Water, scissors mode

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. 

This could be due to the contribution to this intensity from other species such as water molecules bound to the carboxyl groups. Also, the intensity around 1400 cm" is further complicated by the presence of several possible bands (e.g., deformation of COOH, scissoring modes of CH2, stretching modes of COO ). A quantitative differentiation of the ratio of carboxylic to carboxylate from these intensities was difficult, owing to the presence of multiple peaks in this region. 

Fig. 14 Potential dependence of the wave number for the in-plane deformation (scissor mode) of water adsorbed at Pt(l 11) [44, 45).

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. 

For instance, water has both vibrational and rotational modes. One vibrational mode is the scissor movement of the hydrogens toward and away from each other. A rotational mode is the spinning of the hydrogens about an imaginary axis through oxygen. Both these modes of motion are shown in figure 1.13.1. 

So far, we constructed molecules and reasoned their 3D structures in such a fashion that some of you may get the impression that these objects are motionless. The fact is that molecules are very much restless and perform many motions all at the same time. Scheme 9.10 shows the motions that a small molecule like water performs all at once. The molecule vibrates in three modes (Scheme 9.10a) one is a symmetric stretch where the two bonds stretch and shrink in the same direction, and the other is an antisymmetric stretch wherein one bond stretches while the other is shrinking, and finally the two O—H bonds perform a scissoring motion, closing and opening the HOH angle. Additionally, the molecule as a whole also performs rotations around three different axes. 

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