Proton transfer energies

Voogd, J., J. L. Derissen, and F. B. van Duijneveldt. 1981. Calculation of Proton-Transfer Energies and Electrostatic Lattice Energies of Various Amino Acids and Peptides Using CNDO/2 and Ab Initio SCF Methods. J. Am. Chem. Soc. 103, 7701-7706. 

This chapter describes the characteristics of the fluorescence emission of an excited molecule in solution. We do not consider here the photophysical processes involving interactions with other molecules (electron transfer, proton transfer, energy transfer, excimer or exciplex formation, etc.). These processes will be examined in Chapter 4. 

Figure 8. Qualitative potential energy surface proposed to explain the clustering of methoxide onto methanol and the "slow" rate of symmetric proton transfer. Energies in parentheses have units of kcal mol . ...

For the intermolecular proton transfer in the dimers, similar behavior and structures as in the previously described pyrrolo-pyrroles are found, being the nonconcerted TS the true states, with only one imaginary frequency, and Cs symmetry. The proton transfer energy barriers of the dimers range between 59... 

Table 1. Proton affinities and proton transfer energies in the vapour phase (kJ/mol)...

For most vapor-phase complexes of type (6) the Coulomb interaction compensates only part of the proton transfer energy AE and we expect to find neutral type complexes (NC). The only candidates for ionic complexes (IC) or intermediate cases are the associations of strong bases with strong acids, e.g. the complexes between ammonia or aliphatic amines and HC1, HBr or HJ (Table 1). 

For comparison to these predictions from experimental results [59, 60], Colson et al. [46, 65] studied the proton transfer reactions in GC and AT base pairs in their radical cationic and anionic states using HF/3-21G and HF/6-31+G(d)//3-21G levels of theory (see Figure 20-2). Their calculated proton transfer energies (difference between the total energies of the ionized radical base pairs before and after proton transfer) at HF/3-21G level of theory correlated very well with... 

Table 20-3. Calculated proton transfer energies (kcal/mol) along with experimental values...

TABLE 2. Proton transfer energies in the Im...HSMe dyad as calculated by ab initio molecular orbital theory for various models (kJ/mol). Im = imidazole, Me methyl, AE is the difference between energies of the ion-pair and neutral forms (after Bolis et al., 1979)... 

Several studies on CD complexes with aromatic molecules using steady-state and nanosecond spectroscopy have been reported. These studies aimed to understand the photophysical and photochemical behavior of organic guests such as fluorescence and phosphorescence enhancement, excimer/exciplex formation, photocleavage, charge and proton transfer, energy hopping, and cis-trans photo-... 

Tab. 22.2 Proton transfer energy, A pj, deprotonation energy, E yp, hypothetical binding energy of NH4 on the deprotonated zeolite surfaces, E p(SH ), and energy of ammonia adsorption, ad( 3) ( J for Bronsted sites in different zeolite frameworks [13].

Carbenium ions can be formed by proton transfer from the Bronsted site to an unsaturated hydrocarbon which requires a negative proton transfer energy, Eq. (22.4). 

Fig. 22.10(c) shows the following surprising results (i) The predicted energy of adsorption (70 kj mol i at 0 K) is of the same order of magnitude as estimates based on experiments for related molecules (50-63 kJ moh ). (ii) With respect to isobutene in the gas phase separated from the zeolite, the tert-butyl cation is much less stable (-17 kJ moTi) than the isobutoxide (-48 kJ moh ). The reason is that dispersion contributes substantially less to the stabilization of the tert-butyl cation than to the stabilization of the adsorption complex or the isobutoxide. As result, the proton transfer energy increases from 24 kJ moH (DFT) to 59 kJ moh (MP2/DFT) and it seems very unlikely that the fert-butyl cation will be detected in zeolites, even as a short-lived species. 

Owing to the relatively mild ionization process, the dominant ions formed include protonated molecular ions, molecular ion adducts, and cluster ions similar to those described for FAB (section 9.2.1.6). A number of different chemical and physical mechanisms have been proposed to explain ion formation, including gas-phase photoionization, ion-molecule reactions, disproportionation, excited state proton transfer, energy pooling, thermal ionization, and desorption of preformed ions [37]. The choices of ma-... 

Fig. 43. Calculated potential energy siuface for the adsorption of a single water molecule on a zeolitic Bronsted site (bridging OH group) AD adsorption energy, PT proton transfer energy (in kJ mol ), showing the hydroxonimn ion as a transition structure and not in an energy min-immn [655,656]...

The PILs either undergo a boiling point, Tb, or decompose, 7d, upon heating, as mentioned in section 2.2. PILs with a large proton-transfer energy will decompose before reaching... 

Table 9.4. Barriers to intramolecular proton transfer (energy differences between the Q and C2V structures) in malonaldehyde calculated in different appproximations...

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