J. Phys. Chem. 1995,99, 12379-12381
12379
Dissociative Attachment as a Probe of the Distance Dependence of Intramolecular Electron Transfer D. M. Pearl and P. D. Burrow* Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588
J. J. Nash and H. Morrison Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
D. Nachtigallova and K. D. Jordan*" Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: January 24, 1995; In Final Form: June 9, I995@
The dissociative attachment (DA) cross section for C1- production in 3, a molecule with ethylenic and C-C1 groups separated by about 5.8 A, is measured and found to be 6.5 times smaller than that for 2, in which the two functional groups are separated by only 3.4 A. As in 2, the energy of the peak in the DA cross section coincides with that for electron attachment into the 3t* orbital. These results show that the DA process can be used to probe the distance dependence of the electronic coupling between remote functional groups of gas-phase molecules. For 3, unlike 2, the DA spectrum displays a high-energy shoulder in addition to the main peak. We interpret this in terms of two competing DA channels: one arising from direct electron capture into the C-C1 o* orbital and the other involving a n*,o*coupling mechanism. In the latter process, the intramolecular electron transfer takes place in a time comparable to that required for the C-Cl bond length to stretch from its equilibrium value to that at the crossing point of the anionic surfaces.
- -
In a recent study' we showed that the dissociative attachment (DA) process (Rx eRXR X-) can be used to probe intramolecular electron transfer. Specifically, we found that the cross section for producing C1- under low-energy electron impact is about 70 times larger in 2 than in 1. The
+
1
+
3
2
enhancement of the C1- production in 2 (compared to 1) was interpreted in terms of a diabatic picture in which the electron is initially captured into a n* orbital localized on the ethylenic group, followed by an electron jump to the a* orbital localized on the C-Cl group and subsequent dissociation. In contrast, in 1, DA necessarily follows direct electron capture into the C-Cl a* orbital. In the gas phase, the anion states of 1 and 2 are subject to autodetachment, and the longer lifetime of the n* anion relative to that of the a* anion in the vertical attachment region is a crucial factor contributing to the enhanced C1- production in 2.' (The molecules considered here lack symmetry, and the n* and a*labels are used to designate anion states with the odd electron mainly localized on the ethylenic and C-Cl groups, respectively.) Here we show that the DA process can also be used to probe the distance dependence of the electronic coupling in appropriate bichromophoric molecules. This is accomplished by measuring the DA cross section of 3 and comparing it to that of 2. To the extent that the electron capture cross sections, Franck-Condon factors, and the anion lifetimes are similar in 2 and 3, then the major difference in the DA cross sections should be proportional
' On sabbatical leave at the University
of Utah.
* To whom correspondence should be addressed.
@
Abstract published in Advance ACS Ahstrucrs, August 1, 1995.
0022-3654/95/2099- 12379$09.00/0
to (1 - exp(-uHel2)), where Hel is the electronic coupling between the diabatic m* and a d * states, in which coupling between the localized n* and a* orbitals is suppressed, and a depends on the nuclear velocity and slopes of the diabatic potentials.* In the weak coupling limit, the DA cross section should be proportional to He]*. The energies and estimates of the lifetimes of the anion states are obtained from electron transmission spectra (ETS).3 The experimental techniques for measuring the electron transmission spectra and the DA cross sections have been described e l ~ e w h e r e . ~To . ~ aid in interpreting the results, we have also carried out molecular orbital calculations to estimate the magnitude of the a d * , n d * coupling in 3. The electron transmission spectrum of 3 shows peaks due to anion formation at 1.26 and 2.2 eV, whereas that of 2 has peaks at 1.10 and 2.78 eVS5 In both 2 and 3 the lower-lying anion state is due to vertical electron capture into the predominantly n* orbital and the higher-lying state to vertical electron capture into the predominantly C-Cl a* orbital. The lower energy of the n* anion state of 2 relative to that of 3 is consistent with stronger intramolecular electronic coupling in the former molecule (although part of the energy difference is due to the difference in the inductive interaction in the two molecules). The DA cross section for 3, presented in Figure 1, has a maximum value of 7.45 x cm2 at an energy of 1.35 eV, about 0.1 eV above the energy of the n* anion state in the electron transmission spectrum. This cross section is about 6.5 times smaller than that in 2, but is still an order of magnitude larger than that in 1, in which DA proceeds directly through occupation of the a* orbital. The DA spectrum of 3 displays a shoulder near 1.8 eV and has an overall full width at halfmaximum (fwhm) of 1.1 eV, which is about 0.4 eV greater than the width of the n* anion state observed in the electron transmission spectrum. In contrast, the DA spectrum of 2 does 0 1995 American Chemical Society
Letters
12380 J. Phys. Chem., Vol. 99, No. 33, 1995 7
9
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Figure 1. Dissociative attachment cross section of 3 as a function of
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not have a discernible high-energy shoulder, and the width of the peak in the DA cross section is essentially identical to that of the n* anion state in the electron transmission spectrum. These observations suggest that more than one process is contributing to the C1- production in 3. We attribute the main peak in the DA cross section of 3 to the n*,u* coupling mechanism described above and the small shoulder at higher energy to C1- formation from the "direct" process, in which electron capture directly forms the u* anion state.6 As further evidence in support of this interpretation, we note that the shoulder in the DA spectrum of 3 occurs at an energy close to that of the peak of the DA cross section in the saturated molecule 1. In the latter case, the resulting C1- must be due to direct electron capture into the C-Cl CJ*orbital. With the assignments made above, the DA peaks resulting from electron capture into the C-C1 o* orbital of 1 and 3 lie about 0.65 eV below the corresponding peaks in the electron transmission spectra. The reason for this shift is well understood in terms of the large electron detachment rate of the C-C1 u* anion states in the vertical attachment region. We have used a simple Koopmans' theorem (KT)7procedure to calculate, as a function of the C-Cl distance, the energies of the CJ*and n* anion states in both the diabatic and adiabatic representations.8 In this procedure, Hartree-Fock (HF)calculations are carried out on the neutral molecule, and the energies of the adiabatic anion states are estimated by adding (for each geometry) the energies of the two lowest unfilled orbitals to the HF energy of the ground state of the neutral m o l e ~ u l e . ~ The diabatic curves were obtained by transforming the Fock matrices to a localized orbital basis set and deleting matrix elements involving either the localized n* or u* orbitals. Diagonalization of these modified Fock matrices gives the energies of the "dressed" n* and C-Cl u* orbitals which are admixed with the localized orbitals associated with the bridge, but not with one another. The energies of the diabatic anion states are obtained by adding the energies of these so-called "dressed" orbitals to the HF energy of the neutral molecule. Figure 2 shows the diabatic and adiabatic potential energy curves of the lowest two anion states of 2 and 3 calculated using the STO-3G basis The curves have been shifted downward by the amount required to bring the calculated n* vertical attachment energy into agreement with experiment. In both 2 and 3 the diabatic curves cross at a C-Cl distance about 0.1 A greater than that in the equilibrium geometry of the neutral molecule. The electronic couplings determined from one-half the splittings between the adiabatic curves at the crossing point of the diabatic curves (at which point the splittings are at their minimum values) are 0.68 eV for 2 and 0.20 eV for 3. The s4
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Figure 2. Potential energy curves of the n* and u* anion states of 2 and 3 in both the adiabatic (solid curves) and diabatic (dashed curves) representations as a function of the C-Cl bond length. The equilibrium bond lengths of the neutral molecules are denoted by arrows. At C-Cl distances shorter than the crossing point, the predominantly n* anion lies lower in energy; at larger distances, the predominantly o* anion is lower in energy.
square of the ratio of the calculated electronic couplings, [Hel(2)/He1(3)I2, is 11.6, which is only in fair agreement with the value of 6.5 found for the ratio of the observed DA cross sections of 2 to 3. There are several factors that could contribute to the discrepancy between theory and experiment, including the breakdown of the assumption of weak coupling, particularly, in 2. The error due to the use of the KT approximation is expected to be small as it has been demonstrated previously that relaxation and correlation contributions are not very important for describing the electronic coupling through norbornyl b ~ i d g e s . ' ~Both - ' ~ through-space (TS) and through-bond (TB)I4 mechanisms contribute to the coupling in 2 and 3, with natural bond orbitalI5calculations indicating that, at the crossing points of the diabatic curves, the former dominates in 2 and the latter in 3.16 In this paper we have demonstrated that the DA process can be used to probe the distance dependence of the electronic coupling in chromophore-bridge-chromophore systems. In addition, we have found that in 3 DA apparently occurs via two distinct mechanisms, with the dominant channel involving u*,n*coupling and the secondary channel being due to electron capture into the u* orbital, followed by direct dissociation. From the widths of the resonance peaks and the absence of anionic vibrational structure in the ET spectra, the lifetimes of the n* anion states of 2 and 3 are estimated to be at least as long as 5 x s but unlikely to be more than a factor of 2 longer. This mean lifetime is shorter than the time required for the C-C1 bond to stretch from its equilibrium value to its value at the crossing between the anionic diabatic potential energy surfaces (estimated to be on the order of 2 x s). Nevertheless, the DA process takes place through the small fraction of anions that survive to reach this crossing point and, subsequently, the crossing point between the neutral and anionic surfaces.
Acknowledgment. This research was supported with Grants CHE-8922601 (D.M.P., P.D.B., D.N., and K.D.J.) and CHE93 11828 (J.J.N. and H.M.) from the National Science Foundation. K.D.J. thanks Prof. Jack Simons for valuable discussions and for his hospitality and support during a sabbatical stay at the University of Utah, where this manuscript was prepared.
J. Phys. Chem., Vol. 99, No. 33, 1995 12381
Letters References and Notes (1) Pearl, D. M.; Burrow, P. D.; Nash, J. J.; Momson, H.; Jordan, K. D. J. Am. Chem. SOC.1994, 115, 9876. (2) Newton, M. D. Chem. Rev. 1991, 91, 761 and references therein. (3) Sanche, L.; Schulz, G. J. Phys. Rev. A 1972, 5, 1672. (4) Pearl, D. M.; Burrow, P. D. J . Chem. Phys. 1994, 101, 2940. ( 5 ) Momson, H. A,; Singh, T. V.; de Cardenas, L.; Severance, D.; Jordan, K. D.; Schaefer, W. J . Am. Chem. SOC. 1987, 108, 3862. ( 6 ) “Direct” here is used in a diabatic sense. In an adiabatic picture, the production of C1- from the upper state requires surface jumping. (7) Koopmans, T. Physica 1934, 1, 104. (8) This approach is an extension of that used previously to study electronic coupling in symmetric bichromophoric systems. See: PaddonRow, M. N.; Wong, S. S.;Jordan, K. D. J. Am. Chem. SOC. 1990, 112, 1710. Jordan, K. D.; Paddon-Row, M. N. Chem. Rev. 1992, 92, 395. Naleway, C. A.; Curtiss, L. A.; Miller, J. R. J. Phys. Chem. 1991, 95, 8434. Liang, C.; Newton, M. J. Phys. Chem. 1992, 96, 2855. (9) The calculations were performed with the Gaussian 92 program: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, A.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart,
J. J. P.; Pople, J. A. Gaussian, Inc.: Pittsburgh,’PA, 1992. (10) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J . Chem. Phys. 1969,51, 2657. (1 1) Calculations performed with the split-valence 3-21G basis set [Binkley, J. S.; Hehre, W. J.; Pople, J. A. J . Am. Chem. SOC. 1980, 102, 9391 give approximately the same o*h* couplings as those carried out using the STO-3G basis set, thereby providing evidence that the STO-3G basis set is indeed suitable for calculating the couplings in 2 and 3. (12) Kim, K.; Jordan, K. D.; Paddon-Row, M. N. J . Phys. Chem. 1994, 98, 11053. (13) Jordan, K. D.; Paddon-Row, M. N. J. Phys. Chem. 1992,96, 1188. (14) Hoffmann, R.; Imamura, A.; Hehre, W. J. J. Am. Chem. SOC.1968, 90, 1499. (15) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (16) The procedure used for determining the TS and TB contributions to the net electronic coupling is described in ref 8 and has been applied previously to the chloronorbomenes in ref 17. (17) Nash, J. J.; Carlson, D. V.; Jordan, K. D.; Kasper, A. E.; Love, D. E.; Morrison, H. J . Am. Chem. SOC. 1993, 115, 8969. JP950245B
