Polarization Labelling Spectroscopy

A very powerful method for the assignment of complex molecular spectra is the polarization labelling technique , which is based on a combination of polarization spectroscopy and optical double resonance. This method employs two different lasers (see Fig.10.42). The output beam from the first laser is 

Evaluation of the cross sections (see [10.57]) shows that the form, sign, and magnitude of the signals depend on the kind of polarization of the pump wave and on the value of aJ and aJ for pump and probe transition. This gives 

J = 28 - J = 28 1) appear as double resonance signals while at 0.1 torr collision-induced satellites are generated 

This technique is particularly useful if the upper state is perturbed and the assignment by conventional spectroscopy is impeded [10.61]. 

The second probe beam may be a broadband laser. If the polarization signal is detected on a photoplate behind a spectrograph, many transitions from the labelled level can then be detected simultaneously [10.61a]. 

---

B. HemmerUng, R. Bombach, W. Demtroder, N. Spies, Polarization labelling spectroscopy of molecular li2 Rydberg states. Z. Phys. D 5,165 (1987)... 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

Later, the experimental evidence for square cyclobutadiene was called into question. Krantz reported the photolysis of bicyclopyranone in which the carbon atom eliminated as CO2 was labeled with C. One important infrared band that had been assigned to a vibration of square planar cyclobutadiene in earlier studies was altered by the isotopic change, suggesting that this band was due to CO2 trapped with the cyclobutadiene in the rigid rare gas matrix. Thus, the experimental data did not answer the question of the structure of cyclobutadiene. Later work on the theoretical determination of the infrared spectrum of cyclobutadiene ° and further matrix isolation spectroscopy experiments, including the use of polarized IR spectroscopy... 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Fluorescence spectroscopy is also particularly well-suited to clarify many aspects of polymer/surfactant interactions on a molecular scale. The technique provides information on the mean aggregation numbers of the complexes formed and measures of the polarity and internal fluidity of these structures. Such interactions may be monitored by fluorescence not only with extrinsic probes or labelled polymers, but also by using fluorescent surfactants. Schild and Tirrell [154] have reported the use of sodium 2-(V-dodecylamino) naphthalene-6-sulfonate (SDN6S) to study the interactions between ionic surfactants and poly(V-isopropylacrylamide). 

Recently, it was shown that ss-NMR spectroscopy can be used to determine the conformation of EpoB in the solid state [116]. The method relies on the measurement of intramolecular short H-H distances (in the range 1.8-3.0 A) from 2D CHHC correlations under MAS [117]. Regarding the sample preparation, a small amount of 13C labeled compound was diluted with EpoB with natural abundance of carbon isotopes. This reduces the signal to noise but, on the other hand, it excludes contributions from intermolecular H-H polarization transfer. Under these conditions, all CHHC cross-peaks result from intramolecular polarization transfer and reflect intramolecular interproton distances. 

The different spatial orientation of the right-hand product compared to starting material, indicated schematically in Eq. (12), has been followed (70) using polarized spectroscopy and photolysis for Cr(CO)s where there is no chemical "label in the molecule. Both of these rearrangement processes are described by potential surfaces akin to the well-known cis <-> trans ethylene interconversion (71). Figure 13 illustrates schematically for the square pyramidal/trigonal bipyram-idal case an electronic ground state trapped in one of the two wells and... 

These case studies illustrate the power of CIDNP spectroscopy. Short-lived paramagnetic intermediates can be identified because their EPR spectrum remains frozen in as a polarization pattern of the nuclear spins in much longer-lived secondary species, and the pathways of their subsequent reactions can be traced out because these polarizations behave as nearly ideal labels. As the examples have shown, transformations of radical pairs into other radical pairs, with or without the participation of a third molecule as a scavenger, and transformations of biradicals can all be investigated by this method, which yields information that is often inaccessible by other techniques. 

---