Infrared band shape

The dipole autocorrelation function, , defined previously. The full-time dependence of this function for liquid carbon monoxide has been successfully determined experimentally from Fourier inversion of infrared band shapes.2,15 In fact, this was one of the reasons this system was studied. This function has also been successfully evaluated in terms of models of the molecular reorientation process.58 s memory function, KD(t), is defined by... 

Experimental data pertinent to the vibrational predisaociation mechanism of two types of van der Waals complex are presented and discussed. First, variations in the infrared band shape for excitation of the ethylene out-of-plane wag, Vy, in the series of molecules CjH tHF, C2H Ne are discussed in terms of structure and relaxation mechanisms. Second, rotationally resolved laser excited fluorescence spectra for NeBr2 and NeCl2 are presented. There is a strong dependence of decay rate on molecular structure.. Relaxation... 

Infrared Band Shapes by Fuzzy Logic and Partial Cross Correlation Functions. 

The surfaces obtained by cleaving single crystals of NaCl are clearly (001), and a variety of techniques show them to be remarkably defect-free. However, the nanocrystallites produced by sublimation methods, while they appear cubic, their surfaces are poorly defined [18]. The diffuse infrared bands of CO adsorbed to these crystallite faces is suggestive of the heterogeneity of the surfaces [53]. Water adsorbed onto NaCl crystallites yields infrared band shapes [54,55] quite distinct from those we shall discuss in a later section for thin film water on defined NaCl (001) faces. 

An interesting point is that infrared absorptions that are symmetry-forbidden and hence that do not appear in the spectrum of the gaseous molecule may appear when that molecule is adsorbed. Thus Sheppard and Yates [74] found that normally forbidden bands could be detected in the case of methane and hydrogen adsorbed on glass this meant that there was a decrease in molecular symmetry. In the case of the methane, it appeared from the band shapes that some reduction in rotational degrees of freedom had occurred. Figure XVII-16 shows the IR spectrum for a physisorbed H2 system, and Refs. 69 and 75 give the IR spectra for adsorbed N2 (on Ni) and O2 (in a zeolite), respectively. 

Whether the molecule is a prolate or an oblate asymmetric rotor, type A, B or C selection mles result in characteristic band shapes. These shapes, or contours, are particularly important in gas-phase infrared spectra of large asymmetric rotors, whose rotational lines are not resolved, for assigning symmetry species to observed fundamentals. 

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). 

Breuillard C., Ouillon R. Infrared and Raman band shapes and dynamics of molecular motions for N20 in solutions v3 band in CCL and liquid SF6. Mol. Phys. 33, 747-57 (1977). 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

The combination of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) provides an effective in-situ approach for studying the electrode-electrolyte interface. The extreme sensitivity to surface species of SERS is well known. By using polarization modulation of the infrared beam for IRRAS, the complete band shape is obtained without modulating the electrode potential. 

Coleman et al. 2471 reported the spectra of different proportions of poly(vinylidene fluoride) PVDF and atactic poly(methyl methacrylate) PMMA. At a level of 75/25 PVDF/PMMA the blend is incompatible and the spectra of the blend can be synthesized by addition of the spectra of the pure components in the appropriate amounts. On the other hand, a blend composition of 39 61 had an infrared spectrum which could not be approximated by absorbance addition of the two pure spectra. A carbonyl band at 1718cm-1 was observed and indicates a distinct interaction involving the carbonyl groups. The spectra of the PVDF shows that a conformational change has been induced in the compatible blend but only a fraction of the PVDF is involved in the conformational change. Allara M9 250 251) cautioned that some of these spectroscopic effects in polymer blends may arise from dispersion effects in the difference spectra rather than chemical effects. Refractive index differences between the pure component and the blend can alter the band shapes and lead to frequency shifts to lower frequencies and in general the frequency shifts are to lower frequencies. 

Liquids. For a long time, the study of infrared absorption by liquids and solutions has been a convenient way of determining rotovibrational spectra. The goal has often been to just determine peak frequencies, without paying much attention to the band shapes. In more recent years, attention has been devoted to a study of the shapes of vibrational bands and the dynamics of the molecules in the liquid. Only a crude understanding of the dynamics exists which is based on often highly simplified models of real liquids. 

O. Tanimoto. Band shape of the collision induced infrared absorption by rare gas mixtures. Prog. Theoret. Phys., 33 585, 1965. 

Sheppard and Yates (62) also studied the degree of rotation in physically adsorbed methane. The shape of infrared stretching bands is modified by rotation of the molecule so they calculated the band shapes expected for (1) no free rotation, (2) one degree of rotational freedom—rotation about an axis perpendicular to the surface—and (3) three degrees of rotational freedom. Comparison of the shapes of the calculated and observed bands... 

Although a number of infrared bands can be used to establish that a micellar shape change has occurred, it is difficult to determine the actual shape unambiguously from the spectroscopic data alone. We therefore make use of micelle aggregation numbers and solution rheological properties, which depend on micelle size and shape, for correlation with the structural information (packing) provided by the FTIR spectra. 

Fig. 12. Infrared optical absorption bands observed in ethylenediamine solutions (A) spread in band shape of er observed by pulse radiolysis, (B) spread in band shape observed in alkali metal solutions. [Adapted with permission from W. A. Seddon and J. W. Fletcher, Journal of Physical Chemistry, 84, 1104 (1980). Copyright 1980 American Chemical Society.]...

Turner JJ. Infrared vibrational band shapes in excited states. Coord Chem Rev 2002 230 213-24. 

We first check the influence of water on the infrared spectrum of HAc H20-CC14 solution. In the absorption spectrum of CC14 with water (reference sample), the two free water modes va and vs are located at 3,709 and 3,618 cm-1 respectively, and are in agreement with Bulmer [35]. In the lower panel (a) of Figure 21.3 we have shown the spectrum of HAc H20 in CC14 (solid line) and compared with that of reference sample (dotted line) in the whole measured region. The band shape of the reference is well suited to absorption background of the HAc H20 spectrum for correction at the present experimental condition. 

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