Determination of the tautomeric composition of adenine in the gas phase by vibrational spectroscopy methods: I. Analysis of IR spectra

The tautomeric composition of isolated adenine has been analyzed using computational IR spectroscopy. A comparison with experimental data has demonstrated that, in addition to adenine-N₉H, which dominates in the quantitative content, two more tautomers that have the N₉H imino and N₇H amino forms exist in the isolated state and in the gas phase.     

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

Influence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (I_D/I_G), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.     

Electron resonance spectra of NCS and vibronically-excited NCO in the gas phase

Near 25 °C, ab initio calculations of the zero-density viscosity of helium gas η _He have an uncertainty of approximately 0.001%, which is 1/40th of the uncertainty of the best measurements. The uncertainties of the published calculations for neon and argon are probably much larger. This paper presents new measurements of the viscosities of neon, argon, and krypton at 25 °C made with a capillary viscometer that was calibrated with helium. The resulting viscosity ratios are η _Ne/η _He?=?1.59836?±?0.00037, η _Ar/η _He?=?1.13763?±?0.00030, and η _Kr/η _He?=?1.27520?±?0.00040. The argon ratio agrees with a recent, unpublished calculation to within the combined uncertainty (measurement plus calculation) of 0.032%. The neon ratio is smaller than the calculated value by 0.13%.     

Quantum-mechanical calculations of the intensity distribution in spectra of adenine tautomers: II. Two-photon absorption spectra

We have performed quantum-mechanical calculations of the two-photon absorption spectra of three tautomeric forms of adenine in the gas phase and of four forms in the aqueous solution. Based on a comparison of the results of these calculations with experimental data, the occurrence of three structures, Ade-N₉,N₁H⁺, Ade-N₉H (am), and Ade-N₇H(am), in aqueous solution of adenine has been confirmed.     

Electronic and vibrational Raman spectroscopy of Nd0.5Sr0.5MnO3 through the phase transitions

Raman scattering experiments have been carried out on single crystals of Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 320–50 K, covering the paramagnetic insulator-ferromagnetic metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We understand the existence of diffusive response in the ferromagnetic phase to the coexistence of the different electronic phases. The frequency and linewidth of the phonons across the transitions show significant changes, which cannot be accounted for only by anharmonic interactions.     

Raman scattering spectra and electrical conductivity of thin silicon films with a mixed amorphous-nanocrystalline phase composition: Determination of the nanocrystalline volume fraction

Raman spectra and electrical conductivity of thin films of hydrogenated silicon with mixed amorphous-nanocrystalline phase composition have been studied. It is shown that interpretation of experimental data in terms of percolation theory permits one to determine the integrated Raman-scattering cross-section ratio of the nanocrystalline to amorphous phase and to obtain a quantitative estimate of the volume fraction of each phase. Fiz. Tverd. Tela (St. Petersburg) 39, 1348–1353 (August 1997)     

Study of the phase transitions in ammonium bromide by Raman spectroscopy

A complete Raman scattering study of an oriented NH₄Br single crystal has been carried out and new results are reported for temperature between 45 to 410°K. The study includes measurements and interpretations of the Raman spectral bandwidths, intensities and frequencies of the lattice and internal modes as a function of temperature. Because of the ability of generating a single domain crystal, unambiguous assignments of the Raman active modes are made. From the present data together with those presented previously significant information on the behavior of the ordering and crystal structure associated with the phase transitions in NH₄Br is obtained.     

Decoupling multimode vibrational relaxations in multi-component gas mixtures: Analysis of sound relaxational absorption spectra

Decoupling the complicated vibrational-vibrational (V-V) coupling of a multimode vibrational relaxation remains a challenge for analyzing the sound relaxational absorption in multi-component gas mixtures. In our previous work [Acta Phys. Sin. 61 174301 (2012)], an analytical model to predict the sound absorption from vibrational relaxation in a gas medium is proposed. In this paper, we develop the model to decouple the V-V coupled energy to each vibrational- translational deexcitation path, and analyze how the multimode relaxations form the peaks of sound absorption spectra in gas mixtures. We prove that a multimode relaxation is the sum of its decoupled single-relaxation processes, and only the decoupled process with a significant isochoric-molar-heat can be observed as an absorption peak. The decoupling model clarifies the essential processes behind the peaks in spectra arising from the multimode relaxations in multi-component gas mixtures. The simulation validates the proposed decoupling model.     

Determination of titanium content in pyrope by Raman spectroscopy

The titanium content of pyrope garnet can be quantified using the intensity of a Raman band at about 830 cm⁻¹ that is normalized to the 363 cm⁻¹ band using a spectrometer‐specific calibration using 10 to 15 chromian pyropes from Bohemia, Czech Republic. An accuracy of 0.025 wt% could be achieved for TiO₂ contents between 0.17 and 0.67 wt% TiO₂ with a Raman spectrometer with a spectral resolution of better than 3.9 cm⁻¹. The technique can be used in petrological and gemmological studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of the Suzuki phase in doped alkali halides by Raman spectroscopy

The symmetries and frequencies of the Raman active modes of the Suzuki phase in the systems 6NaCl:CdCl₂; and 6NaCl:MnCl₂, have been calculated. Three of the four peaks allowed by symmetry (A_1g and 2F_g) agree to within 25% in position with the experimental values presented here and with other experimental results. The fourth peak (E_g) seems to be associated with a very soft mode, which explains why this peak has not been observed. The features of the Raman spectrum depend mainly upon the chlorine ions.     

Vibrational spectra and structure of polyene compounds. II. Band intensity in the vibrational spectra of linear polyenes

Analysis of infrared intensities shows that a polyene molecule has a system of conjugated and polar carbon bonds. The most intense bands in the Raman spectra are connected with vibrations involving the whole chain and a maximum change in the polarizability.     

Far infrared laser magnetic resonance spectra of the ClSO and FSO radicals in the gas phase

The detection of the free radicals ClSO and FSO by the technique of far infrared laser magnetic resonance is reported. Observations of the former have been made with several laser lines; the spectra are complicated and have not been assigned but are attributed to ClSO on the basis of several chemical tests. The FSO radical was generated by the reaction of carbon disulphide with the products of a discharge through CF₄ + O₂. Spectra have been observed with laser lines at 394, 419 and 513 μm. Using the results of a subsequent microwave study of FSO, the rotational transitions involved in these spectra have been identified. The spectrum at 513 μm has been studied in detail and is shown to arise through level anticrossings between the levels 14_5,Kc (the lower levels involved in the transition) and 15_4,Kc+2. All aspects of this spectrum are reproduced quantitatively by the parameters determined from the microwave spectrum. It is thought that the other two FSO spectra arise in a similar manner. Such anticrossing signals are likely to be widespread in LMR experiments, causing intense narrow lines to occur at high magnetic fields even for molecules with weakly coupled electron spin.     

Monitoring the liquid phase concentration by Raman spectroscopy in a polymorphic system

In this work, Raman spectroscopy was successfully used for the quantitative determination of the liquid phase concentration in an aqueous polymorphic system of D‐mannitol. An extensive study has initially been performed to identify the influence of the solid state, e.g. particle size, particle amount, and different polymorphs, on the intensity of the characteristic Raman solute signal. It was found that the existence of solid phase can decrease Raman intensity, and this influence is more significant when the suspension density is higher, e.g. with smaller size and larger amount of particles. Based on this information, a large number of samples were examined by Raman spectroscopy in the form of clear solutions and suspensions. The spectral preprocessing and partial least squares (PLS) regression were then used to relate the solute concentrations to these spectral data, independent of solid state. Several PLS calibration models were developed with different treatments to the spectral data, and the optimized strategy was finally demonstrated. Particularly, a reference peak at 578 cm⁻¹ related to the sapphire in the Raman probe window was innovatively applied to reduce the influences from the equipment and other external variations, with which the full‐spectrum PLS model was seen to give more stable results rather than partial spectral regions. The optimized model was subsequently applied to predict the liquid phase concentration in a multiphase multicomponent dynamic process, the solvent mediated polymorphic transformation (SMPT) of mannitol, and it was shown that the offline measurements and the predicted values were mainly in agreement with one another. Copyright © 2015 John Wiley & Sons, Ltd.     

Electronic spectra of aliphatic carbonyl compounds by electron impact spectroscopy: II. Saturated ketones

Electron impact energy loss spectra of acetone, 2-butanone, methyl isobutyl ketone, methyl isopropyl ketone, and methyl tertiarybutyl ketone have been recorded at an impact energy of 100 eV and a scattering angle of two degrees. Rydberg assignments have been made using quantum defects and term values obtained by reference to ionization potentials measured by photoelectron spectroscopy. Substituent effects on Rydberg orbital energies are discussed using Taft σ* values.     

Background-free coherent Raman spectroscopy by detecting the spectral phase of molecular vibrations

We propose and demonstrate a new approach to subtracting high nonresonant background in coherent anti-Stokes Raman scattering spectroscopy. The method is based on the retrieval of the spectral phase of molecular vibrations using the technique of frequency-resolved optical gating of Raman scattering. In the presence of high nonresonant background the retrieved phase corresponds directly to the background-free spectrum of the coherent Raman response.     

Investigation of size-driven phase transition in bismuth titanate nanocrystals by Raman spectroscopy

Raman spectroscopy was used to investigate the lattice dynamics and structural transformations in bismuth titanate (Bi₄Ti₃O₁₂) nanocrystals prepared by a chemical coprecipitation technique. The crystal structure of the samples of different grain sizes was determined by X-ray-diffraction analysis. The evolution of the Raman spectrum with grain size was characterized by an intensity decrease, a broadening of the line width, a frequency shift, and the disappearance of the Raman mode. The results revealed the appearance of a size-driven phase transition from orthorhombic to tetragonal phases at a critical size of 44 nm. This result is quite consistent with the X-ray-diffraction measurement and differential thermal analysis. The origin was attributed to the grain-size effect and explained by the surface-energy mechanism. Received: 26 June 2002 / Accepted: 18 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: msz@nju.edu.cn     

Raman and IR spectra of crystalline phosphine in the γ phase

The γ phase of crystalline phosphine has been studied using IR and Raman spectroscopy. The observations include both pure phases, PH₃ and PD₃, and many of the modes of PH₃, PH₂D and PHD₂ dilute in PD₃ as well as of PD₃, PD₂H and PDH₃ dilute in PH₃. The phase transition β → γ is very slow for PH₃, but much more rapid for PD₃.Evidence is adduced pointing to the existence of inequivalent sites in this crystal, but none of a number of suggested models for the unit cell is entirely consistent with the numbers of multiplet components of the internal modes, nor with the numbers of external (lattice) modes observed in the IR and Raman.