A Post-HF Approach to the Sunscreen Octyl Methoxycinnamate

Octyl methoxycinnamate (OMC) is a commercial sunscreen with excellent UVB filter properties. However, it is known to undergo a series of photodegradation processes that decrease its effectiveness as UVB filter. In particular, the trans (E) form - which is considered so far the most stable isomer - converts to the cis (Z) form under the effect of light. In this work, by using post-Hartree-Fock approaches (CCSD, CCSD(t) and CCSD+T(CCSD)) on ground state OMC geometries optimized at the MP2 level we show that the cis and trans form of the gas-phase OMC molecule have comparable stability. Our results suggest that the cis form is stabilized by intra-molecular dispersion interactions, leading to a folded, more compact structure than the trans isomer.<br>

A Post-HF Approach to the Sunscreen Octyl Methoxycinnamate

Octyl methoxycinnamate (OMC) is a commercial sunscreen with excellent UVB filter properties. However, it is known to undergo a series of photodegradation processes that decrease its effectiveness as UVB filter. In particular, the trans (E) form - which is considered so far the most stable isomer - converts to the cis (Z) form under the effect of light. In this work, by using post-Hartree-Fock approaches (CCSD, CCSD(t) and CCSD+T(CCSD)) on ground state OMC geometries optimized at the MP2 level we show that the cis and trans form of the gas-phase OMC molecule have comparable stability. Our results suggest that the cis form is stabilized by intra-molecular dispersion interactions, leading to a folded, more compact structure than the trans isomer.<br>

Benchmarking of Density Functionals for Z-Azoarene Half-Lives via Automated Transition State Search

Molecular photoswitches use light to interconvert from a thermodynamically stable isomer into a meta-stable isomer. Chemists and materials scientists have applied photoswitches in photopharmacology, catalysis, and molecular solar thermal (MOST) materials. Visible-light-absorbing photoswitches are attractive because the relatively low-energy light minimizes undesired photochemical reactions and enables biological applications. Designing ideal photoswitches requires long-lived metastable states; predicting their half-lives with theory is difficult because it requires locating transition structures. We now report the EZ-TS code, which automates the prediction of rate constants for the thermal Z → E isomerization. We leverage EZ-TS to automate the location of the favored transition structure and to comprehensively benchmark the performance of 140 model chemistries against the experimental rate constants of 11 azoarenes. We used 28 density functionals [local spin density approximation, generalized gradient approximation, meta-GGA, hybrid GGA, hybrid meta-GGA], and five basis sets [6-31G(d), 6-31+G(d,p), 6-311+G(d,p), cc-pvdz, and aug-cc-pvdz]. The hybrid GGA functionals performed the best of all tested functional classes. We demonstrate that the mean absolute errors of 14 model chemistries approach chemical accuracy, and mPWPW91/6-31+G(d,p) achieves chemical accuracy and should be used with EZ-TS.<br>

Benchmarking of Density Functionals for Z-Azoarene Half-Lives via Automated Transition State Search

Molecular photoswitches use light to interconvert from a thermodynamically stable isomer into a meta-stable isomer. Chemists and materials scientists have applied photoswitches in photopharmacology, catalysis, and molecular solar thermal (MOST) materials. Visible-light-absorbing photoswitches are attractive because the relatively low-energy light minimizes undesired photochemical reactions and enables biological applications. Designing ideal photoswitches requires long-lived metastable states; predicting their half-lives with theory is difficult because it requires locating transition structures. We now report the EZ-TS code, which automates the prediction of rate constants for the thermal Z → E isomerization. We leverage EZ-TS to automate the location of the favored transition structure and to comprehensively benchmark the performance of 140 model chemistries against the experimental rate constants of 11 azoarenes. We used 28 density functionals [local spin density approximation, generalized gradient approximation, meta-GGA, hybrid GGA, hybrid meta-GGA], and five basis sets [6-31G(d), 6-31+G(d,p), 6-311+G(d,p), cc-pvdz, and aug-cc-pvdz]. The hybrid GGA functionals performed the best of all tested functional classes. We demonstrate that the mean absolute errors of 14 model chemistries approach chemical accuracy, and mPWPW91/6-31+G(d,p) achieves chemical accuracy and should be used with EZ-TS.<br>

A Post-HF Approach to the Sunscreen Octyl Methoxycinnamate

Octyl methoxycinnamate (OMC) is a commercial sunscreen with excellent UVB filter properties. However, it is known to undergo a series of photodegradation processes that decrease its effectiveness as UVB filter. In particular, the trans (E) form - which is considered so far the most stable isomer - converts to the cis (Z) form under the effect of light. In this work, by using post-Hartree-Fock approaches (CCSD, CCSD(t) and CCSD+T(CCSD)) on ground state OMC geometries optimized at the MP2 level we show that the cis and trans form of the gas-phase OMC molecule have comparable stability. Our results suggest that the cis form is stabilized by intra-molecular dispersion interactions, leading to a folded, more compact structure than the trans isomer.<br>

Experimental and Computational Investigation of the Bond Energy of Thorium Dicarbonyl Cation and Theoretical Elucidation of Its Isomerization Mechanism to the Thermodynamically Most Stable Isomer, Thorium Oxide Ketenylidene Cation, OTh+CCO

Collision-induced dissociation (CID) of [Th,2C,2O]+ with Xe is performed using a guided ion beam tandem mass spectrometer (GIBMS). The only products observed are ThCO+ and Th+ by sequential loss of...

Relative thermodynamic stability of the [C,N,O] linkages as an indication of the most abundant structures in the ISM

Context. Most of the compounds detected in the interstellar medium (ISM) that possess isomers correspond to the thermodynamically most stable isomer of a given chemical formula. Using the minimun energy principle (MEP) as a pragmatic tool is particularly efficient for less than six atoms, but for larger systems combinatorial analysis gives an intractable numbers of isomers. Aims. To make the MEP more applicable, we look for a chemical sieve to filter the thermodynamic data needed to determine which isomers of complex organic molecules (COMs) have higher chances of being present in the ISM. To do so, we investigate whether the nature of the [C,N,O] elementary fragment can be determinant for the stabilization of COMs, taking C2H3NO as a case study. Methods. We employed standard quantum chemistry methods to determine the ordering of fragments and derivatives on the energy scale. Density functional theory treatments were systematically performed, together with high-level coupled cluster calculations to refine relative energies. Results. For C2H3NO we find methylisocyanate CH3NCO, which is a compound that was very recently detected in the ISM, to be the most stable isomer in a corpus of 40 isomers of lowest energy. In neutral form we find the stability ordering of NCO > ONC; the same is true for the negative ion. Attachments of R=H, CH3, C2H5, HC2, H2CCH, and C6H5 and metals Al and Mg to the nitrogen atom of the NCO fragment provide the most stable isomers. The energy differences between the successive isomers on the energy scale R-NCO, R-OCN, R-CNO, and R-ONC are of the same order of magnitude for all carbonaceous R. Conclusions. Combining the criterion of most stable linkage with the MEP concept should greatly reduce the window of potential targets to be searched for in the ISM. Compounds containing the NCO linkage should be preferential targets of future research.

A Barrierless Pathway Accessing the C9H9 and C9H8 Potential Energy Surfaces via the Elementary Reaction of Benzene with 1-Propynyl

The crossed molecular beams reactions of the 1-propynyl radical (CH3CC; X2A1) with benzene (C6H6; X1A1g) and D6-benzene (C6D6; X1A1g) were conducted to explore the formation of C9H8 isomers under single-collision conditions. The underlying reaction mechanisms were unravelled through the combination of the experimental data with electronic structure and statistical RRKM calculations. These data suggest the formation of 1-phenyl-1-propyne (C6H5CCCH3) via the barrierless addition of 1-propynyl to benzene forming a low-lying doublet C9H9 intermediate that dissociates by hydrogen atom emission via a tight transition state. In accordance with our experiments, RRKM calculations predict that the thermodynamically most stable isomer – the polycyclic aromatic hydrocarbon (PAH) indene – is not formed via this reaction. With all barriers lying below the energy of the reactants, this reaction is viable in the cold interstellar medium where several methyl-substituted molecules have been detected. Its underlying mechanism therefore advances our understanding of how methyl-substituted hydrocarbons can be formed under extreme conditions such as those found in the molecular cloud TMC-1. Implications for the chemistry of the 1-propynyl radical in astrophysical environments are also discussed.