Physical and Chemical Tests 10-1 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.

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 Spectroscopy 10-2 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.

Defining Spectroscopy 10-2 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.

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 to 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. This is the basis of NMR.

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.

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 Resonance 10-3 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.

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.

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.

In a hypothetical scan of CH 2 ClF in a 70,500-G magnet, the following spectrum would be observed:

High-resolution NMR spectroscopy can differentiate nuclei of the same element. In the NMR spectrum of ClCH 2 OCH 3 at 70,500-G from 0 to 300 MHz, one peak would be observed for each element present. Using high-resolution NMR spectroscopy, the region around each of these peaks can be expanded and additional spectral details can be observed.