Water infrared active modes

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. 

Unlike the water molecule, coition dioxide has no dipole moment How is it posible Tor any of its vibrational modes to be infrared active ... 

The most common protic solvent is water. It is also one of the most complex from the point of view of vibrational spectroscopy because of its highly structured nature. Since water is a triatomic, non-linear molecule it has three vibrational modes, which are illustrated in fig. 5.13. The Vj mode is the symmetrical stretch V2 is the bending mode and V3 is the asymmetrical stretch. All three vibrational modes for water are active in the infrared because they involve changes in the dipole moment. Activity in the Raman spectrum requires that the polarizability of the molecule changes during vibration. Analysis of this aspect of molecular properties is more difficult but it shows that all three modes are also Raman active. A summary of the frequencies of these vibrations for H2O, and the isotopes D2O, and HOD determined from gas phase spectra are given in table 5.7. 

A molecular vibration is infrared active (i.e., excitation of the vibrational mode can be measured via an IR spectrum absorption) only if it results in a change in the dipole moment of the molecule. The three vibrations of water (Table 4.12) can be analyzed in this way to determine their infrared behavior. 

A water molecule (Figure 11.72) has modes of vibration similar to those shown for the carbon dioxide molecule. However, it is a V-shaped molecule rather than a linear molecule. All the vibration modes shown for the water molecule are infrared active. In a bent molecule the changes in bond dipole for symmetric stretching are not in opposite directions. They do not, therefore, cancel out vectorially and, since there is a change in the overall dipole moment of the molecule, infrared absorption occurs. 

Solution Only Nj and O2 do not possess vibrational modes that result in a change of dipole moment, so COj, H2O, and CH4 are infrared active. Not all the modes of complicated molecules are infrared active. For example, a vibration of CO2 in which the O-C-O bonds stretch and contract symmetrically is inactive because it leaves the dipole moment unchanged (at zero). A bending motion of the molecule, however, is active and can absorb radiation. It follows that the continued release of COj and CH4 into the atmosphere can contribute to climate change. Water also contributes, but it is already present in large amounts. 

In a similar manner, the vibrational modes of coordinated water molecules, such as wagging, twisting and rocking (which cannot occur in lattice water molecules) become infrared active, the resulting bands occurring in the region 880-650 cm (11.36-15.38 pm). (For large water clusters see reference 36.)... 

Computer-interfaced FT-IR instruments operate in a single-beam mode. To obtain a spectrum of a compound, the chemist first obtains an interferogram of the background, which consists of the infrared-active atmospheric gases, carbon dioxide and water vapor (oxygen and nitrogen are not infrared active). The interferogram is subjected to a Fourier transform, which yields the spectrum of... 

It is important to appreciate that Raman shifts are, in theory, independent of the wavelength of the incident beam, and only depend on the nature of the sample, although other factors (such as the absorbance of the sample) might make some frequencies more useful than others in certain circumstances. For many materials, the Raman and infrared spectra can often contain the same information, but there are a significant number of cases, in which infrared inactive vibrational modes are important, where the Raman spectrum contains complementary information. One big advantage of Raman spectroscopy is that water is not Raman active, and is, therefore, transparent in Raman spectra (unlike in infrared spectroscopy, where water absorption often dominates the spectrum). This means that aqueous samples can be investigated by Raman spectroscopy. 

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%. 

TABLE 38. The OH-stretch frequencies (in cm ) of phenol-waters complexes calculated at the HF/A and MP2/A (in parentheses) computational levels. Infrared intensity is in kmmol Raman (R) activity in A amu. Partial contributions are evaluated as the ratio of total displacements. The contribution of the first reported mode is referred to 100%... 

With respect to cellulose, the 0-H groups of cellulose and those of adsorbed water are dominant in many of the spectral features in the infrared spectra. In contrast, the skeletal C—C and C—O bonds and the C—H bonds dominate the Raman spectra. A further simplification in the Raman spectra results from the circumstance that the selection rules forbidding activity of overtone and combination bands are more rigidly adhered to than is the case in the infrared spectra, so that the bands observed in the Raman spectra are usually confined to the fundamental modes of the molecules under investigation. ... 

The infrared spectrum of fumed silica after vacuum activation at 150°C for 1 h (A-150) is identical to that observed after simple evacuation for 1 h at ambient temperatures [6,19]. Experiments with a vacuum microbalance have confirmed that there was insignificant mass loss upon going from 1-h evacuation at 22°C to further evacuation at 150°C for 1 h. Therefore, the spectrum shown in Figure 24.1 A can be considered to be that of an as received hydroxylated fumed silica that contains no adsorbed water. The infrared spectrum of P-150 is almost identical to that observed after evacuation at ambient temperature for 1 h, except that there was about 5% decrease in the intensity of the broad 3520-cm feature upon going to 150°C. The microbalance indicated a small decrease in mass of the sample upon going from 22°C vacuum treatment to 150°C (discussed later). After evacuation of either silica for 1 h at 22°C, there was no indication of an infrared band at 1620 cm characteristic of the bending mode of H2O thus, the ambient temperature vacuum treatment was sufficient to remove all adsorbed water. Therefore, the loss of mass of P-22 upon heating to 150°C can probably be attributed to the... 

The mechanism of the activation of H2O2 by TS-1 and related catalysts has been the subject of much research using spectroscopic and computational techniques. This has centred on the nature of the active site and its mode of reaction with H2O2, solvents and the organic substrates. Work to elucidate the structure of the active site has concentrated on the coordination chemistry of the titanium. X-ray and neutron diffraction studies, coupled with X-ray absorption, infrared and Raman spectroscopies, give evidence that most of the Ti(IV) in calcined TS-1, in the absence of any adsorbate molecules, is in tetrahedral coordination. Upon addition of one molecule of water, one of the Ti-OSi bonds is hydrolysed and the titanium adopts tetrahedral coordination as Ti(0Si)30H. Addition of a further water molecule gives rise to a pentaco-ordinated titanium. ... 

As a final qualitative example. Tables XXIV and XXV contain calculations on the water dimer. One interesting feature of hydrogen-bonded dimers is that the infrared intensity of the proton donor molecule is significantly enhanced, often by an order of magnitude. " This is clearly seen in the (H20)2 calculation. The behaviour of the Raman intensities is less well known, though there have been calculations on (H20)2. It seems that the effect of the hydrogen-bond formation are much less marked than in the IR case. The intermolecular modes have very little Raman activity. 

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