NOTES
Metastable Transitions in the Mass Spectrum of Iron Pentacarbonyl
by Robert E. Winters and Jerome H. Collins
2057
Le., FeO+, some FeC,O,-1+ doubly charged ions.
several
Table I : Summary of 70-ev Monoisotopic Mass Spectrum of Iron Pentacarbonyl
The Procter & Gamble Company, Miami Valley Laboratories, Cincinnati, Ohio .@8S9 (Received December 87, 1966)
relative abundance--Singly Doubly charged charged
7 %
Ionic species
One of the best pieces of experimental evidence in favor of the concept of consecutive unimolecular decompositions contributing to the mass spectra of polyatomic molecules is the detection of metastable transitions for each successive step in the breakdown pattern.’ Recently,Z it has been proposed that transition metal carbonyl ions decompose by a series of consecutive unimolecular reactions involving successive loss of neutral carbon monoxide groups. However, no direct evidence, Le., metastable transitions, supporting this proposal has been reported. The purpose of this note is to present the results of an investigation of the metastable transitions observed in the mass spectrum of iron pentacarbonyl.
species, and
25.4 30.2 16.5 0.63 30.2 1.4 100.0 3.3 11.1 85.7
Fe(CO)j Fe(CO)4 Fe(C0h FeC302 Fe(C0)z FeCzO FeCO FeO FeC Fe
0.02 1.9 4.2 0.45 6.1 0.89 3.1 0.12 0.47 2.1
Ten groups of metastable transitions were observed in the mass spectrum of Fe(C0)5. The decomposition reactions, relative abundances, and apparent masses’
Experimental Section The measurements were made with an Atlas CH-4 mass spectrometer. The mass spectrum was scanned magnetically with a constant 3-kv ion-accelerating potential. Study of the metastable transitions was made using a 70-ev1 BO-pa ionizing current for electron bombardment and using an electron multiplier for ion detection. Under these conditions, metastable transitions of the order of 0.001% of the base peak were detectable. Pressure in the ion source was less than 10-5 torr. For this work the sample reservoir and the inlet line to the mass spectrometer were maintained a t room temperature. The ion source was held a t 200 f 5 ” . Commercial iron pentacarbonyl was purified by several vacuum distillation steps. No ion currents attributable to impurities were noted in the mass spectrum.
Results and Discussion Mass spectrometric studies of Fe(CO)5 have been reported previously.2-s The mass spectrum, however, is complicated by the presence of four iron isotopes.6 The result of the reduction of the experimental 70-ev cracking pattern of iron pentacarbonyl to a monoisotopic representation is shown in Table I. Identification of the metal carbonyl ions was made both by mass and isotopic abundance. A number of ionic species, not previously o b ~ e r v e d , ~were - ~ found in this study,
Table 11: Metastable Transitions in the 70-ev Mass Spectrum of Iron Pentacarbonyl % Metastable transitions
+ + +
Fe(CO)b+ + Fe(CO)4+ CO Fe(CO)d+ + Fe(CO),+ CO Fe(CO)3++ Fe(CO)z+ CO FeCa02 + + FeC20+ CO CO Fe(CO)Z+ + FeCO+ CO FeC20+ + FeC + CO FeCO+ + F e +
+
+
+ +
+ +
Fe(C0)42+-c Fe(CO)r2+ CO Fe(C0)32++ Fe(C0)22+ CO CO Fe(CO)22++ FeC02+
+
relative abundance
Apparent mass (n*) Cslcd Exptl
1.460 0.668 0.092 0.003 0.093 0.011 0.004
144.00 116.67 89.60 74.32 63.00 48.17 37.35
144.0 116.7 89.6 74.3 63.0 48.1 37.3
0.075 0.065 0,060
58.34 44.80 31.50
58.3 44.8 31.5
(1) H. M. Rosenstock and M. Krauss in “Mass Spectrometry of Organic Ions,” F. W. McLafferty, Ed., Academic Press, New York, N. Y., 1963,pp 1-64. (2) R. E.Winters and R. W. Kiser, Inorg. Chem., 3 , 699 (1964); 4, 157 (1965);J. Phys. Chem., 69, 1618 (1965). (3) F. W. Aston, “Mass Spectra and Isotopes,” 2nd ed, Edward Arnold and Co., London, 1942,p 148. (4) R. Baldock and J. R. Sites, U. S. Atomic Energy Commission V-761,Technical Information Services, Oak Ridge, Tenn., 1951. (5) A. Foffani, 5. Pignataro, B. Cantone, and F. Grasso, Z . Physik. Chem. (Frankfurt), 45, 79 (1965). (6) R. W. Kiser, “Introduction to Mass Spectrometry and Its Applications,” Prentice-Hall, Ino., Englewood Cliffs, N. J., 1965, p 292.
Volume 70, Number 6
June 1966
2058
NOTES
Acknowledgment. The authors wish to thank Dr. D. A. Nicholson for purification of the iron pentacarbonyl used in this study. ~
~~
(7) Calculations of the apparent masses for the singly and doubly charged transitions were made using the standard equation cited in ref 6 , p 124. (8) Metastable transitions are observed for each of the four iron isotopes. All calculations, however, were made using the major isotope (56) of iron. (9) A similar decomposition scheme has been suggested for W(CO),* + breakdown (R.E. Winters and R. W. Kiser, J . Phys. Chem., 70, 1680 (1966)). (10) J. H. Reynon, G. R. Lester, and A. E. Williams, ibid., 63, 1861 (1959). (11) A. S. Newton and A. F. Sciamanna, J . Chem. Phys., 40, 718 (1964). (12) K. E. McCulloh, T. E. Sharp, and H. M. Rosenstock, ibid., 42, 3501 (1965).
m/e
-
Mass Spectrometric Method for the Determination of the Activity Coefficient of
Figure 1. Magnetic scan of mass regions 144 and 116 for Fe( C0)s. T h e diffuse peaks correspond to the metastable transitions noted.
Ammonia in Aqueous Salt Solutions1
of the metastable ions are summarized in Table 11. The two most abundant metastable transition patterns8 are shown in Figure 1. The sharp peaks at m / e 142 and 114 are Fe68(C0)3+and Fe68(CO)2+,respectively. Each of the metastable transitions observed involves the loss of a neutral carbon monoxide from the metal carbonyl ions. In the case of the singly charged Fe(CO), + transitions, the decomposition scheme recently proposed by Winters and Kiser,* Le., a series of consecutive unimolecular reactions Fe(C0)5+ -+-Fe(C0)4+ + . . . .--) Fe+
(1)
involving successive removal of neutral CO groups is substantiated. Metastable transitions also were observed for the loss of carbon monoxide from FeC302+ and FeC20+ions. The metastables observed in the decomposition of Fe(C0),2+ suggest that the doubly charged transition metal carbonyl ions also lose neutral CO groups in a successive manner.9 Beynon, et ~ 1 have . reported ~ ~ ~ similar transitions for the dissociation of doubly charged ions in the mass spectrum of 2-hydroxyanthraquinone. Recent studies1l,l2 of the decomposition of ion indicate CO+ and O+ as the product ions. Evidence for this type of transition was sought in this study but was not found. The Journal of Physical Chemistry
by Richard A. Durstj2 Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 021 67
Paul G. Schmidt13and Irwin Feldman3 Department of Chemistry, Pomona College, Claremont, California 91 711 (Received January lot 1966)
Experimental techniques for the determination of activity coefficients of nonelectrolytes have been primarily limited to four methods : solubility, distribution, cryoscopy, and vapor p r e ~ s u r e . ~Solubility, while a simple and precise method, is easily applicable only to nonelectrolytes having low solubilities. Distribution is also a relatively simple method, but it is often difficult to find a reference solvent that is sufficiently immiscible with water and for which the distribution ratio of the nonelectrolyte is such as to give adequate accuracy in the activity coefficient determination. Although very precise, cryoscopy is more complex and the temperature of the experiment is (1) This work supported by the National Science Foundation under Grant No. GE.6158. (2) Visiting Assistant Professor of Chemistry, Pomona College (1964-1965), and author to whom reprint requests should be directed. (3) Supported by NSF Undergraduate Science Education Program (GE-6158). (4) F. A. Long and W . F. McDevit, Chem. Rev., 51, 119 (1952).
