Showing papers by "Richard J. Saykally published in 1994"

Many-body effects in intermolecular forces.

Abstract: The authors provide a review and literature survey of many-body effects in intermolecular forces. Topics include experimental methods, theoretical methods, many-body effects in atomic systems, and many-body effects in aqueous and nonaqueous molecular systems.

Dynamics of Structural Rearrangements in the Water Trimer

Abstract: The internal dynamics of the hydrogen bonding network of the water trimer are investigated by tunable far-infrared laser spectroscopy. New intermolecular vibrations have been measured at 87.1 [(HzO),] and 98.1 cm-’ [(DzO),]. Symmetry restrictions produce an exact oblate symmetric rotor pattern in the spectrum, even though theory predicts the trimer structure to be an asymmetric near-planar ring. In addition, each rovibrational transition is split into a quartet. A group theoretical treatment identifies two classes of structural rearrangements to account for these effects. There is considerable current interest in the spectroscopy and dynamics of small water clusters. Experimental and theoretical investigations of these species are motivated by the quest for a detailed understanding of the intermolecular forces and dynamics of the hydrogen bonding networks that operate in the condensed phases of water and in many biological systems.’ Numerous spectroscopic? ab initio,3 and empirical studies4 have addressed the intermolecular dynamics and potential energy surface of the water dimer. A similarly detailed characterization of the water trimer will enable a quantitative comparison with the dimer that could contribute significantly to the understanding of macroscopic systems. For instance, comparison of the water dimer and trimer ab initio intermolecular potential energy surfaces (IPS) predicts that non-pairwise additive forces (“three-body” interactions that can occur in the trimer but not the dimer) contribute 10% of the total binding energy of the trimer. Other predicted “three-body” effects are shorter 0-0 distances and higher average intermolecular vibrational frequencies in the trimer than in the dimer. Similarities in internal dynamics of the dimer and trimer can suggest mechanisms for important processes, such as proton transfer, that occur in the condensed phases of water. Tunable far-infrared laser vibration-rotation-tunneling spectroscopy (FIRVRTS) has emerged as a powerful new tool for addressing such subjects.’ Pugliano and Saykally’ (PLS) recently reported the first detailed experimental study of the cyclic water trimer. In that work, an intermolecular vibration of (DzO), was measured near 89.6 cm-1. This band displayed a strongly perturbed nearsymmetric top rotational pattern with each rovibrational transition split into a quartet. Crude estimates of the 0-0 distances were made by assuming three point masses of 20 amu and adjusting their separations for optimal agreement with the reported rotational constants. The spectral splittings were interpreted as resulting from isomerization tunneling among 96 identical frameworks (48 pairs of enatiomers) via three pathways: (1) “flipping” Aktract published in Aduance ACS Abstracts, March 1, 1994. (1) Saykally, R. J.; Blake, G. A. Science 1993, 259, 1570 and references therein. (2) Fraecr,G.T.Int.Reu.Phys. Chem. 1991,10,189andreferencestberein. ( 3 ) Smith, B. J.; Swanton, D. J.; Pople, J. A,; Scbaefer, H. F., III; Radom, L. J. Chem. Phys. 1990, 92, 1240. Niesor, U.; Corongin, G.; Clementi, E.; Kneller, G.; Bbattacbaraya, D. J. Phys. Chem. 1990,94, 7949. (4) Reimen, J. R.; Watts, R. 0.; Klein, M. L. Chem. Phys. 1982,64,95 and references therein. Cieplak, P.; Kollman, P.; Lybrand, T. J. Chem. Phys. 1990,92,6755. Jorgensen, W. L.; Chandrasekhar, J.; Madura, J.; Impey, R.; Klein, M. J. Chem. Phys. 1983, 79, 926. Townsend, M.; Morse, M.; Rice, S . A. J. Chem. Phys. 1983, 79, 2496. ( 5 ) F’ugliano, N.; Saykally, R. J. Science 1992, 257, 1937. OOO2-7863/94/1516-3507$04.50/0 of a single free hydrogen from one side of the ring to the other; (2) a motion that effectively results in a Cz rotation of a single monomer about its symmetry axis; and (3) a concerted motion that reverses the sense (yclockwise” or “counterclockwise” [cwH.s.0~) of the hydrogen bonding network around the ring. That work precipitated a number of sophisticated theoretical calculations of the trimer structure, vibrational frequencies, and interconversion tunneling dynamics.611 Of these, the calculations by Fowler and SchaefeI.6 are done at the highest level. All highlevel ab initio calculations agree that the lowest energy structure is that shown in Figure 1, and that the flipping motion is nearly free; moreover, all disagree with the crudely estimated 0-0 distances. Wales’’‘’ elegant treatment of the tunneling dynamics identified three low-energy reaction paths on the twelvedimensional IPS and estimated and associated splittings within a high barrier approximation. Schiitz et al.” carried out a detailed treatment of just the three flipping coordinates, giving special attention to the implications of the very low barrier to flipping. The highest-level ab initio calculations performed to date are those of Fowler and Schaefer.6 In this paper, we report the measurement of two new intermolecular vibrations of FIRVRTS. The Berkeley tunable far infrared spectrometer systems employed in this work have been described in detail elsewhere.13 A total of 284 VRT transitions rotationally assigned to a C-type band of (H2O)o and 57 transitionsassigned toan a-type bandof (DzO)3 weremeasured. The transitions of each isotopomer were fit to a symmetric top hamiltonian and the determined molecular constants are listed in Table 1. Portions of the (H2O)s and (D2O)s data are displayed in Figure 2.

The unidentified interstellar infrared bands: PAHs as carriers?

Abstract: Infrared emission spectra of gas-phase naphthalene and pyrene have been measured in the range of 3 to 7.5 micrometers with ultraviolet laser desorption-excitation and a spectroscopic technique featuring single-photon counting in the infrared. The spectra were compared with the unidentified infrared emission bands that are observed in many astronomical objects. Marked discrepancies between those observations and the laboratory emission spectra in the wavelengths and relative intensities of principal spectral features led to the conclusion that small neutral unsubstituted polycyclic aromatic hydrocarbons cannot be the carriers of the unidentified infrared emission bands.

Infrared laser spectroscopy of the linear C13 carbon cluster

Abstract: The infrared absorption spectrum of a linear, 13-atom carbon cluster (C13) has been observed by using a supersonic cluster beam-diode laser spectrometer. Seventy-six rovibrational transitions were measured near 1809 wave numbers and assigned to an antisymmetric stretching fundamental in the 1 sigma g+ ground state of C13. This definitive structural characterization of a carbon cluster in the intermediate size range between C10 and C20 is in apparent conflict with theoretical calculations, which predict that clusters of this size should exist as planar monocyclic rings.

Spectroscopic determination of the intermolecular potential energy surface for Ar–NH3

Abstract: The three‐dimensional intermolecular potential energy surface (IPS) for Ar–NH3 has been determined from a least‐squares fit to 61 far infrared and microwave vibration–rotation–tunneling (VRT) measurements and to temperature‐dependent second virial coefficients. The three intermolecular coordinates (R,θ,φ) are treated without invoking any approximations regarding their separability, and the NH3 inversion–tunneling motion is included adiabatically. A surface with 13 variable parameters has been optimized to accurately reproduce the spectroscopic observables, using the collocation method to treat the coupled multidimensional dynamics within a scattering formalism. Anisotropy in the IPS is found to significantly mix the free rotor basis functions. The 149.6 cm−1 global minimum on this surface occurs with the NH3 symmetry axis nearly perpendicular to the van der Waals bond axis (θ=96.6°), at a center‐of‐mass separation of 3.57 A, and with the Ar atom midway between two of the NH3 hydrogen atoms (φ=60°). The po...

Far infrared vibration‐rotation‐tunneling spectroscopy and internal dynamics of methane–water: A prototypical hydrophobic system

Abstract: Thirteen vibration‐rotation‐tunneling (VRT) bands of the CH4–H2O complex have been measured in the range from 18 to 35.5 cm−1 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–H2O. The eigenvalue spectrum has been calculated with a variational procedure using a spherical expansion of a site–site ab initio 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 quantitative agreement. These observations suggest that both monomers undergo nearly free internal rotation within the complex.

Far-IR Vibration-Rotation-Tunnelling Spectroscopy of the Water Trimer

Abstract: Tunable far-infrared laser vibration–rotation–tunnelling spectroscopy (FIRVRTS) has been used to measure several intermolecular vibrations in the water trimer. Together with ab initio and group theoretical results, these data characterize the structure and especially the tunnelling dynamics of this system in considerable detail.

Characterization of silicon-carbon clusters by infrared laser spectroscopy: the nu 3(sigma u) band of linear Si2C3.

Abstract: The nu 3(sigma u) fundamental vibration of 1 sigma g+ Si2C3 has been observed using a laser vaporization-supersonic cluster beam-diode laser spectrometer. Forty rovibrational transitions were measured in the range of 1965.8 to 1970.9 cm-1 with a rotational temperature of 10-15 K. A least-squares fit of these transitions yielded the following molecular constants: nu 3(sigma u)=1968.188 31(18) cm-1, B"=0.031 575 1(60) cm-1, and B'=0.031 437 4(57) cm-1. These results are in excellent agreement with recent Fourier transform infrared (FTIR) measurements of Si2C3 trapped in a solid Ar matrix [J. Chem. Phys. 100, 181(1994)] and with ab initio calculations [J. Chem. Phys. 100, 175 (1994)] which suggest cumulenic-like bonding for Si2C3, analogous to the isovalent C5 carbon cluster.

Non-additive intermolecular forces from the spectroscopy of van der Waals trimers: far-infrared spectra and calculations on Ar2-DCl

Abstract: The far-infrared vibration-rotation spectrum of the out-of-plane DCl bending band of Ar2-DCl is observed around 36·0 cm-1. The experimental bending frequency, rotational constants and hyperfine coupling constants are compared with the results of calculations employing both pairwise-additive and non-additive interaction potentials. As found previously for Ar2-HCl, there are substantial discrepancies between the experimental results and calculations employing a pairwise-additive potential. To explain the discrepancy it is necessary to include a non-additive term that arises from the interaction of the permanent multipoles of the DCl monomer with an overlap-induced quadrupole on Ar2. The new spectra should prove very valuable in a future determination of the non-additive contribution to the potential.

The nature of monomer inversion in the ammonia dimer

Abstract: A model is presented for calculating the splittings due to umbrella inversion of the monomers in (NH3)2. Input to the model are the six‐dimensional dimer bound state wave functions for rigid monomers, calculated previously [E. H. T. Olthof, A. van der Avoird, and P. E. S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. This model is based on first‐order (quasi) degenerate perturbation theory and adaptation of the wave functions to the group chain G36⊆G72⊆G144. The umbrella inversion splittings depend sensitively on the intermolecular potential from which the bound state wave functions are obtained. A complete interpretation of the observed splitting pattern [J. G. Loeser, C. A. Schmuttenmaer, R. C. Cohen, M. J. Elrod, D. W. Steyert, R. J. Saykally, R. E. Bumgarner, and G. A. Blake, J. Chem. Phys. 97, 4727 (1992)] and quantitative agreement with the measured splittings, which range over three orders of magnitude, are obtained from the potential that reproduces the far‐infrared spectrum of (NH3)2 and the dipole m...

Vibration–rotation–tunneling spectroscopy of Ar–NH3

Abstract: Eight new intermolecular vibrational states of Ar–NH3 have been observed by tunable far infrared laser vibration–rotation–tunneling spectroscopy (FIRVRTS) between 28 and 45 cm−1. These states provide a detailed measure of the angular‐radial coupling in the intermolecular potential energy surface (IPS) and permit the dependence of the IPS on rotation about the NH3 symmetry axis to be quantitatively assessed for the first time. The determination of a complete three‐dimensional IPS for the complex is described in the accompanying paper.