Proton affinity of ammonia

The proton affinity of ammonia, PA(NH3), is the enthalpy change for reaction (6) at the temperature 298.15 K. The results from several ab initio computations are compiled in Table 4. Geometries and vibrational frequencies were obtained at the B3LYP/6-31G(d) level and used to compute ZPEs and enthalpy content (see Section 7). The electronic energy of is zero. 

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The standard enthalpy of this reaction is equal to the difference PA(K) — PA(B). Thus, the determination of PA(K) requires PA B). The proton affinity of B will rely, in turn, on the proton affinity of some other molecule and so on. A scale of relative values of proton affinities is thus built. To derive absolute data, a reliable anchor must be found. The one most frequently used is the proton affinity of ammonia, /M(NH3), which is now accepted to be 853.6 kJ mol-1 [67]. This is in excellent agreement with the result of a benchmark calculation by Martin and Lee,P (NH3) = 853.1 1.3 kJmol-1 [70],... 

Also the value reported by Haney and Franklin of 186 1 kcal/mol is in agreement. The latter workers determined the proton affinity of ammonia as 207 kcal/mol which is approximately 21 kcal/mol larger than that for phosphine. The greater basicity of ammonia as compared to phosphine is showi by the difference of about 20 pH units in their relative basicities in aqueous solutions. The difference in the basicities of the aqueous solutions of 23-32 kcal/mol, which is comparable to that in the gas phase, leads to the somewhat surprising conclusion that solvent effects appear to play an unessential part in the relative basicities of PH3 and NH3 in aqueous solutions. The proton affinities of HjO and H2S, 164 and 170 kcal/mol, respectively, are in the reverse order. 

A comparison of calculated and measured proton affinities (basicities) of nitrogen bases relative to the proton affinity of ammonia as a standard is provided in Table 6-17. The calculations correspond to the usual theoretical models, and the experimental data derive from equilibrium measurements in the gas phase. The data span a large range the proton affinity of the strongest base examined, quinuclidine, is some 27 kcal/mol greater than that of the weakest base, ammonia. 

The enthalpy change of the reaction in Equation (1.18) is minus the proton affinity of ammonia, -P(NH3,g). This could be calculated from the thermochemical cycle shown in Figure 1.58, provided the lattice energy of ammonium chloride is known. 

FIGURE 1.58 Thermochemical cycle for the calculation of the proton affinity of ammonia. 

Calculate a value for the proton affinity of ammonia using the cycle in Figure 1.58... 

Until this point, results appear to be rather divergent, possibly because these studies were performed on SMA dissolved in various solvents and solvation might modify the distribution of electrons in the Si-C-N unit. In order to circumvent this problem, gas phase studies and theoretical approaches were developed. In the gas phase (ion cyclotron resonance mass spectroscopy), trimethylsilylmethyldimethylamine was reported to be more basic than the analogous neopentyldimethylamine from proton affinity measurements based upon proton affinity of ammonia (201.0 kcal/mol) 227.1 and 225.8 kcal/mol respectively.32 Conversely, the ionization potential of MSMA indicates a basicity lower than that of its carbon analog and the authors emphasize the fact that this result is opposite... 

The classical example of the use of lattice energies to determine proton affinities is the case of the ammonium ion. Grimm (46) pointed out that the following cycle could be used to obtain the proton affinity of ammonia. 

The proton affinity of ammonia is therefore given by the relation... 

Proton affinities of ammonia and some of its alkyl and fluoro derivatives (in kcal/mol)... 

Del Bene JE (1993) Proton affinities of ammonia, water, and hydrogen fluoride and their anions a quest for the basis-set limit using the Dunning augmented correlation-consistent basis sets. J Phys Chem 97 107-110... 

Very often the chemical properties of a molecule or of a functional group are governed by a few electrons. For instance, the donor properties and proton affinity of ammonia can be understood from the character of its lone pair bonding properties of an alkyl radical depend on its unpaired electron. As these molecules or functional groups are basic entities in chemistry, it can be conjectured that they could be described by fragment pseudopotentials quite similar to the atomic pseudopotentials associated with the cores of the atoms. 

Proton affinity of ammonia, PA(NH3) (in kJmoE ), computed using a variety of theories and basis sets [geometries, ZPEs, and thermal functions from B3LYP/6-31G(d)]... 

Gronert has shown that additional diffuse functions included into the G2 basis scheme can improve the calculated acidities. A mean absolute deviation of 2.1 kJ mol" is observed for calculation of the acidity of the first- and second-row hydrides, compared with 3.3 kJ mol" with G2. Martin and Lee" have examined the proton affinity of ammonia at significantly higher levels than G2 and find results which are in complete agreement with the G2 prediction. 

This is either an (M + 1)+ or an (M - -18)+ ion, depending upon the relative proton affinities of the analyte and ammonia. The molecular weight is therefore either 221 or 204 Da. Since the molecule contains an even number of nitrogen atoms, it must have an even molecular weight. The molecular species must therefore be (M - -18)+ and so the molecular weight is 204 Da. 

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... 

Table 6-17 Proton Affinities of Nitrogen Bases Relative to Ammonia ... 

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 ... 

One should expect that the true proton-affinity of an ammonia molecule is lower than the value given above also approximately by 20 kcal. 

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