Methane proton affinity

The proton affinity of the trimethylsilylmethyl anion was also calculated from measurements of the electron affinity of the trimethylsilylmethyl radical80. This gave a proton affinity of 25.7 kcal mol-1 for MejSiCFL, relative to methane. This compares well with the calculated value for H3SiCH2- given earlier. 

Brinkman and coworkers81 obtained a value of around 30 kcal mol 1 from a similar gas-phase study for the same proton affinity relative to methane. Calculations at the 6-311+G(d,p) level gave a value of 25 kcal mol 1 (for H3SiCH2-). The proton affinities of (H3Si)2CH- and (H3Si)3C- relative to methane were calculated to be 47 and 66 kcal mol-1, respectively. The experimental proton affinity of (Me3Si)2CH- was found to be approximately 43 kcal mol-1. 

Computational methods have also been used frequently to estimate the thermodynamic stabilities of superelectrophiles. These calculations have often involved the estimation of barriers to gas phase dissociation or deprotonation, and the proton affinities of conventional electrophilic intermediates. Other useful studies have calculated the heats of reactions for isodesmic processes. An interesting example of these calculations comes from a study of the protoacetyl dication (Cf COH2"1- ).42 In calculations at the 6-31G //4-31G level of theoiy, the protoacetyl dication (83) is estimated to react with methane by hydride abstraction with a very favorable... 

Because the nonbonding orbital is occupied, stability increases with s character, the converse of the situation for carbocations. The order of stability of carbanions is sp < sp < sp. The relative stability of gas phase carbanions can be assessed by the energy of their reaction with a proton, which is called proton affinity. The proton affinities of the prototypical hydrocarbons methane, ethene, and ethyne have been calculated at the MP4/6-31+G level/ The order is consistent with the electronegativity trends discussed in Section 1.1.5, and the larger gap between sp and sp, as compared to sp and sp, is also evident. The relative acidity of the hydrogen in terminal alkynes is one of the most characteristic features of this group of compounds. 

Table 3.16. Gas Phase Proton Affinity of Substitnted Methanes (in kcal/mol)...

W. A. Chupka and J. Berkowitz, Photoionization of methane ionization potential and proton affinity of CH4, J. Chem. Phys. 54, 4256-4259 (1971). 

Fig. 2.7 Basicities of some molecules, calculated by the CBS-4 method. The proton affinities refer to the acidity of the conjugate acids. Methane is extremely weak as a base and the methyl anion is extremely strong...

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. 

A long-chain fatty alcohol of molecular mass 242 Da is ionized via positive-ion chemical ionization with methane, isobutane, hydrogen, water, or methanol as the reagent gas. The two major ions in the spectrum are miz 243 and 225 (the loss of water from the protonated molecule). Predict how the ratio of ion abundances of m/z 243 versus m/z 225 will change with those different reagent gases. Rationalize your answers with a reasonable explanation. The proton affinity of this alcohol is 200 kcalmoD ... 

Thus, there is usually less fragmentation with isobutene than with methane. The ammonium ion can transfer a proton only to compounds with a higher proton affinity than ammonia, mainly nitrogen-containing compounds. As with El, the sample is introduced from a reservoir, from a heated probe, or as the eluent from a GC. 

CI-MS is a powerful tool for steric structure identification that often allows differentiation between isomers. In addition, compound selectivity or sensitivity can be increased by choice of reagent gases with different proton affinities and ion-molecule reaction properties. However, in practice, only few reagent gases are used such as methane or isobutene. Occasionally, ammonia with a higher proton affinity than methane is employed to enhance selectivity. Methane and argon are most frequently used in electron capture negative ionization (ECNI). 

The gas-phase reaction of [MnMe(CO)s] with proton donors BH+ leads to the ions [Mn(CO)5]+ and [MnHMe(CO)5]+. The ion [MnHMe(CO)5]+ is only observed with bases B having lower proton affinity than those giving [Mn(CO)6]+. Thus protonation can occur at the Mn—Me bond leading directly to methane with little or no activation energy, while protonation at Mn gives the kinetically stable protonated alkyl complex. 

The proton affinities of reagent gases vary a lot. For example, the proton affinity of methane during formation of protonated species (CH5+) was found to be 131.6 kcal moP [13,16]. In order to cause ionization, the proton affinity of the analyte must be higher than that of... 

Another common reagent gas used in Cl is isobutane (1-C4H10). It possesses an even higher proton affinity (196 kcal moP ) than methane [16,17]. In this case, the ionization process is described by the following reactions ... 

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