Protons theoretical studies

By using this approach, it is possible to calculate vibrational state-selected cross-sections from minimal END trajectories obtained with a classical description of the nuclei. We have studied vibrationally excited H2(v) molecules produced in collisions with 30-eV protons [42,43]. The relevant experiments were performed by Toennies et al. [46] with comparisons to theoretical studies using the trajectory surface hopping model [11,47] fTSHM). This system has also stimulated a quantum mechanical study [48] using diatomics-in-molecule (DIM) surfaces [49] and invoicing the infinite-onler sudden approximation (lOSA). 

Another principal difficulty is that the precise effect of local dynamics on the NOE intensity cannot be determined from the data. The dynamic correction factor [85] describes the ratio of the effects of distance and angular fluctuations. Theoretical studies based on NOE intensities extracted from molecular dynamics trajectories [86,87] are helpful to understand the detailed relationship between NMR parameters and local dynamics and may lead to structure-dependent corrections. In an implicit way, an estimate of the dynamic correction factor has been used in an ensemble relaxation matrix refinement by including order parameters for proton-proton vectors derived from molecular dynamics calculations [72]. One remaining challenge is to incorporate data describing the local dynamics of the molecule directly into the refinement, in such a way that an order parameter calculated from the calculated ensemble is similar to the measured order parameter. 

While most synthetic examples of this cyclization have involved protonation of divinyl ketones to give 3-hydroxy-1,4-pentadienyl cations, theoretical studies suggest that the cyclization would occur even more readily with alternative substituents at C-3. °... 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

Theoretical studies aimed at rationalizing the interaction between the chiral modifier and the pyruvate have been undertaken using quantum chemistry techniques, at both ab initio and semi-empirical levels, and molecular mechanics. The studies were based on the experimental observation that the quinuclidine nitrogen is the main interaction center between cinchonidine and the reactant pyruvate. This center can either act as a nucleophile or after protonation (protic solvent) as an electrophile. In a first step, NH3 and NH4 have been used as models of this reaction center, and the optimal structures and complexation energies of the pyruvate with NH3 and NHa, respectively, were calculated [40]. The pyruvate—NHa complex was found to be much more stable (by 25 kcal/mol) due to favorable electrostatic interaction, indicating that in acidic solvents the protonated cinchonidine will interact with the pyruvate. 

N3-coordinated complexes containing platinum group metal ions have also been synthesized and studied (56,60,61). Steric hindrance has been used to direct binding to N3 in a series of Pd- and Pt-containing complexes of 6,6,9-trimethyladenine (60). Platinum modification was found to have a pronounced effect on the basicity of the adenine moiety. The protonation constants (log Kh values) for the twofold protonation of 6,6,9-trimethyladenine are 4.15 and -0.75, with the initial protonation occurring at N1 followed by N7. The equivalent values for the formation of [Pt(dien)TMA-A3)H]3+ and [Pt(dien)TMA-2V3)H2]4+ are 0.3 and —1.2, respectively. Moreover, the site of initial protonation was found to be N7 (Fig. 19). These observations are supported by theoretical studies (62). 

J(PQ) and J(PN)). Trends in J(P170) have been used to study the protonation of phosphoryl tribromide, phosphoryl trifluoride and dif1uorophosphoric acid. Nitric acid had a greater protonation power than Hammett acidity functions indicated.77 This coupling has been the subject of a semiempirical sum over states theoretical study.7 ... 

Shukla MK, Leszczynski J (2005) Excited state proton transfer in guanine in the gas phase and in water solution a theoretical study. J Phys Chem A 109 7775-7780... 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

Pomes, R. Roux, B., Theoretical study of H+ translocation along a model proton wire, J. Phys. Chem. 1996,100, 2519-2527... 

The close correspondence between the properties of Mu in Si as determined by /u,SR and pLCR and those for the AA9 center produced by implanting hydrogen in silicon shows that Mu in silicon and the AA9 center are isostructural and in fact almost identical. They are neutral isolated bond-centered interstitials. Numerous theoretical studies support this conclusion. The observation of such similar centers for muonium and hydrogen supports the generalization that hydrogen analogs of many of the muonium centers exist. Of course, this assumes that the effects of the larger zero-point vibration of the muon relative to the proton do not make a major contribution to structural differences. The p-SR experiments, reinforced by theory, demonstrate that another structure also exists for muonium in silicon, called normal muonium or Mu. This structure is metastable and almost certainly is isolated neutral muonium at a tetrahedral interstitial site. 

The theoretical study of the structure of propene was then used as a model to calculate the effect of the structure on the proton affinity, and later to predict the acidity of similar systems such as cycloalkenes46. Deformation of the CCC angle as a function of the stability of the anion was probed, and the results were in agreement with the acidities of the hydrogens of propene. The allylic protons were found to be more acidic than the vinylic ones, which is in contrast to the results of Grundler47. 

Tortonda, F. R., Pascual-Ahuir, J. L., Silla, E. and Tunon, I. Solvent effects on the thermodynamics and kinetics of the proton transfer between hydronium ions and ammonia. A Theoretical study using the continuum and the discrete models, J. Phys. Chem., 99 (1995), 12525-12531... 

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