Calculation of proton chemical

If we use the ligand-specific parameters for the calculations of proton chemical shifts of allenes from Table 8, the proton chemical shifts of ketene imines (89,92) mfiy be estimated to a good approximation by Equation 75, which takes into consideration that the substituent effects in ketene imines are generally larger than those in allenes. 

Calculations of proton chemical shifts that used several common density functionals and basis sets on a test set of compounds together with Unear scaling of the results recommended WP04/cc-pVDZ(rms error 0.115 ppm, m = 1.020, ft = 31.84), WP04/6-31G (rms error 0.12 ppm, m = 1.033, ft = 32.02), B3LYP/6-31G (rms error 0.13 ppm. 

Abraham, R. J. (1999) A model for the calculation of proton chemical shifts in non-conjugated organic compounds. Prog. NMR Spectrosc., 35, 85-152. 

To verify the mechanism presented, the quantum-chemical calculations of proton affinity, Aa, were carried out for modifiers, since the corresponding experimental data are quite rare. The calculations were performed for isolated molecules, since the properties of species in the interlayer space are probably closer to the gas phase rather than the liquid. The values of Ah were calculated as a difference in the total energy between the initial and protonated forms of the modifier. Energies were calculated using the TZV(2df, 2p) basis and MP2 electron correlation correction. Preliminarily, geometries were fully optimized in the framework of the MP2/6-31G(d, p) calculation. The GAMESS suite of ah initio programs was employed [10]. Comparison between the calculated at 0 K proton affinities for water (7.46 eV) and dioxane (8.50 eV) and the experimental data 7.50 eV and 8.42 eV at 298 K, respectively (see [11]), demonstrates a sufficient accuracy of the calculation. 

With the advances in experimental solid-state NMR and computational resources (both software and computing power), it is now possible to use both the experimental and computational results (sometimes in a complementary way) to study biologically important macromolecules such as proteins. The quantum-chemical computation (particularly by density functional theory) of NMR parameters in solids has found important application in protein structure determination.30-36 Tesche and Haeberlen37 calculated the proton chemical shift tensor of the methyl groups in dimethyl terephthalate and found the theoretical results were in good agreement with the multiple pulse experiment. 

Jain, R. Bally, T. Rablen, P. R. Calculating accurate proton chemical shifts of organic molecules with density functional methods and modest basis sets, J. Org. Chem. 2009, 74,4017-4023. 

Though both NH and CH2 are ortho, para-directing, we should expect NH to win as it has a Icr. pair of electrons. This would favour the first structure, but is not evidence. Calculation of > expected chemical shifts of protons in the two structures gives a more reliable answer. For exampi the two marked hydrogen atoms are predicted to come at ... 

This result is in stark contrast to a study of Moyna et alf who applied nearly the same formalism for calculating the proton chemical shifts [Eq. (40)]. For the tripeptide under investigation only a limited set of intra-residue proton distances was available. The definition of structure was therefore greatly improved when the proton chemical shift constraints were switched on. The chemical shift refinement reduced the rmsd for the backbone atoms from 1.2 A to 0.2 A, and it revealed a single set of conformers with both peptide bonds in trans conformation. The shift constraints drove the molecule energetically uphill by 3.9 kcal/mol but produced a well-defined minimum within the energy hyper-surface. Obviously, chemical shift constraints will produce well-defined structures when other constraints are not available from experiment. 

The first measurements of the dependence of proton chemical shifts 8 on temperature were performed using ethanol, and observing the shift of the OH proton with respect to the methyl and methylene protons. The first extensive study" of hydrogen-bonding mediated chemical shifts showed that, in the gas phase at very low pressures, the chemical shift measured at a particular temperature is essentially liquid phase, two factors alter the chemical shift from the value measured in the gas phase. The first factor is the change in bulk susceptibility caused by the change in density. The second factor is the association shift , which is the difference between the calculated and measured chemical shifts. The calculated value is given by ... 

L. F. Pacios and P. C. Gomez, Dependence of calculated NMR proton chemical shifts on electron density properties in proton-transfer processes on short strong hydrogen bonds, J. Phys. Chem. A 108, 11783-11792 (2004). 

Because of the proton s chemical importance and its favorable characteristics for NMR detection, the overwhelming bulk of experimental investigation and correlation of chemical shifts has been centered on the proton. However, the situation regarding the determination of proton shielding anisotropies has been unsatisfactory in many respects. Similarly, the field of ab initio theoretical calculations of proton shielding tensors has, until very recently, enjoyed little success. Both these factors are related to the fact that the proton chemical shifts are comparatively small and hence influenced strongly by secondary factors such as neighboring-atom electron distribution and medium effects. 

Quantum Chemical Calculations of Proton Transfer in AN, ADN, and HAN Clusters... 

Summary The -l-cyclopropyl-2-(triisopropylsilyl)ethyl cation is formed by protonation of -l-(triisopropylsilyl)ethenyl-cyclopropane with FSOsH/SbFs at -135°C and was characterized by H- and C-NMR spectra in SO2F2/SO2CIF solution at -105°C. Quantum chemical DFT ab initio calculations of NMR chemical shifts and spin-spin coupling constants were performed for the model structures E- and Z-1-cyclopropyl-2-(trimethylsilyl)ethyl cations. The calculated NMR data of the -l-cyclopropyl-2-(trimethylsilyl)ethyl cation are in good agreement with the experimental data and show that this p-silyl-substituted secondary cyclopropylmethyl carbocation is static on the NMR time scale in the observed temperature range. 

Calculation of proton affinity in a solvent by quantum chemical methods is a very difficult problem, if at all solvable. Suppose two calculations are done, one where the proton is placed near A, and one where it is placed near one of the water molecules (Figure 9.2). The difference between the two ground state energies should be taken. In both calculations, the geometry should be optimized. But geometry optimization can only lead to the lowest enthalpy situation. The geometry of the other state is not well-defined. 

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