26 research outputs found
Research of molybdenum carbide by raman spectroscopy
Molybdenum carbides crystalline phases are well-known polymorphs with useful technological applications including sensors, electronics, and catalysis. According to Mo-C known phase diagram, several polymorphs can exist under ambient conditions. One of the promising synthesis techniques is DC arc plasma. Nowadays, recent trends focus on the non-vacuum arcing procedure in ambient conditions which is possible due to carbon monoxide generation during the synthesis process. This phenomenon as a result of graphite electrodes usage can prevent the oxidation of the synthesis products. As an advantage of this method should be noted the possible cost benefits through the lower energy consumption, also the productivity can be increased by our approach. In this contribution, the arc plasma method is investigated for the crystalline molybdenum carbides synthesis. According to the X-ray diffraction results, the Mo2C and Mo1.2C0.8 crystalline phases were synthesized. Raman spectroscopy confirms the presence and high crystallinity of these MoC phases. This work shows an inexpensive and promising way to obtain molybdenum carbides with potential in optoelectronics, environmental, and energy applications
Comment on “Molecular Mechanism of Dioxin Formation from Chlorophenol based on Electron Paramagnetic Resonance Spectroscopy”
Anthrone and Related Hydroxyarenes: Tautomerization and Hydrogen Bonding
The
keto–enolization of hydroxyl-substituted naphthols and
9-anthrols has been investigated by means of CBS-QB3 calculations.
An excellent agreement between experiment and theory is found for
the energetics for the anthrone (<b>5</b>) ⇌ anthrol
(<b>6</b>) equilibrium, with an enthalpy of tautomerization,
Δ<sub>t</sub><i>H</i>, of 3.8 kcal mol<sup>–1</sup>. In contrast, 1-naphthol is the preferred tautomer with a Δ<sub>t</sub><i>H</i> = −9.0 kcal mol<sup>–1</sup>. Substitution of the hydrogens at the adjacent carbons by hydroxyl
groups leads to the formation of strong intramolecular hydrogen bonds
within a six-membered ring in the enones and the enols. Due to the
difference in the intramolecular hydrogen bond enthalpy, Δ<sub>HB</sub><i>H</i><sub>intra</sub>, the equilibrium shifts
further to the enone. Thus, for 1,8-dihydroxy-anthrone (anthralin,
dithranol) Δ<sub>t</sub><i>H</i> increases to 12.7
kcal mol<sup>–1</sup> with an enol/enone ratio of 10<sup>–10</sup>. The solvent effect on the <b>5</b> ⇌ <b>6</b> equilibrium has been quantified by considering the formation of
intermolecular hydrogen bond(s), leading to an acidity parameter α<sub>2</sub><sup>H</sup> for anthrol of
0.42. It is shown that the hydrogen bond donating ability of bulk
methanol is greatly attenuated through the formation of cyclic oligomers.
The benzylic and phenolic bond dissociation enthalpies for anthrone
up to anthralin suggest some antioxidant potency but the precise (radical)
mechanism of action remains uncertain
Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation
The
thermokinetics for the tautomerization of a series of methylenedihydroacenes
to the corresponding methylacenes (toluene to 6-methylpentacene) have
been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene
does the methylenedihydro form predominate at room temperature. The
obtained equilibrium ratios are consistent with various theoretical
methods, but the agreement with the scarce experimental data is only
qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene
in an inert solvent may proceed by means of a reverse radical disproportionation
reaction (RRD) as the rate-determining step. The benzylic BDE(C–H)s
and the hydrogen atom affinities (HA) of the tautomers have been used
to calculate the reaction enthalpy, Δ<sub>RRD</sub><i>H</i>. It appears that the <i>E</i><sub>a,RRD</sub> is substantially
higher than Δ<sub>RRD</sub><i>H</i>. This implies
that the opposite reaction (and the tautomer forming step), a radical–radical
disproportionation (RD), is an activated process. This is an often
ignored or overlooked kinetic feature. The consequence is that although
the RRD reaction may be kinetically feasible at elevated temperatures,
the products are not the tautomers but rather dimers stemming from
radical–radical recombination reactions, with <i>p</i>-isotoluene as a clear exception. It is further shown that the RRD
self-reaction of phenalene is too slow at 298 K, despite claims to
the contrary
Comment on “Persistent Room‐Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry” and on “Persistent Radical Pairs between N‐Substituted Naphthalimide and Carbanion Exhibit p K
Why Quantum-Thermochemical Calculations Must Be Used with Caution to Indicate lsquoa Promising Lead Antioxidantrsquo
NRC publication: Ye