Conflicting Observations Resolved by a Far IR and UV/Vis Study of the NO3 Radical

Codeposition of NO and O ₂ diluted in Ne at 6 K yield a ON???OO complex that exhibit strong UV absorption. This complex is converted into NO₃ radicals by UV irradiation, and is regenerated by radiation of visible light (see spectra).

     

Definitive evidence for monoanionic binding of 2,3-dihydroxybiphenyl to 2,3-dihydroxybiphenyl 1,2-dioxygenase from UV resonance Raman spectroscopy, UV/Vis absorption spectroscopy, and crystallography

Ultraviolet resonance Raman spectroscopy (UVRRS), electronic absorption spectroscopy, and X-ray crystallography were used to probe the nature of the binding of 2,3-dihydroxybiphenyl (DHB) to the extradiol ring-cleavage enzyme, 2,3-dihydroxybiphenyl 1,2-dioxygenase (DHBD; EC 1.13.11.39). The lowest lying transitions in the electronic absorption spectrum of DHBD-bound DHB occurred at 299 nm, compared to 305 nm for the monoanionic DHB species in buffer. In contrast, the corresponding transitions in neutral and dianionic DHB occurred at 283 and 348 nm, respectively, indicating that DHBD-bound DHB is monoanionic. These binding-induced spectral changes, and the use of custom-designed optical fiber probes, facilitated UVRR experiments. The strongest feature of the UVRR spectrum of DHB was a Y8a-like mode around 1600 cm(-1), whose position depended strongly on the protonation state of the DHB. In the spectrum of the DHBD-bound species, this feature occurred at 1603 cm(-1), as observed in the spectrum of monoanionic DHB. Raman band shifts were observed in deuterated solvent, ruling out dianionic binding of the substrate. Thus, the electronic absorption and UVRRS data demonstrate that DHBD binds its catecholic substrate as a monoanion, definitively establishing this feature of the proposed mechanism of extradiol dioxygenases. This conclusion is supported by a crystal structure of the DHBD:DHB complex at 2.0 A resolution, which suggests that the substrate's 2-hydroxyl substituent, and not the 3-hydroxyl group, deprotonates upon binding. The structural data also show that the aromatic rings of the enzyme-bound DHB are essentially orthogonal to each other. Thus, the 6 nm blue shift of the transition for bound DHB relative to the monoanion in solution could indicate a conformational change upon binding. Catalytic roles of active site residues are proposed based on the structural data and previously proposed mechanistic schemes.     

Limitations on the use of UV/Vis spectroscopy for the evaluation of conjugation effectiveness

A detailed study on N,N-dimethylanilino donor-substituted cyanoethynylethenes shows that there is no correlation between the effectiveness of the donor-acceptor conjugation pathway and the lowest-energy transition in the UV/Vis absorption spectrum.     

The rapid identification of organic colorants by UV/vis spectroscopy

A rapid UV/vis spectroscopic method for the identification of organic colorants by the use of Gaussian analysis is presented. Sets of parameters are obtained for the screening with a low number of data. An optical distance using line positions and intensities is defined as a measure for the similarity of UV/vis spectra.     

IR,Raman, and UV/Vis Spectra of Corannulene for Use in Possible Interstellar Identification

The spectroscopic characterization of corannulene (C₂₀H₁₀) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography‐mass spectrometry (GC‐MS). During a high‐performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time‐dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium.     

Probing the glycosidic linkage: UV and IR ion-dip spectroscopy of a lactoside

The beta(1-->4) glycosidic linkage found in lactose is a prevalent structural motif in many carbohydrates and glycoconjugates. Using UV and IR ion-dip spectroscopies to probe benzyl lactoside isolated in the gas phase, we find that the disaccharide unit adopts only a single, rigid structure. Its fully resolved infrared ion-dip spectrum is in excellent agreement with that of the global minimum structure computed ab initio. This has glycosidic torsion angles of phi(H) (H1-C1-O-C4') approximately 180 degrees and psi(H) (C1-O-C4'-H4') approximately 0 degrees which correspond to a rotation of approximately 150 degrees about the glycosidic bond compared to the accepted solution-phase conformation. We discuss the biological implications of this discovery and the generality of the strategies employed in making it.     

The rearrangements of naphthylnitrenes: UV/Vis and IR spectra of azirines, cyclic ketenimines, and cyclic nitrile ylides

Ar matrix photolysis of 1- and 2-naphthyl azides 3 and 4 at 313 nm initially affords the singlet naphthyl nitrenes, (1)()1 and (1)()2. Relaxation to the corresponding lower energy, persistent triplet nitrenes (3)()1 and (3)()2 competes with cyclization to the azirines 15 and 18, which can also be formed photochemically from the triplet nitrenes. On prolonged irradiation, the azirines can be converted to the seven-membered cyclic ketenimines 10 and 13, respectively, as described earlier by Dunkin and Thomson. However, instead of the o-quinoid ketenimines 16 and 19, which are the expected primary ring-opening products of azirines 15 and 18, respectively, we observed their novel bond-shift isomers 17 and 20, which may be formally regarded as cyclic nitrile ylides. The existence of such ylidic heterocumulenes has been predicted previously, but this work provides the first experimental observation of such species. The factors which are responsible for the special stability of the ylidic species 17 and 20 are discussed.     

Generation and characterization of the selenocysteinyl radical: direct evidence from time-resolved UV/Vis, electron paramagnetic resonance, and Fourier transform infrared spectroscopy

The selenocysteinyl radical 1 has been generated for the first time by laser flash photolysis (lambda(exc) = 266 nm) of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2 and of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3 in acetonitrile and characterized by time-resolved (TR) UV/Vis, Fourier transform infrared (FTIR), and electron paramagnetic spectroscopy in combination with theoretical methods. A detailed product study was conducted using gas chromatography and one- and two-dimensional NMR spectroscopy. In the case of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3, the (9-fluorenylideneamino)oxycarbonyl moiety serves as a photolabile protection group providing a "caged selenocysteinyl radical" suitable for biophysical applications. Cleavage of the diselenide bridge or the selenium-carbonyl bond by irradiation is possible in high quantum yields. Because of the lack of a good IR chromophore in the mid-IR region, the selenocysteinyl radical 1 cannot be monitored directly by TR FTIR spectroscopy. TR UV/Vis spectroscopy revealed the formation of the selenocysteinyl radical 1 from both precursors. The selenocysteinyl radical 1 has a lifetime tau approximately 63 mus and exhibits a strong band located at lambda(max) = 335 nm. Calculated UV absorptions of the selenocysteinyl radical (UB3LYP/6-311G(d,p)) are in good agreement with the experimental results. The use of TR UV/Vis spectroscopy permitted the determination of the decay rates of the selenocysteinyl radical in the presence of two quenchers. The product studies demonstrated the reversible photoreaction of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2. Products of the photolysis of the "caged selenocysteinyl radical" precursor 3 are dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2, carbon dioxide, and some further smaller fragments. In addition, the photodecomposition of the (9-fluorenylideneamino)oxycarbonyl moiety produced 9-fluorenone-oxime 4, 9-fluoren-imine 5, and 6 and 7 as products of the dimerization of two 9-fluorenoneiminoxy radicals 8.     

Structural investigation of high-valent manganese-salen complexes by UV/Vis,Raman, XANES,and EXAFS spectroscopy

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)Mn(III)Cl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)Mn(III)Br], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn(III) complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximately 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as approximately +IV by a comparative study of Mn(III) and Mn(V) reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s-->3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max)=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn(IV) central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.     

In situ IR,NMR, EPR,and UV/Vis Spectroscopy: Tools for New Insight into the Mechanisms of Heterogeneous Catalysis

The development of new solid catalysts for use in industrial chemistry has hitherto been based to a large extent upon the empirical testing of a wide range of different materials. In only a few exceptional cases has success been achieved in understanding the overall, usually very complex mechanism of the chemical reaction through the elucidation of individual intermediate aspects of a heterogeneously catalyzed reaction. With the modern approach of combinatorial catalysis it is now possible to prepare and test much more rapidly a wide range of different materials within a short time and thus find suitable catalysts or optimize their chemical composition. Our understanding of the mechanisms of reactions catalyzed by these materials must be developed, however, by spectroscopic investigations on working catalysts under conditions that are as close as possible to practice (temperature, partial pressures of the reactants, space velocity). This demands the development and the application of new techniques of in situ spectroscopy. This review will show how this objective is being achieved. By the term in situ (Lat.: in the original position) is meant the investigation of the chemical reactions which are taking place as well as the changes in the working catalysts directly in the spectrometer.     

Early events in the photochemistry of aryl azides from femtosecond UV/Vis spectroscopy and quantum chemical calculations

The photochemistry of para- and ortho-biphenylyl azides and 1-naphthyl azide was studied by ultrafast spectroscopy. In every case, the singlet azide second excited states were observed by transient absorption spectroscopy and were found to have lifetimes of hundreds of femtoseconds. The decay of the S(2) states of the azides was accompanied by the growth of transient absorption of the corresponding singlet nitrenes. The intermediate S(1) state of the azides could not be observed due to its low instantaneous concentration resulting from fast fragmentation and nitrene formation. Quantum chemical calculations predict that the S(2) state of the azide is bound and that there is a much lower barrier toward arylnitrene formation from the S(1) state of the azide. Vibrational cooling of para-biphenylnitrene (11 ps) was experimentally observed. The lifetime of singlet ortho-biphenylnitrene was 16 ps in acetonitrile and was not affected by perdeuteration of the aryl ring. The lifetime of singlet 1-naphthylnitrene is 12 ps in acetonitrile at ambient temperature.     

Gas-phase models of gamma turns: effect of side-chain/backbone interactions investigated by IR/UV spectroscopy and quantum chemistry

The conformations of laser-desorbed jet-cooled short peptide chains Ac-Phe-Xxx-NH2 (Xxx=Gly, Ala, Val, and Pro) have been investigated by IR/UV double resonance spectroscopy and density-functional-theory (DFT) quantum chemistry calculations. Singly gamma-folded backbone conformations (betaL-gamma) are systematically observed as the most stable conformers, showing that in these two-residue peptide chains, the local conformational preference of each residue is retained (betaL for Phe and gamma turn for Xxx). Besides, beta turns are also spontaneously formed but appear as minor conformers. The theoretical analysis suggests negligible inter-residue interactions of the main conformers, which enables us to consider these species as good models of gamma turns. In the case of valine, two similar types of gamma turns, differing by the strength of their hydrogen bond, have been found both experimentally and theoretically. This observation provides evidence for a strong flexibility of the peptide chain, whose minimum-energy structures are controlled by side-chain/backbone interactions. The qualitative conformational difference between the present species and the reversed sequence Ac-Xxx-Phe-NH2 is also discussed.     

Ni(N2)4 revisited: an analysis of the Ni-N2 bonding properties of this benchmark system on the basis of UV/Vis, IR and Raman spectroscopy

Matrix isolation of Ni atoms in an N2 matrix leads to the formation of Ni(N2)4. This compound, being isoelectronic with the well known Ni(CO)4, represents an important bench-mark system. It has been characterised experimentally by UV/Vis, IR and Raman spectra. The vibrational spectra give evidence for both a1 modes, three of the four t2 modes, and one of the two e modes of the Td symmetric molecule. The experimental data obtained for Ni(14N2)4 and Ni(15N2)4 were used to determine the valence force constants f(Ni-N) and f(N-N), which are compared with those derived for Ni(N2) and for the corresponding carbonyl complexes Ni(CO) and Ni(CO)4. In addition, several overtones and combination modes of Ni(N2)4 were observed for the first time, providing further valuable information about the bond properties. The data allow for the first time a direct estimate of the Ni-N2 bond energy in Ni(N2)4 (120 kJ mol(-1)), that compares with a value of 148 kJ mol(-1) determined by the same method for Ni(CO)4.     

Performance of a fast fiber based UV/Vis multiwavelength detector for the analytical ultracentrifuge

The optical setup and the performance of a prototype UV/Vis multiwavelength analytical ultracentrifuge (MWL-AUC) is described and compared to the commercially available Optima XL-A from Beckman Coulter. Slight modifications have been made to the optical path of the MWL-AUC. With respect to wavelength accuracy and radial resolution, the new MWL-AUC is found to be comparable to the existing XL-A. Absorbance accuracy is dependent on the light intensity available at the detection wavelength as well as the intrinsic noise of the data. Measurements from single flashes of light are more noisy for the MWL-AUC, potentially due to the absence of flash-to-flash normalization in the current design. However, the possibility of both wavelength and scan averaging can compensate for this and still give much faster scan rates than the XL-A. Some further improvements of the existing design are suggested based on these findings.     

Ion selectivity of crown ethers investigated by UV and IR spectroscopy in a cold ion trap

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M(+)?B15C5 and M(+)?B18C6 (M = Li, Na, K, Rb, and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na(+)?B15C5 and K(+)?B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating an optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.     

Elucidating the Mechanism of Working SnO2 Gas Sensors Using Combined Operando UV/Vis,Raman, and IR Spectroscopy

SnO₂ is the most widely used metal oxide gas‐sensing material but a detailed understanding of its functioning is still lacking despite its relevance for applications. To gain new mechanistic insight into SnO₂ gas sensors under working conditions, we have developed an operando approach based on combined UV/Vis, Raman, and FTIR spectroscopy, allowing us for the first time to relate the sensor response to the concentration of oxygen vacancies in the metal oxide, the nature of the adsorbates, and the gas‐phase composition. We demonstrate with the example of ethanol gas sensing that the sensor resistance is directly correlated with the number of surface oxygen vacancies and the presence of surface species, in particular, acetate and hydroxy groups. Our operando results enable an assessment of mechanistic models proposed in the literature to explain gas sensor operation. Owing to their fundamental nature, our findings are of direct relevance also for other metal oxide gas sensors.     

Application of UV–Vis,near-infrared and mid-infrared spectroscopy to the study of Mn-bearing humites

The combination of electronic and vibrational spectra has been applied to correlate the spectral properties, with composition, structure and cation substitutions such as Mn, Fe, Ca and Zn for Mg in humites: norbergite, alleghanyite, leucophoenicite and sonolite with increasing number of silicate layers, 1, 2, 3 and 4. The observation of two broad bands in the visible range, near 550 and 450 nm (18 180 and 22 220 cm⁻¹) and one sharp band around 410 nm (24 390 cm⁻¹) is characteristic of Mn²⁺ in alleghanyite and leucophoenicite. The study of UV–Vis (electronic) spectral features confirms Mn as a major substituent in these two samples. Cation impurities like Zn and Ca as revealed from EDX analysis might be the cause for the absence of Mn-type spectrum in sonolite. The first observation is the near-infrared spectra of all four minerals in the first fundamental overtone OH-stretching mode are different and each mineral is characterized by its NIR spectrum. The feature in the range 7180–6600 cm⁻¹ [1393–1515 nm or 1.39–1.52 μm] corresponds to the overtones of OH stretching vibrational modes of the humite groups observed in their IR spectra over the range, 3680–3320 cm⁻¹. The infrared spectra of the hydrous components of OH and SiO₄ groups in the mineral structure act as an aid to distinguish the minerals of the humite mineral group. A band at 541 cm⁻¹ is assigned to MnO stretching mode.     

Investigation of secondary structure elements by IR/UV double resonance spectroscopy: analysis of an isolated beta-sheet model system

An isolated beta-sheet model system is investigated in a molecular beam experiment by means of mass- and isomer-selective IR/R2PI double resonance spectroscopy as well as ab initio and DFT calculations. As the exclusive intermolecular assembly, a beta-sheet motif is formed by spontaneous dimerization of two isolated peptide molecules. This secondary structure is produced from the tripeptide model Ac-Val-Tyr(Me)-NHMe without any further environment to form the binding motif which is analyzed by both the characteristic amide A and I vibrations. The experimental and theoretical investigations yield the assignment to an antiparallel beta-sheet model. The result of this detailed spectroscopic analysis on an isolated beta-sheet model indicates that there are intrinsic properties of a beta-sheet structure which can be formed without a solvent or a peptidic environment.     

Lifting of the degeneracy in semibullvalenes by remote and direct substituents: a quantitative study using variable-temperature carbon-13 NMR spectroscopy

A series of six 1,5-(ethylmethyl)semibullvalenes (1a <==> 1a', 2 <==> 2', 3 <==>3') and two 4(2)-substituted semibullvalenes (4 <==> 4'), each undergoing Cope equilibria between nondegenerate valence tautomers, was investigated by carbon-13 NMR spectroscopy at a range of temperatures in several different solvents. Gompper's treatment of substituent perturbation was extended, specifically accounting for the effects of the substituents on chemical shifts, to allow the determination of the thermodynamic parameters for these skewed equilibria. These new treatments were used to determine the population difference (f - f ') between the valence tautomers and the perturbation thermodynamic quantities DeltaH(P), DeltaS(P), and DeltaG(P). The slow-exchange limit was reached for the parent 1,5-(ethylmethyl)semibullvalenes 3a <==> 3a' from which it was established that the preferred valence tautomer is 3a with the ethyl group on the cyclopropane ring. Despite considerable effort, the slow-exchange limit could not be reached in any of our other remotely substituted semibullvalenes. Provided that the ethyl group always prefers the cyclopropyl position as in 3a, the 1-ethyl-5-methylsemibullvalenes 1a, 2, and 3 are more stable by DeltaH(P) = 0.7-1.7 kJ mol(-1) than their valence tautomers 1a', 2', and 3'. In the directly substituted semibullvalenes (4 left harpoon ovet right harpoon 4'), the preferred valence tautomers 4a and 4b have the bromine atom or the nitrile group on the vinyl position (C(4)) rather than on the cyclopropane ring (C(2)) and are more stable than 4a' and 4b' by DeltaH(P) = 4.8 and 7.0 kJ mol(-1), respectively.