J. Phys. Chem. 1987, 91, 6445-6447 functions affords a 2%reduction in Ah(19F). Table I1 contains the R O H F / D Z predictions and an estimate of the effect of electron correlation from a single-excitation CI calculation. While the SECI-ROHF/DZ value for the anisotropic coupling is in close accord with experiment, the isotropic value is again about 25% too large. I t is suprising that the inclusion of double excitations from the R O H F reference in addition produces the much larger value of 520 G reported by Nguyen and Ha.' One possible reason for this behavior is that, while double excitations may be important on energetic grounds, spin-polarization effects on the major doubly excited configurations will only be recovered by considering triple excitations from the R O H F wave function. However, the magnitude of the dominant (presumably ROHF) C I state in the calculation of Nguyen and H a argues against this explanation. It is also possible that the truncation of the CI list on energetic grounds inadvertently omits configurations important for the hyperfine properties. To test this, further spin-restricted calculations have been performed including all double excitations from the R O H F / D Z reference mentioned above and from an ROHF/DZP wave function constructed by using a five-component d-function. Results from these calculations are also included in Table 11. The isotropic coupling constant now fortuitously agrees with experiment.
6445
Matrix Model One extremely simplified model to test the effect of the neon environment has been tried. A pair of neon atoms have been located along the extension of the F-F bond axis, equidistant from the center of mass of the fiied F2frame. Changes in the hyperfine parameters as the neon-neon distance is diminished have been calculated at the UHF/DZ level. The F-F internuclear distance was held fixed at the UHF/DZ equilibrium geometry, 1.9467 A, and the neon atoms were also described by a double-l: basis. The isotropic coupling at fluorine decreases as the matrix is compressed, varying with deformation energy in an approximately linear fashion, with slope -0.7 G/(kcal/mol), over a range of neon(proximal) fluorine distances from 1.5-2.5 A. On the other hand, the dipolar interaction is slightly increased as the neon-neon distance is contracted, with slope +0.2 G/(kcal/mol). Matrix effects on this order are much more in line with intuitive picture of neon as an inert medium in which to study gaslike molecular parameters.
Acknowledgment. The research described herein has been supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. D. M. Chipman is thanked for useful advice and assistance with the spin-restricted programs.
Geometrical Structure of C3+ A. Faibis? E. P. Kanter,* L. M. Tack, E. Bakke, and B. J. Zabransky Argonne National Laboratory, Argonne, Illinois 60439 (Received: September 1 1 , 1987)
Results of Coulomb explosion experiments are presented which provide the first experimental evidence for the geometrical structure of the small carbon cluster ion C3+. These results indicate that the ground state of this molecule forms a bent structure. For comparison, measurements were also made of the carbon geometries within the molecules C3H3+and C3H4+. The former is found to form a predominantly cyclic structure while the latter exhibits a linear geometry in agreement with suggestions of previous experimental and theoretical results.
In recent years, there has been a considerable interest in the study of gas-phase atomic clusters. Among the motivations for this work is the fact that such clusters provide a bridge between molecular and condensed matter physics and thus provide an opportunity to study the structural and electronic rearrangements involved in this transition. While there now exists a substantial body of work on the subject,' all of the experimental structural information to date is indirect. Typically, such experiments involve studies of changes in stability with increasing size through measurements of relative abundances, ionization potentials, or fragmentation energies. Recently, more sophisticated spectroscopic techniques have provided some information on electronic states and further offered hopes of probing the nuclear vibrational motions within such systems.2 Nevertheless, some of the most important findings (such as shell structure) still remain controversial because of the incomplete nature of the body of data. Although C3+is considered to be the fundamental building block of the larger carbon c l ~ s t e r s the , ~ geometry of this molecule is unknown. Several authors have reported results of a b initio calculations in which the optimized geometry of the ion is deduced, assuming a linear configuration," in agreement with the structure of the neutral C3 molecule.5 In this Letter, we report the results of a series of measurements exploiting the Coulomb explosion method (CEM) to study the geometric structure of the C3+ ion. Our results indicate a cyclic structure for this ion. Further measurements of the carbon geometries within the molecules Present address: Weizmann Institute of Science, Rehovot, Israel.
0022-3654/87/2091-6445.$01SO/O
C3H3+and C3H4+strongly suggest that the former is also cyclic while the structure of the latter is clearly not equilateral. The CEM is a technique which provides direct information on the nuclear densities within a molecule by imaging individual molecules. The technique, which has been described in detail elsewhere: involves the foil-induced dissociation of a fast (MeV) beam of molecules. Through their mutual Coulomb repulsion, the resulting highly charged atomic ions convert their Coulomb energy (which can be several hundred electronvolts) into kinetic energy of relative motion. For a molecule containing N atoms, measurements of the 3N velocity components after the explosion provides information on the 3N spatial components within the original molecule.'^* The molecular ions were formed by bombardment of allene (C3H4)gas, at a pressure of -50 mTorr, by -9-35-eV electrons. (1) See e.&;PDMS and Clusters; Hilf, E. R., Kammer, F., Wien, K., Eds.; Springer-VeFlag: Berlin, 1987. (2) Chesnovsky, 0.; Yang, S.; Pettiette, C. L.; Craycraft, M. J.; Liu, Y.; Smalley, R. E. Chem. Physl Lett. 1987, 138, 119. (3) Brown, W. L.; Freeman, R. R.; Raghavachari, K.; Schluter, M. Science 1981, 235, 860. (4) Kuhnel, W.; Gey, E.; Spangenberg, H.-J. Z . Phys. Chem. (Leipzig) 1982, 263, 641. (51 Plesser. I.; Vaaer, Z.: Na'aman, R. Phvs. Rev. Lett. 1986, 56. 1559. (6) Gemmell, D. 3. Chem. Rev. 1980, 80, 301 and references therein. (7) Kanter, E. P.; Vager, Z.; Both, G.; Zajfman, D. J. Chem. Phys. 1986, 85, 7481. ( 8 ) Vager, Z.; Kanter, E. P.; Both, G.; Cooney, P. J.; Faibis, A,; Koenig, W.; Zabransky, B. J.; Zajfman, D. Phys. Reu. Lett. 1986, 57, 2793.
0 1987 American Chemical Society
6446
The Journal of Physical Chemistry, Vol. 91, No. 26, 1987
For each molecular species that we studied, the electron bombarding energy was adjusted to lie just above (-0.5 eV) the measured threshold for production of the ion of interest. The ions were then accelerated by the Argonne 4.5-MV Dynamitron accelerator to an energy of 4.5 MeV, magnetically mass analyzed, and then, after an - 8 - ~flight time, dissociated in a thin (-30-A) Formvar film. The resulting carbon fragment ions were charge state analyzed by deflection in a uniform electric field. After a flight path of -6 m, the ions were detected, in triple coincidence, by the MUPPATS detector ~ y s t e m . ~Depending on the original spatial orientation of the molecule before the explosion, the ions can be separated by up to several centimeters on the detector surface and several tens of nanoseconds temporally. For each molecule, this system records the x and y coordinates where each fragment ion impinges on the surface of the detector (with 160-pm position resolution) as well as the relative time of arrival of each ion (600-ps time resolution). From this information, we deduce the three components of the velocity (relative to the molecular center of mass) of each fragment ion and therefore record nine parameters per molecule. Because of their differing deflections in the electrostatic field after the target, the various charge states of carbon ions are dispersed across the detector surface. After correction for this deflection, we deduce the three body-fixed coordinates v,, ub, and u,, representing the lengths of the three relative velocity vectors for the carbon ions after dissociation. In this Letter, we discuss only the symmetry features observed in the coordinate system defined by these velocities, herein described as "V-space". To further simplify the analysis, we restrict our attention here to molecules in which all three carbon dissociation fragments emerge as C2+ ions, and thus, aside from the differing final velocities acquired due to structural features, these fragments are all equivalent. For the protonated ions (C3H3+and C3H4+) we neglect the effect of the protons on the V-space geometries. For molecules such as these, with symmetric proton geometries, the net momentum carried by the protons is small, as evidenced by our experimental data, and can be neglected for qualitative comparisons of the carbon geometries. A more detailed analysis of the spatial geometry of C3+is currently in progress and will be reported elsewhere.1° To compare the carbon geometries of these molecules in Vspace, we first construct the ordered sides u, Iub Iu,. Consider the quantity
This quantity is limited to the range 1 IR, I2, corresponding to the extremes of linear (R, = 1) and equilateral ( R , = 2) geometries. Figure l shows the distribution in this quantity derived from the V-space data for C3+. While it is not possible to deduce the spatial geometries of these molecules directly from the V-space data without further complex analysis, comparisons of such distributions for different structures provide a very rapid and simple means of recognizing similarities in the stereochemistries of such molecules. There are several molecules in the series C3H,: ( n = 1-4) for which there exist previous experimental and theoretical information concerning the probable geometries. The structure of C3H4+ produced via electron impact ionization on allene has been studied by laser photodissociation" and photoion-photoelectron coincidence investigation^.^^-^^ These results suggest that although the (9) Faibis, A,; Koenig, W.; Kanter, E. P.; Vager, Z. Nucl. Instrum. Methods Phys. Res., Sect. B 1986, 813, 673. ( ! O ) Faibis, A,; Kanter, E. P.; Natanson, G.; Tack, L.; Vager, 2..to be
published. (11) Van Velzen, P. N. T.; Van der Hart, W. J. Org. Mass Spectrom. 1981, 16, 237. (12) Rosenstock, H. M.; McCulloh, K. E. Int. J. Mass Spectrom. Ion Phvs. 1977. 25. 321. i 1 3 ) Stkkbauer, R.; Rosenstock, H. M. Int. J. Mass Spectrom. Ion Phys. 19711 - - . -, -27 , 185 ... (14) Parr, A. C.; Jason, A. J.; Stockbauer, R.; McCulloh, K. E. Int. J . Mass Spectrom. Ion Phys. 1979, 30, 319.
Letters
I .o
1.2
1.4 R,
-
I .6
1.8
2.0
Figure 1. Measured distributions for the ratio R,, as described in text, for the carbon fragment ions (all C2+)resulting from the dissociation of 4.5-MeV C3+,C3H3+,and C,H4+. The ordinate represents the actual number of molecules for the case of C3+. The data for C3H4+are normalized to yield the same integral as the C3+data while the distribution for C3H3+has been arbitrarily scaled. Error bars represent the effect of sampling statistics only. The lines are drawn to guide the eye.
protons can rearrange to form different structural isomers, the carbon chain remains linear. The experimental evidence for the linear structure is further supported by ab initio calculations by Frenking and Schwarz.Is Similarly, there have also been studies of C3H3+produced by electron impact on allene (and also propyne). ICR experiment^'^^'^ have proposed that both cyclic and linear isomers are produced. The branching ratio for producing the two isomers is about 2:l (cyc1ic:linear) for impact on propyne and assumed to be similar for the case of allene. The ab initio calculations of Radom et a1.'* predict the cyclic species to be the more stable by at least 60 kcal/mol. For comparison, Figure 1 also shows the R, distribution obtained for C3H4+by considering only the carbon geometry. Here we find the R, distribution shifted toward unity, implying a more linear geometry. This observation is consistent with the results described above for this ion and the known structure of the neutral allene species. Additionally, we have studied the same distribution for C3H3+. As can be seen from Figure 1, this distribution is more ringlike, again consistent with previous results, and quite similar to the distribution we obtain for C3+. The C3+distribution, which is peaked near R, = 2,implies a geometry in which the two smallest sides are equal to the largest side, characteristic of a ring structure. We must stress that quantitative conclusions about the spatial geometries cannot be drawn from these simple distributions in V-space, and in particular the shifts of these distributions away from R, = 2 do not necessarily imply nonequilateratilty in the original structures. Nevertheless, these comparisons do provide strong evidence that the geometry of the C3+ion is more like the carbon geometry within the cyclic C3H3+than that within the linear C3H4+molecule. While it is conceivable that these results could be due to the observation of highly excited vibrational motions resulting in distorted geometries, the clear lack of any linear geometries (R,= 1) in the C3+data ensemble makes this a very remote possibility. Furthermore, the good agreement we find between our electron ionization thresholds and those reported by the other author^'^ strongly suggests that we are indeed observing the ground-state structures of these molecules. Additionally, experiments with C3+ions prepared in a duoplasmatron source fed with CH, yielded identical results. (15) Frenking, G.; Schwarz, H. Int. J . Mass Spectrom. Ion Phys. 1983, 52, 131.
(16) Ausloos, P. J.; Lias, S. G. J . Am. Chem. Soc. 1981, 103, 6505. (1 7) Smith, D.; Adams, N. G . Int. J . Mass Spectrom. Ion Processes 1987, 76, 307 and references therein. (18) Radom, L.; Hariharan, P. C.; Pople, J. A,; Schleyer, P. v . R. J . Am. Chem. SOC.1976, 98, 10.
6441
J. Phys. Chem. 1987, 91, 6447-6449 More extensive exploration of the potential surface seems to indicate that the C3+ molecule is indeed bent,20in agreement with these observations. Further analysis of our data is under way (19) RosenstWk, H. M.; Draxl, K.;Steiner, B, W.; Herren, J. T.J . phys,
Chem. Ref, Data 1977, 6, Suppl. No. 1. (20) Raghavachari, K., private communication.
which should provide more detailed information on these structures.
Acknowledgment. We thank Dr. W. L. Brown for suggesting the importance of this problem and Dr. K. Raghavachari for his preliminary with us' This work was by the U.S.Department of Energy, Office of Basic Energy Sciences, under Contract W-3 1-109-ENG-38.
Photodlssociatlon of Mass-Selected Solvated Metal Ions:
Sr+(H,O)
M. H.Shen, J. W. Winniczek? and J. M. Farrar* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: September 16, 1987) The photodissociation spectrum of Sr+(H,O) in the visible region between 412 and 500 nm is presented. Hydrated Sr+species are produced by injection of Sr+ from a thermionic emitter into the flow from a pulsed supersonic expansion of H 2 0 seeded in an inert carrier gas. This method produces a broad distribution of Sr+(H,O), clusters. Following mass selection, the resulting beam of Sr+(H20)is merged with a counterpropagating pulsed laser beam. Sr+ daughter ions are detected with a second mass spectrometer as a function of photon wavelength. The spectrum in this region shows three bands which we tentatively assign as arising from predissociation of Sr+(H20)through excited electronic states which correlate asymptotically with a molecule of H 2 0 and Sr+ in the 2Pl/2and 2P3/2states.
Introduction The study of clusters has become a very active area in chemistry.' Much of the interest in studying loosely bound aggregates of atoms, molecules, and ions arises from the fact that such systems exhibit properties intermediate between the gas and condensed phases. Such properties have been exploited to investigate chemical reactions occurring within and possibly on the surfaces of clusters,24 as well as to understafid the transition from isolated atoms and molecules to bulk materials. Interesting topics contained within this latter subject include the nature of critical phenomena and studies of chemical reactions between "bare" reagents in the gas phase contrasted with the same reactions occurring in condensed phases. The fundamental phenomenon of solvation is also an important topic in cluster science, and numerous thermodynamic and structural studies of solvated species, particularly ions, have appeared in the l i t e r a t ~ r e . ~ - ~ Ionic clusters afford a particularly interesting collection of systems for study Io because they bridge the gap between bare, isolated ions and ionic solids and electrolyte solutions; in addition, the interaction between a charge and the permanent and induced moments of surrounding molecules leads to interaction energies which may be as large as 30 kcal mol-', a number which approaches covalent bond energies. Because space charge effects reduce number densities of ionic clusters to values many orders of magnitude smaller than for uncharged clusters, conventional spectroscopic probes of ionic clusters have not yet been applied widely to structural and dynamical questions. Despite this apparent disadvantage in studying ionic clusters, their charge allows them to be mass-selected, thus providing unambiguous correlations of physical and chemical properties with their masses. The availability of tunable lasers and the ability to perform mass analysis on charged particles have made photodissociation spectroscopy a powerful probe of ionic cluster structure and dynami c ~ . ' * - ' ~By placing a well-defined amount of energy in a cluster such that it fragments, the measurement of photodissociation cross sections as a function of photon energy, branching ratios for competitive channels, and energy and angular distributions for photofragments a t fixed energy input affords a detailed look at the dynamics of photofragmentation. In this Letter, we present Present address: Brookhaven National Laboratory, Chemistry Department, Upton, NY 11973.
0022-3654/87/2091-6447$01.50/0
the first results of new techniques for producing and probing mass-selected solvated metal ions, with emphasis on the photodissociation cross section for Sr+(H20) in the visible region of the spectrum.
Experimental Section The method we have chosen for the production of gas-phase solvated metal ions is a variation of the supersonic nozzle expansion method which has become a standard technique for the production of cluster^.'^ In our method, we produce a pulsed jet of water vapor seeded in H2 at a backing pressure of 3 atm and a partial pressure of H 2 0 vapor of 0.03 atm. The pulsed value is a commercially available piezoelectrically driven device15 operated with a pulse width of 100 ps; the nozzle size is 0.5 mm. A short distance ( 2-3 mm) downstream from the nozzle, we inject low-energy Sr+ cations from a thermionic emitter16transversely into the flow. Ionic clusters begin to develop at this point in the expansion and are subsequently extracted into the injection optics of a 60° N
(1) Castleman, Jr., A. W.; Keesee, R. G. Chem. Reu. 1986, 86, 589. Jortner, J. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 188. (2) Garvey, J. F.; Bernstein, R. B. J. Phys. Chem. 1986, 90, 3577. (3) Schriver, K. E.; Camarene, A. M.;Hahn, M. Y.;Paguia, A. J.; Whetten, R. L. J. Phys. Chem. 1987, 91, 1786. (4) Gough, T. E.; Mengel, M.; Rowntree, P. A.; Scoles, G. J. Chem. Phys. 1985,83, 4958. (5) Hager, J.; Wallace, S.C. J. Phys. Chem. 1984, 88, 5513. (6) Kebarle, P. Annu. Rev. Phys. Chem. 1977, 28, 445. (7) Tang, I. N.; Lian, M. S.;Castleman, Jr., A. W. J. Chem. Phys. 1976, 65, 4022. (8) Mark, T. D.; Peterson, K. I.; Castleman, Jr., A. W. Nature (London) 1980, 285, 5764. Peterson, K.I.; Mirk, T. D.; Keesee, R. G.; Castleman, Jr., A. W. J. Phys. Chem. 1984, 88, 2819. (9) Kochanski, E.; Constantin, E. J. Chem. Phys. 1987, 87, 1661. (10) Mark, T. D.; Castleman, Jr., A. W. Adu. A t . Mol. Phys. 1985, 20, 65. (11) Kim, H.-S.; Jarrold, M. F.; Bowers, M. T. J. Chem. Phys. 1986,84, 4882. (12) Castleman, Jr., A. W.; Hunton, D. E.; Lindeman, T. G.; Lindsay, D. N. Int. J . Mass Spectrom. Ion Phys. 1983, 47, 199. (13) Johnson, M. A.; Alexander, M. L.; Lineberger, W. C. Chem. Phys. Lett. 1984, 112, 285. (14) Searcy, J. Q.;Fenn, J. B. J. Chem. Phys. 1974, 61, 5282. (1 5) Lasertechnics Model LPV. (16) Blewitt, J. P.; Jones, E. J. Phys. Reu. 1936, 50, 464.
0 1987 American Chemical Society
