Vibrational Spectrum of Water

The magnitudes of vibrational motions shown are greatly exaggerated. At room temperature atoms move only about 10% away from their equilibrium positions. 

Molecules vibrate at characteristic frequencies, which depend both on the difficulty of the motion (the so-called force constant) and on the masses of the atoms involved. The more difficult the motion and the lighter the atomic masses, the higher the vibrational frequency. For a diatomic molecule the vibrational frequency is proportional to  

Display water as a ball-and-spoke model. How many different vibrations are there Explain. One after the other, animate these vibrations. For each, record the vibrational frequency and provide a description of the atomic motions. What appears to be easier (lower frequency), motions primarily associated with bond stretching or with angle bending  

Repeat the analysis with deuterium oxide (D2O). Are the vibrational frequencies the same, larger or smaller than those in water Rationalize your observations. Are the changes in vibrational frequencies greatest for bond stretching or angle bending motions  

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The difficulties in interpreting the vibrational spectrum of water are obvious. Because of the broad nature of the bands, the two stretching modes Vj and V3 overlap. In addition, overtones complicate the assignment of bands. Thus, the first overtone of the V2 bending mode, 2v2, lies close in frequency to the stretching modes Vj and V3. For these reasons, there are advantages in studying HOD, a molecule for which the vibrational frequencies are quite different. As a result,... 

The vibrational spectrum of water in simple complexes, solutions, and ices has been the subject of extensive experimental and theoretical studies [7, 74-77, 84-132],... 

I. Benjamin, Phys. Rev. Lett., 73, 2083 (1994). Vibrational Spectrum of Water at the Liq-uidWapor Interface. 

We now present one of the many examples of interfacial vibrational spectroscopy using SFG. Figure Bl.5.15 shows the surface vibrational spectrum of the water/air interface at a temperature of 40 °C [83]. Notice that... 

Figure 5-10 Partial MM3 Output as Related to the Vibrational Spectrum of H2O. The experimental values of the two sti etching and one bending frequencies of water are 3756, 3657, and 1595 cm. The IR intensities are all very strong (vs).

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

The differences in selection rules between Raman and infrared spectroscopy define the ideal situations for each. Raman spectroscopy performs well on compounds with double or triple bonds, different isomers, sulfur-containing and symmetric species. The Raman spectrum of water is extremely weak so direct measurements of aqueous systems are easy to do. Polar solvents also typically have weak Raman spectra, enabling direct measurement of samples in these solvents. Some rough rules to predict the relative strength of Raman intensity from certain vibrations are [7] ... 

In Section V the reorientation mechanism (A) was investigated in terms of the only (hat curved) potential well. Correspondingly, the only stochastic process characterized by the Debye relaxation time rD was discussed there. This restriction has led to a poor description of the submillimeter (10-100 cm-1) spectrum of water, since it is the second stochastic process which determines the frequency dependence (v) in this frequency range. The specific vibration mechanism (B) is applied for investigation of the submillimetre and the far-infrared spectrum in water. Here we shall demonstrate that if the harmonic oscillator model is applied, the small isotope shift of the R-band could be interpreted as a result of a small difference of the masses of the water isotopes. 

R. Sankari, M. Ehara, H. Nakatsuji, Y. Senba, K. Hosokawa, H. Yoshida, A. De Fanis, Y. Tamenori, S. Aksela, K. Ueda, Vibrationally resolved O Is photoelectron spectrum of water, Chem. Phys. Lett. 380 (2003) 647. 

The vibrational spectrum of H+ is even harder to interpret. Absorption increases at all frequencies in the infrared and the already broad water bands get broader, but not symmetrically. The additions to the water bands have been interpreted as the new bands of the HsO+ unit in H+ (Falk and Gigufere, 1957). The suggested frequencies are shown in Table 9. On the other hand it has been suggested that the rapid proton shifts from one oxygen to another precludes a band spectrum for that unit in water (Ackermann, 1961) and its absorption has been... 

Since PM-IRRAS is insensitive to the strong IR absorption of water vapor, it has proved to be an efficient way to study the conformation and orientation of protein molecules because only important bands arising from the monolayer are observed [72,97-103], The first in situ study of the protein conformation by PM-IRRAS technique was reported by Dziri et al. [97]. The vibrational spectrum of acetylcholinesterase (AChE) at the air-water interface in its free form and bound to either its substrate or organophosphorus (OP) inhibitor was measured. PM-IRRAS spectra collected during compression of the AChE... 

Vchirawongkwin V, Rode BM (2007) Solvation energy and vibrational spectrum of sulfate in water -an ab initio quantum mechanical simulation. Chem Phys Lett 443 152... 

Water is only moderately suitable as a solvent for IR spectroscopy, since even thin layers of water give rise to a broad IR spectrum with a strong background (Fig. 4.1-21 A). Although the Raman spectrum of water exhibits the same bands, these are usually much weaker than those of the solute. In D2O (Fig. 4.1-2IB), all vibrational frequencies are shifted, so that overlapping of the solvent bands with those of the solute can be avoided. 

We know already that the chosen computational methods accurately describe the properties of pheuol, particularly its vibrational spectrum. The frequeucies of the OH stretchiug vibrations of phenol and water molecule are collected in Table 35. It is interesting to note that the HF/A frequency of 4118 cm" assigned to the vqh stretching vibration of bare phenol corresponds to its highest frequency. Therefore, it can be treated as the most accepting mode of phenol. Moreover, this frequency lies between the frequencies of the vi (4070 cm" ) and (4188 cm" ) OH-stretching vibrational modes of water molecules (equation 40),... 

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