Molecular Dynamics Simulation Of Translational And Rotational Diffusion Of Liquid Isoquinoline
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Date
2010-12
Authors
Ahmad, Norariza
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Journal ISSN
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Publisher
Universiti Sains Malaysia
Abstract
Molecular dynamics (MD) simulations of isoquinoline in liquid phase were performed at several temperatures in the range of 300 to 365 K in order to investigate the evolution of translational and rotational diffusions with temperature. The intermolecular interactions are modeled using Coulombic plus Lennard-Jones potentials. The Lennard-Jones parameters are taken from the Optimized Potential of Liquid Simulation – All Atoms (OPLS-AA) force field. The computed vaporization enthalpies vary from 16.01 to 14.86 kcal.mol-1 from 300 to 365 K and are within 10.5-11.5 % of the experimental studies. Translational (Dtrans) and rotational (Drot) coefficients were computed using velocity (vacf) and angular velocity autocorrelation functions (avacf), respectively. The calculated Dtrans from vacf was found to be larger, 0.0061 to 0.0472 Ǻ2.ps-1, than the values calculated from mean square displacement (MSD), 0.0066 to 0.0500 Ǻ2.ps-1 with the larger difference observed at lower temperature. The rotational diffusion coefficients in the x, y and z axes computed as a function of temperature exhibited quite different behaviours. Drot,z/Drot,y only changed from 2.23 to 2.32. Between 300 and 365 K a larger ratio was observed for Drot,z/Drot,x and the value varies from 1.20 to 1.43. Reorientational correlation times, τ1 and τ2, of each C-H vector were obtained from fitting the correlation function to a biexponential function. The NMR longitudinal relaxation times T1 of 13C were determined from τ2, giving the activation energy for higher and lower temperatures equal to 5.62 and 7.32 kcal.mol-1 respectively for the four CH
vectors parallel to one inplane-inertia axis. The Ea are 5.69 to 7.17 kcal.mol-1, respectively for the three other CH vectors. The investigations of the dynamic properties of translational and reorientational motion as well as T1 relaxation times yielded two Arrhenius plots which cross at about 320 to 325 K. This observation was understood to be unrelated to the structural transition, but linked with the natural temperature dependence of the dynamical properties of the systems.
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Keywords
Molecular dynamics simulation of translational , rotational diffusion of liquid isoquinoline