Organic Chemistry,7th Edition L.G.Wade,Jr. induced magnetic field Chapter 13 Nuclear Magnetic Resonance Spectroscopy Copyright 2010 Pearson Education,Inc
Chapter 13 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Nuclear Magnetic Resonance Spectroscopy
Introduction NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1H ■13C ■15N ■19F ■31P Chapter 13 2
Chapter 13 2 Introduction • NMR is the most powerful tool available for organic structure determination. • It is used to study a wide variety of nuclei: ▪ 1H ▪ 13C ▪ 15N ▪ 19F ▪ 31P
Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged nucleus generates a magnetic field. (current) spinning proton loop of current bar magnet Copyright2010 Pearson Prentice Hall,Inc. Chapter 13 3
Chapter 13 3 Nuclear Spin • A nucleus with an odd atomic number or an odd mass number has a nuclear spin. • The spinning charged nucleus generates a magnetic field
External Magnetic Field twist Bo S N Bo Bo N N N N lower energy higher energy more stable less stable Copyright2010 Pearson Prentice Hall,Inc. An external magnetic field(Bo)applies a force to a small bar magnet,twisting the bar magnet to align it with the external field. The arrangement of the bar magnet aligned with the field is lower in energy than the arrangement aligned against the field. Chapter 13 4
Chapter 13 4 External Magnetic Field • An external magnetic field (B0) applies a force to a small bar magnet, twisting the bar magnet to align it with the external field. • The arrangement of the bar magnet aligned with the field is lower in energy than the arrangement aligned against the field
Alpha-spin State and Beta-spin State. (externally applied field) a-spin state B-spin state (lower energy) (higher energy) Copyght010 Pearson Prentice Ha Inc The lower energy state with the proton aligned with the field is called the alpha-spin state. The higher energy state with the proton aligned against the external magnetic field is called the beta- spin state. Chapter 13 5
Chapter 13 5 Alpha-spin State and Beta-spin State. • The lower energy state with the proton aligned with the field is called the alpha-spin state. • The higher energy state with the proton aligned against the external magnetic field is called the betaspin state
Proton Magnetic Moments B spin higher energy △E Bo a spin lower energy external field Bo increases No Field Magnetic Field spins in all directions spins aligned with the field (a) no energy difference or against the field(B) Copyright 2010 Pearson Prentice Hall,Inc. Chapter 13 6
Chapter 13 6 Proton Magnetic Moments
Two Energy States A nucleus is in resonance when it is irradiated with B state radio-frequency photons having energy equal to the energy difference between the spin states. Bo hv=△E Under these conditions,a proton in the alpha-spin state can absorb a photon a state and flip to the beta-spin Copyright2010 Pearson Prentice Hall.Inc state. Chapter 13
Chapter 13 7 Two Energy States • A nucleus is in resonance when it is irradiated with radio-frequency photons having energy equal to the energy difference between the spin states. • Under these conditions, a proton in the alpha-spin state can absorb a photon and flip to the beta-spin state
△E and Magnet Strength Energy difference is directly proportional to the magnetic field strength. △E=hv=YhBo 2元 Gyromagnetic ratio,y,is a constant for each nucleus(26,753 s-1gauss-1 for H). In a 14,092 gauss field,a 60 MHz photon is required to flip a proton. Low energy,radio frequency. Chapter 13 8
Chapter 13 8 E and Magnet Strength • Energy difference is directly proportional to the magnetic field strength. E = h = h B0 2 • Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for H). • In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. • Low energy, radio frequency
Magnetic Shielding If all protons absorbed the same amount of energy in a given magnetic field,not much information could be obtained. But protons are surrounded by electrons that shield them from the external field. 。 Circulating electrons create an induced magnetic field that opposes the external magnetic field. Chapter 13 9
Chapter 13 9 Magnetic Shielding • If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. • But protons are surrounded by electrons that shield them from the external field. • Circulating electrons create an induced magnetic field that opposes the external magnetic field
Shielded Protons electrons shielding effective effective field field 70.458 70.459 300 MHz 300MH 300MH absorbs does not absorbs absorb Bo Bo Bo 70.459 gauss 70,459 gauss 70,460 gauss naked proton shielded proton stronger applied field absorbs at 70.459 G feels less than 70,459 G compensates for shielding Copyright 2010 Pearson Prentice Hall.Inc. A naked proton will absorb at 70,459 gauss. A shielded proton will not absorb at 70,459 gauss so the magnetic field must be increased slightly to achieve resonance. Chapter 13 10
Chapter 13 10 Shielded Protons • A naked proton will absorb at 70,459 gauss. • A shielded proton will not absorb at 70, 459 gauss so the magnetic field must be increased slightly to achieve resonance