1/2 and -1/2 state It is the small excess of nuclei in the -1/2 that produce NMR signal Common NMR nuclei Protons, 1H 13C 15N 19F 31P Sensitivity depends on natural isotopic abundance and g DE = g?B0 , bigger magnet, greater sensitivity Precession of nuclear dipoles FT pulse Radiofrequency generator A short, intense pulse generates a magnetic field in the x-y plane (excites all nuclei) M0 of the nuclei interacts with the magnetic field produced by the pulse. Tips M0 off axis Θ = gB1tp tp – length of pulse, 90? pulse Vector Illustration of the pulse Relaxation T1 spin-lattice (relaxing back to precessing about the z axis) T2 spin-spin (fanning out) Induced current in coil After pulse, nuclei begin to precess in phase in the x-y plane Packet of nuclei induce current in RF coil Relaxation is measured by monitoring the induced coil → FID (→ FT) NMR spectrum FID Noise reduction and increasing resolution Apodization: Multiply the free-induction decay (FID) by a decreasing exponential function which mathematically suppresses the noise at long times. Other forms of apodization functions can be used to improve resolution or lineshape. Zero filling Chemical Shift Shielding Electrons have spin, produce local B environments Protons in different electronic environments experience different B, different precessing frequencies, DE = hu Chemical shift proportional to size of magnet ppm {(s-s0)/s0}*106 Spin-Spin Coupling Adjacent nuclei have a 50/50 chance of being spin up (+1/2) or spin down (-1/2) Each produce a small magnetic field that is either with or against B0 1 adjacent proton CHOCH3 CH3 is a doublet at frequencies