Water spectra polarization

Potassium dihydrobis(l-pyrazolyl)borate, m.p. 171-172°C., is a white crystalline solid, highly soluble in water and polar solvents. It may be recrystallized from anisole, but with large solubility losses. The infrared spectrum has a complicated BH2 stretch multiplet in the 2200-2500-cm.-1 range. It is very soluble in water and alcohols, but the ligand undergoes slow solvolysis, and solutions should be made up just before use. 

We remark that in simple liquids only one band arises, usually centered at several hundredths of cm-1. On the contrary, the water spectrum, which covers up to 1000 cm-1, is two-humped. Its high-frequency part, bordering with the IR range, is to a certain extent similar to the spectrum of simple polar liquids, while a specific low-frequency part, with the absorption maximum near 200 cm-1, is typical of water/ice. The latter maximum, arising undoubtedly due to the existence of hydrogen bonds (HB), is lacking in simple liquids. 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

Fig. IV-13. Example of a p-polarized reflection spectrum from Ref. [154] for a stearyl alcohol monolayer on water. The dashed line is the baseline to be subtracted from the spectra. [Reprinted with permission from Joseph T. Buontempo and Stuart A. Rice, J. Chem. Phys. 98(7), 5835-5846 (April 1, 1993). Copyright 1993, American Institute of Physics.]...

Figure Bl.5.15 SFG spectrum for the water/air interface at 40 °C using the ssp polarization combination (s-, s- and p-polarized sum-frequency signal, visible input and infrared input beams, respectively). The peaks correspond to OH stretching modes. (After [ ].)...

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

The peak in the UV VIS spectrum of acetone [(CH3)2C=0] corresponding to the transition appears at 279 nm when hexane is the solvent but shifts to 262 nm in water Which is more polar the ground electronic state or the excited stated... 

FIG. 27 Sum frequency generation spectra in ssp polarization of a deuterated water (D2O) film on mica as a function of the relative humidity (RH) at room temperature (296 K). Above 40% RH, the spectrum is very similar to that of ice. The free OD stretching mode can be seen above 91% RH. The bottom trace corresponds to the spectrum of the bulk waver/vapor interface. (From Ref. 72.)... 

The availability of the purified transporter in large quantity has enabled investigation of its secondary structure by biophysical techniques. Comparison of the circular dichroism (CD) spectrum of the transporter in lipid vesicles with the CD spectra of water-soluble proteins of known structure indicated the presence of approximately 82% a-helix, 10% ) -turns and 8% other random coil structure [97]. No / -sheet structure was detected either in this study or in a study of the protein by the same group using polarized Fourier transform infrared (FTIR) spectroscopy [98]. In our laboratory FTIR spectroscopy of the transporter has similarly revealed that... 

FIG. 9 Vibrational sum frequency spectrum in the OH mode region of the neat air-water interface at different temperature for the fundamental visible and infrared beams respectively s- and p-polar-ized and the SFG beam s-polarized. (From Ref. 120, copyright American Physical Society.)... 

Fluorescent probes are divided in two categories, i.e., intrinsic and extrinsic probes. Tryptophan is the most widely used intrinsic probe. The absorption spectrum, centered at 280 nm, displays two overlapping absorbance transitions. In contrast, the fluorescence emission spectrum is broad and is characterized by a large Stokes shift, which varies with the polarity of the environment. The fluorescence emission peak is at about 350 nm in water but the peak shifts to about 315 nm in nonpolar media, such as within the hydrophobic core of folded proteins. Vitamin A, located in milk fat globules, may be used as an intrinsic probe to follow, for example, the changes of triglyceride physical state as a function of temperature [20]. Extrinsic probes are used to characterize molecular events when intrinsic fluorophores are absent or are so numerous that the interpretation of the data becomes ambiguous. Extrinsic probes may also be used to obtain additional or complementary information from a specific macromolecular domain or from an oil water interface. 

The solvation dynamics of the three different micelle solutions, TX, CTAB, and SDS, exhibit time constants of 550, 285, 180 ps, respectively. The time constants show that solvent motion in these solutions is significantly slower than bulk water. The authors attribute the observed time constants to water motion in the Stern layer of the micelles. This conclusion is supported by the steady-state fluorescence spectra of the C480 probe in these solutions. The spectra exhibit a significant blue shift with respect the spectrum of the dye in bulk water. This spectral blue shift is attributed to the probe being solvated in the Stern layer and experiencing an environment with a polarity much lower than that of bulk water. 

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