Vibrational population lifetimes of polyatomic molecules in liquids

CH-stretching modes were first excited by picosecond infrared pulses and the generated excess population was monitored by anti-Stokes scattering of subsequent ultrashort probe pulses. Experimental data are reported on five molecules: CHCl₃, CH₂Cl₂, CH₃CCl₃, CH₃CH₂OH, and CH₃I in the neat liquid and/or in solutions of CCl₄. The observed time constants vary between 1 and 100 ps depending upon the individual molecule and surrounding. Theoretical calculations show that rotational coupling, Fermi resonance, Coriolis coupling, and resonance energy transfer can strongly effect the vibrational population lifetime. The relevance of these processes is quite different for the various molecules investigated.     

Oxidation of HCHO and CH3OH on a pt electrode studied by polarization modulation infrared spectroscopy

Infrared spectra of adsorbed CO decomposed from HCHO and CH₃OH under a constant electrode potential were observed. Oxidation processes of HCHO and CH₃OH and the frequency shifts of adsorbed CO are discussed.     

Single sideband doppler-free double resonance stark spectroscopy using an acousto-optic modulator

An acousto-optic modulator driven at a single radio frequency Ω_RF, is used to modulate a cw single mode CO₂ laser oscillating at frequency Ω_L, producing a single sideband at Ω_L-Ω_RF. This system is used to demonstrate optical-optical double resonance using both co-propagating and counter propagating beams. Doppler-free Stark spectra of the molecules CH₃OH, CF₃I, H₂CS and CH₂NH are used to illustrate the technique.     

29Si and 13C NMR studies of organosilicon chemistry. V—Sugar derivatives

²⁹Si spectra have been obtained for four trimethylsilylated hexopyranoside sugars, and the resonance due to the CH₂OSiMe₃ group at position 5 has been assigned by experiments involving gated decoupling at a single ¹H frequency.     

Investigation of electronic structure in chlorine-substituted methanes

The electronic structure, dipole moments, and nuclear magnetic resonance and nuclear quadrupole resonance constants of the CH₄, CCl₄, CHCl₃, CH₂Cl₂, and CH₃Cl molecules were calculated by the Parr-Pariser-Pople (PPP) and Wolfsberg-Helmholz (WH) methods, with self-consistency of the charges on the atoms. Conclusions were reached on the applicability limits of these methods. The calculated values were compared with experimental data.     

Ideal-gas heat capacities of dimethylsiloxanes from speed-of-sound measurements and ab initio calculations

A two-pronged approach has been used to obtain accurate ideal-gas heat capacities of cyclic and linear dimethylsiloxanes that are useful for thermodynamic modeling of several processes involving these compounds. Acoustic resonance measurements were made on gas-phase octamethylcyclotetrasiloxane (D₄, [(CH₃)₂–Si–O]₄) and decamethylcyclopentasiloxane (D₅, [(CH₃)₂–Si–O]₅) over the temperature range 450–510 K. These new data, along with previously published molecular vibrational frequency data for hexamethyldisiloxane (MM, [(CH₃)₃–Si–O_1/2]₂), were used to develop an appropriate frequency scaling factor that can be used with ab initio frequency calculations to produce reliable ideal-gas heat capacities as a function of temperature. Ideal-gas heat capacities for both cyclic [(CH₃)₂–Si–O]_n (with 3≤n≤8$3\le n\le 8$) and linear (CH₃)₃–Si–O–[(CH₃)₂–Si–O]_n–Si–(CH₃)₃ (with 0≤n≤5$0\le n\le 5$) siloxanes over a wide range of temperatures were determined with the ab initio method.     

Transmission window in the νs (OD) band of hydrogen-bonded monodeuteromethanol

A study of the temperature evolution of the ν_s (OD) band of self-associated CH₃OD in solution shows clearly that the structure in this band is due to Fermi resonance with an infrared inactive overtone or combination. The degree of mismatch between the two interfering modes can be varied continuously over a substantial frequency range simply by changing the temperature.     

Propylene Radical Copolymerization

Unlike ethylene, which is quite active in radical poly- and co-polymerization, the next member of the olefinic row–propylene (Pr)–is much less efficient in such reactions. Thermodynami-cally, the polymerization of Pr is permitted as is evidenced by reactions with Ziegler-Natta catalysts (The free energy change δG for the conversion of liquid Pr into amorphous poly-Pr at 25°C is -12.2 kcal/mol [1].), but the strong chain transfer activity of the Pr-monomer prevents formation of high molecular weight polymers via radical mechanism. Pr easily gives up H″ atoms with the formation of inactive and probably resonance stabilized allyl radicals CH₂[sbnd]CH[sbnd]CH₂ .(The high resonance stabilization of the Pr-radi-cal 'CH₂ CH[dbnd]CH₂, formed after separation of the H″ atom, should not be confused with a very low resonance stabilization of the Pr-radical 'CH₂ [sbnd]CH[sbnd]CH₃, formed after opening of the double bond. The Q–e scheme deals with the second type of radical and Pr is characterized by a low resonance factor, Q_pr 0.002.) As a result, Pr radical homopolymerization gives low-molecular weight polymer (even at a pressure of 15,000 atm the molecular weight of poly-Pr is only ～3000 [2, 3]).     

Selectivity in resonant vibrational excitation by e− impact (0.3–4.5 eV) on formaldehyde,acetaldehyde and acetone

Vibrational excitation by e⁻ impact via low-energy resonances has been investigated in acetaldehyde and acetone and compared with similar results in formaldehyde. Despite the large number of vibrational modes involved, the three systems exhibit a selective excitation of only several modes. Besides an important excitation of the CO stretch modes (dominant in H₂CO), excitation of CH stretch, CH₃ stretch and CH₃ deformation are also observed in CH₃CHO and (CH₃)₂CO. Interpretation of the energy loss spectra is given in terms of recently developed symmetry considerations together with the character of the LUMO occupied by the extra electron to form the transitory negative ion (resonance). Differential cross sections versus electron energy are presented for elastic and several inelastic processes. Weak oscillations (of the “boomerang” type) are observed on the inelastic cross sections for acetaldehyde, whereas no structure appears for acetone. This is in contrast with the pronounced oscillations observed for H₂CO, and reveals a shorter lifetime for the CH₃CHO⁻ and (CH₃)₂CO⁻ resonant states, compared to H₂CO⁻.     

Vibrational analysis of molten poly(ethylene glycol)

The infrared absorption of poly(ethylene glycol) was measured in the molten state. Characteristic bands of the molten state were identified. Normal vibrations and frequency distributions were treated for various conformation models with CH₂CH₂O repeat units. The infrared absorption peaks of the molten state closely correspond to the frequency distribution peaks of the TGT conformation with gauche O? CH₂? CH₂? O groups, although infrared bands due to trans O? CH₂? CH₂? O groups are also observed. Vibrational assignments of the infrared bands and Raman lines were made on the basis of potential energy distributions.     

Methanol Adsorption on TiO2 Film Studied by Sum Frequency Generation Vibrational Spectroscopy (cited:2)

A broadband infrared surface sum frequency generation vibrational spectroscopy (SFG-VS) and an in situ UV excitation setup devoted to studying surface photocatalysis have been constructed. With a home-made compact high vacuum cell, organic contaminants on TiO₂ thin lm surface prepared by RF magnetron sputtering were in situ removed under 266 nm irradiation in 10 kPa O₂ atmosphere. We obtained the methanol spectrum in the CH₃ stretching vibration region on TiO₂ surface with changing the methanol pressure at room temperature. Features of both molecular and dissociative methanol, methoxy, adsorbed on this surface were resolved. The CH₃ symmetric stretching vibration frequency and Fermi resonance of molecular methanol is red-shifted by about 6?8 cm^-1 from low to high coverage. Moreover, the recombination of dissociative methanol and H on surfaces in vacuum was also observed. Our results suggest two equilibria exist: between molecular methanol in the gas phase and that on surfaces, and between molecular methanol and dissociative methanol on surfaces.     

Emission spectrum of CH3S radical

The emission spectrum of the CH₃S radical is obtained in the photolysis of CH₃SCH₃ by using a xenon resonance lamp. The emission is also obtained in the photolysis of CH₃SSCH₃ at ca. 200 nm irradiation as well as at 147.0 nm irradiation. The excitation threshold to produce CH₃S fluorescence in the photolysis of CH₃SSCH₃ is determined to be 202 ± 3 nm or 6.14 ± 0.09 eV. The ratios of electronic quenching rate to fluorescence rate of CH₃S^* with N₂, H₂, D₂ and CH₄ are determined.     

Dissociation of Physisorbed Halomethane Molecules by KrF Excimer Laser Radiation

The competitive processes of photolysis and desorption of CF₂Cl₂, CCl₄, CHF₂Cl, CHCl₃, CH₂Cl₂, CH₂I₂, and CH₃I halomethane molecules physisorbed on fused silica were experimentally studied. These processes were induced by the absorption of high-intensity UV radiation from a KrF excimer laser by these molecules. It was shown that a common feature in the behavior of all of these compounds is nonlinear dependence of desorption on radiation intensity, typical nonlinearity indices being very high: n = 5–7. One-photon dissociation was carried out in the adsorbed state of CCl₄, CH₂I₂, and CH₃I molecules absorbing radiation with a wavelength of 248 nm and in a gas phase. The photofragments are characterized by kinetic energies lower than 0.2 eV. The multiphoton fragmentation of the adsorbed molecules was observed and discussed.     

The relative basicity of sulfur containing esters

The relative gas phase proton affinities for an amide, ester, and thiolester have been established as CH₃CONHCH₃ > CH₃COSCH₃ > CH₃COOCH₃ using ion cyclotron resonance techniques. A dithioester is more basic than the thiolester: CH₃CSSCH₃ > CH₃COSCH₃. d-Orbitals are unimportant in the electronic structure of thiolesters.     

Magnetic resonance study of carbon encapsulated Ni nanoparticles

Magnetic resonance study of six samples consisting of carbon encapsulated nickel nanoparticles or carbon nanotubes ended with such nickel nanoparticles was carried out at room temperature. Samples of Ni/C were prepared by carburization of nanocrystalline nickel by ethylene (C₂H₄) and methane (CH₄). Hydrocarbons decomposition on nickel nanoparticles was done at temperatures 500, 600 and 700°C. Magnetic resonance spectra of samples designated as CH₄/500, CH₄/600, CH₄/700, C₂H₄/500, C₂H₄/600 and C₂H₄/700 were obtained by Bruker E 500 spectrometer. The integrated intensities of the resonance spectra were correlated with the carburization conditions (temperature, type of hydrocarbon) during samples preparation. A core-shell model of the investigated samples allowed rough estimation of appropriate shell sizes.      

Degradation processes in the cellulose/<Emphasis Type="Italic">N</Emphasis>-methylmorpholine-<Emphasis Type="Italic">N</Emphasis>-oxide system studied by HPLC and ESR. Radical formation/recombination kinetics under UV photolysis at 77 K

Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC) and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature, radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide type radicals ∼CH₂–NO^•–CH₂∼ and/or ∼CH₂–NO^•–CH₃∼ at the first and methyl ^•CH₃ and formyl ^•CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g., Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO.     

An Analysis of Low Frequency Discharge in a CH3SiCl3-Ar-H2 Mixture by Optical Emission Spectroscopy and Actinometry

Low frequency (100 kHz) discharge in Ar-H₂ and CH₃SiCl₃-Ar-H₂ mixtures was studied to obtain information on the processes involved in plasma deposition of Si_xC_y:H films from CH₃SiCl₃-Ar-H₂ plasma once the properties of Ar-H₂ plasma are known. The plasmas were studied using optical emission spectroscopy. The addition of small amounts of nitrogen to the plasma mixtures also permitted the use of an actinometry technique. First, plasma parameters (electron density and temperature) and actinometric concentrations of atomic hydrogen in an argon–hydrogen plasma were investigated as a function of the hydrogen content in the feed. Second, the emission intensities of Si, Si⁺, CH, H, Ar and Ar⁺ species produced in the CH₃SiCl₃-Ar-H₂ discharge were analysed as a function of time following the introduction of CH₃SiCl₃ (methyltrichlorosilane, MTCS) to the argon–hydrogen plasmas with various proportions of the feed gasses. The results reveal a rapid decay of the Si-excited state number density versus time, while those of Si⁺ and CH fell off more slowly. The emission of atomic silicon was believed to be a result of electron impact dissociative and excitation processes occurring in the bulk of the discharge, whereas the Si⁺ and CH seemed to originate mainly from products of sputtering of the growing film surface. The fragmentation of the MTCS associated with HCl formation and enhanced atomic hydrogen production as a result of HCl dissociation are proposed. Variations in the radical densities of H and CH₃ were determined using an actinometry technique. The results indicate a significant role for H₂ in gas-phase reactions occurring in the CH₃SiCl₃-Ar-H₂ plasma, as well as in gas-surface interactions, leading to competition between deposition and chemical sputtering of already deposited material.     

Synthesis Gas Production from CO2-Containing Natural Gas by Combined Steam Reforming and Partial Oxidation in an AC Gliding Arc Discharge

In this study, a technique of combining steam reforming with partial oxidation of CO₂-containing natural gas in a gliding arc discharge plasma was investigated. The effects of several operating parameters including: hydrocarbons (HCs)/O₂ feed molar ratio; input voltage; input frequency; and electrode gap distance; on reactant conversions, product selectivities and yields, and power consumptions were examined. The results showed an increase in either methane (CH₄) conversion or synthesis gas yield with increasing input voltage and electrode gap distance, whereas the opposite trends were observed with increasing HCs/O₂ feed molar ratio and input frequency. The optimum conditions were found at a HCs/O₂ feed molar ratio of 2/1, an input voltage of 14.5?kV, an input frequency of 300?Hz, and an electrode gap distance of 6?mm, providing high CH₄ and O₂ conversions with high synthesis gas selectivity and relatively low power consumptions, as compared with the other processes (sole natural gas reforming, natural gas reforming with steam, and combined natural gas reforming with partial oxidation).     

M-dependence of collisional transfer of rotational energy in methanol

An M-resolved microwave double resonance experiment on methanol is described. Relative signal intensities and M-selection rules in pure CH₃OH are consistent with a dipole—dipole collisional interaction, while those for CH₃OHHe and CH₃OHH₂ mixtures indicate more complex interactions.     

Electrochemical materials science: calculation vs. experiment as predictive tools in tailoring intrinsically conducting polythiophenes

A series of 3-(p-X-phenyl)thiophene monomers (X = −H, –CH₃, –OCH₃, –COCH₃, –COOC₂H₅, –NO₂) was electrochemically polymerized to furnish polymer films that could be reversibly reduced and oxidized (n- and p-doped). The oxidation potentials of the monomers and formal potentials of the n- and p-doping processes of polymers were correlated with resonance and inductive effects of the substituents on the phenyl ring as well as the ionization potentials of the monomers calculated via density function theory, which correspond to the energies for generating radical cations during oxidative processes.