3849
may be used with either (6) or (7) to obtain k3 = (3.4 1.2) X 10-'0cm3 molecule-' sec-'.
f
Conclusions Ferrocene is amenable to study in the ion source of a mass spectrometer at pressures up to 1.5 X Torr. The fragment ions Fe+ and FeC5H5+ both undergo rapid charge transfer with neutral ferrocene. These reactions are exothermic for ground-state reactants. The binuclear adduct Fe2(C5H6)g+is formed from reaction of FeC5H5+and ferrocene and does not undergo further reaction under the present conditions. Charge transfer involving FeC5H6+ occurs about six times as fast as the competing adduct formation. The rate constants for reaction of Fe+ and FeC6H5+with ferrocene are about twice those predicted by the polarization theory of Gioumousis and Ste~enson,~' using
the relationship
where e is the charge on the ion, a is the molecular polarizability of ferrocene,S and p is the reduced mass of the reacting system. Considering the limitations of the theory,I8 the agreement is good and perhaps fortuitous, but it suggests no significant steric or energetic barrier to these reactions. Further studies of gaseous ion chemistry of metal complexes should provide useful information about reactions and chemical species not occurring or not easily studied in condensed or neutral systems. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
(17) G. Gioumousis and D. P. Stevenson, J . Chem. Phys., 29, 294
(18) L. Friedman, Annu. Rev. Phys. Chem., 19,273 (1968).
( 19 58).
Low-Temperature Chemical Ionization Mass Spectrometry of Boron Hydrides. The Proton Affinities of Diborane and Tetraborane( 10) Robert C.Pierce and Richard F.Porter*
Contributionfrom the Department of Chemistry, Cornel1 University,Ithaca, New York 14850. ReceivedJanuary 8,1973 Abstract: Low-temperature chemical ionization spectra of B2HE,B4H10,and B6H8in methane have been obtained. Protonation of BzHeto yield B2H7+is observed in CH4 between 79 and 86°K. The proton affinity of B2HE is determined to be 147 f 4 kcal/mol. In the B2H&H4 and B2HrH1 systems &&+ is formed by addition of B*Hs+to BsHs between 80 and 140°K and between 80 and 115"K,respectively. From the observed heat of formation of Ball+, the proton affinity of B4H10 is calculated to be 144 i 5 kcal/mol. For the B4Hlo-CHr and BsH8-CH4 systems ion interconversions are studied as a function of temperature. These results indicate the relative ordering of the zero point energies of several BdHn+and BsHn+ions.
In a
recent publication' we reported the results of a chemical ionization study of the common boron hydrides with a number of ionizing reagents. These results showed that B5HS and BaHlo are moderately strong bases. Independently, Johnson, et al., have isolated the B6&+ cation in a strong acid medium. Chemical ionization of B2H6, B4H10, and B5H11 by CH6+ at source temperatures of 100-200" leads t o hydride abstraction from the boron hydride with the formation of (M - I)+ ions. Only an estimate of the proton affinity of diborane could be obtained from these measurements. It is possible that failure to I)+ ion in these experiments was due to detect the (M thermodynamic instability of the ion at the operating source temperature or to complicated kinetic factors in the proton transfer mechanism. To examine these problems further we have modified our experimental technique for study of these processes at low tempera-
+
(1) J. Solomon and R. F. Porter, J . Amer. Chem. SOC.,94,1443 (1972). (2) H. D. Johnson, 11, V. T. Brice, G. L. Brubaker, and S. G. Shore, J . Amer. Chem. SOC.,94, 6711 (1972).
tures. The results of this investigation have led to a better understanding of the nature of the proton transfer process in these systems and have provided information on the proton affinities of BzHa and B4H10. Experimental Section The design of the low-temperature chemical ionization (CI) source used in this study is illustrated in Figure 1. The ion box and inlet tube were constructed of 304 stainless steel. Source pressures were measured with a McLeod gauge through a side-arm connection into the source cavity. An ion repeller was located 1.2 cm from the ion exit hole. For most experiments the source field strength was maintained at either 10 or 23 V/cm (in this range of field strengths the extent of chemical ionization was seen to vary but not the essential processes of interest). Rhenium filaments were used, and the electron accelerating energy was fixed at 300 V. The source was sandwiched between two heavy copper blocks fitted with refrigeration coils. With liquid nitrogen as coolant it was possible to attain source temperatures between 79 and 310°K (the ambient temperature with hot filament). Temperatures were measured with a chromel-alumel thermocouple spot-welded to the source. The thermocouple voltage was monitored continuously on a Varian Model No, G-2010 recorder. The source pressure was controlled by adjusting the inlet gas flow rate with a Nupro
Pierce, Porter
/ Chemical Ionization Mass Spectrometry of Boron Hydrides
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0.2Model No. SS-4M needle valve. The establishment of temperature equilibrium between the gases and the walls of the ionization chamber in this type of source has been considered by Beggs and 0Field.3 The arguments presented in their paper are generally 1 I I I I I I I I applicable to the work described here. 180 200 220 240 260 280 300 320 The mass analyzer was assembled from an Extranuclear Model (TOK 1 270-9 quadrupole mass filter and a particle detector consisting of a 24-stage Mg-A1 electron multiplier located on axis with the quadFigure 2. Temperature dependence for the B5H,,+ ions: source rupole. The vacuum housing was mounted to a 4-in. diffusion pressure (Pa = 0.07 mm); sample composition, CHI:B5H9 = pump equipped with a 4-in. "Cryosorb" (Granville Phillips) stain200: 1. Temperature dependence for the BsH,D,+ ions: source less steel baffle. Residual pressure in the quadrupole region pressure (P. = 0.06 mm); sample composition, CD4:BbHg = reached about mm when the source pressure of CH4 was 0.80 1GQ:l. mm. One operational advantage of the quadrupole configuration is the low ion accelerating voltage required (between 5 and 10 V). In studies of methane-boron hydride mixtures some experiprofiles in Figure 2. Also shown in Figure 2 is a breakdown of the mental restrictions are imposed by the properties of the boron comintensities of deuterium labeled products observed in studies of a pound. For B4Hlumixtures the lowest operating temperature was BaHo-CD4 mixture. The H-D isotope distributions in the ion about 140°K due to condensation of B4HI0on the walls of the source. products resulting from loss of one or two hydrogen molecules A problem that is frequently encountered in chemical ionization of from B5HoD+for several temperatures are given in Table I. Based methane-boron hydride mixtures is a notable decrease in operational sensitivity from that experienced with pure hydrocarbon gases. This effect is apparently due to the formation of solid Table I. Isotopic Composition of Ions from the Decomposition deposits on the exterior surface of the source as the gases exit of BsHeD+Produced by CI of BsH9in C D P from the slit aligned with the hot filament. All hydrocarbon gases (except CD4) used in these experiments b a H i D '/ I B ~ H '1~ D were Matheson reagent grade. Methane-d4 (minimum isotopic T , OK haH,' IB~H~+ purity of 99 atom D) was supplied by Merck Sharp and Dohme. 300 2.8 1 .4 The boron hydrides, B2H6,4B2D6,4and B4H1~,5 were prepared by 273 3.1 1.2 standard literature procedures. Pentaborane(9) was obtained 273 3.5 1.3 from the Callery Chemical Co. Purity of the boron hydrides was 1.7 242 2.9 checked by infrared and mass spectra and by glpc. Sample mix2.0 217 2.8 tures of hydrocarbon-boron hydride were prepared by transfer of Av ratio +0.5 +0.5 an appropriate quantity of hydrocarbon to a bulb containing a (obsd) 3.0 1.5 known pressure of boron hydride. -0.2 -0.3 B5H9-CH4 System. Chemical ionization of B5Hs in methane Statistical ratio 4.0 1.5 yields B ~ H I oas + the major product at source pressures of the order (calcd) of 0.5 mm. At low source pressures of the order of 0.05 mm, fragmentation processes leading to BjHa+ and B5H6+become important. P, = 0.06mmandBsHg:CD4 = 1:lW. For a source pressure of 0.06 mm and a gas composition BsHg :CH, = 1 :200 the ratio ~ Z ( ( B ~+H, / X ~ I
