A computational study of isomeric [2.2]cyclophanes, namely [2.2]paracyclophane 1, [2.2]metacyclophane 2, and [2.2]metaparacyclophane 3, has been carried out. For 1, geometry optimizations performed by various methods at different basis sets showed that MP2/6-31+G(d,p) and B3PW91/6-31+G(d,p) provide the best results in comparison to the X-ray data. Compound 1 has D(2) symmetry with distorted bridges. A conformational search was performed for [2.2]cyclophanes 2 and 3. Each cyclophane exists in two conformations which have different energies in the case of 3 but are degenerate in the case of 2. Relative energies and strain energies at the bridges follow the same order, indicating that the relief of bridge tension and repulsion between pi clouds are determining factors for the stability of [2.2]cyclophanes. Through a decomposition of strain energy, it can be concluded that both the rings or the bridges can absorb strain, but it depends on the conformer of butane that is considered in the calculation of SE(br). Changes in aromaticity of these compounds were evaluated by NICS and HOMA and were compared with benzene and xylenes dimers as models. Despite distortions from planarity and shortening and lengthening of the C-C bonds relative to the mean, the phenyl rings are aromatic. NICS suggests a concentration of electronic density between the rings as a result of bridging process. Computed MK, NPA, and GAPT charges were compared for the isomeric cyclophanes. The GIAO chemical shifts were calculated and indicate that 1 has a larger diamagnetic anisotropy than the other isomers.

Tribocharged polymers display macroscopically patterned positive and negative domains, verifying the fractal geometry of electrostatic mosaics previously detected by electric probe microscopy. Excess charge on contacting polyethylene (PE) and polytetrafluoroethylene (PTFE) follows the triboelectric series but with one caveat: net charge is the arithmetic sum of patterned positive and negative charges, as opposed to the usual assumption of uniform but opposite signal charging on each surface. Extraction with n-hexane preferentially removes positive charges from PTFE, while 1,1-difluoroethane and ethanol largely remove both positive and negative charges. Using suitable analytical techniques (electron energy-loss spectral imaging, infrared microspectrophotometry and carbonization/colorimetry) and theoretical calculations, the positive species were identified as hydrocarbocations and the negative species were identified as fluorocarbanions. A comprehensive model is presented for PTFE tribocharging with PE: mechanochemical chain homolytic rupture is followed by electron transfer from hydrocarbon free radicals to the more electronegative fluorocarbon radicals. Polymer ions self-assemble according to Flory-Huggins theory, thus forming the experimentally observed macroscopic patterns. These results show that tribocharging can only be understood by considering the complex chemical events triggered by mechanical action, coupled to well-established physicochemical concepts. Patterned polymers can be cut and mounted to make macroscopic electrets and multipoles.

An analysis of the electron density, obtained by B3PW91/6-31+G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) for [2,2]cyclophanes isomers, [2.2]paracyclophane, anti-[2.2]metacyclophane, syn-[2.2]metacyclophane, and [2.2]metaparacyclophane, was made through natural bond orbitals (NBO), natural steric analysis (NSA), and atoms in molecules (AIM) methods and through analysis of frontier molecular orbitals (MOs). NBO indicates that all compounds present through-bond interactions, but only the conformers of [2.2]metacyclophane present significant through-space interactions. The last interactions are observed in AIM analysis and by the plots of MOs. AIM indicates that these through-space interactions are closed-shell ones, and they stabilize the conformers. In contrast, all isomers present through-bond and through-space repulsive interactions. In addition, the atomic properties, computed over the atomic basins, showed that the position of the bridges and the relative displacement of the rings can affect the atomic charges, the first atomic moments, and the atomic volumes.

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Diketopiperazines (DKPs) corresponding to cyclic dipeptides have been reported to exhibit antimicrobial, antitumor, antimutagenic and antiviral properties. These compounds are commonly isolated from microorganisms and sponges and from a variety of tissues and body fluids. In this work, we used electrospray ionization tandem mass spectrometry (ESI-MS/MS) to investigate the fragmentation of a series of DKPs previously isolated from Aspergillus fumigatus, which exhibit the same structural core. Loss of CO directly from the protonated molecule was found to be a fragmentation process common to all the compounds analyzed. However, our results revealed a series of ions that are diagnostic for the substituents at C(4) and C(9). In order to rationalize the differences in the fragmentation pathways of substituted and nonsubstituted DKPs, the relative Gibbs energies (DeltaG) of the product ions and intermediate ions were estimated using the B3LYP/6-31 + + G(d,p) model. The data reported here can be used for the structural elucidation of DKPs from low sample amounts, as an alternative to NMR.

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Electrospray ionization mass spectrometric analysis of lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone) was accomplished in order to elucidate the gas-phase dissociation reactions of this important biologically active natural product. The occurrence of protonated and cationized species in the positive mode and of deprotonated species in the negative mode was explored by means of collision-induced dissociation (CID) experiments. For the protonated molecule, the H(2)O and C(4)H(8) losses occur by two competitive channels. For the deprotonated molecule, the even-electron rule is not conserved, and the radicalar species are eliminated by formation of distonic anions. The fragmentation mechanism for each ion was suggested on the basis of computational thermochemistry. Atomic charges, relative energies, and frontier orbitals were employed aiming at a better understanding of the gas-phase reactivity of lapachol. Potential energy surfaces for fragmentation reactions were obtained by the B3LYP/6-31+G(d,p) model.