Investigating the accuracy of density functional methods for molecules in electric fields.

The use of oriented external electric fields (OEEFs) as a potential tool for catalyzing chemical reactions has gained traction in recent years. Electronic structure calculations using OEEFs are commonly done using methods based on density functional theory (DFT), but until now, the performance of DFT methods for calculating molecules in OEEFs had not been assessed in a more general scope. Looking at the accuracy of molecular geometries, electronic energies, and electric dipole moments compared to accurate coupled-cluster with perturbative triples data, we have investigated a wide variety of density functionals using different basis sets to determine how well the individual functionals perform on various types of chemical bonds. We found that most functionals accurately calculate geometries in OEEFs and that small basis sets are sufficient in many cases. Calculations of electronic energies show a significant error introduced by the OEEF, which the use of a larger basis set helps mitigate. Our findings show that DFT methods can be used for accurate calculations in OEEFs, allowing researchers to make full use of the advantages that they bring.

Synthesis and crystal structure of 2-(anthracen-9-yl)-1-(tert-butyl-dimethyl-sil-yl)-3,6-di-hydro-1λ4,2λ4-aza-borinine.

The title compound, C24H30BNSi (I), is an asymmetric 1,2,3,6-tetra-hydro-1,2-aza-borinine consisting of a BN-substituted cyclo-hexa-diene analog with a B-anthracenyl substituent. A ring-closing metathesis with subsequent substitution of the obtained BCl 1,2-aza-borinine using anthracenyl lithium yielded the title compound I. The asymmetric unit (Z = 8) belongs to the ortho-rhom-bic space group Pbca and shows an elongated N-C bond compared to previously reported BN-1,4-cyclo-hexa-diene [Abbey et al. (2008 ▸) J. Am. Chem. Soc. 130, 7250-7252]. The primarily contributing surface inter-actions are H⋯H and C⋯H/H⋯C (as elucidated by Hirshfeld surface analysis) which are dominated by van der Waals forces. Moreover, the non-aromatic BN heterocycle and the protecting group exhibit intra- and inter-molecular C-H⋯π inter-actions, respectively, with the anthracenyl substituent.

Using oriented external electric fields to manipulate rupture forces of mechanophores.

Oriented external electric fields (OEEFs) can facilitate chemical reactions by selectively weakening bonds. This makes them a topic of interest in mechanochemistry, where mechanical force is used to rupture specific bonds in molecules. Using electronic structure calculations based on density functional theory (DFT), we investigate the effect of OEEFs on the mechanical force required to activate mechanophores. We demonstrate that OEEFs can greatly lower the rupture force of mechanophores, and that the degree of this effect highly depends on the angle relative to the mechanical force at which the field is being applied. The greatest lowering of the rupture force does not always occur at the point of perfect alignment between OEEF and the vector of mechanical force. Using natural bond orbital analysis, we show that mechanical force amplifies the effect that an OEEF has on the scissile bond of a mechanophore. By combining methods to simulate molecules in OEEFs with methods applying mechanical force, we present an effective tool for analyzing mechanophores in OEEFs and show that computationally determining optimal OEEFs for mechanophore activation can assist in the development of future experimental studies.

Alliacane-Type Secondary Metabolites from Submerged Cultures of the Basidiomycete Clitocybe nebularis.

Seven sesquiterpenoids, named nebucanes A-G (1-7), featuring a rare alliacane scaffold with unprecedented furan or pyrrole functions, were isolated from the fermentation broth of Clitocybe nebularis. Their structures were established on the basis of 1D/2D NMR spectroscopic analyses, HR-(+)-ESIMS spectra, and comparison of measured and calculated CD spectra for determination of the absolute configuration. Assessing the biological activities, nebucane D (4) exhibited antifungal effects against Rhodotorula glutinis, while nebucane G (7) displayed significant cytotoxicity against MCF-7 and A431 cell lines.

Can a Finite Chain of Hydrogen Cyanide Molecules Model a Crystal?

When calculating structural or spectroscopic properties of molecular crystals, the question arises whether it is sufficient to simulate only a single molecule or a small molecular cluster or whether the simulation of the entire crystal is indispensable. In this work we juxtapose calculations on the high-pressure structural properties of the (periodic) HCN crystal and chains of HCN molecules of finite length. We find that, in most cases, the behavior of the crystal can be reproduced by computational methods simulating only around 15 molecules. The pressure-induced lengthening of the C-H bond in HCN found in calculations on both the periodic and finite material are explained in terms of orbital interaction. Our results pave the way for a more thorough understanding of high-pressure structural properties of materials and give incentives for the design of materials that expand under pressure. In addition, they shed light on the complementarity between calculations on periodic materials and systems of finite size.

Mechanisms of methyl formate production during electron-induced processing of methanol-carbon monoxide ices.

The formation of methyl formate (CH3OCHO) upon electron irradiation of mixed ices of carbon monoxide (CO) and methanol (CH3OH) has been monitored by post-irradiation thermal desorption spectrometry (TDS). The energy dependence of the product yields obtained with electron energies between 3 and 18 eV was studied. These energies are characteristic of secondary electrons that are released in vast numbers under the effect of ionizing radiation. Our results reveal that the reactions leading to methyl formate are initiated by a number of different electron-molecule interactions that produce CH3O˙ radicals. Dissociative electron attachment (DEA) to CH3OH around 5.5 eV and neutral dissociation (ND) above 7 eV release CH3O˙ radicals that can add to CO to initiate a reaction sequence leading to formation of methyl formate. Around 10 eV, DEA to CO yields an oxygen radical anion that reacts with CH3OH to also produce CH3O˙ radicals. Alternatively, CH3OH can also release H˙ radicals upon both DEA and ND. These can also add to CO to form HCO˙ radicals as an intermediate to formaldehyde (H2CO), which was also investigated to unravel the reaction mechanisms leading to formation of methyl formate. The recombination of HCO˙ and CH3O˙ as minority radical species is considered as an alternative but less probable pathway to the formation of methyl formate. To the best of our knowledge, this is the first study showing considerable contributions of DEA to the formation of methyl formate in CH3OH containing ices. Thus, our study has important implications for current astrochemical models.