Direct Spectroscopic Observation

In atomic spectroscopy, to experimentally observe the spin-sublevel structures is quite easy. However, for large organic molecules and for metal complexes, to detect the splitting directly in the luminescence is difficult because of a number of reasons. 

In order for the sublevel luminescence to be observed, the population of the individual sublevels should be sufficiently large. Thus, the energy separation among sublevels should be of the order of kT or less. However, even if the temperature is of the order of kT, the inhomogeneous broadening still may prevent the resolution of the spectrum, and thus the bands from different sublevels are 

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Basche T, Kummer S and Brauchle C 1995 Direct spectroscopic observation of quantum ]umps of a single molecule Nature 373 132-4... 

It should be emphasized that, in contrast to the cases of secondary alkyl cations (Olah et al., 1964 Saunders et al., 1968) and tertiary alkyl cations (Olah et al., 1964 Brouwer and Mackor, 1964), the evidence for the existence of primary alkyl cations as distinct species has been only indirect, because they have escaped direct spectroscopic observation so far. 

A high specific interfacial area and a direct spectroscopic observation of the interface were attained by the centrifugal liquid membrane (CLM) method shown in Fig. 2. A two-phase system of about 100/rL in each volume is introduced into a cylindrical glass cell with a diameter of 19 mm. The cell is rotated at a speed of 5000-10,000 rpm. By this procedure, a two-phase liquid membrane with a thickness of 50-100 fim. is produced inside the cell wall which attains the specific interfacial area over 100 cm. UV/VIS spectrometry, spectro-fluorometry, and other spectroscopic methods can be used for the measurement of the interfacial species and its concentration as well as those in the thin bulk phases. This is an excellent method for determining interfacial reaction rates on the order of seconds. 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

Basche, T., Kummer, S. and Brauchle, C. Direct spectroscopic observation of quantum jumps of a single molecule, Nature, 373 (1995), 132-134... 

To put things into perspective, we. can broadly classify these analytical methods into bulk, dry surface, and in situ interfacial techniques. This chapter focuses on the last category, illustrating two in situ techniques used to study anion binding at the goethite (a-FeOOH)-water interface titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy. In fact, CIR-FTIR could prove to be extremely powerful, since it allows direct spectroscopic observation of ions adsorbed at the mineral-water interface. 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

The last years have seen a rapid development of the direct spectroscopic observation of stable carbonium ion intermediates in strongly acidic solutions. Thus many alkylcarbonium ions, previously suggested... 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

Because reactive intermediates (RIs) undergo facile intra- and/or intermolecular reactions, they are, by definition, short lived. As described in the other chapters in this volume, the transient namre of RIs provides a challenge to their direct spectroscopic observation. When the spectra of a putative RI can be obtained, it may not be certain that the observed spectra actually belong to the RI. 

The pHPZC of ferric hydroxide surfaces is about 8 [127], so aqueous Pb2+ should be electrostatically repelled from these surfaces at pH values less than 8. However, as seen in Figure 7.6(a), the Pb2+ present in this aqueous solution is sorbed essentially completely to ferric hydroxide surfaces at pH 6. This behavior suggests that Pb2+ forms direct chemical bonds to these surfaces in order to overcome the repulsive electrostatic forces below the pHpzc of ferric hydroxide. This conclusion based on macroscopic uptake data has been confirmed by direct spectroscopic observation using X-ray absorption fine structure (XAFS) spectroscopy under in situ conditions (i.e., with aqueous solution in contact with a-FeOOH surfaces at ambient temperature and pressure) [133,134]. These studies showed that the aquated Pb(II) ion forms dominantly inner-sphere, bidentate complexes on a-FeOOH surfaces. 

The reactivity of aromatic side chains to undergo dealkylation is in line with the stability of the corresponding carbocations. This indicates the possible involvement of carbocations in dealkylation, which was proved to be the case. The intermediacy of the rm-butyl cation in superacid solution was shown by direct spectroscopic observation.228,229 Additional proof was provided by trapping the ferf-butyl cation with carbon monoxide during dealkylation 230... 

Fluorescence quenching (248) and direct spectroscopic observation of the redox participants establishes the importance of primary photochemical electron exchange. 

Photolysis of 6 in a formaldehyde-doped argon matrix (0.6%) yielded also silene 7 as the major product but also minor quantities of other products. Warming of the matrix to 30-35 K allowed the direct spectroscopic observation of the reaction between 7 and formaldehyde. The intensity of the IR absorptions of these compounds disappeared to a large extent, and in addition to 8 a new product, which could be identified... 

Shafirovich V, Cadet J, Gasparutto D, Dourandin A, Fluang W, Geacintov NE (2001) Direct spectroscopic observation of 8-oxo-7,8-dihydro-2 -deoxyguanosine radicals in double-stranded DNA generated by one-electron oxidation at a distance by 2-aminopurine radicals. J Phys Chem B 105 586-592... 

Although the transient spectrum of a radical-ion pair was recorded in a picosecond-nanosecond time domain in the flash photolysis of Co(III) alkylcobalamins,123 there has been no direct spectroscopic observation of the LMCT excited states. The observed photochemical behavior may be simply described by the following sequence of events (1) absorption of radiation produces a Franck-Condon excited state, which (2) rapidly loses its excess vibrational energy (k > 10ns ) to form the thermalized excited state, followed by (3) product formation and internal conversion to the ground state. The existence of LMCT excited states with a finite lifetime in the... 

Tamao, Y., and Blakely, R. L., 1973, Direct spectroscopic observation of an intermediate formed from deoxyadenosylcobalamin in ribonucleotide reduction. Biochemistry 12 24034. 

Grimes, C.C. and Brown, T.R. (1974). Direct spectroscopic observation of electrons in image-potential states outside liquid helium, Phys. Rev. Lett. 32, 280-283. 

Direct spectroscopic observation of the postulated diradical intermediates has not been possible so far. Thus, multiphoton infrared laser excitation of tetramethyldioxetane in the gas phase failed to detect diradical intermediates with lifetimes greater than about 5 nsec.Picosecond spectroscopy limited the lifetime of a diradical intermediate, if formed, to less than about lOpsec in the 264-nm pulsed photolysis of tetramethyldioxetane in acetonitrile, using a mode-locked neodymium... 

In general, the information bearing on the nature of the CO2 primary photodlssociative steps is of two kinds (i) indirect inference based upon analysis of final products following the completion of secondary reactions, and (li) direct spectroscopic observation of primary and intermediate products. Both approaches have made major contributions to the present level of understanding of the CO2 photochemistry which has been summarized recently by Welge (45), and Preston and Cvetanovlc (46) for wavelengths longer than 100 nm. 

The first direct spectroscopic observation of the highly anharmonic nature of ring puckering in a small ring molecule was made by Danti4 who found part of the progression for oxetane at 90-140 cm-1 in far infrared absorption. This work... 

In recent years the pioneering work of Olah, Saunders, Brouwer, and Hogeveen has made possible the direct spectroscopic observation of many carbocations20. One remarkable development from such studies has been the conclusion that many carbocations, such as 2,3,3-trimethyl-2-butyl, which can be captured in solvolysis without equilibration17, undergo very rapid equilibration under stable ion conditions. Such equilibration often cannot be frozen out even at temperatures as low as -150 °C. Under such stable ion conditions it appears that rapid equilibration is the norm. 

In the absence of surface-active species, the surface charge of metal oxides is due solely to the adsorption/desorption of protons. The ions of an inert electrolyte remain at some distance from the surface and affect the surface charging indirectly. In most models, proton adsorption and desorption is interpreted as the protonation and deprotonation of discrete surface sites. Direct spectroscopic observation of these surface sites is difficult, and it does not give clear and... 

Laser flash photolysis is one of the most efficient methods for the direct spectroscopic observation of free radicals and for monitoring the kinetics of formation and decay in real-time. This method is an extension of conventional flash photolysis method [26] that was invented by Norrish and Porter in 1949, and who were awarded by the Nobel Prize in 1967. We have used this approach to investigate the generation and reactions of free radicals with DNA. In this technique, a laser light pulse is used to produce short-lived intermediates in solution contained in an optical cuvette, and the kinetics of their formation and decay are monitored by transient absorption spectroscopy. The apparatus we used is shown in Figure 4.1. 

Although, the authors were unable to determine the multiplicity of aroylnitrenes unambiguously by direct spectroscopic observation, the authors concluded that the sum total of their data left little doubt that aroylnitrenes have singlet ground states. ° On the basis of this finding, they pro-... 

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