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  • 1
    Electronic Resource
    Electronic Resource
    The nuclear quadrupole coupling constants and the structure of the para–para ammonia dimer (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8693-8703 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Expressions are derived for the nuclear quadrupole splittings in the E3 and E4 (para–para) states of (NH3)2 and it is shown that these can be matched with the standard expressions for rigid rotors with two identical quadrupolar nuclei. The matching is exact only when the off-diagonal Coriolis coupling is neglected. However, the selection rules for rotational transitions are just opposite to those for the rigid rotor. Hyperfine splittings are measured for the J=2←1 transitions in the E3 and E4 states with ||K||=1; the quadrupole coupling constants χaa=0.1509(83) MHz and χbb−χcc=2.8365(83) MHz are extracted from these measurements by the use of the above mentioned correspondence with the rigid rotor expressions. The corresponding results are also calculated, with and without the Coriolis coupling, from the six-dimensional vibration–rotation–tunneling (VRT) wave functions of (NH3)2, which were previously obtained by Olthof et al. [E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. From the comparison of χaa with the measured value it follows that the semiempirical potential and the resulting VRT states of Olthof et al. are very accurate along the interchange (cursive-thetaA,cursive-thetaB) coordinate. From χbb−χcc it follows that this potential is probably too soft in the dihedral angle γ¯=γA−γB, which causes the torsional amplitude to be larger than derived from the experiment. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468972
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  • 2
    Electronic Resource
    Electronic Resource
    Structure, internal mobility, and spectrum of the ammonia dimer: Calculation of the vibration–rotation-tunneling states (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8430-8442 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have obtained a potential for (NH3)2 by calculating the six-dimensional vibra- tion–rotation-tunneling (VRT) states from a model potential with some variable parameters, and adjusting some calculated transition frequencies to the observed far-infrared spectrum. The equilibrium geometry is strongly bent away from a linear hydrogen bonded structure. Equivalent minima with the proton donor and acceptor interchanged are separated by a barrier of only 7 cm−1. The barriers to rotation of the monomers about their C3 axes are much higher. The VRT levels from this potential agree to about 0.25 cm−1 with all far-infrared frequencies of (NH3)2 observed for K=0, ||K||=1, and ||K||=2 and for all the symmetry species: Ai=ortho–ortho, Ei=para–para, and G=ortho–para. Moreover, the dipole moments and the nuclear quadrupole splittings agree well with the values that are observed for the G states. The potential has been explicitly transformed to the center-of-mass coordinates of (ND3)2 and used to study the effects of the deuteration on the VRT states. The observed decrease of the dipole moment and the (small) changes in the nuclear quadrupole splittings are well reproduced. It follows from our calculations that the ammonia dimer is highly nonrigid and that vibrational averaging effects are essential. Seemingly contradictory effects of this averaging on its properties are the consequence of the different hindered rotor behavior of ortho and para monomers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468105
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  • 3
    Electronic Resource
    Electronic Resource
    The nature of monomer inversion in the ammonia dimer (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8443-8454 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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 moment and nuclear quadrupole splittings of (NH3)2 and (ND3)2. The umbrella inversion splittings of (ND3)2 are predicted.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468106
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  • 4
    Electronic Resource
    Electronic Resource
    Tunneling dynamics, symmetry, and far-infrared spectrum of the rotating water trimer. II. Calculations and experiments (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 8051-8063 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: With the Hamiltonian derived in the preceding paper and the ab initio potentials of T. Bürgi, S. Graf, S. Leutwyler, and W. Klopper [J. Chem. Phys. 103, 1077 (1995)] and of J. G. C. M. van Duijneveldt-van de Rijdt and F. B. van Duijneveldt [Chem. Phys. Lett. 237, 560 (1995)], we calculate the pseudo-rotation tunneling levels in a rotating water trimer. The internal motions are treated by a three-dimensional discrete variable representation and the Coriolis coupling with the overall rotation is included. Also the effects of donor tunneling are included, by introducing semi-empirical coupling matrix elements. New experimental data are presented for the c-type band at 87.1 cm−1 in (H2O)3, which show that specific levels in the donor tunneling quartets of this band are further split into doublets. With the results of our quantitative calculations and the model of the preceding paper we can understand the mechanisms of all the splittings observed in the earlier high-resolution spectra of (H2O)3 and (D2O)3, as well as these new splittings, in terms of pseudo-rotation tunneling, donor tunneling and Coriolis coupling. An unambiguous assignment is given of all the bands observed and analyzed. The ab initio potential of the Van Duijneveldts yields accurate energies of the lower pseudo-rotation levels, the potential of Bürgi et al. performs better for the higher levels. With our analysis we can deduce from the spectra that donor tunneling involves inversion of the trimer. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472661
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  • 5
    Electronic Resource
    Electronic Resource
    Tunneling dynamics, symmetry, and far-infrared spectrum of the rotating water trimer. I. Hamiltonian and qualitative model (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 8034-8050 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Hamiltonian is derived for the rotating water trimer with three internal motions—the rotations of the monomers about their hydrogen bonds. We obtain an expression of the kinetic energy operator, which is a non-trivial extension of earlier heuristic forms used for the non-rotating trimer. The Coriolis coupling operator between the single-axis monomer angular momenta and the overall trimer rotation is given for the first time. To analyze the effects of the tunneling and Coriolis splittings on the energy levels of the trimer, we introduced a qualitative model for the pseudo-rotation and donor tunneling. By perturbation theory and application of the permutation-inversion groups G6 and G48 we obtain algebraic expressions for the splittings due to pseudo-rotation and donor tunneling, respectively. The pseudo-rotation does not produce any internal angular momentum and does not yield first order Coriolis splitting, but in second order the Coriolis coupling lifts various degeneracies and gives rise to observable J-dependent splittings. Donor tunneling splits every pseudo-rotation level into a quartet and those levels in this quartet that belong to the three-dimensional irreps of G48 into doublets. For J(approximately-greater-than)0 a rather complex pattern of larger (for the internal states with G6 labels k=±1 and ±2) and smaller (for the levels with k=0 and k=3) splittings is obtained, especially for the substates with |K|=1 which are Coriolis coupled to the K=0 substates. The results of calculations in the companion paper, together with the model introduced in the present paper, will be used to interpret all the tunneling splittings observed in high-resolution spectra of (H2O)3 and (D2O)3. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472660
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  • 6
    Electronic Resource
    Electronic Resource
    Vibration and rotation of CO in C60 and predicted infrared spectrum (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 832-847 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the Hamiltonian for the vibrations and rotations of CO inside a freely rotating or fixed C60 molecule and we calculate its eigenstates from an atom–atom model potential. The ensuing level structure can be understood in terms of three basic characteristics. (i) Simultaneous rotations of CO and its position vector R, which give rise to a rotational structure similar to that of free CO. The effective rotational constants differ considerably, however. (ii) Splittings of the levels by the icosahedral field of C60 which perturb the regular rotational structure, because they are of the same order of magnitude as the rotational spacings. (iii) Large frequencies associated with the (nearly harmonic) vibrations of CO against the hard walls of the C60 cage: 209 cm−1 for the radial excitation and 162 cm−1 for the twofold degenerate libration. These vibrations give a rovibrational level structure similar to that of a linear triatomic molecule, the radial excitation resembles a bond stretch (Σ) state, the libration a Π-bending state. From the eigenstates we calculate the line strengths of the electric dipole transitions allowed by the icosahedral symmetry. Additional (approximate) selection rules are found, and the infrared spectrum of CO@C60 is predicted. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470809
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