The Power of Nanomatrices[JU1]

Transcription

1 SCIENCE S PERSPECTIVE: MOLECULAR SPECTROSCOPY The Power of Nanomatrices[JU] Giacinto Scoles and Kevin K. Lehmann Supersonic free jets and matrix isolation are the two most commonly adopted methods for probing isolated molecules at low temperatures. Several new techniques are currently being developed to overcome some of their limitations (-[JU]). Three recent papers by Nauta and Miller (-0) and a paper by Weber et al. on page xxxx of this issue () illustrate the power of one of these techniques, nanomatrix isolation spectroscopy, for isolating and characterizing clusters that are not accessible by other techniques. The results shed light on important issues such as ion solvation. In helium nanodroplet isolation (HENDI) spectroscopy, a beam of tiny He droplets of only 000 to 00,000 atoms is passed through a pickup cell containing a low pressure vapor of the substance to be examined (). After picking up dopant molecules, the droplets return very rapidly to their "equilibrium" temperature of 0. K (see panel B in the figure) and carry the dopants into the next chamber, where they are interrogated spectroscopically (, ). Two years ago, we argued that such superfluid helium nanodroplets () may be the ultimate spectroscopic matrix isolation medium (). First, the temperature to which a large molecule can be cooled in these nanodroplets () is lower than in a free jet expansion. Second, HENDI spectra suffer less from matrix site-dependent perturbations and uncontrollable complex formation than other forms of matrix isolation spectroscopy. Has the new method kept its promises? In our view, the answer is an unqualified yes. The papers by Nauta and Miller (-0) and by Weber et al. () demonstrate the fast progress in this area. Nauta and Miller work with helium nanodroplets, whereas Weber et al. use much smaller nanomatrices of argon (just a few atoms), but the papers share the idea of interrogating a cold molecule by "matrix spectroscopy in the gas phase" (). Weber et al s discovery that the perturbations induced by argon nanomatrices on negative ions are also weak[ju], brings with it the possibility of mass selection, a clear advantage for the study of complex multicomponent mi x- tures. Previous work has shown selective production of the high spin states of alkali dimers and trimers by HENDI (). Nauta and Miller's work demonstrates the wider applicability of the method for the preparation of metastable molecular complexes in liquid helium nanodroplets, with much higher selectivity than other matrix isolation methods. For polymers of strongly dipolar molecules, HCN () and HCCCN (), only the hydrogen bonded linear chains are observed in HENDI spectroscopy, despite the fact that for the longer chains, cyclic or antiparallel chains are lower in energy. Only the latter are observed in a supersonic jet. This self assembly of a unique, higher energy form is due to permanent dipole-dipole interactions which dominate at long range, steering the molecules towards a local energy minimum in the droplets. The rapid cooling provided by the liquid environment is likely to be critical in preventing the energy released upon hydrogen bond formation (see panel D in the figure ) from annealing the polymer to its global minimum. The size of the linear chain that can be assembled turns out to be limited by the droplet size. More recent work by Nauta and Miller (0) deals with water clusters formed by the same technique. Clusters up to the hexamer, formed by slow supersonic expansion in a slit jet, were previously studied by Saykally and coworkers (). This work, combined with theoretical studies (0), provided rich detail about the interactions of water molecules, but to date only the lowest energy isomer of each cluster could be probed. As in a previous study of the trimer [JU](), Nauta and Miller find that the OH stretching vibrational spectra of the (H O) N N=- complexes observed by HENDI spectroscopy are the same (except for improved resolution due to lower temperature) as the corresponding transitions of complexes produced by jet expansion. In contrast, the hexamer (N=) has a very different spectrum. In the jet expansions, the hexamer forms a cage structure believed to be the global minimum structure. The HENDI spectrum for the hexamer is instead consistent with a cyclic, chiral, hydrogen bonded structure that is the same as the basic hexagonal subunit in bulk ice. This structure has also been predicted to be present as transient structural motif in liquid water. Thus, the ability to observe this particular isomer on the complex energy landscape of the water hexamer is important for building a bridge between the microscopic and the bulk properties of water. Why is the cyclic metastable structure selectively formed in the nanodroplets? Contrary to HCN, even the water dimer deviates from a linear structure, and the formation of the cyclic trimer requires only a small distortion of the hydrogen bonds. The other cyclic clusters form by sequential insertion of water molecules in an environment where energy dissipation is fast and facile[ju]. Out of plane isomerizations are presumed not to occur thermally because of large barriers, and cannot be tunneled into because of the need for large changes in the heavy atom positions. This suggests that formation of metastable isomers should be the rule for HENDI self assembly. We and our coworkers have found that the dimers of (CH)SiCCH formed in He nanodroplets are also linear symmetric tops, despite the fact that a slipped antiparallel structure is likely to be to be the lowest energy isomer. Water is also the main protagonist of the paper of Weber et al. (), who demonstrate that argon nanomatrix isolation spectroscopy can be used to chara cterize the solvent structure in a complete hydration shell around an anion, in this case O -. The presence of the Ar atoms, which are easily evaporated, provides a convenient way to cool the sizable solvation energy released when the anion is hydrated. Without such cooling, the hydrated species are fluctional, likely sampling several isomeric structures, which results in diffuse infrared spectra carrying limited structural information. For complexes with an incomplete hydration shell, the spectra SCIENCE VOL. xxx galley printed March, 000 For Issue Date:????

2 remain diffuse even when the complexes are cooled by Ar evaporation. In contrast, the OH stretching fundamental spectrum for the tetrahydrated O - shows well defined structure, in excellent agreement with that predicted for a highly symmetric hydrogen-bonded structure. As in the Nauta and Miller work, the OH stretch is particularly sensitive to the structural properties of the complex due to its strong dependence on the degree and cooperativity of hydrogen bonding. If one neglects the possible difficulties of assembling superoxide tetrahydrate in liquid helium nanodroplets, the use of Ar is, in principle, less favorable than that of He because of the larger perturbations and the warmer temperatures present in Ar nanomatrices. However, the observed linewidths for the tetrahydrate were sufficiently narrow compared to those of the less hydrated complexes to enable reliable structural assignment. With improved spectral resolution, one can foresee observation of rotational or rotational-tunneling structure, which would allow more detailed comparison of experiment with theory for these important molecular species that are the cornerstones of macroscopic aqueous solvation. Little is known about the size and nature of chemical reaction barriers smaller than ~0. Kcal/mol (). HENDI spectroscopy may be useful here, because the low temperature environment is expected to suppress almost all reactions with an appreciable barrier, leading instead to the formation of van der Waals complexes indicative of the presence of even a small barrier. This may enable chemists to selectively choose the direction of approach in those bimolecular encounter in which the barrier [JU]has a strong angular dependence[ju]. Product branching ratios should also be affected by the control of reactant approach, even when the barrier is not low. HENDI may also be used to study isomers of complex organic molecules. Toennies et al. have shown in a recent paper () that the number of tyrosine and tryptophan isomers detected by HENDI is smaller than that detected in the warmer but still cold environment of a supersonic free jet (). Photon-induced annealing of the isomers of these and other, larger, molecules should teach us a great deal about the intramolecular dynamics of complex molecules. References.W. C. Stwalley and He Wang, J. Mol. Spec., ()..D. R. Willey et al., J. Chem. Phys., ()..J. D. Weinstein et al., Nature,, ()..B. Tabbert, H. Gunther, G. zu Putlitz, J. Low Temp. Phys. 0, (); M. Takami, Comments At. Mol. Phys., ()..S. I. Kanorsky and A. Weis, Adv. Atom. Mol. Opt. Phys., ()..T. Momose et al., J. Chem. Phys. 0, 0 (); S. Tan et al., J. Chem. Phys., ()..J. P. Toennies and A. F. Vilesov, Annu. Rev. Phys. Chem., ().. T. E. Gough et al., J. Chem. Phys., ().. F. Stienkemeier, J. Higgins, W. E. Ernst, G. Scoles, Phys. Rev. Lett., ().. S. Goyal, D. L. Schutt, G. Scoles, Phys. Rev. Lett., ().. a. S. Grebenev, J. P. Toennies, A. F. Vilesov, Science,, 0 (); b. M. Hartmann et al., Phys. Rev. Lett., ().. K. K. Lehmann and G. Scoles, Science,, 0 ().. K. Nauta and R. E. Miller, Science, ().. K. Nauta and R. E. Miller, Farad. Discuss., (). 0. K. Nauta and R. E. Miller, Science, (000).. Weber and Johnson, Science, XXXX (000).. T. E. Gough, D. G. Knight, G. Scoles, Chem. Phys. Lett., ().. J. Higgins et al., J. Phys. Chem. A0, 0 (); J. Higgins et al., Science,, ().. K. Liu, M. G. Brown, R. J. Saykally, J. Phys. Chem. 0, () and references therein. 0. M. G. Brown et al., J. Chem. Phys., ().. R. Fröchtenicht, M. Kaloudis, M. Koch, F. Huisken, J. Chem. Phys. 0, ().. I. R. Sims and I. W. M. Smith, Ann. Rev. Phys. Chem., 0 ().. A. Lindinger, J. P. Toennies, A. F. Vilesov, J. Chem. Phys. 0, ().. L. A. Philips, S. P. Webb, S. J. Martinez, III, G. R. Flemming, D. H. Levy, J. Am. Chem. Soc. 0, ().. We would like to thank their students and co-workers for useful discussions and their contributions. In particular we are indebted to Carlo Callegari who drafted this figure. Assembling clusters in cold nanodroplets. A water molecule approaches a liquid helium nanodroplet that contains another molecule (A). Upon impact and solvation, energy is released by evaporation (B). The two molecules are pulled together inside the droplet by long range forces (C). The energy liberated in forming the bimolecular complex is released again by evaporation (D). The cold complex is now ready for spectroscopic interrogation (E) (). CREDIT CARLO C ALLEGARI, PRINCETON SCIENCE VOL. xxx galley printed March, 000 For Issue Date:????

3 MS Field Code: chemistry Web Field Code: chemistry Highwire Abstract (to be posted online with a link to the full text): To come - not written yet SCIENCE VOL. xxx galley printed March, 000 For Issue Date:????

4 Dear Dr. Scoles, Your text galleys follow. Please look them over carefully for any errors that may have been introduced during editing. Fax back any corrections you may have within hours of receipt of your galley to Elizabeth Lenox at (0)--. Please also check the separate page of editorial queries keyed to notations in the text; my queries are shown as [JU], [JU], etcetera. Answers to these should be included with your proof corrections. The art for your commentary is not included with this galley. Our artist will contact you separately about it. Legibility is likely to be a problem when you fax your corrections written on the galley. Therefore, in addition to the marked galley, please include a typed list of your corrections, and also this list to me at juppenbrink@science-int.co.uk. Thank you for your excellent contribution to the Perspectives section. Sincerely, Julia Uppenbrink Senior Editor, SCIENCE juppenbrink@science-int.co.uk (please contact Elizabeth Lenox, for additional information) SCIENCE VOL. xxx galley printed March, 000 For Issue Date:????

5 Page: [JU] I would prefer a one-line title for length reasons. I do not like this one much - still thinking - maybe you can think of a snappy title? Page: [JU] There are too many references. Please reduce by about ten if at all possible. (I think some of these early ones can go.) Page: [JU] The fact that interactions are weak for uncharged species has not really been introduced yet. Mention above somewhere? Page: [JU] Unclear: do Huisken and collaborators do HENDI? Page: [JU] Okay? Page: [JU] The small barrier? Page: [JU] You could cite a recent paper in Science here, Skouteris et al, November, "The Role of van der Waals Interactions in the Cl+HD Reaction"