MCB 124 Lecture 4 on NMR Fall 2010

MCB 124 Lecture 4 on NMR Fall 2010 - = exp(- hB o /kT) Most...

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Unformatted text preview: = exp(- hB o /kT) Most nuclei "spin" about an axis of rotation in a manner analogous to electron spin*. The spinning nuclear positive charge generates a magnetic dipole. NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY Randomly Oriented Nuclear Dipoles E=h Nuclear Dipoles in a Magnetic Field B B 2 Larmor equation N U N L E = hB 0 h = hB The frequency of absorption is dependent on and B o . is a constant for each diff erent kind of nucleus ( 1 H, 2 H, 13 C, 31 P, etc.) The ratio of population of dipoles in the two energy states is given by = .999999 = h = h/2 *Nuclei with both even mass and atomic number, like 12 C, do not spin and give no NMR signal. 1 ~ Nuclei with large (such as 1 H) have higher sensitivity and absorb at higher frequency than nuclei with smaller (such as 13 C, 31 P). = exp(- E/kT) MCB 124: PROTEIN NMR The energy required for a nuclear spin transition is very small, about 1x10-25 Joules. This corresponds to low frequencies in the electromagnetic spectrum. low energy high energy low frequency high frequency UV Visible Infrared M i c r o w a v e s Radiowaves T V X-rays -rays The low energy/frequency of NMR means it is a very insensitive but very "gentle" technique. 10 11 MHz 10 10 MHz 10 8 MHz 10 2 MHz 10 14 MHz Comparison of sensitivity and frequency for diff erent nuclei in a fi xed fi eld B o =11.75T 50 125 202 500 15 N 31 P 1 H 13 C , sensitivity Thus for 1 H, experimental sensitivity = (1)(1)=1 For 13 C experimental sensitivity =(.01)(.01)= 1x10-4 The relative sensitivity is a function of . By convention, the relative sensitivity of 1 H = 1. For 31 P experimental sensitivity =(.05)(1) = 5x10-2 frequency, MHz Experimental sensitivity = (relative sensitivity)(fractional natural abundance) 2 H 77 NMR 2 Nuclear spins are eff ected by their molecular environment in several ways. 1) The surrounding distribution of electrons slightly alters the local magnetic fi eld experi- enced by each nucleus giving rise to chemical shift. Chemical shift is the change in peak position of a nucleus due to its surrounding distribution of electrons. In addition, nuclear spins can interact with one another in two ways. 2) Through intervening chemical bonds. This is a scalar interaction and is termed J coupling or spin-spin coupling. C C H H 3) Through space. This is a dipolar interaction and is termed an NOE interaction. H H 3 The Larmor equation predicts that all nuclei of each isotope would absorb at the same frequency, but this is not the case due to chemical shift. The distribution of electrons around each nucleus produces slightly diff erent magnetic fi elds for each nucleus in a diff erent molecular (chemical) environment....
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MCB 124 Lecture 4 on NMR Fall 2010 - = exp(- hB o /kT) Most...

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