Energy Isomers Research Articles

DFT investigations of the unusual reactivity of 2‐pyridinecarboxaldehyde in base‐catalyzed aldol reactions with acetophenone

AbstractThe aldol reaction is recognized as an archetypal method to form carbon‐carbon bonds. Base‐catalyzed aldol reactions of aryl aldehydes with aryl methyl ketones typically produce condensation products (chalcones). However, 2‐pyridinecarboxaldehye, 2‐quinolinecarboxaldehyde, and 2‐ and 3‐fluorobenzaldehyde undergo tandem aldol‐Michael reactions with aryl enolates to yield conjugate addition products. To elucidate the different reactivities of aldol products formed in the reactions of benzaldehyde and 2‐pyridinecarboxaldehyde with the sodium enolate of acetophenone, density functional theory (DFT) calculations were performed to compare the energies of conformational isomers of the intermediates formed in the reaction, the energies of the corresponding condensation products, and the kinetic barriers to dehydration and conjugate addition. Computational results support the formation of the trans isomer of the condensation product in base‐catalyzed aldol reactions of either benzaldehyde or 2‐pyridinecarboxaldehyde with acetophenone. The susceptibility of the condensation product to conjugate addition is determined to result from the lower LUMO energy of trans‐1‐phenyl‐3‐(2‐pyridinyl)‐2‐propen‐1‐one. Consequently, the kinetic barrier for conjugate addition of a second equivalent of enolate with the condensation product of 2‐pyridinecarboxaldehyde is found to be significantly lower. Similar results were obtained for the reactions of fluorobenzaldehydes with aryl enolates.

Exploring Cage‐warped Rotation Isomers of C20 Fullerene: MP2 and Density Functional Calculations

Using the density functional methods B3LYP, M06‐2X, PBE1PBE, B3LYP D3, M06‐2X D3, and PBE1PBE D3, and the ab initio MP2 calculations, the optimized atomic structures and energies of C20 cage rotation isomers without a constraint were obtained. The reaction path between the two rotational isomers was then studied, in which there are three local stable atomic structures. One is the most stable fullerene structure with the D2h point group staggered between the upper and bottom pentagons (ST) and the other is the second lowest energy rotational isomer with the Cs point group eclipsed between the upper and bottom pentagons (EC) with a torn fullerene structure (M1) between ST and EC. The first transition structure T1 between the ST and the M1, and the second transition structure T2 between the M1 and the EC were then obtained along the reaction coordinate between these three structures. Vibration analysis of all optimized structures including the transition structures without a constraint was carried out. For each of the rotational isomers obtained by our calculations, thermodynamic stability was explained with the relative energy difference, and then the kinetic stability was analyzed as the HOMO–LUMO energy difference.

A Quadratic Pair Atomic Resolution of the Identity Based SOS-AO-MP2 Algorithm Using Slater Type Orbitals

We report a production level implementation of pair atomic resolution of the identity (PARI) based second-order Møller–Plesset perturbation theory (MP2) in the Slater type orbital (STO) based Amsterdam Density Functional (ADF) code. As demonstrated by systematic benchmarks, dimerization and isomerization energies obtained with our code using STO basis sets of triple-ζ-quality show mean absolute deviations from Gaussian type orbital, canonical, basis set limit extrapolated, global density fitting (DF)-MP2 results of less than 1 kcal/mol. Furthermore, we introduce a quadratic scaling atomic orbital based spin-opposite-scaled (SOS)-MP2 approach with a very small prefactor. Due to a worst-case scaling of , our implementation is very fast already for small systems and shows an exceptionally early crossover to canonical SOS-PARI-MP2. We report computational wall time results for linear as well as for realistic three-dimensional molecules and show that triple-ζ quality calculations on molecules of several hundreds of atoms are only a matter of a few hours on a single compute node, the bottleneck of the computations being the SCF rather than the post-SCF energy correction.

Water dimer isomers: interaction energies and electronic structure.

The energetic and electronic structure of various water dimer isomers has been explored through DFT methodology. Six different possible water dimers come in two broad categories, planar and non-planar. In each of the categories, three distinct topologies (i) linear, (ii) ring and (iii) bifurcated, have been obtained. The linear dimer has the highest interaction energy, followed by the ring dimer and then comes the bifurcated dimer. For each of these type, a planar dimer having all six atoms organized in a single plane come very close to, but has a slightly higher energy than the corresponding non-planar counterpart. Bader's atoms in molecules (AIM) theory, reduced density gradient (RDG) method and non-covalent interaction (NCI) analysis reveal that the electron density distribution among the interacting water molecules correlates exactly in the sequence of interaction energies of different isomers of water dimer. Graphical abstractVarious possible water dimer isomers.

Теоретическое изучение изомеризации 1-амино-4-фениламино-9,10-антрахинона

This paper presents the results of computer simulation of tautomeric transformations of the molecule 1-amino-4-phenylamino-9,10-anthraquinone. It is known from the literature that the presence of substituents in the 1,4-position of anthraquinone-9,10 leads to various tautomeric transformations, with a shift in the absorption maximum and the appearance of absorption bands in the red wave region in electronic spectra. Both the 1-amino-4-hydroxyanthraquinone described in the literature and the 1-amino-4-phenylamino-9,10-anthraquinone are characterized by two types of prototropic tautomerism – keto-enol and amino-imine. Quantum-chemical modeling contributes to the calculation of the relative energies of tautomers and isomers, the barriers of their interconversions, as well as finding their structural parameters. The aim of this study was to study the mechanism of the formation of tautomers during hydrogen transfer in the molecule of 1-amino-4-phenylamino-9,10-anthraquinone, as well as the formation of isomers during migration of the OH group. The calculations were performed using the Gaussian09 program. To study of various tautomers of 1-amino-4-phenylamino-9,10-anthraquinone, the B3LYP method with the def2TZV basis was used. A search was conducted for transition states during hydrogen transfer and OH group migration. The descent along the reaction path was calculated to confirm that the transition state is in the path of the desired reaction. The minima corresponding to the starting material and product were localized. The activation enthalpies of the studied reactions were calculated. Migration of the OH group in the 1-amino-4-phenylamino-9,10-anthraquinone molecule leads to the formation of 4-phenylamino-9-amino-1,10-anthraquinone. As the calculation shows, the keto-form of 1-amino-4-phenylamino-9,10-anthraquinone is energetically more profitable than all the isomers studied in this work, including the enol form. The smallest difference in total potential energies is 23.7 kJ/mol between the initial ketone form of 1-amino-4-phenylamino-9,10-anthraquinone and the last transformation structure – the 4-phenylamino-9-amino-1,10-anthraquinone molecule.

Multiresponse Kinetic Modeling of Heat-Induced Equilibrium of β-Carotene cis-trans Isomerization in Medium-Chain Triglyceride Oil.

The kinetics and mechanism of the stepwise cis-trans isomerization reactions of all-trans-β-carotene dissolved in MCT (medium-chain triglyceride) oil at temperatures in the range of 80-160 °C have been analyzed using multiresponse modeling. Quantitation of the cis-isomers was performed using HPLC-DAD and quantitation at the reaction isosbestic point at 421 nm. Multiresponse kinetic modeling using the Bayesian criterion was initially performed at 120 °C to determine the best model. Subsequently, the reparametrized Arrhenius equation was used to calculate the activation energies of all reactions. The equilibrium constants for the individual isomerization reactions were determined from the rate constants and the final product distributions. The enthalpies and entropies of the isomerization reactions were determined from the temperature dependence of the equilibrium constants. The 13-cis and 13,13'-di-cis isomers were found to be the fastest formed isomers followed by the 9-cis, 9,13-di-cis, and 13,15-di-cis isomers, where the latter was found to be formed from 13-cis and not the 15-cis isomer. The relative free energies of the β-carotene isomers were determined as all-trans < 13-cis < 9-cis < 13,13'-di-cis < 9,13-di-cis ≈ 15-cis < 13,15-di-cis. The entropic contribution of each reaction was found to play an important role in the ordering. It is concluded that the β-carotene system is quite labile at temperatures ranging from 80 to 160 °C and resulting in equilibrium distributions of the cis-trans isomers.

Masses of ground and isomeric states of In101 and configuration-dependent shell evolution in odd- A indium isotopes

We report first precision mass measurements of the $1/2^-$ isomeric and $9/2^+$ ground states of $^{101}$In. The determined isomeric excitation energy continues a smooth trend of odd-$A$ indium isotopes up to the immediate vicinity of $N=50$ magic number. This trend can be confirmed by dedicated shell model calculations only if the neutron configuration mixing is considered. We find that the single particle energies are different for different states of the same isotope. The presented configuration-dependent shell evolution, type II shell evolution, in odd-$A$ nuclei is discussed for the first time. Our results will facilitate future studies of single-particle neutron states.

Energy of the ^{229}Th Nuclear Clock Isomer Determined by Absolute γ-ray Energy Difference.

The low-lying isomeric state of ^{229}Th provides unique opportunities for high-resolution laser spectroscopy of the atomic nucleus. We determine the energy of this isomeric state by taking the absolute energy difference between the excitation energy required to populate the 29.2-keV state from the ground state and the energy emitted in its decay to the isomeric excited state. A transition-edge sensor microcalorimeter was used to measure the absolute energy of the 29.2-keV γ ray. Together with the cross-band transition energy (29.2 keV→ground) and the branching ratio of the 29.2-keV state measured in a recent study, the isomer energy was determined to be 8.30±0.92 eV. Our result is in agreement with the latest measurements based on different experimental techniques, which further confirms that the isomeric state of ^{229}Th is in the laser-accessible vacuum ultraviolet range.

Energy-Screened Many-Body Expansion: A Practical Yet Accurate Fragmentation Method for Quantum Chemistry.

We introduce an implementation of the truncated many-body expansion, MBE(n), in which the n-body corrections are screened using the effective fragment potential force field, and only those that exceed a specified energy threshold are computed at a quantum-mechanical level of theory. This energy-screened MBE(n) approach is tested at the n = 3 level for a sequence of water clusters, (H2O)N=6-34. A threshold of 0.25 kJ/mol eliminates more than 80% of the subsystem electronic structure calculations and is even more efficacious in that respect than is distance-based screening. Even so, the energy-screened MBE(3) method is faithful to a full-system quantum chemistry calculation to within 1-2 kJ/mol/monomer, even in good quality basis sets such as aug-cc-pVTZ. These errors can be reduced by means of a two-layer approach that involves a Hartree-Fock calculation for the entire cluster. Such a correction proves to be necessary in order to obtain accurate relative energies for conformational isomers of (H2O)20, but the cost of a full-system Hartree-Fock calculation remains smaller than the cost of three-body subsystem calculations at correlated levels of theory. At the level of second-order Møller-Plesset perturbation theory (MP2), a screened MBE(3) calculation plus a full-system Hartree-Fock calculation is less expensive than a full-system MP2 calculation starting at N = 12 water molecules. This is true even if all MBE(3) subsystem calculations are performed on a single 40-core compute node, i.e., without significant parallelization. Energy-screened MBE(n) thus provides a fragment-based method that is accurate, stable in large basis sets, and low in cost, even when the latter is measured in aggregate computer time.

Robust and accurate hybrid random-phase-approximation methods.

A fully self-consistent hybrid dRPA (direct random phase approximation) method, named sc-H[γ]dRPA, is presented with γ = 1/3. The exchange potential of the new method contains a fraction γ of nonlocal Hartree-Fock-like exchange besides the exact local Kohn-Sham (KS) exchange potential. The sc-H[γ]dRPA method, in contrast to a straightforward self-consistent dRPA method within the KS formalism, does not suffer from convergence problems for systems with small eigenvalue gaps. Moreover, the sc-H[γ]dRPA method yields distinctively more accurate reaction, isomerization, and transition state energies than other dRPA approaches, e.g., the frequently used non-self-consistent dRPA method using orbitals and eigenvalues from a KS calculation with the exchange-correlation potential of Perdew, Burke, and Ernzerhof (PBE). The sc-H[γ]dRPA method outperforms second-order Møller-Plesset perturbation theory and coupled cluster singles doubles methods while exhibiting a more favorable scaling of computational costs with system size. A value of γ = 1/3 is shown to be a good choice also for a dRPA@PBE[γ] method, which is a non-self-consistent dRPA method using orbitals and eigenvalues from the hybrid PBE0 method with an admixture of γ = 1/3 of exact exchange instead of the 25% of the PBE0 functional. The dRPA@PBE[γ] method yields reaction, isomerization, and transition state energies that are as good as the sc-H[γ]dRPA ones but is computationally simpler and more efficient because it does not require the self-consistent construction of the dRPA correlation potential. The direct sc-H[γ]dRPA, on the other hand, in contrast to all standard density-functional methods, yields qualitatively correct correlation potentials.

X-ray pumping of the 229Th nuclear clock isomer.

The metastable first excited state of thorium-229, 229mTh, is just a few electronvolts above the nuclear ground state1-4 and is accessible by vacuum ultraviolet lasers. The ability to manipulate the 229Th nuclear states with the precision of atomic laser spectroscopy5 opens up several prospects6, from studies of fundamental interactions in physics7,8 to applications such as a compact and robust nuclear clock5,9,10. However, direct optical excitation of the isomer and its radiative decay to the ground state have not yet been observed, and several key nuclear structure parameters-such as the exact energies and half-lives of the low-lying nuclear levels of 229Th-remain unknown11. Here we present active optical pumping into 229mTh, achieved using narrow-band 29-kiloelectronvolt synchrotron radiation to resonantly excite the second excited state of 229Th, which then decays predominantly into the isomer. We determine the resonance energy with an accuracy of 0.07 electronvolts, measure a half-life of 82.2 picoseconds and an excitation linewidth of 1.70 nanoelectronvolts, and extract the branching ratio of the second excited state into the ground and isomeric state. These measurements allow us to constrain the 229mTh isomer energy by combining them with γ-spectroscopy data collected over the past 40 years.

Massive-Parallel Implementation of the Resolution-of-Identity Coupled-Cluster Approaches in the Numeric Atom-Centered Orbital Framework for Molecular Systems.

We present a massive-parallel implementation of the resolution of identity (RI) coupled-cluster approach that includes single, double, and perturbatively triple excitations, namely, RI-CCSD(T), in the FHI-aims package for molecular systems. A domain-based distributed-memory algorithm in the MPI/OpenMP hybrid framework has been designed to effectively utilize the memory bandwidth and significantly minimize the interconnect communication, particularly for the tensor contraction in the evaluation of the particle-particle ladder term. Our implementation features a rigorous avoidance of the on-the-fly disk storage and excellent strong scaling of up to 10 000 and more cores. Taking a set of molecules with different sizes, we demonstrate that the parallel performance of our CCSD(T) code is competitive with the CC implementations in state-of-the-art high-performance-computing computational chemistry packages. We also demonstrate that the numerical error due to the use of RI approximation in our RI-CCSD(T) method is negligibly small. Together with the correlation-consistent numeric atom-centered orbital (NAO) basis sets, NAO-VCC-nZ, the method is applied to produce accurate theoretical reference data for 22 bio-oriented weak interactions (S22), 11 conformational energies of gaseous cysteine conformers (CYCONF), and 32 isomerization energies (ISO32).

Importance of van der Waals Descriptions on Accurate Isomerization Energy Calculations of Thiourea Compounds: LCgau-BOP+LRD Method.

We have previously reported that, whereas conventional density functional theory (DFT) functionals have provided poor calculations on the alkane isodesmic reaction energy and isomerization reaction energy of organic molecules that include C, N, and O atoms, our developed long-range corrected (LC)- and LC including Gaussian attenuation (LCgau)-DFT + local response dispersion (LRD) functionals, which can accurately calculate inter- and intramolecular weak interactions, give accurate isomerization energies on these reactions. In this work, we found that B3LYP-D3, LC-ωPBE-D3, and ωB97XD, known for their good descriptions of weak interaction calculations, fail to reproduce the isomerization reaction energies of the molecules that include the S atom, such as methyl-thiourea, ethyl-thiourea, and propyl-thiourea. In contrast, LC- and LCgau-BOP+LRD functionals provide isomerization reaction energies that are very close to those produced by highly accurate wave function methods. These results show that an accurate description of the intramolecular weak interaction between the alkyl group and the S atom, unlike in the case of urea, is significant to reproduce the correct energy of the molecules with an alkyl group and S atom.

Molybdenum complexes with citrate revisited. A mononuclear [MoVOCl4(H2O)]− ion as a new synthetic entry

Products of the reaction of a mononuclear molybdenum(V) compound, (PyH)5[MoVOCl4(H2O)]3Cl2, with citric acid (IUPAC name: 2-hydroxypropane-1,2,3-tricarboxylic acid, abbreviated as H4cit) were characterized by spectroscopic techniques and X-ray structure analysis. When the supply of oxygen was limited, molybdenum retained the initial oxidation state and a tetranuclear Mo(V) compound formed, (MeNC5H5)4[MoV4O8(cit)2]·2H2O (1) (MeNC5H5+ = N-methylpyridinium cation). Otherwise, oxidation of metal to +6 state took place resulting in {(C6H5)4P}[MoVIO2Cl(H2cit)] (3), (PyH)2[MoVIO2(H2cit)2]·Py (4), (PyH)4[{MoVIO2(Hcit)}2(µ2-O)]·2Py (5), (PyH)4[MoVI4O11(Hcit)2]·2H2O·1/2CH3OH (6) and (PyH)6[MoVI4O11(Hcit)2]Cl2·H2O·3/2CH3OH (7) (Py = pyridine, C5H5N; PyH+ = pyridinium cation, C5H5NH+). In one instance, (PyH)2[MoVOCl2(Hcit)]0.39[MoVIO2Cl(Hcit)]0.61·CH3CN (2), a compound that is a solid mixture of Mo(V) and Mo(VI) species was obtained. In all complexes, the citrate coordinated to molybdenum in a chelating manner with at least two oxygen atoms that belonged to deprotonated OH and α-carboxyl groups. In {MoVIO2}2+ species, the two trans positions of the Mo=O moieties were occupied with carboxylate oxygens. In order to assess stabilities of different orientations of citrate ligands with respect to Mo=O, the isomeric forms of [MoVIO2Cl(H2cit)]− and [MoVIO2(H2cit)2]2−, the anions of 3 and 4, were subjected to DFT calculations. Results reveal that the relative energies of isomers are close to one another. A particular orientation of citrate does not seem to have a determining impact. As the case of the [MoVIO2(H2cit)2]2− isomers, which gain stability by intramolecular hydrogen bonds, shows, other factors are at work. For the metal–metal bonded {MoV2O4}2+ compounds, comparison with the known complexes suggests that a concept of a preferential orientation of citrate, i.e., with the alkoxide oxygen situated at the site trans to Mo=O, is not valid.

The heavier pnictogen and chalcogen analogues of isocyanic and cyanic acids and their dimers: A high level ab initio study

AbstractThe CCSD(T)/cc‐pVTZ//CCSD/cc‐pVTZ method is used to determine the geometries and energetics of the isomers HXCY vs HY─CX (XN, P, As; YO, S) and their dimers from chain dimerizations and head‐to‐head or head‐to‐tail [2 + 2] cyclodimerizations. The HO─CX structures with CX triple bonds lie at energies at least 18.5 kcal/mol above their HXCO isomers. However, the energy differences between the HXCS and HS─CX isomers are found to be particularly small, especially in the [H,P,C,S] and [H,As,C,S] systems. For (HNCY)2, the lowest energy dimers are the chain isomers, which lie ~11 kcal/mol below the lowest energy cyclic dimers aNO containing a NCNC ring and cNS containing a NCSC ring. Formation of the remaining dimers through dimerization from two monomers is predicted to be endothermic and thus thermodynamically disfavored. However, the energies of the chain isomers in the other (HXCY)2 (XP, As; YO, S) series are higher than those of the corresponding isomeric lowest energy cyclodimers. For (HXCO)2 (XP, As), the lowest energy structures are the head‐to‐head dimers hPO and hAsO containing a C─C─XX ring. For (HXCS)2 (XP, As), the lowest energy structures are the head‐to‐head dimers gPS and gAsS with a CCXS ring.

Free energy of conformational isomers: The case of gapped DNA duplexes.

Liquid-crystalline phases in all-DNA systems have been extensively studied in the past and although nematic, cholesteric and columnar mesophases have been observed, the smectic phase remained elusive. Recently, it has been found evidence of a smectic-A ordering in an all-DNA system, where the constituent particles, which are gapped DNA duplexes, resemble chain-sticks. It has been argued that in the smectic-A phase these DNA chain-sticks should be folded as a means to suppress aggregate polydispersity and excluded volume. Nevertheless, if initial crystalline configurations are prepared in silico with gapped DNA duplexes either fully unfolded or fully folded by carrying out computer simulations one can end up with two different phases having at the same concentration and temperature the majority of gapped DNA duplexes either folded or unfolded. This result suggests that these two phases have a small free energy difference, since no transition is observed from one to the other within the simulation time span. In the present manuscript, we assess which of these two phases is thermodynamically stable through a suitable protocol based on thermodynamic integration. Our method is rather general and it can be used to discriminate stable states from metastable ones of comparable free energy.

Thermochemistry of phosphorus sulfide cages: an extreme challenge for high-level ab initio methods

The enthalpies of formation and isomerization energies of P4Sn molecular cages are not experimentally (or theoretically) well known. We obtain accurate enthalpies of formation and isomerization energies for P4Sn cages (n = 3, 4, 5, 6, and 10) by means of explicitly correlated high-level thermochemical procedures approximating the CCSD(T) and CCSDT(Q) energies at the complete basis set (CBS) limit. The atomization reactions have very significant contribution from post-CCSD(T) correlation effects and, due to the presence of many second-row atoms, the CCSD and (T) correlation energies converge exceedingly slowly with the size of the one-particle basis set. As a result, these cage structures are challenging targets for thermochemical procedures approximating the CCSD(T) energy (e.g., W1-F12 and G4). Our best enthalpies of formation at 298 K (∆fH°298) are obtained from thermochemical cycles in which the P4Sn cages are broken down into P2S2 and S2 fragments for which highly accurate ∆fH°298 values are available from W4 theory. For the smaller P4S3 and P4S4 cages, the reaction energies are calculated at the CCSDT(Q)/CBS level and for the larger P4S5, P4S6, and P4S10 cages, they are obtained at the CCSD(T)/CBS level. Our best ∆fH°298 values are − 94.5 (P4S3), − 108.4 (α-P4S4), − 98.7 (β-P4S4), − 126.2 (α-P4S5), − 126.1 (β-P4S5), − 112.7 (γ-P4S5), − 144.7 (α-P4S6), − 153.9 (β-P4S6), − 134.4 (γ-P4S6), − 136.3 (δ-P4S6), − 118.7 (e-P4S6), and − 215.4 (P4S10) kJ mol−1. Interestingly, we find a linear correlation (R2 = 0.992) between the enthalpies of formation of the most stable isomers of each molecular formula and the number of atoms in the P4Sn cages. We use our best ∆fH°298 values to assess the performance of a number of lower-cost composite ab initio methods. For absolute enthalpies of formation, G4(MP2) and G3(MP2)B3 result in the best overall performance with root-mean-square deviations (RMSDs) of 10.6 and 12.9 kJ mol−1, respectively, whereas G3, G3B3, and CBS-QB3 result in the worst performance with RMSDs of 27.0–38.8 kJ mol−1. In contrast to absolute enthalpies of formation, all of the considered composite procedures give a good-to-excellent performance for the isomerization energies with RMSDs below the 5 kJ mol−1 mark.

First principles study of structural and optical properties of [formula omitted] isomers

In this work we undertake a comprehensive numerical study of the ground state structures and optical absorption spectra of isomers of B12 cluster. Geometry optimization was performed at the coupled-cluster-singles-doubles (CCSD) level of theory, employing cc-pVDZ extended basis sets. Once the geometry of a given isomer was optimized, its ground state energy was calculated more accurately at the coupled-cluster-singles-doubles along with perturbative treatment of triples (CCSD(T)) level of theory, employing larger cc-pVTZ basis sets. Thus, our computed values of binding energies of various isomers are expected to be quite accurate. Our geometry optimization reveals eleven distinct isomers, along with their point group, and electronic ground state symmetries. We also performed vibrational frequency analysis on the three lowest energy isomers, and found them to be stable. Therefore, we computed the linear optical absorption spectra of these isomers of B12, employing large-scale multi-reference singles-doubles configuration-interaction (MRSDCI) approach, and found a strong structure-property relationship. This implies that the spectral fingerprints of the geometries can be utilized for optical detection, and characterization, of various isomers of B12. We also explored the stability of the isomer with the structure of a perfect icosahedron, with Ih symmetry. In bulk boron icosahedron is the basic structural unit, but, our vibrational frequency analysis reveals that it is unstable in the isolated form. We speculate that this instability could be due to Jahn-Teller distortion because five-fold degenerate HOMO orbitals in Ih structure are unfilled.

Ab initio structure and vibration-rotation dynamics of the formyl and isoformyl cations, HCO+/HOC.

Accurate structure and potential energy surface of the formyl and isoformyl cation system, HCO+/HOC+, in its ground electronic state X̃ 1Σ+ have been determined from ab initio calculations using the coupled-cluster approach in conjunction with the correlation-consistent basis sets up to septuple-zeta quality. Both the isomers are confirmed to be linear at equilibrium, with the total energy minimum of HOC+ lying 14 120 cm-1 above that of HCO+ and the HCO+ → HOC+ isomerization energy barrier being 26 870 cm-1 (in the Born-Oppenheimer approximation). The equilibrium structural parameters for HCO+ are estimated to be re(HC) = 1.0919 Å and re(CO) = 1.1058 Å, whereas those for HOC+ are estimated to be re(HO) = 0.9899 Å and re(CO) = 1.1544 Å. The vibration-rotation energy levels were predicted for various isotopologues using a variational approach and compared with the experimental data. For the spectroscopically well characterized formyl cation, the observed vibration-rotation energies and the effective rotational constants are reproduced to within about 2.3 cm-1 and 1.7 MHz, respectively. The role of the core-electron correlation, higher-order valence-electron correlation, scalar relativistic, and adiabatic effects in determining the structure and vibration-rotation dynamics of both the isomers is discussed.

Spanning solar spectrum: A combined photochemical and thermochemical process for solar energy storage

Storage in the form of chemical energy is crucial for efficient utilisation of solar energy. In recent years, solar photon-induced molecular isomerization energy storage, in which solar energy can be directly converted and stored as chemical energy through internal molecular isomerization reactions, has received increasing interest. It is a closed cycle without any CO2 emission. However, the absorption of reactants cannot cover the full spectrum of solar radiation, and only the ultraviolet and a portion of the visible spectrum of solar energy can be stored. Moreover, some absorbed photons are dissipated as heat, leading to an increase in the reaction temperature and a decrease in the solar photochemical efficiency. To address these problems, a new energy storage system which integrates the photochemical process with thermochemical process has been proposed to convert the full spectrum of solar energy into chemical energy. Concentrated sunlight enters the photochemical device. Norbornadiene derivatives present in the photochemical devices can be isomerized to the related quadricyclanes by absorbing the ultraviolet-visible light photons. Some absorbed photons are simultaneously stored in the chemical bonds of the quadricyclanes. The unabsorbed photons corresponding to the visible-infrared spectrum are transmitted to thermochemical reactors, providing heat for methanol decomposition. This portion of the solar energy is stored in the form of chemical energy of the products (H2 and CO). The heat dissipated in the photochemical process is transferred to the thermochemical reactors for methanol decomposition, resulting in a higher solar chemical efficiency. Results show that the average solar chemical efficiencies corresponding to the design condition and off-design condition are 75.38% and 49.78%, respectively. The results of comparison of the thermochemical performances verify that the proposed system is superior to both the single photochemical system and the single thermochemical system.