Real-time monitoring of nucleation-growth cycle of carbon nanoparticles in acetylene plasmas

Quantum cascade laserabsorption spectroscopy was used to measure the absolute concentration of acetylene in situ during the nanoparticle growth in Ar + C2H2 RF plasmas. It is demonstrated that the nanoparticle growth exhibits a periodical behavior, with the growth cycle period strongly dependent on the initial acetylene concentration in the chamber. Being 300 s at 7.5% of acetylene in the gas mixture, the growth cycle period decreases with the acetylene concentration increasing; the growth eventually disappears when the acetylene concentration exceeds 32%. During the nanoparticle growth, the acetylene concentration is small and does not exceed 4.2% at radio frequency (RF) power of 4 W, and 0.5% at RF power of 20 W. An injection of a single acetylene pulse into the discharge also results in the nanoparticlenucleation and growth. The absorption spectroscopy technique was found to be very effective for the time-resolved measurement of the hydrocarbon content in nanoparticle-generatingplasmas.

Cavity-enhanced laser spectroscopic studies of vibrational overtones of acetylene

This thesis contains five experimental spectroscopic studies that probe the vibration-rotation energy level structure of acetylene and some of its isotopologues. The emphasis is on the development of laser spectroscopic methods for high-resolution molecular spectroscopy. Three of the experiments use cavity ringdown spectroscopy. One is a standard setup that employs a non-frequency stabilised continuous wave laser as a source. In the other two experiments, the same laser is actively frequency stabilised to the ringdown cavity. This development allows for increased repetition rate of the experimental signal and thus the spectroscopic sensitivity of the method is improved. These setups are applied to the recording of several vibration-rotation overtone bands of both H(12)C(12)CH and H(13)C(13)CH. An intra-cavity laser absorption spectroscopy setup that uses a commercial continuous wave ring laser and a Fourier transform interferometer is presented. The configuration of the laser is found to be sub-optimal for high-sensitivity work but the spectroscopic results are good and show the viability of this type of approach. Several ro-vibrational bands of carbon-13 substituted acetylenes are recorded and analysed. Compared with earlier work, the signal-to-noise ratio of a laser-induced dispersed infrared fluorescence experiment is enhanced by more than one order of magnitude by exploiting the geometric characteristics of the setup. The higher sensitivity of the spectrometer leads to the observation of two new symmetric vibrational states of H(12)C(12)CH. The precision of the spectroscopic parameters of some previously published symmetric states is also improved. An interesting collisional energy transfer process is observed for the excited vibrational states and this phenomenon is explained by a simple step-down model.

Nitrogenase activity (acetylene reduction) associated with leaves of different cultivars of paddy

An investigation was conducted to study the levels of nitrogen fixation on the leaf or sheath surfaces of four cultivars of paddy plants by using acetylene reduction technique. Varying levels of positive nitrogenase activity were observed on all the leaf surfaces. Sheath of IET 1991 cultivar showed a higher rate of fixation than the leaf surface. All the nitrogen-fixing organisms on the leaf or sheath surfaces belonged to the genus Beijerinckia. There was no correlation between the bacterial density and the level of fixation. Scanning electron microscopic data revealed that the upper surface of IET 1991 leaf was highly silicified and the microflora was either scanty or nil while the lower surface appeared quite different and harboured more micro-organisms. Similarly, the inner surface of sheath was devoid of silicification and showed the presence of micro-organisms.

Nitrogenase activity (acetylene reduction) associated with leaves of different cultivars of paddy (Oryza sativa L.)

An investigation was conducted to study the levels of nitrogen fixation on the leaf or sheath surfaces of four cultivars of paddy plants by using acetylene reduction technique. Varying levels of positive nitrogenase activity were observed on all the leaf surfaces. Sheath of IET 1991 cultivar showed a higher rate of fixation than the leaf surface. All the nitrogen-fixing organisms on the leaf or sheath surfaces belonged to the genus Beijerinckia. There was no correlation between the bacterial density and the level of fixation. Scanning electron microscopic data revealed that the upper surface of IET 1991 leaf was highly silicified and the microflora was either scanty or nil while the lower surface appeared quite different and harboured more micro-organisms. Similarly, the inner surface of sheath was devoid of silicification and showed the presence of micro-organisms.

Vibration-rotation pattern in acetylene. II. Introduction of Coriolis coupling in the global model and analysis of emission spectra of hot acetylene around 3 mu m

A high temperature source has been developed and coupled to a high resolution Fourier transform spectrometer to record emission spectra of acetylene around 3 mu m up to 1455 K under Doppler limited resolution (0.015 cm(-1)). The nu(3)-ground state (GS) and nu(2)+nu(4)+nu(5)(Sigma(+)(u) and Delta(u))-GS bands and 76 related hot bands, counting e and f parities separately, are assigned using semiautomatic methods based on a global model to reproduce all related vibration-rotation states. Significantly higher J-values than previously reported are observed for 40 known substates while 37 new e or f vibrational substates, up to about 6000 cm(-1), are identified and characterized by vibration-rotation parameters. The 3 811 new or improved data resulting from the analysis are merged into the database presented by Robert et al. [Mol. Phys. 106, 2581 (2008)], now including 15 562 lines accessing vibrational states up to 8600 cm(-1). A global model, updated as compared to the one in the previous paper, allows all lines in the database to be simultaneously fitted, successfully. The updates are discussed taking into account, in particular, the systematic inclusion of Coriolis interaction.

Characterization of circumstellar carbonaceous dust analogues produced by pyrolysis of acetylene in a porous graphite reactor

Carbon particles synthesized by acetylene pyrolysis in a porous graphite reactor have been investigated. The intimate chemical and physical structures of the particles were probed by proton nuclear magnetic resonance spectroscopy, infrared Fourier transform spectroscopy and X-ray diffraction. The analysis points towards a chemical structure composed of soluble low-mass aromatics surrounding small insoluble larger aromatic islands bridged by aliphatic groups. The diffraction profile indicates that the particles are mostly amorphous with small crystalline domains of not, vert, similar6.5 Å composed of a few stacked graphene layers. The properties of these particles are compared with these obtained with other types of production methods such as laser pyrolysis and combustion flames. The results are briefly discussed in the context of the evolution of infrared interstellar emitters. Possible uses of the reactor are proposed.

Are Metallocene-Acetylene (M = Ti, Zr, Hf) Complexes Aromatic Metallacyclopropenes?

The bonding nature of metallocene acetylene complexes Cp2M(eta(2)-H3SiC2SiH3) 1M and CP2M (eta(2)- HC2H) 1M' (M = Ti, Zr, Hf) wits studied by density functional theory method. It is found that this acetylene complex has indeed it metallacyclopropene moiety with two in-plane M-C sigma-bonds and one out-of-plane pi-bond interacting with the metal center, resulting in the formation of it delocalized three-center and two-electron (3c-2e) system. Along with its delocalized out-of-plane bonding, this complex has been characterized its aromatic on the basis of the computed stabilizing energy and negative nucleus-independent chemical shifts (NICS). The aromatic stabilization increases from Ti to Zr and Hf, and this is because of the increased charge separation between the CP2M fragment and the H3SiC2SiH3 (also HC2H) unit. The decrease of the M-C bond length from Zr to Hf is ttributed to the increased s character of both M and C hybridization of the M-C a-bonds.

Experimental and Theoretical Studies of Unusual Four-Membered Metallacycles from Reactions of Group 4 Metallocene Bis(trimethylsilyl)-acetylene Complexes with the Sulfurdiimide Me3SiN=S=NSiMe3

The use of the sulfurdiimide RN=S=NR' (R = R' = SiMe3, 3) in reactions with group 4 metallocene bis(trimethylsilyl)-acetylene complexes of the type [Cp2M(L (eta(2)-Me3Si-C2SiMe3)] (1: M = Ti, no L; 2: M = Zr, L = pyridine) has led to the formation of four-membered metallacycles 4M containing the group 4 metal, nitrogen and sulfur. DFT calculations performed on compound 4Ti indicate that this complex is best described as a sigma-complex with cyclic delocalisation of the ring electrons. Moreover, pseudo-Jahn-Teller distortion plays a significant role in stabilising this complex.

Reactions of Titanocene Bis(trimethylsilyl)acetylene Complexes with Carbodiimides: An Experimental and Theoretical Study of Complexation versus C-N Bond Activation

The reaction of the low valent metallocene(II) sources Cp'Ti-2(eta(2)-Me3SiC2SiMe3) (Cp' = eta(5)-cyclopentadienyl, 1a or eta(5)-pentamethylcyclopentadienyl, 1b) with different carbodiimide substrates RN=C=NR' 2-R-R' (R = t-Bu; R' = Et; R = R' = i-Pr; t-Bu; SiMe3; 2,4,6-Me-C6H2 and 2,6-i-Pr-C6H3) was investigated to explore the frontiers of ring strained, unusual four-membered heterometallacycles 5-R. The product complexes show dismantlement, isomerization, or C-C coupling of the applied carbodiimide substrates, respectively, to form unusual mono-, di-, and tetranuclear titanium(III) complexes. A detailed theoretical study revealed that the formation of the unusual complexes can be attributed to the biradicaloid nature of the unusual four-membered heterometallacycles 5-R, which presents an intriguing situation of M-C bonding. The combined experimental and theoretical study highlights the delicate interplay of electronic and steric effects in the stabilization of strained four-membered heterometallacycles, accounting for the isolation of the obtained complexes.

A computational study of heat transfer, soot production and transport in an acetylene-air laminar diffusion flame in the field of a line vortex

The present work involves a computational study of soot formation and transport in case of a laminar acetylene diffusion flame perturbed by a co nvecting line vortex. The topology of the soot contours (as in an earlier experimental work [4]) have been investigated. More soot was produced when vortex was introduced from the air si de in comparison to a fuel side vortex. Also the soot topography was more diffused in case of the air side vortex. The computational model was found to be in good agreement with the ex perimental work [4]. The computational simulation enabled a study of the various parameters affecting soot transport. Temperatures were found to be higher in case of air side vortex as compared to a fuel side vortex. In case of the fuel side vortex, abundance of fuel in the vort ex core resulted in stoichiometrically rich combustion in the vortex core, and more discrete so ot topography. Overall soot production too was low. In case of the air side vortex abundan ce of air in the core resulted in higher temperatures and more soot yield. Statistical techniques like probability density fun ction, correlation coefficient and conditional probability function were introduced to explain the transient dependence of soot yield and transport on various parameters like temperature, a cetylene concentration.

A computational study of flame dynamics on soot production and transport for an acetylene-air laminar diffusion flame in the field of a convecting line vortex

The present work involves a computational study of soot formation and transport in case of a laminar acetylene diffusion flame perturbed by a convecting line vortex. The topology of the soot contours (as in an earlier experimental work [4]) have been investigated. More soot was produced when vortex was introduced from the air side in comparison to a fuel side vortex. Also the soot topography was more diffused in case of the air side vortex. The computational model was found to be in good agreement with the experimental work [4]. The computational simulation enabled a study of the various parameters affecting soot transport. Temperatures were found to be higher in case of air side vortex as compared to a fuel side vortex. In case of the fuel side vortex, abundance of fuel in the vort ex core resulted in stoichiometrically rich combustion in the vortex core, and more discrete soot topography. Overall soot production too was low. In case of the air side vortex abundance of air in the core resulted in higher temperatures and more soot yield. Statistical techniques like probability density function, correlation coefficient and conditional probability function were introduced to explain the transient dependence of soot yield and transport on various parameters like temperature, a cetylene concentration.

Non-covalent C-Cl center dot center dot center dot pi interaction in acetylene-carbon tetrachloride adducts: Matrix isolation infrared and ab initio computational studies

Non-covalent halogen-bonding interactions between n cloud of acetylene (C2H2) and chlorine atom of carbon tetrachloride (CCl4) have been investigated using matrix isolation infrared spectroscopy and quantum chemical computations. The structure and the energies of the 1:1 C2H2-CCl4 adducts were computed at the B3LYP, MP2 and M05-2X levels of theory using 6-311++G(d,p) basis set. The computations indicated two minima for the 1:1 C2H2-CCl4 adducts; with the C-Cl center dot center dot center dot pi adduct being the global minimum, where pi cloud of C2H2 is the electron donor. The second minimum corresponded to a C-H...Cl adduct, in which C2H2 is the proton donor. The interaction energies for the adducts A and B were found to be nearly identical. Experimentally, both C-Cl center dot center dot center dot pi and C-H center dot center dot center dot Cl adducts were generated in Ar and N2 matrixes and characterized using infrared spectroscopy. This is the first report on halogen bonded adduct, stabilized through C-Cl center dot center dot center dot pi interaction being identified at low temperatures using matrix isolation infrared spectroscopy. Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) analyses were performed to support the experimental results. The structures of 2:1 ((C2H2)(2)-CCl4) and 1:2 (C2H2-(CCl4)(2)) multimers and their identification in the low temperature matrixes were also discussed. (C) 2015 Elsevier B.V. All rights reserved.