Vibronic Spectra of Jet-Cooled 2-Aminopurine center dot H2O Clusters Studied by UV Resonant Two-Photon Ionization Spectroscopy and Quantum Chemical Calculations

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine center dot H2O. monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift delta nu of the S-1 <- S-0 transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm(-1)), while that of isomer B is much larger (delta nu = 889 cm(-1)). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H2O acting as an acceptor to a N-H or -NH2 group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy D-e = 56.4 kJ/mol. Isomers B and C are H-bonded between the -NH2 group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest (1)pi pi* states of A and B in excellent agreement with the observed 0(0)(0) bands; also, the relative intensities of the A and B origin bands agree well with the calculated S-0 state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited (1)pi pi* state to the lower-lying (1)n pi* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the (1)pi pi* state of B is planar and decoupled from the (1)n pi* state. These observations agree with the calculations, which predict the (1)n pi* above the (1)pi pi* state for isomer B but below the (1)pi pi* for both 9H-2AP and isomer A.

Low-lying excited states and nonradiative processes of 9-methyl-2-aminopurine

he UV spectrum of the adenine analogue 9-methyl-2-aminopurine (9M-2AP) is investigated with one- and two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm−1 resolution in a supersonic jet. The electronic origin at 32 252 cm−1 exhibits methyl torsional subbands that originate from the 0A′′1 (l = 0) and 1E ″ (l = ±1) torsional levels. These and further torsional bands that appear up to 000+230 cm−1 allow to fit the threefold (V 3) barriers of the torsional potentials as ∣∣V′′3∣∣=50 cm−1 in the S 0 and ∣∣V′3∣∣=126 cm−1 in the S 1 state. Using the B3LYP density functional and correlated approximate second-order coupled cluster CC2 methods, the methyl orientation is calculated to be symmetric relative to the 2AP plane in both states, with barriers of V′′3=20 cm−1 and V′3=115 cm−1. The 000 rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis 1ππ* excitation. The residual 25% c-axis polarization may indicate coupling of the 1ππ* to the close-lying 1 nπ* state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated 1 nπ oscillator strength is only 6% of that of the 1ππ* transition. The 1ππ* vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm−1. The methyl torsion and the low-frequency out-of-plane ν′1 and ν′2 vibrations are strongly coupled in the 1ππ* state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the 1ππ* spectrum of 9H-2-aminopurine [S. Lobsiger, R. K. Sinha, M. Trachsel, and S. Leutwyler, J. Chem. Phys.134, 114307 (2011)]. From the Lorentzian broadening needed to fit the 000 contour of 9M-2AP, the 1ππ* lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.

Bathorhodopsin structure in the room-temperature rhodopsin photosequence: picosecond time-resolved coherent anti-Stokes Raman scattering.

Structural changes in the retinal chromophore during the formation of the bathorhodopsin intermediate (bathoRT) in the room-temperature rhodopsin (RhRT) photosequence (i.e., vision) are examined using picosecond time-resolved coherent anti-Stokes Raman scattering. Specifically, the retinal structure assignable to bathoRT following 8-ps excitation of RhRT is measured via vibrational Raman spectroscopy at a 200-ps time delay where the only intermediate present is bathoRT. Significant differences are observed between the C=C stretching frequencies of the retinal chromophore at low temperature where bathorhodopsin is stabilized and at room temperature where bathorhodopsin is a transient species in the RhRT photosequence. These vibrational data are discussed in terms of the formation of bathoRT, an important step in the energy storage/transduction mechanism of RhRT.

Caracterização e aplicação analítica de eletrodos modificados com sistemas porfirínicos supramoleculares

Estudos com eletrodos modificados foram conduzidos utilizando dois sistemas porfirínicos supramoleculares diferentes. O primeiro foi baseado na modificação de eletrodo de carbono vítreo com uma porfirina de níquel tetrarrutenada, [NiIITPyP{RuII(bipy)2Cl}4]4+. A modificação do eletrodo foi realizada por meio de sucessivos ciclos voltamétricos em meio alcalino (pH 13), gerando um eletrodo com característica similar a eletrodos modificados com α-Ni(OH)2. A caracterização química do filme formado foi realizada através das técnicas de voltametria cíclica, ressonância paramagnética eletrônica, espectroscopia eletrônica por reflectância e espectroscopia Raman com ensaio espectro-eletroquímico. Os resultados sugerem a formação de um polímero de coordenação, [µ-O2-NiIITPyP{RuII(bipy)2Cl}4]n, composto por subunidades porfirínicas ligadas entre si por pontes µ-peroxo axialmente coordenadas aos átomos de níquel (Ni-O-O-Ni). O crescimento do filme apresentou dependência da alcalinidade do meio pela formação do precursor octaédrico [Ni(OH)2TRPyP]2+ em solução, pela coordenação de OH- nas posições axiais do átomo de níquel. O processo de eletropolimerização indicou a participação de radical hidroxil, gerado por oxidação eletrocatalítica da água nos sítios periféricos da porfirina contendo o complexo de rutênio. O mesmo eletrodo foi aplicado como sensor eletroquímico para análise amperométrica de ácido fólico em comprimidos farmacêuticos. O sensor foi associado a um sistema de Batch Injection Analysis (BIA) alcançando considerável rapidez e baixo limite de detecção. Para as análises das amostras também foi proposto um método para a remoção da lactose, que agia como interferente. O segundo estudo envolveu a modificação de eletrodos de carbono vítreo com diferentes hemoglobinas, naturais (HbA0, HbA2 e HbS) e sintéticas (Hb-PEG5K2, αα-Hb-PEG5K2 e BT-PEG5K4), para a avaliação da eficiência na redução eletrocatalítica de nitrito mediada por FeI-heme. Os filmes foram produzidos pela mistura de soluções das hemoglobinas com brometo de didodecildimetiltrimetilamônio (DDAB), aplicados nas superfícies com consecutiva evaporação, formando filmes estáveis. Os valores de potencial redox para os processos do grupo heme e a sua associação com a disponibilidade do grupo na proteína foram avaliados por voltametria cíclica. Os valores das constantes de velocidade, k, para redução de nitrito foram obtidos por cronoamperometria em -1,1 V (vs Ag/AgCl(KCl 3M)) que foram utilizados para estudo comparativo entre as espécies sintéticas para eventual aplicação clínica.

Promoter effect of sodium in graphene-supported Ni and Ni–CeO2 catalyst for the low-temperature WGS reaction

The low temperature water–gas shift (WGS) reaction has been studied over Ni–CeO2/Graphene and Ni/Graphene. The catalysts were prepared with 5 wt.% Ni and 20 wt.% CeO2 loadings, by deposition-precipitation employing sodium hydroxide and urea as precipitating agents. The materials were characterized by TEM, powder X-ray diffraction, Raman spectroscopy, H2-temperature-programmed reduction and X-ray photoelectron spectroscopy (XPS). The characterization and the reaction results indicated that the interaction between the active species and the support is higher than with activated carbon, and this hinders the reducibility of ceria and thus the catalytic performance. On the other hand, the presence of residual sodium in samples prepared by precipitation with NaOH facilitated the reduction of ceria. The catalytic activity was highly improved in the presence of sodium, what can be explained on the basis of an associative reaction mechanism which is favored over Ni-O-Na entities.

Nuclear magnetic resonance spectroscopy and ultrasound

The work described in this thesis is directed to the examination of the hypothesis that ultrasound may be used to perturb molecular motion in the liquid phase. These changes can then be detected by nuclear magnetic resonance (NMR) in spin-lattice and spin-spin relaxation times. The objective being to develop a method capable of reducing the pulsed NMR acquisition times of slowly relaxing nuclei. The thesis describes the theoretical principles underlying both NMR spectroscopy and ultrasonics with particular attention being paid to factors that impinge on testing the above hypothesis. Apparatus has been constructed to enable ultrasound at frequencies between 1 and 10 mega-hertz with a variable power up to 100W/cm-2 to be introduced in the NMR sample. A broadband high frequency generator is used to drive PZT piezo-electric transducer via various transducer to liquid coupling arrangements. A commercial instrument of 20 kilo-hertz has also been employed to test the above hypothesis and also to demonstrate the usefulness of ultrasound in sonochemistry. The latter objective being, detection of radical formation in monomer and polymer ultrasonic degradation. The principle features of the results obtained are: Ultrasonic perturbation of T1 is far smaller for pure liquids than is for mixtures. The effects appear to be greater on protons (1H) than on carbon-13 nuclei (13C) relaxation times. The observed effect of ultrasonics is not due to temperature changes in the sample. As the power applied to the transducer is progressively increased T1 decreases to a minimum and then increases. The T1's of the same nuclei in different functional groups are influenced to different extents by ultrasound. Studies of the 14N resonances from an equimolar mixture of N, N-dimethylformamide and deuterated chloroform with ultrasonic frequencies at 1.115, 6, 6.42 and 10 MHz show that as the frequency is increased the NMR signal to noise ratio decreases to zero at the Larmor frequency of 6.42 MHz and then again rises. This reveals the surprising indication that an effect corresponding to nuclear acoustic saturation in the liquid may be observable. Ultrasonic irradiation of acidified ammonium chloride solution at and around 6.42 MHz appears to cause distinctive changes in the proton-nitrogen J coupling resonance at 89.56 MHz. Ultrasonic irradiation of N, N-dimethylacetamide at 2 KHz using the lowest stable power revealed the onset of coalescence in the proton spectrum. The corresponding effect achieved by direct heating required a temperature rise of approximately 30oC. The effects of low frequency (20 KHz) on relaxation times appear to be nil. Detection of radical formation proved difficult but is still regarded as the principle route for monomer and polymer degradation. The initial hypothesis is considered proven with the results showing significant changes in the mega-hertz region and none at 20 KHz.

High pressure and low temperature study of ammonia borane and lithium amidoborane

Hydrogen has been considered as a potentially efficient and environmentally friendly alternative energy solution. However, one of the most important scientific and technical challenges that the "hydrogen economy" faces is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH3NH 3), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%).However, a fairly high temperature is required to release all the hydrogen atoms, along with the emission of toxic borazine. Recently research interests are focusing on the improvement of H2 discharge from ammonia borane (AB) including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques. Till now the detailed information about the bonding characteristics of AB is not sufficient to understand details about its phases and structures. ^ Elemental substitution of ammonia borane produces metal amidoboranes. Introduction of metal atoms to the ammonia borane structure may alter the bonding characteristics. Lithium amidoborane is synthesized by ball milling of ammonia borane and lithium hydride. High pressure study of molecular crystal provides unique insight into the intermolecular bonding forces and phase stability. During this dissertation, Raman spectroscopic study of lithium amidoborane has been carried out at high pressure in a diamond anvil cell. It has been identified that there is no dihydrogen bond in the lithium amidoborane structure, whereas dihydrogen bond is the characteristic bond of the parent compound ammonia borane. It has also been identified that the B-H bond becomes weaker, whereas B-N and N-H bonds become stronger than those in the parent compound ammonia borane. At high pressure up to 15 GPa, Raman spectroscopic study indicates two phase transformations of lithium amidoborane, whereas synchrotron X-ray diffraction data indicates only one phase transformation of this material. ^ Pressure and temperature has a significant effect on the structural stability of ammonia borane. This dissertation explored the phase transformation behavior of ammonia borane at high pressure and low temperature using in situ Raman spectroscopy. The P-T phase boundary between the tetragonal (I4mm) and orthorhombic (Pmn21) phases of ammonia borane has been determined. The transition has a positive Clapeyron slope which indicates the transition is of exothermic in nature. Influence of nanoconfinemment on the I4mm to Pmn2 1 phase transition of ammonia borane was also investigated. Mesoporus silica scaffolds SBA-15 with pore size of ~8 nm and MCM-41 with pore size of 2.1-2.7 nm, were used to nanoconfine ammonia borane. During cooling down, the I4mm to Pmn21 phase transition was not observed in MCM-41 nanoconfined ammonia borane, whereas the SBA-15 nanocondfined ammonia borane shows the phase transition at ~195 K. Four new phases of ammonia borane were also identified at high pressure up to 15 GPa and low temperature down to 90 K.^

Raman Analysis of Dilute Aqueous Samples by Localized Evaporation of Submicroliter Droplets on the Tips of Superhydrophobic Copper Wires

Raman analysis of dilute aqueous solutions is normally prevented by their low signal levels. A very general method to increase the concentration to detectable levels is to evaporate droplets of the sample to dryness, creating solid deposits which are then Raman probed. Here, superhydrophobic (SHP) wires with hydrophilic tips have been used as supports for drying droplets, which have the advantage that the residue is automatically deposited at the tip. The SHP wires were readily prepared in minutes using electroless galvanic deposition of Ag onto copper wires followed by modification with a polyfluorothiol (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol, HDFT). Cutting the coated wires with a scalpel revealed hydrophilic tips which could support droplets whose maximum size was determined by the wire diameter. Typically, 230 μm wires were used to support 0.6 μL droplets. Evaporation of dilute melamine droplets gave solid deposits which could be observed by scanning electron microscopy (SEM) and Raman spectroscopy. The limit of detection for melamine using a two stage evaporation procedure was 1 × 10^-6 mol dm^-3. The physical appearance of dried droplets of sucrose and glucose showed that the samples retained significant amounts of water, even under high vacuum. Nonetheless, the Raman detection limits of sucrose and glucose were 5 × 10^-4 and 2.5 × 10^-3 mol dm^-3, respectively, which is similar to the sensitivity reported for surface-enhanced Raman spectroscopy (SERS) detection of glucose. It was also possible to quantify the two sugars in mixtures at concentrations which were similar to those found in human blood through multivariate analysis.

Estudo da homogeneidade e discriminação de madeiras através de espectroscopia de imagens, imagens digitais e ferramentas quimiométricas

Tese (doutorado)–Universidade de Brasília, Instituto de Química, Programa de Pós-Graduação em Química, 2016.

High Pressure and Low Temperature Study of Ammonia Borane and Lithium Amidoborane

Hydrogen has been considered as a potentially efficient and environmentally friendly alternative energy solution. However, one of the most important scientific and technical challenges that the “hydrogen economy” faces is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH3NH3), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%).However, a fairly high temperature is required to release all the hydrogen atoms, along with the emission of toxic borazine. Recently research interests are focusing on the improvement of H2 discharge from ammonia borane (AB) including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques. Till now the detailed information about the bonding characteristics of AB is not sufficient to understand details about its phases and structures. Elemental substitution of ammonia borane produces metal amidoboranes. Introduction of metal atoms to the ammonia borane structure may alter the bonding characteristics. Lithium amidoborane is synthesized by ball milling of ammonia borane and lithium hydride. High pressure study of molecular crystal provides unique insight into the intermolecular bonding forces and phase stability. During this dissertation, Raman spectroscopic study of lithium amidoborane has been carried out at high pressure in a diamond anvil cell. It has been identified that there is no dihydrogen bond in the lithium amidoborane structure, whereas dihydrogen bond is the characteristic bond of the parent compound ammonia borane. It has also been identified that the B-H bond becomes weaker, whereas B-N and N-H bonds become stronger than those in the parent compound ammonia borane. At high pressure up to 15 GPa, Raman spectroscopic study indicates two phase transformations of lithium amidoborane, whereas synchrotron X-ray diffraction data indicates only one phase transformation of this material. Pressure and temperature has a significant effect on the structural stability of ammonia borane. This dissertation explored the phase transformation behavior of ammonia borane at high pressure and low temperature using in situ Raman spectroscopy. The P-T phase boundary between the tetragonal (I4mm) and orthorhombic (Pmn21) phases of ammonia borane has been determined. The transition has a positive Clapeyron slope which indicates the transition is of exothermic in nature. Influence of nanoconfinemment on the I4mm to Pmn21 phase transition of ammonia borane was also investigated. Mesoporus silica scaffolds SBA-15 with pore size of ~8 nm and MCM-41 with pore size of 2.1-2.7 nm, were used to nanoconfine ammonia borane. During cooling down, the I4mm to Pmn21 phase transition was not observed in MCM-41 nanoconfined ammonia borane, whereas the SBA-15 nanocondfined ammonia borane shows the phase transition at ~195 K. Four new phases of ammonia borane were also identified at high pressure up to 15 GPa and low temperature down to 90 K.

Online impedance spectroscopy estimation of a battery

Electrochemical impedance spectroscopy (EIS) is a helpful tool to understand how a battery is behaving and how it degrades. One of the disadvantages is that it is typically an 'off-line' process. This paper investigates an alternative method of looking at impedance spectroscopy of a battery system while it is on-line and operational by manipulating the switching pattern of the dc-dc converter to generate low frequency harmonics in conjunction with the normal high frequency switching pattern to determine impedance in real time. However, this adds extra ripple on the inductor which needs to be included in the design calculations. The paper describes the methodology and presents some experimental results in conjunction with EIS results to illustrate the concept.

Estudo por espectroscopia micro-Raman dos mecanismos de separação de fase em vidros fosfatos de metais de transição

Glass-ceramics are prepared by controlled separation of crystal phases in glasses, leading to uniform and dense grain structures. On the other hand, chemical leaching of soluble crystal phases yields porous glass-ceramics with important applications. Here, glass/ceramic interfaces of niobo-, vanado- and titano-phosphate glasses were studied by micro-Raman spectroscopy, whose spatial resolution revealed the multiphase structures. Phase-separation mechanisms were also determined by this technique, revealing that interface composition remained unchanged as the crystallization front advanced for niobo- and vanadophosphate glasses (interface-controlled crystallization). For titanophosphate glasses, phase composition changed continuously with time up to the equilibrium composition, indicating a spinodal-type phase separation.

Knoop microhardness and FT-Raman evaluation of composite resins: influence of opacity and photoactivation source

The aim of this study was to evaluate the degree of conversion by Knoop microhardness (KHN) and FT-Raman spectroscopy (FTIR) of one nanofilled (Filtek Supreme-3M-ESPE [FS]) and one microhybrid composite (Charisma-Heraeus-Kulzer [CH]), each with different opacities, namely enamel, dentin, and translucent, which were photo-activated by a quartz-tungsten-halogen lamp (QTH) and a light-emitting diode (LED). Resin was bulk inserted into a disc-shaped mold that was 2.0 mm thick and 4 mm in diameter, obtaining 10 samples per group. KHN and FTIR values were analyzed by two-way ANOVA and Tukey's tests (α = 0.05). Nanofilled resin activated by a LED presented higher microhardness values than samples activated by a QTH for dentin opacity (p < 0.05). The microhybrid resin showed no differences in KHN or FTIR values with different activation sources or opacity. The nanofilled dentin and enamel resins showed lower FTIR values than the translucent resin. The KHN values of the translucent resins were not influenced by the light source.

Spectral Region Optimization for Raman-Based Optical Biopsy of Inflammatory Lesions

Objective: The biochemical alterations between inflammatory fibrous hyperplasia (IFH) and normal tissues of buccal mucosa were probed by using the FT-Raman spectroscopy technique. The aim was to find the minimal set of Raman bands that would furnish the best discrimination. Background: Raman-based optical biopsy is a widely recognized potential technique for noninvasive real-time diagnosis. However, few studies had been devoted to the discrimination of very common subtle or early pathologic states as inflammatory processes that are always present on, for example, cancer lesion borders. Methods: Seventy spectra of IFH from 14 patients were compared with 30 spectra of normal tissues from six patients. The statistical analysis was performed with principal components analysis and soft independent modeling class analogy cross-validated, leave-one-out methods. Results: Bands close to 574, 1,100, 1,250 to 1,350, and 1,500 cm(-1) (mainly amino acids and collagen bands) showed the main intragroup variations that are due to the acanthosis process in the IFH epithelium. The 1,200 (C-C aromatic/DNA), 1,350 (CH(2) bending/collagen 1), and 1,730 cm(-1) (collagen III) regions presented the main intergroup variations. This finding was interpreted as originating in an extracellular matrix-degeneration process occurring in the inflammatory tissues. The statistical analysis results indicated that the best discrimination capability (sensitivity of 95% and specificity of 100%) was found by using the 530-580 cm(-1) spectral region. Conclusions: The existence of this narrow spectral window enabling normal and inflammatory diagnosis also had useful implications for an in vivo dispersive Raman setup for clinical applications.

Fluorescence and reflectance spectroscopy for identification of atherosclerosis in human carotid arteries using principal components analysis

Objectives: The aim of this work was to verify the differentiation between normal and pathological human carotid artery tissues by using fluorescence and reflectance spectroscopy in the 400- to 700-nm range and the spectral characterization by means of principal components analysis. Background Data: Atherosclerosis is the most common and serious pathology of the cardiovascular system. Principal components represent the main spectral characteristics that occur within the spectral data and could be used for tissue classification. Materials and Methods: Sixty postmortem carotid artery fragments (26 non-atherosclerotic and 34 atherosclerotic with non-calcified plaques) were studied. The excitation radiation consisted of a 488-nm argon laser. Two 600-mu m core optical fibers were used, one for excitation and one to collect the fluorescence radiation from the samples. The reflectance system was composed of a halogen lamp coupled to an excitation fiber positioned in one of the ports of an integrating sphere that delivered 5 mW to the sample. The photo-reflectance signal was coupled to a 1/4-m spectrograph via an optical fiber. Euclidean distance was then used to classify each principal component score into one of two classes, normal and atherosclerotic tissue, for both fluorescence and reflectance. Results: The principal components analysis allowed classification of the samples with 81% sensitivity and 88% specificity for fluorescence, and 81% sensitivity and 91% specificity for reflectance. Conclusions: Our results showed that principal components analysis could be applied to differentiate between normal and atherosclerotic tissue with high sensitivity and specificity.