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“…The presence of the two weak internal hydrogen bonds in the C 2 v monomer compared to the
single internal hydrogen bond in the C s monomer results in an increased stability of about
4–8 kJ/mol. 22,28,29,38−41 This interpretation of the presence
of these two monomers at these ratios can also be deduced from all
other spectral ranges shown in Figure 3 and
is demonstrated in the Supporting Information.…”
Twenty
years ago two different polymorphs of carbonic acid, α-
and β-H2CO3, were isolated as thin, crystalline
films. They were characterized by infrared and, of late, by Raman
spectroscopy. Determination of the crystal structure of these two
polymorphs, using cryopowder and thin film X-ray diffraction techniques,
has failed so far. Recently, we succeeded in sublimating α-H2CO3 and trapping the vapor phase in a noble gas
matrix, which was analyzed by infrared spectroscopy. In the same way
we have now investigated the β-polymorph. Unlike α-H2CO3, β-H2CO3 was regarded
to decompose upon sublimation. Still, we have succeeded in isolation
of undecomposed carbonic acid in the matrix and recondensation after
removal of the matrix here. This possibility of sublimation and recondensation
cycles of β-H2CO3 adds a new aspect to
the chemistry of carbonic acid in astrophysical environments, especially
because there is a direct way of β-H2CO3 formation in space, but none for α-H2CO3. Assignments of the FTIR spectra of the isolated molecules unambiguously
reveal two different carbonic acid monomer conformers (C2v and Cs). In contrast to the earlier study on α-H2CO3, we do not find evidence for centrosymmetric
(C2h) carbonic acid dimers
here. This suggests that two monomers are entropically favored at
the sublimation temperature of 250 K for β-H2CO3, whereas they are not at the sublimation temperature of 210
K for α-H2CO3.
“…The presence of the two weak internal hydrogen bonds in the C 2 v monomer compared to the
single internal hydrogen bond in the C s monomer results in an increased stability of about
4–8 kJ/mol. 22,28,29,38−41 This interpretation of the presence
of these two monomers at these ratios can also be deduced from all
other spectral ranges shown in Figure 3 and
is demonstrated in the Supporting Information.…”
Twenty
years ago two different polymorphs of carbonic acid, α-
and β-H2CO3, were isolated as thin, crystalline
films. They were characterized by infrared and, of late, by Raman
spectroscopy. Determination of the crystal structure of these two
polymorphs, using cryopowder and thin film X-ray diffraction techniques,
has failed so far. Recently, we succeeded in sublimating α-H2CO3 and trapping the vapor phase in a noble gas
matrix, which was analyzed by infrared spectroscopy. In the same way
we have now investigated the β-polymorph. Unlike α-H2CO3, β-H2CO3 was regarded
to decompose upon sublimation. Still, we have succeeded in isolation
of undecomposed carbonic acid in the matrix and recondensation after
removal of the matrix here. This possibility of sublimation and recondensation
cycles of β-H2CO3 adds a new aspect to
the chemistry of carbonic acid in astrophysical environments, especially
because there is a direct way of β-H2CO3 formation in space, but none for α-H2CO3. Assignments of the FTIR spectra of the isolated molecules unambiguously
reveal two different carbonic acid monomer conformers (C2v and Cs). In contrast to the earlier study on α-H2CO3, we do not find evidence for centrosymmetric
(C2h) carbonic acid dimers
here. This suggests that two monomers are entropically favored at
the sublimation temperature of 250 K for β-H2CO3, whereas they are not at the sublimation temperature of 210
K for α-H2CO3.
“…It should be noted that the C=O, O–O, and C–O bond lengths are 1.222 Å, 1.453 Å, and 1.366 Å at all levels of theory. This is consistent with the reported geometries in the literature 44 . Figure 15 Optimized geometries for EHP with CO 2 , CH 4 , SO 2 gases at the B3LYP/6-311++G(3df,3pd).…”
Section: Resultssupporting
confidence: 93%
“…It should be noted that the C=O, O-O, and C-O bond lengths are 1.222 Å, 1.453 Å, and 1.366 Å at all levels of theory. This is consistent with the reported geometries in the literature 44 . www.nature.com/scientificreports/ Pathway E1 represents the reaction of the EHP with CH 4 , along with the impact of photochemical reactions on the barrier.…”
Section: The Potential Energy Diagram (Ped) For Criegee Intermediate supporting
A detailed computational study of the atmospheric reaction of the simplest Criegee intermediate CH2OO with methane has been performed using the density functional theory (DFT) method and high-level calculations. Solvation models were utilized to address the effect of water molecules on prominent reaction steps and their associated energies. The structures of all proposed mechanisms were optimized using B3LYP functional with several basis sets: 6-31G(d), 6-31G (2df,p), 6-311++G(3df,3pd) and at M06-2X/6-31G(d) and APFD/6-31G(d) levels of theory. Furthermore, all structures were optimized at the B3LYP/6-311++G(3df,3pd) level of theory. The intrinsic reaction coordinate (IRC) analysis was performed for characterizing the transition states on the potential energy surfaces. Fifteen different mechanistic pathways were studied for the reaction of Criegee intermediate with methane. Both thermodynamic functions (ΔH and ΔG), and activation parameters (activation energies Ea, enthalpies of activation ΔHǂ, and Gibbs energies of activation ΔGǂ) were calculated for all pathways investigated. The individual mechanisms for pathways A1, A2, B1, and B2, comprise two key steps: (i) the formation of ethyl hydroperoxide (EHP) accompanying with the hydrogen transfer from the alkanes to the terminal oxygen atom of CIs, and (ii) a following unimolecular dissociation of EHP. Pathways from C1 → H1 involve the bimolecular reaction of EHP with different atmospheric species. The photochemical reaction of methane with EHP (pathway E1) was found to be the most plausible reaction mechanism, exhibiting an overall activation energy of 7 kJ mol−1, which was estimated in vacuum at the B3LYP/6-311++G(3df,3pd) level of theory. All of the reactions were found to be strongly exothermic, expect the case of the sulfur dioxide-involved pathway that is predicted to be endothermic. The solvent effect plays an important role in the reaction of EHP with ammonia (pathway F1). Compared with the gas phase reaction, the overall activation energy for the solution phase reaction is decreased by 162 and 140 kJ mol−1 according to calculations done with the SMD and PCM solvation models, respectively.
“…These results can be regarded as slight improvements on the values reported very recently. 14 It should be noted that the relevant quantity if discussing equilibria is, of course, the free energy of reaction, which is only partly given by the reaction energies discussed above. It should be kept in mind that the free energy of dimerization is more positive than the reaction energy due to the reduction of two molecules to one and is expected to be strongly temperature dependent.…”
Section: Stability and Energetics Of The Dimermentioning
confidence: 99%
“…The latter dimer, however, remains the energetically most stable one at the CCSD(T)/aug-cc-pVDZ//MP2/6-311++G** level of theory. In 2011, Wu et al 14 In this work we aim to improve on the aforementioned results, especially concerning the infrared absorption spectra for gas-phase carbonic acid species and the energetics of carbonic acid dimer (referring for the rest of this paper to the one composed of two cis-cis monomers). The quantum chemical methods that are used are described in section 2.…”
We calculate harmonic frequencies of the three most abundant carbonic acid conformers. For this, different model chemistries are investigated with respect to their benefits and shortcomings. Based on these results we use perturbation theory to calculate anharmonic corrections at the ωB97XD/aug-cc-pVXZ, X = D, T, Q, level of theory and compare them with recent experimental data and theoretical predictions. A discrete variable representation method is used to predict the large anharmonic contributions to the frequencies of the stretching vibrations in the hydrogen bonds in the carbonic acid dimer. Moreover, we re-investigate the energetics of the formation of the carbonic acid dimer from its constituents water and carbon dioxide using a high-level extrapolation method. We find that the ωB97XD functional performs well in estimating the fundamental frequencies of the carbonic acid conformers. Concerning the reaction energetics, the accuracy of ωB97XD is even comparable to the high-level extrapolation method. We discuss possibilities to detect carbonic acid in various natural environments such as Earth's and Martian atmospheres
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