A fluorescent cage receptor for the detection of choline in pseudophysiological medium is described. Not only does this capsule complex choline with an association constant greater than 9.9 × 104 M–1 ...in buffered medium but it is also selective toward acetylcholine.
The structures, energies, and rearrangements of imidoylnitrenes H–C(NH)–N, H2N–C(NH)–N, Ph–C(NH)–N, H–C(NPh)–N, and MeO–C(NCN)–N (10a–e) are investigated at DFT and CASPT2 levels of theory. ...Imidoylnitrenes are potentially formed by pyrolysis or photolysis of azides, tetrazoles (6, 6′), or sydnones. Unlike most acylnitrenes, the imidoylnitrenes 10 have triplet ground states. The first excited states are the open-shell singlets (OSSs), lying between ca. 4 and 20 kcal mol–1 above the triplets at the CASPT2 level. The second excited states are the closed-shell singlets (CSSs), lying >50 kcal mol–1 higher in energy. The OSS imidoylnitrenes can ring-close to 1H-diazirines 9 with very low activation energies (2–12 kcal mol–1), and the 1H-diazirines can then rearrange to nitrile imines 8 with activation energies of 37–48 kcal mol–1. Conversely, nitrile imines generated directly by pyrolysis or photolysis of 2,5-substituted tetrazoles 6 can rearrange to 1H-diazirines 9 and imidoylnitrenes 10 with activation energies of 37–60 kcal mol–1. Finally, the imidoylnitrenes 10 can rearrange to carbodiimides 11 with modest activation barriers of 12–20 kcal mol–1. Calculated vibrational data, UV–vis spectra, and spin densities in the triplet states are also reported, and zero-field splitting parameters |D/hc| in the range 0.9–1 cm–1 and nonzero |E/hc| values are predicted.
Chemical reactions involving quantum mechanical tunneling (QMT) increasingly attract the attention of scientists. In contrast to the hydrogen‐tunneling as frequently observed in chemistry and ...biology, tunneling solely by heavy atoms is rare. Herein, we report heavy‐atom tunneling in trifluoroacetyl nitrene, CF3C(O)N. The carbonyl nitrene CF3C(O)N in the triplet ground state was generated in cryogenic matrices by laser (193 or 266 nm) photolysis of CF3C(O)N3 and characterized by IR and EPR spectroscopy. In contrast to the theoretically predicted activation barriers (>10 kcal mol−1), CF3C(O)N undergoes rapid rearrangement into CF3NCO with half‐life times of less than 10 min and unprecedentedly large 14N/15N kinetic isotope effects (1.18–1.33) in solid Ar, Ne, and N2 matrices even at 2.8 K. The tunneling disappearance of CF3C(O)N becomes much slower in the chemically active toluene and in 2‐methyltetrahydrofuran at 5 K.
Tunnel vision: Trifluoroacetyl nitrene (CF3C(O)N) in the triplet ground state was generated and characterized. In contrast to the theoretically predicted activation barrier of more than 10 kcal mol−1, the nitrene undergoes a fast 1,2‐CF3 shift into singlet trifluoromethyl isocyanate (CF3NCO) through heavy‐atom tunneling in cryogenic matrices (2.8–23.0 K).
In this paper the aggregation of asphaltenes is studied for two asphaltene molecule families, namely, PA3 and CA22 analogues, on the basis of the work of Schuler et al. (J. Am. Chem. Soc., 2015, 137, ...9870). The chemical characteristics of these molecules were screened by changing the heteroatoms on the backbone and the lateral chain-ends. These molecules were mixed together with different relative concentrations, and for the first time, the aggregation of different asphaltenes was determined using molecular dynamics simulations (MDS). The results show that the interaction energies vary for different heteroatom arrangement within a given structure and depend on the type of asphaltene. Moreover, we showed that the chain-ends have a crucial role on this phenomenon.
We present molecular dynamics simulations (MDS) for interpreting the molecular aggregation of four different asphaltene molecular models. These simulations are based on recent small-angle X-ray ...scattering (SAXS) and small-angle neutron scattering (SANS) experiments from Eyssautier and co-workers Eyssautier J. ; Levitz P. ; Espinat D. ; Jestin J. ; Gummel J. ; Grillo I. ; Barre L. Insight into asphaltene nanoaggregate structure inferred by small angle neutron and X-ray scattering. J. Phys. Chem. B 2011, 115 (21), 6827−6837, 10.1021/jp111468d, which proposed a discoidal asphaltene nanoaggregate structure of 0.67 nm height and its chemical composition. Basically, we have investigated the sulfur atom position in the asphaltene structure. π–π is the main type of contact, and it seems to be responsible for the parallel orientation of molecules. This geometry is also corroborated by the cosine of angles between the polyaromatic cores. However, the polar side chain is also important to the aggregation, because it occurs in the systems during all of the simulations. Three systems, namely, systems 1T, 2T, and 4T, are in good agreement with experimental work, indicating an average height lower than 0.8 nm. Concerning the place occupied by the sulfur atom in the molecule, we have found that, when it is grafted directly to the conjugated core, no effect can be deducted from the stability of dimers and trimers and all of the other studied parameters. This is not the case when sulfur is located in the lateral chain, where it has a particular affinity with oxygen and hydrogen atoms of the acid chain ends of neighboring molecules. Consequently, we suggest that these configurations of the sulfur atom are more likely to be problematic in the oil industry.
Fullerene derivatives have been ubiquitous as an electron-accepting material in organic photovoltaic solar cells (OSCs). We consider whether and why traces of PCBM oxidation products should be seen ...as electronic defects impairing the performance of OSCs. Thin PCBM deposits were first illuminated under ambient air for a few minutes, thus revealing the extraordinary easiness of oxidizing PCBM. The charge transfer in polymer:PCBMox bulk heterojunctions was then studied. As a result of a few minutes of PCBM photooxidation, the electron transfer from the polymer to two types of PCBMox species was shown to occur at the expense of the transfer to pristine PCBM. Such modifications to the molecular structure of PCBM and to the charge transfer at the nanoscale were finally correlated with a dramatic loss in the device’s photovoltaic performance at the macroscale. This study clearly indicates the need to integrate photooxidation-resistant electron-accepting materials into OSCs to extend their lifetime.
Direct Observation of Carbamoylnitrenes Li, Hongmin; Wan, Huabin; Wu, Zhuang ...
Chemistry : a European journal,
June 1, 2016, Letnik:
22, Številka:
23
Journal Article
Recenzirano
As the prototype Curtius rearrangement reaction, carbamoyl azide decomposes into aminoisocyanate and molecular nitrogen. However, the key intermediate carbamoylnitrene was previously undetected, even ...though the decomposition of carbamoyl azides has been studied frequently since its discovery in the 1890s. Upon ArF laser (λ=193 nm) photolysis, the stepwise decomposition of the two simplest carbamoyl azides H2NC(O)N3 and Me2NC(O)N3, isolated in solid noble gas matrices, occurs with the formation of the corresponding carbamoylnitrenes H2NC(O)N and Me2NC(O)N. Both triplet species are characterized for the first time by combining matrix‐isolation IR spectroscopy and quantum‐chemical calculations. Subsequent visible‐light irradiations cause efficient rearrangement of these nitrenes into the respective aminoisocyanates.
Rearrange it! Elusive carbamoylnitrenes, R2NC(O)N (R=H, Me), assumed since the 1890s to be key intermediates in the Curtius rearrangement of carbamoyl azides, have been generated and observed directly for the first time by IR spectroscopy following ArF laser matrix photolysis of the corresponding azides (see figure).
Large-scale introduction of Organic Solar Cells (OSCs) onto the market is currently limited by their poor stability in light and air, factors present in normal working conditions for these devices. ...Thus, great efforts have to be undertaken to understand the photodegradation mechanisms of their organic materials in order to find solutions that mitigate these effects. This study reports on the elucidation of the photodegradation mechanisms occurring in a low bandgap polymer, namely, Si-PCPDTBT (poly(4,4′-bis(2-ethylhexyl)dithieno3,2-b:2′,3′-dsilole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl). Complementary analytical techniques (AFM, HS-SPME-GC-MS, UV–vis and IR spectroscopy) have been employed to monitor the modification of the chemical structure of the polymer upon photooxidative aging and the subsequent consequences on its architecture and nanomechanical properties. Furthermore, these different characterization techniques have been combined with a theoretical approach based on quantum chemistry to elucidate the evolution of the polymer alkyl side chains and backbone throughout exposure. Si-PCPDTBT is shown to be more stable against photooxidation than the commonly studied p-type polymers P3HT and PCDTBT, while modeling demonstrated the benefits of using silicon as a bridging atom in terms of photostability.
The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar ...matrices at cryogenic temperatures (Ph-CNN-H 6, Ph-CNN-CH3 17, Ph-CNN-SiMe3 23, Ph-CNN-Ph 29, Ph3C-CNN-CPh3 34, and the boryl-CNN-boryl derivative 39). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm–1 have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe3 23, PhCNNPh 29, and boryl-CNN-boryl 39). Nitrile imines with IR absorptions below ca. 2200 cm–1 are more likely to be allenic (e.g., HCNNH 1, PhCNNH 6, HCNNPh 43, PhCNNCH3 17, and Ph3C-CNN-CPh3 34). All nitrile imines isomerize to the corresponding carbodiimides both thermally and photochemically. Monosubstituted carbodiimides isomerize thermally to the corresponding cyanamides (e.g., Ph-NCN-H 5 → Ph-NH-CN 8), which are therefore the thermal end products for nitrile imines of the types RCNNH and HCNNR. This tautomerization is reversible under flash vacuum thermolysis conditions.
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