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Polymers
Published in Bryan Ellis, Ray Smith, Polymers, 2008
Decomposition Details: The observed evolution of D2 from the deuterio-deriv. of PSE, as the primary gaseous product at 250- 400°C, where crosslinking of the polymer occurs, suggests that loss of H2 from the Si is a key step in the crosslinking process. A reaction pathway is postulated for the crosslinking and pyrolysis of PSE in which both 1,1-H2 elimination and intramol. H-transfer reactions lead to highly reactive silylene intermediates; these insert into Si-H bonds of neighbouring polymer chains forming Si-Si bonds which rapidly rearrange to Si-C bonds at these temps. to form Si-C interchain cross-links. The cross-links prevent extensive fragmentation of the polycarbosilane network as the temp. is increased further to the range (420°C) where homolytic bond cleavage occurs at an appreciable rate, leading to free radicals. These free radical processes are presumably the main mechanisms at higher temps. ( >475°C) where extensive rearrangement of the Si/C network structure is evidenced by solid-state NMR spectro- scopy. Further heating of the polymer to 1000°C leads to the formation of SiC in high yield (approx. 85%) [1,10]
Active Thermochemical Tables: the thermophysical and thermochemical properties of methyl, CH3, and methylene, CH2, corrected for nonrigid rotor and anharmonic oscillator effects
Published in Molecular Physics, 2021
Following the ATcT split-species concept [78,86], methylene is in ATcT represented as three chemical species: equilibrated methylene, CH2 (equilib.), triplet-only methylene, CH2 (triplet), and singlet-only methylene, CH2 (singlet). From the thermodynamic viewpoint, CH2 (equilib.) corresponds to ‘ordinary’ methylene that is normally tabulated in standard thermochemical compilations [3,6]. The ground state of CH2 is a triplet, X 3B1, the first excited state is a singlet, a 1A1. CH2 (equilib.) is assumed to be thermally fully equilibrated over all accessible electronic states. CH2 (singlet) uses a 1A1 as its ground state, and is assumed to be equilibrated only over singlet electronic states, as if the triplet states were not available. The original cause célèbre for introducing the split-species concept is, in fact, methylene itself, based on the long-standing notion that singlet and triplet methylene have different reactivities (and thus kinetic rate constants), have a propensity to react differently (e.g. abstraction vs. insertion), and consequently have different lifetimes, and should thus be treated separately in chemical models [84]. (Parenthetically, a similar situation occurs for the silicon analogue, silylene, SiH2, with the difference that the singlet-triplet splitting is inverted in comparison to methylene. The point is that the excited triplet SiH2 can be experimentally demonstrated to remain unrelaxed to the singlet ground state even after repeated collisions [87].)
Embedded equation-of-motion coupled-cluster theory for electronic excitation, ionisation, electron attachment, and electronic resonances
Published in Molecular Physics, 2021
Valentina Parravicini, Thomas-C. Jagau
In all computations except those for charge transfer states, the environment B consists of a varying number (1–5) of water molecules, whereas we take different small molecules as fragment A: formaldehyde (CHO), methanol (CHOH), ethylene (CH), methylene (CH), fluoromethylene (HCF), silylene (SiH), dinitrogen (N), and carbon monoxide (CO). These systems are small enough to be treated as a whole by EOM-CCSD and the comparison between full and embedded EOM-CCSD is the centrepiece of our work. Furthermore, we report transition energies obtained with computationally cheaper alternatives: ionisation and electron-attachment energies are calculated with ΔDFT using the maximum overlap method (MOM) [91] and for excited states TD-DFT calculations are carried out within the Tamm-Dancoff approximation (TDA/TD-DFT) [92,93]. As concerns charge transfer, we investigate two systems: HCl (A) embedded in 5 HO (B), where charge transfer takes place in the HCl molecule, and the donor-acceptor complex NHF where we take the donating NH molecule as fragment A and the accepting F molecule as fragment B.
A perturbative approach to multireference equation-of-motion coupled cluster
Published in Molecular Physics, 2021
Marvin H. Lechner, Róbert Izsák, Marcel Nooijen, Frank Neese
To assess the accuracy of ground-state energy calculation, we chose to compute the singlet–triplet gaps on methylene and silylene. Despite not being the focus of EOM-like methods, the ground state calculations on the singlet and triplet geometries demonstrate that the singlet–triplet gap is reproduced accurately to within 0.7 kcal/mol, an error comparable to that of the fic-NEVPT2 method at 1.3 kcal/mol. This finding is also supported by the quality of the total energies for the diatomic test systems, where the MR-EOMPT method reproduces the FCI-quality total energies better than fic-NEVPT2.