David W. Muenow
970
troimetric Evidence for the Gaseous Silicon Oxide Nitride Molelcinle and Its Heat of Atomization David W. Muenow Chemistry Department, University of Hawaii, Honoluiu, Hawaii 96822 (Received January 5 , 7973) Putilication costs assisted by The University o f Hawaii
'The gaseous silicon oxide nitride molecule Si2NO(g) has been observed with a high-temperature mass spectrometer. Its heat of formation and atomization energy are calculated to be AHTO298 -- -38.1 f 10.3 kcal mol-l and AHaloms'= 427.4 f 17.3 kcal mol-I, respectively.
Introduction Silicon oxynitride, Si2N20, has been observedl as a, mineral in metemiles of the enstatite chondrite type. The presence of this mineral in a meteorite is of importance in ternis off origin since it provides an estimate for the conditions of the environment from which it formed.2 As part of a mass spectrometric study on materials at high temperature the gaseous molecule SizNO has been observed and its heat of atomization obtained. Previously, only one other gaseous. nietd nitride oxide (NUO)3 has been reported. The lack 01' molecules of this type in part derives from the ease of oxidation of the corresponding solid metal nitride and 1,he high stability of the nitrogen molecule. Results and Discussion Measurements were performed with a 60" sectoi -field, 15.8-cm radius-of-curvature, first-order, direction-focusing mass spectrome ter equipped with a high-temperature Knudsen effusion cell. Ions are produced by an electron impact ion source, and after mass analysis detected and amplified with an electron multiplier and vibrating reed electrometer. The instrument i s differentially pumped with two high-speed all-metal vapor diffusion pumps and liquid nitrogen traps and baffles. Details of the basic cell assembly have been previously described.4 The effusion cell was machined from tantalum and fitted with a graphite crucible and lid. Heating was accomplished by radiation from a tungsi,en loop resistor wire surrounding the cell, and temperatures measured with a Pt-Pt-10% Rh thermocouple peened into its base. Over the temperature range 1677-1768°K the following isomolccular equilibrium was studied SizN(g) + SiO(g) = SizNO(g) + Si(g)
(1)
The graphite crucible was loaded with small chips of Si3N4? powdered silica, boron nitride, and silicon metal. The presence of B N provides a convenient source of nitrogen a t high temperatures; metallic silicon ensures a reducing atmosphlere. With the cell heated to 1725°K and using 26-V ionizing electrons the main peaks in the mass spectrum were found to be Si23, Si+, Nz+, 3i2N+, Si&O+, and S O + . A weak peak at m l e 54 was also observed and is tentatively identified with SiCN+, previously reported ,5 Assignments were based upon appearance potentials, shutterability, L] nd isotopic-abundance calculation!s. ApThe Journal of Physical Chemistry, Val. 77, No. 7, 7973
pearance potentials were measured by the vanishing-current method using Si+ (AP = 8.2 eV) and HzO+ (AP = 12.6 eV) as standardse6The values 11.3 f 0.5 eV for SiO+ ( m l e 44) and 9.5 f 0.5 eV for S t N + ( m l e 70) agree closely with those reported p r e v i o ~ s l yand ~ , ~ suggest these ions to be parents. The value 10.8 f 0.5 eV for Si2NO-t ( m / e 86) indicates the ion is formed by direct ionization of SizNO(g) and not by dissociative ionization of some higher molecular weight species. No shutterable peaks of mass greater than those for the isotopes of SizNO-'- ( m / e 86, 87, and 88) were observed. Since aluminum oxide i s a common impurity in nitrides of boron and silicon and can easily be accommodated in solid solution with Si3N4 it was suspected that a portion of measured ion currents for mass peaks m / e 86 and 70 (assigned to SizNO+ and SizN+, respectively) might be due to AlzOz+ and A120+ which produce peaks a t these same masses. Neither the isotopic abundances nor previous appearance potential measurements9 for AlzOz+ (9.9 f 0.5 eV) and AlzO+ (7.9 f 0.2 eV) support this view. Temperature-dependent, ion-intensity data for Si+, Si2N+, SizNO+, and SiO+ were obtained using 25-V ionizing electrons at several temperatures in the range 16771768°K and were used to calculate the equilibrium constant, K,, for reaction I. Instrumental sensitivity was determined using the silver-calibration technique.l* From JANAFll free energy functions for Si(g) and SiO(g), estimated values for SizN(g) 1-72 cal deg-I mol-I a t 1725"M, ref 298"]8 and SizNO(g) [-BO cal deg-l mol-l a t 1725"K, ref 298", by analogy with AlZOz(g)], and the equilibrium constants one derives the corresponding heats of reaction by the third-law method. The results are given in Table 1. Using the average heat of reaction 1 AH298O p=. 1.7 f 3.3 kcal mol-I, the dissociation energy of SiO(g), 0298' = (1) C. A. Anderson, K. Keil, and 6 . Mason, Science, 146, 256 (1964). (2) W. R. Ryali, and A. Muan, Science, 165, 1363 (1969). (3) K . A. Gingerich, Naturwissenschaften, 24, 646 (1967). (4) D. W. Muenow and R. T. Grimiey, Rev. Sci. Instrum., 42, 455 (1971). (5) D. W. Muenow and J. L. Margrave, J. Phys. Chem., 94, 2577 (1970). (6) R. W. Kiser, "Introduction to Mass Spectrometry and its Applications," Prentice-Hall, Englewood Cliffs, N. J., 1962. (7) D. L. Hildenbrand and E. Murad, J. Chem. Phys., 51, 807 (1969). (6) K. F. Zmbov and J. L. Maigrave, 'J. Amer. Chem. Soc., 89, 2492 (1967). (9) J. Drowart, G. DeMaria, R. P. Burns, and M. G. Inghram, J . Chem. Phys., 32, 1366 (1960). (10) J. L. Margrave, Ed., "The Characterization of High Temperature Vapors," Wiley, New York, N. Y . , 1967, pp 222-227. (1 1) "JANAF Thermochemical Tables," Dow Chemicai Go., Midland, Mich., 1968.
Communications to the Editor
971
TABLE I: Equilibrium Data for Heat of Reaction 1 -AJGT'
-
Temp.
"K
-LogKDa
1677
8.92
1698
0.24 0.88
f 727 f 743 i 768
"_"I
HZW ] I T , deg- mol
(3) (3) (3) (3) (3)
0.97 0.42
Av
AHzw'. kcai mol-
-
2.0
-3.2 I .a
2.5 -1.9
third-law 1.7 f 3.3
a Assumed relative ionization cross sections and multiplier gains cancel. Relative intensities (1727'K, 25 eV) for SizNO+, Si', SizN+, and respectively. 1.00, 5 X IO-*, and 1 X SiQ+ are 2 X
192.3 i 2 kcal m0l--1,1~ the heat of formation of SizN(g), A N f 0 z y 8 = 93 :k 5 kcal mol-f,8 and the dissociation energy of 0&), DSW" = 119 kcal mo1-l,l2 one calculates for the heat of formation of SizNQ(g), AHf0298 = -38.1 f 10.3 kcal mol-$. Combined with the dissociation energies of Nz(g) and O&), D29s0 = 226 f 211 and 119 kcal
~
O
mol-l,12 respectively, and the heat of sublimation of silicon, AHs0298 = 108.4 f 3 kcal mol--1,13 leads to the atomization energy of SiZNQ(g), AHatomS"= 427.4 f 17.3 kcal mol-1. This may be compared with an estimated value of 444 f 10 kcal mo1-I using 104 kcal mol-1 as the lower limit for the mean Si-0 bond energy (from the matrix-isolated molecule Si303 with proposed planar-cyclic structure)l* and 118 f 5 kcal mo1-l for the mean Si-N bond energy in SizN(g) [AHatoms' = 236 f 10 kcal mol-l]. Favorable agreement suggests a cyclic rather than linear structure for SizNO(g).
Acknowledgment. This work was supported by grants from the Research Corporation and the Hawaii Institute of Geophysics. (12) L. Brewer and G. M. Rosenblatt, Advan. High Teemperature Chem., 2, 20 (1969). (13) H. L. Schick, "Thermodynamics of Certain Refractory Compounds," Voi. I , Academic Press, New York, N. Y., 4966, p 158. (14) J. S. Anderson and J. S. Ogden, J. Chem. Phys,, 51,4189 (1969).
~ ATIONS M ~ TO~THEI EDITOR
Direct Observation of the Dibromide Radical Anion Oxidation of Tris(bipyridyl)ruthenium(l I). Evidence for a Triplet-to-Triplet Energy Transfer Mechanism in1 the Photosensitized Redox Decomposition of Cobalt( I l l ) Substrates' Puh//catfon costs a s m i e d by ,'he Nationai Science Foundation
Sir: It has very recently been proposed2 that the Ru(hip y P + photosensitized redox decomposition of cobalt suhstrates such as Co(NH&,Br2+ and Co(HEDTA)Cl- proceeds predominately by means of electron transfer to these oxidants from the thermally equilibrated charge transfer to ligand triplet excited state, Ru(bipy)sP+ (3CT),3 rather than by means of triplet-to-triplet energy transfer as we had earlier proposed for the case of Co(HEDTA)Cl- .4 The evidence cited for the former mechanism appears to be the observation that some Ru(bipy)s3+ is a reaction product under some reaction conditions.2 We have carefully examined the formation af Ru(bipy)s3+ in the Ru(bipy)a2+ sensitized redox decompositions of Co(NH&Br2+, Co(EDTA)- , and Co(MEDTA) x- (X = C1, Br, or NO2); the full report of these studies will be presented el~ewhere.~c In the present communication we wish to point out that B product analysis is not sufficient to establish a mechanistic
hypothesis in systems as complex as these, that radical oxidations of Ru(bipy)32+ are often very rapid and sometimes observable, and that examination of evidence bearing on the sensitization mechanism indicates that the mechanistic hypothesis of Gafney and AdamsonZCdoes not apply to the systems we have investigated. We have focussed on Co(NH&,Br2+ as any acceptor substrate, €or purposes of the present report, since the Br2- radical anion is a readily formed, easily detected, well-characterized5 product of the photoredox decomposition of this substrate and since the Br2- radical is a powerful and facile oxidant6.7 which in principle should be able to oxidize Ru(bipy)s2+. (1) Partial support of this research by the National Science Foundation (Grant No. GP 24053) is gratefully acknowledged. (2) (a) A. W. Adamson, "Proceedings XIV lnternaticnal Conference on Coordination Chemistry," Toronto, June, 1972, p 240; (b) Abstracts 164th National Meeting of the American Chemical Society, New York, N.Y., August, 1972, INORG 2; (c) H. D. Gafney and A. W. Adamson, J. Amer. Chem. Soc., 94,8238 (1972). (3) (a) J. N. Demas and G. A. Crosby, J. Mol. Specfrosc., 26, 72 (1968); (b) F. E. Lytle and D. M. Hercules, J. Amer. Chem. SOC., 91, 253 (1969). (4) (a) P. Natarajan and J. F. Endicott, J. Amer. Cnem. SOC.,94, 3635 (1972); (b) ibid;, in press: (c) J. Phys. Chem., suomitted for pubiication. (5) J. K. Thomas, Advan. Radiat. Chem., 1, 103 (1960). (6) S. D.Malone and J. F. Endicott, J. Phys. Chem., 76, 2223 (1972) (7) W. H.Woodruff and D. W. Margerum, Inorg. Chem., in press. The Journal of Physical Chemistry, Vol. 77, No. 7. 1973
