tailieunhanh - Lecture Organic chemistry: Chapter 13 - L. G. Wade, Jr.

Chapter 13 - Nuclear magnetic resonance spectroscopy. In this chapter, students will be able to understand: Use the chemical shifts, splitting patterns, and integrations shown in a proton NMR spectrum to propose structures for possible compounds; use the number of peaks and their chemical shifts in a 13C NMR spectrum to determine the number of types of carbon atoms in the compound and what functional groups they might represent;. | Chapter 13 ©2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Nuclear Magnetic Resonance Spectroscopy Chapter 13 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 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. Chapter 13 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. Chapter 13 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 . | Chapter 13 ©2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Nuclear Magnetic Resonance Spectroscopy Chapter 13 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 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. Chapter 13 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. Chapter 13 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 beta-spin state. Chapter 13 Proton Magnetic Moments Chapter 13 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. Chapter 13 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. Chapter 13 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