March 20, 1963
COMMUNICATIONS TO THE EDITOR
A SYNTHESIS O F XENON DIFLUORIDE NOT INVOLVING USE O F ELEMENTAL FLUORINE
Sir: Although the existence of noble gas fluorides has excited intense interest in recent months, each of the preparative heretofore described has involved manipulation of elemental fluorine. This Laboratory is engaged currently in the exploration of new approaches to the synthesis of noble gas compounds by techniques involving neither fluorine handling nor high pressure or high temperature instrumentation. Among the pathways to xenon fluoride synthesis currently being explored here are the use of high voltage electric and microwave powered discharges in mixtures of xenon and a variety of perfluorinated species, and the vacuum ultraviolet photolysis of these mixtures using xenon and krypton resonance lamps. Among the fluorination reagents being evaluated are CF4 and SiF4. These approaches were suggested by a considerable volume of literature pertaining to noble gas photosensitized reactions,j and by the frequent observation of ionic species containing xenon in mass spectrometric studies.6 Abstraction reactions involving various ionic and electronically excited species of xenon, and simple perfluorinated species, seemed a promising route to the formation of xenon difluoride. The well known 1470 A. line of xenon, for example, corresponds to an available energy of 194.5 kcal. An estimate of the C-F bond dissociation energy in CF4, for example, is 121 kcal.' and the stripping of a second fluorine atom, to produce CF2, would be expected to require substantially less energy. Further, CF2 appears to be a relatively stable species capable, however, of elimination by polymeri~ation.5,~ We wish to report, a t this stage, the demonstrated possibility of routinely successful syntheses of XeF2 using a high voltage discharge in approximately equimolar mixtures of xenon and CF4. The apparatus consisted of a simple U-tube immersed in a refrigerant bath, and equipped with neon-sign electrodes, appropriate flow regulating valves and a high vacuum sampling train. The discharge was powered by a commercial neon-sign transformer operated near, or somewhat above, its rated capacity of 6000 v. a t 120 ma. The Pyrex discharge tube, having an approximate diameter of 25 mm. and a distance between electrodes of 360 mm., was refrigerated with Dry Ice and acetone, thus maintaining the walls of the reaction zone a t temperatures above the boiling points of the reactants. In a typical experiment, two I.-atm. of reaction mixture are permitted to flow through the discharge zone over periods of 1-2 hr. Upon terminating the discharge, a white microcrystalline or amorphous material is found, collected largely in the bottom of the U-tube. After distillation, a white, nonvolatile organic residue remains in the discharge region. The volatile component, condensed in the sampling train, appears to contain a t least two fractions condensable a t -78'. The more volatile of these can be removed by flash evacuation a t room temperature. The amounts of this material have been insufficient for physical characterization. (1) C. L. Chernick, e l ai., Science, 138, 136 (1962).
(2) J. H . Holloway and R. D . Peacock, Proc. Chem. Soc. (London), 389 (1962) (3) J. L. Weeks, C. L. Chernick and M. S. Matheson. J. A m . Chem. Soc., 84, 4612 (1962). (4) A . V. Grosse, A. D . Kirshenbaum, A. G. Streng and L. V. Streng, i b i d . , 86, 360 (1963). ( 5 ) See, for exnmple: J. R. Dacey and J. W. Hodgins, Can. J . Res., 28, 173 (1950). (6) See, for example: M. S. B. Munson, F. H . Field and J. L. Franklin, J . Chem. Phys., 37, 1790 (1962). (7) T. L. Cottrell, "The Strengths of Chemical Bonds," Butterworths Scientific Publications, London, 1958, p. 275. (8) W. Mahler, Inorg. Chem., 1, 230 (1963).
823
The remaining fraction on standing undergoes spontaneous distillation with recrystallization at room temperature, forming clear, colorless, well formed crystals adhering t o the walls of the storage vessel. Typical yields have been 50-150 mg. It must be emphasized that no serious effort has been made to optimize discharge conditions or to prefluorinate the sampling train. Mass spectrographic analysis of these crystals was performed with a Consolidated Electrodynamics Model 21-103C instrument. The mass spectrum demonstrated conclusively that the crystals were the expected XeF2. Peaks corresponding to XeFz+, X e F i , X e + and Xe++ were observed, having the correct isotopic distribution. Trace quantities corresponding t o still higher masses were observed also. It is noteworthy that the first injection of vapor into the mass spectrograph sampling manifold resulted merely in deactivation of interior surfaces and contaminants, producing as a consequence the mass spectrum of xenon in correct isotopic distribution. Analogous phenomena had been observed earlier, typically in distillation of the XeFz in the un-prefluorinated glass vacuum system. The infrared spectrum of the vapor phase in equilibrium with these crystals was recorded a t approximately 24", using a 10-cm. gas cell equipped with KBr windows. A Beckman IR-4 spectrometer employing CsBr optics was used for these measurements. The vapor spectrum was found to exhibit a band with P and R components centered a t 549 crn.-l and 564 cm.-', respectively, The previous observation by D. F. Smithg that greater intensity is associated with the R branch is confirmed. In addition, an extremely weak absorption was observed a t 520 cm.-l. It may be significant to note that Smith has estimated a frequency of 515 cm.-I for the infrared inactive fundamental, vl, on the basis of an apparent combination band a t 1070 ern.-'. Acknowledgment.-We wish to thank Professor G. J. Mains for the mass spectrographic analysis. (9) D . F.Smith, J. Chem. P h j s . , S8, 270 (1963) MELLON ISSTITUTE DOLPHUS E. MILLIGAN 13, PENNSYLVANIA D. RICHARD SEARS PITTSBURGH RECEIVED F E B R U A R13, Y 1963
NON-CHLOROPHYLLOUS PIGMENTS OF
CHLOROBI LTM THIOSULFA TOPHIL U M CHLOROBIUMQUINONE
Sir : The anaerobic, photosynthetic bacterium, Chlorobium thiosulfatophilum, differs from all other known photosynthetic systems in having several unique chlorophy1ls.l This induced us to examine other pigments of this bacterium which may be related to its photosynthetic and oxidative phosphorylating systems, viz., the carotenes and the quinones. This organism had been reported2 to contain y carotene, pro-y-carotene, and rubixanthin. Also, Fuller, et al., 3 reported that C. thiosu[fatophilum (strain L) is devoid of coenzyme Q and plastoquinone, but detected a substance, Xmax 254 mk, which they suggested was related to plastoquinone. We have extracted' dried cells of C. thiosulfatophilum (strain PM) and chromatographed the non-chlorophyllous pigments on Decalso. A carotene fraction was eluted with isooctane and established as y-carotene, identical with an authentic sample4 by its visible (1) J. H. Mathewson, W . R. Richards and H. Rapoport, J. A m . Chem. Soc., 86, 364 (19631, and references therein. (2) T. W. Goodwin and D. G. Land, Biochem. J.,6 2 , 553 (1956). (3) R . C. Fuller, R . M . Srnillie, N . Rigopoulos and V. Yount, Arch. Biochem. Biophys., 96, 197 (1961). (4) W e are indebted t o Dr. G . McKinney for this sample.
824
Vol. 85
COMMUNICATIONS TO THE EDITOR TABLE I
KEVERSED-PHASE
PAPER
cHROMATOGRAPHYQ
OF cHLOROBIUM-
SOME OTHERNATURAL QUINOSES
QUINONE ASD
Quinone
R F ~
Chlorobiumquinone
0.46
Ultraviolet fluorescencee
Blue X'iolet .34 Violet Coenzyme Ql; .24 Red .17 Red Plastoquinoned a On Whatman 3 M M paper impregnated with silicone 550. * Descending technique with propanol:water, 4: 1. Seen with an ultraviolet lamp. On subsequent development with neotetrazolium chloride each spot became blue-violet. Generously provided by Dr. K. Folkers
,44
absorption spectra and by circular paper chromatography on alumina paper.5 Elution with isooctane-5yo ether gave a vitamin K which was identical with an authentic sample6 of vitamin K2(35)by m.p. (54O),
e
Structure I has been assigned to chlorobiumquinone on the basis of the following evidence
ACH3 v
C
H
CH3
= C-CHzI
CH3 [CH2- CH=C-CHI]B I -H
A
v
I
0
0
I1 Chlorobiumquinone has the molecular formula C45H6202 by element analysis (found, C, 84.6; H, 9.3) and spectrophotometric molecular weight determina-
TABLE I1
S.M.R. ABSORPTIOK~ OF CHLOROBIUMQUINONE, VITAMINK2(30), PLASTOQUINONE AND COENZYME Qlo
Structural element
H
Chlorobiumquinone
Vitamin Kz(ao)
7.9(m)* [4Ic
7.9(m)
Plastoquinone
Coenzyme
Qio
O
...
... 0 (chain)
-CH=C
paper chromatography and infrared and ultraviolet absorption. Continued elution with isooctane+% ether gave another quinone fraction. The yield was low, but was improved substantially when the cells were extracted with 70% acetone-30yo water, 0.03 M in ferricyanide. This material has hE,t,O,H254 mp (sh 263 mk) which, with sodium borohydride, disappears into a broad inflection on powdered polyethyla t 251 m ~ Chromatography . ~ ene and crystallization from acetone and petroleum ether gave this new quinone, chlorobiumquinone, m.p. 50-51 O, clearly distinguished from other natural quinones by paper chromatography (Table I). ( 5 ) Procedure of Dr. E. A. Shneour, for whose assistance w e are grateful. ( 6 ) 0. M e r , R . Ruegg, L. H. Chapard-dit-Jean, A. Winterstein a n d 0. Wiss, Heiv. Chim. Acta, 41, 786 (1958). We t h a n k Dr. M e r for samples of vitamins Ktcao) a n d Kz(ss). (7) C/. reference 3.
tions (see below). In the infrared (KBr), it shows absorption essentially identical with that of vitamin K2(36), plastoquinone, and coenzyme Qlo a t 1660 and 1610 cm.-', characteristic of a 1,4-quinone, and a t 875, 795, and 755 cm. -l, characteristic of the all-trans-polyisoprenoid chains. Also, peaks a t 700 and 690 cm.-', a t 720 and 690 corresponding to those in vitamin K2(35) em.-', are assigned to the four adjacent aromatic protons. These infrared assignments indicated a 1,4naphthoquinone with a polyisoprenoid side-chain. e mir However, the ultraviolet spectrum [ h ~ : ~ t S n249 ( E 16,300)] is different from that of the various vitamins K . The new chromophore was established as a vinyl1,4-naphthoquinone by comparison with a synthetic sample of 2-methyl-3-vinyl-l,4-naphthoquinone (II),' k280.3ta"s 249 mp ( E 16,200). Comparison of extinc(8) G. Manecke a n d W. Storck, B e y , 94, 300 (1961).
March 20, 1963
COMMUNICATIONS TO THE EDITOR
825
very important that there be available a theoretical approach capable of treating such compounds and semiquantitatively predicting their stabilities. We wish to report that an adaptation of the simple molecular orbital theory is applicable to the problem, and to give the results of calculations of the stabilities of some inert gas halides. The energy of an electron in a localized two-center molecular orbital, $ = N ( + A A+G), is given by the equation6 w = % (1 $. x)gA (1 - x2)'"@ f % ( 1 - x)qC (1) where x = (1 - X z ) , / ( l A*) is defined as the bond ionicity, qa and qG are coulomb integrals for the halogen 9 and the central atom, respectively, and p is the exchange integral; 2 W measures the energy of the procCH3 ess l/nGA, -+ l/nG"+ A+ 2e-. From W and data available in the literature, the energy of formation of gaseous GA, from the elements is calculated. The coulomb integrals are approximated by expressions conI11 taining the bond ionicity, the ionization potentials and 0 the valence state preparation energy (v.s.p.e.)of the inert gas atom, the ionization potential and electron affinity CH3 I of the halogen, the G-A bond distance, and a geometric [cHz-C H ~ -C H - ic--H ~ factor.6 The G-A exchange integral is approximated 0 by the p for the appropriate interhalogen compound, using the equation of Pauling and Sherman,' and corIV recting for v.s.p.e. of the inert gas atom and van der ( E 15)900))268 (20,000), 335 (250). Both compounds Waals repulsion. have identical n.m.r. absorptionlo a t 6 1.6 for the Cg, Xenon tetrafluoride has a square planar configuraC, methylenes; 6 2.3 for the C6, CS methylenes; 6 2.0 tion,8a and XeFz is linear.8b The same geometries for the C2-CH3; 6 1.2 for the side-chain methylenes and were assumed for other possible inert gas halides GA, methenes; and 6 0.8 for the side-chain methyls. and GA,. The experimental valuesa of R were used for Chlorobiumquinone (I) is unique among natural the Xe compounds. Bond lengths of the other inert vitamins K in having a double bond conjugate with the gas halides were estimated from the Pauling covalent quinone'' and one carbon less than the normal multipleradiig of the halogen A and of the halogen adjacent to of-five carbons in the side chain. Although a vinyl the inert gas in the periodic table. Other estimates naphthoquinone has not been found previously, it has were obtained from the known bond 1engths'O of the been postulated as an intermediate in oxidative phoshalides of elements immediately preceding the inert gas phorylation.'? in the periodic table. Since for the tetrahalides the The absence of coenzyme Q and plastoquinone in C. calculation is rather sensitive to the exact value asfhiosulfatophilurn is of interest, as is the fact that addisumed for R,calculations were made for a range of tion of a mild oxidizing agent during the extraction reasonable values. The valence states were assumed to substantially increases the yield of chlorobiumquinone. be p2dsfor the tetrahalides and ps for the dihalides, and This is not the case for the accompanying vitamin the v.s.p.e. values were estimated from spectroscopic K2(35),and suggests that chlorobiumquinone is present data. mostly in a reduced form. Best values of the energies and ionicities are shown in (9) hI. Tishler, I,. F. Fieser a n d N . L . Wendler, J . A m . Chem. SOC.,62, Table I. The model predicts considerable stability for 28063 (1940). the known compound XeF4, and somewhat lower stai10) I n deuteriochloroform with internal tetramethylsilane. (11) A quinone recently detected spectrophotometrically in Sarcina lrcteu bility for XeF,. Krypton tetrafluoride is expected to may be of this t y p e [ D . H. L. Bishop, K. P. P a n d y a a n d H . K. King, Bioexist, while KrF2, XeC14and ArF4 are marginal. ( h e m . J . 83, GO6 (19G2)I. The method predicts the intuitively expected increase (12) I. Chmielewska, Biochim. Biophys. Acta, 39, 170 (1960). in stability of the halides with increasing atomic num(13) Fellow of t h e Consejo Nacional d e Investigaciones Cientificas y TPcnicas de la Republica Argentina, 1962. ber of the inert gas, and greater stability for fluorides DEPARTMEKT OF CHEMISTRY BENJAMIN FRYDMAN' than for the corresponding chlorides. More quantitaUSIVERSITYOF CALIFORNIA HENRYRAPOPORT tively, a change in the central atom from xenon to krypBERKELEY, CALIFORXIA ton or even argon is expected to entail a smaller deRECEIVED JANUARY 11, 1963 crease in stability than a change of ligand from fluorine to chlorine. Calculations were not made for bromides BOND ENERGIES AND IONIC CHARACTER OF INERT GAS and iodides, since the trends in the calculated results HALIDES' strongly suggest that none of these would be stable. Sir: A fluoride of radon has been prepared.? Insufficient Much interest has been aroused by the recent demoninformation about the ionization potentials of radon is ~ t r a t i o n ~that - ~ a t least ssme of the so-called inert (6) R . G. Pearson, S. Yamada and H . B. G r a y , Pvoc. 7th I C C C , G.5 (1962); gases are able to form true chemical compounds. It is R . G. Pearson a n d H. B. G r a y , Inorg. Chem., in press.
tion coefficients supports the molecular formula of chlorobiumquinone. All these structural assignments were confirmed and further features established by a detailed study of chlorobiumquinone's n.m.r. spectrum in comparison with those of vitamin Kncao),plastoquinone and coenzyme Qlo, presented in Table 11. Thus structure I is uniquely defined for chlorobiumquinone. Further confirmation was found in a comparison of octadecahydrochlorobiumquinone (111) (C46HaoOz; Found, C, S2.9; H , 11.8) with hexahydrovitamin K1 (IY).$ In the ultraviolet, I11 has X ~ ~ ~ ' 260 " " e mp ( E 16,(100)268 (20,100), 333 (250) ; IV absorbs a t 260 mp
+
+
+
+
(1) Acknowledgment is made t o the National Science Foundation for support of this research. ( 2 ) H . H . Claassen, H. Selig a n d J. G . M a l m , J . A m . Chem. Soc., 84, 3593
(1962).
(3) C . L. Chernick, el ul., Science, 188, 136 (1962). (4) P. R . Fields, L. Stein a n d M. H . Zirin, J. A m . Chcm. S O L ,84, 4164 (1962). (5) J. L. Weeks, C. L. Chernick a n d M . S. Matheson, ibid., 84, 4612 (19G2).
+
(7) L. Pauling a n d J. Sherman, J . A m . Chem. SOL.,69, 1450 (1937). (8) (a) J. A. Ibers a n d W. C . Hamilton, Science, 139, 106 (1963); (b) H . Levy and P. A. Agron, .I. A m , Chem. SOC.,8 8 , 241 (1963); we t h a n k Dr. W . C. Hamilton for communicating these results t o u s prior t o publication. (9) L, Pauling, "The N a t u r e of t h e Chemical Bond," Cornell University Press, I t h a c a , pi. Y.,1960, p. 224. (10) "Interatomic Distances," T h e Chemical Society, London, 1958. (11) C. E . Moore, "Atomic Energy Levels," K a t l . Bur. S t d . Circ. 467, Washington, D. C., 1949, 1952, 1958.
