Spin_Relaxation_10

Spin_Relaxation_10 - Spin Relaxation BCMB/CHEM 8190 T1, T2,...

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Spin Relaxation BCMB/CHEM 8190
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T 1 , T 2 , NOE (reminder) T 1 is the time constant for longitudinal relaxation - the process of re-establishing the Boltzmann distribution of the energy level populations of the system following perturbation T 2 is the time constant for transverse relaxation - loss of phase coherences of the nuclear dipoles in the transverse plane The Nuclear Overhauser Effect is the change in intensity for a signal (resonance) when the equilibrium spin populations of a different spin are perturbed
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What are the origins of T 1 and T 2 relaxation and the Nuclear Overhauser Effect (NOE)? Key: A Fuctuating interaction is capable of causing a transition -just like an rf pulse. H (t) = -B 1 (t) γ Ix P ββ -> αβ |< ββ | γ B 1 ( I x1 + I x2 )| αβ >| 2 ; I x = ( I + + I - )/2 But, B 1 (t) is natural in origin (tumbling of molecules) Note: H (t) must have both correct spin operator to connect states and a Fuctuation at the right frequency, Δ E = h υ Some sources of interaction: -chemical shift anisotropy -dipole-dipole (nucleus-nucleus or nucleus-electron) -nuclear quadrupole - electric ±eld gradient -others….
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Chemical Shift Anisotropy (CSA) Chemical shifts arise from electronic shielding of the nucleus -shielding depends on orientation of the molecule with respect to B 0 -the orientation dependent chemical shift differences or range is called the CSA -in solution, rapid reorientation results in averaging of the chemical shift Rapid molecular reorientation results in local, Fuctuating magnetic ±elds (magnitude and direction) -these local Fuctuating ±elds lead to energy level transitions, just like applied ±elds
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Spin_Relaxation_10 - Spin Relaxation BCMB/CHEM 8190 T1, T2,...

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