Water vibrational spectrum

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).

Polyatomic molecules vibrate in a very complicated way, but, expressed in temis of their normal coordinates, atoms or groups of atoms vibrate sinusoidally in phase, with the same frequency. Each mode of motion functions as an independent hamionic oscillator and, provided certain selection rules are satisfied, contributes a band to the vibrational spectr um. There will be at least as many bands as there are degrees of freedom, but the frequencies of the normal coordinates will dominate the vibrational spectrum for simple molecules. An example is water, which has a pair of infrared absorption maxima centered at about 3780 cm and a single peak at about 1580 cm (nist webbook). 

As the molecule vibrates it can also rotate and each vibrational level has associated rotational levels, each of which can be populated. A well-resolved ro - vibrational spectrum can show transitions between the lower ro-vibrational to the upper vibrational level in the laboratory and this can be performed for small molecules astronomically. The problem occurs as the size of the molecule increases and the increasing moment of inertia allows more and more levels to be present within each vibrational band, 3N — 6 vibrational bands in a nonlinear molecule rapidly becomes a big number for even reasonable size molecules and the vibrational bands become only unresolved profiles. Consider the water molecule where N = 3 so that there are three modes of vibration a rather modest number and superficially a tractable problem. Glycine, however, has 10 atoms and so 24 vibrational modes an altogether more challenging problem. Analysis of vibrational spectra is then reduced to identifying functional groups associated... 

The complexity of the physical properties of liquid water is largely determined by the presence of a three-dimensional hydrogen bond (HB) network [1]. The HB s undergo continuous transformations that occur on ultrafast timescales. The molecular vibrations are especially sensitive to the presence of the HB network. For example, the spectrum of the OH-stretch vibrational mode is substantially broadened and shifted towards lower frequencies if the OH-group is involved in the HB. Therefore, the microscopic structure and the dynamics of water are expected to manifest themselves in the IR vibrational spectrum, and, therefore, can be studied by methods of ultrafast infrared spectroscopy. It has been shown in a number of ultrafast spectroscopic experiments and computer simulations that dephasing dynamics of the OH-stretch vibrations of water molecules in the liquid phase occurs on sub-picosecond timescales [2-14],... 

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... 

Just as for other properties of H-bonded systems, the water dimer has been the subject of perhaps the greatest scrutiny to its vibrational spectrum. Curtiss and Pople s seminal work ... 

In Figure 29, the spectrum of an oil-sand sample shows the fundamental C-H peaks at 3.5 xm. From the two peaks in this region, one could determine the aromatic-aliphatic ratio of the hydrocarbons present in the sample. The fundamental water vibration is at approximately 3 xm (this peak would be substantially larger in a conventional emulsion sample), and the fundamental vibrations due to clays are at approximately 2.8 xm. The shape of the clay peaks indicates that kaolinite and a small amount of swelling clays such as bentonite are present in this sample. 

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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