17O NMR Chemical Shifts of Polyoxides in Gas Phase and in Solution

Quantum Chemical Approaches to the Calculation of NMR Parameters: From Fundamentals to Recent Advances

Quantum chemical methods for the calculation of indirect NMR spin–spin coupling constants and chemical shifts are always in progress. They never stay the same due to permanently developing computational facilities, which open new perspectives and create new challenges every now and then. This review starts from the fundamentals of the nonrelativistic and relativistic theory of nuclear magnetic resonance parameters, and gradually moves towards the discussion of the most popular common and newly developed methodologies for quantum chemical modeling of NMR spectra.

Tropospheric oxidation of methyl hydrotrioxide (CH3OOOH) by hydroxyl radical

We have employed high level theoretical methods to investigate the oxidation of methyl hydrotrioxide by hydroxyl radical, which is of interest in atmospheric chemistry research. The reaction can proceed by abstraction of either the terminal hydrogen atom of OOH group producing CH3O, O2 and H2O, or one hydrogen atom of the CH3 group forming H2CO, HO2 and H2O. The rate constants for both reactions at 298 K are computed to be 4.7 × 10-11 and 2.1 × 10-12 cm3 molecule-1 s-1, respectively, that is, the abstraction of terminal hydrogen atom of the OOH group is about 22 times faster than that of a hydrogen atom of the CH3 group. The rate constant for the overall CH3OOOH + OH reaction is computed to be 4.9 × 10-11 cm3 molecule-1 s-1. Our calculations predict branching ratios between 99.0 and 93.9% for the formation of methoxy radical plus molecular oxygen and water, and between 1.0 and 6.1% for the formation of formaldehyde plus hydroperoxyl radical and water, in the 225-325 K temperature range. The lifetime of CH3OOOH in the troposphere is predicted to range from of 1.8 hours at 225 K, up to 3.9 hours at 275 K and decreasing to 0.2 hours at 310 K. CH3OOO and CH2OOOH radicals have been also investigated.

Environmental effects on the spectroscopic properties of gallic acid: a combined classical and quantum mechanical study

The solvation of gallic acid (in water and acetonitrile) is studied by means of its spectroscopic properties. IR, UV, and NMR spectra are predicted by using various solvation models obtained in terms of both purely classical and density functional approaches. Comparison with experiments is used to validate solvation models. Hydrogen-bond and long-range (or bulk) effects are evaluated by comparing different solvation models. A continuum-only approach, a purely discrete, and a mixed continuum/discrete approach based on quantum-mechanical and classical molecular-dynamics solute-solvent clusters are tested.

Solvent Effects on NMR Isotropic Shielding Constants. A Comparison between Explicit Polarizable Discrete and Continuum Approaches

The gas-to-aqueous solution shifts of the 17O and 13C NMR isotropic shielding constants for the carbonyl chromophore in formaldehyde and acetone are investigated. For the condensed-phase problem, we use the hybrid density functional theory/molecular mechanics approach in combination with a statistical averaging over an appropriate number of solute-solvent configurations extracted from classical molecular dynamics simulations. The PBE0 exchange-correlation functional and the 6-311++G(2d,2p) basis set are used for the calculation of the shielding constants. London atomic orbitals are employed to ensure gauge-origin independent results. The effects of the bulk solvent molecules are found to be crucial in order to calculate accurate solvation shifts of the shielding constants. Very good agreement between the computed and experimental solvation shifts is obtained for the shielding constants of acetone when a polarizable water potential is used. Supermolecular results based on geometry-optimized molecular structures are presented. We also compare the results obtained with the polarizable continuum model to the results obtained using explicit MM molecules to model the bulk solvent effect.

Systematic studies on the computation of nuclear magnetic resonance shielding constants and chemical shifts: The density functional models

We present a systematic density functional investigation on the prediction of the 13C, 15N, 17O, and 19F NMR properties of 23 molecules with 21 density functionals. Extensive comparisons are made for both 13C magnetic shieldings and chemical shifts with respect to the gas phase experimental data and the best CCSD(T) results. We find that the OPBE and OPW91 exchange-correlation functionals perform significantly better than some popular functionals such as B3LYP and PBE1PBE, even surpassing, in many cases, the standard wavefunction-based method MP2. Further analysis has been performed to explore the individual role played by various exchange and correlation functionals. We find that the B88 and PBE exchange functionals have a too strong tendency of deshielding, leading to too deshielded magnetic shielding constants; whereas the OPTX exchange functional performs remarkably well. We claim that the main source of error arises from the exchange functional, but correlation functional also makes important contribution. We find that the correlation functionals may be grouped into two classes. class A, such as LYP and B98, leads to deshielded NMR values, deteriorating the overall performance; whereas class B, such as PW91 and PBE, generally increases the absolute shieldings, which complements the exchange functionals, leading to improved results in the calculation of NMR data.

The Ozonation of Silanes and Germanes: An Experimental and Theoretical Investigation

Ozonation of various silanes and germanes produced the corresponding hydrotrioxides, R3SiOOOH and R3GeOOOH, which were characterized by 1H, 13C, 17O, and 29Si NMR, and by infrared spectroscopy in a two-pronged approach based on measured and calculated data. Ozone reacts with the E-H (E = Si, Ge) bond via a concerted 1,3-dipolar insertion mechanism, where, depending on the substituents and the environment (e.g., acetone-d6 solution), the H atom transfer precedes more and more E-O bond formation. The hydrotrioxides decompose in various solvents into the corresponding silanols/germanols, disiloxanes/digermoxanes, singlet oxygen (O2(1delta(g))), and dihydrogen trioxide (HOOOH), where catalytic amounts of water play an important role as is indicated by quantum chemical calculations. The formation of HOOOH as a decomposition product of organometallic hydrotrioxides in acetone-d6 represents a new and convenient method for the preparation of this simple, biochemically important polyoxide. By solvent variation, singlet oxygen (O2(1delta(g))) can be generated in high yield.

Thermal and solvent effects on the NMR and UV parameters of some bioreductive drugs

15N NMR chemical shifts and n-->pi* electronic transition energy for metronidazole (1) has been calculated and compared with experimental data. A detailed computational study of 1 is presented, with special attention to the performance of various theoretical methods for reproducing spectroscopic parameters in solution. The most sophisticated approach involves density functional based on the Car-Parrinello molecular dynamics simulations of 1 in aqueous solution (BP86 level) and averaging chemical shifts and deltaE(n-->pi*) over snapshots from the trajectory. In the NMR and UV calculations for these snapshots (performed at the B3LYP level), a small number of discrete water molecules are retained, and the remaining bulk solution effects are included via a polarizable continuum model (PCM). A good agreement with experiment is also obtained using static geometry optimization and NMR computation of pristine 1 employing a PCM approach. Further theoretical predictions are also reported for 17O NMR and deltaE(n-->pi*) of three hydroxycinnamic acid derivatives, which suggest that it is essential to incorporate the dynamics and solvent effects for NMR and UV calculations in the condensed phase.