J. BERKOWITZ, D. A. BAFUSAND T. L. BROWN
1380
when: T and 0 are defined by equations 11-8 and 11-12>and R = @,/K (43) The relatiolnships T(b) and B(b) for the different hypotheses are given in Part II.2 The value of R may be determined most conveniently by comparing a projection strip (log I ) , of the experimental curve g(log I ) with a family of normalized curves log R (log I);, calculated for the appropriate hypothesis by means of equation 42, cf. p. 1377, and Figs. 3 and 4. The position of the strip on the log I axis must be compatible with the value of (log I log I ) obtained using the plot log D (log I ) . The required pa,rameters may be obtained from equations 40, 41 and 43. The values are best checked by substitution into equations 3, 11-8 and 11-11 to give the :set T,0, b. The primary data then can be recalculahed using the relationships 13 = b ( 1 T)a n d S = b ( l + 6) cf. equations 11-7 and 11-10. Imidazole.-The above method was applied to Anderson, ‘Duncan and Rossotti’s measurements16 of B(S) for imidazole in carbon tetrachloride a t 18”. The plots laog D (log I ) and (log I ) could be described using either Hypothesis I with log pz = 2.38, log K = 2.80 and log bo = -3.725 (on the molar scale) or Hypothesis I1 with log p2 = 2.45, log K = 3.02 and log bo = -3.73 (see Fig. 4). Although the Hypothesis I parameters are in ex-
+
(16) V . M. W. Anderson, J. L. Duncan and F. J. C . Rossotti, J . Chem. Soc., 2165 (1961).
Vol. 65
0-
L o g R.
1 -
-2
-
-3 0
0 4
1
I
-0 8
Log
I
LO9
1
I
-0 4
’
Fig. 4.-Imidazole in carbon tetrachloride a t 18’. Kormalized curves log R (log I&for Hy othesis 11. With projection strip (log 1); superimpose8 in the position corresponding to log R = -0.57 and (log I - log I ) = -0.71.
cellent agreement with those obtained previously16 from the functions B(b) and S(b), the Hypothesis I1 parameters are preferred since they give a somewhat better description of the primary data B ( S ) a t high concentrations. Acknowledgments.-We are grateful to the Earl of Moray Endowment and to Imperial Chemical Industries, Ltd., for the provision of calculating machines .
THE MASS SPECTRUM OF ETHYLLITHIUM VAPOR’ BY JOSEPH BERKOWITZ, Argonne National Laboratory, Argonne, Illinois
D. A. BAFUSAND THEODORE L. BROWN Il’oyes Chemical Laboratory, University of Illinois, Urbana, Illinois Received February 88,1961
The saturai,ed vapor of ethyllithium has been analyzed tiy use of a mass spectrometer. Mass peaks corresponding to L t R , 1 ( n = 1, 2 , 3 , 4,5, 6) were observed. The LisR6+ ant1 LicRJ+peaks had appearance potentials 3-4 e.v. lower than any of the others, and were thus assumed to be the only parent ions. Corroborative evidence for this conclusion w a obtained ~ by using a double-oven to analyze the undersaturated vapor. The results point to hexamer and tetramer as the predominant species in ethyllithium vapor.
__
+
Introduction A survey of the metal alkyls, excluding the transition metals, reveals that these compounds fall into two classes. In one category may be placed those metal alkyls in which the bond between the alkyl group a.nd the metal atom is a normal, two-center covalent bond. This class of compounds is typified by mercury dimethyl, lead tetraethyl, etc. A second class of compounds embraces those in which the observed structures or properties can be accounted for only in terms of multicenter bonds. (1) Work performed under the auspices of the U. S. Atomio Energy Cornmiasion and the Air Form O 5 c e of Scientific Research. Air Reaearch and Development Command, Contract No. A F 49(638)-466.
These are generally referred to as electron-deficient compounds. The term “electron-deficient” is applied more generally to all compounds in which it is necessary to postulate the existence of multicenter bonds, and SO includes, in addition to such metal alkyls as aluminum trimethyl and beryllium dimethyl, compounds such as the boron hydrides. Rundle2 has stated that the common feature of all of these electron-deficient compounds is the presence of an atom “with more low-energy orbitals than valence electrons combined with atoms or groups containing no unshared electron pairs.” The decision as to what constitutes low-energy orbitals is not always so obvious, but in the case of (2) R. E. Rundle, J . Phvs. Chem., 61, 45 (1957).
August, 19Gl
MASSSPECTRUM OF ETHYLLITHIUM VAPOR
1381
metals from the first three groups, such as Be, Mg, t)emperaturevapor species.11J2 The sample of ethyllithium prepared by methods described previous1y.B All operaAl, Li, etc., it presumably includes all of those was tions involving the sample vials and the sample cell, includatomic orbitals with the same principal quantum ing transfer to the mass spectrometer, were conducted under number. From the structures of those electron- an inert atmosphere. Two different sample cells were employed. The initial deficient compounds of the first three groups which utilized a tantalum Knudsen cell rather similar are presently known, it appears that the metal atom experiments in construction to those described previously.llJ2 In these uses all of its lovu-energy orbitals in forming the exploratory experiments, no attempt was made to measure multicenter bonds. the temperature of the cell accurately. When it became apparent that polymeric vapor species The lithium alkyls form an interesting group of involved, a further study was made with the aid of a compounds in many respects. Their physical prop- were double oven (see Fig. 1). In this arrangement, one chamerties, as now k~nown,~ provide strong evidence for the association of LiR units when the lithium EFFUSION ORIFICE SAMPLE I N L E T alkyl is dissolved in organic solvents. Studies of 9 the freezing point depression of benzene with ethyllithium solute would appear to indicate a molecular weight about six times the formula weight,4-6 although results a t variance with this have been reported.’ Infrared spectral studies directed to the identification of the bands due to associated and POROUS THERMOCOUPLE unassociated species have been reported by a group NICKEL FRIT CAVITY of Russian workers in a series of papers.8-l0 They THERMOCOUPLE have investigated the spectra of methyl- and ethylCAVITY lithium as Kujol mulls of the solids, as solutions in Fig. 1.-Double-oven apparatus. benzene, cyclohexane and hexane, and in the vapor state. Bands due to the unassociated C-Li vibra- ber contained the sample; the vapor diffused through a tions have been assigned for the compounds in solu- porous metallic frit and a long channel into a second chamber, which in turn contained the effusion orifice. In this tion and in the vapor; other bands in the solutions manner, thp effect of temperature on the vapor composition, and in the solids, but which were absent in the at constant total pressure, could be explored. Alternatively, spectra of the vapor phase, were assigned to asso- the expected behavior with reduced pressure at constant temperature, in accordance with LeChatelier’s principle, ciated species. could be used to check the fragmentation patterns and mass By examining the similarities and differences ob- assignments. In both types of experimente, the vapor efserved in the infrared spectrum of ethyllithium in fusing from the Ihudsen orifice could be distinguished from the vapor phase, in various organic soIvents, and in the background gas by observing the difference in ion intenthe crystalline state, Shigorin and co-workers8-10 sity when the molecular beam wm intercepted by a manually operated “shutter” plate. In the double-oven experiment, concluded that ethyllithium was associated as temperatures a t various points were measured by bolting or hexamer and dimer in solution, but probably was not spot-welding five thermocouples (Pt-Pt,lO% Rh) to the associated in the vapor. The determination of the nickel oven. The details of construction and heatmg of molecular species existing in a vapor on the basis of this oven will be presented in a later publication. An unsteadiness in the ion beam intensity was encountered infrared spectra alone is at best a hazardous task; after approximately three hours of heating the cell. This for a molecular system as complex as ethyllithium could be traced to a corresponding alteration in the intensity the problem is compounded. To add to the un- of the ionizing electron beam. Upon removal of the ethylcertainty of the conclusions of Shigorin and co- lithium source and overnight pumping, the unsteadiness One is led to conclude that free lithium formed workers, they reported a vapor pressure for ethyl- vanished. by decompofiition of ethyllithium in tke ionization chamber lithium of CLZ. 5 mlm. a t 70-80”, whereas previous provided leakage paths between the electron-emitting filainvestigators had obtained to m ~ . ~ ment and the ionization chamber. This behavior precluded The investigation reported below was undertaken the conducting of accurate temperature-variation experiin order to test these conclusions, and hopefully to ments. provide some insight into the bonding of lithium Results with the alk,yIs. The major ion peaks observed in the single-oven experiment, and their relative intensities when using Experimental Methods I n order to obtain independent information regarding the 75-volt electrons for ionization, are shown in Table constituents of ethyllithium in the vapor phase, mass spec- I. It should be emphasized that these relative intrometric analysis of the vapor was undertaken. The in- tensities have been corrected for the contribution strument employed was a 12-inch radius of curvature, 60’ single-focussing mass spectrometer a t Argonne National due to background. Because of the fact that the Laboratory, which had been developed for the study of high lithium-bearing species are condensable, the background contribution was less than 10% in most in(3) G. E. Coatss. “Organa-Metallic Compounds,” Methuen Br Co., stances. I n addition to the ion intensities reported, Ltd., London, 2nd Edition, 1960. the isotopic contributions of Li6 and CI3 were also (4) W. Schlenk and J . Iioltz, Ber., SO, 272 (1917). observed on adjacent peaks, and confirmed the ( 5 ) F. Hein and H. Schramm, 2. physik. Chenk., 8151, 234 (1930). assignments. These ion peaks are attributed to t h a (6) T. L. Brown axid M. T. Rogem, J . A m . Cham. Soc., 79, 1859 (1957). process
1
(7) K. B, Piotrovsky and hl. P. Ronina, Doklady Akad. Nauk, S.S.S.R., 115,737 (1957). (8) A. N. Rodinow, D. N. Shigorin and T. V. Talalaeva, ibid., 123, 113 (1958). (9) A. N. Rodinow, V. N. Vacaleva, T. V. Talalaeva, D. N. Shigorin, E. N. Guryanova and K. A. Kocheskov. ibid., 126, 562 (1959). (10) D. N. Shigorin, Spectrochim. Acto, 14, 198 (1959).
Li,R,
+ e- --+ I,i%R;-l + It + 2e-
(1)
(11) W. A. Chupka and M. G. Inghram, J . Phys. Chem., 19, 100 (1 955). (12) W. A. Chupka, J. Berhowitz and C. F. Giese, J . Chem. Plrys., SO, 827 (1959).
J. BEHKOWITZ, D. A. BAFUSAND T. L. BROWN
1382
1-01. 65
where R is the C2H6radical. I n these experiments, the intensity of the parent ion Li,R, + was negligibly small, ie., within the uncertainty of background corrections. This kind of behavior toward ionizat.ion has been observed in alkali halides, la as well as UF4, UF6, UFs,14 totally halogenated methanes, l5 a,nd such organometallic compounds as dimethylmercury and tet.raethyllead.I5
molecular species, and all other ion peaks were due to more severe fragmentation of these molecules than given by eq. 1. This hypothesis subsequently was tested in the double-oven experiment. In principle, it is possible to deduce both the fragmentation patternli#l8and the relative cross s e c t i ~ n s l ~for ~ ’ ionization ~ of the molecular species by using a double-oven arrangement. I n the present instance, the instability in TABLE I“ emission caused by the ethyllithium beam (see RELATIVE ION INTENSITIES O F N A J O R P E A K S I N THE i\ilASS Experimental Methods) together with the small SPECTRUM O F S-TURATED ETHYLLITHIUM VAPOR temperature range available made the fine control Relative necessary for this experiment difficult to attain. Inn intensity Mass No. It was possible, however, to observe a shift in the 18f LieRst 24 ratios of ion intensities as a result of superheating 151 Li6RI 1.3 the vapor in the upper (effusion) oven. The mass 115 LiR3 47.5 spectrum observed under these conditions is shown 79 LisRz 15 in Table 111. I n Table IV, the ratio of various 43 LizR 100 “fragment” peaks to the “parent” peaks 187 and 7 Li 14 115 are compared for the single- and double-oven a Data were obtained with an electron energy of 75 e.v. Ion intensities have been corrected for secondary electron experiments. These ratios suggest that most of the fragment peaks are to be attributed to the tetramer. emission at the first dynode of the electron multiplier. This is particularly marked for mass 43, although the The measured appearance potential of these effect is also evident for masses 79 and 7. ionic peaks, is shown in Table 11. Absolute values TABLE 111” were obtained by comparing the measured values with the appearance potential of Hg, obtained in RELATIVEIONINTENSITIES OF MAJORPEAKS IN THE MASS the same rim. SPECTRUM OF UNDERSATURATED ETHYLLITHIUM VAPOR +
+
+
+
+
Mass
TABLEIIa
No.
APPEARANCEPOTENTIAL OF MAJOR IONS IN THE MASS E~PECTRUM OF ETHYLLITHIUM VAPOR Mass No.
187 151 115 79 43
Appearance potential, e.v.
Inn
Lias 7.7 f 0 . 5 Lis& 12.5 f 0.5 LhRs 8 . 0 f0 . 5 Li3R2+ 11.7 f 0 . 5 Li2R+ 11.7 f 0.5 I Li + 14 f 2 5 Absolut,e values of the appearance potentials have been obtained by comparison with the appearance potential of Hgaz. +
+
+
I
Relative intensity
Inn
187 LisRS + 15.1 1.3 151 LhR, 115 LLR3 + 46.5 79 LiaRs + 14.2 43 LizR+ 100 7 Li 17.4 Data were obtained with 70 e.v. electrons. Electron multiplier amplification was not needed. +
+
(1
The use of a double oven has shifted the ratio of hexamer to tetramer from 0.505 to 0.325. This shift is in the direction to be expected by LeChatelier’s principle.
These appearance potentials clearly differentiate TABLE IV between masses 187 and 115, on the one hand, and all RATIOOF IONINTENSITIES FOR SINGLEAND DOUBLE-OVEN of the other ion peaks, on the other. It is most EXPERIMENTS probable that the observed differential in appearance Single Oven Double oven potentials of 3 4 e.v., is associated with the breakA, Relative to mass 187 (hexamer) ing of a t least one bond. If all of the ion peaks ;\1151/M187 0.054 0.086 listed in Ta.bles I and I1 were formed by reaction 1, M79/M 187 0.626 0.941 one would expect the electron removed by the M43/M187 4.16 6.63 ionization process to be approximately equally Mf/M187 0.584 1.15 bound in each of the parent polymer molecules. B. Relative to mass 115 (tetramer) Such behavior is observed among all of the alkali halide polymers.13~16 In the current experiments, M 151/N 115 0.027 0.028 only and Li4R3+conformed to this behavior. M79/M115 0.316 0.306 Hence, it was tentatively concluded that the hexaM43/M 115 2.10 2.15 mer and t,etra,mer of ethyllithium were the major M7/M115 0.295 0.374 (13) J. Berkowitz and W. 4. Chupka, ibid., 29, 653 (1958). (14) (a) L. 0.Gilpatrick, R. Baldnck and J. R . Sites, Oak Ridge National Laboratory Report No. 1376; “Mass Spectrometer Investigation of UFI.” ( b ) E. H. S. Burhop, H. S. W. hlassey and C. Watt, in Nationd Nuclear Energy Series, Div. I, Vnl. 5,p. 145 (1949)(McGrawHill, New York) edit. by A. Gnthrie and R. K. Wakerling. (15) American Petroleum Institute Research Project 44, ”Mass Spectral Data.’’ Serial N o s . 401,603,694,695,699,700,701. Petroleum Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pennsylvania. (16) L. Friedman, J . Chem. Phys., 23, 477 (1955).
An attempt was made to estimate the relative importance of hexamer and tetramer in the saturated vapor by comparing the corresponding “parent” ion intensities a t 12 e.v. bombarding voltage, where fragmentation was minimized. Under these (17) L. N. Gorochov. Vestnik Moskovskovo Uniaersiteta. 231 (1958). (18) J. Berkowitz. H. A. Tasman and W. A. Chupks, to be published. (19) T. J. Milne, J . Chem. P h y s , 28, 717 (1958)
August, 1961
MASSSPECTRUM OF ETHYLLITHIUM VAPOR
1383
conditions, the ratio of hexamer to tetramer was densed phases was evidence for unassociated mole1.09, using electron multiplier detection. The cules in the vapor. The presence of hexamer and tetramer suggests a measured relative efficiency of secondary electron production reduced this to 0.82. The relative ioni- ring structure, rather than a linear configuration, zation efficiency of these molecules is very likely in since the latter would be expected to also yield other such a dii.ection as to reduce this figure further, per- multiples of the basic monomer unit. The prefhaps to as lorn as 0.55. Finally, the known isotopic erential fragment ation of the CzHs radical during correction would raise this number to 0.65. ionization suggests that Li is bonded to more than It is possible to estimate the total vapor pressure one atomic center, while CzHs is not. The very from the results of the double-oven experiments. high appearance potential of Li+ in these experiThe sensitivity of the mass spectrometer with the ments is further evidence for this conclusion. One identical oven assembly and geometric arrangement is tempted to speculate that the structures of the has been tested recently at known temperatures hexamer and tetramer involve an inner lithium and pressures with LiF, LiC1, and LiBr.I8 These core surrounded by ethyl radicals. experiments also provided a measure of the pressure Most studies of the depression of the freezing drop across the frit between the two ovens. An point by ethyllithium in benzene, and in particular average of several such calculations yielded Plower/ those which have been performed with solutes of oven pure, recrystallized ethyllithium, indicate a degree Pupper = 31.3. This result, when combined with the of association in the range 5-7. There is no evioven calculated ionization cross section ratios of Otvos dence from any of these studies that the degree of and Stevenson,20enables one to compute the vapor association is concentration dependent, although pressure in the lower oven, which is in equilibrium it cannot be said that the results to date have with its condensed phase a t a known temperature. been sufficiently precise and sensitive to afford At 87", the total pressure was computed to be 7.8 X much assurance on this point. It does seem quite possible, however, that either one or both of the mm. It should be added that attempts were made to species present in the vapor of ethyllithium is also observe higher polymers than the hexamer, but the predominant species in solution. In this conthe fact that no signal was observed above back- nection it is noteworthy that ethyllithium apparently possesses a dipole moment in benzene.21 If ground indicated a concentration less than 0.01%. the solute is associated to hexamer in benzene soluConclusions tion, the dipole moment is computed to be a t least The mass spectrometric results for ethyllithium 1.50 D. show that the saturated vapor consists predomiAcknowledgment.-The authors wish to thank nantly of a hexamer and a tetramer in roughly equal Dr. William A. Chupka, Argonne National Laboraconcentrations a t 80-95". This result is contrary tory, for the use of the mass spectrometer, and Mr. to the conclusions of Shigorin and co-workers,8-10 Dean W. Dickerhoof, Department of Chemistry, who inferred that the difference in the observed in- University of Illinois, for assistance with the prepfrared spectra of ethyllithium in the vapor and con- aration of ethyllithium. (20) J. W. Otvos and D. P. Stevenson, J. Am. Chem. Soc., 78, 546 (1956).
(21)
IT. T. Rogers and T. L. Brown, J . Phgs. Chem.,
61, 336 (1967).
