A p d 20, 1959
MASS SPECTRA OF CHROMYL CHLORIDE, CHUlROFLUORlDE A N D
1787
FOX ATOMIC RESEARCE AND DEPARTMENT OF CHBMXSTRY. IOWA STATE WORK WAS PERFORMED IN TEE AXES LABOUTOXYOF THE ATOMIC ENEXCY COUOUSSXON]
[CONTXIBUTION No. 643 FROM TEE -1 COLLEGE.
FLUORIDE
u. s.
The Mass Spectra of Chromyl Chloride, Chromyl Chlorofluoride and Chromyl Fluoride BY GERALD D. FLESCH AND HARRY J. SVEC Rscrtrvg~ JUNE 2.1958
The results of a mass spectrometric study of CxQCl,, CxQCIF and CxQPt are reported. The fragmentation patterns of each compound in the normal and dimeric mass range and the appearance potentials of the positive ion fragments of CrOtClt and CxQF2 are given. A method of taking appearance potential data is presented which results in the appearance potential curves of all fragments having nearly identical slopes and also permits taking data when the ion currents are changing slowly.
Introduction Chromium is one of the elements on which no work has been done to determine whether or not the isotope abundance is dependent on geological deposition. T h e element is widely distributed throughout the world principally as the ore, chromite, which generally is found in pre-Cambrian deposits. The volatility of the easily prepared and well known compound CrOzC12simplifies the sample handling problem associated with the mass spectrometry. A mass spectrometer capable of handling corrosive gases of high mass was available in our laboratory and so chromite ore samples were collected from well characterized deposits throughout the world.' No knowledge of the behavior of chromyl halides in the mass spectrometer was available, so a study of the fragmentation pattern of CrO,Cl, was undertaken to determine the best ion fragments for isotope abundance work and the extent of possible interferences. The parent ion appeared the most logical to use for isotope abundance measurements. However, large discrepancies in the isotope abundances from those expected to have been present (using known C1 and 0 abundances and measured Cr+ abundances) were observed when the parent ions were employed in the measurement. This caused doubt concerning the validity of such measurements. Since these discrepancies could only have been caused by ion fragments of a possible dimeric species, the heavy mass region was investigated and dimeric chromium oxychloride was discovered. This also gave rise to a fragmentation pattern and the interference observed in the parent monomer ions. In looking for a more suitable compound, chromyl fluoride (CrOzF2) was prepared and its normal and dimeric fragmentation patterns were studied. This led to the chance discovery of chromyl chlorofluoride, Cr02C1F, which was found in the mass spectrometer, and presumably was due to reaction between Cr02F2 and adsorbed Cr02C12. Attempts to explain the difference in fragmentation patterns between the similar chromyl compounds made a study of the appearance potentials of the various fragments advisable. The results of these studies are presented in this paper. Results of the isotope abundance work will be presented in a future publication. Experimental All measurements were made on a 1SO degree mass spectrometer with a five inch radius (Consolidated Electrodynamics Corporation mass spectrometer, model CEC 21(1) The cooperation of Dr. \V. H. Hartford of the Mutual Chemical Company in gathering the samples IS acknowledged.
220). It was m d i e d t o permit magnetic as well as electrostatic scanning. Magnetic scanning was used exclusively in these measurements. The ionizing current was 150 pa., and the xans were made using 1600 volt ions. Electron accelerating voltages were measured with a Rubicon Potentiometer. Type 2700, placed across a 1: 100 voltage divider. Argon was used as the calibrating gas in the appearance potential measurements. Because of the corrosive nature of the gases, a constant gas flow into the analyzer was difficult t o achieve. This resulted in data for the appearance potential work which was not constant and subject to considerable spread. The difficulty was overcome by measuring the ion currents at some reference electron accelerating voltage immediately before a measurement a t the desired electron voltage, thus enabling corrections to be made for any possible change in gas flow during any particular data-collecting experiment. The electron accelerating voltage for this instrument was supplied from a ten-turn helical potentiometer placed across a 150 volt voltage-regulator tube. To facilitate the change from one electron voltage to another, a second helical PO-' tentiometer and a switching device (Fig. 1) were added t o SHIELD
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the filament circuit. One potentiometer was adjusted to some reference voltage. The second potentiometer was then varied to supply any desired voltage. The switching device allowed the operator t o choose either potentiometer as the source of voltage for electron acceleration. In taking data, the variable voltage was adjusted to the desired value, the filament was switched to the reference voltage and the ion current recorded. Then the filament was switched to the variable voltage and the resulting second ion curreiit recorded. The ratio of the ion current at the variable voltage to that a t the reference voltage tvas plotted against the changing voltage to give the ionization efficiency curve. The actual method employed in taking data was according to the following procedure. The reference voltage was set a t 50 volts and the ion current ratios were recorded nt five-volt intervals from this value. A plot of these datn yielded a rough curve. A new reference point was chnsen a t the upper end of the straight line portion of the curve at the point where the curl-c' started to deviate from the straight line. \\.-ith this new reference point, more precise data were taken at smaller accelerating voltuge intervals. The curves in Fig. 2 were obtained by this method. This method of presenting data makes it possible to compare the general characteristics of ionization efficiency
GERALD D. FLESHAND HARRYJ. SVEC
1788
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Fig. 2.-Ionization
efficiency curves for positive ion Fragments of chromyl chloride and chromyl fluoride.
curves on the same scale without resorting to any mathematical manipulation other than that involved in taking the ratios. I t is also possible to make comparisons with similar curves obtained for standard substances, such as argon, since the plots are generally parallel. Extrapolation of the straight line portions of the curvesz in Fig. 2 gives values for the intercept relative to a value of 15.7 e.v. for argon. These values are listed in Table I.
t o be determined from the equilibrium mixture of the three compounds. To accomplish this, the complete spectrum of the mixture was first determined. Purity of the spectrum was established by taking the currently accepted isotope abundances of Cr, 0, C1 and F and calculating the expected ion current peak heights of all possible fragments of each isotopic species. These were then compared with the observed peak heights. Where several fragments had the same mass, the contribution of each fragment was deterTABLE I mined by the use of simultaneous linear equations. The INTERCEPT POTENTIALS OF THE CHROMYL HALIDEPOSITIVE peak heights so calculated for each mass agreed with those observed within 2'% of the observed peak height. The IONFRAGMENTS Fluoride Chloride TABLE I1 Intercept Intercept Ion potential," e.v. Ion potential," e.v. FRAGMENTATION PATTERNS FOR THE CHROMYL HALIDES CrOG 12.6 f 0 . 3 CrOzFz 14.0 f 0.2 Fluoride Chlorofluoride Chloride AbunAbunAhunCrOzCl 13.9 f . 3 16.3 f . 3 CrOzF Ion dance Ion dance Ion dance 1 9 . 8 f .2 CrOz 1 5 . 2 =t . 5 CrOz CrOzF2 47 CrOZClF 100 CrOzCl? 100 15.8 f . 2 1 6 . 8 f .4 CrOClz CrOF2 Cr0zF 23 CrOzCl 61 CrOzCl 51 CrOCl 17.0 f . 3 1 9 . 8 f .2 CrOF 1 CrOzF 1 CrOz 12 CrOz 24.4 f . 2 CrO 21.4 f .2 CrO CrOz 1 CrClz 18.2 f . 2 14.8 f .2 CrFz CrOFz 40 CrOClF 13 CrOClz 1 CrCl 22.2 & . 2 2 1 . 7 f .4 CrF CrOF 21 CrOCl 35 CrOCl 25 Cr 26.7 f . 2 30.8 f .2 Cr 6 CrOF 6 CrO 14 Limits set down in this table represent the precision CrO obtained for several measurements on each of the ion fragCrO 9 ments. All data have been corrected to an intercept value CrFZ 100 CrClF 13 CrClz 4 of 15.7 e.v. for argon. CrCl 20 CrCl 12 CrF 18 Fragmentation Patterns.-The fragmentation patterns CrF 4 Cr 33 29 of the three chromyl compounds were determined a t an Cr Cr 30 electron accelerating voltage of 70 volts and an ion accelerating voltage of 1600volts. The results are shown in Table 11. The fluoride and chloride patterns were obtained from the amounts of each fragment ion present were taken to be those pure compounds. The pattern for the mixed halide was calculated. From the fragmentation pattern of the fluoride obtained from an equilibrium mixture3 according t o the and chloride, the contribution of each of their fragment ions was known. These were subtracted from the measured equation amounts of each ion fragment of the mixture. The residue CrO?Ch CrO?Fe = 2CrOyClF was assumed t o be the amount of fragment ions formed from Because of this equilibrium, the chlorofluoride cannot be the chlorofluoride. The values listed for the chlorofluoirde prepared in pure form and so its fragmentation pattern had in Table I1 were obtained from a mixture whose ion currents were in the ratio ( 2 ) R. H. Vought, Phys. Reu., 71, 93 (1947). CrOzF~:CrOrCIF:CrO~Cl~: : i : l 8 :13. ( 3 ) G . I).Flesch and H. J. Svec, THISJOURNAL, 80, 3189 (19%).
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MASSSPECTRA OF CHROMYL CHLORIDE, CHLOROFLUORDE AND FLUORIDE
April 20, 1959
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Fig. 3.-Ionization
efficiency curves of C r + and CrOF+ of chromyl fluoride a t various pressures.
fluoride and mixture, respectively. Experiments to determine the nature or origin of the compounds causing these ions were inconclusive. The data strongly suggest that dimeric compounds exist as distinct molecular species mixed with the monomers. Ionization Efficiency Measurements.-The ionization efficiency curves for many of the ion fragDimeric Species.-The dimer mass region of ments shown in Fig. 2 are similar to those for the pure chloride, the pure fluoride and a mixture CHz+ observed in the spectruni of methane.4 of the two had the ions and abundances listed in Thus one is led to suspect that several processes may be responsible for the formation of these ions. Table 111. These were identified from the mass The voltage separation of the two breaks should be a measure of the difference in potential of the two TABLE 111 FRAGMENTATION PATTERNS OF CHROMYL HALIDEPOSITIVE processes responsible for the ions. In some cases, especially the Cr+ curves, i t is difficult to assign IONSI N THE DIMERREGION any reasonable processes to the different breaks -Chloride-Fluorider-CblorofluorideAbunA bunAbunbecause of the large voltage differences (15-1s e.v. Ion dance IOU dance Ion (lance for Cr+). However, a study of the pressure deCrzO~F4 100 Cr20SF2 12 CrzOICla 100 pendence for the curves of Cr+, CrOf and CrCI+ CrrOsF 10 CrzOIClz 45 of the chloride and Cr+, CrO+, CrF+ and CrOFf Crt04F3 75 Crt0,CI 52 of the fluoride showed that two distinct breaks were CrtO4F2Cl 100 CrzO~C13 3 significant only for CrOF+. In the others, the ion CrzOaFClz 70 CrzO,Cl 12 currents a t the lower electron voltages were relaCrtOXla 16 CrOCL 30 tively constant over a wide range of pressure. This CrZOlF2 14 indicates that the source of these ions is not due to Cr2OrClz 14 direct electron bombardment of the sample gas or Crt03F~ 6 any contained gaseous impurity. Since there was Cr202F* 16 no background a t the masses involved, the ions and isotopic variations of the ions. The total probably resulted from an apparent zero-order reacdimer current was about 0.1, 0.1 and 0.3y-of the (4) C. A. McDowell and J. W. Warren, Disc. F a r a d a y Sur , "Hydrototal positive ion current of pure chloride, pure carbons," 10, 53 (1951).
They were checked by calculating and measuring the ion currents of a mixture whose ratio was 16:8: 1. The average error between calculated and observed ion currents was _ ~ ... ~the. ~ less than 3%. Two peculiarities are noteworthy in the cracking patterns. The CrFn+ion is twice as abundant as the parent, CrOZF2+, and the abundances of the chloride fragments of the chlorofluoride are larger than those of the corresponding fluoride fragments.
17!)0
WARRENL. REYNOLDS AND ROGERH. WEISS
tion of tlic sample gas with something in the sample iiilct system, ion source or analyzer system of the spcvtronieter. If adsorption of some species is the priiiiary process on which these ions depend and the adsorbing surface is only partially covered a t low imssurcs, a first-order pressure dependence is expected. At higher sample pressures the surface should be coiiipletely covered, with the resulting effect being the apparent zero-order dependence observed. Support of this thrsis was given by an expcriiiient in which the sample gas pressure in the source was greatly reduced. z\t pressures froin of the normal operating pressure, the ion to currents a t lower electron accelerating voltages varied directly as the pressure. I t is interesting to note that the initial break of the ionization efficiency curves for all the ion fragiiiriits exhibiting an apparent double break was 12.5 & 0.5 volt for fluoride fraginents and 11.5 f 0.5 \. olt for the chloride fragmiits. This sinal1 variation suggests that these fmginent ions riiay be of t 1icrni:il origin and not priinarily due to electron I)oiiibardincnt of gascous niolecules. Fibwre 3 shows the Crf and CrOF+ curves as tlicy appeared :it various pressures. The upper nuiiibcr of each C r + curve is the relative ion current fur 45-volt electrons. The lower number is the value a t 25 electron volts. The number on the CrOF+ curve is the relative ion current for 45volt electrons. I t would be teiiiptiiig td iiiterpret these results
Vol. 81
in terms of the bond strengths in molecular or ionic chromyl halides. However, the paucity of ionization potential data for CrO+ or CrX+, coupled with the uncertainties involved in interpreting such data as presented in Figs. 2 and 3, make any such attempts tenuous. It is apparent that no simple assumptions can be made which represent the processes occurring in the mass spectrometer ion source. While the results listed in Table I appear to have some consistency, their meaning in terms of thermodynamic properties in these molecules is clouded by the several likely processes which inay be occurring. Attempts to measure ionization efficiencies of the chlorofluoride fragment ions were unsuccessful, except for the parent ion. The value of 14.0 & 0.2 volt was obtained for the appearance potential from a measurement on a mixture containing very little chloride. The aniount of chlorofluoride present was large enough to give reliable results, and the very small amount of CrOC12+ from the chloride did not interfere. I t was thought that the appearance potential curves for some of the ions of a mixture of all three compounds might exhibit two or more “breaks” because of the probable difference in appearance potential of the ions which are common fragments of two or all of the compounds. In the course of work reported here, no such breaks were observed on any mixture. AMES,IOWA
[CON.I.KII~III 10s VKOM n i u SCIIOOL OP CIIEMISTKY, UNIVERSITYoi? MINNESOTA]
Iron(II1) Complexes in Non-aqueous Solvents. I. The Solvolysis and Chloride Complex Constants in N-Methylacetamide BY
\\‘ARREN
L. REYNOLDS A N D ROGERH. Wmss RECEIVED OCTOBER 31, 1958
~~~cctr~,~ili~~toiiictric I I I I ~ : I S U ~ C I I I ~ I>~ivl~lctl I~S v;ilucs of 1.4 x 10-3 mole IiLcr-1 for tlie first sulvolysis constant of Fc+Ja i d 7.5 x 1 1 t z ;tilt1 I:{ litcr IIII)IL~--’ for tlir stcl)n.isc f~~riiiatiori constants of 1;eCl + * atid PeCla+, rcspectivcly, iii S-iiictli)lucetariiide. ~ t;Hi:; ~ iiip o f I‘eS+’ (wliere S - is the solvent anion), PcCl +* and FcCl?+were found to bc ‘flie iiio1:ir e~tiiictione ~ r l l i c i c i;it over tlic l . x x 1 1 1 3 , 1 .$I x 1 0 3 : i r i d $1 3 x 1 0 3 . rvspectivcly. T h c average latcnt heat of vaporization of S-iiictl~~lacelaiiiiiic t eii~p~r:itiirc interval f r ~ m i115 to 26’ wits 14.9 kc:il. mole-1.
Experiiiierits, preliiiiinary to an investigation of
+
the kirictics of the Fe(1I) Fe(II1) isotope exchange rcnction i n the solvent N-methyl‘acetamide ( X l l L l ) ,showecl that Fe+$formed complexes with the :inions o f various strong miueral acids in this solvelit. In 1;ig. 1 are showii spectra of anhydrous FeCl:, tlissolved iri NLI.1 in the presence of 0.1 ill
The solvent NMA was chosen because it could bc obtained in relatively anhydrous condition (water concentration less than 0.002 Jl) and because of its high dielectric constant.
Experimental Reagents.-Iron(II1)
chlt~ ide was prel)nred by p;issiiig
liytlr~cliloric.~)cn~Iiloric anti sulfuric acids and in dry Clr gas a t rooin teiril)er;iture through iroii powdcr in tlie tlic. absciicc (Jf aiiy adtlcti acid. Presence of watcr presence of a trace of moisture. The product w a s :iii:ilyzed iodonictrically and foiiiid t u coiitain 96. 1 1 1 ) t o a t ivast one iiiolar had no :tppreciahle effect on main iinpurit:~probably was watcr; tlic :ilnotlllt o f water t I i i w spectra. ilp1xirently coiiiplexes between iritrdiiccd with the 1’cCl3 in ~i~;iking a 0.1 ruillirnolar solu~ ~l’c(II1) was tlius 1iiuc11less than the ~niriiriiuii~ w:itcr I;c +:$ aiid tile various ;inions arc formed which have t i o of which could be detected by a Karl Fisclicr titration. tliiTcrcii t nbsorl)tioii spectra. ’Therefore, before content Hydrochloric acid solutions were prelxircd by Imssing d r y t lie rate coilstants for the isotoiiic cwhingc of iron I-ICI gas’ into NRIA. ‘l‘lic solutions were ~tu11t1:rrdizedl ) v I)c,twcc.ii l;c(l I ) : i i i t l tlic varioiis I;e(III) c o i i ~ p l c ~ sdilutiiig ail aliqiiot a t Ic.i.;t twifi,ld with witer mltl titr;itiiil: ;is iiidic:ttor. wit11 standxrd K:iOlI using ~~licnol~~lith;ilciii c.o111(1 I x tlc.tc.riiiiiivt1, it was iicsccss:try to cli.tcriiiiiw NaCl solutiuiis were prepired by dissolviug allthe associatioil coiistaiits oi soiiic’ o f lhcsc coiiiplcxcs _Standard _ ~ a i i d to clc.tcriiiiiie the solvolysis coiistmt of I’e + 3 ( I ) “lni,rganic Synthmes,” Vol. I , XIcCraw-Irxll I3ook C o . , I n c , i i i NA1.l. N e w York. N. Y . , p. 117.
