Tabla De Espectroscopia Infrarroja
Páginas: 12 (2925 palabras)
Publicado: 24 de septiembre de 2012
Online edition for students of organic chemistry lab courses at the University of Colorado, Boulder, Dept of Chem and Biochem. (2002)
u
Chapter 15
Infrared Spectroscopy: Theory
An important tool of the organic chemist is Infrared Spectroscopy, or IR. IR spectra are acquired on a special instrument, called an IR spectrometer. IR is used both to gather information about the structure ofa compound and as an analytical tool to assess the purity of a compound. IR spectra are quick and easy to run, and since the teaching labs are fortunate enough to have three of these instruments, students are encouraged to acquire spectra of compounds made in the laboratory throughout the first and second semesters of the lab course.
15.1 The Electromagnetic Spectrum
Infrared refers to thatpart of the electromagnetic spectrum between the visible and microwave regions. Electromagnetic spectrum refers to the seemingly diverse collection of radiant energy, from cosmic rays to X-rays to visible light to microwaves, each of which can be considered as a wave or particle traveling at the speed of light. These waves differ from each other in the length and frequency, as illustrated in Figure15.1. Frequency, ν (nu), is the number of wave cycles that pass through a point in one second. It is measured in Hz, where 1 Hz = 1 cycle/sec. Wavelength, λ (lambda), is the length of one complete wave cycle. It is often measured in cm (centimeters). Wavelength and frequency are inversely related:
c c ν = --- and λ = -λ ν
Eq. 1
where c is the speed of light, 3 x 1010 cm/sec Energy isrelated to wavelength and frequency by the following formulas:
hc E = h ν = -----
Eq. 2
λ
where h = Planck’s constant, 6.6 x 10–34 joules-sec Note that energy is directly proportional to frequency and inversely proportional to wavelength. The IR region is divided into three regions: the near, mid, and far IR (see Figure 15.2). The mid IR region is of greatest practical use to the organicchemist. This is the region of wavelengths between 3 x 10–4 and 3 x 10–3 cm. Chemists prefer to work with numbers which are easy to write; therefore IR spectra are sometimes reported in µm, although another unit, ν (nu bar or wavenumber), is currently preferred.
The Electromagnetic Spectrum15 Infrared Spectroscopy: Theory 155
Online edition for students of organic chemistry lab courses at theUniversity of Colorado, Boulder, Dept of Chem and Biochem. (2002)
10-12 cosmic rays, 10-12 to 10-10cm 10-10 gamma rays, 10-10 to 10-8cm 10-8 X-rays, 10-8 to 10-6cm
wavelength
10-6
purple blue green red
far ultraviolet, 10-6 to 2x10-5cm ultraviolet, 2x10-5 to 3.8x10-5cm visible, 3.8x10-5 to 7.8x10-5cm infrared, 7.8x10-5 to 3x10-2cm
energy
10-2
0
microwave, 3x10-2 to 102cm102 radiofrequency, .6x102 to 5x102cm 103
λ
Figure 15.1 The electromagnetic spectrum.
A wavenumber is the inverse of the wavelength in cm:
1 ν = -λ
Eq. 3
where ν is in units of cm–1, λ is in units of cm and now:
E = hc ν
Eq. 4
In wavenumbers, the mid IR range is 4000–400 cm–1. An increase in wavenumber corresponds to an increase in energy. As you will learn later, this is aconvenient relationship for the organic chemist. Infrared radiation is absorbed by organic molecules and converted into energy of molecular vibration. In IR spectroscopy, an organic molecule is exposed to infrared radiation. When the radiant energy matches the energy of a specific molecular vibration, absorption occurs. A typical IR spectrum is shown below. The wavenumber, plotted on the X-axis, isproportional to energy; therefore, the highest energy vibrations are on the left. The percent transmittance (%T) is plotted on the Y-axis. An absorption of radiant energy is therefore represented by a “trough” in the curve: zero transmittance corresponds to 100% absorption of light at that wavelength.
The Electromagnetic Spectrum15 Infrared Spectroscopy: Theory
156
Online edition for...
u
Chapter 15
Infrared Spectroscopy: Theory
An important tool of the organic chemist is Infrared Spectroscopy, or IR. IR spectra are acquired on a special instrument, called an IR spectrometer. IR is used both to gather information about the structure ofa compound and as an analytical tool to assess the purity of a compound. IR spectra are quick and easy to run, and since the teaching labs are fortunate enough to have three of these instruments, students are encouraged to acquire spectra of compounds made in the laboratory throughout the first and second semesters of the lab course.
15.1 The Electromagnetic Spectrum
Infrared refers to thatpart of the electromagnetic spectrum between the visible and microwave regions. Electromagnetic spectrum refers to the seemingly diverse collection of radiant energy, from cosmic rays to X-rays to visible light to microwaves, each of which can be considered as a wave or particle traveling at the speed of light. These waves differ from each other in the length and frequency, as illustrated in Figure15.1. Frequency, ν (nu), is the number of wave cycles that pass through a point in one second. It is measured in Hz, where 1 Hz = 1 cycle/sec. Wavelength, λ (lambda), is the length of one complete wave cycle. It is often measured in cm (centimeters). Wavelength and frequency are inversely related:
c c ν = --- and λ = -λ ν
Eq. 1
where c is the speed of light, 3 x 1010 cm/sec Energy isrelated to wavelength and frequency by the following formulas:
hc E = h ν = -----
Eq. 2
λ
where h = Planck’s constant, 6.6 x 10–34 joules-sec Note that energy is directly proportional to frequency and inversely proportional to wavelength. The IR region is divided into three regions: the near, mid, and far IR (see Figure 15.2). The mid IR region is of greatest practical use to the organicchemist. This is the region of wavelengths between 3 x 10–4 and 3 x 10–3 cm. Chemists prefer to work with numbers which are easy to write; therefore IR spectra are sometimes reported in µm, although another unit, ν (nu bar or wavenumber), is currently preferred.
The Electromagnetic Spectrum15 Infrared Spectroscopy: Theory 155
Online edition for students of organic chemistry lab courses at theUniversity of Colorado, Boulder, Dept of Chem and Biochem. (2002)
10-12 cosmic rays, 10-12 to 10-10cm 10-10 gamma rays, 10-10 to 10-8cm 10-8 X-rays, 10-8 to 10-6cm
wavelength
10-6
purple blue green red
far ultraviolet, 10-6 to 2x10-5cm ultraviolet, 2x10-5 to 3.8x10-5cm visible, 3.8x10-5 to 7.8x10-5cm infrared, 7.8x10-5 to 3x10-2cm
energy
10-2
0
microwave, 3x10-2 to 102cm102 radiofrequency, .6x102 to 5x102cm 103
λ
Figure 15.1 The electromagnetic spectrum.
A wavenumber is the inverse of the wavelength in cm:
1 ν = -λ
Eq. 3
where ν is in units of cm–1, λ is in units of cm and now:
E = hc ν
Eq. 4
In wavenumbers, the mid IR range is 4000–400 cm–1. An increase in wavenumber corresponds to an increase in energy. As you will learn later, this is aconvenient relationship for the organic chemist. Infrared radiation is absorbed by organic molecules and converted into energy of molecular vibration. In IR spectroscopy, an organic molecule is exposed to infrared radiation. When the radiant energy matches the energy of a specific molecular vibration, absorption occurs. A typical IR spectrum is shown below. The wavenumber, plotted on the X-axis, isproportional to energy; therefore, the highest energy vibrations are on the left. The percent transmittance (%T) is plotted on the Y-axis. An absorption of radiant energy is therefore represented by a “trough” in the curve: zero transmittance corresponds to 100% absorption of light at that wavelength.
The Electromagnetic Spectrum15 Infrared Spectroscopy: Theory
156
Online edition for...
Leer documento completo
Regístrate para leer el documento completo.