Vibrational Analysis of Paraelectric–Ferroelectric Transition of LiNbO3: An Ab-Initio Quantum Mechanical Treatment

The phase transitions between paraelectric (PE) and ferroelectric (FE) isomorph phases of LiNbO3 have been investigated quantum mechanically by using a Gaussian-type basis set, the B3LYP hybrid functional and the CRYSTAL17 code. The structural, electronic and vibrational properties of the two phases are analyzed. The vibrational frequencies evaluated at the Γ point indicate that the paraelectric phase is unstable, with a complex saddle point with four negative eigenvalues. The energy scan of the A2u mode at −215 cm−1 (i215) shows a dumbbell potential with two symmetric minima. The isotopic substitution, performed on the Li and Nb atoms, allows interpretation of the nontrivial mechanism of the phase transition. The ferroelectric phase is more stable than the paraelectric one by 0.32 eV.

A Spectroscopic Paradox: The Interaction of Methanol with ZSM-5 at Room Temperature

The adsorption of methanol in HZSM-5 at low temperatures has long been regarded as an associative process involving hydrogen bonding to the acidic zeolite hydroxyl groups. Recent studies employing inelastic neutron scattering spectroscopy (INS) have reported that complete dissociation to methoxylate the zeolite occurs at 298 K, and infrared evidence for a partial dissociation at 298 K has also been described. Here we investigate the apparent contradictions between different techniques, using a combination of INS, infrared spectroscopy and solid-state NMR spectroscopy, including isotopic substitution experiments. Different possible explanations are proposed and considered; we conclude that at room temperature methanol is very largely associatively adsorbed, although the presence of some small extent (>1%) of methoxylation cannot be ruled out.

Routes involving no free C2 in a DFT-computed mechanistic model for the reported room-temperature chemical synthesis of C2.

<p>Recent lively debates about the nature of the quadruple bonding in the diatomic species C₂ have been heightened by recent suggestions of molecules in which carbon may be similarly bonded to other elements. The desirability of having methods for generating such species at ambient temperatures and in solution in order to study their properties may have been realized by a recent report of the first chemical synthesis of free C₂ itself under mild conditions. The method involved unimolecular fragmentation of an alkynyl zwitterion<b> 2</b> as generated from the precursor <b>1</b>, resulting in production and then trapping of free C₂ at ambient temperatures rather than the high temperature gas phase methods normally employed for C₂ generation. Here, alternative mechanisms are proposed for this reaction based on DFT calculations involving bimolecular 1,1- or 1,2-iodobenzene displacement reactions from <b>2</b> directly by galvinoxyl radical, or hydride transfer from 9,10-dihydroanthracene to <b>2</b>. These mechanisms result in the same trapped products as observed experimentally, but unlike that involving unimolecular generation of free C₂, exhibit calculated free energy barriers commensurate with the reaction times observed at room temperatures. The relative energies of the transition states for 1,1 <i>vs</i> 1,2 substitution provide a rationalisation for the observed isotopic substitution patterns. The same mechanism also provides an energetically facile path to polymeric synthesis of carbon rich species by extending the carbon chain attached to the iodonium group, eventually resulting in formation of amorphous carbon and discrete molecules such as C₆₀.</p><div><div><p><br></p></div></div>

No Free C2 Is Involved in the DFT-Computed Mechanistic Model for the Reported Room-Temperature Chemical Synthesis of C2.

<p>Trapping experiments were claimed to demonstrate the first chemical synthesis of the free diatomic species C₂ at room temperatures, as generated by unimolecular fragmentation of an alkynyl iodonium salt precursor. Alternative mechanisms based on DFT energy calculations are reported here involving no free C₂, but which are instead bimolecular 1,1- or 1,2-iodobenzene displacement reactions from the zwitterionic intermediate <b>11</b> by galvinoxyl radical, or by hydride transfer from 9,10-dihydroanthracene. These result in the same trapped products as observed experimentally, but unlike the mechanism involving unimolecular generation of free C₂, exhibit calculated free energy barriers commensurate with the reaction times observed at room temperatures. The relative energies of the transition states for 1,1 <i>vs</i> 1,2 substitution provide a rationalisation for the observed isotopic substitution patterns and the same mechanism also provides an energetically facile path to polymerisation by extending the carbon chain attached to the iodonium group, eventually resulting in formation of species such as amorphous carbon and C₆₀.</p><div><br><div><p><br></p></div></div>

No Free C2 Is Involved in the DFT-Computed Mechanistic Model for the Reported Room-Temperature Chemical Synthesis of C2.

<p>Trapping experiments were claimed to demonstrate the first chemical synthesis of the free diatomic species C₂ at room temperatures, as generated by unimolecular fragmentation of an alkynyl iodonium salt precursor. Alternative mechanisms based on DFT energy calculations are reported here involving no free C₂, but which are instead bimolecular 1,1- or 1,2-iodobenzene displacement reactions from the zwitterionic intermediate <b>11</b> by galvinoxyl radical, or by hydride transfer from 9,10-dihydroanthracene. These result in the same trapped products as observed experimentally, but unlike the mechanism involving unimolecular generation of free C₂, exhibit calculated free energy barriers commensurate with the reaction times observed at room temperatures. The relative energies of the transition states for 1,1 <i>vs</i> 1,2 substitution provide a rationalisation for the observed isotopic substitution patterns and the same mechanism also provides an energetically facile path to polymerisation by extending the carbon chain attached to the iodonium group, eventually resulting in formation of species such as amorphous carbon and C₆₀.</p><div><br><div><p><br></p></div></div>