Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance Chapter 21
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Electromagnetic Radiation Electromagnetic radiationElectromagnetic radiation: light and other forms of radiant energy Wavelength ( )Wavelength ( ): the distance between two consecutive identical points on a wave Frequency ( )Frequency ( ): the number of full cycles of a wave that pass a point in a second Hertz (Hz)Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read “per second”)
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Electromagnetic Radiation Wavelength
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Molecular Spectroscopy Molecular spectroscopyMolecular spectroscopy: the study of which frequencies of electromagnetic radiation are absorbed or emitted by substances and then correlating these frequencies with specific types of molecular structure
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Molecular Spectroscopy We study two types of molecular spectroscopy
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Molecular Spectroscopy Nuclear magnetic resonance (NMR) spectroscopyNuclear magnetic resonance (NMR) spectroscopy: a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of hydrogens using 1 H-NMR spectroscopy carbons using 13 C-NMR spectroscopy phosphorus using 31 P-NMR spectroscopy
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Spin States Nuclei of 1 H and 13 C, isotopes of the two elements most common to organic compounds, have a spin and behave as if they are tiny bar magnets 12 C and 16 O do not have a nuclear spin and do not behave as tiny bar magnets thus, nuclei of 1 H and 13 C are quite different from those of 12 C and 16 O
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Spins within a collection of 1 H and 13 C atoms, nuclear spins are completely random in orientation when placed in a strong external magnetic field, interactions between their nuclear spins and the applied magnetic field are quantized, and only two orientations are allowed by convention, nuclei with spin +1/2 are aligned with the applied field and in the lower energy state; nuclei with spin -1/2 are aligned against it and in the higher energy state
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Spins For 1 H and 13 C nuclei in an applied field.
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Spins In an applied field strength of 7.05T (present-day superconducting electromagnets) the difference in energy between nuclear spin states for 1 H is approximately cal/mol, which corresponds to electromagnetic radiation of 300 MHz 13 C is approximately cal/mol, which corresponds to electromagnetic radiation of 75 MHz
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance when nuclei in the lower energy state are irradiated with radio frequency of the appropriate energy, energy is absorbed and the nuclear spin is “flipped”
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance ResonanceResonance: the absorption of electromagnetic radiation by a nucleus and the flip of its nuclear spin from a lower energy state to a higher energy state the instrument used to detect this flip of nuclear spin records it as a signal If we were dealing with 1 H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1 H would produce a signal for all 1 H the same is true of 13 C nuclei
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance But hydrogens in organic molecules are not isolated from all other atoms; they are surrounded by electrons electrons have spin and thereby create local magnetic fields; these local magnetic fields are several orders of magnitude weaker than the applied field at the molecular level, the local magnetic fields shield hydrogens from the applied field the effective magnetic field experienced by a 1 H nucleus is the applied field less the local magnetic field
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance The differences in resonance frequency among 1 H in organic molecules are very small the difference in resonance frequencies for hydrogens in CH 3 Cl compared to CH 3 F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance It is customary to measure the resonance frequency (signal) of individual nuclei relative to the 1 H and 13 C resonance frequencies (signals) of tetramethylsilane (TMS)
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nuclear Magnetic Resonance Chemical shift ( )Chemical shift ( ): the shift in ppm of an NMR signal from the signal of TMS for a 1 H-NMR spectrum, signals are reported by how far they are shifted from the resonance signal of the 12 equivalent hydrogens in TMS for a 13 C-NMR spectrum, signals are reported by how far they are shifted from the resonance signal of the one carbon in TMS
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. NMR Spectrometer Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, a radio-frequency detector, and a sample tube The sample is dissolved in a solvent, most commonly CDCl 3 or D 2 O, and placed in a sample tube which is then suspended in the magnetic field and set spinning Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. NMR Spectrum DownfieldDownfield: the shift of an NMR signal to the left on the chart paper UpfieldUpfield: the shift of an NMR signal to the right on the chart paper
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Equivalent Hydrogens Equivalent hydrogensEquivalent hydrogens: have the same chemical environment Molecules with 1 set of equivalent hydrogens give 1 NMR signal 2 or more sets of equivalent hydrogens give a different NMR signal for each set
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Signal Areas Relative areas of signals are proportional to the number of hydrogens giving rise to each signal all modern NMR spectrometers electronically integrate and record the area of each signal
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Chemical Shift - 1 H-NMR
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Chemical Shift - 1 H-NMR
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Signal Splitting (n + 1) PeakPeak: a unit into which an NMR signal is split; doublet, triplet, quartet, etc. Signal splittingSignal splitting: splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens (n + 1) rule:(n + 1) rule: the 1 H-NMR signal of a hydrogen or set of equivalent hydrogens is split into (n + 1) peaks by a nonequivalent set of n equivalent neighboring hydrogens
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Signal Splitting (n + 1) ProblemProblem: predict the number of 1 H-NMR signals and the splitting pattern of each
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 13 C-NMR Spectroscopy Each nonequivalent 13 C gives a different signal A 13 C signal is split by an 1 H bonded to it according to the (n + 1) rule In the most common mode of recording a 13 C spectrum, called the hydrogen-decoupled mode, this signal splitting is eliminated in order to simplify the spectrum in the hydrogen-decoupled mode, all 13 C signals appear as singlets
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Chemical Shift - 13 C-NMR
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Alkanes: all 1 H-NMR signals fall in the narrow range of C signals fall in the considerably wider range of Alkenes: vinylic hydrogens typically fall in the range the sp 2 hybridized carbons of alkenes give 13 C-NMR signals in the range , which is downfield from the signals of sp 3 hybridized carbons
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Alcohols: the chemical shift of the hydroxyl hydrogen is variable. It normally falls in the range , but may be as low as 0.5. hydrogens on an sp 3 hybridized carbon adjacent to the -OH group are deshielded by the electron- withdawing inductive effect of the oxygen and their signals appear in the range
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Benzene and its derivatives all six hydrogens of benzene are equivalent and their 1H-NMR signal appears as a sharp singlet at 7.27 hydrogens attached to a substituted benzene ring appear in the region in 13 C-NMR spectroscopy, carbon signals of aromatic rings appear in the range
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Amines the chemical shift of amine hydrogens, like those of hydroxyl hydrogens, is variable and may be found in the region because of rapid exchange of amine hydrogens, spin- spin splitting between amine hydrogens and hydrogens on the adjacent -carbon are prevented, and amine hydrogens generally appear as singlets
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Aldehydes and ketones aldehyde hydrogens give an 1 H-NMR signal in the range hydrogens in the alpha-carbon to an aldehyde or ketone carbonyl group typically appear in the region the carbonyl carbon of aldehydes and ketones are readily identifiable in 13 C-NMR spectroscopy by the position of their signals between
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Carboxylic acids the chemical shift of the carboxyl hydrogen is so large ( ), even large than that of the aldehyde hydrogen ( ) that it serves to distinguish carboxyl hydrogens from most other types of hydrogens the carbonyl carbon of carboxylic acids appears in an 13 C-NMR spectrum in the region
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Interpreting NMR Spectra Esters hydrogens on the alpha-carbon to the carbonyl group of an ester are slightly deshielded and give signals at hydrogens on carbon attached to the carbon of the ester oxygen are more strongly deshielded and give signals at the carbonyl carbon of an ester appears in a 13 C-NMR spectrum in the region
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Index of H Deficiency Index of hydrogen deficiency (IHD)Index of hydrogen deficiency (IHD): the sum of the number of rings and pi bonds in a molecule To determine IHD, compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds the molecular formula of the reference hydrocarbon is C n H 2n+2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Index of H Deficiency for each atom of a Group VII element (F, Cl, Br, I) added to the reference hydrocarbon, subtract one H no correction is necessary for the addition of atoms of Group VI elements (O,S) to the reference hydrocarbon for each atom of a Group V element (N, P) added to the reference hydrocarbon, add one hydrogen
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. NuclearMagneticResonance End Chapter 21