Positive-Ion
Reactions in M o n o s i l a n e - W a t e r
2841
Mixtures
well as the rotation of -CH2, and, therefore, the intensity ratio, h ( M I =: O)/h(MI = -I), again becomes small and approaches 2. Acknoaledgments. We are indebted to Professor Yoshida Ogino and Mr. Akira Igarashi for helpful discussions and for supplying the adsorbents. This work was supported by a grant from the Japanese Ministry of Education. This financial support is gratefully acknowledged.
References aind Notes (1) J. H. Lunsford, Advan. Catai., 22, 265 (1972). (2) M. Shimizu, H. D. Gesser, and M. Fujimoto, Can. J. Chem., 47,
1375 (1969). (3) Y. Ono and T. Keii, J. Phys. Chem., 72, 2851 (1968). (4) S. Kubota, M. lwaizumi, and T. Isobe, Bull. Chem. SOC.Jap.. 44, 2684 (1971). (5) R. J. Kokes, J. Phys. Chem., 66,99 (1962). (6) (a) V. 6 . Kasanskii, G. 6 . Pariiskii, I . V. Aleksandrov, and G. M. Zhidomirov, Solid State Phys. (USSR), 5, 649 (1963);(b) J. Turk-
evich and Y. Fujita, Science, 152, 1619 (1966);(c) G. B. Gurbutt, H. D. Gesser, and M. Fujimoto, J. Chem. Phys., 48,4605 (1968). (7) N. Takezawa and H. Kobayashi, J. Catal., 25, 179 (1972). (8) L. Pauiing, J. Chem. Phys., 51, 2767 (1969). (9) (a) R. W. Fessenden and R. H. Shuler, J. Chem. Phys., 43, 2704 (1965);(b) J. Cooper, A. Hudson, and R. A. Jackson, Mol. Phys., 23, 209 (1972);(c) A. Begum, J. H. Sharp, and M. C. R. Symons, J. Chem. Phys., 53, 3756 (1970);(d) A. Begum, S. Subramanian, and M. C. R. Symons, J. Chem. SOC.A, 918 (1970). (IO) R. S.Eachus and M. C. R. Symons, J. Chem. SOC.A, 790 (1968). (11) J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbitai Theory," McGraw-Hill, New York, N.Y., 1970. (12) H. M. McConnell, J. Chem. Phys., 24, 764 (1956). (13) J. Higuchi, J. Chem. Phys., 39,3455 (1963). (14) The value 54,.7G was observed as the I 3 C hyperfine splitting constant of SiOCH2 at room temperature by the use of 13C enriched methanol. This value is larger than that of the hydroxymethyl radical (R. W. Fessenden, J. Phys. Chem., 71, 74 (1967))and it is nearly the same as that of CH2F.lZa For the CH2F radical, it has been shown that the proton hyperfine splitting constant is negative (D. L. Beveridge, P. A. Dobosh, and J. A. Pople, J. Chem. Phys.. 48, 4802 (1968)).The 13C hyperfine splitting of the MgOCH2 radical could not be observed because of large line broadening caused by the molecular motion more hindered than the SiOCH2 radical. (15) R. Lefebvreand J. Maruani, J. Chem. Phys., 42, 1480 (1965).
Positive-Ion Reactions in Monosilane- Water Mixtures' T. M. H. Cheng and F. MI. Lampe* Davey Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received July 23. 1973) Publicat,on costs assisted by the U.S. Atomic Energy Commission
Tandem mass spectrometry has been used to investigate the ion-molecule reactions occurring in gaseous mixtures of monosilane and water. Reaction rate constants were obtained by direct comparison with the known rate constant for hydride ion transfer from SiH4 to SiHZT. As in other ion-molecule studies of monosilane the principal reaction is one of hydride ion transfer from monosilane to ions derived from water. SiH4D+ is observed to be formed in an endothermic process from collision of ground-state D 3 0 t with SiH4 and in an exothermic process involving an excited state(s) of D 3 0 + . Some implications of the results to the aqueous acid hydrolysis of monosilane are discussed.
Introduction one may expect some of the ionic processes involved in the Subsequenl to the discovery*,3 and chara~terization~-~Oliquid-phase hydrolysis of monosilane to be of significance in the radiation chemistry of gaseous monosilane-water of ion-molecule reactions in methane and to the accepmixtures. As a part of a general program of study of the tance of their fundamental role in the radiation chemistry radiolytic behavior of silanes, an investigation of the radiof methane,"-28 Olah and his c o w o r k e r ~ ~ ~demon.3~ olysis of monosilane-water mixtures is in progress in our strated the importance of similar reactions and ionic laboratory. As an aid to the interpretation of the radiolyproducts in strong acid solutions of methane. Thus in the sis experiments. we have carried out a tandem mass speccase of methane a direct correspondence between the trometric investigation of the positive-ion chemistry of chemistry of ionized gases and that of liquid solution has monosilane-water mixtures. This paper constitutes a rebeen established. port of our findings. One should expect a similar situation to obtain in the case of monosilane. Since the pioneering researches of Experimental Section Stock and his coworker^.^^-^^ it has been known that monosilane reacts with water under alkaline or acidic conThe tandem mass spectrometer used in these studies ditions to yield. ultimately, hydrogen in amounts essenpermits the injection of mass-selected reactant ions. havtially equal to twice that contained in the monosilane. ing kinetic energies variable down to about 1 eV, into a The overall reaction is thought to proceed cia silanol and collision chamber containing the neutral reactant moledisiloxane intermediate~3I-3~ but the necessity of having cule. The apparatus, which has been described previousalkaline or acidic conditions indicates that ionic reactions ly,35 consists of two quadrupole mass filters separated by are of importance. By analogy with the case of methane, the collision chamber and ion lenses. In all experiments The Journal of Physicai Chemistry, Vol. 77, No. 24, 1973
T. M. H. Cheng and F. W. Lampe
2842
the mass filters were mounted in the “in-line” configura tion. Retarding field measurements indicate that the energy spread of the reactant ion beam entering the collision chamber is about 1 eV. Satisfactory operation of the apparatus was checked by examination of the energy dependence of the cross section for the reaction O+ i- N2 NOf 4- N (1) over the range 1-27 eV (laboratory energy). The depen denre of the cross section on kinetic energy of O+ was found to be identical, within experimental error. to that found by Giese.36 In addition to identification of the product ions formed in collisions of electron-impact-produced ions of water with monosilane and vice versa, the various relative reaction cross sections were studied as a function of kinetic energy in the range of 2-15 eV. As described previously,37.38 the shapes of the cross section us. energy curves were used to differentiate between exothermic and endothermic reactions; the cross sections of endothermic reactions generally rise from zero a t the energy threshold to a broad maximum several eV above threshold, while cross sections for exothermic processes decrease for all values of ion kinetic energy. Phenomenological rate constants of exothermic reactions were determined by direct comparison of relative reaction cross sections a t 2.5 eV ion energy (Tab) with the cross section for reaction 2, L‘ZZ -+
siH,+ i- S ~ H , -+ SiH,+
+ Sill,
(2)
The rate constant of (2) has been previously determined35 as 2.5 f 0.3 x 1Q-IQcm3/sec. Collision chamber pressures Torr, a pressure sufficiently low were of the order of to preclude the observation of third-order processes. Ionization chamber pressures of the order of 1-5 X Torr were employed. Monosilane was purchased from the J. T. Baker Chemical Co It was subjected t o several freeze-pump-thaw cycles and checked mass spectrometrically for satisfactory purity before use. Deuterium oxide was obtained from Merck of Canada and used as received.
Results and Discussion 1. Xature o,f the Elementary Reactions, The ion-molecule reactions found to be exothermic on the basis of the dependence of reaction cross section on energy are tabulated in Table I. Also shown in Table I are the standard enthalpy changes calculated for the various reactions from available thermochemical data,3Q,40 and phenomenological rate constants a t 2.5-eV (lab) ion kinetic energy. In all cases except two the neutral products are written arbitrarily as those that give maximum exothermicity. In the two exceptions, the exothermicity is reduced from the maximum possible by the 0 - H and H-H bond dissociation energies, respectively. Qny may note immediately that the most rapid reaction of all ions with monosilane is hydride abstraction leading to the SiHs+ ion, an observation consistent with other s t ~ d i e s ~ of ~ 9the 3 ~reactions of ions with SiH4. The reactions observed to be endothermic. on the basis of the cross-section dependence on energy. are depicted in Table II. Also shown in Table I1 are the threshold energies observed for the reactions, the energies at which the maximum in the cross section occurs, and the values of the cross sections a t the maximum relative to that for reaction 2 a t an ion kinetic energy of 2.5eV. The Journal of Physical Chemistry. Vol. 77, No. 24, 7973
In the remainder of this section we discuss the charac teristics of the reactions that occur when the various reactant pairs are brought together. (a)OD+ + SiH4. When OD+ ions are injected in Si&, reactions 3-8 (Table I) are observed as exothermic processes, in that the cross sections decrease with increasing kinetic energy over the entire range studied, namely, 1.310.2 eV in the center-of-mass system. Reaction 9 (Table 11) is observed as an endothermic process a t relative kinetic energies above 1.9 eV; however, the intensities of the product HSiODt ( m / e 47) were so small and the energy dependence such in the range of detection that no threshold, other than an upper limit, could be obtained. In the case of reactions 7-9, the mass of the product ion does not determine the formula since one does not know whether the hydrogen and/or deuterium is bound to silicon or oxygen. By comparison with the reaction of OH+ with SiH4, we have established the numbers of hydrogen atoms and deuterium atoms in the product ions of 7-9 and we have written the formulas with the arbitrary assumption that no intramolecular exchange occurs between hydrogen and deuterium originally bound to silicon and oxygen. As shown in Table I, the enthalpy changes calculated from available thermochemical data39.40 for 3-7 are in agreement with our observations from the cross-sectionkinetic energy dependence that these reactions are exothermic. As will be discussed later. the observed thresholds of (24) and (%), shown in Table II, permit an evaluation of the standard enthalpy of formation of SiOD+ and the value obtained leads to the exothermic enthalpy change for (7) shown in Table I. The standard enthalpy of formation of HZSiOD+ is unknown and cannot be evaluated from our data. We can therefore say only that the enthalpy change of ($) is at most zero. Reactions 6 and 7 are exothermic by 140 and 142 kcal, respectively, if the neutral products are written as HDO and 2Kz, respectively. While this much energy could be shared between the ionic and neutral products, dissociation of the neutrals is energetically possible and we have arbitrarily written these reactions assuming such dissociation occurs. Hydrogen atom abstraction from SiH4 by OD4 with formation of HDQ+ and SiH3 is an exothermic process and 1s analogous to reaction 10, shown in Table I. Although a search for HDO- formation in the energy range of 2-5 eV (lab) was made, we were unable to detect it. ( b ) DzOf SiH4. The exothermic reactions 10-17, shown in Table I, and the endothermic reactions 18 and 19, shown in Table 11, are observed to occur when DzQ+ ions are injected into SiH4. The structures of the ionic products of 13-17 were arrived a t by comparison of mass shifts when HZ0+ was substituted for DzQ+ and with the arbitrary assumption that silicon and oxygen do not undergo intramolecular exchange of hydrogen and deuterium atoms. The DzO+ and HzO+ comparison was not able to differentiate between the products HSiQDz + and &SiOD+. and we are not really able to choose between them. However, we have a slight preference for HSiODZ+ because the alternative product H3SiQD+ corresponds to the molecular ion of H3SiOD, an unstable molecule which has never been isolated.41 The kinetic energy dependence of the cross section of (lo), namely, hydrogen atom abstraction from SiH4, is interesting. As shown in Figure 1, the reaction is exothermic a t low kinetic energies and can only be as written in Table I. However, the cross section goes through a minimum
+
2843
Positive-Ion Reactions in Monosilane-Water Mixtures TABLE I: Exothermic Ion-Molecule Reactions in SiH4-D20 Mixtures
AH",kcal
Reaction no.
Reaction
3 4
OD+ SiH4-W HDO -I- Hp 4- H OD+ SiH4 -SiH+ HDO H2 OD+ SiH4 SiH2+ HDO -k H OD+ 4- SiH4 SiH3+ 4- OD 4- Hb OD+ f SiH4 + S O D + H2 H -k Hb OD+ SiH4 HzSiOD+ HZ D20+ SiH4 HDp0+ 4- SiH3 D 2 0 + 4- SiH4 SiH2+ D20 HZ D 2 0 + SiH4 SiHsi D20 H DpO+ SiH4 SiOD+ HD H2 -k H D20+ SiH4- HSiOD+ 4- H2 HD D20+ -I-SiH4 -H2SiOD+ HD -I- H HSiODp+ Hz H (or H3SiOD+ -4- HD) D 2 0 + SiH4 Dp0+ SiH4 H2Si0Dpf -k H2 D3O+* SiH4 SiH3+ D20 HD
+ +
----
+
5
6 7 8 10 11 12 13 14 15 16 17 2Od 21d 22 23 26
+ + + + + +
-29 - 72 -44 - 20 - 26 20 - 47 -17 -9 26
+ + + + + + + + + + + + + + + + + + + + + D 3 0 + * + SiH4 SiH4D* + D20 SiH3+ + H20b -* H2SiOH- + H2 SiH2+ -+ D20 SiODp+ f H2 SiH+ + D20 SiOD2+ + H
----
a Ion kinetic energy is
2.5 eV (lab). *See text,
C
Refers to ground-state reactants.
1010
x
k , a cm3/sec
0.21 f 0.03 0.62 f 0.08 1.4 f 0.2 1.8 f 0.3 1.7 f 0.3 0.86 f 0.2 0.79 f 0.1 1.9 f 0.3 2.1 f 0.3 0.12 & 0.02 0.033 f 0.003 0.32 f 0.04 0.019 & 0.005 0.039 f 0.01 >1.1 f 0.26 >0.19 f 0.03* 0.058 f 0.08 0.053 f 0.08 0.12 f 0.02
+
QO
%O -5
