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J. Phys. Chem. 1991, 95,4928-4929
in eq 2 extends over the occupied and unoccupied MO of the bridge (excluding the core orbitals) with orbital energies b,. C1, and C,,, are coefficients for the atomic orbitals a t the end atoms of the bridge. c is the orbital energy of the transferring donoracceptor orbital and vl and q2 are matrix elements between this orbital and the end atomic orbitals of the bridge. The summation includes only bridge-dependent quantities and may conveniently be called the transfer capability of the bridge. Equation 2 agrees rather well with experimental results in some cases studied.28 According to eq 2 an intervening aromatic group has theoretically the potential of enhancing the ET rate in a protein since its A orbitals extend over a large volume of space. The coefficients are large at the end atoms and the energy gap between occupied A and empty A* orbitals is smaller than between u and u* MO. lAl will then be large by eq 2 provided the aromatic groups are as parallel as possible and with a large overlap to other parts of the ET pathway. A wave function decreases as ex ( r(201/*), where I is the ionization energy in atomic units. 34,3? -It is reasonable to assume that the ionization energy is about 0.35 au ( N 10 eV). This (34)Handy, N.C.; Marron, M. T.; Silverstone, H. J. Phys. Rev. 1969, 180, 45. ( 3 5 ) Morrell, M.M.; Parr, R. G.; Levy, M. J . Chem. Phys. 1975,62,549.
corresponds to a decrease as e x p ( 4 . 8 9 ) for the overlap. Since A may be assumed to be proportional to the overlap, a decrease as exp(-l.7r) for Az may be expected, or, if r is measured in A, as exp(-Pr), where 0 = 1.7 X 1.89 H 3.2 A-‘. This means a very fast decrease with distance between aromatic or other groups. Thus aromatic groups have to be close to each other or bridge parts of the peptide chain in the same way as hydrogen bonds. Hydrogen bonds between chains are important in promoting ET since they bring the chains together with a larger overlap and at the same time bring in the H 1s orbital as an additional bridge orbital. In spite of the rather negative conclusions regarding the importance of aromatic groups in E T in the present case, we may still conclude that the latter groups in fortunate cases can be localized in such a way that the electronic factor is much increased. In particular, one may suspect that biological E T systems, where the reaction rate for a particular ET step is rate determining for the whole process, will evolve in such a way that the rate is optimized. One way of doing this is to use aromatic side groups as bridges.20 At the time being there are very few, if any, proven cases, however. Acknowledgment. We are grateful to N F R (the Swedish Natural Science Research Council) for support.
Direct Evldence for Mn SubstiMion in a Framework Site in MnAPO-11 Molecular Sleve from the Adsorbed Water Coordination Conflguratlon Deduced by Electron Spin Echo Modulation Analysls Cuillaume Brouet, Xinhua Chen, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: January 25, 1991; I n Final Form: April 8, 1991)
Electron spin echo modulation analysis of 0.1 mol 95 Mn (relative to P) in MnAPO- 1 1 with adsorbed DzO shows two deuterium at 0.24 nm and two at 0.36 nm from Mn. This suggests that two waters hydrate an Mn0, configuration with a D 4 bond orientation for the waters as expected for a negatively charged site. When Mn substitutes for AI in the AlPO,-I 1 framework it forms a negatively charged site, whereas Mn would form a positively charged site in an extraframework position. Thus the spin echo analysis indicates that Mn at low concentration substitutesin a framework position of MnAPO-11 during synthesis.
AIP04-1 1 molecular sieve belongs to the new class of microporous crystalline aluminophosphates that were first synthesized by Wilson et al.lJ Addition of various metal ions into the synthesis mixture produces metal-ion-incorporated aluminophosphate molecular sieves, which are named M~AF”s.’*~MnAPO-l l refers to Mn-incorporated AlP04-11. Because the location of transition-metal species in such molecular sieves can play a vital role in catalytic processes, it is of some significance to determine the location of Mn in MnAPO-I 1. The two chemically different types of sites are extraframework and framework sites of which extraframework sites are the most common in molecular sieves. It has been suggested that Mn is incorporated for A1 in the MnAPO-1 1 framework based on chemical analysis (P/Al > 1) and the observation of Bronsted acidity e n h a n ~ e m e n t . ~However, ,~ (I) Wilson, S.1.;Lok. B. M.; Flanigen, E. M. US. Patent 4 310 440, 1982. (2) Wilson, S.T.;Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J . Am. Chcm. Soc. 1982,104, 1146-1147. (3) Wilson, S.T.;Flanigen, E. M. US.Patent 4 567 029, 1986. (4)Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. In New Developments in Zeolfte Science and Technology; Murakami, Y.,Lijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986;pp 103-1 12. 0022-3654/91/2095-4928S02.50/0
this evidence is rather indirect. We report here direct spectroscopic evidence for Mn incorporation for A1 in the AlP04-l 1 framework based on the orientation of coordinated water molecules from electron spin rmnance (ESR) and electron spin echo modulation (ESEM) measurements. MnAPO-11 was first synthesized with 10 mol 8 Mn, relative to P, as indicated in Example 75 of the patent.) At such a high Mn concentration, the ESR spectrum of Mn(I1) is broad with unresolved hyperfine lines and is assigned to predominately extraframework Mn. This result is comparable to that of MnAPO-5.6 In contrast, hydrated, calcined MnAPO-11 with only 0.1 mol % Mn gives an ESR spectrum with well-resolved hyperfine lines fully consistent with that from divalent Mn.’** However, the ESR spectrum alone does not provide sufficient information for determining the site of Mn in MnAPO-11. (5) Pluth, J. J.; Smith, J. V.; Richardson, W. R., Jr. J . Phys. Chem. 1988, 92,2734-2738. (6)Goldfarb, D.Zeolites 1989,9,509-5 15. (7)Barry, T.1.; Lay, L. A. J. Phys. Chem. Solids 196627, 1821-1831. (8)White, L. K.;Szabo, A.; Carkner, P.; Chasten, N. D. J . Phys. Chem. 1977,81,1420-1424.
0 1991 American Chemical Society
4929
J. Phys. Chem. 1991, 95, 4929-4931 D /Q--
~
D
0.8-
f C
r = 0.27~~
.-t 06c 0 .- 0.4-
-EXPT
--- CALC
Zb I 2
0.2 .
0
I I
2
3
4
5
T, PS
Figure 1. Three-pulse ESEM signal of 0.1 mol % Mn in MnAPO-11 recorded at 4 K. The best simulation with parameters is also shown.
ESEM spectroscopy provides complementary information to ESR regarding the local environment of paramagnetic specie^.^ Simulation of ESEM signals yields the type and number of interacting magnetic nuclei and their interaction distance and isotropic hyperfine c ~ u p l i n g .The ~ MnAPO-11 sample for ESEM measurement was prepared by (1) evacuating calcined MnAPO-11 at 350 O C overnight and (2) exposing the sample to D 2 0 vapor a t the room temperature vapor pressure for 15 h. The ESEM signals were recorded at 4 K on a home-built ESE spectrometer.I0 Three-pulse echoes were recorded with a u/2-77r/2-T+/2 pulse sequence as a function of T. The value of T was selected to be 0.27 p s to minimize 27AIm o d ~ l a t i o n .Two-pulse ~ glitches were eliminated by phase cycling." The ESEM signal recorded for MnAPO-11 with adsorbed DzO is shown in Figure 1. Simulations assuming all interacting deuteriums to be equidistant from Mn did not yield a reasonable fit to the signal. The best fit required a two-shell model with deuteriums located at two different distances. The best fit parameters are two deuteriums at 0.24 nm and two other deuteriums a t 0.36 nm. The shorter interaction distance is too short for a normal geometry of D 2 0 bonded to a transition metal cation in which the metal ion coordinates to the oxygen; for Mn coordinated to the oxygen of D 2 0 along the bisector of the D-0-D angle the Mn-D distance is at least 0.28 nm.l2 However, the short in-
Figure 2. Proposed model of Mn coordination as Mn0, in MnAPO-11 hydrated with D20. 0- indicates a framework oxygen. From the ESEM data the Mn-D distance is 0.24 nm and the distance from Mn to the remote deuteriums is 0.36 nm.
teraction distance to D is similar to that found for water coordinated to small negative ions. For both hydrated electrons and hydrated 0, the water is coordinated with a D-0 bond orientation and the ESEM data is best fit with a two-shell model."*" Furthermore, for a D-0 bond orientation and the gas-phase geometry of water the Mn distance to the remote deuterium of water is about 0.36 nm as found in the ESEM analysis. Mn in an extraframework site is clearly positive, but substitution of Mn(I1) for AI(II1) in the framework effectively generates a negatively charged site. This substitution may be compensated by the equivalent replacement of Al(II1) by P(V) or more probably by extraframework cation^.^ Water coordinated to Mn(1I) in such a framework site is expected to react to this negatively charged environment and adopt a dipole-oriented or bond-oriented configuration accordingly. Figure 2 illustrates a bond-oriented structure where the water deuterium is near the plane of three oxygens of an M n 0 4 configuration. The bond-oriented configuration is possibly a consequence of packing forces in the framework, but it also has precedent for small negative ions.13*" Thus, the deduced coordination configuration for adsorbed water around Mn in MnAPO-11 provides strong spectroscopic evidence for Mn substitution in a framework site. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program. We thank D. Goldfarb for helpful discussions.
(9) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (IO) Narayana, P. A.; Kevan, L. Magn. Reson. Rev. 1983, 7 , 234. ( 1 1) Fault, J. M.;Schweiger, A,; Brauschweiler,L.; Forrer, J.; Ernst, R.
Sass, C. E.; Kevan. L. J. Phys. Chem. 1989, 93, 4669-4674. Narayana, P. A.; Bowman, M. K.; Kevan, L.; Yudanov, V. F.; Tsvetkov, Yu. D. J. Chem. Phys. 197563, 3365-3311. (14) Narayana, P.A.; Suryanarayana, D.; Kevan, L. J. Am. Chem. Soc.
J . Magn. Reson. 1986, 66, 14.
1982, 104,3552-3555.
(12) (13)
Irregular Oscillations In the Condensation Kinetics of CClsF 2.Cheng, I. P. Hamilton,* and H. Teitelbaum* Department of Chemistry, The University of Ottawa, Ottawa, Canada KIN 6N5 (Received: January 28, 1991; In Final Form: April 19, 1991) We observe complicated behavior in the condensation kinetics of CC13F;in particular, the refractive index gradient oscillates as a function of time. In the simplest instance, nucleation occurs via steps in which the newly formed clusters must be stabilized, and the mechanism is present for self-catalysis of dissociation and self-inhibition of association, which could lead to such oscillations. We have been able to represent the essential elements of our observations using a simple phenomenological expression: a differential-delay equation. Recently, we commenced an experimental program designed to measure the initial rate of homogeneous nucleation of certain gases.' Our experimental technique consists of shockampressing ( 1 ) Carruthers, C.; Francoeur, K.;Teitelbaum. H.In Shock Tubes and Waues;Gronig, H . , Ed.; VCH Verlag: Weinheim, 1988; p 327.
a pure gas which is saturated close to its equilibrium vapor pressure. The resulting supersaturated gas then begins to nucleate, forming clusters of increasing size en route to condensation. Shock compression is always accompanied by shock heating, and hence by an increased equilibrium vapor Pressure, So most gases be made to condense in this way. However, for some chloro-
0022-3654/91/2095-4929$02.50/00 1991 American Chemical Society
