Ethane proton affinity

The overall order of alkane acidities deduced from the decomposition of silicon anions is the same as that obtained from the decomposition of alkoxides " " . The ethyl anion has the highest proton affinity, i.e., ethane is the weakest acid. The acidities of propane (secondary hydrogen) and of cyclobutane are both lower than that of methane, but all other alkanes are more acidic. The acidity of methane is particularly enhanced by a cyclopropyl substituent and by a t-butyl group. 

The 4-31 + G basis set used to calculate the proton affinities in Table 2 is then about the minimum acceptable by today s standards. Theory agrees with experiment in predicting that ethane and cyclobutane are the two least acidic alkanes. However, theory also predicts all the open-chain alkyl anions to have higher proton affinities than CH3", in disagreement with experiment One possible reason for this difference is... 

Proton transfer mass spectrometiy (PTR-MS) was developed in 1995 at the University of Innsbruck by Werner Lindinger and his collaborators [48]. PTR-MS was able to utilize the ability of the H3O+ ion to transfer its proton (PA(H20) = 691 kJ mol ) to any analyte in air that has a higher proton affinity. Proton transfer reactions are invariably fast when exoergic and occnr at the collision rate [49], HjO is readily formed from a discharge in moist air it does not react with the bnlk constituents of air and is therefore a good indicator of the presence of any volatile species that may be present in a sample. Many of the organic molecnles that are important analytes in medicine, foods, and the environment have proton affinities (PAs) higher than that of H2O. Notable exceptions are the small alkanes, ethane, and acetylene. 

The values for proton affinity and basicity listed are for a temperature of 298 K. For some specific molecules, such as H2O, CO2 and ethane, there is a considerable array of experimental and theoretical data to justify an estimated error margin of 1-3 kj mol . For most other molecules the error margins are somewhat larger, with Hunter and Lias estimating a... 

Most widely used methods are based on (i) the competition of protons with the bound metal ions for thiolate ligands, i.e., the pH stability of the metal-thiolate clusters or (ii) the competition with a metal chelator. In the former case the pH stability of the Cd-thiolate clusters is followed by absorption spectroscopy of the CysS-Cd(II) LMCT band at 250 nm. From the apparent p/Ca values the apparent stability constants of the clusters can be derived [103]. The apparent p/sTa values are determined either by taking the pH values of half-maximum absorbance or using a non-linear curve fit of the pH plot [104]. The other method is based on the competition for a single metal ion between the chelator 5F-BAPTA (1,2-bis-(2-amino-5-fluorophenoxy)ethane-A,W,A ,A -tetraacetic acid) and the protein followed by NMR spectroscopy [71]. Although this method was established for the zinc metalloforms of MTs, its applicability to cadmium metalloforms has also been demonstrated [105]. In this case, by analogy with zinc finger proteins a lower affinity for mixed Cys/His coordination of Cd(II) in MTs compared to sole Cys coordination has been shown. 

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