Infrared Spectroscopy - Systems Chemistry

Infrared Spectroscopy - Systems Chemistry

Infrared spectroscopy (IR spectroscopy) is a technique that focuses on the infrared region of the electromagnetic spectrum, which possesses longer wavelengths and lower frequencies than visible light. This wide array of methodologies, primarily absorption spectroscopy, allows for the identification and study of various chemical substances. A frequently used laboratory device that harnesses this technology is the Fourier Transform Infrared (FTIR) spectrometer.

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The infrared section of the electromagnetic spectrum is typically broken down into three categories: near-infrared, mid-infrared, and far-infrared. These divisions are based on their proximity to the visible spectrum. Near-infrared, with higher energy levels, spans approximately 14000-4000 cm-1 (0.8-2.5 µm wavelength) and is capable of exciting overtone or harmonic vibrations. Mid-infrared, covering about 4000-400 cm-1 (2.5-25 µm), helps in examining fundamental vibrations and the related rotational-vibrational structure. Far-infrared, ranging from 400-10 cm-1 (25-1000 µm), is adjacent to the microwave region, featuring low energy and suitable for rotational spectroscopy. These classifications are conventional and loosely based on molecular or electromagnetic characteristics.

Theory

Infrared spectroscopy leverages the principle that molecules absorb specific frequencies, which are indicative of their structure. These absorbed frequencies, known as resonant frequencies, match the vibration frequency of the bond or group in the molecule. Factors determining these energies include the shapes of the molecular potential energy surfaces, atomic masses, and vibronic coupling.

Under the Born-Oppenheimer and harmonic approximations, when the molecular Hamiltonian for the electronic ground state is akin to a harmonic oscillator near the equilibrium molecular geometry, resonant frequencies are defined by the normal modes of the molecular electronic ground state potential energy surface. Although, initially, these frequencies can be associated with bond strength and the masses of the atoms at each end. Consequently, the vibration frequency can be linked to specific bond types.

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For a vibrational mode in a molecule to be "IR active," it must involve changes in the permanent dipole. Molecules can vibrate in several ways, each termed as a vibrational mode. Linear molecules possess 3N - 5 vibrational modes, whereas nonlinear molecules have 3N - 6 vibrational modes, also known as vibrational degrees of freedom. For instance, a nonlinear molecule like H2O would have 3 x 3 - 6 = 3 vibrational degrees of freedom or modes.

Simple diatomic molecules feature only one bond and a singular vibrational band. In symmetric molecules like N2, this band is not observed in the IR spectrum but rather in the Raman spectrum. However, unsymmetrical diatomic molecules such as CO absorb in the IR spectrum. Complex molecules, with numerous bonds, exhibit intricate vibrational spectra, resulting in multiple peaks in their IR spectra.

The atoms in a CH2 group, commonly found in organic compounds, can vibrate in six distinct ways: symmetric stretching, antisymmetric stretching, scissoring, rocking, wagging, and twisting:

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