Vibrational relaxation of CH3I in the gas phase and in solution

Transient electronic absorption measurements reveal the vibrational relaxation dynamics of CH(3)I following excitation of the C-H stretch overtone in the gas phase and in liquid solutions. The isolated molecule relaxes through two stages of intramolecular vibrational relaxation (IVR), a fast component that occurs in a few picoseconds and a slow component that takes place in about 400 ps. In contrast, a single 5-7 ps component of IVR precedes intermolecular energy transfer (IET) to the solvent, which dissipates energy from the molecule in 50 ps, 44 ps, and 16 ps for 1 M solutions of CH(3)I in CCl(4), CDCl(3), and (CD(3))(2)CO, respectively. The vibrational state structure suggests a model for the relaxation dynamics in which a fast component of IVR populates the states that are most strongly coupled to the initially excited C-H stretch overtone, regardless of the environment, and the remaining, weakly coupled states result in a secondary relaxation only in the absence of IET.     

Differential and total (e,2e) cross sections of simple polyatomic molecules

In this paper, we present a theoretical approach to calculate differential and total ionization cross sections of polyatomic molecules by fast electron impact. More exactly, we have studied the ionization of ammonia (NH(3)) and methane (CH(4)) molecules, and previous results concerning the H(2)O molecule ionization are reported for comparison. The calculations are performed in the distorted wave Born approximation without exchange by employing the independent electron model. The molecular target wave functions are described by linear combinations of atomic orbitals. To describe the interaction between the inactive target electrons and the slow ejected electron, we have introduced a distortion via an effective potential calculated for each molecular orbital. The present theoretical calculations agree well with a large set of existing experimental data in terms of multiple differential and total cross sections.     

Vibrational energy flow rates for cis- and trans-stilbene isomers in solution

Transient electronic absorption following excitation of the first C-H stretching overtone (2nu(CH)) or a C-H stretch-bend combination (nu(CH) + nu(bend)) monitors the flow of vibrational energy in cis-stilbene and in trans-stilbene. Following a rapid initial rise as energy flows into states interrogated by the probe pulse, the absorption decays with two time constants, which are about a factor of 2 longer for the cis-isomer than for the trans-isomer. The decay times for cis-stilbene are tau2(cis) = (2.6 +/- 1.5) ps and tau3(cis) = (24.1 +/- 2.1) ps, and those for trans-stilbene are tau2(trans) = (1.4 +/- 0.6) ps and tau3(trans) = (10.2 +/- 1.1) ps. The decay times are essentially the same in different solvents, suggesting that the relaxation is primarily intramolecular. The two decay times are consistent with the sequential flow of energy through sets of coupled states within the molecule, and the difference in the rates for the two isomers likely reflects differences in coupling among the states arising from the different structures of the isomers. The similarity of the time evolution following excitation of the first C-H overtone at 5990 cm(-1) and the stretch-bend combination at 4650 cm(-1) is consistent with a subset of states, whose structure is similar for the two vibrational excitation energies, controlling the observed flow of energy.     

Intercalation of hydrotalcites with hexacyanoferrate(II) and (III)—a thermoRaman spectroscopic study

Raman spectroscopy using a hot stage indicates that the intercalation of hexacyanoferrate(II) and (III) in the interlayer space of a Mg, Al hydrotalcites leads to layered solids where the intercalated species is both hexacyanoferrate(II) and (III). Raman spectroscopy shows that depending on the oxidation state of the initial hexacyanoferrate partial oxidation and reduction takes place upon intercalation. For the hexacyanoferrate(III) some partial reduction occurs during synthesis. The symmetry of the hexacyanoferrate decreases from O_h existing for the free anions to D_3d in the hexacyanoferrate interlayered hydrotalcite complexes. Hot stage Raman spectroscopy reveals the oxidation of the hexacyanoferrate(II) to hexacyanoferrate(III) in the hydrotalcite interlayer with the removal of the cyanide anions above 250 °C. Thermal treatment causes the loss of CN ions through the observation of a band at 2080 cm⁻¹. The hexacyanoferrate (III) interlayered Mg, Al hydrotalcites decomposes above 150 °C.     

Floating-potential dielectrophoresis-controlled fabrication of single-carbon-nanotube transistors and their electrical properties

We present a floating-potential dielectrophoresis method used for the first time to achieve controlled alignment of an individual semiconducting or metallic single-walled carbon nanotube (SWCNT) between two electrical contacts with high repeatability. This result is significantly different from previous reports, in which bundles of SWCNTs were aligned between electrode arrays by a conventional dielectrophoresis process where the results were only collected from the control electrode regions. In this study, our alignment focus is not only on the regions of the control electrodes but also on those of the floating electrodes. Our results indicate that bundles of carbon nanotubes along with impurities were first moved into the region between two control electrodes while individual nanotubes without impurities were straightened and aligned between two floating electrodes. The measurements for the back-gated nanotube transistors made by this method displayed an on-off ratio and transconductance of 10(5) and 0.3 microS, respectively. These output and transport properties are comparable with those of nanotube transistors made by other methods. Most importantly, the findings in this study show an effective way to separate individual nanotubes from bundles and impurities and advance the processes for site-selective fabrication of single-SWCNT transistors and related electrical devices.     

Determination of the absolute configuration of the sex pheromone of the obscure mealybug by vibrational circular dichroism analysis

Comparison of theoretical and experimental vibrational circular dichroism (VCD) spectra of an enantiopure synthetic sample of the obscure mealybug sex pheromone allowed the determination of the absolute configuration of the insect's pheromone.     

A versatile synthesis of unsymmetrical 3,3'-bioxindoles: stereoselective Mukaiyama aldol reactions of 2-siloxyindoles with isatins

A new synthesis of 3,3'-bioxindoles is reported that is well suited for the preparation of unsymmetrical structures. In the key step, 3-hydroxy-3,3'-bioxindoles are constructed by Mukaiyama aldol reaction of 2-siloxyindoles with isatins. These tertiary carbinols are formed in high diastereoselectivities, with substitution at various positions of the isatin and the 2-siloxyindole being tolerated.     

Sonochemical and thermal redox reactions of triphenylmethane and triphenylmethyl carbinol in nitrobenzene

The reaction of triphenylmethane and triphenylcarbinol with nitrobenzene under thermal or ultrasonic activation was studied. It was shown beyond doubt that the thermal reaction of the aforementioned systems at 210 degrees C occurs through electron transfer. The sonochemical reactions occur at 40 degrees C, although slowly, while heating at the same temperature leaves the system unchanged. Electron transfers are also involved but an unexpected reductive process was evident.     

Assembly of Terpenoid Cores by a Simple,Tunable Strategy

Oxygenated, polycyclic terpenoid natural products have important biological activities. Although total synthesis of such terpenes is widely studied, synthetic strategies that allow for controlled placement of oxygen atoms and other functionality remains a challenge. Herein, we present a simple, scalable, and tunable synthetic strategy to assemble terpenoid‐like polycycloalkanes from cycloalkanones, malononitrile, and allylic electrophiles, abundantly available reagent classes.     

A New Route for the Synthesis of Alkali Polyphosphate from Economical Starting Materials: Preparation and Characterization of Sodium Cyclotriphosphate

Abstract

Thermal synthesis of sodium cyclotriphosphate (SCTP) – Na₃P₃O₉ was investigated in the temperature range of 150 °C to 750 °C using sodium chloride (NaCl) and 85 wt% orthophosphoric acid (H₃PO₄) as economical starting materials. Reaction temperature had a crucial impact on the chloride elimination rate and the formation of SCTP. The best result was obtained at 600 °C with 96% of elimination of the initial chloride as hydrochloric acid and 84% of selectivity in SCTP. At lower temperatures, residual chloride contents were high. At higher temperatures (650 °C and 750 °C), SCTP was melted and transformed into glassy products.