Isotropic and anisotropic NMR chemical shifts in liquid water: a sequential QM/MM study (original) (raw)
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The Journal of Physical Chemistry B, 2007
The temperature dependencies of NMR shifts in the critical region of two coexisting phases have been simulated using statistical thermodynamics and graph-theory consideration of equilibrium processes of molecular association. Microparameters of magnetic screening of various water and water/pyridine structures used in the statistical averaging have been evaluated by density functional theory calculations (PBE1PBE and B3PW91 functionals in the 6-311++G** basis set). The gauge-including atomic orbital (GIAO) approach has been applied to ensure gauge invariance of the results. Solvent effects were taken into account by a polarized continuum model (PCM). NMR shifts "order parameters" (∆δ) |δ +δ-|) and "diameters" (φδ) |(δ + + δ-)/2δ C |, where δ + , δ-, and δ C are the chemical shifts of coexisting phases and at the critical point respectively) have been calculated in each case close to the lower critical solution point (T L) and processed using linear regression analysis of ∆δ ∼ |T-T L | and φδ ∼ |T-T L | in the log-log plot. It has been shown that the critical index can be evaluated with high precision from the slope of ∆δ) f(T-T L) at any realistic set of model input parameters. The slope of diameter has been found to depend on both input and R values. The obtained φδ slopes (0.58-0.63) are very close to 2 values. The results are discussed within the concept of complete scaling. Results of simulation are compared and supported by experimental NMR data for water/2,6-lutidine, acetic anhydride/n-heptane, and acetic anhydride/cyclohexane systems.
Recent progress in understanding chemical shifts
Solid State Nuclear Magnetic Resonance, 1996
In the past three or four years computer hardware and software developments have reached the stage where the nuclear magnetic resonance (NMR) spectra of many molecular systems can now be accurately evaluated. Detailed analysis of chemical shifts may soon become a routine part of solid (and liquid) state NMR structure prediction in chemistry and biology, and this Article covers the development of the topic from its earliest beginnings.
The Journal of Physical Chemistry B, 1997
The gas-to-liquid chemical shifts of water have been calculated by combining molecular dynamics simulations and quantum chemically derived shielding polarizabilities. The use of a force field based on intermolecular perturbation theory ensures that the electric fields are adequately modeled. The experimental proton shift and its temperature dependence are reproduced, but the oxygen shift lacks higher order terms such as the linear field-gradient contribution. Shifts arising from the difference in the gas phase and liquid geometries of the water molecule have been estimated and discussed.
The Journal of Physical Chemistry, 1993
The maximum entropy (ME) and additive potential (AP) methods of determining the angular distribution functions, p (o , x) , from the partially-averaged dipolar couplings obtained from the NMR spectra of liquid crystalline samples are compared. Here o represents the orientation of the mesophase director in a molecular frame, and x represents bond rotational motion. It is emphasized that these two methods are fundamentally different. Thus, the model-independent ME analysis can determine only p~c (w , x) and p~c (x) , which are dependent on the potential of mean torque, Ucxt(o,x), and the subscript LC denotes that these distributions are for the liquid-crystalline phase. The AP method, which is model-dependent, can also determine p~c (o , x) and p~c (x) but in addition yields pi&), the distribution of the internal angular coordinate in an isotropic phase, which depends on Snt(x), an effective, mean conformational energy. The advantages of applying both the ME and AP methods to analyzing the same set of dipolar couplings is illustrated by the case of 4-nitro-l-(@,@,@trifluoroethoxy)benzene dissolved in two nematic solvents. The ME analysis reveals that motion about the r i n g 4 and O-C(H2) bonds is cooperative, which was then used to guide the choice for the form of &(x) in the treatment of the same data by the AP method.
Journal of Chemical Theory and Computation, 2010
We present here a method that can calculate NMR shielding tensors from first principles for systems with translational invariance. Our approach is based on Kohn-Sham density functional theory and gauge-including atomic orbitals. Our scheme determines the shielding tensor as the second derivative of the total electronic energy with respect to an external magnetic field and a nuclear magnetic moment. The induced current density due to a periodic perturbation from nuclear magnetic moments is obtained through numerical differentiation, whereas the influence of the responding perturbation in terms of the external magnetic field is evaluated analytically. The method is implemented into the periodic program BAND. It employs a Bloch basis set made up of Slater-type or numeric atomic orbitals and represents the Kohn-Sham potential fully without the use of effective core potentials. Results from calculations of NMR shielding constants based on the present approach are presented for isolated molecules as well as systems with one-, two-and three-dimensional periodicity. The reported values are compared to experiment and results from calculations on cluster models.
The Journal of Organic Chemistry
NMR chemical shifts have been experimentally measured and theoretically estimated for all the carbon atoms of (1R,3S,4S,8S)-p-menthane-3,9-diol in chloroform solution. Theoretical estimations were performed using a combination of molecular dynamics simulations and quantum mechanical calculations. Molecular dynamics simulations were used to obtain the most populated conformations of the (1R,3S:4S,8S)-p-menthane-3,9-diol as well as the distribution of the solvent molecules around it. Quantum mechanical calculations of NMR chemical shifts were performed on the most relevant conformations employing the GIAO-DFT formalism. A special emphasis was put in evaluating the effects of the surrounding solvent molecules. For this purpose, supermolecule calculations were performed on complexes constituted by the solute and n chloroform molecules, where n ranges from 3 to 16. An excellent agreement with experimental data has been obtained following this computational strategy.
Solid State Nuclear Magnetic Resonance, 1998
This paper presents results from applying different point charge models to take into account intermolecular interactions to model the solid state effects on the 15 N NMR chemical shifts tensors. The DFT approach with the BLYP gradient corrected exchange correlation functional has been used because it can include electron correlation effects at a reasonable cost and is able to reproduce 15 N NMR chemical shifts with reasonable accuracy. The results obtained with the point charge models are compared with the experimental data and with results obtained using the cluster model, which includes explicitly neighboring molecular fragments. The results show that the point charge models can take into account solid state effects at a cost much lower than the cluster methods. q 1998 Elsevier Science B.V.