1680
group in contact with the carbon surface. The agreement between the experimentally obtained values and that in column a leaves no doubt that the propane is horizontally oriented on the surface in the monolayer.
NOTES
Carbonyl Ions1
Experimental Section Mass spectra were measured with approximately 70-ev electrons on a Bendix Model 12-100 time-offlight mass spectrometer. The instrumentation has been described previously. l1 The solid tungsten hexacarbonyl was vaporized from a lt5.2-cm stainless steel tube connected directly to the ion source of the mass spectrometer. This arrangement allowed external heating of the sample in order to increase its vapor pressure.
by Robert E. Winters and Robert W. Kiser
Results and Discussion A comparison of the singlyg and doubly charged ions
Doubly Charged Transition Metal
Department of Chemistry, K a n s a s State University, M a n h a t t a n K a n s a s 66604 (Received November 39, 1966)
The majority of positive ions formed in the ion source of the mass spectrometer bear a single charge. It is possible, however, to obtain doubly charged positive ions if in the initial ionization process two electrons are removed from the molecule. The absolute intensity of 3-2 ions is generally found to be much lower than +1 ions.2 Mohler, et u Z . , ~ have suggested that decomposition of doubly charged hydrocarbon ions to give two singly charged ions is the more probable process for the + 2 ion breakdown scheme. No direct relationship between the intensity of the singly and doubly charged species is observed in hydrocarbon^.^^^ In fact, not infrequently a rather intense doubly charged species is observed whereas the ion at twice its m / e value is quite smalL4 The first evidence of a doubly charged ion decomposing into a second doubly charged species and a neutral fragment was reported by Beynon, et uL6 Meyerson and Vander Haar’ then found nine more instances where doubly charged ions disrupted to a neutral particle and a different doubly charged ion. This type of fragmentation has also recently been reported8 for azulene and naphthalene parent molecule 2 ions. In order to examine the mode of decomposition of the doubly charged transition metal carbonyl ions, we have studied the 70-ev mass spectrum of tungsten hexacarbonyl. Earlier investigationg of this monomeric carbonyl was unable to provide satisfactory data on the +t2 ions due to the compound’s low vapor pressurelo under the experimental conditions. We have now modified the inlet to the mass spectrometer in order to increase the sample pressure in the ion source. The results are interpreted in terms of successive unimolecular decomposition Of the doubly charged ions by loss of neutral CO groups.
+
T h e Journal of Physical Chemistry
formed upon electron impact of tungsten hexacarbonyl is shown in Figure 1. The most intense ion in each spectrum is arbitrarily assigned an intensity of 10O.l2 The intensities of all other peaks are given values relative to the most intense peak. Contrary to previous observations4 that no direct relationship exists between singly and doubly charged ions, a definite relation appears to exist between such ions from this metal carbonyl. Also, only the most intense singly charged M(CO)z species were found as + 2 ions in the mast3 spectra of Ni(CO)4 and Fe(CO)5,13and Cr(CO)6 and I I O ( C O ) ~ .Such ~ close similarity in the mass spectra suggests that the mode of forination of the doubly charged fragmentation ions is very similar to the process proposed earlierg for the singly charged ions, i e . , elimination of neutral CO groups. Such a clecomposition scheme would require considerable charge to be associated with the metal atom. How(1) This report is based on work performed under contract with the U. S. Atomic Energy Commission, Contract No. AT(ll-1)-751. with Kansas State University. Abstracted from the Ph.D. dissertation of R. E. Winters, Kansas State University, 1965. (2) F. H. Field and J. L. Franklin, “Electron Impact Phenomena and the Properties of Gaseous Ions,” Academic Press, Inc., New York, N. Y., 1957. (3) F. H. Mohler, V. H. Dibeler, and R. M . Reese, J . Chem. Phys., 22, 394 (1954). (4) J. H. Beynon, “Mass Spectrometry and Its Application to Organic Chemistry,” Elsevier Publishing Co., Amsterdam, 1960. (5) K. Bieman, “Mass Spectrometry: Organic Chemical Applications,” McGraw-Hill Book Co., Inc., New Tork. N. Y., 1962. (6) J. H. Beynon, G. R. Lester, and A. E. l~illiams,J . P h y s . Chem., 63, 1861 (1959). (7) S.Meyerson and R. W.Vander Haar, J . Chem. P h y s . , 37, 2458 (1962). E. Wacks, i b i d . , 41, 3195 (1964). (8) R. J. Van Brunt and -M. (9) R. E. Winters and R. W.Kiser, Inorg. Chem., 4 , 157 (1965). (10) T. N. Rezukhina and V. V. Shvyrev, Veslik M o s k o u . U n h . , 7 ( 6 ) , Ser. Fiz.-Mat. i Estestven. N a u k , (4), 41 (1952). (11) E. J. Gallegos and R. W. Kiser, J . Am. Chem. SOC.,83, 773 (1961); J . P h y s . Chem., 65, 1177 (1961). (12) The actual intensities of the singly charged ions are 4.27 times the intensities of the doubly charged ions.
(13) R. E. Winters and R. W. Kiser, Inorg. Chem., 3, 699 (1964)
NOTES
1681
M(CO),2+ +M(C0),-l2+ M(C0)2+ +A P +
+ CO (1)
+ co
ever, the second ionization potentials of several transition metal carbonyl species (Table I) strongly suggest that the second electron removed upon electron impact is an electron from an orbital largely associated with the metal atom. The first ionization potentialsg~'O of these metal carbonyls have been reported also as being very near the ionization potentials of the metals. Note that within experimental error the second ionization potential of the RI(CO), species corresponds to the second ionization potential of the respective metal atom.
I
Table I: Comparisons of the Second Ionization Potentials of Some M(CO), Species and the Metals Second ionization potential, ev"
M(CO)z species
Ni(C0)S Fe( CO) Cr( CO) Mo( CO) Mo( C0)z Mo(CO)a
17.6 f1 . 0 1 6 . 2 f2 . 0 17.3 & 1.0" 1 6 . 4 f0 . 6 15.2 f 0 . 6 15.4 f1 . 2
Metal
Second ionization potential, evb
Ni Fe
Cr MO
18.15 16.18 16.49 15.717 16. I5*
'
a See ref 9 and 13. C. E. Moore, "Atomic Energy Levels, ' National Bureau of Standards Circular 467, U. S. Government D. It. Bidinosti and Printing Office, Washington, D. C. N. S. M d n t y r e , University of Western Ontario, personal communication, 1965. a AI. A. Catalan and F . R. Rico, A n a l e s Real SOC.Espan. Fis. Quim. (hladrid), 48A, 328 (1952).
Doubly-Charged Ions
unimolecular decomposition of doubly charged ions. Such ca.lculations are now underway in these laboratories. ~~~~
~~
~
~
(14) R. E. Winters and J. H. Collins, private communication: a paper describing this work has been submitted to J . P h y s . Chem.
(15) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring, Proc. Natl. Acad. Sci. U . S., 38, 667 (1952); M. Vestal, A. L. Wahrhaftig, and W. H. Johnston, J . Chem. Phys., 37, 1276 (1962).
3 - 100
B
BO
Singly-Charged Ions
-
The Nature of the Acidic Sites on Silica-Alumina.
A Revaluation of the Relative Absorption Coefficients of Chemisorbed Pyridine
I
I
Figure 1. Mass spectra of doubly and singly charged ions from tungsten hexacarbonyl.
by Michael R. Basila and Theodore R. Kantner
I n a recent infrared spectroscopic study' of pyridine
(PY) adsorbed on a synthetic silica-alumina (SA), we It appears, therefore, that the doubly charged transition metal carbonyl molecule ions decompose in a manner similar to that previously proposed for the singly charged positive ions, Le., by a series of consecutive unimolecular decompositions. The metastable transitions corresponding to such processes in both the singly and doubly charged ions have recently been observed.14 It is anticipated that the quasi-equi1ibrium theory of mass spectra>15may find new applications in the
estimated the ratio of Lewis to Brqhsted acid sites to be -1 from the intensities of bands characteristic of the coordinately bonded (LPY) and protonated (BPY) chemisorbed pyridine a t 1450 and 1545 cm-l, respectively. This estimate was based on relative absorbtion coefficients derived by assuming identical absorption coefficients for the v l g a mode which occurs at 1490 cm-1 for both chemisorbed species. Subsequent work (1) M. R. Basila, T. R. Kantner, and K. H. Rhee, J. P h y s . Chem., 68,3197 (1964).
Volume 70,Number 6
M a y 1966
