D. A. BAFUS,E. J. GALLEGOS, A N D R. W. KISER
2614
An Electron Impact Investigation of Some Alkyl Phosphate Esters
by Donald A. Bafus,' Emilio J. Gallegos,2and Robert W. Kiser3 Contribution from the Department of Chemistry, Kansas State University, Manhattan, Kansas 66502,
and Chevron Research Company, Richmond, California (Received ,Votember 29, 1965)
Partial mass spectra of dimethyl methylphosphonate, trimethyl phosphate, and triethyl phosphate are reported. I n addition, the appearance potentials, precise determinations of ionic masses, and observed metastable transitions are reported for these compounds. Ionization and dissociation processes are assigned on the basis of energetic and highresolution data. Heats of formation of the gaseous ionic species determined from the energetic data are in agreement with literature values. The molecular ionization potentials of dimethyl methylphosphonate, trimethyl phosphate, and triethyl phosphate were determined to be 10.48, 10.77, and 10.06 ev, respectively.
Introduction Interest has been shown recently in the mass specalkyltrometric studies of phosphine and dipho~phine,~ substituted phosphine^,^ and alkyl-substituted phosphite esters6p7 and phosphonate~.~However, only a few mass spectrometric studies have been carried out on the alkyl phosphate esters. The mass spectrum of triethyl phosphate has been reported by both McLafTerty8 and Q ~ a y l eand , ~ Quayle has also reported the mass spectra of tri-n-butyl phosphate and tris(1-chloro-Zpropyl) thionopho~phate.~It was found that during ionization and dissociation considerable hydrogen atom rearrangement occurs producing some interesting gaseous ionic species (e.g., HP02+, HzPOl+, HPOSf, H*P04+, etc). However, no attempt was made to determine the appearance potentials for these molecules. I t is of interest to examine also the high-resolution mass spectra of related molecules, with precise determinations of ionic mass, since it is possible that other atomic combinations can account for the same nominal m/e values observed (e.g., CH4PO+instead of POZ+and CH5P02+ instead of HP03+, etc). In addition, the appearance potentials of the principal positive ions in the mass spectra of these compounds could be helpful in determining what atomic combinations are present at the threshold of formation of the fragment ions a t nominal m/e values of interest and thereby give insight into the processes of molecular ion fragmentation. We report our methods of study and the results observed in the mass spectrometric investigation of three alkyl phosphate esters. With the exception of the The J O U T n d of Physical Chemistry
mass spectrum of triethyl phosphate, the mass spectra, appearance potentials, precise ionic mass determinations, and metastable transitions reported herein are new mass spectral data.
Experimental Section The instrument used to determine the appearance potentials and mass spectra was a Bendix nlodel 12-100 time-of-flight mass spectrometer described elsewhere.1° The precise mass determinations and metastable transitions were obtained with an AEI AIS-9 double-focusing high-resolution mass spectrometer of Nier ge0metry.l' The AIS-9 was also used to check the ionization potentials determined with the time-of-flight instrument. The triethyl phosphate used was Eastman Kodak Yellow Label and was used without further purification. The trimethyl phosphate and dimethyl methyl~~
~
~
(1) U. 8. Naval Radiological Defense Laboratory, San Francisco,
Calif. (2) Chevron Research Co. (3) Kansas State University. (4) Y. Wada and R. UT.Kiser, Inorg. Chem., 3, 174 (1964). (5) Y. Wada and R. W. Kiser, J . Phys. Chem., 68, 2290 (1964). (6) Y. Wada, Doctoral Dissertation, Kansas State University, Manhattan, Kan., iiug 1964, and references cited therein. (7) J. L. Occolowitz and G. L. White, Anal. Chem., 35, 1179 (1963) (8) F. W. McLafferty, ibid., 23, 306 (1956). (9) A. Quayle, Advan. Mass Spectry., 365 (1959). (10) E. J. Gallegos and R. W. Kiser, J . Am. Chem. SOC.,83, 773 (1961); J . Phys. Chem., 65, 1177 (1961). (11) High-resolution studies were carried out on this instrument a t Chevron Research Co.
ELECTRON IMPACT INVESTIGATION OF ALKYLPHOSPHATE ESTERS
phosphonate used were obtained from Aldrich Chemical Co. and also were used without further purification. I n all cases volatile impurities were observed upon introduction of the samples to the mass spectrometer; but after prolonged pumping with the vacuum system of the mass spectrometer, these impurities disappeared, at which time the mass spectrum and appearance potentials were recorded. Subsequently, a gas chromatographic check on purity of the trimethyl and triethyl phosphate was carried out, and they were found to be, by means of this technique, free of impurities. The purity of dimethyl methylphosphonate was determined mass spectrometrically, and the sample was found to contain about 5 mole yo trimethyl phosphate. However, the appearance potentials measured for dimethyl methylphosphonate indicate very strongly that this impurity presented no difficulties. The different series of metastable ions observed from the two compounds also indicates no serious difficulties were experienced. The low vapor pressure of these compoundse necessitated the use of calibrants for the electron energy scale other than the noble gases. Mercury (IP = 10.43 ev12) background from the diffusion pump was used to calibrate the scale for triethyl phosphate. The vapor pressures of dimethyl methylphosphonate and trimethyl phosphate were sufficient to “flood” mercury from the ion source but still not sufficient to be able to use the rare gases. For these compounds a small amount of water (IP = 12.59 e v 9 was introduced into the sample prior to the introduction of the sample into the mass spectrometer. The samples exhibited no deleterious effects from the introduction of water. Appearance potentials were determined from the ionization efficiency curves using the technique of extrapolated voltage differences reported by Warren14 and the technique of Lossing, Tickner, and Bryce.15 The energy compensation method16 was used to check the determinations of ionization potentials. The mass spectra were recorded a t a nominal electron energy of 70 v. The precise masses of unknown ions reported in this study were determined using the fragment ions of heptacosafluorotributylamine, (C4F&N, and were generally within 5 ppm of the values calculated from tabulated values of nuclidic masses. The precise masses for the fragment ion from (C4F&N have been reported elsewhere. l7
Results and Discussion The mass spectra and appearance potentials determined for the alkyl phosphates in this study are shown in Tables 1-111. The error values of the appearance
2615
potentials indicate a one standard deviation precision of triplicate determinations. The mass measurements shown in Table IV for metastable transitions have an uncertainty of one in the last digit given. The probable processes which give rise to the observed metastable transitions are indicated in Table IV; these data were used, insofar as possible, to aid in the prediction of the probable processes listed in Tables 1-111. The molecular heats of formation used in the thermochemical calculations are: (CH30)3PO, -239.4 kcal mole-l;l* (CH30)2CH3P0,-211.9 kcal mole-l (estimated by the method of Franklinle) ; and (C2H,O)sPO, - 285.1 kcal mole-1.20 Heats of formation employed for other gaseous species are: H, 52.1;21v22CHI, 33.2;2112a CH4, -17.9;21 C2H3, 64;24 C2H4, 12.5;;, H20, -57.8;21 CO, -26.4;21 CH20, -27.1;’l CHa0, 5;26 C2H30, 8;24 CzH40,-39.8;25 and C2H50,-8.527 kcal mole-l. Trimethyl Phosphate. The similarity of the ionization potential of this molecule, 10.77 ev, to that observed for methanol, 10.88 ev,28suggests that the electron removed upon ionization is a nonbonding electron of the oxygen bound to both a carbon and phosphorus, rather than either a nonbonding electron of the oxygen bound only to the phosphorus or one of the bonding electrons. The fragment ions observed for this molecule are similar to those suggested by Wadas and (12) C. E. Moore, “Atomic Energy Levels,” National Bureau of Standards Circular 467, Vol. 3, U. S. Government Printing Office, Washington, D . C., 1958. (13) K. Watanabe, J. Chem. Phys., 26, 542 (1957). (14) J. W. Warren, Nature, 165, 810 (1960). (15) F. P. Lossing, A. W. Tickner, and W. A. Bryce, J. Chem. Phys., 19, 1254 (1951). (16) R. W. Kiser and E. J. Gallegos, J. Phys. Chem., 66, 947 (1962). (17) AEI Technical Information Sheet A169. (18) ASTIA, AD84063, National Bureau of Standards No. 68219, Dec 31, 1955, 102 pp. (19) J. L. Franklin, Ind. Eng. Chem., 41, 1070 (1949). (20) S. B. Hartley, W. S. Holmes, J. K. Jacques, M. F. Mole, and J. C. McCoubrey, Quart. Rev. (London), 17, 204 (1963). (21) D . D . Wagman, W. H . Evans, I. Halow, V. B. Parker, S. M. Bailey, and R. H. Schumm, “Selected Values of Chemical Thermodynamic Properties. Part 1,” National Bureau of Standards Technical Note 270-1, U. S. Government Printing Office, Washington, D . C., Oct 1 , 1965. (22) H. A. Skinner and G. Pilcher, Quart. Rev. (London), 17, 264 (1963). (23) B. E. Knox and H. B. Palmer, Chem. Rev., 61, 247 (1961). (24) M. Szwarc, ibid., 47, 75 (1950). (25) F. D. Rossini, D . D. Wagman, W. H. Evans, S. Levine, and I
Jaffe, “Selected Values of Chemical Thermodynamic Properties,’ National Bureau of Standards Circular 500, U. 6. Government Printing Office, Washington, D. C., Feb 1, 1952. (26) R. H. Martin, F. W. Lampe, and R. W. Taft, J. Am. Chem. Soc., 88, 1353 (1966).
(27) P. Gray and A. Williams, Chem. Rev., 59, 239 (1959). (28) J. D. Morrison and A. J. C. Nicholson, J. Chem Phys., 20,1021 (1952).
Volume 70, Number 8 August 1966
D. A. BAFUS,E. J. GALLEGOS, AND R. W. KISER
2616
~~
Table I: Partial Mass Spectrum and Appearance Potentials of the Principal Positive Ions of Trimethyl Phosphate RACa t
a
m/e
70 v
15 29 31 45 47 48 65 79 80 95 109 110 139 140
100.0 36.6 16.7 1.3 22.0 2.0 5.0 26.6 17.3 22.1 28.0 86.3 5.4 14.0
See Discussion.
-AP,
AXf(ion), kcal/mole
Probable process
ev-
CaHs04P
18.6 f 0 . 3 17.3 f0.4 17.3 rt 0 . 4 14.0 rt 0 . 2 19.6 rt 0 . 3
.-+
1 5 . 1 i0 . 3 15.1 f 0 . 2 13.9 f 0 . 4 14.4 zk 0 . 3 (14.3)" 11.9 zk 0 . 2 10.77 rt 0.30'
' 10.73 ev was measured us. Xe with the MS-9.
+ + + + + + + + + + + + + + + + + + + +
...
CH3+ ? CHO+ ? -+ CHaO+ f ? -+ CzHsO+ ? -+ PO+ 2CH20 CHaO 2H -+ HPO' H 2CHz0 CHaO -+ HzOzP' 2CH20 CH1 + CH40zP+ 2CHz0 H -+. CHsOzPf 2CH20 -+ CHaOsP+ CHzO CH3 -+ CzHeOaP' CHzO H + CzH70aP+ CHzO .-+ CaHsOaP+ H -t CaH904P+ -+
...
... 158
... 130 111 135
87 (65)" 62
...
9
Relative abundance.
Table I1 : Partial Mass Spectrum and Appearance Potentials of the Principal Positive Ions of Dimethyl Methylphosphonate
a
mle
RACa t 70 v
15 29 31 45 47 48 63 79 80 93 94 109 123 124
82.5 39.8 16.9 6.0 47.6 3.0 13.3 100.0 21.7 38.6 88.0 28.3 3.4 15.7
-+
16.9 f 0 . 2 1 3 . 4 i0 . 3 (11.9)" 12.5 i 0.1 11.5f0.2 1 3 . 3 zk 0 . 3 10.48 zk 0.20'
-+
' 10.43 ev measured us. Xe with the MS-9.
Occolowitz and White7 and, where comparison can be made, the heats of formation of these fragment ions are in general agreement with those reported by Wada. Mass measurements on mle 79 and 80 show that these masses arise from CH402P+and CH602P+,respectively, rather than PO3+ and HP03+ and are the same ions which were observed by Occolowitz and White7 for wile 79 and 80 in trimethyl phosphite. The mass measurements also show that m/e 45 is due to CzHsO+ rather than PCHz+; therefore, the m/e 45 ion must arise by rearrangement. The metastable ions observed in the mass spectrum of this molecule arise exclusively by the elimination of a neutral CHzO species. As can be seen from Table IV, the probable processes listed yield values for the The Journal of Physical Chemistry
+ + + + + + + + + + + + + + + + + + + + +
C3Hs03P + CH3+ ? -+ CHO+ ? CH30f ? -+. C2Hs0+ ? -+ PO+ 2CHz0 CHa 2H + POH+ 2CH20 CH3 H + CHdOP+ 2CH20 H -+ CHa02P+ CHzO CH3 -+ CHr,OzP+ CO CH4 + CzHeOzP+ CHz0 H + CzH?OzP+ CHzO + CzHeOaPf CHI .-+ CaHsOaP+ H CaHsOaP+
16.8& 0.1 18.9 f 0 . 8 13.8i0 . 2 16.3 i 0 . 1 19.4 f 0 . 3
See Discussion; measured us. Xe with the MS-9.
AHf(ion), kcal/mole
Probable process
AP, ev
...
...
... ... 152
... 180 91 (108)o 51 80 62
... 30
Relative abundance.
metastable masses which are in excellent agreement with those observed experimentally. Using these processes as guide lines, a decomposition scheme of the parent-molecule ion can be suggested which is compatible also with the energetic requirements. This scheme may be represented as
140+
110+ +80+ , \
4
95+ --+ 65+
2617
ELECTRON IMPACT INVESTIGATION OF ALKYLPHOSPHATE ESTERS
Table I11 : Partial Mass Spectrum and Appearance Potentials of the Principal Positive Ions of Triethyl Phosphate AHdion), kcal/mole
RA* a t d e
70 v
29 45 81 82 83 99 109 110 111 125 126 127 137 138 139 153 155 167 181 182
25.0 45.4 78.3 47.0 14.4 100.0 49.3 9.2 18.2 19.7 5.7 47.0 11.4 10.6 8.3 4.2 68.0 6.1 2.5 26.0
AP, ev
Probable process
+
Cd%504P -* CZH5' ? -* CzH5O+ ? + HzOaP' CzH3 2CzH4 H20 + H303P+ CzH40 2CzH4 -* H&P+ 2CzH4 CzHaO + H404P+ CzHa 2CzH4 -* CzHeO&?+ Cz&O f CzH4 CzH,OsP+ CzH40 CzH4 -* CzHsOaP' CHs CO CzH4 + CzHaOaP+ 2CzH4 H -* CzH704P+ 2CzH4 4 CzHs04P+ CzH4 C2H3 -* C4Hio0aPf CzH50 -* C4HiiOaP' CzHiO C1Hiz03P+ CH3 CO + C4HioOaP+ CzH4 H 4 C4Hi204P+ CzH3 -* GHiz04P+ CHa CeHir04P+ H -* CeH&4Pf
18.9 f 0 . 1 16.8 f 0 . 2 17.8 f0.3 14.4 f 0 . 2 14.5 f 0 . 3 14.3 f0.3 15.5 f 0 . 4 13.5 f 0 . 3 13.6 f 0 . 2 13.5 f0 . 3 12.8 f 0.2 12.7 f 0 . 2 12.7 f0.2 11.6 f 0 . 4 12.3 i 0 . 2
-+
-+
+ + + + + + + + + + + + + + + + + + + + + + + + + + +
+ +
11.5 i 0 . 3 11.9 i 0 . 3
-+
10.06
0.27"
* 10.02 f 0.20 ev measured us. Xe with the MS9.
+
... ... 94 62 16 -44 68 54 9 - 51 - 15 -69 16 22 -8
... -84 -44
...
- 53
* Relative abundance.
Table IV : Observed Metastable Transitions in Trimethyl Phosphate, Dimethyl Methylphosphonate, and Triethyl Phosphate Molecule
(CHs0)aPO
(CHaO)&HaPO
Metastable masExptl
Probable processesa--
Calcd
86.41 58.17 57.26
86.43 58.18 57.26
CaHs04P+ (140) CzH?OsP+(110) CzHeOaP' (109)
71.27 66.39 57.26 42.68
71.26 66.39 57.26 42.68
CaHgOaP+ (124) CzH?OzP+(94) CzHeOaP+ (109) CzHeOzP+ (93)
132.0 129.3 115.7 104.6 104.1 102.1 88.6 77.2 66.3 61.1 54.7
132.01 129.33 115.69 104.64 104.06 102.12 88.64 77.17 66.27 61.13 54.68
-+
--t -+
+
-* -+
+
+ + + C?H?OzP+(94) + CHiO (30) CHdOzP+ (79) + CHa (15) CHaOzP+ (79) + CHzO (30) CHiOP+ (63) + CH20 (30) CaHinOnP+ (155) + CzH3 (27) CaHioOiP+ (153) + C2H4 (28) C4Hi208P+ (139) + CO (28) CaHiiOsP+ (138) + CzHaO (44) CzHe04P+ (127) + C2H4 (28) CzHe04P+ (125) + C2H4 (28) C?HeOaP+ (111) + CzH4 (28) H404P' (99) + CzH4(28) H20aP' (81) + Hz0 (18) HaOsP+ (82) + CzH4 (28) &OO,P+(83) + CzHaO (43)
CzH7OaP+ (110) CHzO (30) CHsO?P+ (80) CHIO (30) CH40zP+ (79) CHz0 (30)
CdIia04Pf (182) CSHI~OOIP+ (181) 6 CsHiz04P+ (167) CdIisoiP+ (182) C4HizOdP+ (155) -+ C4HioO4P+ (153) + C4Hiz03P+ (139) C2&04P+ (127) + + HaOiP+ (99) CzH?OaP+ (110) + CzHtOaP+ (126) + -+
-+ -+
-+
* Including nominal masses of ionic and neutral species.
The solid arrows indicate paths for which metastable processes are observed, while the broken arrows indicate paths which are suggested by the energetic considerations. Since all ions formed from this molecule have a t least
one rearrangement involved in their formation and no heats of formation are available for phosphorus radicals, no estimation of ionization potentials for these radicals can be made. However, the AHr(P0) of -5.8 kea1 mole-' 2 9 is known, and combining this with AHr(PO)+ Volume 70, Number 8 August 1966
D. A. BAFUS,E. J. GALLEGOS, AND R. W. KISER
2618
of 155 i= 3 kcal mole-l (the average of the values for this ion shown in Tables I and 11) yields a value of I(P0) = 7.0 ev. This appears to be a reasonable value when it is compared with the ionization potential of 9.25 ev for NO.30 Wilkinsonsl has estimated I ( P 0 ) N 8 ev. From the trends in the A 0 and BO ionization potentials, where A is an element of the second period and B is an element of the third period, I(P0) s 7 ev may be estimated. The AHr(C&03P+) of 65 f 3 kcal mole-l used to estimate the appearance potential of m / e 109 for this molecule is an average of the values given in Tables I1 and 111. The estimated value of 14.3 ev appears to be a reasonable value when the difference in appearance potentials of m / e 109 and 110, 2.4 ev, is compared to the difference of 2.0 ev observed in triethyl phosphate (see Table 111). I n addition, this difference is just the bond dissociation energy, D(C2HB02PO+-H), not too dissimilar to the values listed by Field and Franklin32for some oxygen-containing ions. This appearance potential was also determined on the MS-9 using xenon as the calibrant and found to be 14.1 ev, in good agreement with the calculation. Therefore, the value of 65 k 3 kcal mole-’ may be taken as a “best” value for AHt (C~HaOd’+) Dimethyl Methylphosphonate. There are two possible structural configurations for a molecule of the composition (CH&,03P
0 CH3-O-P-O-CH3
I/ I
and CH3-O-P-O-CH3
I
O-CHI
I
CH3 I1
I n I the phosphorus is in a trigonal environment, while the phosphorus in I1 is in a tetrahedral environment. Structure I1 might be expected to have an ionization potential very similar to that of trimethyl phosphate, but I might be expected to be somewhat lower, since a nonbonding electron on phosphorus is available for ionization. Comparing two compounds whose geometries are known, POCla and PC&, it is found that the ionization potential for POC& is about 1 higher than that for P C h 6 Since dimethyl methylphosphonate has . an ionization potential 1.66 ev larger than trimethyl phosphite, structure I1 is preferred for dimethyl methylphosphonate, and I for trimethyl phosphite. An investigation Of trimethyl phosphite in the MS-9 gave a molecular ionization potential for this cornpound Of 8.92 ev; this is only slightly greater than the Of 8*82ev determined by Wadas’ In addition to the same four metastables observed for dimethyl The Journal of Physical Chemistry
methylphosphonate, the mass spectrum of trimethyl phosphite exhibits another metastable at 95.81, representing the dissociation of 124+ to 109+ with the loss of a methyl group (it is calculated that this metastable should occur at 95.80). I n addition, synthetic methodP are well documented for the preparation of (R0)zR’PO (11) from (R0)3P (I) by the reaction (R0)3P
+ R’X + (R0)zR’PO + R X
As was the case for trimethyl phosphate, the metastable masses calculated from the probable processes shown in Table IV are in excellent agreement with those observed experimentally. A decomposition scheme for the parent-molecule ion can be proposed using the metastable processes as a guide which is consistent with energetic considerations. This scheme may be represented as 123+
93+
63+ --+ 48+ --+ 47+
fl /
124+ --f 94f +79+ ‘\
4
/
109+
The significance of the solid and broken arrows is the same as employed above for trimethyl phosphate. Only one fragment ion, m / e 109, arises by simple bond cleavage from the parent-molecule ion. m / e 109 arises by the loss of a m / e 15 moiety, CH3, and hence could arise by cleavage of two different bonds (see structure I1 above). One bond, the P-C bond, has a bond dissociation energy of 65 kcal mo1e-1,18 while the other bond, a C-0 bond, has a bond dissociation energy of about 84 kcalm0le-1.~~Since the P-C bond energy is less than that of the C-0 bond, the P-C bond is the one considered to be broken; this leads to an ionization potential for C2HeOsP of approximately 10.5 ev when the usual equation A(Ri+)
2 D(Ri - R2)
+ I(&)
(1)
is used. A(Rl+) was taken as 13.3 ev (see Table 11). This value is about the same as that observed for the ionization potential of the parent and is estimated to be (29) R. L. Potter and V. N. DiStefano, J . Phya. Chem., 65, 849 (1961).
.
,
(30) K.Watanabe, F.Marmo, and E. C. Y. Inn., Phya. Rev., 91,436 (1953)* (31) P.G. Wilkinson, Aatrophya. J . , 138,778 (1963). (32) F. H.Field and J. L. Franklin, “Electron Impact Phenomena and the Properties of Gaseous Ions,” Academic Press Inc., New York, N. Y., 1967. (33) G. M. Kosolapoff, “Organophosphorus Compounds,” John Wiley and Sons, Inc., New York, N. Y., 1950. (34) T. L. Cottrell, “The Strengths of Chemical Bonds,” 2nd ed, Butterworth and CO. Ltd., London, 1958.
2619
ELECTRON IMPACT INVESTIGATION OF ALKYLPHOSPHATE ESTERS
high about 1 ev when compared to the ionization potentials estimated for other phosphorus radicals (see below). The appearance potential for m / e 80 shown in Table I1 was measured vs. xenon on the MS-9 and gives a value of 108 kcal mole-' for AHr(CHbOzP+) which does not compare well with the value in Table I for m / e 80; however, it does compare favorably with the 114 kcal mole-' reported by Wada for this same ion from dimethyl phosphite.6 This leaves sufficient doubt as to the best value for the heat of formation of CHEOzP+ and/or the structure of this ion that additional studies are planned in these laboratories. Triethyl Phosphate. The general features of the mass spectrum are in agreement with that reported by McLafferty.8 The base peak reported by McLafferty was m / e 155, while the base peak reported here with both the MS-9 and the time-of-flight units was m / e 99. Since m / e 99 was reported by McLafferty to have a relative abundance of 93% of m / e 155, the noted difference is not considered to be significant and is probably due to instrumental factors. The ionic species shown in Table 111, and confirmed by precise mass measurements, are in complete agreement with those suggested by McLaff erty. High-resolution studies of nominal m / e 63 (not shown in Table 111) and 29 showed that two ions are present at each mass. Precise mass measurements at 70 ev on each of these species showed that m / e 63 consists of CHBOP+and POz+, and m / e 39 consists of CzH5+ and CHOf. As the electron energy was decreased the low mass component of each nominal mass, i.e., POz+ and CHO+, rapidly diminished in the mass spectrum. At the threshold, m / e 63 and 29 consist of CHIOP+ and C2H5+, respectively. This result is not unexpected. The low mass components of each peak arise as the result of more severe fragmentation; Le., they require more energy for their formation than do the high mass component of each nominal mass. The large number (17) of metastable ions observed in the mass spectrum, of which 11 of the more intense ones are given in Table IV along with the probable processes, is not surprising in that seven metastable ions are observed in the negative ion mass spectrum of C2H602C12P.35 As was observed for the methyl compounds above, the metastable ions arise predominantly as the result of rearrangement processes rather than simple bond cleavage. The major neutral species eliminated in the metastable transitions is m / e 28, which is the same as that observed for many hydrocarbon mole-
cules.s2 Using energetic considerations and the probable processes for the formation of the metastable ions, in Table IV, the following decomposition scheme of the parent-molecule ion is suggested. 181+ +153+ -+-125+ 167+ ---+ 139+ --+ 111+ ' .x 155+ -3 127+ +99+ -3 81+ 182+-, --3r 154+ --+ 126+ --f 83' + \=L , 138+ - - f 110' 82+ x 137+ ---+ 109+
&
The significance of the solid and broken arrows is as mentioned earlier. The remaining six metastable ions and their associated probable processes, although not reported here, indicate that branching occurs from the secondary ions and further complicates the decomposition scheme. Assuming the above decomposition scheme to be correct, m / e 167 and 137 are both formed by simple bond rupture. Thus it is possible to estimate the ionization potentials of C5HlZ04Pand C4H1003Pusing eq 1 and the m / e 167 appropriate bond dissociation energies. arises from the rupture of a carbon-carbon bond for which a value of 84 kcal mole-' 34 is taken. This, together with the appearance potential of 11.9 ev gives an 1(CbHu04P)= 8.3 ev, which is probably within kO.5 ev, since the values for D(C-C) in various molecules vary from 73 to 93 kcal m01e-l.~~m / e 137 arises from the rupture of a P-0 bond for which Hartley, et aZ.,'-'O give a value of 4.0 ev. This value combined with the appearance potential of 12.7 ev gives a value of 8.7 ev for 1(C4Hl0O3P). The ionization potential of triethyl phosphate of 10.06 ev is about 0.5 ev below 10.60 ev observed for ethanol36 and suggests that the electron removed on ionization is a nonbonding electron of the oxygen bound to carbon and phosphorus (see discussion concerning trimethyl phosphate above). Acknowledgments. This work was supported in part by the U. S. Atomic Energy Commission under Contract No. AT(11-1)-751 and in part by Melpar, Inc., under Subcontract No. SU-571466-64 with Kansas State University. The authors wish to acknowledge gratefully the cooperation and assistance provided by the Chevron Research Company. (35) B. L. Donally and H. E. Carr, Phys. Rer., 93, 111 (1954). (36) T. Kambara and I. Kanomata, Proc. Phys. SOC.Japan, 4, 71 (1949).
Volume 70, iVumber 8 August 1966
