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RISS 인기검색어
- 검색키워드
- 저자 : Gaigeot, Marie-Pierre (검색결과 2 건)
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Gaigeot, Marie-Pierre,Cimas, Alvaro,Seydou, Mahamadou,Kim, Ju-Young,Lee, Sungyul,Schermann, Jean-Pierre American Chemical Society 2010 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.132 No.51
<P>The role of water in the structural change of nicotine from its inactive form in the gas phase to its bioactive form in aqueous solution has been investigated by two complementary theoretical approaches, i.e., geometry optimizations and molecular dynamics. Structures of the lowest-energy nicotineH(+)-(H(2)O)(n) complexes protonated either on the pyridine (inactive form) or pyrrolidine (active form) ring have been calculated, as well as the free-energy barriers for the proton-transfer tautomerization between the two cycles. These structures show chains of 2-4 water molecules bridging the two protonation sites. The room-temperature free-energy barrier to tautomerization along the minimum-energy path from the pyridine to the pyrrolidine cycle drops rapidly when the number of water molecules increases from 0 to 4, but still remains rather high (16 kJ/mol with four water molecules), indicating that the proton transfer is a rather difficult and rare event. We compare results obtained through this explicit water molecule approach to those obtained by means of continuum methods. Car-Parrinello molecular dynamics (CPMD) simulations of the proton-transfer process in bulk with explicit water molecules have been conducted at room temperature. No spontaneous proton transfers have been observed during the dynamics, and biased CPMD simulations have therefore been performed in order to measure the free-energy profile of the proton transfer in the aqueous phase and to reveal the proton-transfer mechanism through water bridges. The MD bias involves pulling the proton from the pyridine ring to the surrounding bulk. Dynamics show that this triggers the tautomerization toward the pyrrolidine ring, proceeding without energy barrier. The proton transfer is extremely fast, and protonation of the pyrrolidine ring was achieved within 0.5 ps. CPMD simulations confirmed the pivotal role played by the water molecules that bridge the two protonation sites of nicotine within the bulk of the surrounding water.</P>
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Spezia, Riccardo,Cimas, Alvaro,Gaigeot, Marie-Pierre,Salpin, Jean-Yves,Song, Kihyung,Hase, William L. The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.33
<P>In this paper we report different theoretical approaches to study the gas-phase unimolecular dissociation of the doubly-charged cation [Ca(urea)]<SUP>2+</SUP>, in order to rationalize recent experimental findings. Quantum mechanical plus molecular mechanical (QM/MM) direct chemical dynamics simulations were used to investigate collision induced dissociation (CID) and rotational–vibrational energy transfer for Ar + [Ca(urea)]<SUP>2+</SUP> collisions. For the picosecond time-domain of the simulations, both neutral loss and Coulomb explosion reactions were found and the differences in their mechanisms elucidated. The loss of neutral urea subsequent to collision with Ar occurs <I>via</I> a shattering mechanism, while the formation of two singly-charged cations follows statistical (or almost statistical) dynamics. Vibrational–rotational energy transfer efficiencies obtained for trajectories that do not dissociate during the trajectory integration were used in conjunction with RRKM rate constants to approximate dissociation pathways assuming complete intramolecular vibrational energy redistribution (IVR) and statistical dynamics. This statistical limit predicts, as expected, that at long time the most stable species on the potential energy surface (PES) dominate. These results, coupled with experimental CID from which both neutral loss and Coulomb explosion products were obtained, show that the gas phase dissociation of this ion occurs by multiple mechanisms leading to different products and that reactivity on the complicated PES is dynamically complex.</P> <P>Graphic Abstract</P><P>Chemical dynamics and statistical analysis differentiate reaction pathway kinetics between neutral loss and Coulomb explosion in unimolecular [Ca(urea)]<SUP>2+</SUP> dissociation. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp41379e'> </P>
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