2996
T. MCALLISTER AND F. P. LOSSING
Free Radicals by Mass Spectrometry.
XLI.
Ionization Potential
and Heat of Formation of PH, Radical’
by T. McAllister2and F. P. Lossing Division of Pure Chemistry, National Research Council of Canuda, Ottawa, Canada
(Received March 3,1969)
Monoenergetic electron-impact measurements on PH2 radicals generated thermally from benzylphosphine giveI(PHz) = 9.83 V. The appearance potential A(PH2+/PH3)is found to be 13.47 V, leading to aNr(PH2+)= 259.8 =k 2 kcal/mol, AHt(PH2) = 33.1 rt 2 kcal/mol, and D(PH2-H) = 83.9 3 kcal/mol. Introduction As pointed out by Friswell and Gowenlock in a review article, the thermochemical and kinetic information concerning phosphino and alkylphosphino radicals is fragmentary and in some cases rather speculative. The only available value for the ionization potential of the phosphino radical, I(PH2) = 9.28 V, has been derived4 indirectly from appearance potentials of the PH2+fragment ion in the spectra of PHBand P*H+ In a recent review of the mass spectrometry of phosphorus hydrides, Fehlner and Called have drawn attention to a number of discrepancies in the appearance-potential data for PH2+, particularly for A(PHZ+/PzH4) as measured by various authors.4-0 These discrepancies appear to arise in part from the instability of PZH4 in conventional ion sources, as evidenced by large observed differences in the fragmentation pattern for PzH4found in normal and “collision-free” sampling. In view of this difficulty it seems rather probable that the values of I(PH2) and AHf(PH2) derived from appearance potentials in PZH4 may be seriously in error. I n the present work we report measurements of the ionization potential of PHz by impact of “monoenergetic” electrons on PHz radicals generated by thermal decomposition of benzylphosphine. Measurements of A(PHz+/PHa)and I(PHa) by this technique are also reported. A value for the ionization potential of (CH&P radical has also been obtained, but is of lower precision. Experimental Section For comparison purposes, two sets of electron-impact experiments were carried out. I n one set the measurements of ionization thresholds were made using a 90” magnetic sector mass spectrometer with a modified Nier-type ion source, incorporating a fused-silica capillary furnace for generation of free radicals at Torr and millisecond contact times.’ The second set of measurements was made using a two-stage doublehemispherical electron-energy selector, combined with a quadrupole mass filter. This instrument has been described recently.8 It was fitted with a similar lowThe Journal of Physical Chemistry
pressure silica furnace, taking suitable precautions to minimize electrical and magnetic interference with the monoenergetic electron beam. Due to the narrow (0.07 V) energy half-width of this instrument compared to that of the conventional ion source, the accuracy of measurement of ionization thresholds is greatly increased. In both instruments the thermal decomposition of benzylphosphine vapor9 was found to produce satisfactory yields of PH2 (and benzyl) radicals, starting at a furnace temperature of 650”. The radical yields became approximately constant by 850”. No products of phosphino radical decomposition or combination, such as phosphorus or phosphorus hydrides other than PH3, were detected even up to 1000”. The presence of free PH2 radicals over this range was confirmed by treating PHz with CH3 radicals produced simultaneously from the pyrolysis of ethyl nitrite. The radical combination product CH3PHz was formed in abundance. Attempts to produce the dimethylphosphino radical (CH3)zP from the thermal decomposition of tetramethyldiphosphinelo were less successful. A small yield of (CH3)zP radical was observed a t 650°, along with products at m/e 76 and 1.5,which were presumably (CH&P and CH3 radicals, respectively. The addition of phenyl
(1) Issued as NRCC Contribution No. 10911. (2) National Research Council of Canada Postdoctorate Fellow, 1966-1968. (3) N. J. Friswell and B. G. Gowenlock, Advan. &ee Badical Chem., 2, 1 (1967). (4) F. E. Saalfeld and H. J. Svec, Inorg. Chem., 3, 1442 (1964). ( 5 ) T. P. Fehlner and R. B. Callen, in “Mass Spectrometry in Inorganic Chemistry,” Advances in Chemistry, No. 72, American Chemical #ociety, Washington, D. C., 1968, Chapter 13, p 181. (6) E‘. Wada and R . W. Kiser, Inorg. Chem., 3, 174 (1963). (7) I. P. Fisher, J. B. Homer, and F. P. Lossing, J. Am. Chem. SOC., 87, 957 (1965). (8) K, Maeda, G. P. Semeluk, and F. P. Lossing, Int. J . Mass Spectrom. Ion Phys., 1, 395 (1968). (9) L. Horner, H . Hoffman, and P. Beck, Chem. Ber., 91, 1583 (1958). (10) H . Niebergall and B. Langenfeld, ibid., 9 5 , 64 (1962).
2997
FREERADICALS BY MASSSPECTROMETRY
." a
a 1
P
5 E-
30-
0
a
-
'0
a
X
2
a
I,
8
20-
0 1
60 40 20
0 ELECTRON ENERGY (VOLTS)
Figure 1. Threshold ionization efficiency curves for PHI radical and Kr standard, obtained with monoenergetic electrons.
radicals, produced by the simultaneous decomposition of t-butyl perbenzoate, produced new peaks a t m/e 138 and 123, corresponding to the parent and parent-minusCHs peaks of the combination product (CH&PCeHb. The yield of (CHS)~Pradicals obtainable, although sufficient to allow an approximate measurement of the ionization potential in the 90" sector mass spectrometer, was too small t o allow the more accurate monoenergetic electron measurement to be made. An attempt to increase the radical-ion current in the latter instrument by increasing the reactant pressure in the ionization chamber above 2 X 10-5 Torr was found to cause the onset of gross instability in the electron multiplier, presumably as a result of the action of phosphorus on the dynode surfaces.
Results and Discussion The ionization efficiency curve for PH2 radical measured using the monoenergetic-electron beam is shown in Figure 1, together with a curve for the krypton standard run simultaneously. The part of the curve shown appears to consist of two nearly linear segments joined by a short curved portion. The extrapolation of the lower segment to zero ion current appears fairly unambiguous, and displays a foot due to the electron energy spread which is about the same as for the krypton curve. The average of several measurements as in Figure 1 gives I(PH2) = 9.83 V with an average deviation of 50.02 V. There is no way to prove from Figure 1 that I(PH2) = 9.83 V is necessarily the adiabatic ionization potential. The sharp onset does suggest that it is, however. Using the 90" sector mass
Figure 2. Threshold curve for production of PH2+ fragment ion from PH,.
.Ot
40 20 0 ELECTRON ENERGY (VOLTS)
Figure 3. Threshold ionization efficiency curve for the parent ion of I",. The structure shown by this curve appears to be reproducible and is probably real.
spectrometer with conventional energy spread, and treating the PH2 onset data by the usual semilogarithmic methodrr I(PH2) = 9.96 V was obtained. The relatively small difference between the value obtained by the two methods, 0.13 V, is consistent with the near-linearity of the PH2 ionization efficiency curve near the onset. I(PH2) = 9.83 V is not consistent with the indirect value of 9.28 derived from PH2+appearance potentials in PH, and P2H4,probably for the reason cited earlier. Volume '78,Number 9 September 196'9
T. MCALLISTER AND F. P. LOSSING
2998 The onset for PH2+ ion production from PHI, measured with the monoenergetic-electron beam, is shown in Figure 2. The curve appears to be segmented a t the foot, and can be extrapolated to zero ion current with reasonable confidence to give A(PH2+) = 13.47 f 0.05 V. This is slightly higher than 13.2 k 0.2 V found in earlier ~ o r k . ~ Using - ~ the 90" sector instrument A (PHZ+) = 13.6 V was obtained. The ionization efficiency curve for the parent ion of PHa measured with the monoenergetic beam is shown in Figure 3. It appears to consist of nearly linear segments at intervals of about 0.15 V. Whether this corresponds to vibrational progressions in PH3+ is not known. The extrapolated threshold for I(PH3) is 9.97 V, in good agreement with the photoionization value of 9.98 V,ll and in reasonable agreement with the R P D measurement6 of 10.05 k 0.05 V. Heat of formatioil 2nd bond dissociation energy data can be derived from these three ionization thresholds as follows: (1) From AHf(PHa) = 1.3 kcal/mol12 and I(PH3) = 9.97 V, one obtains aHr(PH,+) = 231.2 kcal/mol (1 eV = 23.061 kcal/mol). (2) From A (PHz+/PHs) = 13.47 V, one obtains AHf (PH2+) = 259.8 kcal/mol. (3) From AHt(PHz+) = 259.8 kcal/mol and I(PH2) = 9.83 V, one obtains AHf(PH2) = 33.1 kcal/mol. (4) From AHf(PHz) = 33.1 kcal/mol, AHt(PH8) = 1.3 kcal/mol, and AH,(PzH4) = 5.0 kcal/mo1,12one obtains D(PH2-H) = 83.9 kcal/mol and D(PHz-PH2) = 61.2 kcal/mol. These results and the earlier data are summarized in Table I.
The Journal of Physical Chemistry
Table I: Summary of Electron Impact and Heat of Formation Data --Threshold This work
Process
+ + +
+ + + +
-
energy, V Other
PHI e 4 PHa+ 2e 9.97 f 0.02 9.9811 10.056 PH3 e PH2+ H 28 13.47 0.05 13.268s 13.44 9.83 rt 0.02 9.28' PH2 e -+ PHzf 2e (CH&P e 4 (CH&P+ 2e -8.3 ...
+
*
+
Entity
-----AHf, Thia work
koal/mol-----Other
PH3 + PHa + PHI
231.2 & 1 259.8 =I=2 33.1 rt 2
2336 2546 39. 04
83.9 k 2 61.2 4 80.7 Et 3
90.34 74.14 73 I86
Bond dissociation energy
D(PH2- H ) D ( P H z PHz) D(PHa+- H )
As explained above, a precise value for the ionization potential of (CH&P radical could not be obtained. An approximate value of 8.3 V was measured using the 90" sector instrument and the usual semi-logarithmic data treatment,
(11) W. C. Price and T. R. Passmore, Disc. Faraday Soc., 35, 232 (1963). (12) S. R. Gunn and L. G. Green, J . Ph.ys. Chern., 65, 779 (1961).
