Browsing by Subject "HYDROGEN-BOND"

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  • Luecke, Ana-Luiza; Wiechmann, Sascha; Freese, Tyll; Nieger, Martin; Foeldes, Tamas; Papai, Imre; Gjikaj, Mimoza; Adam, Arnold; Schmidt, Andreas (2018)
    Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subsequent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids. Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in the absence and presence of water. (C) 2018 Elsevier Ltd. All rights reserved.
  • Lattouf, Elie; Anttalainen, Osmo Antero; Kotiaho, Tapio; Hakulinen, Hanna Idamaria; Vanninen, Paula; Eiceman, Gary Alan (2021)
    Gas phase reactions between hydrated protons H+(H2O)(n) and a substance M, as seen in atmospheric pressure chemical ionization (APCI) with mass spectrometry (MS) and ion mobility spectrometry (IMS), were modeled computationally using initial amounts of [M] and [H+(H2O)(n)], rate constants k(1) to form protonated monomer (MH+(H2O)(x)) and k(2) to form proton bound dimer (M2H+(H2O)(z)), and diffusion constants. At 1 x 10(10) cm(-3) (0.4 ppb) for [H+(H2O)(n)] and vapor concentrations for M from 10 ppb to 10 ppm, a maximum signal was reached at 4.5 mu s to 4.6 ms for MH+(H2O)(x) and 7.8 mu s to 46 ms for M2H+(H2O)(z). Maximum yield for protonated monomer for a reaction time of 1 ms was similar to 40% for k(1) from 10(-11) to 10(-8) cm(3).s(-1), for k(2)/k(1) = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M2H+(H2O)(z)] to [MH+(H2O)(x)] using excessive levels of [H+(H2O)(n)], even for [M] > 10 ppb, as commonly found in APCI MS and IMS measurements. Ion losses by collisions on surfaces were insignificant with losses of
  • Ren, Hao; Qiu, Xing-Ping; Shi, Yan; Yang, Peng; Winnik, Francoise M. (2019)
    A series of azopyridine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (similar to 5000 <M-w <20 000 g mol(-1), polydispersity index 1.25 or less) were prepared by reversible addition fragmentation chain-transfer polymerization of NIPAM in the presence of a chain-transfer agent that contains an AzPy group and an n-dodecyl chain. In cold water, the polymers form nanoparticles (5.9 nm <R-h <10.9 nm) that were characterized by light scattering (LS), H-1 NMR diffusion experiments, and high-resolution transmission electron microscopy. We monitored the pH-dependent photoisomerization of C12-PN-AzPy nanoparticles by steady-state and time-resolved UV-vis absorption spectroscopy. Azopyridine is known to undergo a very fast cis-to-trans thermal relaxation when the azopyridine nitrogen is quaternized or bound to a hydrogen bond donor. The cis-to-trans thermal relaxation of the AzPy chromophore in an acidic nanoparticle suspension is very fast with a half-life tau = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (tau = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (tau > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, H-1 NMR spectroscopy, and LS analysis, suggest that in acidic medium, the nanoparticles consist of a core of assembled C12 chains surrounded by a shell of hydrated PNIPAM chains with the AzPy(+) end groups preferentially located near the particle/water interface. In neutral medium, the shell surrounding the core contains AzPy groups H-bonded to the amide hydrogen of the PNIPAM chain repeat units. At pH 10.0, the amide hydrogen binds preferentially to the hydroxide anions. The AzPy groups reside preferentially in the vicinity of the C12 core of the nanoparticles. The morphology of the nanoparticles results from the competition between the segregation of the hydrophobic and hydrophilic components and weak attractive interactions, such as H-bonds between the AzPy groups and the amide hydrogen of the PNIPAM repeat units.
  • Ryding, Mauritz J.; Giuliani, Alexandre; Patanen, Minna; Niskanen, Johannes; Simoes, Grazieli; Miller, Glenn B. S.; Antonsson, Egill; Jokinen, Tuija; Miron, Catalin; Björneholm, Olle; Hansen, Klavs; Borve, Knut J.; Uggerud, Einar (2014)