Advanced Propellant Chemistry

4 Estimated Stability of Perfluoroammonium Ion and Its Salts J. N O R T O N W I L S O N

Downloaded by CORNELL UNIV on September 28, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0054.ch004

Shell Development Co., Emeryville, Calif.

The heat of formation of NF ion is estimated at 245 ± 20 kcal./mole from thermochemical correlations and from failure to detect NF as product of an ion-molecule reaction between NF and NF in a mass spectrometer. The ion NF H was detected as a product of a related reaction under similar conditions; its heat of formation appears to be less than 225 kcal./mole. Estimates of the heat of formation of some salts of NF have been made by means of the Kapustinskii approximation for lattice energies. For this purpose a correlation was developed between known "thermochemical radii" of tetrahedral ions and their van der Waals radii. It is concluded that the perchlorate, sulfate, and fluoride salts will be unstable relative to likely decomposition products. +

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' T P h e hypothetical salt, N F C 1 0 , would clearly be an excellent oxidizing agent if it could be made. This paper presents some estimates con cerning the stability of the ion, N F + , and of its salts. O u r first concern is to estimate the heat of formation of the perfluoroammonium ion i n the gas phase. A rough but simple estimate can be made on the assumption that the dissociation energy of a fluorine atom from N F + is about the same as the average bond energy i n either N F + or N F . From the known heats of formation and ionization potentials of N F and N , and the heat of forma tion of F , we obtain for the dissociation N F + = N + + 3 F an average bond energy of 76.3 db 2 kcal./mole; the corresponding bond energy i n N F is 66.5 ± 0.6. The first of these values leads to a heat of formation for N F + of 216.9 ± 8.5 and the second to 226.4 db 7 kcal./mole. The ther mochemical data used i n this and subsequent computations are listed i n the Appendix together with their sources. A more realistic estimate can be made by examining trends i n the dissociation energy of a fluorine atom from the series of molecules C F , 4

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30

Holzmann; Advanced Propellant Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

4.

Perfluoroammonium Ion

WILSON

C F , C F and the isoelectronic series N F + , N F + , ( N F + ) . The rele vant data are assembled i n the Appendix; the trends are illustrated in Figure 1. The drop i n dissociation energy between C F and C F is equal to that between N F + and N F + within the relatively broad limits of ex perimental error; a line parallel to that connecting the experimental points for C F and C F is shown intersecting the line connecting the points for N F + and N F + . F r o m Figure 1, the most likely value for the dissocia tion energy of F from N F + is estimated to lie in the range 50-55 k c a l . / mole. A substantial range of uncertainty is indicated by lines extending to the right from the upper and lower limits of error at the N F point; these are drawn with the maximum and minimum slopes, respectively, of 3

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140

Figure 1.

Dissociation energies of F from isoelectronic series CF , CF CF,;NF +, NF + NF,+ X

S>

g

S

9


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ADVANCED PROPELLANT

CHEMISTRY

the lines that can be drawn to connect the C F and C F points within their ranges of error. The dissociation energy D ( N F + — F ) is thus estimated as 50 ± 20 kcal./mole and the heat of formation of N F + as 244 =h 26 k c a l . / mole. If A f / / ( N F + ) were 236 ± 10 kcal./mole, then from the known heats of formation of N F , N F , and N F + the following ion-molecule reaction would be athermal: 3

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NF

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+

+ NF, = N F

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+

+ NF

2

A search has therefore been made for this reaction i n a mass spectrometer ( Consolidated model 21-103A). Observations were made at the partial pressure of 200 microns H g of N F in the sample reservoir, with the ioniza tion chamber operating at 260° C. and with 70 volt ionizing electrons. N o formation of N F + was observed, though i n an experiment with C D under similar conditions, the ion C D + was clearly detected. In a similar experiment with C D and N F each at 200 microns partial pressure, C D + and N F D + were clearly observed, but no trace of N F was found. If failure to find N F + is caused by the endothermicity of the reaction written above, then ΔΗ/( N F * ) is greater than 225 kcal./mole. Observation of the ion N F D + suggests, on the other hand, that either or both of the following ion-molecule reactions is exothermic 3

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+

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CH GH

-f N F

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+ NF

+

= GH + NF H

+

3

= CHs +

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+

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NF H 3

+

The first of these would be athermal if A # / ( N F H + ) = 225 ± 6 k c a l . / mole; the second, if the value were 220 =fc 3. The heat of formation of N F H + is thus very likely less than 230 kcal./mole; this implies a dissocia tion energy D ( N F + — H ) > 97 kcal./mole. The dissociation energy of H from the ions N H + to N H + is known to fall i n the range 120-135 k c a l . / mole. It seems reasonable then to conclude that the heat of formation of N F + is greater than 225 kcal./mole and probably less than 260 k c a l . / mole; a value around 240 kcal./mole seems not unlikely. This implies that dissociation of N F + to N F + and F should be endothermic by 50 db 25 kcal./mole, and dissociation to N F + + F endothermic by 38 db 24 k c a l . / mole. The increase of standard entropy i n the latter dissociation is esti mated about 45 e.u.; this w i l l contribute —13.5 kcal./mole to the standard free energy of dissociation at 300°K. It is thus not unlikely that the ion N F + can be prepared and observed i n the gas phase by a suitable ionmolecule reaction. Let us turn now to the question of the lattice energy of salts of N F . For tetrahedral ions such as this one, the simplest approach, though an approximate one, is that proposed many years ago by Kapustinskii (10). 3

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WILSON

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H e assumed that for salts made up of combinations of spherical or tetrahedral ions the lattice energy could be well approximated b y assigning to the crystal structure (usually unknown) a Madelung constant equal to that of sodium chloride and estimating the repulsive contribution to the lattice energy by a Born-Mayer expression similar to that which holds approximately for the alkali halides. These assumptions lead to the following expressions for the lattice energy U:

= 290.2 η

(

L

-

μ — Madelung constant = 1.7475 for N a C l n = Number of ions per formula ν = Ionic charge in units of electronic charge

R^RZ)

^ - / f o r m u l a wt.

R = Effective ionic radius ("thermochemical radius") ρ = Born-Mayer repulsion parameter (exponential repulsive potential).

2.7

Figure 2.

Correlation of thermochemical radius R with sum of bond distance R(B—X) and van der Wools radius R(X) in tetrahedral ions k


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CHEMISTRY

This expression has turned out to be remarkably useful i n correlating the heats of formation of the salts of tetrahedral ions, provided suitable values are assumed for the "ionic radii" R and R _ . Kapustinskii and his co workers recognized that these quantities are not necessarily equal to the packing radii of the ions i n the actual structure of the crystal; conse quently, they have come to be known as thermochemical radii. The ther mochemical radius and heat of formation for a tetrahedral ion are nor mally determined from Equation 1 and the known heats of formation of two of its salts. +

I n order to apply Equation 1 to the hypothetical salts of N F + it is necessary to estimate a thermochemical radius for that ion. W e have found that a fairly good correlation exists for a number of symmetrical tetrahedral ions BX4 between the thermochemical radius R and the sum of (a) the internuclear distance R ( B — X ) between the central atom of the ion and one of its ligands and ( b ) the van der Waals radius, R«,(X) of the ligand. This correlation, shown i n Figure 2, is described approxi mately by:


4

k

# * ( Β Χ - ) = (0.75 ± 0.07) A . + (0.55 τ 0.024) ( Λ ( Β Χ ) + # ( X ) ) 4

η

(2)

W

with van der Waals radii 1.35 and 1.41 A . assigned to F and O , respec tively. The form of this correlation testifies to the artificial character of the thermochemical radii R . k

The N — F distance i n N F is reported to be 1.37 A (23); the N - C distance i n the approximately tetrahedral complex ( C H ) N : B F 3 is re ported as 1.50 Α., about 0.03 A . larger than i n trimethylamine, (23). A recent x-ray crystallographic study of ( C H ) N + B r - gave 1.50 ± 0.02 A . also as the N — C distance i n the tetramethylammonium ion (8). W e therefore take the N — F distance in N F + as 1.40 A . F r o m this and E q u a tion 2 we obtain a thermochemical radius of 2.26 A . for perfluoroammo nium ion. 3

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Table I.

Stability of Hypothetical Energy Units, kcal./gm. mole

Anion AHf(g) Symbol FCIO4SO42

BFi~

60.7 65 88 151 426 407 6

e

Lattice Energy (Kapustinskii) 147 116 352 118 118

Salt AHf(c) 37 33 41 -13 -299 -280

db 20 ± 20 ±20 ± 20 db 20 ± 20

Probable Decomposition Products NF + F 3

2

N F , + FCIO4 2 N F , + F 0 + SO* NF + F + BF 2

3

2

• From A H , ( F ) and electron affinity (Appendix, Tables A , B ) . * From detailed lattice energy calculations by Altschuller (1 ).


â

4.


WILSON

Stability of Some Perfluoroammonium

35

Salts

The results obtained by using Equation 1 are summarized i n Table I. The heats of formation given for the isolated anions were computed from the standard heat of formation of the corresponding potassium salt b y means of the Kapustinskii approximation. The value thus derived from F - (using a radius of 1.33 A . ) falls within 5 kcal./mole of that from the heat of formation and measured electron affinity of F . T h e value for B F - differs b y 19 kcal./mole from that obtained i n a detailed latticeenergy calculation b y Altschuller ( J ) ; his value is very close to that de rived by Kapustinskii and Yatsimirskii ( I I ) by modifying the Kapustin skii equation. The heats of formation given for the hypothetical salts of N F in Table I were obtained b y adding the heat of formation computed for the anion to the value 245 ± 20 for the heat of formation of N F + and sub tracting the lattice energy of the salt as obtained from the Kapustinskii approximation. The heat of decomposition was obtained b y comparing this heat of formation with the sum of the heats of formation of the likely decomposition products. F o r the fluoride, perchlorate, and sulfate salts decomposition is predicted to be exothermic by a much larger margin than the estimated uncertainty of the prediction; these salts should be unstable at one atmosphere and any temperature. F o r the fluoroborate the conclusion is less clear. F o r this salt, the most reasonable estimate of the heat of decomposition probably lies between the two estimates given, and closer to the second estimate than to the first since the Kapustinskii approximation i n the simple form used here tends to underestimate the lattice energy (24). W e therefore conclude that the fluoroborate also may well be unstable at one atmosphere and any temperature. E v e n if the heat of decomposition of the fluoroborate is positive b y as much as 10 kcal./mole, which seems unlikely, stability of the crystal would be limited to low temperatures b y the high entropy of dissociation. T h e sum of the standard entropies of the likely products, N F + F + BFe, is


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Perfluoroammonium Salts

Decomposition

AH

f

-29.7

Advanced Propellant Chemistry

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