Infrared spectroscopy molecular motions

Both infrared and Raman spectroscopy provide infonnation on the vibrational motion of molecules. The teclmiques employed differ, but the underlying molecular motion is the same. A qualitative description of IR and Raman spectroscopies is first presented. Then a slightly more rigorous development will be described. For both IR and Raman spectroscopy, the fiindamental interaction is between a dipole moment and an electromagnetic field. Ultimately, the two... 

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms iavolved ia the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of iaertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting ia very high specificity. The vibrational spectmm of any molecule is unique, except for those of optical isomers. Every molecule, except homonuclear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption ia the iafrared. Several texts treat iafrared iastmmentation and techniques (22,36—38) and thek appHcations (39—42). 

FTIR spectroscopy exploits infrared (IR) absorption in the mid-IR (400-4000 cm ) region and provides information on the molecular chemistry of materials and interactions (17). IR spectra in this region arise from molecular motions within a chemical bond, and a comprehensive review of the pharmaceutical applications of FTIR can be found elsewhere (18). 

A standard experimental probe of this motion is infrared spectroscopy. We may use the results of Sections 7,5 and 8.2.3 to examine the effect of interaction with the thermal environment on the absorption lineshape. The simplest model for the coupling of a molecular system to the radiation field is expressed by a term —fi S in the Hamiltonian, where is the molecular dipole, and (t) is the oscillating electric field (see Section 3.1). For a one-dimensional oscillator, assuming that /r is proportional to the oscillator displacement from its equilibrium position and taking cos((uZ), we find that the coupling of the oscillator to the thermal environment and the radiation field can be modeled by Eq. (8.31) supplemented by a term (F/ni where F denotes the radiation induced driving force. We can... 

One should remember that the atoms and molecules in liquids are not motionless but in a state of flux determined by the intermolecular interactions and temperature. From the study of microwave spectroscopy discussed in chapter 4, it was found that rotational diffusion processes in liquids are characterized by relaxation times the order of a few picoseconds. When a liquid is irradiated with UV or visible radiation which involves frequencies greater than 10 Hz, the atoms and molecules appear to be motionless because the frequency of the electromagnetic radiation is much greater than that associated with molecular motion (10 Hz and lower). The same is true for infrared spectroscopy except in the... 

Infrared and Raman spectroscopy are important analytical tools used to investigate a wide variety of molecules in the solid, liquid, and gas states, and yielding complementary information about molecular structure and molecular bonds. Both methods supply information about resonances caused by vibration, vibration-rotation, or rotation of the molecular framework, but because the interaction mechanism between radiation and the molecule differs in the two types, the quantum-mechanical selection rules differ as well. Therefore, not all of the molecular motions recorded by one type of spectroscopy will necessarily be recorded by the other. The geometrical configuration of the molecule and the distribution of electrical charge within that configuration determine which molecular motions may appear in each type of spectrum. 

ATR-FTIR spectroscopy (attenuated total reflectance Fourier transform infrared spectroscopy)—IR spectroscopy uses the absorption of infrared radiation to probe the vibrational frequency of molecular motions. Attenuated total reflectance method uses a crystal of high refractive index to channel the infrared light (using total internal reflectance) into the crystal and causes only a thin layer of a sample in contact with the exterior of the crystal to be sensitively detected. 

In the following sections of this chapter, we first treat intramoleular vibrations then, in much more detail, phonons using typical examples and finally, very briefly, stochastic rotational motions ( reorientations ) and translational diffusion of molecules. Although the experimental methods for the characterisation of dynamics in molecular crystals are in principle no different from those used to investigate inorganic crystals, we shall briefly describe inelastic neutron diffraction, Raman scattering, infrared and far-infrared spectroscopy, as well as NMR spectroscopy to the extent necessary or useful for the specific understanding of molecular and lattice... 

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