CHAPTER 10 Using Nuclear Magnetic Resonance Spectroscopy to Deduce Structure

Physical and Chemical Tests10-1 Purification: Chromatography Distillation Recrystallization Comparison to known compounds: Melting point Boiling point … many other properties When the properties of an unknown purified substance match those in the literature for a known compound, the identity and structure of the substance are still not known with certainty. Many new substances are newly synthesized for the first time and their properties are not in the literature. 2

Elemental analysis reveals the gross composition of the sample. Chemical tests identify the functional groups present. For larger molecules, knowledge of the composition and functional groups present in a substance are not enough to determine the chemical structure of the substance. For instance, the alcohol C 7 H 16 O:

Defining Spectroscopy10-2 Spectroscopy is a technique for analyzing the structure of molecules, usually based on how they absorb electromagnetic radiation. Four types are most often used in organic chemistry: Nuclear Magnetic Resonance spectroscopy (NMR) Infrared spectroscopy (IR) Ultraviolet spectroscopy (UV) Mass spectroscopy (MS) NMR spectroscopy of C and H provides the most detailed information regarding the atomic connectivity of a molecule. 4

Defining Spectroscopy10-2 Molecules undergo distinctive excitations. Electromagnetic radiation can be described as a wave having a wavelength (λ), a frequency (ν) and a velocity (c). The speed of light in a vacuum is 3 x cm s -1 or 3 x 10 8 m s -1. The units of wavelength must match those used for the speed of light. The units of frequency are cycles s -1 (or just s -1 ) or Hertz (Hz).

Molecules absorb energy in discrete packets called “quanta”. A quanta of electromagnetic radiation is referred to as a photon. The energy of a photon is determined by the frequency of the incident radiation: E = hν When a photon of energy is absorbed by a molecule, it causes electronic excitation or mechanical motion to occur. The electronic excitations and motions of a particular molecule are also quantized so only certain frequencies of radiation are able to be absorbed. An analysis of the frequencies of electromagnetic radiation absorbed by a molecule provides information about the arrangement of the atoms in the molecule. 6

The lowest energy state of a molecule is called the ground state. Absorption of electromagnetic radiation causes the molecule to move to an excited state. The difference in energy between the excited state and the ground state must be exactly equal to the energy of the photon absorbed.

Absorption of X-rays results in the promotion of electrons from inner atomic shells to outer ones (electronic transitions). This requires X-ray energies greater than 300 kcal mol -1. UV and visible absorption excites valence shell electrons, typically from a filled bonding to an unfilled antibonding orbital. This involves energies between 40 and 300 kcal mol -1. IR absorption causes bond vibration excitation: 2 ~10 kcal mol -1. Microwave radiation excites bond rotations: ~10 -4 kcal mol -1. Radiowaves, in the presence of a magnetic field, produces alignment of nuclear magnetism: ~10 -6 kcal mol -1  basis of NMR. 8

In this diagram, frequency is specified in units of wavenumbers, defined as 1/λ, which is the number of waves per centimeter. Wavenumbers are used to specify energy in infrared spectroscopy.

A spectrometer records the absorption of radiation. Continuous Wave Spectrometry (CW) Radiation of a specific wavelength (UV, IR, NMR, etc.) is generated and passes through a sample. The frequency of the radiation is continuously changed and the intensity of the transmitted beam is detected and recorded. Frequencies that are absorbed by the sample appear as peaks deviating from a baseline value. 10

Fourier Transform Spectroscopy (FT) A much faster technique. A pulse of electromagnetic radiation covering the entire spectrum under scrutiny (NMR, UV, IR) is used to obtain the whole spectrum instantly. The pulse may be applied multiple times and the results accumulated and averaged, which provides for very high sensitivity. The signal measured is actually the decay, with time, of the absorption event. This signal is then mathematically transformed using a Fourier transform, producing the more familiar frequency versus absorption plot.

Proton Nuclear Magnetic Resonance10-3 Nuclear spins can be excited by the absorption of radio waves. Many nuclei can be thought of as spinning on their axes, either clockwise or counterclockwise. One such nucleus is the hydrogen nucleus: 1 H. A 1 H nucleus is positively charged and its spinning motion generates a magnetic field. In the presence of an external magnetic field, H 0, the magnetic field of the hydrogen nucleus can be oriented either with H 0 (lower energy) or against H 0 (higher energy). These two states are called  and  spin states, respectively. 12

The difference in energy between the  and  states depends directly on the external magnetic field strength, H 0. 21,150 G 90 MHz 42,300 G 180 MHz 70,500 G300 MHz The actual energy difference is small. At 300 MHz, the energy difference for a proton is about 3 x kcal mol -1. Because the energy difference is so small and the equilibrium between the two states is so fast, the numbers of nuclei in the two states are nearly equal, however, a slight excess will be in the  state because of the external magnetic field. When electromagnetic radiation having the same energy as energy difference strikes the nucleus, the electromagnetic radiation is absorbed and the slight excess of nuclei in the  state is reduced.

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Many nuclei undergo magnetic resonance. In general, nuclei composed of an odd number of protons ( 1 H and its isotopes, 14 N 19 F, and 31 P) or an odd number of neutrons ( 13 C) show magnetic behavior. If both the proton and neutron counts are even ( 12 C or 16 O) the nuclei are non-magnetic.

Using NMR Spectra to Analyze Molecular Structure 10-4 The position of an NMR absorption of a nucleus is called its chemical shift. Chemical shifts depend upon the electron density around a nucleus and are thus controlled by the structural environment of the nucleus. The NMR chemical shifts provide important clues for determining the molecular structure of a chemical compound. 16

The position of an NMR signal depends on the electronic environment of the nucleus. In the high-resolution 1 H NMR spectrum of chloro(methoxy)methane above, two separate resonance absorptions of hydrogen are observed. These absorptions reflect the differing electronic environments of the two types of hydrogen nuclei present. Electrons in the bonds connecting the hydrogen atoms to the molecule affect the NMR absorptions.

Bound hydrogens are connected to a molecule by orbitals whose electron density varies: Bond polarity Hybridization of the attached atom Presence of electron withdrawing/donating groups The electrons in these orbitals are affected by the external magnetic field, H 0, in such a way as to generate a small local magnetic field, h local, opposing H 0. The total magnetic field seen by the hydrogen nucleus is the sum of these two fields and is thus reduced. The hydrogen nucleus is said to be shielded from H 0 by its electron cloud. 18

The degree of shielding of a nucleus depends upon its surrounding electron density. Adding electrons increases shielding. Removing electrons causes deshielding. Shielding causes a displacement of an NMR peak to the right in the spectrum (shifted upfield). Deshielding causes a displacement to the left (shifted downfield).

Chemically equivalent hydrogens in a molecule all have identical electronic environments and therefore show NMR peaks at the same position. In the NMR spectrum of 2,2-dimethyl-a-propanol, there are three different peaks due to absorptions by: Nine equivalent methyl hydrogens on the butyl group (most shielded); One hydrogen on the OH; Two equivalent methylene hydrogens. 20

The chemical shift describes the position of an NMR peak. Rather than reporting the exact frequency of each resonance in an NMR spectrum, we measure frequencies relative to an internal standard, tetramethylsilane, (CH 3 ) 4 Si. To remove the effect of differing applied magnetic fields using different spectrophotomers, the frequencies relative to tetramethylsilane are divided by the frequency of the spectrometer. This yields the chemical shift (δ), a field-independent number measured in ppm.

For (CH 3 ) 4 Si, δ is defined as The spectrum above would be reported as: 1 H NMR (300 MHz, CDCl 3 ) δ = 0.89, 1.80, 3.26 ppm 22

Functional groups cause characteristic chemical shifts. Each type of hydrogen in a molecule has a chemical shift which depends upon its chemical environment.

The absorptions of alkane hydrogens occur at relatively high field. Hydrogens close to an electron withdrawing group (halogen or oxygen) are shifted to relatively lower field (deshielding). The more electronegative the atom, the more the deshielded methyl hydrogens are relative to methane. 24

Multiple substituents exert a cumulative effect. The deshielding influence of electron withdrawing groups diminishes rapidly with distance.

Hydroxy, mercapto, and amino hydrogens absorb over a range of frequencies. The absorption peak of the proton attached to the heteroatom may be relatively broad. This variability of chemical shift is due to hydrogen bonding and depends upon: Temperature; Concentration; Presence of H-bonding species such as water (moisture). When line broadening is observed, it usually indicates the presence of OH, SH, or NH 2 (NHR) groups. 26

Tests for Chemical Equivalence10-5 Generally, chemically equivalent Hs have the same chemical shift. To see chemically equivalent nuclei, symmetry operations are checked to decide on the expected NMR spectrum. Rotational symmetry results in equivalent protons when the group of protons is rapidly rotating, as in a methyl group.

Conformational interconversion may result in equivalence on the NMR time scale. In the case of the rapid rotation of the methyl group in chloroethane, or the rapid conformation flip in cyclohexane, the observed chemical shifts are the averages of the values that would be observed without the rapid rotation or flip. 28

In the case of cyclohexane, the single line in the NMR spectra at δ = 1.36 ppm at room temperature becomes two lines at a temperature of -90 o C, one at δ = 1.12 ppm for the six axial hydrogens and one at δ = 1.60 for the six equatorial hydrogens. At this temperature, the conformational flip of the benzene is slower than the NMR time scale. In general, the lifetime of a molecule in an equilibrium must be on the order of one second to allow its resolution by NMR.