J . Phys. Chem. 1985,89, 4867-4868
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Molecular Beam Electric Deflection Study of the Hydrogen-Bonded Clusters (H,O), (CH,OH),,
and (C,H,OH), Bruce D. Kayt*and A. W. Castleman, Jr.** Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: February 1 , 1985; In Final Form: May 28, 1985)
The molecular beam electric deflection technique was employed to study the homomolecular hydrogen-bonded cluster systems (H20)N. ( C H @ H ) , and (C2H50H)w Focusing experiments were made for values of N ranging up to 17 for the case of water and methanol, and up to 13 for clusters of ethanol, thereby extending the range of earlier studies’s2made for cluster sizes of 6, 4, and 3, respectively. In agreement with the results of Dyke, Muenter, and co-workers, the results indicate that the dimers of these species are polar due to the formation of highly directional hydrogen bonds. The trimer and larger clusters are nonpolar, suggesting that these species have cyclic, closed-ring structures.
Introduction Investigation of the formation and properties of van der Waals and similar weakly bound cluster molecules provides a unique way of following the molecular details of the continuous course of change of a system from the gaseous to the condensed state. One useful technique in this regard is molecular beam electric deflection which can be used to determine whether or not a given beam species has a polar structure.l-’ Thus, the technique provides some clue of structure even in the case of large clusters7 where high-resolution electric resonance spectroscopy3 is not feasible. In recent years, considerable attention has been directed to the study of van der Waals molecules. However, most studied have involved weakly bound small clusters comprised of rare gas atoms (Ar, Ne, Xe) bound to simple molecules (HCl, C12, N2, COz) or in a few cases dimers among polyatomic species.8 While data from such systems provide fundamental information about the dispersive interactions responsible for the formation and stability of van der Waals complexes, they do not address large hydrogen-bonded systems of interest in elucidating the formation of the condensed phase. In the present study we examine the polarity of large homomolecular hydrogen-bonded clusters of H20, C H 3 0 H , and C2H50H. The present investigation of the water clusters has been extended to ones comprised of 17 molecules, enabling comparison with the findings of Dyke and Muenter’ for clusters as large as the hexamer. In the case of water, we were especially interested in determining whether the formation of ion pairs predicted by Stillinger and David9 for water clusters at N 2 14 might manifest in a dramatic change in deflection. The results of the electric deflection experiments for the alcohol systems provide comparison with work by Odutola et al? made for clusters up to the tetramer.
moved in and out of the beam axis by means of a “push-pull” type feedthrough located at the field entrance. The purpose of the beam obstacle is to block on-axis beam molecules since the electric field gradient vanish along the field axis. Thus, on axis molecules cannot be focused and only contribute to unnecessary background in the focusing experiments. In the absence of an electric field the beam obstacle blocks more than 95% of the primary beam from reaching the detector. Detection is accomplished via electron impact ionization employing a quadrupole mass spectrometer (Finnigan 750) aligned perpendicular to the beam axis. The procedure for performing an electric deflection experiment is (a) produce a beam of the species of interest; (b) block the beam with the beam obstacle; (c) apply voltage to the quadrupole field; (d) search for an increase in beam intensity at the detector (refocused beam); (e) change the applied voltage and search for refocused beam. The observation of a refocused beam indicates that the species is polar. The absence of refocusing indicates a nonpolar species or a polar species of insufficient polarity to be focused in the apparatus. A lower limit for the minimum dipole moment of being refocused in this apparatus is e ~ t i m a t e d ’ ~toJ ~be -0.05 D for the case of a rigid symmetric top molecule. For all practical purposes, molecules having dipole moments less than this are nonpolar. In situations where a species displays no detectable focusing, it is advantageous to remove the beam obstacle and apply voltage to the rod in the “straight-through” (obstacle removed) configuration. If a species is nonpolar the field will induce a dipole due to the polarizability, and the nonpolar species will defocus in the “straight-through” configuration. Such polarization defocusing will manifest itself as an attenuation of the “straight-through” ~
~~
(1) Dyke, T.; Muenter, J. J. Chem. Phys. 1972, 57, 5011. Experimental Section (2) Odutola, J. A.; Viswanathan; Dyke, T. R. J. Am. Chem. SOC.1979, The details of the apparatus have been discussed previo~sly.6*~J~ 101,4787. (3) Janda, K.; Steed, J.; Novick, S.;Klemperer, W. J . Chem. Phys. 1977, Basically, it consists of five major regions: the cluster source, the 67, 5162; Novick, S.; Janda, K.; Klemperer, W. J . Chem. Phys. 1976, 65, nozzle exhaust chamber, the differential pumping chamber, the 5115; Novick, S.;Davies, P.; Harris, S.;Klemperer, W. J . Chem. Phys. 1973, deflection chamber, and the detection chamber. The deflection 59, 2273; Harris, S.; Novick, S.;Klemperer, W.; Falconer, W. J . Chem. Phys. 1974, 61, 2273; Harris, S.; Novick, S.;Klemperer, W. J . Chem. Phys. 1974, chamber houses the key elements of the electric deflection ap60, 3208. paratus, including the inhomogeneous electrostatic deflection field (4) Viswanathan, R.; Dyke, T. R. J . Chem. Phys. 1982, 77, 1166. and a beam obstacle. The inhomogeneous electric field is an (5) Odutola, J.; Muenter, J. J. Chem. Phys. 1978, 68, 5663. electrostatic quadrupole geometry, 57.15 cm in length; it is com(6) Kay, B. D.; Hofmann-Sievert, R.; Castleman, A. W., Jr. Chem. Phys., submitted for publication. prised of four 0.476-an-diameter stainless-steel rods whose centers (7) Sievert, R.; Cadez, I.; Van Doren, J.; Castleman, A. W., Jr. J. Phys. are evenly spaced on a 0.894-cm-diameter circle. This configuChem. 1984,88,4502. ration corresponds to an internal radius of 0.209 cm. These (8) Peterson, K. I.; Klemperer, W. J. Chem. Phys. 1984, 80, 2439. dimensions correspond to geometries which most accurately (9) Stillinger, F. H.; David, C. W. J . Chem. Phys. 1980, 73, 3389. (10) Sievert, R.; Castleman, A. W., Jr. J . Phys. Chem. 1984, 88, 3329; represent the true hyperbolic fields using circular rods.’IJ2 The Keesee, R. G.; Sievert, R.; Castleman, A. W., Jr. Ber. Bunrenges. Phys. Chem. beam obstacle is a 1.8-mm-diameter ceramic rod which can be 1984, aa, 273. (11) Freidburg, Z . Phys. 1951, 130, 493. (12) Willard, H.; Merritt, L.; Dean, J. “Instrumental Methods of Present address: Sandia National Laboratories, Albuquerque, NM 87185. Analysis”; Van Nostrand: New York, 1974; 5th ed, p 476. *Experimentalphases of this work were undertaken at the Department of (13) Kay, B. D. Ph.D. Thesis, University of Colorado, Boulder, CO, 1982. Chemistry, Chemical Physics Laboratory, CIRES, University of Colorado, (14) Townes, C. H.; Schawlow, A. L. “Microwave Spectroscopy”; Boulder, CO 80309. McGraw-Hill: New York. 1955.
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beam intensity when a voltage is applied to the rods.
Results and Discussion Findings of the previously reported experiments' indicate that the monomer ( H 2 0 ) and dimer ((H20)2)are polar and that the higher polymer [(H20),, N = 3-61 are nonpolar. In the present study, the quadrupole electric deflection of water clusters was extended to cluster sizes containing up to 17 water molecules. As expected, both H 2 0 and ( H 2 0 ) 2were found to focus. The observed strong focusing of (H20)2, detected as the protonated monomer [H+(H,O)], at low voltage is indicative of a first-order Stark effect; this species was also found to focus in the hexapole fieldI3 as expected. The quadrupole focusing of H 2 0 is much weaker, indicative of a second-order Stark effect; it did not display focusing in the hexapole field.I3 The strong focusing of ( H 2 0 ) 2 is consistent with the calculated s t r u c t ~ r e ' ~which J ~ has been verified by molecular beam electric resonance spectroscopy." Structural calculation^^^^^* performed on higher polymers predict cyclic structures for clusters containing five or more waters. There is no agreement as to whether the trimer and tetramer have open chain or closed ring structures. The results of the present quadrupole electric deflection experiments showed that the species (H20), with N = 3 to 17 do not focus; rather they show defocusing ranging from about 5 to 10%when the beam obstacle is removed and the field is on. Such defocusing indicates that these clusters are nonpolar and is strong evidence favoring the predicted cyclic structures. [It is worthy of note that the odd members of the acetic acid clusters were found to display focusing.' The results enabled definite assignments for N ranging to 4 and indicated the presence of higher-order clusters having polar structures.] N o evidence for the predicted9 formation of ion pairs was obtained, perhaps because polarizability effects dominate the focusing behavior. Simple primary alcohols interact through hydrogen bonds similar to those between water molecules. Recent ab initio mo(15) Del Bene, J.; Pople, J. J. Chem. Phys. 1970, 52, 4858; Owicki, J.; Shipman, L.; Scheraga, H. J . Phys. Chem. 1975, 79, 1794. (16) Kistenmacher, H.; Popkie, H.; Clementi, E.; Watts, R.J. Chem. Phys. 1974,60,4455. Kistenmacher, H.; Lie, G.; Popkie, H.; Clementi, E. J. Chem. Phys. 1974, 61, 546. (17) Dyke, T.; Mack, K.; Muenter, J. J. Chem. Phys. 1977, 66, 498. (18) Hankins, D.; Moskowitz, J.; Stillinger, F. J . Chem. Phys. 1970,53, 4544.
lecular orbital calculations on the menthanol dimer19 suggest that the dimer should be bound by a near linear hydrogen bond and, thus, be a polar species. Similar calculations on higher methanol polymersZosuggest cyclic nonpolar structures similar to those for water. 5 ~ 1 8 Both the monomer and dimer of methanol were found to exhibit refocusing characteristic of polar structures. The results of the quadrupole electric deflection experiment on the species ( C H , O H ) , N = 3 to 17, show that these species rather strongly defocus (about 7-14%), consistent with the theoretically predictedI9 nonpolar cyclic structures and the earlier finding that the trimer and tetramer are nonpolar.2 While no ab initio calculations on ethanol clusters presently exist, on heuristic grounds these species are expected to have structures similar to those for methanol. The detected ion clusters had the stoichiometry H+(C2HsOH),; the distribution also displays an intensity drop between H+(C2H50H)4and H+(C2HSOH)s. In accord with the methanol results, the quadrupole electric deflection of ethanol monomer and dimer also showed these species to exhibit refocusing characteristics of polar structures. Again, all clusters (C2HSOH), ( N = 3-13) display strong defocusing (1O-17%), an indication of nonpolar cyclic structures. Study of the electric deflection of water clusters has been extended in the present work to a degree of aggregation of 17 molecules. In agreement with earlier structures' for clusters up to the hexamer, and in accord with p r e d i c t i o n ~ , ~the ~ Jpresent ~ findings are consistent with the clusters having a cyclic structure. The results of the molecular beam electric deflection experiments performed on clusters of methanol, and ethanol, indicate that, in agreement with the situation for the smaller clusters found by Odutola et a1.,2the higher polymers are nonpolar supporting the cyclic closed ring structures predicted the~retically.'~
Acknowledgment. The authors gratefully acknowledge the US. Army Research Office, Grant No. DAAG-29-79-0133, which supported the experimental phases of the work, and Grant No. DAAG29-82-K-0160, which supported the later phases of writing enabling publication. Registry NO. HZO, 7732-18-5; CH3OH, 67-56-1; CZHSOH, 64-17-5. (19) Del Bene, J. J . Chem. Phys. 1971,55,4633. (20) Curtiss, L. J . Chem. Phys. 1977, 67, 1144.
A Comparative Study of Organic Counterion Binding to Micelles with the Fourier Transform NMR Self-Diffusion Technique Mikael Jansson* and Peter Stilbs Institute of Physical Chemistry, Uppsala University, S - 751 21 Uppsala, Sweden (Received: February 25, 1985; In Final Form: June 13, 1985) Multicomponent NMR self-diffusion measurements on D 2 0 solutions of decylammonium acetate, chloroacetate, and dichloroacetate are reported. The association behavior of ionic surfactant systems, as evaluated for a comparison of individual component diffusion data (quantified in terms of the degree of counterion binding, the cmc, and the nonmicellar amphiphile concentration above the cmc), was found to be distinctly sensitive to the counterion character. The cmc decreases in the series CH,COO-, CH2C1COO; and CHC12C00- while the degree of counterion binding increases rather strongly. In competitive ion-binding experiments in mixed surfactant systems the differences in ion binding are dramatically amplified.
Introduction N M R based pulsed-gradient spin-echo (PGSE) self-diffusion measurements have recently emerged as a powerful tool for the investigation of counterion binding in polyelectrolyte systems.'-" (1) Lindman, B.; Puyal, M.-C.; Kamenka, N.; RymdBn, R.; Stilbs, P. J . Phys. Chem. 1984, 88, 5048. (2) Lindman, B.; Kamenka, N.; Puyal, M.-C.; Brun, B.; Jonsson, B. J . Phys. Chem. 1984, 88, 5 3 .
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With the Fourier transform modification of the technique (FTPGSE)one can simultaneously monitor the self-diffusion coefficients of several components and, through a comparative procedure, conveniently quantify aggregation and substrate- and ion-binding processes in solution. Recent investigations in this (3) Stilbs, P.; Lindman, B. J. Phys. Chem. 1982, 85, 2587. (4) Stilbs, P.; Lindman, B. J . Magn. Reson. 1982, 48, 132.
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