Polarized light spectroscopy

In 1956 McCrone founded McCrone Associates, a private analytical laboratory in which the principal analytical technique employed was polarized light spectroscopy. Over the years he and his staff learned to visually identify over 30000 particles (McLafferty 1990). McCrone Associates speciahzed in the identification of polymorphs, asbestos samples, airborne impurities, among others. McCrone recently endowed a chair of chemical microscopy to Cornell University, his Alma Mater. 

We have studied the low-light B800-850 complexes (isolated and in membranes) with polarized light spectroscopy (room temperature and 77K CD, LD, absorption and fluorescence depolarization) to get a better understanding of the pigment organization in these complexes. 

I.r. laser techniques have been used to study [Fe(CO)4] in a nitrogen matrix at 20 K. Surprisingly, the quantum yield for intramolecular isomerization is greato-than the quantum yield for formation of [Fe(CO)4(N2)]. Photolysis of matrix isolated [Mo(CO)s] in the presence of Na at 20 K affords [Mo(CO)5Na)]. Under similar conditions, the analogous chromium and tungsten complexes have been detected by i.r. and Raman techniques. The photochemistry of [O(C0)5] and related species in matrices has been studied via polarized light spectroscopy. A mechanism has been proposed which accounts for almost all the experimental data on matrix-isolated [M(CO)s] (M=Cr, Mo, or W), [M(CO)5(Na)], and [M(CO)4(CS)]. Reactions occur via the following steps ... 

The varying actual orientation of molecules adsorbed at an aqueous solution-CCU interface with decreasing A has been followed by resonance Raman spectroscopy using polarized light [130]. The effect of pressure has been studied for fatty alcohols at the water-hexane [131] and water-paraffin oil [132] interfaces. 

An interesting aspect of two-photon spectroscopy is that some polarization infonnation is obtainable even for randomly oriented molecules in solution by studymg the effect of the relative polarization of die two photons. This is readily done by comparing linearly and circularly polarized light. Transitions to A states will absorb linearly polarized light more strongly than circularly polarized light. The reverse is true of transitions to B ... 

Guttler F, Sepiol J, Plakhotnik T, Mitterdorfer A, Renn A and Wild U P 1993 Single molecule spectroscopy fluorescence excitation spectra with polarized light J. Lumin. 56 29-38... 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Circular dichroism (c.d.) spectroscopy measures the difference in absorption between left- and right-circularly polarized light by an asymmetric molecule. The spectrum results from the interaction between neighboring groups, and is thus extremely sensitive to the conformation of a molecule. Because the method may be applied to molecules in solution, it has become popular for monitoring the structure of biological molecules as a function of solvent conditions. 

Of the visible spectroscopic techniques, CD spectroscopy has seen the most rapid and dramatic growth. The far-UV circular dichroism spectrum of a protein is a direct reflection of its secondary structure [71]. An asymmetrical molecule, such as a protein macromolecule, exhibits circular dichroism because it absorbs circularly polarized light of one rotation differently from circularly polarized light of the other rotation. Therefore, the technique is useful in determining changes in secondary structure as a function of stability, thermal treatment, or freeze-thaw. 

In ellipsometric spectroscopy, an elliptically polarized light is allowed to reflect on the interface and the change in ellipticity and phase angle are determined from complex reflectivity. 

UV-visible (UV-vis) spectroscopy detects valence electron transitions. One may detect the electronic state of metal ions from d-d transitions. Identification of unusual ligands—that is, Cu(II)-SR, Fe(III)-OPh, Fe(III)-0-Fe(III)—may be possible. UV-vis spectroscopy on single crystals using polarized light may yield geometric information. 

In the case of synthetic optically active polymers, the intuitive meaning of a CD signal intensity is very similar to that of UV spectroscopy, with the additional dimension of the subtracted absorption between left and right circularly polarized light.37 Absorption of light obeys the Beer-Lambert law, and thus CD intensity is defined as Ae = eL - eR = (AL - AR)/cl, where Ae is the... 

Vukjovic et al.199 recently proposed a simple, fast, sensitive, and low-cost procedure based on solid phase spectrophotometric (SPS) and multicomponent analysis by multiple linear regression (MA) to determine traces of heavy metals in pharmaceuticals. Other spectroscopic techniques employed for high-throughput pharmaceutical analysis include laser-induced breakdown spectroscopy (LIBS),200 201 fluorescence spectroscopy,202 204 diffusive reflectance spectroscopy,205 laser-based nephelometry,206 automated polarized light microscopy,207 and laser diffraction and image analysis.208... 

Enantiomers have identical chemical and physical properties in the absence of an external chiral influence. This means that 2 and 3 have the same melting point, solubility, chromatographic retention time, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR) spectra. However, there is one property in which chiral compounds differ from achiral compounds and in which enantiomers differ from each other. This property is the direction in which they rotate plane-polarized light, and this is called optical activity or optical rotation. Optical rotation can be interpreted as the outcome of interaction between an enantiomeric compound and polarized light. Thus, enantiomer 3, which rotates plane-polarized light in a clockwise direction, is described as (+)-lactic acid, while enantiomer 2, which has an equal and opposite rotation under the same conditions, is described as (—)-lactic acid. 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

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