Spectroscopic signatures

The least energetic intermediates, those with the slowest reactions, can often be directly detected during the course of a reaction. They can then be identified by their spectroscopic signatures. Actually, if the detection technique has sufficient sensitivity... 

For di- and polynuclear gold(I) complexes vith two Au(I) centers held in close proximity, a lower energy 5denergy from its mononuclear counterpart is a spectroscopic signature [2, 7, 44]. Excitation to the 5da —> 6pG transition gives rise to a [5da, 6pa] excited state having a formal... 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

Further studies were carried out with halocarbene amides 34 and 357 Although again no direct spectroscopic signatures for specifically solvated carbenes were found, compelling evidence for such solvation was obtained with a combination of laser flash photolysis (LFP) with UV-VIS detection via pyridine ylides, TRIR spectroscopy, density functional theory (DFT) calculations, and kinetic simulations. Carbenes 34 and 35 were generated by photolysis of indan-based precursors (Scheme 4.7) and were directly observed by TRIR spectroscopy in Freon-113 at 1635 and 1650 cm , respectively. The addition of small amounts of dioxane or THF significantly retarded the rate of biomolecular reaction with both pyridine and TME in Freon-113. Also, the addition of dioxane increased the observed lifetime of carbene 34 in Freon-113. These are both unprecedented observations. 

In either neat dioxane or THF, carbene-ether ylides are observed as a broad IR absorption band between 1560 and 1610 cm , distinct from the IR bands of the free carbenes. With discrete spectroscopic signatures for the free carbene and its corresponding ether ylides, TRIR spectroscopy was used to confirm that the effects described above with dilute ether in Freon-113 were due to specific solvation of the carbene (Scheme 4.6, Reaction 2) rather than a pre-equilibration with the coordinating solvent (Scheme 4.6, Reaction 3) or reactivity of the ylide itself (Scheme 6, Reaction 4). In Freon-113 containing 0.095M THF simultaneous TRIR observation of both the free carbene (x = ca. 500 ns) and the carbene-THF ylide (x = ca. 5ps) was possible7 The observation that lifetimes of these species were observed to be so different conclusively demonstrates that the free carbene and the carbene-THF ylide are not in rapid equilibrium and that Reaction 3 of Scheme 4.6 is not operative. By examining the kinetics of the carbene 34 at 1635 cm directly in Freon-113 with small amounts of added dioxane, it was observed that the rate of reaction with TME was reduced, consistent with Reaction 2 (and not Reaction 4) of Scheme 4.6. 

It has been known for some time that the spectroscopic signature of Ob-vacs can be healed by exposure to 02 [42-46], In addition, Epling etal. [47] show that temperature-programmed desorption (TPD) spectra of water and ammonia are perturbed when the surface is predosed with 02. This implies that oxygen is left on the surface in some form when Ob-vacs are healed by 02, As such, Epling et al. proposed that one Ob-vac is healed per 02 molecule with the other O atom being adsorbed at a Ti5c site (Oad), a dissociation mechanism supported by theoretical calculations [48, 49]. 

In general, the -dependent variations of natural atomic charges in dative bonds are significantly larger than those in covalent bonds. Indeed, the Q (R) variations in dative bonds resemble those in ionic bonds (cf. Fig. 2.9), to which they are evidently related by similarities in donor-acceptor character. The strong AQ /AR dependence tends to be associated with enhanced infrared vibrational intensity and other spectroscopic signatures characteristic of ionic bonding. 

HB-3) pronounced three-center character, with distinctive 2 Jab geminal spin couplings, IR vibrational couplings, and other spectroscopic signatures ... 

NMR is the most fundamental molecular specific probe of diffusion. Polymer motions and the spectroscopic signature of a given nucleus can be unambiguously related to a particular morphological domain. The size and time scale of the experiments are such that the fundamental hopping events of diffusing molecules can be sampled. 

Monomeric hemes possess a mirror plane and are hence achiral (151). Incorporation of the heme macrocycle into the anisotropic protein matrix distorts the heme environment, inducing a circular dichroism spectrum (57, 152, 153). From the design standpoint, the presence of an induced heme CD spectrum qualitatively confirms intimate communication between the heme and the surrounding protein matrix, which indicates the heme is most likely specifically bound. This spectroscopic signature serves as a first indication that the heme resides within the designed protein scaffold and has been used by various groups to... 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

However, not all reactive intermediates are kind enough to provide spectroscopic signatures that allow their immediate and unambiguous identification, and it is therefore often necessary to compare those signatures to ones obtained by means of modeling calculations (the reader may note that with this we leave the realm of forensic analogy that we have perhaps already stretched too far). In fact, many recent matrix isolation studies owe their success to the tremendous advances in the field of computational chemistry, and to the increased availability of the hard- and software required to carry out such calculations. This simation provides an opportunity for much creative work in the field of reactive intermediates, but it also implies an obligation on the part of those who use such methods to apply them with due care and circumspection. 

Besides self trapping two alternative explanations, Fermi resonance and conformational substates, have been previously discussed as well [2]. In a recent study [6] we compared the 2D-IR spectrum of ACN with those of two molecular systems, which show the same splitting in the amide I band, and which were chosen as simple representatives of the alternative mechanisms. The three 2D-IR spectra differ completely, albeit in a well understood way. Based on the 2D-IR spectroscopic signature Fermi resonance and conformational sub-states can be definitely excluded as alternative explanations for the anomalous spectra of ACN. The 2D-IR spectrum of the amide I mode in ACN, on the other hand, can be naturally explained by self-trapping, as dicussed above. 

Beyond the binary systems. Spectroscopic signatures arising from more than just two interacting atoms or molecules were also discovered in the pioneering days of the collision-induced absorption studies. These involve a variation with pressure of the normalized profiles, a(a>)/n2, which are pressure invariant only in the low-pressure limit. For example, a splitting of induced Q branches was observed that increases with pressure the intercollisional dip. It was explained by van Kranendonk as a correlation of the dipoles induced in subsequent collisions [404]. An interference effect at very low (microwave) frequencies was similarly explained [318]. At densities near the onset of these interference effects, one may try to model these as a three-body, spectral signature , but we will refer to these processes as many-body intercollisional interference effects which they certainly are at low frequencies and also at condensed matter densities. 

The spectroscopic signatures of excited states of derivatives of the conjugated polymer poly (p-phenylene vinylene) have been widely investigated both... 

Nesbitt, D.J. (1988b). Spectroscopic signature of floppiness in molecular complexes, in The Structure of Small Molecules and Ions, ed. Naaman, R. and Vager, Z. (Plenum, New York). 

A vibrational spectrum provides something like a structural fingerprint of matter because it is characteristic of chemical bonds in a specific molecule. Therefore, imaging based on vibrational spectroscopic signatures, such as Raman scattering and IR absorption, provides a great deal of molecular structural information about the target under study. 

Despite the difficulty imposed by the attenuation and dispersion of the atmosphere on subpicosecond THz pulses, there has been some effort to extract spectroscopic signatures of explosives, such as RDX, from standoff ranges up to 30 m [91], However, the measured spectral absorption peak of RDX artificially broadens, due to the absorption and dispersion in the atmosphere, as the distance to the target increases. This broadening at standoff distances suggests that spectroscopic identification of explosives such as RDX might be problematic because the apparent spectral shape cannot be directly compared to a spectral standard curve. To circumvent the atmospheric attenuation and dispersion... 

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