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Organic Chemistry Text Book (CHEM 3401 and 3402)

14.1 Introduction of Molecular Spectroscopy

The spectroscopic techniques described below do not provide a three-dimensional picture of a molecule, but instead yield information about certain characteristic features. A brief summary of this information follows: 

•   Mass Spectrometry: Sample molecules are ionized by high energy electrons. The mass to charge ratio of these ions is measured very accurately by electrostatic acceleration and magnetic field perturbation, providing a precise molecular weight. Ion fragmentation patterns may be related to the structure of the molecular ion.
•   Ultraviolet-Visible Spectroscopy: Absorption of this relatively high-energy light causes electronic excitation. The easily accessible part of this region (wavelengths of 200 to 800 nm) shows absorption only if conjugated pi-electron systems are present.
•   Infrared Spectroscopy: Absorption of this lower energy radiation causes vibrational and rotational excitation of groups of atoms. within the molecule. Because of their characteristic absorptions identification of functional groups is easily accomplished.
•   Nuclear Magnetic Resonance Spectroscopy: Absorption in the low-energy radio-frequency part of the spectrum causes excitation of nuclear spin states. NMR spectrometers are tuned to certain nuclei (e.g. 1H, 13C, 19F &  31P). For a given type of nucleus, high-resolution spectroscopy distinguishes and counts atoms in different locations in the molecule. 

Electromagnetic radiation, as you may recall from a previous chemistry or physics class, is composed of oscillating electrical and magnetic fields. Visible light is electromagnetic radiation. So are the gamma rays that are emitted by spent nuclear fuel, the x-rays that a doctor uses to visualize your bones, the ultraviolet light that causes a painful sunburn when you forget to apply sun block, the infrared light that the army uses in night-vision goggles, the microwaves you use to heat up your frozen burritos, and the radio-frequency waves of your cell phone signal.

While the speed of a wave in the ocean can vary, the speed of electromagnetic waves – commonly referred to as the speed of light – is a constant, approximately 300 million meters per second, whether we are talking about gamma radiation, visible light, or FM radio waves. Electromagnetic radiation is defined by its wavelength, which is the distance between one wave crest to the next.

Because electromagnetic radiation travels at a constant speed, each wavelength corresponds to a given frequency, which is the number of times per second that a crest passes a given point. Longer waves have lower frequencies, and shorter waves have higher frequencies. Frequency is commonly reported in hertz (Hz), meaning ‘cycles per second’, or ‘waves per second’. The equivalent standard unit for frequency is s-1.

When talking about electromagnetic waves, we can refer either to wavelength or to frequency - the two values are inversely proportional:

where  (Greek 'lambda') is wavelength,  (Greek ‘nu’) is frequency in s-1 (or Hz), and c is the speed of light, a constant value of 3.0 x 108 m·s-1.

Electromagnetic radiation transmits energy in discreet quantum 'packages' called photons. Shorter wavelengths (and higher frequencies) correspond to higher energy. High energy radiation, such as gamma radiation and x-rays, is composed of very short waves – as short as 10-16 m. Longer wavelengths are far less energetic, and thus are less dangerous to living things. Visible light waves are in the range of 400 – 700 nm (nanometers, or 10-9 m), while radio waves can be several hundred meters in length.

The energy of a particular wavelength of electromagnetic radiation can be expressed as: equation 4.2 E = hc/h  where E is energy in kJ/mol of photons and h is 3.99 x 10-13 kJ·s·mol-1, a number known as Planck’s constant.

Using equations 4.1 and 4.2, we can calculate, for example, that visible red light with a wavelength of 700 nm has a frequency of 4.29 x 1014 Hz, and an energy of 171 kJ per mole of photons.

The full range of electromagnetic radiation wavelengths is referred to as the electromagnetic spectrum.