红外光谱学

Infrared spectroscopy characterizes covalent bonds by measuring which frequencies of IR radiation a molecule absorbs. When IR light passes through a sample, specific bonds vibrate at their resonance frequency, causing absorption. Each bond type absorbs within a characteristic frequency range with a distinctive peak shape, allowing chemists to identify unknown compounds by matching observed peaks to known bond signatures.

A bond's resonance frequency depends on bond strength, bond length, the identity of involved atoms, and the electronic environment. For example, conjugated bonds vibrate at different frequencies than non-conjugated bonds. The vibrational mode—the oscillation pattern of atoms within the molecule—also affects resonance frequency. These factors combine to give each molecule a unique, characteristic IR spectrum.

The most common vibrational modes in IR spectroscopy are stretching and bending. Linear molecules have 3N minus 5 vibrational modes, while non-linear molecules have 3N minus 6 vibrational modes, where N is the number of atoms. These different modes produce distinct peaks in the IR spectrum, enabling detailed analysis of molecular structure and functional groups.

Attenuated total reflectance (ATR) places a pure sample directly on a crystal surface without prior processing. IR light reflects off the crystal's underside into a detector, with absorbed frequencies reflecting more weakly. This method eliminates the need to suspend solids in oil or press them into pellets, making sample preparation faster and simpler for solid and liquid samples.

An interferometer blocks all but a few wavelengths at any given time, and an IR detector measures light intensities for each setting. Once data is collected across the desired frequency range, Fourier transform processes this raw data into a recognizable spectrum. This mathematical transformation converts interferometer measurements into the characteristic peak patterns used for compound identification.

Step-scan FTIR monitors protein samples with temporal resolution on the order of tens of nanoseconds, tracking molecular vibrations of peptide groups. Protein conformational changes affect these vibrations, causing shifts in absorbance spectra. Data can be presented as individual spectra or 3D plots of intensity, frequency, and time, allowing researchers to observe protein dynamics in real time.

FTIR spectroscopy combined with microscopy generates IR absorbance images where each pixel corresponds to a complete IR spectrum. This approach determines molecular composition with excellent spatial resolution and can display tissue images at different frequencies to visualize the distribution of specific molecule types throughout the tissue sample.