Water symmetric stretch

It is apparent from Fig. 4 that the normal modes of vibration of the water molecule, as calculated from the eigenvectors, can be described approximately as a symmetrical stretching vibration (Mj) and a symmetrical bending vibration... 

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. 

In Figure 9.9a, a CARS image of a protein flake of casein suspended in water is taken at 2930 cm, a Raman shift that overlaps with the CHj-symmetric stretch... 

Fig. 3.21 Normal modes of vibration of [he water molecule (a) symmetrical stretching mode. A,i (b> bending mode. <4, (c) andsyiranetncal stretching mode. Bt, and their transformations under C symmetry operations.

Consider next the water molecule. As we have seen, it has a dipole moment, so we expect at least one IR-active mode. We have also seen that it has CIt, symmetry, and we may use this fact to help sort out the vibrational modes. Each normal mode of iibratbn wiff form a basis for an irreducible representation of the point group of the molecule.13 A vibration will be infrared active if its normal mode belongs to one of the irreducible representation corresponding to the x, y and z vectors. The C2 character table lists four irreducible representations A, Ait Bx, and B2. If we examine the three normal vibrational modes for HzO, we see that both the symmetrical stretch and the bending mode are symmetrical not only with respect to tbe C2 axis, but also with respect to the mirror planes (Fig. 3.21). They therefore have A, symmetry and since z transforms as A, they are fR active. The third mode is not symmetrical with respect to the C2 axis, nor is it symmetrical with respect to the ojxz) plane, so it has B2 symmetry. Because y transforms as Bt, this mode is also (R active. The three vibrations absorb at 3652 cm-1, 1545 cm-1, and 3756 cm-, respectively. 

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...

FIGURE 3.13 The three normal vibrational modes of water. For the top mode (the symmetric stretch) both O-H bonds are extended or compressed at the same time. For the middle mode (the antisymmetric stretch) one O-H bond is extended when the other is compressed. The bottom mode is called the bend. In every case the hydrogen atoms move more than the oxygen, because the center of mass has to stay in the same position (otherwise the molecule would be translating). For a classical molecule (built out of balls and perfect springs) these three modes are independent. Thus, for example, energy in the symmetric stretch will never leak into the antisymmetric stretch or bend modes. 

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. 

Fundamental vibrations involve no change in the center of gravity of the molecule. The three fundamental vibrations of the nonlinear, triatomic water molecule are depicted in the top portion of Figure 2.1. Note the very close spacing of the interacting or coupled asymmetric and symmetric stretching compared with the far-removed scissoring mode. 

The (1,1-diphenyl) phosphonitrile fluoride trimer is a colorless crystalline solid that melts at 68.5-69.5°C. It can be recrystallized from n-pentane, n-heptane, petroleum ether, or absolute methanol. It is also soluble in diethyl ether, carbon disulfide, and chloroform, but it is insoluble in and not attacked by water. The infrared spectrum shows a strong phosphorus-nitrogen stretching mode at 1250-1265 cm."1. Strong bands at 914-920, 900-906 cm."1 and at 812-820 cm."1 are associated, respectively, with phosphorus-fluorine asymmetric and symmetric stretching modes. 

Since the symmetry coordinates of water are good approximations of the normal vibrations, the pictorial representations are applicable to them as well. Indeed, the three normal modes of Figure 5-4 are the same as the symmetry coordinates we just derived. The A symmetry stretching mode is called the symmetric stretch while the B2 mode is the antisymmetric stretch. 

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