YASCOWADAASD ROBERT W. KISER
2290
A Mass Spectrometric Study of Some Alkyl-Substituted Phosphines'
by Yasuo Wada and Robert W. Kiser Department of Chemistry, K a n s a s State University, Manhattan, K a n s a s
(Received A p r i l $3, 1964)
The appearance potentials of positive ions in the mass spectra of trimethylphosphine, triet hylphosphine, monomet hylphosphine, and monoet hylphosphine are reported. Assignments of the probable processes of ionization and dissociation made are consistent with the observed energetics from the electron impact data. The molecular ionization potentials of trimethylphosphine,' triethylphosphine, monomethylphosphine, and monoethylphosphine were determined to be 8.60, 8.27, 9.72, and 9.61 e.v., respectively. The molecular ionization potentials of these compounds were in good agreement with those calculated using the group orbital method (8.35 e.v. for triethylphosphine, 9.53 e.v. for monomethylphosphine, and 9.40 e.v. for monoethylphosphine). The ionization potential of methinophosphide, HCP, is derived to be 13.0 =k 0.6 e.v.
Introduction We have studied previously the phosphorus hydrides phosphine and diphosphine by using mass spectrometric methods.2 In the course of further investigation of phosphorus compounds, we have extended our studies to the alkyl-substituted phosphines, trimethyl-, triethyl-, monomethyl-, and monoethylphosphine. In this paper we report the information obtained on the above compounds, including the mass spectral cracking patterns and appearance potentials for the principal positive ions. From the measured appearance potentials, probable processes are postulated for the formation of the various positive ions consistent with the energetic dats. Experimental The experimental data reported here were obtained using a Bendix time-of-flight mass spectrometer. The instrumentation has been described previously.a The mass spectra were obtained a t a nominal electron energy of 70 e.v. Appearance potentials were determined by the extrapolated voltage difference method.4 The technique of Lossing, Tickner, and Bryce6 and the energy compensation method6 also were used. Calibration of the voltage scale in the determination of ionization and appearance potentials was accomplished by mixing xenon with the compound being investigated. The samples of trimethylphosphine and triethylThe Journal of Physical Chemistrg
phosphine were prepared similar to the method described by Hibbert.' The prepared trialkylphosphines were finally separated from the reaction mixture as the silver iodide complex.8 This complex is very stable and provided significant aid in the necessary handling processes. The samples of pure trialkylphosphines were prepared by the simple thermal decomposition of the trialkylphosphine-silver iodide complex in a vacuum system followed by fractional distillation steps using liquid nitrogen as the refrigerant. The samples of monomethylphosphine and monoethylphosphine were prepared both in a similar manner9 and by the same method.1° 4 slight modification, (1) This work was supported in part by the U. S. Atomic Energy Commission under Contract No. AT(l1-1)-751 with Kansas State University. Portion of a dissertation submitted by Y . Wada to the Graduate School of Kansas State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) Y. Wada and R. W. Kiaer, Inorg. Chem., 3, 174 (1964). (3) E. J. Gallegos and R. W. Kiser, J . Am. Chem. Sac., 83, 773 (1961). (4) J. W. Warren, Nature, 165, 810 (1950). ( 5 ) F. P. Lossing, A. W. Tickner, and W. A. Bryce, J . Chem. P h y s . , 19. 1254 (1951). (6) R. w. Kiser and E. J. Gallegos, J . P h y s . Chem., 66, 947 (1962). (7) H. Hibbert, Ber., 39, 160 (1906). (8) F. G. Mann, A. F. Wells, and D. Purdie, J . Chem. Soc., 1828 (1937); F. G. Mann and A. F. Wella, ibid.,708 (1938). (9) N. Davidson and H. C. Brown, J . Am. Chem. Soc., 64, 719 (1942). (io) R. I. Wagner and A. B. Burg, ibid., 75, 3869 (1953).
&fASS S P E C T R O M E T R I C S T U D Y O F
ALKYL-SUBSTITUTED PHOSPHINES
2291
using an inert atmosphere of nitrogen, was made so as to conduct the reaction under atmospheric pressure rather than in a vacuum system. The prepared samples were then fractionally distilled until reasonably pure samples were obtained.
Results The partial mass spectral cracking patterns and the clastograms which were obtained are given in Fig. 1-4. Experimentally determined mass spectra and appearance potentials for the principal positive ions formed from the compounds investigated are given in columns 2 and 3 of Tables I-IV. Peaks a t m/e = 14, 16, 28, and 32 have been omitted because of unknown contributions from nitrogen and oxygen. The probable processes for the formation of the various ions and the heats of formation consistent with the proposed proc-. esses are presented in colunns 4 and 5 of Tables I-IV.
H2PCH3
4p
4,O
3.0
IO
I
I
d W
? c
?
(0
2.0
3p
40
5.0
6,O
7.0
30
60
70
80
90
(m/d
Figure 2. , 70-e.v. mass spectra of monomethyl- and monoethylphosphine.
W
Yz
dz
a
3
0.2
20
40
80
60
IO 0
I
O0.IOt
20
m
30
40
50
I20
(m/e)
Figure 1. 70-e.v. mass spectra of trimethyl- and triethylphosphine.
/
,
/
-
-
/ /
The heat .of formation of trimethylphosphine in the gas phase is -23.2 kcal./mole.ll Since the heats of formation of triethyl-, monomethyl-, and monoethylphosphine were not available in the literature, we deduced the values from the known values for' trimethylphosphine and phosphine (1.3 kcal./niole12) to be -38 kcal./mole, -7 kcal./mole, and - 12 kcal./mole, respectively, using the method of Franklin.la Other
0 IO
20
30
40
50
ELECTRON ENERQY
Figure 3.-Clastograms
70
60
80
E
(ex)
for trimethyl- and triethylphosphine.
(11) L. H . Long and J. F. Sackman, T r a n s . Faraday SOC.,53, 1606 (1957). (12) S. R. Gunn and L. G.. Green, J. Phgs. Chem., 65, 779 (19F1); S. R. G u m , ibid., 68, 949 (1964). (13) J. L. Franklin, Ind. Eng. Chem., 41, 1070 (1949).
Volume 68,Number 8
August, 1964
YASUOWADAAXD ROBERT W. KISER
2292
Table I : Appearance Potentials and Heats of Formation of the Principal Ions from Trimethylphosphine
m/e
Relative abundance
15 34 35 43 44 45 46 47 57 58 59 60 61 62 75 76 77
15.2 0.8 0.7 1.1 12.1 56.4 10.3 11.4 32.8 11.8 66.2 9.4 100.0 4.1 20.0 78.2 3.0
Appearance potential, e.v.
AHf (ion), kcal./mole
Probable process
P(CHa)a + CHa+ 14.2 f0 . 2 13.2 f0 . 3 1 8 . 4 f0 . 2 16.1 f0 . 4 14.0 f0 . 3 1 4 . 7 f0 . 2 1 6 . 7 f0 . 2
+ (1)
+ + + + + + + + + + + + + + +
PH4+ CH3 CzHz +PC+ CH3 Hz CH4 -+ P C H + 2CHa Hz + PCHz+ 2CHa H + PCHa+ CzHa Hz -+ HPCHa+ CHa CHz + PCzHc;+ CHI 2H2
+
14.0 f0 . 2
+
11.7 f0 . 2
+
10.2 f0 . 5 8.60 f0 . 2
+ CH3 + Hz P(CHa)z+ + CH3
P(CHz)z+
(HaC)zPCHz+ -L P(CHa)a+ -+
+H
218 267 337 232 287 216 330 268 215 160 175
Table 11 : Appearance Potentials and Heats of Formation of the Principal Ions from Triethylphosphine
m/e
34 35 45 46 47 57 58 59 60 61 62 63 75 76 89 90 103 118
Relative abundance
6.6 1.1 29.0 4.7 9.8 46.9 20.7 45.7 14.5 73.0 100.0 5.0 19.5 8.1 14.1 75.6 22.8 52.7
Appearance potential, 0.v.
14.7f0 . 2 14.7 f0 . 3 19.1 f 0 . 5 17.9 f0 . 5 15.8 f0 . 2 16.5 f0 . 3 16.7 f0 . 2 1 6 . 0 f0 . 2 13.4 f0 . 5 14.0 f0 . 2 12.7 f 0 . 2 13.8 f0 . 5 11.4 f 0 . 3 1 0 . 7 f0 . 3 12.0 f 0 . 2 8.27 f O . 2 4
Probable process
PCeHis
+
+ 3CzH4 + CzH3 + 2CzH4 P C + + 2CzH6 + CHa PCHa+ + CZH6 + CzH4 + CHa HPCHa+ + CzHs + CzH4 + CHz PCzHz+ + CzHs + CzH4 + 2Hz PCzHs+ + 2CzH5 + Hz PCzH4+ + 2CzHs + H PCZHE++ C2H4 + Ci" HPCzHfi+ + C2H5 + CzHz + Hz €"a+
+ PH4+ -+
+
-+ + + -+ -+
+ -+
HzPCzHa+
+ 2CzH4
+ (HsCz)PCH3+
-+
-L +
-+
+ CzHs + CHz
+ +
P(CzHs)z+ CzH5 HP(CzH5)z+ CzHa (H&z)zPCHz+ CHI P(CzHfi)a+
+
AH1 (ion), kcal./mole
264 212 29 5 313 224 309 303 235 279 209 230 190 203 196 207 153
Table I11 : Appearance Poteqtials and Heats of Formation of the Principal Ions from Monomethylphosphine"
a
m/e
Relative abundance
Appearance potential, 0.v.
Probable process
15 43 44 45 46 47 48
79.7 8.3 42.6 56.6 100.0 15.8 63.7
14.8f 0.2 1 4 . 5 f0 . 3 14.7 f0 . 3 14.7 f 0 . 2 12.2 f 0 . 2 11.6f0.12 9 . 7 2 f0.15
H2PCHa + CHa+ PHz .-+ P C + H 2Hz + PCH+ 2H2 + PCHz+ H Hz + PCHa+ Hz + HPCHa+ H -+ HzPCHa+
Peaks of m/e = 31, 32, 33, and 34, due to PHa impurities, have been omitted.
The Journal of Physical Chemistry
+ + + + + + + +
AHf (ion), kcal./mole
309 275 332 280 274 208 217
MASSSPECTROMETRIC STUDYOF ALKYL-SUBSTITUTED PHOSPHINES
2293
Table IV : Appearance Potentials and Heats of Formation of the Principal Ions from Monoethylphosphine
m/e
Relative abundance
Appearance potential, e.v.
34 43 44 45 46 47 57 58 59 60 61 62
53.2 25.3 14.4 35.2 10.9 19.7 50.7 35.2 19.4 18.7 13.2 100.0
1 1 . 2 f0 . 2 12.0 f 0 . 3 1 3 . 1 f0 . 5 12.7f0 . 4 12.0 f 0 . 2 12.2 f0 . 2 15.8f0 . 3 1 2 . 9 j=0 . 4 12.9 f0 . 3 12.0 f 0 . 2 12.0 f0 . 3 9.47 f 0.5
HLPCHS
AHr (ion), kcal./mole
Probable process
+
PCZH:, + PH3' CZH4 + PC+ CH3 2Hz -+ PCH'. CH4 Hz + PCHz+ CH3 HZ + PCH3+ CH4 -+ HPCH3+ CH3 + PCzHT+ 2% H + PCzH3+ 2Hz -+ PCzHa+ H HZ + PCzH5+ Hz + HPCzHj+ H + HzPCzH6
234 233 308 249 283 238 300 29 1 238 265 213 206
+ + + +
+ + + + + + + + + + + +
1
Table V : Molecular Ionization Potentials of the Alkyl-Substituted Phosphines --Ionization Calculated
Molecule
12,99" 10.20b 1.55" 1.53c (8.60) 8.35 9.02 8.87 9.53 9.40 a
potential, e.v.-Measured
8 . 6 0 , c 9 2d 8 . 27c 9.7d 9.72O 9,6lC
These parameters are given by Franklin, ref. 15, and Price, * See ref. 2. 0 This work. d See ref. 17.
et al., ref. 16.
IO
20
30
40
50
ELECTRON ENERGY
60
70
80
90
(e~.l
Figure 4. Clastograms for monomethyland monoethylphosphine.
heats of formation employed in our thermochemical calculations were those tabulated by Ga1leg0s.l~ The molecular ionization potentiels of the alkylsubstituted phosphines investigated are summarized in Table V and compared with the calculated values using the group orbital m e t h ~ d . ' ~ ?It' ~is seen that the ionization potentials determined experimentally are in good agreement with the calculated values.
Discussion m/e = 34. This ion can only be PH3+ and is observed from both triethylphosphine and monoethyl-
phosphine. It is not significant in the mass spectrum of either of the methyl-substituted phosphines. Assuming thttt the neutral fragments are as listed in Tables I1 and IV, the heat of formation of this ion is calculated to be 264 kcal ./mole in the triethylphosphine study and 234 kcal./mole in the nionoethylphosphine study. Ahrt(PH3+)from PH3is 237 k ~ a l . / i i i o I e . ~ ~ ~ ~ m/e = 35. The ion must be PH4+ in trimethyland triethylphosphines. The appearance potentials of this interesting ion are determined to be 14.2 e.v. from triniethylphosphine and 14.7 e.v. froin triethylphosphine. Assuming the neutral fragiiients are C2H2 CH3, and C2H3 2CzH4,respectively, AHf(PH4+)
+
+
(14) E. J. Gallegos, "Mass Spectrometric Investigation of Saturated Heterocyclics," Doctoral Dissertation, Kansas State University, Manhattan, Xan., January, 1962. (15) J. L. Franklin, J . Chem. Phys., 2 2 , 1304 (1954). (16) W. C . Price, R. Bralsford, P. V. Harris, and R. G. Ridley,
Spectrochim. Acta, 14, 45 (1959). (17) J. Fischler and i M . Halmann, J . Chem. SOC.,31 (1964).
Volume 68, Number 8 August, 1Q64
2294
is calculated to be 218 kcal./mole and 212 kcal./mole. Taking the average value of AHr(PH4f) = 215 j= 3 kcal./mole, the proton affinity of PH3 is determined to be 153 kcal./mole. Wendlandt’* has reported a value of 207 j= 10 kcal./mole for the proton affinity of PH3. Vetchinkin, et d , l Q have reported 194 f 7 kcal./mole for the proton affinity of NH3, whereas 227 kcal./mole is given for the proton affinity of NH3 by Wendlandt.’* Lampe, Franklin, and Field20 have reported 167 kcal./mole as the lower limit for the proton affinity of HZO. From the fact that PH3 is a weaker base than ”3, the proton affinity of PH3 is expected to be a little lower than 167 kcal./mole. m/e = 43. This ion may be PCf in all cases. AHf(PC+) is calculated to be 267 kcal./mole from P(CH3)3, and 275 kcal./mole in the HzPCH3 case, assuming that the neutral fragments are as shown in Tables I-V. The value of 275 kcal./mole of H2PCH3 is likely too high. Note that this ion is analogous to CN +. m/e = 44. This ion is PCH+. From the appearance potentials obtained, A H ~ ( P C H +is ) determined to be 337 kcal./mole in P(CH3),, 332 kcal./mole in H2PCH3, and 308 kcal./mole in HzPCzH~. This ion, the analog of HCN+, is discussed separately below. m/e = 45. This ion, PCHz+, is observed in all compounds with rather high relative abundance. The AHf(PCH2f) is calculated to be 232 kcal./mole for P(CH3)3, 280 kcal./mole for H2PCH3, and 249 kcal./ mole for HzPCzH6. The values of AHf(PCHz+) are rather poorly spread from 232 to 280 kcal./mole. The ion formation processes may be quite different in each case. m/e = 46. The ion corresponding to this m/e value can only be PCH3+. The appearance potentials for this ion are determined in all compounds. AHf (PCH3f) is calculated to be 287 kcal./mole from P(CHJ3, 313 kcal./mole from P(C2H6)3, 274 kcal./ mole from H2PCH3,and 283 kcal./mole from HzPC2H6. A “best value” is probably 280 kcal./mole. m/e = 47. AHf(HPCH3+)values calculated from the data obtained are 208-238 kcal./mole. However, AHf(HPCH3f) values from PR3 are close to each other and average 220 kcal./mole. m/e = 48. From among the four molecules studied, this ion is produced only from H2PCH3. The process of formation is ionization without dissociation. From the thermochemical data and determined appearance potential, AHf(H2PCH3+)= 217 kcal./mole. m/e = 56. The ion is probably PC2+ and is only observed in the case of the ethyl-substituted phosphines. From the appearance potential data, AHf(PC2+) is calculated to be 254 kcal./mole (from HzPCzH5). The Journal of Physical Chemistry
YASUOWADAAXD ROBERT W. KISER
m/e = 56. The appearance potentials of this ion were determined only in the ethyl-substituted phosphines. The ion is probably HPCzf. AHf(HPC2+) is calculated to be 309 kcal./mole (from H2PC2H6). m/e = 67. Values of the heats of formation of this ion are in the range 300-330 kcal./mole. The ion is fairly intense in each mass spectrum. Thus, the processes of formation of the ions might be quite unique for each case. mie = 58. The ion corresponding to m/e = 58, was studied only in the ethyl-substituted phosphines. This ion is PC2H3+. Assuming that the neutral fragments are as shown in Tables I-V, AHr(PC2H3+)is calculated to be 303 kcal./mole (from P(CzHS)3)and 291 kcal./mole (from HzPC2H6). m/e = 5.9. This ion is observed in P(CH3)3,P(CZH6)3, and HzPCzH6. However, where P(CH2)2+ may be the ion from P(CH3),, the ion, PCzH,+, may be formed from the ethyl-substituted cases. The heats of formation calculated show this reasoning clearly; AHf[P(CH&+] = 268 kcal./mole, while AHf(PCzH4f) is 235 kcal./mole in the P(CzH6)3 case and 238 kcal./ mole in the H z P C Z Hcase. ~ m/e = 60. The ion observed in the ethyl-substituted phosphine spectra is PC2H6+. AHf(PCzH6+)is calculated to be 279 kcal./mole for P(CzHs)3, assuming , 265 that the neutral fragments are CzH6 C Z H ~and kcal./mole for H2PCzH6. m/e = 61. The ion corresponding to m/e = 61 formed in P(CH3), may be P(CH3)2+,which is the most dominant ion in the spectrum, while in P(CZHS)~ and H2PC2H6it is possibly HPCzH6+. If a CH3 is the only neutral fragment in P(CH3)3, AHf[P(CH,)z+] is calculated to be 215 kcal./mole. AHr(HPC2Hsf) is calculated to be 209 kcal./mole from the P ( C Z H ~ ) ~ study and 213 kcal./mole from the IIzPCZHSstudy. m/e = 6d. Appearance potentials were determined for the ion of this m/e only in P(C2HJ3 and HzPCzHs. The ion is probably HzPC2H6+. This ion results from the simple ionization process in HzPCZHS. AHf (HzPCzHb+) calculated is 230 kcal./mole from the P(C2H6)3study and 206 kcal./mole from the HzPCZHS study, The lower value is believed to be more nearly correct. m/e = 75. The ion must be (H~C’)ZPCHZ+ from P(CH3),, and H6CzPCH3+ from P(CzHJ3. AHf[(H3-
+
(18) W.Wendlandt, Science, 122, 831 (1955). (19) S. I. Vetchinkin, E. A. Pshenichnov, and N. D. Sokolov, Z h . Fiz. Khim., 33, 1269 (1959). (20) F. W. Lampe, J. L. Franklin, and F. H. Field, “Kinetics of the Reactions of Ions with Molecules,” in “Progress in Reaction Kinetics,” Pergamon Press, New York, N. Y.,1961.
2295
MASSSPECTROMETRIC STUDYOF ALKYL-SUBSTITUTED PHOSPHISES
C)2PCH2+]is 160 kcal./mole and AHE(H~CZPCH~+) is determined to be 190 kcal./mole. m / e = 76. This ion is only produced upon ionization of P(CH3)3. The ionization potential determined leads to the calculation of AHf[P(CH3)3+]= 175 kcal./ mole. This ion is quite abundant in the mass spectrum of trimethylphosphine, in opposition to the findings of Cullen and Frost.21 m / e = 89. The ion is P(C2HJ2f. Assuming that C2H6 is the neutral fragment, AHf[P(CzH6)2+]is calculated to be 203 kcal./mole. m / e = 90. The ion must be HF’(CzH6)2+. It has a very high abundance. From the determined appearance potential, A H f [ H P ( C ~ H E , ) ~=+ ]196 kcal./ mole is calculated, providing that the neutral fragment is C2H4. Using an estimated heat of formation for HP(CzH6)2 = -25 k.cal./mole, I [ H P ( C Z H ~ )isZ ]found to be 9.6 e.v.. This value is higher than the theoretical value of 8.87 e.v. (see Table V). m / e = 103. AHr[(H6C2)PCHz+]= 207 kcal./mole is calculated, assuming that the neutral fragment is CHs. m/e = 118. P(C2H6)3+is the ion of m / e = 118. From the ionization potential determined, AHf[P(C2H6)3+]is calculated to be 153 kcal./mole. Ionization Potential of Methinophosphide. Gier first prepared the very reactive, pyrophoric methinophosphide (HCP) in 1961.22 Apparently this is the only known compound containing a C=P bond.2z Gier reported the mass spectrum for this molecule and, although the mass spectrum did not aid in distinguishing between the HCP arid HPC structures, the structure of HCP was established by means of infrared studies of HCP. Recently Tylerz3used microwave techniques with both HCP and DCP to verify the structure of HCP and to determine bond lengtihs and rotational constants. Although the mass spectrum was reported by Gier, no appearance potentials were given. In an attempt to add to our knowledge of the properties of this interesting molecule, we have derived a value for the ionization potential of methinophosphide using our appearance potential data for tlhe HCP+ ion de-
termined by electron impact in each of the studied alkylphosphines. Estimating by analogy from nitrogen-containing organic compounds to phosphorus-containing organic compounds, and using Franklin’s approach,I3 a value of AHf(HCP) N 27 kcal./mole is obtained. Coupling this estimate with the average value of AHr(HCP+) = 326 f 13 kcal./mole, the ionization potential of methinophosphide is calculated to be 13.0 f 0.6 e.v. It is interesting to compare this derived value for I(HCP) to experimental values ranging from 13.7 to 13.9 e.v.24-28 for I(HCN). Using this estimated AHf(HCP) and taking D(H-CP) = D(H-CN) = 114 k ~ a l . / m o l e ,a~value ~ of AHf(CP) N 89 kcal./mole is calculated, which in turn leads to a value of D(C=P) = 158 kcal./mole. This last result is nearly the same as the value of Dq(C=P) = 6.9 e.v. given by Herzberga30 Taking the average value of AHf(CP+) = 268 f 22 kcal./mole and using AHf(CP) =I 89 kcal./mole, the ionization potential of C P is calculated to be 7.8 f 1 e.v. This value may be compared to I(CK) = 14.2-14.6 It is seen that a direct electron impact study of methinophosphide is desirable. (21) W. R. Cullen and D. C. Frost, Can. J . Chem., 40, 390 (1962). (22) T. E. Gier, J . Am. Chem. SOC.,83, 1769 (1961). (23) J . K. Tyler, J . Chem. P h y s . , 40, 1170 (1964). (24) T . N. Jewett, P h y s . Rev., 46, 616 (1934). (25) J. D. Morrison and A. J. C. Nicholson, J . Chem. Phys., 20, 1021 (1952). (26) B. C. Cox, Ph.D. Thesis, University of Liverpool, 1953 (reported by J . D. Craggs and H. s. W. Massey, Handbuch P h p i k , (I) 37, 314 (1959)). (27) C. J. Varsel, F. A. Morrell, F. E. Resnik, and W. A. Powell, A n a l . Chem., 32, 182 (1960). (28) K. Watanabe, T. Nakayama, and J. R. Mottl, J . Quant. Spectry. Radiatize Transfer, 2, 369 (1962). (29) T. L. Cottrell, “The Strengths of Chemical Bonds,” 2nd Ed., Butterworth and Co. Ltd. London, 1958, p. 271. (30) G. Herzberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules,” 2nd Ed., D. Van Nostrand Co., Inc., New York, N. Y., 1950, p. 523. (31) D. P. Stevenson, J . Chem. P h y s . , 18, 1347 (1950). (32) J. T. Herron and V. H. Dibeler, J . Am. Chem. Soc., 82, 1555 (1960). (33) J. Berkowitz, J . Chem. P h y s . , 36, 2533 (1962).
Volume 68, Number 8
August, IS64
