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J. Phys. Chem. 1996, 100, 12-14
Evidence for the High-Energy (Collinear) van der Waals Isomer of the Acetonitrile Dimer Caroline E. H. Dessent, Jun Kim, and Mark A. Johnson* Sterling Chemistry Laboratory, Yale UniVersity, 225 Prospect Street, P.O. Box 208107, New HaVen, Connecticut 065208107 ReceiVed: October 30, 1995X
While the stable configuration of the acetonitrile dimer has antiparallel dipole moments, recent calculations [Popelier, et al. Faraday Discuss. 1994, 97, 243] reveal a high-energy isomer in which the dipoles are collinearly aligned. We report direct evidence for the existence of this isomer using a strategy in which we enforce the less-favorable orientation of the neutrals by electrostatically binding them to the iodide ion in the I-‚(CH3CN)2 cluster. The collinear dimer is then released by photoexcitation of the dipole-bound excited state of I-‚(CH3CN)2, producing the (CH3CN)2- anion as the dominant ionic photofragment. The fragment is mass selected and stripped of the excess electron in a modest external electric field (∼10 kV/cm), indicating that the anion consists of a diffuse electron as expected for a “dipole-bound” species. This behavior suggests that the force field of the neutral “core” complex supports a minimum in the highly dipolar, collinear configuration.
Introduction The anisotropy of van der Waals bonds between stable solvent molecules is an important microscopic aspect of liquid behavior,1 and in some cases (e.g., HCN) locally stable isomeric forms of dimers and trimers can be generated in a free-jet expansion and characterized in the gas phase.2 The distribution of the isomers, however, is typically not under direct experimental control. Here we are concerned with the case of acetonitrile, an archetypal polar solvent molecule, where cluster generation in a supersonic expansion produces significant quantities of only one isomer of the van der Waals dimer, while calculations of the potential energy surface indicate the existence of two significant isomers.3 The global minimum (M1) corresponds to the previously observed1 structure with antiparallel dipoles, but a second isomer (M2), with collinear head-to-tail dipoles is postulated to exist in a local minimum with about half the binding energy of the global minimum. The collinear form exists in a very shallow minimum, with a barrier for conversion to M1 of only 25 meV. The relative positions of the two calculated minima are displayed in Figure 1 as a function of the angle between the axes of the two molecules. In this paper, we report a direct synthetic route to the high-energy, collinear isomer using cluster ion photodissociation. The key to this synthesis lies in our previous observation4,5 that photoexcitation of ion-polar molecule complexes, I-‚M, just below their electron detachment thresholds accesses excited states that decay into dipole-bound ground-state anions (M-). The process is an efficient route to such labile anions as approximately 50% of the parent complex may be converted into products with reasonably high cross section (∼10-17 cm2). We present a rational synthetic method for generating highdipole-moment neutral dimers by first associating solvent molecules around a halide ion and then photogenerating the ground-state “dipole-bound” anion of this isomeric form. The technique relies on the recognition that in an ion-molecule complex, such as I-‚(CH3CN)2, the dominant intermolecular force is the strong ion-dipole electrostatic potential, which may be powerful enough to lock the solvent molecules into a significantly different configuration than the one they might adopt as bare molecules in the gas phase. If the solvent molecules are aligned in a configuration with a significant X
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0012$12.00/0
Figure 1. Schematic representation of the potential energy surface of the van der Waals dimer (CH3CN)2 as a function of the angle between the acetonitrile molecules. The dashed line corresponds to the location of the dipole-bound anion, (CH3CN)2-, with θc representing the position at which the anionic (dipole-bound) surface crosses the neutral surface. The energies shown on the diagram are relative to the dissociated monomers and refer to structures on the neutral surface. Numerical values are from the calculations of Popelier et al.3
resultant dipole moment, the complex will possess a dipolebound excited state that should decay with production of a ground state dipole-bound cluster ion, representing a synthetic route to novel dipole-bound anions. Almost by definition, these dipole-bound anions will be built on neutral cores with high dipole moments, and electron detachment from the anion should directly produce the neutral, high dipole isomer! Experiment The dipole-bound excited state of I-‚(CH3CN)2 was accessed as described in a previous paper.5 Briefly, the precursor ionmolecule complex is prepared by expanding a mixture containing the ambient vapor pressures of methyl iodide and acetonitrile in several atmospheres of Ar through a pulsed free jet and then © 1996 American Chemical Society
Letters
Figure 2. Photofragment ion mass spectrum resulting from excitation of I-‚(CH3CN)2 at 3.84 eV (the peak of the dipole-bound excited state).
intersecting the expansion with a 1 keV electron beam. The clusters are subsequently mass selected using standard timeof-flight (TOF) mass spectrometry.6 Photoexcitation of I-‚(CH3CN)2 at 3.84 eV produced ionic photofragments that were dispersed in a second TOF spectrometer. One method of establishing that an excess electron is dipole bound involves field detachment or stripping of a fast anion beam.7-9 In our experiment, an electric field is established in a region consisting of three, equally spaced (0.425 cm) grids, which are perpendicular to the ion trajectory. A positive voltage (0-10 kV) is applied to the central grid with the outer grids grounded. Additionally, a fourth grid was placed after this structure, which could be negatively biased (-|0-5| kV) to repel anions, allowing differentiation between fast neutrals and ions. The experimental error on the magnitude of the field is estimated to be (0.5 kV/cm. We define the critical detachment field (or disappearance field) as the point where the intensity of the parent ion approaches zero (or for the case of (CH3CN)2-, the point at which the fraction of neutrals approaches a constant value with increasing field strength). Each data set was recorded several (three or four) times so that the data presented in Figure 3 are averages over the different data sets: The error bars indicate the standard deviation of the different data sets. Results and Discussion Figure 2 displays the relative intensities of the CH3CN- and (CH3CN)2- ions obtained from photoexcitation of I-‚(CH3CN)2 at 3.84 eV, the peak of the “dipole-bound” excited state of the complex.5 Note that the dimer anion is the dominant product. Figure 3 displays the detachment probability (i.e., [total number of neutrals]/[number of neutrals + number of anions]) as a function of the applied electric field for both photofragments. The behavior of CH3CN- (Figure 3a) was studied for comparison with the previous results of Schermann8 and Dunning,9 who observed a sharp onset for detachment close to 10 kV/cm, with the critical detachment field, Fc, occurring close to 13 kV/cm. We also observe Fc at ∼13 kV/cm, but our CH3CN- displays
J. Phys. Chem., Vol. 100, No. 1, 1996 13
Figure 3. Fraction of neutrals produced by field detaching the dipolebound anions (a) CH3CN- and (b) (CH3CN)2- as a function of the electric field strength. The arrows indicate the locations of the critical detachment fields, Fc.
a smoothly increasing detachment probability starting at much lower field strength (3 kV/cm), suggesting that our photoproduced CH3CN- possesses considerable internal excitation compared to the anion produced by Rydberg electron transfer. We have also studied the field detachment behavior of CH3CNwhen it is produced by 3.53 eV excitation of the I-‚CH3CN binary complex. Its field detachment profile is indistinguishable from that of the ion produced from the ternary complex, I-‚(CH3CN)2, indicating that the nature of the internal excitation of the ion is similar for both complexes. The detachment profile of (CH3CN)2-, presented in Figure 3b, also shows a smoothly increasing detachment probability with increasing field strength, which we again believe to be characteristic of an internally excited ion. However, the detachment probability of the dimer ion increases more slowly with increasing field strength compared to the monomer, and the ion does not appear to be completely detached at even 24 kV/cm, the limit of our instrument. There appears to be a reproducible plateau in the detachment probability at ∼0.8 at 19 kV/cm which we assign as Fc for the dimer. It is currently unclear whether the remaining 20% of (CH3CN)2- ions correspond to stable anions that do not contain dipole-bound electrons (and hence could not be field detached at any reasonable electric field) or if they could be detached at moderatley higher fields. While there is no rigorous theory that relates the critical detachment field of a dipole-bound anion to its binding energy, a model which has been used previously to estimate the binding energies of Rydberg electrons10 can be applied to dipole-bound anions. When the long-range potential experienced by the dipole-bound electron, including the external electric field, F, is given by
V(r) ) -(µ‚r/r3) + Fr
(1)
one can relate the critical binding field, Fc, to the electron binding energy, Eb, by the classical expression
14 J. Phys. Chem., Vol. 100, No. 1, 1996
Eb3 )
27 2 µF 4 c
Letters
(2)
where all quantities are in atomic units. For acetonitrile, using Fc ) 13 kV/cm and µ ) 1.54 au gives the electron binding energy as 11 meV. A crude estimate of the size of the dipolebound electron can be obtained by setting11 Eb ) µ/r2, which yields a value of 31 Å (in this estimate, r is the position of a point charge from a point dipole, µ, that corresponds to an interaction energy of Eb given by eq 2). A similar analysis of the (CH3CN)2- data is complicated by the fact that the dipole moment of the neutral van der Waals core is unknown. However, as (CH3CN)2- can be stripped of its excess electron in modest electric fields, it is clearly a dipolebound anion. The observation that the detachment probability increases more slowly with increasing field strength than observed for CH3CN- suggests that the neutral core of the dimer ion has a larger dipole moment than the neutral monomer. For a dipole-bound anion to exist as a stable species, the neutral core on which it is built must also be stable [note that our method of photogenerating dipole-bound ground-state anions is grounded on the repulsive nature of the neutral solvent (monomer or dimer) with respect to the iodine radical].5,12 In a dipole-bound system, the excess electron is too weakly coupled to the dipolar neutral to effect a reorganization of the core or stabilize an otherwise repulsive core structure. Therefore, the neutral core of (CH3CN)2- must be built on a stable neutral acetonitrile dimer. As discussed above, simulations3 produced only two stable configurations for this system (see Figure 1); obviously, as M1 (antiparallel dipoles) has no net dipole moment, it is unable to bind an excess electron, but M2 (collinear headto-tail) has a large net dipole moment that could easily trap a low-energy electron to produce M2- (represented on Figure 1 by the dashed line). Although the linear isomer exists within a shallow well, it is locally stable, and therefore capable of supporting the dipole-bound electron. In this experiment, the linear (CH3CN)2- dipole-bound ion is produced as a consequence of the arrangement of the solvent molecules in the parent ion-molecule complex, I-‚(CH3CN)2. The photochemistry of the complex5 is dominated by an asymmetric structure with a high vertical dipole moment that can be crudely described as I--CH3CN-CH3CN, where photoexcitation leads to the production of (CH3CN)2-. Thus, the dominant ionic cluster isomer corresponds to the solvent molecules being aligned in a manner that would correspond to an extremely minor van der Waals isomer. Finally, the dipolebound electron may be ejected, by either a photon or the application of an electric field, to produce the elusive CH3CNCH3CN isomer, which should be locally stable even after electron detachment.
It is worth noting that the photodetachment of negative ions has previously been used as a route to “unstable” positions on the corresponding neutral surface,13,14 although we are unaware of any other examples where detachment of an anionic cluster would produce a high-energy van der Waals complex. Schermann15 has recently suggested that Rydberg transfer to van der Waals clusters may be a useful tool for distinguishing between different neutral isomers that are produced in a free-jet expansion. Electron attachment produces a charged species that can be subsequently focused, mass selected, and characterized by field detachment. The limitation of this method is that it will only lead to significant amounts of dipole-bound anions from polar van der Waals clusters that are present in the expansion. In fact, Schermann and co-workers8 have studied Rydberg electron transfer to acetonitrile clusters, and although they were able to produce CH3CN- and (CH3CN)3-, they failed to observe (CH3CN)2-, presumably due to the predominance of the antiparallel dimer isomer in the expansion. The advantage of the synthesis described here involving photolysis of an ionmolecule complex is that it accesses van der Waals complexes that are not produced in significant quantities under normal conditions. Acknowledgment. We thank the NSF for providing financial support for this study. References and Notes (1) Buck, U. J. Phys. Chem. 1994, 98, 5190. (2) Jucks, K. W.; Miller, R. E. J. Chem. Phys. 1988, 88, 2196. (3) Popelier, P. L. A.; Stone, A. J.; Wales, D. J. Faraday Discuss. 1994, 97, 243. (4) Dessent, C. E. H.; Bailey, C. G.; Johnson, M. A. J. Chem. Phys. 1995, 102, 6335. (5) Dessent, C. E. H.; Bailey, C. G.; Johnson, M. A. J. Chem. Phys. 1995, 103, 2006. (6) Johnson, M. A.; Lineberger, W. C. Techniques in Chemistry; Farrar, J. M., Saunders, W. H., Eds.; Wiley: New York, 1987; p 591. (7) Haberland, H.; Ludewigt, C.; Schindler, H.-G.; Worsnop, D. R. Phys. ReV. A 1987, 36, 967. (8) Desfrancois, C.; Abdoul-Carime, H.; Khelifa, N.; Schermann, J. P. Europhys. Lett. 1994, 26, 25. (9) Popple, R. A.; Finch, C. D.; Dunning, F. B. Chem. Phys. Lett. 1995, 234, 172. (10) Latimer, C. J. Contemp. Phys. 1979, 20, 631. (11) Mead, R. D.; Lykke, K. R.; Lineberger, W. C.; Marks, J.; Brauman, J. I. J. Chem. Phys. 1984, 81, 4883. (12) Dessent, C. E. H.; Bailey, C. G.; Johnson, M. A. Proc. Yamada Conference XLIII, Structures and Dynamics of Clusters; Universal Academy Press Inc.: Tokyo, 1995. (13) Neumark, D. M. Acc. Chem. Res. 1993, 26, 153. (14) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91, 5974. (15) Schermann, J. P. Proc. Yamada Conference XLIII, Structures and Dynamics of Clusters; Universal Academy Press Inc.: Tokyo, 1995.
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