NOTES
3198
*,FS
=
- (’/&)
(16rPm)
2 ~W~(En^- /Eo)~-’ ~ X c
(nla(pfN)sj[o) (1) YN is the gyromagnetic ratio of nucleus N, 10 )(In)) represents the ground (excited state) of the molecule ) a &function centered on with energy Eo(E,), 8 ( F k ~ is nucleus N, fix is the electron spin operator, and all the other symbols have their usual meaning. After making the “average energy” approximation2 some progress has been made by McConnella using a molecular orbital approach and by Karplus and Anderson‘ using a valence-bond wave function. Recently Pople and Santry6used eq. 1 in conjunction with a molecular orbital approximation. Exact calculations, however, are diilicult? and any general observations that can be made are helpful. Some preliminary results on the basis of symmetry considerationsare reported here. Consider the operator which reflects the electronic states of ethylene in the plane of symmetry which is perpendicular to the C-C axis of the molecule. All the states of this molecule may be characterized as symmetric, IS), or antisymmetric, / A ) , such that RIS) = IS) and RIA) = - [ A ) . Equation 1 may be written as a sum of contributions due to symmetric and antisymmetric states. In the light of this, the direct and indirect 13C-H coupling in ethylene can be written as (o18(pkN>gICln>
~’ICZ-HI
=
Kc(ES Skj
- EO)-’(o16(FEz)fik[S) x
+
(Sl8(?jH~)8jlo) Kc(E.4 Akj
- EO)-’ x
(0 18(%c3f3 IC 1 4 ( AI8(F,EIl) 8 F l s C r C p H ~ =z
KC(E.9 Skj
g,lo>
(2)
- EO)-1(01a(%C8)@IC(@x
+
(Sla(~;Et)&[0) KC(E‘4 Akj
(JIC~ =E 156.4 I ~ C.P.S. and J l ~ a ~=r -2.4 ~ l c.P.s.) imply that the symmetric and antisymmetric contributions are 77.0 and 79.4 c.P.s., respectively. The difference in magnitude and relative sign for the directly and nondirectly bonded I3C-H coupling can be interpreted as the result pf approximately equal contributions from the symmetric and antisymmetric states. Molecular symmetry requires that these contributions add in the case of the former and cancel in the case of the latter. Thus, a minimum of two excited states must be considered when eq. 1is used. This analysis also suggests that in the (‘average energy” approximation7 different A-values should be used in the case of direct and indirect couplings because the symmetric ground state used in such calculations could not be expected to take into account the symmetry properties of all the excited states. Couplings in other molecules with a high degree of symmetry are presently being investigated in this laboratory. Acknowledgment. The author wishes to thank Dr. E. R. Malinowski and Dr. T. J. Dougherty for helpful discussiom and acknowledges the support of the U. S. Army Research Office (Durham), Contract DA-31124-ARO(D)-90. (1) E.Fermi, 2.PhyaiR, 60,320 (1930). (2) N. F. Ramsey, Phys. Reu., 91, 303 (1953). (3) H.M.McConnelI, J. Chem. PhyS., 24, 460 (1956). (4) M. Karplus and D. H. Anderson, ibid., 30, 6 (1959). (5) D.P.Santry and J. A. Pople, Mol. Phys., 8 , 1 (1964). (6) R.M.Lynden-Bell and N. Sheppard, Proc. Roy. SOC.(London), A269, 385 (1962). (7) The “average energy” approximation has also been discussed by M. Karplus, J . Chem. Phys., 33, 941 (1960),and A. D. McLachlan, ibid., 32, 1263 (1960).
- Eo)-’ x
( o l ~ ( % c * ) ~ k I(AlW,FII)@,lO) A) Since the value of a matrix element is invariant under any symmetry operation, it follows that
Ionization Potentials and Mass Spectra of Cy clopentadienylmolybdenum
Dicarbonyl Nitrosyl and
(018(%C*)skIS)= R ( o \ W m J ~ k l S= ) (Ol8(~EP>’BICls)
1,3-Cyclohexadieneiron Tricarbonyl’ (O[8(%c8)8k[A) = lI(Ol8(%cJga[A) = - ( O I ~ ( ~ E J ~ I C I A )
It becomes obvious that the terms in the expansion of and 8 F 1 ~ 8 ~ aare - ~identical 1 in magnitude but have different signs for the antisy-etric contributio~. Denoting the symmetric and antisymmetric contributions by S and A , eq. 2 may be written as
by Robed E. winter8and Robert
w. zser
SFIC,-H1
+A
8 F ~ ~ 2= - ~81
b F ~ c d l - ~=l 8
-A
The data of Lynden-Bell and Sheppards for ethylene The Journal of Physical Chemary
Department Of Chemistry, Kansas State UniVmaity, Manhattan, Kansas 66604 (Received April 13,1966)
Recently, we reported2 mass spectrometric studies of the ionization and dissociation in several transition metal carbonyls. From the measured ionization potentials it was suggested that the electron removed
NOTES
upon ionization may be considered to have been localized on the metal atom rather than in the carbonyl groups. Unimolecular decompositions of the parent molecule ions of these carbonyls by apparent stepwise losses of CO groups were indicated. In order to study these phenomena in more complex systems, we have investigated the mass spectra and ionization potentials for two substituted transition metal carbonyls of varying composition: cyclopentadienylmolybdenum dicarbonyl nitrosyl and 1,3-cyclohexadieneiron tricarbonyl.
Experimental The samples of cyclopentadienylmolybdenum dicarbonyl nitrosyl and 1,3-cyclohexadienein tricarbony1 were kindly provided by Dr. R. B. King of Mellon Institute. Low-voltage mass spectrometry indicated that no volatile impurities were present in the two compounds. The mass spectra and ionization potentials were determined with a Bendix (Model 12-100) time-offlight mass spectrometer. The instrumentation has been described previously.a Mass spectra for each of the compounds were obtained at nominal electron energies of 70 e.v. The experimental ionization efficiency curves for the parent molecule ions were interpreted _using the extrapolated voltage difference method described by Warren4 and the method of Lossing, Tickner, and Bryce? Since the compounds were not sui5ciently volatile to use any of the noble gases for calibration purposes, mercury (from the diffusion pump) and oxygen (from a small air leak) backgrounds in the mass spectrometer were used to calibrate the ionizing voltage scale. Spectroscopic values for the ionization potentials of mercury (10.43 e.v.6) and of oxygen (12.08 e.v.l) were employed for this purpose. Decomposition products were repeatedly cleaned from the ion source and the Wiley magnetic electron multiplier components throughout this study. Also, frequent replacement of tungsten filaments was necessary. The mass spectra were found to remain constant and the ionization potentials were reproducible to within the quoted error limits (one standard deviation) for independent runs.
Results and Discussion Ionization Potentials. The ionization potentials determined for cyclopentadienylmolybdenum dicarbonyl nitrosyl and ]1,3-cyclohexadieneiron tricarbonyl are 8.1 h 0.2 and 8.0 f 0.2 e.v., respectively. The fact that the ionization potentials are lower than those of the substituentsa but only dightly greater than the
3199
ionization potentials of the metals again suggests that ionization subsequent to electron impact involves the removal of an electron associated with the metal atom-possibly from a hybrid molecular orbital involving considerable contribution from the metal atomic orbital. As one would anticipate, therefore, the ionization potentials of these substituted metal carbonyls are comparable to the ionization potentials2 of the corresponding metal carbonyls, e.g., iron pent* carbonyl and molybdenum hexacarbonyl. Mass Spectra. A partial monoisotopic mass spec, trum of the important ion fragments formed when C&&Mo(C0)2NO was sublimed into the ion source is shown in Table I. The isotopic abundances were found to agree, within the experimental error, with the presently accepted values.@ The ion of greatest mass which was observed W ~ L I the parent molecule ion. No indication of association of molecules in the gas phase was detected and there was no evidence of thermal decomposition in the ion source. Fragmentation ions formed by loss of CO groups in preference to cleavage of nitrosyl or cyclopentadienyl groups were found. This is not surprising; the stronger bonds are between the metal and NO or CaHs groups.10 Also, the presence of MoC&16+ ions and the absence of MONO+ions in the mass spectrum indicate that the Mo-csH~ bond is somewhat more diEcult to rupture than the Mo-NO bond. Dissociation involving loss of hydrocarbon aggregates smaller than C& from CsHsMo(CO)zNOwas observed, (1) This work was supported in part by the U. S. Atomic Energy Commission under Contract No. AT(11-1)-751 with Kansas State University; a portion of a dissertation presented by R. E. Winters to the Graduate School of Kansas State University in partial fulfillment of the requirementsfor the degree of Doctor of Philosophy. (2) R. E. Winters and R. W. Kiser, Imrg. Chem., 3, 699 (1964); 4, 157 (1965); J. Phya. Chem., 69, 1618 (1965). (3) E. J. Gallegos and R. W. Kiser, J. Am. C h m . SOC.,83, 773 (1961);J . Phys. C h . ,65, 1177 (1961). (4) J. W.Warren, Nature, 165, 810 (1950). (5) F. P. Lossing, A. W. Tickner, and W. A. Bryce, J. C h . Phgis., 19, 1254 (1951). (6) C. E.Moore, “Atomic Energy Levels,” National Bureau of Standards Circular 467, U. S. Government Printing Office, Washington, D. C., 1958. (7) G. Hersberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules,” D. Van Nostrand Co., Inc., New York, N. Y., 1950. (8) Y. Kaneko, J . Phys. SOC.Japan, 16,1587(1961): I(C0) = 14.11 e.v.; A. G.Harrison, L. R. Honnen, M. J. Dauben, Jr., and F. P. Lossing, J. Am. C h . SOC.,82, 5593 (1960): I(CSHS) = 8.69 e.v.; G. G. Cloutier and H. I. SchilT, J. C h m . Phya., 31, 793 (1959): I(N0) = 9.25 e.v.; W. C. Price and A. D. Walsh, PTOC.Roy. SOC. (London), A179, 201 (1941): I(CsHs) = 8.40 e.v. (9) J. H. Beynon, “Mass Spectrometry and Its Applications to Organic Chemistry,” Elsevier Publishing Co., Amsterdam, 1960, pp. 557,559. (10) C. T. Mortimer, “Reaotion Heats and Bond Strengths,” Pergamon Press, New York, N. Y., 1962,pp. 149-154.
Volume?69,Number 9 September 1986
NOTES
3200
Table I: Relative Abundances of the Principal Positive Ions from Cydopentadienylmolybdenum Dicarbonyl Nitrosyl at 70 e.v.
m/e
249 221 193 167 163
Relative sbundance
Ion
C~H~MO(CO)~NO+ 64.6 C~H~MO(CO)NO+46.2 C~H~MONO 72.4 CaH3MoNO+ 96.3 C~H~MO+ 100.0 +
m/e
137 110 98 81.5
Ion
Relative abundance
CaHaMo+ 95.0 MoC’ 47.1 Mo+ 51.9 CSHSMO*+ 34.1
e.g., the formation of C3H3Mo+. An ion similar to
this, FeCJ&+, has been reportedll to occur in the mass spectrum of ferrocene. The doubly charged MoCgHS2+ ion appears in significant quantity in the mass spectrum of this compound. This ion is very abundant; it is much more abundant than the commonly encountered relative abundances of approximately 0.01 to 0.1% for doubly charged ions in many hydrocarbons.12 Similar observations of the great intensity of doubly charged ions in the mass spectra of the carbonyl compounds were reported in earlier studies of molybdenum hexacarbonyL2 Hoehn, Pratt, Watterson, and Wilkinson13have reported a partial mass spectrum for the perfiuoro derivative of 1,3-cyclohexadieneiron tricarbonyl, i.e., C6FsFe(C0)3. Ions corresponding to CBFsFe(C0)2+,CB FsFeCO+, and C6FsFef were noted; it was suggested that they were formed by a stepwise loss of CO from the parent molecule ion. The most abundant ions detected in the mass spectrum of the analogous CB HsFe(C0)3are shown in Table 11. Ions formed by successive removal of CO groups from the parent molecule ion were detected also in the mass spectrum of this compound. However, two striking differences were noted in the fragmentation of the CGHriron tricarbonyl and the previously studied C6Fs-iron tricarbonyl. First, C6HBFeCO+was formed upon electron impact Table 11 : Relative Abundances of the Principal Positive Ions from 1,3-CyclohexadieneionTricarbonyl a t 70 e.v.
m/e
Ion
Relative abundance
220 192 164 162 134 112
CsHaFe(CO)a+ C&8Pe(CO)2+ CeH8FeCO+ CaHSeCO + CsHBFe+ Fe(CO)a+
1.0 3.5 0.9 2.4 57.8 0.9
The Journal of Physical Chemistry
m/e
84
80 79 78 56 39
Ion
FeCO+
ca,+ CeHs+ Fe+ GHa+
Relative abundance
10.6 1.7 3.2 2.4 100.0 3.0
in addition to the C m e C O + ion fragmented by simple removal of two CO groups. Also, no ion of composition C&BFe+ was detected in this study; however, CJ&Fe+ ion was observed in large abundance. The deficiency of CBF6FeCO+and CeF6Fe+in the mass spectrum of the perfluoro derivativela may be due to the greater strength of the C-F bonds as compared to the C-H bonds in the ionic species. The second striking difference in the cracking patterns for these two related compounds lies in the intensities of the metal carbonyl ions of the type Fe(CO),+. Iron dicarbonyl and iron monocarbonyl ions were found to have a low abundance in the C$-IsFe(CO)8 mass spectrum and no Fe(CO)3+was observed. Since the iron carbonyl ions were the most abundant ions in the cracking pattern of C6F~Fe(CO)3, the low abundance (or absence) of similar ions in the mass spectrum of the hydrogen-containing compound indicatea that the C a r F e + bond is stronger than the C6FgFe+bond. Acknowledgment. The authors wish to express their appreciation to Dr. R. B. King of the Mellon Institute for the gift of the compounds used in this study. (11) F. W.McLafferty, Anal. C h . ,28, 306 (1956). (12) See, for example, F. H. Field and J. L. Franklin, “Electron Impact Phenomena and the F’roperties of Gaseous Ions,” Academic Press Inc., New York, N. Y.,1957, p. 183. (13) H. H.Hoehn, L. Pratt, K. F. Watterson, and G. Wilkinson, J. C h a . Soc., 2738 (1961).
Solubilities of Fluorocarbonsin Cyclohexane and 1,4-Dioxane by J. L. Carson, R. J. Knight, I. D. Watson, and A. G. Williamson Department of Chemistry, University of Otago, Dunedin, New Zedand (Received April 14, 1966)
Konecny and Deal’ have reported solubility measurements for mixtures of 1-hydroperfiuoroheptane with paraffins, aromatics, and polar compounds, which they interpret as indicating that the behavior of C7FISH in paraffins and alkylbenzenes is dominated by the fluorocarbon chain while in polar solvents spec& interactions involving the CH group of C7FlsHstrongly influence the solution behavior. The solubilities of C7F16 and C~FUHin cyclohexane and 1,Pdioxane reported here are in qualitative agree(1) J. 0. Konecny and C. H. Deal, J . Phys. C h a . , 67,504 (1963).
