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 f 1 . 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 Printing Office, Washington, D. C. D. It. Bidinosti and N. S. Mdntyre, 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 . Phys. 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.
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
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 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. Phys. Chem., 68,3197 (1964).
Volume 70,Number 6
M a y 1966
NOTES
1682
has shown that this assumption is incorrect and it is the purpose of this note to revaluate the absorption coefficients. The experimental details of the work are given in our earlier paper.' Lewis-BrZnsted Acid Site Distribution on SA. When a sample of SA containing chemisorbed PY is exposed to HzO vapor a large proportion of the chemisorbed LPY is transformed to chemisorbed BPY.'v2 Inspection of the spectrum reveals that the 1490cm-' band changes in intensity rather than remaining constant as one would expect if the absorption coefficients were identical in the two chemisorbed species. The intensity increase suggests that the BPY species has the greater absorption coefficient. It is possible to derive an expression for the relative absorption coefficients for the two species by substituting the relationships (1)-(3) into (4). E149OL/t1450
=
c = 0.25
(1)
consistent with this expectation. The results shown in Table I indicate that the Lewis sites predominate in all the silica-aluminas, but that a wide range in the ratio of Lewis to Brdnsted sites exists.
Table I : Lewis-Bqnsted Acid Site Distribution in Silica-Aluminas" % Catalyst
AltOa
SA, m*/g
American Cyanamid Triple A Davison F-125 Houdry S-90 Houdry S-36
25
430
6.0
23 12 12
352 42 1
3.8
157
[LPYI/[BPYl
4.2 2.9
The catalyst samples were pretreated by heating from ambient to 500' over a 3-hr period with continuous evacuation, calcining in pure O2for a t least 4 hr at 500", and evacuating overnight at 500'.
([LPYIi - [LPYIz) = -([BPY]i - [BPYlz) (2)
- AJ.450' A1490' - A1490' A1450'
=
lE1450([LPY]I - [LpYlZ)
(3)
=
lei49oL([LPYII - [LPYM 4lei490B([BPY 11 - [BPY 12)
(4)
The resulting expression for the absorption coefficient ratio is E1490B
C(A14j01
L= E1490
-
A1450') - (A1490' - A14902) (5) C(A14501 - A1450')
I n (1)-(5), A signifies the peak absorbance, I is the optical path length, and the superscripts 1 and 2 refer to the absorbancies before and after addition of HzO. The value of C was determined from the spectrum of PY chemisorbed on potassium-poisoned SA where only the LPY species is observed.' The value of E1490B/ E1490L was estimated to be 6.0 f 0.9. The ratio of Lewis to Brfinsted acid sites may be estimated from the following equation.
-[LPY - 1 [SPY]
6.OCA1450 Ai490 - CAi45o
(2)
Using (2), a value of 6.0 is calculated for the ratio [LPY]/[BPY] on SA or in other words one out of every seven chemisorbed pyridine molecules is a protonated species. Other Silica-Aluminas. In the course of our work we have studied several other commercial silicaaluminas. One would not expect the Lewis-Brdnsted acid site distribution to be constant for such materials, especially in view of the variations in the composition and methods of preparation which usually exist between commercial catalysts. Our measurements are The Jourltal of Physical Chemistry
(2) E. P. Parry, J . Catalysis, 2, 371 (1963).
Nuclear Magnetic Resonance Studies of Inorganic Fluorides.
11. Solvent Effects on
J( Si-' gF) in Silicon Tetrafluoride' by T. D. Coyle, R. B. Johannesen, F. E. Brinckman, and T. C. Farrar National Bureau of Standards, Washington, D. C. (Received December 7, 1966)
Several recent have demonstrated the occurrence of solvent and temperature effects on nuclear spin coupling constants in a number of systems. The various mechanisms which may produce detectable changes in coupling constants by altering the average electronic structure of the molecules containing the coupled nuclei have been summarized by Gutowsky and c o - ~ o r k e r s . ~I n this note we report our observa(1) Presented in part at the 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. (2) Part I: R. B. Johannesen, T. C. Farrar, F. E. Brinckman, and T. D. Coyle. J . Chem. Phys., 44, 962 (1966). (3) H. S. Gutowsky, J . Jonas, F. Chen, and R. Meinzer, ibid., 42, 2625 (1965), and references cited therein. (4) K. Kuhlmann and D. M. Grant, J . Phys. Chem., 68, 3208 (1964). (5) S. L. Smith and R. H. Cox, J . Mol. Spectry., 16, 216 (1965). (6) P. Bates, S. Cawley, and 9. S. Danyluk, J . Chem. Phys., 40, 2415, (1964).
