Proton affinity, defined

A more appropriate measure of the inherent basicity of a species on an oxide surface may be its gas-phase proton affinity, defined as the negative of the change in enthalpy for the reaction... 

The four major quantities on which we will focus are atomization energies, electro afiinities, ionization potentials and proton affinities. We ll look at each of them i turn in this section. Note that all of them are conventionally defined to be greater tha... 

The proton affinity is defined as the energy released when a proton is added to a system, computed as the energy difference between the system of interest and the same molecule with one additional proton (H ). For example, the proton affinity of PHj is computed as EfPHj) - E(PH/). 

The original paper defining the Gaussian-2 method by Curtiss, Raghavachari, Trucks and Pople tested the method s effectiveness by comparing its results to experimental thermochemical data for a set of 125 calculations 55 atomization energies, 38 ionization potentials, 25 electron affinities and 7 proton affinities. All compounds included only first and second-row heavy atoms. The specific calculations chosen were selected because of the availability of high accuracy experimental values for these thermochemical quantities. 

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

The proton affinity of any species A in the gas phase, abbreviated by PA(A), is defined as the negative of the standard enthalpy of reaction 4.25 at 298.15 K. The minus sign ensures that proton affinities always have positive values. 

The proton affinity is defined in terms of the hypothetical reaction... 

The proton affinity p of a substance such as NH3 is defined as the change in energy for the reaction... 

The proton affinity of a donor, D, is defined as the enthalpy for the process ... 

Proton affinities can be found in a similar way a proton affinity is defined as the enthalpy change of the reaction shown in Equation (1.17), where a proton is lost by the species, A. 

The most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

Note that the proton affinity (PA) has the opposite sign from the enthalpy of reaction of Eq. 9.47 Proton affinities are always listed as positive numbers despite referring to exothermic reactions (recall the same convention with electron affinities. Chapter 3) Proton affinities may be obtained in a number of ways. The simplest, and most fundamental for defining an absolute scale of proton affinities, is to use a Bom-Haber cycle of the sort ... 

The most widely studied reference acid is the proton. Proton affinity, PA(B), is defined for a base B as the heterolytic bond dissociation energy for removing a proton from the conjugated acid BH+ (equation 20). The homolytic bond dissociation energy D(B+—H) defined by equation 21 is related to PA(B) and the adiabatic ionization potentials IP(H) and IP(B) (equation 22) are derived from the thermochemical cycle shown in Scheme 6. 

Another important question related to substituent effects on gas-phase basicities of thiocarbonyl compounds is whether these substituent effects arise from interactions within the neutral or within the protonated species, or both. It is useful to define the relative proton affinities along the monosubstituted series of compounds by means of the process shown in equation 54. 

Proton affinity data taken from Bartmess and Mclver, 1979. The proton affinity of B is defined as the negative of the enthalpy ... 

This technique is especially valuable in cases where equilibrium measurement is hampered due to low volatility (peptides, zwitterions, salts etc.). Gas phase acidity (GA) and basicity (GB), and proton affinity (PA) are defined in the absolute sense through the following definitions ... 

The general strategy for generating radical adducts to heterocyclic molecules relies on selective protonation of a suitable neutral precursor to prepare a cation of a well-defined structure. The gas-phase acid is chosen so as to attack only the most basic site in the molecule, or alternatively, non-selective protonation can be used to prepare a mixture of ions. For example, protonation of imidazole with NH4+ occurs selectively on the imine nitrogen atom (N-l), which has the highest proton affinity and is the only position that can be protonated by an exothermic reaction (Scheme 22) [239]. 

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