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Electron impact ionisation energies of some halo-cyclotriphosphazenes
J, inorg, nucl. Chem. Vot. 43, pp. 477-480 Pergamon Press Ltd., 1981. Printed in Great Britain
0022-1902/8110301-0477/$02.00/0
ELECTRON IMPACT IONISATION ENERGIES OF SOME HALO-CYCLOTRIPHOSPHAZENES PHILIP CLARE Department of Chemistry, University of Cape Coast, Cape Coast, Ghana and D. BRYAN SOWERBY* Department of Chemistry, University of Nottingham, Nottingham, England
(Received 12 February 1980; receivedfor publication 29 May 1980) Abstraet--Ionisation energies have been determined for N3P3F~X6-nwhere n = 2, 3 and 4 and X = Br, CI or NMe2. using an electron impact ion source and the semi-logarithmicplot method. Results are compared with values for N:~P3X6where X = Br, CI, F or NMe2. The substitution of dimethylaminogroups for fluorines in NaP3F6causes a marked decrease in ionisationenergy. A similar though less pronounced decrease occurs on substitution of fluorine by bromine or chlorine. The appearance energies were determined for several fragment ions. INTRODUCTION The ionisation energies of N3P3BrnCIr-~ were measured by electron impact mass spectrometry and showed a relatively smooth decrease from N3P3C16 to N3P3Brr[I]. The effects of both ring size and the substituent on the ionisation energies of homogeneously substituted cyclophosphazenes were determined for (NPX2)3.4, where X = OCH2CF3, OMe, OPh, NMe2 and Me, by electron impact, and for (NPF2),, n = 3-8, from photoelectron spectral2]. Within the series (NPX2h, the highest ionisation energy is found when X = F and the decrease is in the order F > OCH2CF3 > CI > Br > OMe > OPh > NMe2 > Me. For both chlorides and fluorides (NPX:),, the first ionisation energy was highest for n = 3, and alternated in magnitude for n =4-7. The alternation was explained in terms of the relative energies of two ring-bonding zr-systems: a heteromorphic (out-of-plane) system and a homomorphic (in-plane) system both involving d-orbitals at phosphorus atoms. It was also concluded that ionisation occurs from the highest occupied ring-bonding orbital, rather than from an exocyclic substituent as phosphorus. The exact nature of bonding in phosphazenes and the relationship to measured properties of these molecules has been the subject of many papers and reviews (e.g. 3) and recently Mitchell et al.[4, 5] have re-examined UV photoelectron spectra of N3P3Fr, N4P4Fs and N3P3C16 with the aid of Xa scattered wave calculations. They suggest that the highest filled orbitals are basically homomorphic ring-bonding orbitals, but are out-of-plane with respect to the ring. For the fluorides the highest energy orbital corresponds to a fairly weak-P-N-bonding system, not involving fluorine orbitals, but in N3P3CI6 it corresponds to a combination of - N - P - C I - orbitals which are weakly ring bonding. In the present work, the ionisation energies of N3P3FnX6-,; n = 2, 3, 4, and X = CI and NMe2 and n = 3, 4 for X = Br, are presented with the aim of providing data between the values for N3P3Xr, X = Br, CI, F and NMe2. Results are discussed, bearing in mind the limitations of this method of obtaining and treating the data[6].
*Author to whom correspondence should be addressed.
EXPERIMENTAL Instrumental. An A.E.I. MS 902 mass spectrometer was connected to an isolated Solartron digital voltmeter to measure the electron voltage between the tungsten filament and the trap. The sample was admitted through the heated inlet and the reference gas, xenon, through the cold inlet. The trap current was 20 ~A and the repeller plate was grounded to the source block. The source exit slit and the collector slit were wide open. Sample pressures were adiusted to give reasonable intensities at the collector on the least sensitive collector current scale. The total pressure of sample and reference did not exceed 9 x 10-6 mm of mercury. After stabilisation of the source for at least I hr the ion intensity was measured in the range 0.1-1% of the intensity at 70eV, in increments of 0.1 eV for both the sample and xenon. As the semi-logarithmic plots for sample and reference were linear and usually parallel in this region, a simple computer program was used to fit the best straight lines to each set of sample and reference data by the method of least squares. The ionisation efficiency line of the sample was mathematically corrected to be parallel to the reference line in certain cases, but this typically involved a reduction of less than 0.05eV in the calculated ionisation energy of the sample. Chemicals. Mixtures of cis and trans non-geminalisomers of the mixed halides[7] and of the dimethylaminofluorides[8]were used. The homogeneouslysubstituted compounds N3P3X6where X=CI[9], Br[10], NMe2[II] and F[12] were prepared as described earlier. RESULTS AND DISCUSSION lonisation energies The ionisation energies of the bromofluoro-, chlorofluoro- and dimethylaminofluorotriphosphazenes are presented in Table 1. Redetermined values for N3P3X6 where X = Br, Cl, F and NMe2 are shown for comparison. Three results are generally quoted for each compound and these were used to determine the mean value. Each of these three values was determined on a separate occasion and is itself the average of at least three individual determinations. The r.m.s, deviation of each set of data did not exceed 0.03 eV and this is less than the variation between different occasions as can be seen in Table 1. These results are displayed in Fig. 1, where the ionisation energies of N3P3FsX (X = Br, CI, NMe2)[13] have been included, and are plotted to show their relationship to the number of fluorine atoms, n, in the series of triphosphazenes N3P~FnXr-,. For X = CI and Br the ionisation energies for n = 0-5 lie almost on a straight
477
478
PHILIP CLAREand D. BRYANSOWERBY Table I. Ionisationenergies (eV) Compound
1
2
3
N3P3CI 6
10.05
iO.17
iO.iO
Mean
Literature
iO.ii
10.43(5) iO.27(i), 10.26(2)
N3P3CI4F 2
10.44
10.53
10.48
10.48
N3P3CI3F 3
10.74
10.78
10.76
10.76
N3P3CI2F 4
10.94
10.99
10.97
10.97
N3P3F 6
11.59
11.55
11.61
11.58
N3P3Br2F 4
10.61
10.63
10.64
10.63
N3P3Br3F 3
10.36
10.38
10.36
10.37
9.62
-
-
9.62
N3P3Br 6 N3P3F4(NMe2) 2
8.90
9.02
8.95
8.96
N3P3F3(NMe2) 3
8.64
8.65
8.67
8.65
N3P3F2(NMe2) 4
8.17
8.22
8.15
8.17
7.62
N3P3(NMe2)6
7.61
7.63
N3P3CI3(NMe2) 3
8.58
8.39
Benzene
9.44 15.88
Argon
7.62
-
8.49
-
-
9.44
15.91
-
15.90
11.4(2), 11.42(13)
9.56(1)
7.85(2)
9.24(14) 15.76(14)
that the electron was lost from an in-plane or-orbital where a change in electronegativity of the substituent at phosphorus would be expected to have a direct effect on the radial electron density at a ring nitrogen atom, The most recent discussions[4, 5] indicate the highest occupied orbital is a weakly bonding out-of-plane ~r-orbital in N3P3F6 and a similar orbital containing additional P-C1 character in N3P3C16. Even, so, Fig. 1 shows there is a direct relationship between the energy of the highest occupied molecular orbital and the type and number of substituents on the triphosphazene ring. The trend for the series N~P3F,(NMe2)6-, is most interesting. Firstly, there is a very large decrease in ionisation energy, i.e. 1.8 eV, between the hexafluoride and the pentafluoride. Secondly, the values for n = 5, 4 and 3 are higher than the ionisation energy for dimethylamine (8.24 eV) but lower than this when n = 2 and 0 (and presumably for n = 1). In contrast, for the mixed halides the ionisation energies are all lower than that for the appropriate hydrogen halide, i.e. HF, 15.77eV; HCI, 12.74 eV and HBr, I 1.62 eV [14]. Thirdly, N3PaX3(NMe2)3 X =C1, F have almost the same ion~1C > isation energy. These factors indicate that the formal lone pair on the nitrogen of the dimethylamino group is probably the dominant factor in the determination of the "8 energy of the highest occupied molecular orbital and can o overwhelm the effect of the electronegativity difference ~9 between fluorine and chlorine substituents. On the other hand, there is strong evidence from IR spectroscopy that o ~ "-..~c) the overall strength of ring bonding is greater in the o o trifluoride as the stretching mode vt,, which is basically an asymmetric ring stretching mode, is at 1240cm-' in N3P3F3(NMe2)3 (8), but only 1212 cm-1 in N3P3CI3(NMe2)3 (15). The most basic site in aminophosphanzenes is usually a ring nitrogen atom [16] and 1 I I I I • when protonation occurs, it takes place in the plane of 6 5 4 3 2 1 0 the ring[17]. An amino-substituent in a phosphazene is n u m b e r of f l u o r i n e a t o m s (nl Fig. 1. Ionisation energies in the series N3P~F.X6-. for (a) involved in back-donation from the amine nitrogen lone X = CI, (c) X = Br, and (c) X = NMe2as a functionof the number pair into a phosphorus d-orbital, which thus reduces the strength of in-plane ~r-bonding, and increases the radial of fluorine atoms. line and for X = NMe2 on a curve. The largest decrease in ionisation energy is from N3P3F6 to N3P3FsX in all three series. The absolute magnitude of this effect is uncertain as the values for N3P3FsX were determined elsewhere. However, the relatively smaller differences between ionisation energies when n = 2, 3 and 4 suggests this observation is valid. A similar, relatively large, decrease was observed between N3P3CI6 and N3P3BrCIs, and was related to the effect of the less electronegative bromine atom on the basicity of the ring nitrogen atom [l]. It was then thought
\
Electron impact ionisation energies of some halo-cyclotriphosphazenes electron density at a ring nitrogen atom. Thus there may be no simple relationship between ionisation energy and basicity if an electron is ionised from an out-of-plane ~'-system. For N3P3F6, benzene and argon the vertical ionisation energies measured here by electron impact are 0.1-0.2 eV higher than those measured from photoelectron spectra. This is to be expected considering the high filament temperature and resultant non-homogeneous electron energies employed. However, the value obtained for N3P3CI6, I0.11 eV, is lower than two previous electron impact measurements [I, 2] and all three results are lower than the value for the vertical transition indicated by photoelectron spectroscopy to be at 10.43eV[5] and just above onset of ionisation occurring between 9.9 and 10.0eV. It is surprising that all three electron impact results are so low as this method is usually considered to measure the energy of the vertical transition, although the possibility of determining adiabatic ionisation energies by electron impact mass spectrometry has been discussed[18]. For N3Pa(NMe2)6, the present result is 0.23 eV below the earlier electron impact value[2], but for N~P3 Br6 the values are essentially the same[l]. Bearing in mind the many factors that can contribute to erroneous results, it is unwise to draw conclusions by too close a comparison between results from differen't laboratories[6].
Appearance energies The measurement of appearance energies of fragment ions is essential for certain types of thermochemical studies, and these can only be determined by mass spectrometry. However, calculations with theoretical ionisation efficiency curves indicate that results from the semi-logarithmic method can be up to 0.5 eV higher than the true values[6,19]. Nevertheless, the lack of sen-
sitivity of the spectrometer used precludes application of the more accurate "modified critical slope" method, since readings of.ion current must be taken very close to the region of onset of ionisation. Appearance energies of selected fragment ions are summarised in Table 2. Some values are derived from application of the semi-logarithmic plot method and the remainder, where very weak intensity ions were examined, from the difference in electron voltage between sample and reference at the points corresponding to 0.1% of their intensities at 70 eV. The most interesting and surprising aspect of these results, considering that the PF bond is so much stronger than PBr or PCI bonds, is that the appearance energies of singly charged fragment ions which have lost fluorine from the parent ion are similar to, or even lower than, those arising from loss of one chlorine or bromine from the same parent. Indeed, the appearance energy of N3P3CI4F÷, 10.45eV, is not only lower than that of N3P3CI3F~, but seems to be identical to the ionisation energy of N3P3CI4F2 itself, i.e. 10.48eV, within the reproducibility of these results. This is in marked contrast to the appearance energy of N~P3F~ from N3P3F6, i.e. 16.2eV. This would seem to be a more reasonable +__~ + value if the process occurring is N3P3F6 N3P3F5 + F. since the strength of the PF bond is ca. 5.0 eV and the difference between the appearance energy of N3P3F~ and the ionisation energy of N3P3F6 is 4.65 eV. Loss of bromine or chlorine atoms from the parent ions in this way is supported by the observation of the appropriate metastable ions, whereas loss of fluorine from the parent ions of the mixed halides gives rather weak daughter ions and corresponding metastable ions were not observed[20]. As this casts doubt on the nature of the parent species when loss of fluorine is observed, it is not possible to use these appearance energy values in thermochemical calculations.
Table 2. Appearanceenergies (eV) of selected fragment ions Fragment Ion
Compound
N3P3CI4 F+
N3P3CI4F 2
2a
10.45
N3P3CI3F2 +
N3P3CI4F 2
5
11.78
N3P3CI3F2 +
N3P3CI3F 3
2a
12.10
N3P3CI2F3+
N3P3CI3F 3
7
12.54
N3P3C12F3+
N3P3CI2F 4
2a
12.10
N3P3CIF4+
N3P3CI2F 4
6
12.70
No. of Determinations
Appearance Potential (eV)
N3P3Br3F2 +
N3P3Br3F 3
2a
11.O5
N3P3Br2F3+
N3P3Br3F 3
4
11.24
N3P3Br2F3 +
N3P3Br2F 4
2a
11.67
N3P3BrF4 +
N3P3Br2F 4
6
11.47
N3P3CIs+
N3P3CI 6
5
ii.43 b
N3P3CI4 +
N3P3CI 6
4a
11.78
N3P3F5+
N3P3F 6
3
16.23
a
single reading method; b JINC VoL 43, No. 3--D
11.O6 (I) .
479
remainder by semi-log method.
480
PHILIP CLARE and D. BRYAN SOWERBY REFERENCES
1. G. E. Coxon, T. F. Palmer and D. B. Sowerby, J.. Chem. Soc. (A), 358 (1969). 2. G. R. Branton, C. E. Brion, D. C. Frost, K. A. R. Mitchell and N. L. Paddock, J. Chem. Soc. (A), 151 (1970). 3. D. P. Craig and N, U Paddock, Nonbenzenoid Aromatics p. 273, Vol 2, Academic Press, London (1971). 4. K. A. R. Mitchell, U Noodleman and N. L. Paddock, Chem. Phys. Lett. 47, 265 (1977). 5. K. A. R. Mitchell, L. Noodleman and N. P. C. Westwood, Chem. Phys. Lett. 58,252 (1978). 6. J. H. Beynon, R. G. Cooks, K. R. Jennings and A. J. Ferrer-Correia, Int. J. Mass Spectrom. Ion Phys. 18, 87 (1975). 7. P. Clare and D. B. Sowerby, lnorg. Synth. XVIII, 194 (1978). 8. B. Green, D. 8. Sowerby and P. Clare, J. Chem. Soc (A), 3487 (1971). 9. S. M. Zhivukhin, V. V. Kireev, V. P. Popilin and G. S.
Kolesnikov, Russ. J. lnorg. Chem. 15, 630 0970). 10. G. E. Coxon and D. B. Sowerby, J. Chem. Soc. (A), 1566(1967). 11. R. Keat and R. A. Shaw, J. Chem. Soc. 2215 (1965). 12. F. Seel and J. Langer, Angew. Chem. 68, 461 (1956). 13. D. Bohler and E. Niecke, 4th European Symposium on Fluorine Chemistry, Ljubljana (1972). 14. Handbook of Chemistry and Physics, 55th Edition, C.R.C. Press, Cleveland (1975). 15. R.-Stahlberg and E. Steger, Spectrochim. Acta 23A, 2005 (1967). 16. R. A. Shaw, Pure and Applied Chem. 44, 317 (1975). 17. N. V. Mani and A. J. Wagner, Chem. Commun. 658 (1968). 18. R. A. W. Johnstone and F. A. Mellon, J. Chem. Soc. Faraday II, 68, 1209 (1972). 19. J. L. Occolowitz, B. J. Cerimbele and P. Brown, Org. Mass Spectrom. 8, 61 (1974). 20. P. Clare and D. B. Sowerby, J. Inorg. Nucl. Chem. 43, 467 098O).
0022-1902/8110301-0477/$02.00/0
ELECTRON IMPACT IONISATION ENERGIES OF SOME HALO-CYCLOTRIPHOSPHAZENES PHILIP CLARE Department of Chemistry, University of Cape Coast, Cape Coast, Ghana and D. BRYAN SOWERBY* Department of Chemistry, University of Nottingham, Nottingham, England
(Received 12 February 1980; receivedfor publication 29 May 1980) Abstraet--Ionisation energies have been determined for N3P3F~X6-nwhere n = 2, 3 and 4 and X = Br, CI or NMe2. using an electron impact ion source and the semi-logarithmicplot method. Results are compared with values for N:~P3X6where X = Br, CI, F or NMe2. The substitution of dimethylaminogroups for fluorines in NaP3F6causes a marked decrease in ionisationenergy. A similar though less pronounced decrease occurs on substitution of fluorine by bromine or chlorine. The appearance energies were determined for several fragment ions. INTRODUCTION The ionisation energies of N3P3BrnCIr-~ were measured by electron impact mass spectrometry and showed a relatively smooth decrease from N3P3C16 to N3P3Brr[I]. The effects of both ring size and the substituent on the ionisation energies of homogeneously substituted cyclophosphazenes were determined for (NPX2)3.4, where X = OCH2CF3, OMe, OPh, NMe2 and Me, by electron impact, and for (NPF2),, n = 3-8, from photoelectron spectral2]. Within the series (NPX2h, the highest ionisation energy is found when X = F and the decrease is in the order F > OCH2CF3 > CI > Br > OMe > OPh > NMe2 > Me. For both chlorides and fluorides (NPX:),, the first ionisation energy was highest for n = 3, and alternated in magnitude for n =4-7. The alternation was explained in terms of the relative energies of two ring-bonding zr-systems: a heteromorphic (out-of-plane) system and a homomorphic (in-plane) system both involving d-orbitals at phosphorus atoms. It was also concluded that ionisation occurs from the highest occupied ring-bonding orbital, rather than from an exocyclic substituent as phosphorus. The exact nature of bonding in phosphazenes and the relationship to measured properties of these molecules has been the subject of many papers and reviews (e.g. 3) and recently Mitchell et al.[4, 5] have re-examined UV photoelectron spectra of N3P3Fr, N4P4Fs and N3P3C16 with the aid of Xa scattered wave calculations. They suggest that the highest filled orbitals are basically homomorphic ring-bonding orbitals, but are out-of-plane with respect to the ring. For the fluorides the highest energy orbital corresponds to a fairly weak-P-N-bonding system, not involving fluorine orbitals, but in N3P3CI6 it corresponds to a combination of - N - P - C I - orbitals which are weakly ring bonding. In the present work, the ionisation energies of N3P3FnX6-,; n = 2, 3, 4, and X = CI and NMe2 and n = 3, 4 for X = Br, are presented with the aim of providing data between the values for N3P3Xr, X = Br, CI, F and NMe2. Results are discussed, bearing in mind the limitations of this method of obtaining and treating the data[6].
*Author to whom correspondence should be addressed.
EXPERIMENTAL Instrumental. An A.E.I. MS 902 mass spectrometer was connected to an isolated Solartron digital voltmeter to measure the electron voltage between the tungsten filament and the trap. The sample was admitted through the heated inlet and the reference gas, xenon, through the cold inlet. The trap current was 20 ~A and the repeller plate was grounded to the source block. The source exit slit and the collector slit were wide open. Sample pressures were adiusted to give reasonable intensities at the collector on the least sensitive collector current scale. The total pressure of sample and reference did not exceed 9 x 10-6 mm of mercury. After stabilisation of the source for at least I hr the ion intensity was measured in the range 0.1-1% of the intensity at 70eV, in increments of 0.1 eV for both the sample and xenon. As the semi-logarithmic plots for sample and reference were linear and usually parallel in this region, a simple computer program was used to fit the best straight lines to each set of sample and reference data by the method of least squares. The ionisation efficiency line of the sample was mathematically corrected to be parallel to the reference line in certain cases, but this typically involved a reduction of less than 0.05eV in the calculated ionisation energy of the sample. Chemicals. Mixtures of cis and trans non-geminalisomers of the mixed halides[7] and of the dimethylaminofluorides[8]were used. The homogeneouslysubstituted compounds N3P3X6where X=CI[9], Br[10], NMe2[II] and F[12] were prepared as described earlier. RESULTS AND DISCUSSION lonisation energies The ionisation energies of the bromofluoro-, chlorofluoro- and dimethylaminofluorotriphosphazenes are presented in Table 1. Redetermined values for N3P3X6 where X = Br, Cl, F and NMe2 are shown for comparison. Three results are generally quoted for each compound and these were used to determine the mean value. Each of these three values was determined on a separate occasion and is itself the average of at least three individual determinations. The r.m.s, deviation of each set of data did not exceed 0.03 eV and this is less than the variation between different occasions as can be seen in Table 1. These results are displayed in Fig. 1, where the ionisation energies of N3P3FsX (X = Br, CI, NMe2)[13] have been included, and are plotted to show their relationship to the number of fluorine atoms, n, in the series of triphosphazenes N3P~FnXr-,. For X = CI and Br the ionisation energies for n = 0-5 lie almost on a straight
477
478
PHILIP CLAREand D. BRYANSOWERBY Table I. Ionisationenergies (eV) Compound
1
2
3
N3P3CI 6
10.05
iO.17
iO.iO
Mean
Literature
iO.ii
10.43(5) iO.27(i), 10.26(2)
N3P3CI4F 2
10.44
10.53
10.48
10.48
N3P3CI3F 3
10.74
10.78
10.76
10.76
N3P3CI2F 4
10.94
10.99
10.97
10.97
N3P3F 6
11.59
11.55
11.61
11.58
N3P3Br2F 4
10.61
10.63
10.64
10.63
N3P3Br3F 3
10.36
10.38
10.36
10.37
9.62
-
-
9.62
N3P3Br 6 N3P3F4(NMe2) 2
8.90
9.02
8.95
8.96
N3P3F3(NMe2) 3
8.64
8.65
8.67
8.65
N3P3F2(NMe2) 4
8.17
8.22
8.15
8.17
7.62
N3P3(NMe2)6
7.61
7.63
N3P3CI3(NMe2) 3
8.58
8.39
Benzene
9.44 15.88
Argon
7.62
-
8.49
-
-
9.44
15.91
-
15.90
11.4(2), 11.42(13)
9.56(1)
7.85(2)
9.24(14) 15.76(14)
that the electron was lost from an in-plane or-orbital where a change in electronegativity of the substituent at phosphorus would be expected to have a direct effect on the radial electron density at a ring nitrogen atom, The most recent discussions[4, 5] indicate the highest occupied orbital is a weakly bonding out-of-plane ~r-orbital in N3P3F6 and a similar orbital containing additional P-C1 character in N3P3C16. Even, so, Fig. 1 shows there is a direct relationship between the energy of the highest occupied molecular orbital and the type and number of substituents on the triphosphazene ring. The trend for the series N~P3F,(NMe2)6-, is most interesting. Firstly, there is a very large decrease in ionisation energy, i.e. 1.8 eV, between the hexafluoride and the pentafluoride. Secondly, the values for n = 5, 4 and 3 are higher than the ionisation energy for dimethylamine (8.24 eV) but lower than this when n = 2 and 0 (and presumably for n = 1). In contrast, for the mixed halides the ionisation energies are all lower than that for the appropriate hydrogen halide, i.e. HF, 15.77eV; HCI, 12.74 eV and HBr, I 1.62 eV [14]. Thirdly, N3PaX3(NMe2)3 X =C1, F have almost the same ion~1C > isation energy. These factors indicate that the formal lone pair on the nitrogen of the dimethylamino group is probably the dominant factor in the determination of the "8 energy of the highest occupied molecular orbital and can o overwhelm the effect of the electronegativity difference ~9 between fluorine and chlorine substituents. On the other hand, there is strong evidence from IR spectroscopy that o ~ "-..~c) the overall strength of ring bonding is greater in the o o trifluoride as the stretching mode vt,, which is basically an asymmetric ring stretching mode, is at 1240cm-' in N3P3F3(NMe2)3 (8), but only 1212 cm-1 in N3P3CI3(NMe2)3 (15). The most basic site in aminophosphanzenes is usually a ring nitrogen atom [16] and 1 I I I I • when protonation occurs, it takes place in the plane of 6 5 4 3 2 1 0 the ring[17]. An amino-substituent in a phosphazene is n u m b e r of f l u o r i n e a t o m s (nl Fig. 1. Ionisation energies in the series N3P~F.X6-. for (a) involved in back-donation from the amine nitrogen lone X = CI, (c) X = Br, and (c) X = NMe2as a functionof the number pair into a phosphorus d-orbital, which thus reduces the strength of in-plane ~r-bonding, and increases the radial of fluorine atoms. line and for X = NMe2 on a curve. The largest decrease in ionisation energy is from N3P3F6 to N3P3FsX in all three series. The absolute magnitude of this effect is uncertain as the values for N3P3FsX were determined elsewhere. However, the relatively smaller differences between ionisation energies when n = 2, 3 and 4 suggests this observation is valid. A similar, relatively large, decrease was observed between N3P3CI6 and N3P3BrCIs, and was related to the effect of the less electronegative bromine atom on the basicity of the ring nitrogen atom [l]. It was then thought
\
Electron impact ionisation energies of some halo-cyclotriphosphazenes electron density at a ring nitrogen atom. Thus there may be no simple relationship between ionisation energy and basicity if an electron is ionised from an out-of-plane ~'-system. For N3P3F6, benzene and argon the vertical ionisation energies measured here by electron impact are 0.1-0.2 eV higher than those measured from photoelectron spectra. This is to be expected considering the high filament temperature and resultant non-homogeneous electron energies employed. However, the value obtained for N3P3CI6, I0.11 eV, is lower than two previous electron impact measurements [I, 2] and all three results are lower than the value for the vertical transition indicated by photoelectron spectroscopy to be at 10.43eV[5] and just above onset of ionisation occurring between 9.9 and 10.0eV. It is surprising that all three electron impact results are so low as this method is usually considered to measure the energy of the vertical transition, although the possibility of determining adiabatic ionisation energies by electron impact mass spectrometry has been discussed[18]. For N3Pa(NMe2)6, the present result is 0.23 eV below the earlier electron impact value[2], but for N~P3 Br6 the values are essentially the same[l]. Bearing in mind the many factors that can contribute to erroneous results, it is unwise to draw conclusions by too close a comparison between results from differen't laboratories[6].
Appearance energies The measurement of appearance energies of fragment ions is essential for certain types of thermochemical studies, and these can only be determined by mass spectrometry. However, calculations with theoretical ionisation efficiency curves indicate that results from the semi-logarithmic method can be up to 0.5 eV higher than the true values[6,19]. Nevertheless, the lack of sen-
sitivity of the spectrometer used precludes application of the more accurate "modified critical slope" method, since readings of.ion current must be taken very close to the region of onset of ionisation. Appearance energies of selected fragment ions are summarised in Table 2. Some values are derived from application of the semi-logarithmic plot method and the remainder, where very weak intensity ions were examined, from the difference in electron voltage between sample and reference at the points corresponding to 0.1% of their intensities at 70 eV. The most interesting and surprising aspect of these results, considering that the PF bond is so much stronger than PBr or PCI bonds, is that the appearance energies of singly charged fragment ions which have lost fluorine from the parent ion are similar to, or even lower than, those arising from loss of one chlorine or bromine from the same parent. Indeed, the appearance energy of N3P3CI4F÷, 10.45eV, is not only lower than that of N3P3CI3F~, but seems to be identical to the ionisation energy of N3P3CI4F2 itself, i.e. 10.48eV, within the reproducibility of these results. This is in marked contrast to the appearance energy of N~P3F~ from N3P3F6, i.e. 16.2eV. This would seem to be a more reasonable +__~ + value if the process occurring is N3P3F6 N3P3F5 + F. since the strength of the PF bond is ca. 5.0 eV and the difference between the appearance energy of N3P3F~ and the ionisation energy of N3P3F6 is 4.65 eV. Loss of bromine or chlorine atoms from the parent ions in this way is supported by the observation of the appropriate metastable ions, whereas loss of fluorine from the parent ions of the mixed halides gives rather weak daughter ions and corresponding metastable ions were not observed[20]. As this casts doubt on the nature of the parent species when loss of fluorine is observed, it is not possible to use these appearance energy values in thermochemical calculations.
Table 2. Appearanceenergies (eV) of selected fragment ions Fragment Ion
Compound
N3P3CI4 F+
N3P3CI4F 2
2a
10.45
N3P3CI3F2 +
N3P3CI4F 2
5
11.78
N3P3CI3F2 +
N3P3CI3F 3
2a
12.10
N3P3CI2F3+
N3P3CI3F 3
7
12.54
N3P3C12F3+
N3P3CI2F 4
2a
12.10
N3P3CIF4+
N3P3CI2F 4
6
12.70
No. of Determinations
Appearance Potential (eV)
N3P3Br3F2 +
N3P3Br3F 3
2a
11.O5
N3P3Br2F3+
N3P3Br3F 3
4
11.24
N3P3Br2F3 +
N3P3Br2F 4
2a
11.67
N3P3BrF4 +
N3P3Br2F 4
6
11.47
N3P3CIs+
N3P3CI 6
5
ii.43 b
N3P3CI4 +
N3P3CI 6
4a
11.78
N3P3F5+
N3P3F 6
3
16.23
a
single reading method; b JINC VoL 43, No. 3--D
11.O6 (I) .
479
remainder by semi-log method.
480
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