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Far Infrared Vibration-Rotation-Tunneling Spectroscopy and Internal Dynamics of Methane-Water: a Prototypical Hydrophobic System L

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Far Infrared Vibration-Rotation-Tunneling Spectroscopy and Internal Dynamics of Methane-Water: a Prototypical Hydrophobic System L Far infrared vibration-rotation-tunneling spectroscopy and internal dynamics of methane-water: A prototypical hydrophobic system L. Dore,a) R. C. Cohen,b) C. A. Schmuttenmaer,c) K. L. Busarow, M. J. Elrod, J. G. Loeser, and R. J. Saykally Department of Chemistry, University of California, Berkeley, California 94720 (Received 26 March 1993; accepted 1 October 1993) Thirteen vibration-rotation-tunneling (VRT) bands of the CH4-H,O complex have been mea- sured in the range from 18 to 35.5 cm-’ using tunable far infrared laser spectroscopy. The ground state has an average center of mass separation of 3.70 A and a stretching force constant of 1.52 N/m, indicating that this complex is more strongly bound than Ar-H,O. The eigenvalue spectrum has been calculated with a variational procedure using a spherical expansion of a site-site ab i&o intermolecular potential energy surface [J. Chem. Phys. 93, 7808 ( 1991)]. The computed eigenvalues exhibit a similar pattern to the observed spectra but are not in quantita- tive agreement. These observations suggest that both monomers undergo nearly free internal rotation within the complex. I. INTRODUCTION order terms in the IPS, whereas they are usually neglected when the more typical low order terms are present. Water-hydrophobe interactions govern essential fea- This study and the accompanying microwave (MW) tures of many chemical and biochemical processes. Con- work’ constitute the first spectroscopic studies of this clus- siderable controversy exists in the literature concerning the ter; in fact it is only within the last 3 years that any high correct molecular description of the macroscopic phenom- resolution spectra for methane containing complexes have ena commonly associated with hydrophobic interactions been reported. Legon and co-workers have studied dimers (e.g., phase separation of oil and water, micelle formation, of the type CH,HX [X=F (Ref. 9), Cl (Ref. 9), CN tertiary protein structure),lm3 and the lack of accurate in- (Refs. 10, 1 l), Br (Ref. 12)] and Ohshima and Endo have termolecular potentials for describing the interaction of investigated the internal dynamics of methane within the water with “hydrophobic” molecules certainly constitutes CH4-HCl complex. l3 These studies indicate that methane an impediment to this endeavor. Recent studies of the pro- acts as a proton acceptor. However, microwave spectra totypical system Ar-H,O (Refs. 4 and 5) represent the probe only the average structure in a given VRT state initial phase of ongoing efforts in this laboratory to obtain (usually the ground state), and do not necessarily yield accurate experimental potentials for describing hydropho- structures that have any fundamental relationship to the bic interactions. Numerous vibration-rotation-tunneling intermolecular forces acting in the system. This has been (VRT) states of several isotopes were measured by tunable shown, for example, in the cases of the Ar-H,O (Refs. 6, far infrared laser spectroscopy of Ar-Hz0 clusters formed 7) and Ar-NH3 (Ref. 14) complexes. in a cw planar supersonic expansion. These data were then Theoretical studies of the CH4-HZ0 complex have used in a direct nonlinear least-squares fit to determine the been carried out,‘5-‘7 but there are serious discrepancies full three-dimensional anisotropic intermolecular potential among the results. The ab initio calculations of Szcz#niak surface (IPS) describing the interaction of an Ar atom ef al. I5 support previous theoretical results of Novoa with water.6’7 ef uZ.,16indicating that the global minimum occurs indeed The results of the Ar-H,O studies provide the frame- at the C* * *H-O contact. On the contrary, the global min- work in which to interpret and understand the spectra and imum configuration found by Woon et a1.,17 in their ab dynamics of more complicated systems, such as CH,-H,O. initio study, occurs at the C-H***0 contact. The CH4-HZ0 complex is the simplest hydrocarbon-water Tunable far-infrared (FIR) laser spectroscopy has cluster, and is closely related to Ar-HZ0 since the charge been used to obtain the first high resolution VRT spectrum distribution of the methane is almost spherical, with the of CH,H,O produced in a cw planar supersonic jet. Six octopole as the lowest nonvanishing multipole moment. bands with origins between 18 and 19 cm-’ and seven Therefore, in CH4-HZ0 only high order terms in the mul- bands with origins in the range 25.5-35.5 cm-’ have been tipole expansion of the electrostatic interaction (octopole- rotationally assigned. The bands sample 13 different VRT dipole, hexadecapole-dipole, . ..) are present; thus the spec- levels as upper states, and 8 levels as lower states, which tra of this complex probe the importance of these higher match the observed MW data.* The slow interconversion of different nuclear spin conformers in the supersonic ex- *)Permanent address: Dipartimento di Chimica “G. Ciamician,” Univer- pansion leads to population of at least six different VRT sit& di Bologna, via Selmi 2, 40136 Bologna, Italy. levels of the complex, and the observed high vibrational “Present address: Department of Chemistry, Harvard University, Cam- level density is the result of the slightly hindered internal bridge, Massachusetts 02138. “Present address: Department of Chemistry, University of Rochester, motion of both monomers within the cluster. Rochester, New York 14627. The CH,-H,O system is much more complicated than J. Chem. Phys. 100 (2), 15 January 1994 0021-9606/94/l 00(2)/863/l 4/$6.00 @ 1994 American Institute of Physics 863 Downloaded 30 Jan 2004 to 132.162.161.164. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp 864 Dore et al.: Spectroscopy of methane-water TABLE I. FIR laser lines used in the present study. mixture was expanded continuously through a 100X0.025 mm slit into a chamber maintained at 250 mTorr by a 1200 Lasing gas Frequency ( GHz) e/s Roots system. DCOOD 527.9260 We have scanned from 500 to 1125 GHz. The coverage HCOOH 584.3882 is almost continuous above 900 GHz, but not below this HCOOH 692.9514 frequency. Background subtraction has been implemented HCOOH 761.6083 by taking two scans and turning off the CH, in the second HCOOD 849.8280 CHBOD 939.4940 one; in this way only spectral features requiring CH4 have CH,OD 980.5916 been observed. In all, 950 peaks have been recorded, and CH,OD 1016.8972 we have been able to give an unambiguous rotational as- CHP, 1042.1504 signment for 329 of them, as described in the following CH2DOH 1101.1594 sections. Ill. SYMMETRY any other system for which an anisotropic IPS has been The experimental data allow rigorous assignment of experimentally determined. Detailed experimental inter- the total angular momentum (J), its projection (a) along molecular potential surfaces have been developed for sev- the van der Waals (vdW) axis, and the relative parity of eral two-dimensional systems [Rg-H, (Ref. 18), Rg-HX, connected states. The molecular symmetry (MS) group of X=halogen (Ref. 19)], for the three-dimensional system CH4-H20 will be used for identification and labeling of the Ar-H20,6,7 and for the (quasi) four-dimensional system observed VRT energy levels and to determine the optical Ar-NH3.14 The high symmetry of the CH,-HZ0 cluster, selection rules. The MS group is the set of the feasible compared to (H,O),, makes investigation of the IPS a nuclear permutations, that is, the feasible nuclear permu- reasonable starting point for the development of general tation group, plus the products of these operations with the methods for treating the pair-wise interactions of poly- space-fixed inversion operator; by a feasible permutation atomic molecules, such as the water dimer. Nevertheless, one means an interchange of the hydrogen nuclei of the significant advances in computational approaches for de- cluster that lead to a structure which is separated from the termining the eigenvalues of strongly coupled multidimen- initial structure by an accessible potential barrier. sional systems are still required before a parametrized six- If methane and water are assumed to be nearly free dimensional IPS can actually be determined from the internal rotors within the complex, the MS group for the experimental data. cluster is the direct product of three groups, the nuclear In this work we perform approximate calculations to permutation groups of the two monomers (because all per- obtain the bending VRT levels of the complex, using the mutations are possible), and a group, isomorphic to C’,, site-site potential of Woon et al. l7 expanded in products of formed by identity and inversion operations (this preserves Wigner rotation matrices. A comparison of these results to the handedness of the methane). This group is designated the experimental data indicates that either a less approxi- G4s and its character table is given by Bunker,22 and is mate method or a more reliable potential surface, or both, reproduced in Table II. The numbers 14 represent CH, will be required to obtain a quantitative agreement between hydrogens and the letters a and b represent H20 hydro- theory and experiment. gens. The labels describing the irreducible representations of G4s can be understood by considering a direct product wave function $cH4 8 r,!+r,oo &.,-, . The letters A, E, and F II. EXPERIMENT indicate the degeneracy of the state as well as the symme- try of the internal functions ($cH4) which describe the Both the tunable FIR laser system and the planar su- personic jet have been described in detail previously.20*21 motions of the CH, monomer within the molecule. The Tunable FIR radiation is generated by mixing the output superscripts f refer to the symmetric/antisymmetric of a narrow band ( < 100 kHz), fixed frequency, optically property of the internal rotation functions of the H20 sub- pumped FIR laser with the output of a MW synthesizer or unit ( hZo) with respect to interchange of the hydrogens; its harmonics in a Schottky barrier diode.
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