1586 where yl, y2, and y3 are the activity coefficients of DDQ-ether complex, ether, and DDQ, respectively, and proposing constancy of the ratio y1/y2y~at different donor and acceptor concentrations. The constancy of the ratio of activity coefficients has been proposed previously by other in~estigators.1~Js The electron donor ability of the ethers is in the order of THF > T H P E l,Cdioxane, as judged by K values. This order agrees with the order of basicity reported for these ethers in studies on hydrogen bonding18and iodine c0mp1exes.l~ The equilibrium constant for the 1,4dioxane complex should be twice that of the T H P complex on a statistical basis, as l,.i-dioxane presents two probable points of attack. The TCNE-l,4-dioxane complex is twice as stable as the corresponding T H P complex;12 however, K for DDQ-l,4-dioxane is of similar value to K for the DDQ-THP complex, which suggests that the characteristics of the acceptor molecule and steric factors may be of importance in complexing. (14) H. A. Benesi and J. H. Hiberbrand, J . Amer. Chem. Soc., 71, 2703 (1949). (15) R. 5. Drago, R . L. Carlson. N. J. Rose, and D. A. Wenz, (bid., 83, 3572 (1961). (16) 9. Searles and M. Tamres, ibid., 7 3 , 3704 (1951). (15') S. M. Brandon 0. P., M. Tarnres, and S. Searles. ibid., 8 2 , 2129 (1960).
Negative Ion Formation by Electron Impact in Nitrogen Trifluoride by J. C. J. Thynne Chemistry Department, Edinburgh University, Edinburgh, Scotland (Received July 8 , 1 9 6 8 )
Negative ion formation in nitrogen trifluoride has been previously investigated by Reese and Dibeler,l who observed the ions F- and Fz- (the latter in very low abundance) but not NF-, which is reported here.
Experimental Section The mass spectra were measured using a Bendix timeof-flight mass spectrometer, Model 3015. The pressure in the ion source was normally kept at 5 X mm in order to minimize the possibility of ion formation due to ion-molecule reactions or collisions with secondary electrons, except where a study of the pressure dependence of the ion formation was made. The electron energy was measured on a SoIatron digitaI voltmeter, LM 1619, and the spectra were recorded on a 1-mV Kent potentiometric recorder or a HewlettPackard X-Y recorder, 7035AM. Ionization curves were normally recorded three times and the appearThe Journal of Physical Chemistrg
I
0
S
10 1s Electron energy, V.
20
I
Figure 1. Upper curve, ionization efficiency curve for P ion formation by electron impact in nitrogen trifluoride; middle curve (filled circles), ionization efficiency curve for 3'2- ion formation; lower curve (open circles), ionization efficiency curve for NFion formation.
ance potentials were reproducible usually to better than 3 0 . 2 eV. The electron trap current was maintained constant automatically in the linear range (0-2 PA) of trap current vs. collected ion current. The electron energy scale was calibrated using as a reference the appearance potential of the S- ion from CS2, the maximum value of the resonance peak at 6.4 eV being taken as the critical value.2pa The sample of nitrogen trifluoride had been stared over water and was treated by vacuum distillation at - 160'. A positive ion mass spectrometric analysis agreed with that of Reese and Dibeler,' and no impurities could be detected.
Results and Discussion Using 50-eV electrons the relative abundance of the ions F-, Fz-,NF- was found to be 1000:5.0:0.7, respectively. The three ions are formed over a wide range of energies and for each of them there are at least two appearance potentials indicating that there is more than one mechanism for ion formation in each case. In Figure 1 we show the ion formation (expressed by the ion current) as a function of the electron energy over the range 0-25 eV. Our data above 26 eV are not shown in the figure for convenience of presentation. I n all cases there was a gradual increase in the intensity of the ion current with increasing electron energy, the increase becoming less marked as the electron energy increased. No additional ion-fomation process above 25 eV as evidenced by a break in (1) R. M.Reese and V. H. Dibeler, J . Chem. Phys., 24, 1175 (1956). (2) F. H.Dorman, ibid., 44, 3856 (1966). (3) J. E.Collin, M. J. Rubin-Franskin, and L. d'Or. paper presented at International Mass Spectrometry Conference, Berlin, Sept 1967.
1587
NOTES
the ionization efficiency curve was observed for any of these ions. A. F- Ion Formation. This ion is formed over the whole electron energy range studied and, in agreement with previous observations,l the intensity of the ion a t low electron energies (-2-3 eV) is very much greater than that a t high energies (-20 eV) indicating that the ion-pair formation process is much less extensive than is the dissociative capture process. From the bond energy D(NF2-F) 2.5 eV4 and the electron affinity E ( F ) = 3.6 eV6 it would be expected that the F- ion would be produced by reaction 1 for an ionizing energy near zero, and this would appear to be so from Figure 1.
-
1
NF3
+ NF2
+ e+F-
The peak at 1.7 eV is reasonably close to the minimum enthalpy requirement of 1.2 eV for the reaction 2
NF,
+ e-F-+
NF
+F
and the peak at 3 . 4 eV would appear to correspond to &heprocess 3
NF3
+ e-F-+
N
+ 2F
Reaction 3 would require an energy of about 3 . 5 e V ~ At 13.4 eV there is a sharp increase in the ion current and a small peak having a maximum value at 83.8 eV is obtained. The peak shape suggests that a dissociative capture rather than an ion-pair process is responsible, although this electron energy is much higher than the usual energies a t which capture processes normally occur. Reese and Dibeler' have reported the appearance potential of the NFa+ ion to be 13.2 eV. NF3
+e
4
NF3+
+ 2e
Formation of the NF3+ ion is accompanied by the production of secondary electrons which will have a range of energies, some of which will be sufficiently low to be captured by nitrogen trifluoride which may then undergo dissociation, i.e. NF3
+ e (2)
- 5
( NF3-) *
F- + NF2
where e(2) is a secondary electron. For primary electron capture (reaction l ) , the Fion current will be directly proportional to the pressure of NF, in the ion source, whereas for secondary electron capture by (5) the ion current a (pressure)2. Measurement of the F- ion intensity as a function of the ion source pressure a t 14.8 eV, the energy at which the peak maximum occurs, shows the pressure exponent to be two, suggesting that the onset at 13.4 eV is the result of secondary electron capture.
At 21 eV a break in the ionization efficiency curve is observed and since no decrease in the ion current is noted above this energy it is likely that an ion-pair process is involved. These may be 6
NFs
+ e-F-
+ F+ + N + F F- + N+ + 2F
7
Values of 22.2 and 25 eV have been measured' for A(N+) and A (F+); it seems possible therefore that reaction 7 is the reaction responsible for the small increase in ion current. B. F2- Ion Formation. The observation that the F2ion is formed readily from many fluorinated moleculess suggests that molecular fluorine has a rather high electron affinity. The ion cannot, however, be formed by a simple dissociative process. Figure 1, curve 2 indicates that the ion is formed a t zero electron energy and that a further ionization process has its onset at 3.9 eV, the resonance peak reaching a maximum value a t 5.6 eV. Possible ionformation reactions involved are 8
NF3
+ e+F2- + N F N +F 9
-F2-+
The difference in the energy requirements for reactions 8 and 9 is 3.9 eV; this is in accord with the differences between the two appearance potentials reported above; we therefore suggest that these reactions are responsible. There is an indication of a low crosssection ionisation process a t 19 eV although the process responsible cannot be determined. C . NF- Ion Formation. This ion has not been observed previously. Figure 1, curve 3 indicates that it is formed by a dissociative capture process a t zero electon energy and that the resonance peak has a maximum value at 2.8 eV. The narrow peak, particularly on the high-energy side, suggests that the ion is formed with little or no kinetic energy. The appreciable height of the peak suggests that the NF- ion is formed by a reaction of large cross-section, possibly NFs
+ e-
NF-
+ 2F
Another ionization process occurs a t 23.0 eV; in view of the shape of the ion curve it is likely that this corresponds to the ion-pair process such as
NF3
+ e-
NF-
+ F+ + F
rather than a secondary electron capture process. D. Thermochemical Data. Calculations in this paper (4) A. Kennedy and 0.B. Colburn, J. Chem. Phys., 35, 1892 (1961). (6) I. N. Bakulina and N. I. Ionov, Zh. FiB. K h i m . , 33, 2063 (1959). (6) K. A. G. MacKeil and J. C. J. Thynne, Trans. Faraday SOC., 64, 2112 (1968).
Volume 75, Number 6 M a y 1969
NOTES
1588
are based upon the following data (in kcal mol-’): AHf(N) 113,’ AHJ’(F) 18.9,’ AHf(NFa) -29.7,* AHf(NF2) -8.9,9 AHf(NF) 42,9 AHJ’(F-) -63.2,1° AHf(F+) 420,11 and AHf(N+) 448,11
Acknowledgments. Thanks are due to the Science Research Council for a grant in support of this work, to the referee for helpful comments, and to Professor A. F. Trotman-Dickenson for a gift of the nitrogen trifluoride.
excess ligand, only the ion pair contributes to the epr absorption. I n these conditions the concentration of the inner and outer-sphere complexes may be evaluated from the “residual” signal intensity and K 2 may be easily determined. The disappearance of the broad spectrum of the complex species suggests that the lifetime of the latter . is much longer than the spin relaxation time (7 >> Tz) This is a reasonable assumption in the case of the inner-sphere complex, . in which the rate of ligand exchange should correspond to the rate of exchange of water molecules in the Mn2+ solvation sphere (IC = 1,’. N lo’ sec-l) . If exchange between the free ion and the ion pair is assumed to be a diffusion-controlled reaction, rates of exchange are expected of the order of 10“ sec-l, while relaxation times for both species are of the order of 10-9sec. This corresponds to the rapid exchange limit (T
