بررسي نظري و تجربي ترموديناميك واكنش تشكيل ديمر با مرز پروتون در دستگاه طيف سنج تحرك يوني

چكيده انگليسي :

Thermodynamics Study of Proton bound dimers Formation Reaction inIon Mobility Spectrometry Theoretical and Experimental Zahra Izadi Z Izadinajafabadi@ch iut ac ir October 30 2010 Department of Chemistry Isfahan University of Technology Isfahan 84156 83111 IranDegree M S Language FarsiM Tabrizchi Prof m tabriz@cc iut ac ir H Farrokhpour Assist Prof farrokhphossein@gmail com Abstract Proton bonding is responsible for many important phenomenon in physics chemistry and biology Simple proton bound dimers of the type AH A are often used as model systems to investigate the nature ofintermolecular interaction such as proton bonding In this work an ion mobility spectrometer constructed inour laboratory was used to experimentally study the thermodynamic of proton bond dimer formationMH M MHM at atmospheric pressure The reaction is considered as one of the preliminary steps indissolving protonated molecules In addition ab initio calculation was performed to back the experimentalresults First the geometry of the reactant and products for methyl isobutyl keton MIBK was optimizedusing Gaussian 98 Then the enthalpies and Gibbs free energies were calculated using Hartree Fock methodwith 6 311 G basis set The calculation was then carried out using B3LYP method to obtain moreaccurate results The calculation yielded a 28 5 kcal mol for enthalpy change of reaction In the second part the enthalpy change for reaction was determined by measuring the equilibrium constant at differenttemperatures using Van t Hoff plot To establish equilibrium in the ionization region the sample flow ratewas increased until the reaction quotient Q was independent of the flow rate The relative abundances of themonomer and dimer were obtained from the intensity of their corresponding peaks in the mobility spectrumto be used in calculation of the equilibrium constant K MHM MH M The Van t Hoff plot resulted inabout 5kcal mol for H which is far from the theoretical value of 25kcal mol The reason for such bigdifference was attributed to hydration of ions in atmospheric pressure In fact a series of multiequilibriumreactions rather than a simple reaction happens in IMS A model was proposed to solve this complicatedsystem Based on the results obtained from the calculation a mixture of protonated monomers MH H2O nwith different n s was considered to be in equilibrium with unhydrated dimer MHM with equilibriumconstant of Kn Hence a reduced equilibrium constant of K as KRed 1 pn Kn was defined for the wholereaction as well as a reduced enthalpy change of H Red Yn H n where Yn pn Kn pn Kn and p is thepartial pressure of water Y is the portion of each reaction n in the whole reaction which depends on thewater vapor pressure and temperature The reduced enthalpy is defined as the enthalpy change for formationof one mole dimer from any root Since Hn s have different values and Y is temperature dependent thereduced enthalpy will be strongly affected by temperature so that it may even change its sign to positive atlow temperatures Key WordsProton bound dimer ab initio calculation Ion Mobility Spectrometry multiequilibriumreactions Van t Hoff plot

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بررسي نظري و تجربي ترموديناميك واكنش تشكيل ديمر با مرز پروتون در دستگاه طيف سنج تحرك يوني

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