D. J. KNOWLES, A. J. C. NICHOLSON, AND D. L. SWINGLER
3642
sonable that complex formation with 1-naphthol should reduce this amount by more than half. Consequently, we estimate that release of electrostriction can contribute no more than 1-2 cm3 to the observed 10.2 cm3 for A Boof complex formation. Most of the volume increase appears to result from the peculiar properties of organic solutes in aqueous solution. There is much evidence to suggest that the partial molal volumes of organic solutes with “hydrophobic” exposed groups are substantially smaller iri dilute aqueous solution than in dilute solution in nonpolar solvents, or than the molar volumes of the pure liquid solutes. 14-16 For example, the apparent molar volume of benzene in dilute aqueous solution is 6.4 cm3 less than the molar volume of pure liquid benzene at 250.16 The corresponding volume loss for toluene is 8.57 cm3.17 The extent of the contraction appears to increase with increasing size of the nonpolar portion of the molecule.1s~18Evidently the water molecules are packed more tightly near the nonpolar surfaces of or-
Electron Impact Studies.
ganic solute molecules than in the bulk water region. To explain the positive value of AVO, we then need to postulate only that complex formation between 1naphthol and 3,5-dinitrobenzoate ion reduces the total nonpolar surface area per mole of complex and thus displaces some water molecules into the more loosely packed state characteristic of the bulk water region. The experimental value of A v o , after allowing for release of electrostriction, is quite similar to the volume change when toluene is transferred from dilute aqueous solution to pure liquid toluene, and hence of a plausible magnitude. (14) F. Franks and D. J. G. Ives, Quart. Rev., Chem. Soc., 20, 1 (1966), and references cited. (15) F. Franks in “Physico-Chemical Processes in Mixed Aqueous Solvents,” F. Franks, Ed., Heineman Ltd., London, 1967, p 61. (16) W. L. .Masterton, J . Chem. Phys., 22, 1830 (1954). (17) J. E. Desnoyers and F. M. Ichhaporia, Can. J . Chem., 47, 4639 (1969) (18) F. Franks and J. M. Quickenden, Chem. Commun., 388 (1968). I
11. Stannous Bromide and Stannic Bromide
by D. J. Knowles,* Department of Physical Chemistry, University of Melbourne, Parkville, Victoria, Australia
806g
A. J. C. Nicholson, and D. L. Swingler Division of Chemical Physics, CSIRO, Chemical Research Laboratories, Clayton, Victoria, Australia 8168 (Received April 7, 1070)
Ionizattionefficiency curves are given for the ions produced by electron impact on stannous bromide and stannic bromide. Ionization potentials are given for the parent molecules and derived for some of the radical fragments. Ionization potentials for the group I V halides and subhalides are discussed.
Introduction As part of a study of the vapor-phase reactions of the metal halides, ionization potentials (I.P.) and fragment appearance potentials (A.P.) were needed. A previous paper’ gives data for tin chlorides and this paper gives similar data for tin bromides. Ionization efficiency (I.E.) curves were measured for the ionization of SnBrz and SnBrd by electrons. A discussion of the available IP of the group IV halides MX1-4is given.
used in the previous study of chlorides.’ It has been our experience with E.I. studies of the vapors over heated inorganic halides that the apparent electron energy as measured by the circuit voltmeter increased slowly with time. This most likely reflects a gradual build up of a salt layer on the ion source during experimentation which results in changes in contact and surface potentials. For this reason the I.E. curve of the reference gas was measured frequently during the exper-
Experimental Section * All correspondence should be directed t o D. J. Knowles, Preston Institute of Technology, St. Georges Road, Preston, Victoria, The quadrupole mass spectrometer and Knudsen 3072. cell assembly used have been described p r e v i o u ~ l y . ~ ~Australia ~ (1) A. S. Buchanan, D. J. Knowles, and D . L. Swingler, J . Phys. The experimental procedure employed for the electron Chsm., v3, 4394 (1969). impact (E.1.) measurements was identical with that (2) J. w.Hastie and D. L. Swingler, High Temp. Sci., 1 , 4 6 (1969). The Journal of Physical Chemistry, Vol. 74,No. $0, 1070
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3643
iments and the ion source was electropolished after any one set of measurements. SnBrz (Alfa Inorganics) was contained in a silica-lined Knudsen cell and heated under dynamic vacuum at ca. 200” for 12 hr prior to taking measurements. SnBr4 (Hopkins & Williams) was admitted via a gas leak. The mass spectra of these compounds showed no impurity ions and the isotope patterns agreed with those calculated from the known abundance ratios of tin and bromine. The Knudsen cell was held, to A l ” K , ’ a t temperatures suitable to give adequate ion currents (575°K for SnBrz, room temperature for SnBr4). The ion source temperature was 410°K.
Results I.E. curves are shown in Figure 1 and the results assembled in Table, I. The reactions postulated are numbered in the table and referred to in the text by these numbers. A.P. values were calculated exactly as in ref 1 assuming that all ions are formed in their ground states and carry no excess kinetic energy. The calculations establish that the postulated reactions are reasonable ones. It is recognized that the methods used for
Z
8
determining I.P. and A.P. (semilog plot and “initial break”) can give erroneous values but such values are almost always too high. Knowledge of the upper bound of an A.P. is still useful in disentangling reaction mechanisms, particularly in determining whether products are formed by ion-molecule or molecule-molecule reactions, SnBr+(SnBrz). The A.P. of the SnBr+ fragment from SnBr,, A.P. [SnBr+(SnBrz)], is related to the bond dissociation energy in the molecule, D(BrSn-Br), and the I.P. of the radical SnBr by
A.P. [SnBr+(SnBr)] 2 I.P.(SnBr)
+ D(BrSn-Br) (1)
D(BrSn-Br) is equal to the heat of atomization, AHato, of SnBrz, less D(Sn-Br). AHat”(SnBrz) = 6.8 eV.3 D(Sn-Br) is not known accurately, Feber4 gives 3.2 eV and Gaydons favors 2.0 =k 1.0 eV. The higher value gives a minimum D(BrSn-Br) = 3.6 eV; substituting this value and the upper limit of A.P. [SnBr+(SnBrz)] from Table I gives I.P.(SnBr) < 7.8 eV. Sn+(SnBrz). This is the least abundant ion from SnBrz and gives rather featureless LE. curves. Figure 1 shows a representative example and all curves for this ion showed changes in slope a t 14.5 and 12.8 eV. These are interpreted as due to the onset of reactions 3 and 4 and the signal below 12.8 eV assumed to be due to the ion-pair process 5. SnBr3+(SnBr4). D(BraSn-Br) is not known but estimating it as half the difference between AHat” (SnBr4) and AHato(SnBrz),2.2 eV, should not cause a large error. The A.P. for reaction 7 and the equivalent equation to 1 give I.P.(SnBr3) < 9.5 eV. 8nBrz+(SnBrr). The major reaction producing this ion has its onset around 15.2 eV and corresponds to reaction 8. At lower voltages 9 and 10 contribute and the steeper step-shape of the lowest portion of this curve supports the view that reaction 10 produces an ion pair. SnBr+(SnBri). The I.E. curve for this ion does not show well defined break-points. It seems probable that the onset around 15.0 eV is due to reaction 11. Sn+(XnBr4). This I.E. curve has a relatively small foot” (h.2 eV) compared to that for the same ion from SnC14 (”7 eV).I Reaction 14 seems to be the only one occurring and there was no evidence for reaction 15 or an ion-pair process equivalent to that occurring in the SnCL case.
Discussion The experimental I.P. values of the molecules and radicals MX1--4 (M = group I V metal, X = halogen ELECTRON ENERGY eV (corr.) Figure 1. Ionization efficiency curves: (b) stannic bromide. ,
(a) stannous bromide;
(3) T. L. Cottrell, “The Strengths of Chemical Bonds,” 2nd ed, Butterworths, London, 1958. (4) R. C. Feber, “Heats of Dissociation of Gaseous Halides,” LA3164, TID-4500, UC-4 Chem., 1965. (5) A. G. Gaydon, “Dissociation Energies and Spectra of Diatomic Moleciiles,” 2nd ed, Chapman and Hall, London, 1953.
The JOUTW~ of Physical Chemistrg, Vol. 74, N o . &’031970
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Table I : Appearance Potentials for Ions from SnBr4 and SnBrz Ion
Parent molecule
SnBrz + SnBr +
SnBrz SnBrz
Sn
+
SnBrz
SnBr4+ SnBr3+
SnBr4 SnBr4
SnBrz +
SnBr4
A.P.,eV,
10.0 i 0. 4b 1 1 . 0 f 0.4' 1 4 . 5 f 1.0 12.8 f1 . 0
SnBr4
Sn +
SnBr4
14.1 12.1 10.7
1 0 . 6 f 0.4' 11.3 f 0.4' 15.2 i 1 . 0 13.6 f 1 . 0 1 2 . 0 i1 . 0 15.0 h 1 . 0 13.7 i1 . 0
+ Br- + Br + 3Br + Brz + Br
,(lo) SnBr4 SnBrz+ (11) SnBr4 -* SnBr+ { (12) SnBr4 + SnBr + -+
SnBr +
A.P., eV, oalcd"
measd
14.5 12.5 11.1 15.5 13.5 12.1 18.6 16.6
18.5 i1 . 0
" Using values I.P.(Sn) = 7.3 eV, C. E. Moore, "Atomic Energy Levels," National Bureau of Standards Circular 467, U. S. Government Printing Office, Washington, D. C., 1958. E.A.(Br) = 3.4 eV, H. B. Gray, "Electrons and Chemical Bonding," W. A. Benjamin, New York, N. Y., 1965, p 34. D(Brz) = 2.0 eV,&D(Sn-Br) = 3.2 eV,4 AH,to(SnBr4) = 11.3 eV,s AH,tO(SnBrz) = 6.8 eV,aand I.P. (SnBrz) and (SnBr) from this work. b Semilog treatment. except iodide) can be divided into three groups with fairly well separated IP. I. M X 4 Molecules. Values for these I.P., all 3 10.6 eV, are given in Table I1 together with the values of the halogen molecules. The I.P. of any particular halide is very close to that of the corresponding halogen, of a mixed halide to that of the halogen with the lowest I.P. This is a clear indication that the electron removed on ionization comes from a lone pair orbital on the halogen. To a good approximation the energy of this orbital can be regarded as unaffected by the largely covalent bond between the halogen and the valency saturated metal. This is also true for organic halide^,^ group I11 halides, InCla,' SbCla,eAsCla,' boron halides,* but not for group I halides. 2. M X 2 Molecules and MXa Radicals. Here the I.P. (Table 111)lie between 9 and 10.5 eV and the distinction between halogens has almost disappeared. These I.P. are well above those for the metal atom. The bond angles employed (determined and estimated) for various
Table 11: I.P. for MX4 (M X = Halogen or Hydrogen
=
I.P. exptl,a eV
CF4 CCl4 CClaF CHzClBr SiF4
