Polarizer attachment

Experimentally, commercial steady-state fluorescence spectrometers can be equipped with polarizer attachments, either sheet or Glan-Thompson polarizers. Alternatively, sheet polarizers are usually easily incorporated into the sample cavity in the excitation and emission pathways. Likewise, for time-resolved spectroscopy, polarizers may simply be introduced into the excitation and detection paths. Frequently, the excitation source in time-resolved experiments is a laser which will be inherently polarized. 

Sample Selection and Mounting. The sample should now be safe for a few minutes, and one can proceed to selection and mounting of a crystal. Selection of a suitable crystal is similar to procedures commonly used for chemically stable samples. A sowing needle held with a pin vice, and a sharp razor blade are useful tools. The binocular microscope should have a polarizing attachment to assist in judging quality of crystallinity. 

Table 5.15 Pressure-sensitive adhesives for polarizer attachment ...

The skeleton of the molecule can be drawn showing an oleophilic hydrocarbon part, R, to which is attached a polar hydrophilic part, X ... 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

Film stability is a primary concern for applications. LB films of photopoly-merizable polymeric amphiphiles can be made to crosslink under UV radiation to greatly enhance their thermal stability while retaining the ordered layered structure [178]. Low-molecular-weight perfluoropolyethers are important industrial lubricants for computer disk heads. These small polymers attached to a polar head form continuous films of uniform thickness on LB deposi-... 

C—H bonds are polarized by attached unsaturated carbon substituents. Such groups "activate" the neighbouring CHj, CHp or CH groups in the following order CR=NR > COR > CN > COOR > CR = NR > Ph > CR=CRj. Two activating substituents reinforce each other. 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

To understand the effect of a carbonyl group attached directly to the ring consider Its polarization The electrons m the carbon-oxygen double bond are drawn toward oxy gen and away from carbon leaving the carbon attached to the nng with a partial posi tive charge Using benzaldehyde as an example... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Section 17 2 The carbonyl carbon is sp hybridized and it and the atoms attached to It are coplanar Aldehydes and ketones are polar molecules Nucleophiles attack C=0 at carbon (positively polarized) and electrophiles especially protons attack oxygen (negatively polarized)... 

In spite of being ionic many quaternary ammonium salts dissolve m nonpolar media The four alkyl groups attached to nitrogen shield its positive charge and impart lipophilic character to the tetraalkylammonium ion The following two quaternary ammonium salts for example are soluble m solvents of low polarity such as benzene decane and halo genated hydrocarbons... 

DEPT (Section 13 18) Abbreviation for distortionless enhance ment of polarization ttansfer DEPT is an NMR technique that reveals the number of hydrogens directly attached to a carbon responsible for a particular signal... 

The group moment always includes the C—X bond. When the group is attached to an aromatic system, the moment contains the contributions through resonance of those polar structures postulated as arising through charge shifts around the ring. 

The subscript v is attached to both of these intensities as a reminder that the foregoing analysis is based on the assumption of vertical polarization for the incident light beam. The ratio of these intensities gives the fraction of light scattered per molecule by vertically polarized light ... 

The solubihty of alkylphenols in water falls off precipitously as the number of carbons attached to the ring increases. They are generally soluble in common organic solvents acetone, alcohols, hydrocarbons, toluene. Solubihty in alcohols or heptane follows the generalization that "like dissolves like." The more polar the alkylphenol, the greater its solubihty in alcohols, but not in ahphatic hydrocarbons likewise with cresols and xylenols. The solubihty of an alkylphenol in a hydrocarbon solvent increases as the number of carbon atoms in the alkyl chain increases. High purity para substituted phenols, through Cg, can be obtained by crystallization from heptane. 

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