Isotropic and anisotropic NMR chemical shifts in liquid water: a sequential QM/MM study

Abstract

Apresentamos um estudo QM/MM seqüencial dos deslocamentos químicos gás-líquido da água. Cálculos de química quântica extensivos, usando a teoria do funcional da densidade foram realizados para estruturas do líquido de água, geradas através de simulações de Monte Carlo e Dinâmica Molecular. A dependência do deslocamento químico com os potenciais empíricos utilizados nas simulações, com o tamanho do aglomerado e com o funcional escolhido para os cálculos quânticos foi analisada. Os resultados corrigidos devido ao erro de superposição de base estão em boa concordância com os resultados experimentais, mostrando que um potencial empírico simples associado a um funcional apropriado é capaz de descrever os deslocamentos químicos. Todos os resultados apresentados são estatisticamente convergidos.We present a sequential QM/MM study of the gas-liquid chemical shifts of water. Extensive quantum chemical calculations using density functional theory have been performed for structures of liquid water generated by Monte Carlo and Molecular Dynamic simulations. The dependence of the chemical shifts on the empirical potential used in the simulations, on the cluster size and on the functional chosen for the quantum chemical calculations were analyzed. The results after correcting for basis set superposition errors are in good agreement with the experimental data, showing that a simple empirical potential associated to an appropriate functional is able to describe the chemical shifts. All results presented here are statistically converged. Keywords: sequential QM/MM, water, NMR, chemical shift, DFT IntroductionCalculations of the chemical shielding can reproduce with accuracy the chemical environment of the nuclei in a molecule. The advances in computational resources and the improvement in the theoretical methods provide important refinements to estimate the chemical shielding, such as inclusion of the electron correlation effects and the possibility of studying systems that are more complex than isolated molecules or small clusters. Presently the determination of the chemical shifts for nuclei in proteins, solids and liquids through current methodologies 1-5 are very convincing.An important category of systems whose electronic structure can be investigated through NMR calculations are solvated systems. A solvated molecule experiences the effect of the interaction with its vicinity, such as in hydrogen bonds. 6,7 Several theoretical models were proposed to describe the solvent effects on chemical shielding. [8][9][10][11][12][13][14] The main branches are the self-consistent reaction field methods (SCRF) 15,16 where the solute is placed inside a hollow cavity in a polarizable medium represented by its dielectric constant. In some cases, explicit solvent molecules, forming minimum energy clusters are considered in order to mimic the solvation shells of the solute. 10,17 It was shown that the former method does not work in determining the chemical shift and the latter, though it can serve as a rough approximation, does not serve as...

DIADORIM: a Monte Carlo Program for liquid simulations including quantum mechanics and molecular mechanics (QM/MM) facilities: applications to liquid ethanol

“…Therefore, the development of methodologies to incorporate quantum mechanics corrections in molecular simulation is a subject that has been pursued by many research groups. [9][10][11][12][13][14][15][16][17][18] Nevertheless, despite the extraordinary achievements in computer power computational cost is still a limitation against the use of full ab initio quantum mechanical methods to study large system. To accomplish this need, semiempirical methodologies have been developed to obtain quantum mechanical information of large systems at a feasible computational cost.…”

“…[13][14][15] In some methodologies the quantummotif is not included directly in the trajectory production and the QM calculations are performed a posteriori using molecular arrangements chosen from the classical force field trajectory. 16 Due to the development of quantum chemical methods, different approaches have been used to describe the quantum-motif. [17][18][19] Therefore, differently conceived quantum chemistry methodologies can be merged to build new computer simulation programs aimed to treat large chemical systems in its complexity.…”

The isotropic nuclear magnetic shielding constants of acetone in supercritical water: A sequential Monte Carlo/quantum mechanics study including solute polarization

“…After verifying that they give similar results, we adopt only the B3LYP hybrid functional with the 6-311+ + G͑2d ,2p͒ Gaussian basis set that has shown to be a good theoretical model for the calculation of chemical shifts. 25 As the chemical shifts from normal to SCW might be of the same order as the errors involved in DFT calculations, it is important that the same theoretical level be used in the two thermodynamic conditions. All calculations of the shielding constants have been made using gauge-including atomic orbitals 62,69 ͑GIAO͒ as implemented in the GAUSSIAN 03 package.…”

“…Among the molecular properties, the nuclear shielding constants are very sensitive to changes in the environment and the solvent shift of the shielding constants of a solute molecule constitutes a very useful parameter for probing local modifications of intermolecular interactions in solution. [19][20][21][22][23][24][25] The local magnetic field on the atomic nucleus generated by the response of the electronic structure to an external magnetic field is affected by the solvent. This effect is of great interest in normal fluids but only limited knowledge is available for SC fluids and SCW.…”

“…Finally, we verify that all values reported here are statistically converged. For the QM calculation of the shielding constants, we use density functional theory ͑DFT͒ that offers a good compromise 25,50 between computational cost and accuracy for molecular properties in condensed phase.…”