Ammonia proton affinity

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. 

On the basis of the results by Chen and Her [51] it may be expected, however, that the low-proton-affinity PE matrix will be barely detectable, and thus not interfering, in ammonia DCI-MS of a dissolved PE/additive sample. 

Two other molecules considered in the same context, but with proton affinities larger than ammonia, were pyridine (C5H5N) and trimethylamine (TMA) (CH3)3N whose proton affinities are 220.8 cal mol-1 and 225.1 cal mol-1, respectively.190 Metastable decomposition studies of NH3(C5H5N)mH+ (m = 1-5) yield the following results ... 

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 reaction of pyridine (py) with bare metal ions (except Fe+) has not been studied widely. The reaction of Fe+ produced by electron ionization of Fe(CO)5 with a mixture of two pyridines (108) was used to compare the proton affinities with the Fe+ affinity. A good correlation was observed. The absolute Fe+ affinity of py was determined to be 49 3 kcal mol-1, which is higher than the value of 44 3 kcal mol 1 for the Fe+-NH3 bond dissociation energy (46). Steric problems with ortho substituted pyridines gave lower than expected affinities. The reaction of py and substituted pyridines showed a maximum addition of four pyridines, similar to the GIB experiments with ammonia (46). 

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],... 

In most papers the experimental proton affinity difference between water and ammonia is taken as being equal to 37.5 kcalmol-1. Various calculated values of this gap have appeared. The PA values of ammonia and water were 205.6 and 168 or 168.6 kcalmol-1 respectively (APA = 37 or 37.6 kcalmol-1) according to Dixon and Lias26 the work of Defrees and McLean27 led to similar calculations of P/HN1I3) = 204.0 and PA (H30) = 165.1, with A PA = 38.9 kcalmol-1, although Pople and coworkers28 predicted a A PA = 39.4 kcalmol-1. 

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. 

The gas phase basicity of 3,5-bis(trifluoromethyl)triazole (22) has been determined by Fourier transform ion cyclotron resonance mass spectrometry as —50.2 + 0.8 kJ mol <94JOC1039). The proton affinity related to ammonia was calculated as — 60.5 kJ mol . 

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. 

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

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 ... 

Finally, atomic and molecular proton affinities (PAs) have also been evaluated for various functionals for ammonia, water, acetylene, silane, phosphine, silylene, hydrochloric acid, and molecular hydrogen. For G2 and G3 theories, the mean unsigned error in PAs is 1.1 and... 

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

Reconcile the values of the proton affinities of pyridine (953 kJ mol-1) and ammonia (87 kJ mol"1) with the argument on page 343 concerning the relationship between Kb and electronegativity. The latter argument seems lo go with the conveniional wisdom" raiher than ihc discussion in ihis chapter. Criticize. 

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