Experimental and theoretical studies of tetramethoxy-p-benzoquinone: infrared spectra, structural and lithium insertion properties

International audienceIn the search for low-polluting electrode materials for batteries, the use of redox-active organic compounds represents a promising alternative to conventional metal-based systems. In this article we report a combined experimental and theoretical study of tetramethoxy-p-benzoquinone (TMQ). In carbonate-based electrolytes, electrochemical behaviour of this compound is characterized by a reversible insertion process located at approximately 2.85 V vs. Li+/Li0. This relatively high potential reactivity, coupled with our effort to develop computational methodologies in the field of organic electrode materials, prompted us to complement these experimental data with theoretical studies performed using density functional theory (DFT). Single crystals of TMQ were synthesized and thoroughly characterized showing that this quinonic species crystallised in the P21/n space group. The experimental crystal structure of TMQ was then used to assess various DFT methods. The structural features and vibrational spectra were thus predicted by using as a whole five common density functionals (PBE, LDA, revPBE, PBEsol, B3PW91) with and without a semi-empirical correction to account for the van der Waals interactions using either Grimme's (DFT-D2) or Tkatchenko-Scheffler (TS) scheme. The most reliable combination of the DFT functional and the explicit dispersion correction was chosen to study the Li-intercalated molecular crystal (LiTMQ) with the view of indentifying Li insertion sites. A very close agreement with the experiment was found for the average voltage by using the most stable relaxed hypothetical LiTMQ structure. Additionally, a comparison of vibrational spectra gained either for TMQ molecule and its dimer in gas phase or through periodic calculation was undertaken with respect to the experimentally measured infrared spectra. The topological features of the bonds were also investigated in conjunction with estimates of net atomic charges to gain insight into the effect of chemical bonding and intermolecular interaction on Li intercalation. Finally, π-electron delocalization of both quinone and alkali salts of p-semiquinone were determined using the Harmonic Oscillator model of Aromaticity (HOMA) or aromatic fluctuation index (FLU) calculations

Computational quantum chemistry in initial designs and final analyses

The broad utility of computational quantum chemical studies of molecular properties and their interactions is examined in a number of research projects covering molecular property prediction, chemical structure analyses, and the vibrational spectroscopy of molecular crystals. The broader scopes of the research serve to explore (1) the utility of theoretical studies for providing a greater understanding of molecules and their interactions, (2) the importance of considering the molecular environment in computational studies for understanding the properties of single molecules as they are currently considered experimentally, and (3) the importance of the selection of appropriate levels of theory for addressing specific questions raised from experimental results given the ever-present need to balance chemical accuracy and computational resources. Five research projects are presented in this work, including (1) the study of molecular crystals by periodic density functional theory and inelastic neutron scattering spectroscopy, (2) the electronic coupling behavior of the phenyl- and tropyl-substituted closo -boranes and closo -carboranes, (3) the design of a new class of borane- and carborane-based nonlinear optical materials through purely theoretical means as an example of the rational design of any class of molecules through a coupling of theory and structure-property relationships, (4) a computational study of the isomer energies of the eight known octamolybdates clusters and rationalizations for their rearrangement pathways as identified in their crystal forms, and (5) a computational study of binding interactions and structural features among the alkali organometallic complexes

The low/room-temperature forms of the lithiated salt of 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone: a combined experimental and dispersion-corrected density functional study

International audienceFollowing our first experimental and computational study of the room temperature (RT) form of the tetrahydrated 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone dilithium salt (Li2DHDMQ?4H2O), we have researched the occurrence of hydrogen ordering in a new polymorph at lower temperature. The study of polymorphism for the Li2DHDMQ?4H2O phase employs both experimental (single crystal X-ray diffraction) and theoretical approaches. While clues for disorder over one bridging water molecule were observed at RT (b form), a fully ordered model within a supercell has been evidenced at 100 K (a form) and is discussed in conjunction with the features characterizing the first polymorphic form reported previously. Density functional theory (DFT) calculations augmented with an empirical dispersion correction (DFT-D) were applied for the prediction of the structural and chemical bonding properties of the a and b polymorphs of Li2DHDMQ?4H2O. The relative stability of the two polymorphic systems is evidenced. An insight into the interplay of hydrogen bonding, electrostatic and van der Waals (vdW) interactions in affecting the properties of the two polymorphs is gained. This study also shows how information from DFT-D calculations can be used to augment the information from the experimental crystal diffraction data and can so play an active role in crystal structure determination, especially by increasing the reliability and accuracy of H-positioning. These more accurate hydrogen coordinates allowed for a quantification of H-bonding strength through a topological analysis of the electron density (atoms-in-molecules theory)

Commensurate Urea Inclusion Crystals with the Guest (<i>E,E</i>)‑1,4-Diiodo-1,3-Butadiene

The urea inclusion compound (UIC) with (<i>E,E</i>)-1,4-diiodo-1,3-butadiene (DIBD) as a guest (DIBD:UIC) has been prepared and crystallographically characterized at 90 and 298 K as a rare example of a commensurate, fully ordered UIC. The crystal shows nearly hexagonal channels in the monoclinic space group <i>P</i>2₁/<i>n</i>. The DIBD guest molecules are arranged end-to-end with the nonbonding iodine atoms in van der Waals contact. The guest structure is compared with that for DIBD at 90 K and with computations for the periodic UIC and isolated DIBD molecule