Water, rotational fine structure

Apart from molecular vibrations, also rotational states bear a significant influence on the appearance of vibrational spectra. Similar to electronic transitions that are influenced by the vibrational states of the molecules (e.g. fluorescence, Figure 3-f), vibrational transitions involve the rotational state of a molecule. In the gas phase the rotational states may superimpose a rotational fine structure on the (mid-)IR bands, like the multitude of narrow water vapour absorption bands. In condensed phases, intermolecular interactions blur the rotational states, resulting in band broadening and band shifting effects rather than isolated bands. 

These values of W are then the allowed values for the rotational energy of the unsymmetrical-top molecule. Wang has evaluated the integrals Hue and shown that the secular equation can be further simplified. The application in the interpretation of the rotational fine structure of spectra has been carried out in several cases, including water, hydrogen sulfide, and formaldehyde.4... 

The most common bent triatomic molecule that you encounter daily is H2O. Like SO2, H2O belongs to the C2V point group and possesses three modes of vibration, all of which are IR and Raman active. These are illustrated in Fig. 3.13a which shows a calculated IR spectrum of gaseous H2O. (An experimental spectrum would also show rotational fine structure.) In contrast, the IR spectrum of liquid water shown in Fig. 3.13b is broad and the two absorptions around 3700 cm are not resolved. The broadening arises from the presence of hydrogen bonding between water molecules (see Section 10.6). In addition, the vibrational wavenumbers in the liquid and gas phase spectra are shifted with respect to one another. 

In summary, there is a real benefit in measuring the spectra of VOCs with uiue-solvable rotational fine structure by measuring the spectrum at lower resolution than 2cm . When Av is much greater than 8cm , however, the bands are broadened to the point that they can overlap neighboring bands from other analytes, and the prediction errors increase. For small molecules with resolvable rotational fine struc-mre, the effect of resolution on analytical accuracy depends on the presence of other analytes with nearby absorption bands and the presence of water lines in the same spectral region. 

Recent advances in spectroscopic methods have enabled the water pentamer to be studied experimentally. Infrared cavity ringdown spectroscopy has been used to examine the intramolecular absorption features of the gas-phase water pentamer, which match the spectral features of the pentamer rings in liquid water and amorphous ice (Paul et al., 1999 Burnham et al., 2002). Vibration Rotation Tunnelling (VRT) spectroscopy has been used to provide a more direct probe of the water pentamer intermolecular vibrations and fine structure in liquid water (Liu et al., 1997 Harker et al., 2005). The water pentamer was found to average out... 

Here, A and B are the fine-structure and the rotational constants of OH and is the azimuthal angle out of the rotation plane with

internuclear vector, cos2 and sin2 represent, respectively, lobes in the rotation plane and perpendicular to it. The Cj approach the value 2 in the limit of large j which has the consequence that p(A ) is preferentially oriented in the rotation plane while p A") is preferentially oriented perpendicular to the plane of rotation. Equation (11.15) gives the relative weights of the in-plane and the out-of-plane contributions of the real A-doublet states. They play a vital role in the photolysis of water through the AxBi state (see Section 11.2.2). 

The dielectric properties of tissues and cell suspensions will be summarized for the total frequency range from a few Hz to 20 GHz. Three pronounced relaxation regions at ELF, RF and MW frequencies are due to counterion relaxation and membrane invaginations, to Maxwell-Wagner effects, and to the frequency dependent properties of normal water at microwave frequencies. Superimposed on these major dispersions are fine structure effects caused by cellular organelles, protein bound water, polar tissue proteins, and side chain rotation. 

Figure 21-14 shows visible spectra for 1,2,4,5-tetrazine that were obtained under three different conditions gas phase, liquid phase, and aqueous solution. Notice that in the gas phase, the individual tetrazine molecules are sufficiently separated from one another to vibrate and rotate freely, so many individual absorption peaks resulting from transitions among the various vibrational and rotational states appear in the spectrum. In the liquid state and in solution, however, tetrazine molecules are unable to rotate freely, so we see no fine structure in the spectrum. Furthermore, because frequent collisions and interactions between tetrazine and water molecules cause the vibrational levels to be modified energetically in an irregular way. the spectrum appears as a single broad peak. The trends shown in the spectra... 

In the visible region of the spectrum water vapour is transparent and all further absorptions of interest occur in the infrared or at even longer wavelengths. These are associated with transitions between vibrational levels of the molecule, the fundamental modes for which are shown in fig. 1.4, and have a fine structure dependent upon the rotational levels involved. Since each of the three normal modes has a direct effect upon the dipole moment of the molecule, they aU lead to absorption bands. Because the interatomic potentials have appreciable anharmonic components from terms of cubic or higher order in the displacements, the relation between... 

Study, five are lines from the water discharge laser. The transitions are of two types pure rotational transitions within a given spin component, and fine-structure transitions between the two spin components. The latter are usually weaker than the former they are actually electric dipole forbidden in the case (a) limit. 

Optically active poly(7V-formylpropylenimine) made from L-4-methyloxazoline is isotactic. The barrier to rotation about the N—CHO bond results in two effects (a) fine structure in the C NMR spectrum of the polymer and (b) mutarotation when the polymer is dissolved in water. These effects are absent in the hydrolysis product, polypropylenimine tacticity effects are readily observed in the latter, provided that is is first converted to the polyhydrochloride. 

---