Asymmetric stretching water

FIG. 9 Diagram illustrating the three vibrational modes (31V— 6) of water in the gas phase. (A) The first mode is called bending, in which the water molecule moves in a scissors-like manner. (B) The second is the symmetric stretch, where the hydrogen atoms move away from (or toward) the central oxygen atom simultaneously—i.e., in-phase motion. (C) The third is the asymmetric stretch, in which one hydrogen atom approaches the central oxygen atom, while the other moves away—i.e., out-of-phase motion. 

The analysis of vibration spectra proceeds by the use of normal modes. For instance, the vibration of a nonlinear water molecule has three degrees of freedom, which can be represented as three normal modes. The first mode is a symmetric stretch at 3586 cm , where the O atom moves up and the two H atoms move away from the O atom the second is an asymmetric stretch at 3725 cm where one H atom draws closer to the O atom but the other H atom pulls away and the third is a bending moment at 1595 cm , where the O atom moves down and the two H atoms move up and away diagonally. The linear CO2 molecule has four normal modes of vibration. The first is a symmetric stretch, which is inactive in the infrared, where the two O atoms move away from the central C atom the second is an asymmetric stretch at 2335 cm where both O atoms move right while the C atom moves left and the third and fourth together constitute a doubly degenerate bending motion at 663 cm where both O atoms move forward and the C atom moves backward, or both O atoms move upward and the C atom moves downward. 

Ligands that can coordinate to an active center in an enzyme and prevent coordination by the substrate will tend to inhibit the action of that enzyme. 1 We have seen that azide can occupy the pocket tailored to fit the carbon dioxide molecule. This prevents the latter from approaching the active site. Furthermore, the infrared evidence indicates that the azide ion actually does bind the zinc atom The asymmetric stretching mode of the azide ion is strongly shifted with respect to the free ion absorption. Thus the zinc is inhibited from acting as a Lewis acid towards water with the formation of a coordinated hydroxide ion. Other inhibitors also bind to the metal atom. As little as 4 x I0-6 M cyanide or hydrogen sulfide inhibits the enzymatic activity by 85%. 

Figure 1A shows the FTIR spectrum of a freshly prepared 0.36 M solution of TMMS in 10% water-acetone. The Si—O—C methoxy asymmetric stretch band (1083 cm-1) and the symmetric stretch band (865 cm-1) are clearly identified along with a water band. After a sufficient delay which depends on the solution pH, the Si—O—C bands disappear, as shown in Fig. IB, indicating complete hydrolysis of the methoxy group, and are replaced by the C—O stretch of methanol (1031 cm-1) and the Si—OH stretch (896 cm-1) of the silanol group. After further standing, the Si—OH band is reduced and the Si—O—Si asymmetric stretch (1043 cm-1) is present, as shown in Fig. 1C. Thus, the...

Normal vibrational modes of the water molecule v (symmetric stretch) and V2 (bending) have A symmetry, while V3 (asymmetric stretch) has 2 symmetry. 

Before leaving the discussion of this area, let us consider a specific chemical example. The water molecule has C2V symmetry, hence its normal vibrational modes have A, Ai, B, or B2 symmetry. The three normal modes of H2O are pictorially depicted in Fig. 6.3.1. From these illustrations, it can be readily seen that the atomic motions of the symmetric stretching mode, iq, are symmetric with respect to C2, bending mode, i>2, also has A symmetry. Finally, the atomic motions of the asymmetric stretching mode, V3, is antisymmetric with respect to C2 and This example demonstrates all vibrational modes of a molecule must have the symmetry of one of the irreducible representations of the point group to which this molecule belongs. As will be shown later, molecular electronic wavefunctions may be also classified in this manner. 

As previously, the CH2 stretching vibrations in the 2850-2950-cm" region were the focal point for the lipid-related analyses. Since the area under the curve of an infrared absorbance is directly proportional to the amount of the absorbing species [62,63], the area under the asymmetric stretching band provided an indication of the amount of lipid in the vicinity of each measurement. Similarly, the frequency and bandwidth of these absorbances reported on the conformational order of the SC lipids under study. The water concentration in each successive layer was estimated from a measurement of the area under the O-H stretching band at 2100 cm" . The sequential IR analyses... 

Figure 10. Cylindrical internal reflectance (CIR) infrared spectra of humic and fiilvic acid size fractions A) 3K-1K molecular weight and B) 100-30K molecular weight isolated from water samples from Lake Bradford, Florida. The major bands are assigned as 1) C=0 asymmetrical stretch, 2) metal-bonded carboxyl asymmetrical stretch, 3) free carboxyl asymmetrical stretch, 4) carboxyl sym metrical stretch, and 5) C-C stretch.

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