TOURNAL
J
O F T H E AMERICAN CHEMICAL SOCIETY Registered in U. S. Patent Ofice.
@ Copyright, 1963, b y the American Chemical SocieIy
MAY20, 1963
VOLUME85, NUMBER10
PHYSICAL AND INORGANIC CHEMISTRY [COSTRIBLITION FROM RIAS, 7212 BELLONA AVENUE,BALTIMORE 12, MARYLAND, DEPARTMENT O F CHEMISTRY, THEJOHNS HOPKISS UNIVERSITY, BALTIMORE,MARYLAND, AND CALLERY CHEMICAL COMPASY, CALLERY, PENNSYLVANIA]
Appearance and Ionization Potentials of Selected Fragments from Isotopically Labeled Pentaboranes'" BY JOYCE J. KAUFMAN, W. S. KOSKI,L. J. KUHNSAND SALLY S. WRIGHT RECEIVED APRIL28, 1962 This paper presents appearance potentials of isotopically labeled pentaboranes, B1%H9and BlljDg, and some of their fragment ions as determined by mass spectrometric electron impact measurements. A set of apparently self-consistent ionization potentials for the pentaboranes and various fragments was calculated from these appearance potentials using what little thermochemical bond energy data are available combined with the authors' interpretation of the processes taking place on ionization and fragmentation. A(B6H9) > A(BjD9) is compared to d(B2H6) < d(B2D6) and possible reasons for the reversal of ionization potential differences are discussed. Using an I B M 7090 computer program, monoisotopic fragmentation patterns for normal and deuteriated pentaboranes were calculated from the mass spectra of these compounds at 70 e.v. plot of ion current versus apparent ionizing voltage. Using known spectroscopic values for the internal standard, a correction was determined and applied to the unknown, giving its appearance potential. The accuracy of this method was verified by measuring the appearance potential of C2H6+from C2H6 which checked to within 0.4 e.v., higher than the spectroscopic value (the appearance potentials of ions are usually 0.2-0.3 e.v. higher than spectroscopic ionization potentials). The appearance potential of c&+ was also measured relative to xenon which was used as the internal standard for the BjDg measurements. The extrapolated appearance potentials for C2H6" relative to either krypton or xenon checked each other to within experimental error.
This paper presents appearance potentials of isotopically labeled pentaboranes, B 115H9and B'l5D9, and some of their fragment ions as determined by mass spectrometric electron impact measurements. A set of apparently self-consistent ionization potentials for the pentaboranes and various fragments was calculated from these appearance potentials using what little thermochemical bond energy data are available combined with the authors' interpretation of the processes taking place on ionization and fragmentation. The cracking pattern data were obtained by electron impact of B1'5H9 and B1I6D9,and subsequent measurement of the relative intensities of the molecule ions and fragment ions formed. A program for calculating monoisotopic fragmentation patterns from mass spectral raw data was written for an IBM 7090. Using this program the monoisotopic fragmentation patterns for B5H9 and BsDg were calculated.
TABLE I B11gH9 APPEARANCE POTENTIAL DATA m/e
Method 1 A , (e.v.)
64
10.38 f 0 . 0 5
Ion if
B",Hn+
Experimental The instrument used to measure the appearance potentials of the pentaboranes and their fragment ions has been previously described.lb As an internal standard for Bl1sHg,krypton was used. Since it would have been extremely tedious to match the signals a t 70 e.v. from the krypton calibrating gas and the numerous fragment ions from BllIHg exactly, a method was devised to approximate matching K r values. Data were obtained for various isotopic krypton ions of m / e 80, 82,83,84, and 86, a t a number of different pressures. In each series of runs, the krypton values were plotted against the ionizing voltage and the average ion current a t each voltage was determined for the five krypton ions in terms of per cent of value a t i 0 e.v. From these percentages, values for krypton matching the various ions were estimated. For BjDg fragments, xenon was used as the internal standard. (Fig. 1.) The resulting estimated krypton curves were then plotted with the curves of the respective ions from pentaborane on a semilog
10.54
11.43 f 0 . 1 12.13 f . 3 12.67 f .03 13.01 f . 3 12.26 f 0 . 2 15.07 f . 3 14.06 .1 15.96 f . 5 .2 17.99 1.0 19.97
11.44 12.48 12.69 13.20 12.20 14.50 14.06 15.52 17.59 19.96
* *
(1) (a) Presented in part before t h e Division of Inorganic Chemistry a t the 140th r a t i o n a l American Chemical Society Meeting, September, 1961. This research was supported in part by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, and in part by the Office of Naval Research. Reproduction in whole o r in part is permitted for any purpose of t h e United States Government. (b) J. J. Kaufman, W. S. Koski, L. J. Kuhns and R . W. Law, J . A m . Chem. Soc., 84, 4198 (1962); presented a t t h e 138th National American Chemical Society Meeting, S e w York, N. Y., September, 1960; also O S R Technical Report 2, March 19, 1962, Contract Nonr 3471(00).
70 e.v. value, %
10.39
10.50 f .1
*
B'ljDg B"jD8H (B''rB'OD9) B"sD7 B"5Dj
hlethod 2 A , (e.v.)
(
B11s13pAPPEARANCE POTENTIAL DATA i3 9.i7 f 0 1 9.i9
(10)
*
(10)
72
69 65
9.38 .15 10.98 f . 2 12.92 f .03
9.38 11.14 12.93
(10)
(IO)
This method was then used to obtain the appearance potentials of some ions from pentaborane including: m/e 64, B11aH9(Fig. 2 ) ; m/e 62, Bi1gH1 (Fig. 3); and m / e 65, B11tD9(Fig. 4). In each of these cases the log of the ion current was plotted against the uncorrected ionizing voltage. When the curves for the internal standard gas and the ion of interest are not parallel, the first method cannot be used with any degree of accuracy. Another method used to find appear-
1369
1370
J. KAUFMAN, W. S.KOSKI,L. J. KUHKSASD SALLY S.WRIGHT
JOYCE
Jj2
'
13
'
'
17 1
14 I 15 16 I Ionizing Voltage (uncorrected).
1
1
Vol. 85
'
18 Ionizing Voltage (uncorrected).
Fig. 1.-Calibration curve, krypton. Relative intensity versus ionizing voltage (experimental and estimated).
Fig, 3.--Ion
efficiency curve m / e 62 (B",H?) and krypton
2mri o
I
li
'
'
9 17
'
'
10 13
!
'
11 14
'
'2
1 15
ionizing Voltage luncorrectedl
Fig 9 -Ion
13' 16
A 0
' '
l9
rnie64 ev Krypton ev
'Estimated lrom Fig' I'
efficiency curve m / e 64 (B1l&H9) and krypton
ance potentials is to call the appearance potential of both the ion of interest and the calibrating gas the voltage a t which the observed ion current is equal to some percentage of the ion current at 70 e.v. (usually about 1%). This second method is just as valid as the first method for the ideal cases where both curves are parallel. I t can be seen in Table I that the values of appearance potential obtained by the two methods for m / e 64 and m / e 62 are identical. This second method has the advantage that it can be used for ions where the curves are not parallel. The accuracy is not as good in these cases but at least the method serves to give some indication of the appearance potential of the ion in qnestion.
Fig. 4.-Ion
1 10
xenon
I
I
