Espectroscopia infrarroja
English
Condividere
Panoramica
Fuente: Vy M. Dong y Zhiwei Chen, Departamento de química, Universidad de California, Irvine, CA
Este experimento demostrará el uso de espectroscopia de infrarrojo (IR) (también conocido como Espectroscopía Vibracional) para aclarar la identidad de un compuesto desconocido mediante la identificación de la functional group(s) presente. Espectros IR se obtiene en un espectrómetro de IR usando la reflexión total atenuada técnica de muestreo (ATR) con una cuidada muestra de lo desconocido.
Principi
Un enlace covalente entre dos átomos puede considerarse como dos objetos con masas m1 y m2 que están conectados con un resorte. Naturalmente, este vínculo se extiende y comprime con cierta frecuencia vibracional. Esta frecuencia 
está dada por la ecuación 1, donde k es la constante de fuerza del resorte, c es la velocidad de la luz, y μ es la masa reducida (ecuación 2). La frecuencia se mide típicamente en wavenumbers, que se expresan en centímetros inversos (cm-1).
De la ecuación 1, la frecuencia es proporcional a la fuerza de la primavera e inversamente proporcional a las masas de los objetos. Por lo tanto, C-H, N-H, O-H bonos y tiene estirando más frecuencias que C-C y C O bonos, como el hidrógeno es un átomo de luz. Doble y triple enlaces puede considerarse como resortes más fuertes, por lo que un enlace doble de C-O tiene una frecuencia más alta que de un solo enlace de C-O. Luz infrarroja es la radiación electromagnética con longitudes de onda de 700 nm a 1 mm, que es coherente con las fuerzas de enlace relativo. Cuando una molécula absorbe luz infrarroja con una frecuencia que es igual a la frecuencia natural de vibración de un enlace covalente, la energía de la radiación produce un aumento de la amplitud de la vibración del enlace. Si la electronegatividad (tendencia para atraer electrones) de los dos átomos en un enlace covalente son muy diferentes, una separación de carga produce que se traduce en un momento de dipolo. Por ejemplo, en un enlace doble de C-O (grupo carbonilo), los electrones pasan más tiempo alrededor del átomo de oxígeno que el átomo de carbono porque el oxígeno es más electronegativo que el carbono. Por lo tanto, es un momento de dipolo neto, dando por resultado una carga parcial negativa en el oxígeno y una carga parcial positiva sobre el carbono. Por otro lado, un alquino simétrico no tiene un momento dipolar neto debido a los dos momentos de dipolo individuales en cada lado cancelar mutuamente. La intensidad de la absorción infrarroja es proporcional al cambio en el momento de dipolo cuando el bono estira o comprime. Por lo tanto, un estiramiento del grupo carbonilo se mostrará una banda intensa en el IR, y un alquino interno simétrico va a mostrar un pequeño, si no invisible, banda de estiramiento del enlace C-C triple (figura 1). La tabla 1 muestra algunas frecuencias de absorción característico. La figura 2 muestra el espectro IR de un éster de Hantzsch. Observe el pico a 3.343 cm-1 para el N-H solo enlace y el pico a 1.695 cm-1 para los grupos carbonilo. En este experimento, se utiliza la técnica de muestreo de ATR, donde la luz infrarroja se refleja en la muestra que está en contacto con un cristal ATR varias veces. Por lo general, se utilizan materiales con un alto índice de refracción, como seleniuro de germanio y el zinc. Este método permite examinar directamente los analitos sólidos o líquidos sin más preparación.
Figura 1. Diagrama que muestra C–O doble y C–tramos de enlace triple C y el cambio resultante en el momento de dipolo.
Tabla 1. Frecuencias características de IR de enlaces covalentes en moléculas orgánicas.
Figura 2. Espectro de IR de un éster de Hantzsch.
Procedura
Risultati
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.
Trascrizione
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!