Jan., 1949
MASSSPECTRA OF CYCLIC AND STRAIGHT CHAINFLUOROCARBONS
337
[CONTRIBUTION FROM NATIONAL BUREAUOF STANDARDS]
Mass Spectra of Some Cyclic and Straight Chain Fluorocarbons1 BY FRED L. MOHLER,EVELYN G. BLOOM,J. H. LENGELAND C. E. WISE The Mass Spectrometry Section of the Bureau of Standards has measured the mass spectra of some pure fluorocarbons prepared by George H. Cady of the Chemistry Department of the University of Washington. la This paper presents these measurements in comparison with the mass spectra of the corresponding hydrocarbons. Tables of the detailed mass spectral data have been published in the Catalog of Mass Spectral Data.2
Experimental
TABLE I BOILING POISTSOF FLUOROCARBONS Substance
1 2 3 4 5 6
Cyclo-CsFto n-CsFi2 *-C5F12 Cyclo-CsFpH Cyclo-CsFiz &&Fie
TABLE I1 MASSSPECTRA O F CYCLOHEXANES Sensitivity relative to 43+ of n-butane: C3F6+,1.37 Ion
Mass spectra were obtained with a 180' Consolidated mass spectrometer which has been described in detail elsewhere.3 Recommended procedures were followed in obtaining the spectra except that it was necessary t o use a magnet current in excess of the rated current to record the ions of high molecular weight. A magnet current of 1.3 amperes was used in the range from molecular weight 50 up t o a value exceeding the molecular weight of the molecule. Mass spectra were measured with ionizing potentials of 50 volts and 70 volts but only the 70-volt spectra are reported here. The temperature of the ionization chamtier was automatically maintained a t 245 O and the electron current to the collector (catcher) was automatically held a t 9 microamperes. Relative intensities in the mass spectrum are expressed on a scale of the peak of maximum height taken as 100 and peaks as small as 0.01 can be detected. The sensitivity is commonly expressed in terms of the current a t the maximum peak !n arbitrary units dividcd by the pressure in the gas reservoir. The ratio of this sensitivity to the sensitivity t o normal butane a t 43+ gives a value independent of the arbitrary units used and it is this relative sensitivity that is given here. The resolving power of our mass spectrometer was not sufficient to resolve completely unit mass differences beyond mass 150. This introduces a little'uncertainty in reading the mass peaks of the heavier ions from cyclo-GFsH. The boiling points of the six fluorocarbons measured are listed in Table I. The first three of these compounds were carefully separated from impurities using an 82 theoretical plate fractionating column. The same column was used for cyclo-C5FgH but traces of impurities probably remained. The last two compounds are of good quality but' inferior to the first three. The mass spectrometer analysis shpwed no recognizable impurities in any of the perfluoro compounds. In the spectrum of cyclo-C6FgH
Number
there is a small peak, 0.19% of the maximum peak, near mass 250. This could be C$Flo+or CSF~OH+.The spectrum shows that it cannot be cyclo-CsFlo but a trace of a peduoropentene is possible. Another possibility is that it is C6FloHf from CSF11H. This compound makes an unknown but probably small contribution to the pattern recorded in Table 111.
Boiling point, OC.
. 22.5 29.3 30.1 37.8to 38.2 51 (sublimes) 82.4 to 82.6
(1) Paper presented before the Oregon meeting of the American Chemical Society, September 13-17, 1948. ( l a ) The fluorinated pentanes used for this research were prepared under contract with the Office of Naval Research, Navy Department. (B) Catalog of mass spectral data, American Petroleum Institute, Project 44, National Bureau of Standards. (3) H. W. Washburn, H. F. Wiley, S. M. Rock and C . E,Berry, Ind. and Bng. Chem., A n d . Rd., 16, 64 (1943).
Rel. inten.
Ion
73.9 4.55
CSFiz Fil
CSHi2
Hi1
1.20 22.2 1.69 2.95
0.05 Fe 2 0 . 9 FB 0.16 F7 2.71 Fio
c4-
c4-
HB 100
FB
H, Hs Hs H4 Ha Hz
34.9 6.82 4.80 1.35 3.34 2.23
F7
Fs F6 F4
F3
Fz
Ion
Rel. inten.
G0.70 3.77
c5-
c5-
Hie Hs Hg H7
Rel. inten.
H, Hs
13.6 29.8 Hg 64.9 H4 6.21 H3 2 8 . 4 HZ 2.53 H 1.05
cz0
21.4 3.96 2.06 1.54 0.57 .36
H5 H4 Ha HI
Hz H C
C*F7 Fe Fs F4 F3
F2
F
Rel. inten.
0.02 .48 100 2.53 15.0 2.96 1.07
C*-
11.7 13.2 30.4 4.74
F.j Fa Fz
5.59 29.2 2.14 1.23
5.35 0.42 .08 .05
CFa Fz F C
70.3 5.15 21.8 .03
C-
Ha
Ion
F5
The hydrocarbon mass spectra given for comparison with the fluorocarbons were obtained with National Bureau of Standards Samples of purity 99.85% and better. The tables omit some smaller peaks of less than 2% of the maximum peak.
Results When mass spectra of fluorocarbons are tabulated in the order of increasing or decreasing molecular weight the ions with different numbers of carbon atoms and of fluorine atoms alternate in a rather confusing disorder. For convenient comparison with the more familiar hydrocarbon spectra the ions containing the same number of carbon atoms have been grouped together in the following tables. Cyclohexanes.-Table I1 gives the mass spectra of cyclohexane and the perfiuoro compound. In the hydrocarbon molecule the molecule ion (parent ion) and ions C5H9f, C4Hs+, CzHs+ and CzH3+ are relatively abundant. It is of interest that CH3+ has about the same intensity in cyclics as in molecules containing CHa terminal radicals. In the fluorinated compounds the parent ion peaks are always very small indicating that the more probable ionization process is removal of F-
F. L. MOHLER, E. G. BLOOM,J. H. LENGEL .4ND
338
c. E. WISE
Vol. 71
height of peaks containing an H atom when there are seven or more fluorine atoms. The absence of C4F7+ in the cyclo-C5FgH spectrum shows that the CeFlo+is not from cyclo-C~,Fl~. Table 111shows a very close parallelism between Re1 ions of C5F9Hcontaining H and the corresponding inten. ions of cyclo-CsFlo. The occurrence of C3FeHt but no ion C3F7+from cyclo-CsFlois an exception. 1 ) 11 4 2ti Another exception is the fact that CF+ is large but CH+ is small. This is not surprising however I 1 33 as the CH+ peak is of the same magnitude as is found in all hydrocarbons. 20.7 The perfluoro ions of cyclo-C~FgHdo not cor1.93 respond closely to the ions of CsFlo. The absence 1.34 of ions C6Fsand C4F7is a striking characteristic of the CsFgH spectrum. I t is suggestive that the 2.97 abundance of CF3+ from CjFgH is almost four times the abundance of CFzH+ for this is the u 80 4 priori probability of finding H missing or present 1 . 6 2 on one of the C atoms of cyclo-CSFgH. The 11.0 chance of finding H missing or present on two ad2.39 jacent carbon atoms is 3 to 2 while the ratio of 0.69 C2F4+to C2F3H+ is 2.52 to 2. In ions with more than two carbon atoms there is no evidence for 0 60 such a relation. Normal and 1sopentanes.-Table I V shows 23.4 the mass spectra of the normal pentanes and the 2.62 I n both the perfluoro compounds isopentanes. 1.64 CFs+ is by far the largest peak which is quite unlike the hydrocarbon spectra. The difference in 14.0 the C3F7+peaks is the most conspicuous difference 2.24 in the peduoro spectra while the C4H9+peaks are 0.13 best for djstinguishing the hydrocarbons. The largest peak in each carbon group of perfluoro-w-
TABLE 111 MASSSPECTRA OF CYCLOPENTANES Sensitivity relative to 43+ of n-butane; GF;+ of C,Fio, 1.73; CsFs'of CaF$H, 0.61 Cyclo-CoHio Ion
Re1. inten.
Cyclo-CsF~o I011
Rel, inten.
Cyclo-CsF91-1 Ion
Rel. inren.
e" F g
0
F b
018
e* I1
F* Fa
0.76 74
C: F7
He
H~ H,j HZ H? H
100 28.9 7.33 20.9 3.16 1.42
cgHj H, H3
H2
4.50 3.78 14.7 3.35
Fg F~ F1 Fa F:! F
CuFA F d
Fa Fz
C-
C--
Ha H2
Fa Fz F
3.44 0.51 H .ll a Masked by
.39
loti 1.36 9.65 2.62 1.13 0.24 23.8 1.88 1.28
16.2 5.11 23.3 isotope.
Fa Fs
Fc F F, €7
C, F,
,66 .
