INTRODUCTION
Versão em Português
1- DEFINITION OF SPECTROSCOPY
The history of infrared spectroscopy begins with Sir Isaac Newton (1643-1727).
Sir Isaac Newton
Fig. IRC01 - Newton portrayed by Godfrey Kneller
Although he become famous for his monumental work of the laws of mechanics, Philosophiae Naturalis Principia Mathematica, better known as Principia, published in Latin in 1687, the optics was among his interests early on.
First edition of the book Philosophiae Naturalis Principia Mathematica of Isaac Newton
Fig. IRC02 - Face of the first edition of the book Principia
The first course given by Newton at Cambridge University in 1669 was about optics. His first article published in 1672 and his last book, Opticks, published in English in 1704, were studies about the light.
First edition of the book Opticks of Isaac Newton
Fig. IRC03 - Face of the first edition of the book Opticks
The refraction of light through a prism of glass had already been known before Newton, but he was the first to develop a serie of elaborated and accurated experiments. Newton established mathematical rules for the experimental measurements of the refraction of light.
Isaac Newton's experiment of refraction of light
Fig. IRC04 - Newton studying the refraction of light
In one of the experiments created by Newton, shown below, the light of the sun, indicated by the letter S, comes into a dark room through a hole in the wall and passes through a prism, indicated by P1. A screen with several small holes allows the selection of short bands of light. One of these bands of light passes through the hole, indicated by the letter G, and through a second prism, indicated by P2. This prism does not divides the light into new colors, just produces a selected color spot, which is indicated by the letter M.
Isaac Newton's experiment of refraction of light
Fig. IRC05 - One of Newton's experiments on the refraction of light
The work about optics initiated by Newton was complemented by Frederick William Herschel. It was him who discovered the infrared light.
It is interesting to note that great scientific discoveries can also be made by ordinary people. That was the case of Herschel.
Frederick William Herschel
Fig. IRC06 - Frederick William Herschel
Friedrich Wilhelm Herschel (1738-1822) was born in Hanover, Germany. He played oboe in the military band from Hanover and in 1775 his regiment was sent to England. At that time, the crowns of England and Hanover were united under the throne of George II. This brief visit caused him a good impression and in the following year he quit the regiment and moved to London. Quickly learned English and when he was 19 years old changed his name to Frederick William Herschel. Herschel devoted himself professionally to music and composed numerous musical pieces. His interest, however, was not limited to music; in his spare time Herschel devoted himself to mathematics and astronomy. He built several telescopes in his backyard and in 1871 discovered the planet Uranus.
Frederick William Herschel telescope
Fig. IRC07 - Telescope built in the backyard of Herschel
The solid knowledge of Herschel on mathematics and optics, allowed him to complement the work initiated by Newton. Herschel imagined the existence of other components of the sun light, outside the visible region. Since this region is invisible to the human eye, Herschel created an experiment to detect it. By placing a blackened thermometer bulb in the beam of the light below the red color, he noted that the temperature increased up to the room temperature. Herschel defined that temperature as "reference temperature" and called infrared to that part of the light spectrum.
In March 1800, Herschel made another discovery. By placing a sample in the path of the infrared light, he observed that by changing the part of the spectrum that passed through the sample, to his surprise, in some points the temperature suddenly decreased. Herschel concluded that in these points, the temperature decreased because the sample absorbed infrared light and, for this reason, he defined infrared spectroscopy as the "measurement of light absorption in the infrared".
Herschel also concluded that the absorption was proportional to the difference between the reference temperature and the the temperature at the points in which occured absorption. With this experiment, Herschel created the principle that would be used in the infrared spectrometer.
The development of electronics in the early 20th century, enabled creating devices to automatically generate the frequencies present in the spectrum of infrared sunlight. The first infrared spectrometer fully automated is from 1937 (E. Lehrer, Z. Techn. Phys. 1 942, 23, 169). The first commercial spectrometer, however, would only be released in the 1960s. This spectrometer was known as Michelson interferometer.
A graph generated by a spectrometer, is similar to our fingerprint. As there aren't two people with identical fingerprints, there aren't also two samples with identical spectra and, as a fingerprint is used to identify a person, a spectrum is also used to identify a sample.
The identification of a sample from its infrared spectrum is similar to a puzzle game. Let's consider, with an example, how this game works.
Fig. IRC08 - Spectrum of an unknown sample
The vibrational modes of the molecules are like the parts of a puzzle game. Molecules don't vibrate randomly. They vibrate only in some determined wave numbers. Our challenge is to identify the vibrational modes associated to each peak.
By inspecting the spectrum above, we note the presence of three broad peaks: in 3347, in 1430, and in 662. These peaks are always present in molecules which have a -OH radical.
The peak of wave number equal to 3347 is related to -OH stretch vibrational mode.
Fig. IRC09 - Peak of wave number equal to 3347
The peak of wave number equal to 1430 is related to -OH bend vibrational mode.
Fig. IRC10 - Peak of wave number equal to 1430
The peak of wave number equal to 662 is related to -OH wag vibrational mode.
Fig. IRC11 - Peak of wave number equal to 662
By continuing the inspection of the spectrum, we note the presence of three narrow and sharp peaks : in 2945, in 2833, and in 1460. These peaks are always present in molecules which have a -CH3 radical.
The peak of wave number equal to 2945 is related to -CH3 antisymmetric stretch vibrational mode.
Fig. IRC12 - Peak of wave number equal to 2945
The peak of wave number equal to 2833 is related to -CH3 symmetric stretch vibrational mode.
Fig. IRC13 - Peak of wave number equal to 2833
The peak of wave number equal to 1460 is related to -CH3 antisymmetric bend vibrational mode.
Fig. IRC14 - Peak of wave number equal to 1460
We can yet note the presence of a peak with wave number equal to 1030. This peak is always present in molecules having a carbon-oxygen chemical bond, and is related to carbon-oxygen stretch vibrational mode.
Fig. IRC15 - Peak of wave number equal to 1030
We can then conclude that the molecule has two radicals, -CH3 and -OH, and that these radicals are bonded. This spectrum is, therefore, of a methanol sample.
The sample is identified. The game is over!
2- THE NATURE OF LIGHT
In the first part of this introduction, we said that the refraction of light by a glass prism was known before Newton, and that he was the first to establish mathematical rules, through a series of accurate and precise experiments. We also said that Herschel discovered sun light was formed by infrared light and that, in 1800, he conducted an experiment which had the principle of operation of a spectrometer. Finishing the first part, we showed how an unknown sample can be identified from its spectrum.
Let's now discuss the nature of sunlight to understand the significance of the two axes that appear on a spectrum: the transmittance and the wave number.
However, before discussing the nature of light, we need to mention that in 1801, the german chemist and physicist Johann Wilhelm Ritter, motivated by the work of Herschel discovered the existence of "invisible colors" above the violet color. With this discovery, he determined the spectrum of sunlight.
Who revealed the nature of sunlight was the scottish mathematician and physicist James Clerk Maxwell, when in 1864 he mathematically predicted the existence of electromagnetic waves. According to his theory, the electromagnetic wave was made up of two fields, an electric field and a magnetic field, which laid in orthogonal planes and propagated in space carrying energy. In the two videos below we can see the representation of an electromagnetic wave, in fast motion and in slow motion.
Electromagnetic wave (fast motion)
Electromagnetic wave (slow motion)
The experimental comprovation of Maxwell's theory was made in 1888 by german physicist Heinrich Rudolf Hertz. It was him who first produced an electromagnetic wave in a laboratory. To make sure that in his experiment were being produced electromagnetic waves, he developed a detector. With this experiment, Hertz also established the principle that would be used on radio and television broadcast and for that reason, the electromagnetic waves he discovered are called TV-radio waves.
Hertz experiment
Fig. IRC16 - Experiment of Hertz to transmit and receive electromagnetic waves
The electromagnetic spectrum has been finally determined by german physicist Wilhelm Conrad Roentgen, who discovered the x-ray in 1895, and by french physicist Paul Villard, who discovered the gamma-rays in 1900.
Electromagnetic spectrum
Fig. IRC17 - Electromagnetic spectrum
Visible light, infrared light, micro-waves, TV-radio waves, ultraviolet light, x-rays, and gamma-rays, are electromagnetic waves which can be described by its wavelenght (), the length of one cycle of oscillation, or by its frequency (), the number of cycles that pass a point in one second. The are related by the formula , where c is the speed of light (3x108 ms-1).
The wavelength of a beam of light defines its position in the electromagnetic spectrum. It is often more convenient to use the wavenumber (), defined as the inverse of the wavelength. The energy of the light is related to its wavelength by the formula , where h is Planck's constant (6.62 10-34 J.S).
Radiation in the mid infrared (~4000 – 400 cm-1), corresponds to the vibrational frequencies of molecules. The theory of molecular vibration explains the appearance of infrared spectra.
Infrared region where molecules vibrate
Fig. IRC18 - Vibrational frequencies of molecules
3- THEORY OF MOLECULAR VIBRATION
To be continued ...
4- GENERATION OF AN INFRARED SPECTRUM
To be continued ...