Inquiring 199Hg NMR Parameters by Combining Ab Initio Molecular Dynamics and Relativistic NMR Calculations.

Ab initio molecular dynamics (AIMD) sampling followed by relativistic density functional theory (DFT) 199Hg NMR calculations were performed for Hg organometallic complexes in water, dimethyl sulfoxide, and chloroform. The spin-orbit coupling, a relativistic effect, is a key factor for predicting δ(Hg) and 1J(Hg-C) accurately, in conjunction with a dynamic treatment of the systems. Good agreement between the theoretical and experimental results is reached by adopting implicit (based on a continuum model) and explicit (solvent molecules treated quantum mechanically) solvation models. Broader trends appearing in the experimental data available in the literature are reproduced by the calculations, and therefore, quantum chemistry is able to assist in the assignment and interpretation of 199Hg NMR data. Less pronounced trends, such as changes in the 199Hg chemical shift in different systems with the same atom types bound to Hg, are too weak to be predicted reliably by the current state-of-the-art theoretical methods based on AIMD sampling and relativistic DFT with hybrid functionals for NMR calculations.

13 C NMR as an analytical tool for the detection of carbonic acid and pKa determination.

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. 13 C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO2(aq) and another one for the species H2 CO3 , HCO3 - , and CO3 2- -herein collectively named Hx CO3 x-2 . The chemical shift of this second peak depends on the molar fraction of the three species in equilibrium and has been used to assess the equilibrium between HCO3 - and CO3 2- . The detection of H2 CO3 at low pH solutions is hindered, because of the concurrent liberation of CO2 when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the Hx CO3 x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H2 CO3 . The method employed the formation of frozen layers of NaH13 CO3 and acid solutions within the tube, which are mixed as the tube reaches room temperature. At this point, the tube is already securely sealed, preventing any loss of CO2 to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H2 CO3 was determined as 160.33 ± 0.03 ppm, which is in good agreement with value obtained by DFT calculations combined with Car-Parrinello molecular dynamics. The H2 CO3 pKa value determined by the present method was 3.41 ± 0.03, for 15% D2 O aqueous medium and 0.8 mol/L ionic strength. The proposed method can be extended to studies about analogs such as alkyl carbonic and carbamic acids.

Revisiting NMR through-space J(FF) spin-spin coupling constants for getting insight into proximate F---F interactions.

At present times it is usual practice to mark biological compounds replacing an H for an F atom to study, by means of (19)F NMR spectroscopy, aspects such as binding sites and molecular folding features. This interesting methodology could nicely be improved if it is known how proximity interactions on the F atom affect its electronic structure as gauged through high-resolution (19)F NMR spectroscopy. This is the main aim of the present work and, to this end, differently substituted peri-difluoronaphthalenes are chosen as model systems. In such compounds are rationalized some interesting aspects of the diamagnetic and paramagnetic parts of the (19)F nuclear magnetic shielding tensor as well as the transmission mechanisms for the PSO and FC contributions to (4)JF1F8 indirect nuclear spin-spin coupling constants.

A study of the rotational barriers for some organic compounds using the G3 and G3CEP theories.

The G3, G3CEP, MP4, MP4CEP, QCISD(T), and QCISD(T)CEP methods were applied to study 43 internal rotational barriers of different molecules. The calculated G3 and G3CEP barriers were accurate with respect to those obtained experimentally, typically showing deviations of <0.50 kcal mol⁻¹. The results for the MP4CEP, MP4, QCISD(T), and QCISD(T)CEP calculations were less accurate, and larger deviations of approximately ±1 kcal mol⁻¹ were observed. The accuracy of G3CEP was comparable to that of G3, but a reduction in CPU time of between 5 and 35 % was observed when the dependence of the pseudopotentials on the size of the molecule and atom type was taken into account. The behaviors of the energy components show that these corrections depend on the molecular environment and whether the calculations are performed with all electrons or pseudopotentials. Usually, the predominance of a specific effect follows a distinct pattern when the G3 and G3CEP results are compared. For the G3 calculations, the most important component of the corrected MP4/6-31G(d) rotational energy is ΔE(2df,p). Among the 43 molecules, 29 were dependent on polarization effects, ΔE(2df,p); 19 were dependent on diffuse functions, ΔE +; and 13 depended on the effects of more elaborate basis functions (ΔE(G3large)). Similar behavior was observed for the G3CEP calculations: polarization effects were more important for 25 molecules, followed closely by the effect of diffuse functions for 23 molecules, and finally the effect of large basis sets (19 molecules). ΔE(QCI) correction seldom resulted in significant effects on the G3 and G3CEP calculations.