Espectroscopia infravermelha
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概述
Fonte: Vy M. Dong e Zhiwei Chen, Departamento de Química da Universidade da Califórnia, Irvine, CA
Este experimento demonstrará o uso de espectroscopia infravermelha (IR) (também conhecida como espectroscopia vibracional) para elucidar a identidade de um composto desconhecido identificando o(s) grupo funcional presente. Os espectros de RI serão obtidos em um espectrômetro de RI utilizando a técnica de amostragem total atenuada (ATR) com uma amostra pura do desconhecido.
Principles
Uma ligação covalente entre dois átomos pode ser pensada como dois objetos com massas m1 e m2 que estão conectados com uma mola. Naturalmente, este vínculo se estende e comprime com uma certa frequência vibracional. Esta frequência 
é dada pela Equação 1,onde k é a constante de força da mola, c é a velocidade da luz, e μ é a massa reduzida(Equação 2). A frequência é tipicamente medida em números de ondas, que são expressas em centímetros inversos (cm-1).
Da Equação 1,a frequência é proporcional à força da mola e inversamente proporcional às massas dos objetos. Assim, as ligações C-H, N-H e O-H têm frequências de alongamento mais altas do que as ligações C-C e C-O, já que o hidrogênio é um átomo de luz. Títulos duplos e triplos podem ser considerados como molas mais fortes, de modo que um vínculo duplo C-O tem uma frequência de alongamento maior do que um vínculo único C-O. A luz infravermelha é radiação eletromagnética com comprimentos de onda que variam de 700 nm a 1 mm, o que é consistente com as forças relativas da ligação. Quando uma molécula absorve a luz infravermelha com uma frequência que equivale à frequência vibracional natural de uma ligação covalente, a energia da radiação produz um aumento na amplitude da vibração da ligação. Se as eletronegatividades (a tendência de atrair elétrons) dos dois átomos em uma ligação covalente são muito diferentes, ocorre uma separação de carga que resulta em um momento dipolo. Por exemplo, em uma ligação dupla C-O (um grupo carbonyl), os elétrons passam mais tempo em torno do átomo de oxigênio do que o átomo de carbono porque o oxigênio é mais eletronegativo do que o carbono. Portanto, há um momento de dipolo líquido resultando em uma carga negativa parcial sobre o oxigênio e uma carga positiva parcial sobre o carbono. Por outro lado, uma alkyne simétrica não tem um momento de dipolo líquido porque os dois momentos individuais de dipolo de cada lado se cancelam. A intensidade da absorção infravermelha é proporcional à mudança no momento do dipolo quando a ligação se estende ou comprime. Assim, um trecho de grupo carbonyl mostrará uma faixa intensa no IR, e uma alkyne interna simétrica mostrará uma pequena, se não invisível, banda para alongamento da ligação tripla C-C(Figura 1). A Tabela 1 mostra algumas frequências de absorção características. A Figura 2 mostra o espectro ir de um éster Hantzsch. Observe o pico em 3.343 cm-1 para a ligação única N-H e o pico em 1.695 cm-1 para os grupos carbonyl. Neste experimento, é utilizada a técnica de amostragem ATR, onde a luz infravermelha reflete a amostra que está em contato com um cristal ATR várias vezes. Normalmente, são utilizados materiais com alto índice de refração, como germânio e selenida de zinco. Este método permite examinar diretamente analitos sólidos ou líquidos sem maiores preparações.
Figura 1. Diagrama mostrando C–O duplo eC–Ctriplo laços ea mudança resultante no momento do dipolo.
Mesa 1. Frequências de IR características de ligações covalentes presentes em moléculas orgânicas.
Figura 2. Espectro ir de um éster Hantzsch.
Procedure
Results
Table 2: Appearance and observed IR frequencies of the compounds listed in Figure 3.
| Compound Number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| Appearance | clear liquid | white solid | clear liquid | clear liquid | clear liquid | clear liquid | yellow liquid | white solid | white solid | clear liquid |
| Observed frequencies (cm-1) | 1691, 1601, 1450, 1368, 1266 |
2773, 2730, 1713, 1591, 1576 |
2940, 2867, 1717, 1422, 1347 |
3026, 2948, 2920, 1605, 1496 |
2928, 2853, 1450, 904, 852 |
3926, 3315, 2959, 2120, 1461 |
3623, 3429, 3354, 2904, 1601 |
3408, 3384, 3087, 1596, 1496 |
3226, 2966, 1598, 1474, 1238 |
3340, 2959, 2861, 1468, 1460 |
Applications and Summary
In this experiment, we have demonstrated how to identify an unknown sample based on its characteristic IR spectrum. Different functional groups give different stretching frequencies, which allow the identification of the functional groups present.
As shown in this experiment, IR spectroscopy is a useful tool for the organic chemist to identify and characterize a molecule. In addition to organic chemistry, IR spectroscopy has useful applications in other areas. In the pharmaceutical industry, this technique is used for quantitative and qualitative analysis of drugs. In food science, IR spectroscopy is used to study fats and oils. Lastly, IR spectroscopy is used to measure the composition of greenhouse gases, i.e., CO2, CO, CH4, and N2O in efforts to understand global climate changes.
成績單
Infrared, or IR, spectroscopy is a technique used to characterize covalent bonds.
Molecules with certain types of covalent bonds can absorb IR radiation, causing the bonds to vibrate. An IR spectrophotometer can measure which frequencies are absorbed. This is generally represented with a spectrum of percent IR radiation transmitted through the sample at a given frequency in wavenumbers. In this type of spectrum, the peaks are inverted, as they represent a decrease in transmitted light at that frequency.
The absorbed frequencies depend on the identity and electronic environment of the bonds, giving each molecule a characteristic spectrum. However, each type of bond will absorb IR radiation within a specific frequency range, and will have a common peak shape and absorption strength. Peaks can therefore be assigned to specific bonds, allowing identification of an unknown compound from the IR spectrum.
This video will illustrate the characterization of an unknown organic compound with IR spectroscopy and will introduce a few other applications of IR spectroscopy in organic chemistry.
A covalent bond between two atoms can be modeled as a spring connecting two bodies with masses m1 and m2. This “spring” has a resonance frequency, which, in this case, is the frequency of light corresponding to the quantum of energy needed to excite an oscillation in the bond at that same frequency, but with even greater amplitude.
The resonance frequency of a bond depends on the bond strength and length, the identity of the involved atoms, and the environment. For instance, a conjugated bond will vibrate in a different frequency range than a non-conjugated bond.
The resonance frequency also depends on the vibrational mode, which is the oscillation pattern of the atoms within a molecule. The most common vibrational modes observed by IR spectroscopy are stretching and bending. Linear molecules have 3N minus 5 vibrational modes, where N is the number of atoms, and non-linear molecules have 3N minus 6 vibrational modes.
IR spectrophotometry is primarily performed by shining a broad-spectrum light source through an interferometer, which blocks all but a few wavelengths of light at any given time, onto the sample. An IR detector measures the light intensities for each interferometer setting. Once data has been collected over the desired frequency range, it is processed into a recognizable spectrum by Fourier transform.
The sample can be gaseous, liquid, or solid, depending on the construction of the instrument. For a standard detector, gases and liquids are placed in a cell with IR-transparent windows, and solids are suspended in oil or pressed into a transparent pellet with potassium bromide. The IR light is then directed through the sample to the detector.
An alternate method for solid and liquid samples is attenuated total reflectance, or ATR. In this method, the pure sample is placed in contact with a crystal surface. IR light is then reflected off the underside of the crystal into a detector, with the absorbed frequencies reflecting more weakly. The sample doesn’t need to be processed first, as the light does not travel through it.
Now that you understand the principles of IR spectroscopy, let’s go through a procedure for identifying an unknown organic compound using the ATR sampling technique on an FTIR instrument.
To begin the characterization procedure, turn on the FTIR spectrometer and allow the lamp to warm up to operating temperature.
Ensure that the ATR crystal is clean. Then, with no sample in place, use the spectrometer software to record a background spectrum.
Next, obtain a solid sample of an unknown organic compound and note its appearance. Using a clean metal spatula, carefully place the sample on the crystal surface. Alternatively, for liquid samples, a pipette is used to transfer samples to crystal surface.
Carefully screw down the probe until it locks into place to fix the sample against the crystal surface.
Then, collect at least one IR spectrum of the unknown sample. After data collection has finished and the background has been subtracted, use the analysis tools in the software to identify the wavenumbers of the peaks.
When finished with the spectrometer, remove the sample and clean the probe with acetone. Save the spectra, close the software, and turn off the spectrometer.
In this experiment, the unknown sample may be one of ten organic compounds, each with five characteristic IR peaks. Based on the phase and visual appearance of the unknown, 8 of the possibilities may be eliminated.
The spectrum from the unknown compound shows a wide peak near the 3,300 wavenumber region, indicative of either an -OH or -NH stretching absorption. The peaks to right indicate the presence of carbon-carbon double bonds and carbon oxygen bonds. Of the two remaining compounds, only one has an -OH group so the compound is phenol.
IR spectrophotometry is a widely used characterization tool in biology and chemistry. Let’s look at a few examples.
In this procedure, FTIR spectroscopy performed with the ATR method was used to obtain IR absorbance images of tissue by introducing a microscopy component into the instrument. Each pixel in the image had a corresponding IR spectrum, allowing determination of the molecular composition of the tissue with excellent spatial resolution. The tissue image could also be displayed at different frequencies to visualize the distribution of molecule types throughout the tissue.
The molecular vibrations of peptide groups in a protein are affected by protein conformational changes. By monitoring a protein sample with step-scan FTIR, which has a temporal resolution on the order of tens of nanoseconds, protein dynamics can be monitored via the changes in their absorbance spectra. The data can be presented as individual spectra or as 3D plots of intensity, frequency, and time for peak identification and further analysis.
You’ve just watched JoVE’s introduction to IR spectroscopy. You should now be familiar with the underlying principles of IR spectroscopy, the procedure for IR spectroscopy of organic compounds, and a few examples of how IR spectroscopy is used in organic chemistry. Thanks for watching!