Berry pseudorotation mechanism for the interpretation of the 19F NMR spectrum in PF5 by ab initio molecular dynamics simulations.

For the first time, theoretical evidence that confirms the importance of the Berry pseudorotation process in the interpretation of the 19F NMR spectrum of phosphorus pentafluoride (PF5) is presented. Ab initio molecular dynamics simulations have been performed to generate a large number of configurations used for NMR parameter computations at the density functional theory level. Two different temperatures were set to highlight the effect of pseudorotation process on the NMR spectrum. Average 19F chemical shifts and spin-spin coupling constants calculated for the five fluorine atoms converge towards the NMR equivalence of the five atoms when the Berry pseudorotation mechanism is accounted for.     

Computing the (1)H NMR spectrum of a bulk ionic liquid from snapshots of car-parrinello molecular dynamics simulations.

We have investigated the performance of several computational protocols in predicting the NMR spectrum of a molecular ion in a complex liquid phase such as an ionic liquid. To do this, we computed the proton NMR chemical shifts of the 1-ethyl-3-methylimidazolium cation [emim](+) in [emim][Cl]. Environmental effects on the imidazolium ring proton chemical shifts are quite significant and must be taken into account explicitly. Calculations performed on the isolated imidazolium cation as well as on the [emim][Cl] ion pair grossly fail to reproduce the correct spacing between proton signals. In contrast, calculations performed on clusters extracted from the trajectory of a Car-Parrinello molecular dynamics simulation yield very good results.     

Modeling of heavy-atom-ligand NMR spin-spin coupling in solution: molecular dynamics study and natural bond orbital analysis of Hg-C coupling constants

Ab initio molecular dynamics (MD) and relativistic density functional NMR methods were applied to calculate the one‐bond Hg? C NMR indirect nuclear spin–spin coupling constants (J) of [Hg(CN)₂] and [CH₃HgCl] in solution. The MD averages were obtained as J(¹⁹⁹Hg? ¹³C)=3200 and 1575 Hz, respectively. The experimental Hg? C spin–spin coupling constants of [Hg(CN)₂] in methanol and [CH₃HgCl] in DMSO are 3143 and 1674 Hz, respectively. To deal with solvent effects in the calculations, finite “droplet” models of the two systems were set up. Solvent effects in both systems lead to a strong increase of the Hg? C coupling constant. From a relativistic natural localized molecular orbital (NLMO) analysis, it was found that the degree of delocalization of the Hg 5d_σ nonbonding orbital and of the Hg? C bonding orbital between the two coupled atoms, the nature of the trans Hg? C/Cl bonding orbital, and the s character of these orbitals, exhibit trends upon solvation of the complexes that, when combined, lead to the strong increase of J(Hg? C).     

Insight into Local Structure and Molecular Dynamics in Organic Solid‐State Ionic Conductors

Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion‐conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1‐ethyl‐1‐methylpyrrolidinium tetrafluoroborate ([C₂mpyr][BF₄]) are studied by a combination of quantum chemical calculations and advanced solid‐state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 25° in the pyrrolidinium ring has been observed, even in the low‐temperature phase (?45 °C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.     

Thermal effects and vibrational corrections to transition metal NMR chemical shifts

Both zero-point and classical thermal effects on the chemical shift of transition metals have been calculated at appropriate levels of density functional theory for a number of complexes of titanium, vanadium, manganese and iron. The zero-point effects were computed by applying a perturbational approach, whereas classical thermal effects were probed by Car-Parrinello molecular dynamics simulations. The systematic investigation shows that both procedures lead to a deshielding of the magnetic shielding constants evaluated at the GIAO-B3 LYP level, which in general also leads to a downfield shift in the relative chemical shifts, delta. The effect is small for the titanium and vanadium complexes, where it is typically on the order of a few dozen ppm, and is larger for the manganese and iron complexes, where it can amount to several hundred ppm. Zero-point corrections are usually smaller than the classical thermal effect. The pronounced downfield shift is due to the sensitivity of the shielding of the metal centre with regard to the metal-ligand bond length, which increase upon vibrational averaging. Both applied methods improve the accuracy of the chemical shifts in some cases, but not in general.     

Hyperfine coupling tensors of the benzosemiquinone radical anion from Car-Parrinello molecular dynamics.

Based on Car-Parrinello ab initio molecular dynamics simulations of the benzosemiquinone radical anion in both aqueous solution and the gas phase, density functional calculations provide the currently most refined EPR hyperfine coupling (HFC) tensors of semiquinone nuclei and solvent protons. For snapshots taken at regular intervals from the molecular dynamics trajectories, cluster models with different criteria for inclusion of water molecules and an additional continuum solvent model are used to analyse the HFCs. These models provide a detailed picture of the effects of dynamics and of different intermolecular interactions on the spin-density distribution and HFC tensors. Comparison with static calculations allows an assessment of the importance of dynamical effects, and of error compensation in static DFT calculations. Solvent proton HFCs depend characteristically on the position relative to the semiquinone radical anion. A point-dipolar model works well for in-plane hydrogen-bonded protons but deviates from the quantum chemical values for out-of-plane hydrogen bonding.     

Conformational properties of penicillins: quantum chemical calculations and molecular dynamics simulations of benzylpenicillin

Herein, we present theoretical results on the conformational properties of benzylpenicillin, which are characterized by means of quantum chemical calculations (MP2/6-31G* and B3LYP/6-31G*) and classical molecular dynamics simulations (5 ns) both in the gas phase and in aqueous solution. In the gas phase, the benzylpenicillin conformer in which the thiazolidine ring has the carboxylate group oriented axially is the most favored one. Both intramolecular CH. O and dispersion interactions contribute to stabilize the axial conformer with respect to the equatorial one. In aqueous solution, a molecular dynamics simulation predicts a relative population of the axial:equatorial conformers of 0.70:0.30 in consonance with NMR experimental data. Overall, the quantum chemical calculations as well as the simulations give insight into substituent effects, the conformational dynamics of benzylpenicillin, the frequency of ring-puckering motions, and the correlation of side chain and ring-puckering motions.     

Metal Olefin Complexes: Revisiting the Dewar−Chatt−Duncanson Model and Deriving Reactivity Patterns from Carbon‐13 NMR Chemical Shift

Metal olefin complexes that are ubiquitous intermediates in catalysis are investigated by a detailed analysis of their ¹³C‐NMR chemical shift tensors. This analysis allows evidencing specific electronic features, namely the olefin‐to‐metal σ‐donation and the metal‐to‐olefin π‐backdonation as proposed in the Dewar?Chatt?Duncanson model. Apart from these interactions, the chemical shift tensor analysis reveals an additional ligand‐to‐metal π‐donation of the olefin σ(C=C) orbital in systems with suitably oriented vacant d‐orbitals. This interaction which is not accounted for in the Dewar?Chatt?Duncanson model explains the reactivity of this type of metal olefin complexes towards oxidative cyclization (olefin insertion) and protonolysis.     

Continuous symmetry analysis of NMR chemical shielding anisotropy

Molecular symmetry is a key parameter which dictates the NMR chemical shielding anisotropy (CSA). Whereas correlations between specific geometrical features of molecules and the CSA are known, the quantitative correlation with symmetry--a global structural feature--has been unknown. Here we demonstrate a CSA/symmetry quantitative relation for the first time: We study how continuous deviation from exact symmetry around a nucleus affects its shielding. To achieve this we employed the continuous symmetry measures methodology, which allows one to quantify the degree of content of a given symmetry. The model case we use for this purpose is a population of distorted SiH(4) structures, for which we follow the (29)Si CSA as a function of the degree of tetrahedral symmetry and of square-planar symmetry. Quantitative correlations between the degree of these symmetries and the NMR shielding parameters emerge.     

Experimental and theoretical study of~(13)C shielding tensors in new-style molybdenum complex

~~Experimental and theoretical study of~(13)C shielding tensors in new-style molybdenum complex~~     

Ab initio static and molecular dynamics study of 4-styrylpyridine.

We report an in‐depth theoretical study of 4‐styrylpyridine in its singlet S₀ ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree–Fock (HF), second‐order Møller–Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post‐HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis?trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car–Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.     

13C NMR chemical shifts of carbonyl groups in substituted benzaldehydes and acetophenones: substituent chemical shift increments

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.     

Understanding sterol-membrane interactions part I: Hartree-Fock versus DFT calculations of 13C and 1H NMR isotropic chemical shifts of sterols in solution and analysis of hydrogen-bonding effects

1H and 13C NMR chemical shifts are exquisitely sensitive probes of the local environment of the corresponding nuclei. Ultimately, direct determination of the chemical shifts of sterols in their membrane environment has the potential to reveal their molecular interactions and dynamics, in particular concerning the hydrogen-bonding partners of their OH groups. However, this strategy requires an accurate and efficient means to quantify the influence of the various interactions on chemical shielding. Herein the validity of Hartree-Fock and DFT calculations of the 13C and 1H NMR chemical shifts of cholesterol and ergosterol are compared with one another and with experimental chemical shifts measured in solution at 500 MHz. A computational strategy (definition of basis set, simpler molecular models for the sterols themselves and their molecular complexes) is proposed and compared with experimental data in solution. It is shown in particular that the effects of hydrogen bonding with various functional groups (water as a hydrogen-bond donor and acceptor, acetone) on NMR chemical shifts in CDCl3 solution can be accurately reproduced with this computational approach.     

Quantum mechanical calculations of conformationally relevant 1H and 13C NMR chemical shifts of N-, O-, and S-substituted calixarene systems

QM GIAO calculations of (13)C and (1)H chemical shift values of the ArCH(2)Ar group in N-, O-, and S-substituted calixarene systems were performed with a hybrid DFT functional MPW1PW91 and 6-31G(d,p) basis set. A good reproduction of experimental data was obtained for some representative calixarenes and for a series of simplified calixarene models. This allowed the derivation of chemical shift surfaces versus phi and chi dihedral angles. The applicability of chemical shift surfaces in the study of calixarene conformational features is illustrated.     

The "invisible" 13C NMR chemical shift of the central carbon atom in [(Ph3PAu)6C]2+: a theoretical investigation

The experimental 13C NMR chemical shift of the central carbon atom in the octahedral [(Ph3PAu)6C]2+ cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. However, upon comparison with the 13C NMR properties of the related complex [C{Au[P(C6H5)2(p-C6H4NMe2)]}6]2+, Schmidbaur et al. have assigned a signal at delta=135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.     

Spectroscopic properties in the liquid phase: combining high-level ab initio calculations and classical molecular dynamics.

We present an integrated computational tool, rooted in density functional theory, the polarizable continuum model, and classical molecular dynamics employing spherical boundary conditions, to study the spectroscopic observables of molecules in solution. As a test case, a modified OPLS-AA force field has been developed and used to compute the UV and NMR spectra of acetone in aqueous solution. The results show that provided the classical force fields are carefully reparameterized and validated, the proposed approach is robust and effective, and can also be used by nonspecialists to provide a general and powerful complement to experimental techniques.