ALBERT

Notes: We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric-dipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy-level diagrams for the Ka=0 and Ka=1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G144. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the "donor'' and "acceptor'' roles of the monomers, internal rotation of the monomers about their C3 symmetry axes, and quite unexpectedly, "umbrella'' inversion tunneling.In the Ka=0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In Ka=1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm−1, respectively. In the A+E states, the measured 20.5 cm−1 splitting can result from a difference in "donor'' and "acceptor'' internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that "donor–acceptor'' interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six-dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

Notes: The high resolution infrared spectrum of the C9 cluster has been measured in direct absorption by infrared diode laser spectroscopy of a pulsed supersonic carbon cluster jet. Fifty-one rovibrational transitions have been assigned to the ν6 (σu ) antisymmetric stretch fundamental of the 1Σ+9 linear ground state of C9. The measured rotational constant [429.30(50) MHz] is in good agreement with ab initio calculations and indicates an effective bond length of 1.278 68(75) A(ring), consistent with cumulenic bonding in this cluster. Several perturbations are observed in the upper state, and the upper- and lower-state centrifugal distortion constants are observed to be anomolously large, evidencing a high degree of Coriolis mixing of the normal modes.

Notes: Measurements of the fundamental van der Waals stretching vibration Σ(000,vs=1) ←Σ(000,vs=0) of Ar–H2O [ν0=907 322.08(94) MHz] and a transition from the lowest excited internal rotor state Σ(101,vs=0) to the Σ(101,vs=1) level [ν0=1019 239.4(1.0) MHz] are presented. A simultaneous rotational analysis of the new stretching data with the internal rotor bands observed by us previously [J. Chem. Phys. 89, 4494 (1988)], including the effects of Coriolis interactions, provides experimental evidence for the new assignment of the internal rotor transitions suggested by Hutson in the accompanying paper. Fits to the rotational term values for the vs=0 states are used to derive effective radial potential energy surfaces for each of the Σ internal rotor states. The results show the well depth (153.4 cm−1) of the effective radial potential for the Σ(101,vs=0) level to be approximately 25 cm−1 deeper than that for the Σ(000,vs=0) ground state of the complex, indicating that the former is stabilized considerably more by the anisotropic intermolecular potential energy surface than is the ground state.

Notes: The first accurate measurement of an intermolecular vibration of the water dimer is reported. Five vibration–rotation-tunneling (VRT) bands of the perdeuterated isotope, located near 84 cm−1, have been assigned to the A1/E/B1 tunneling components of the Ka=0←0 and Ka=1←0 subbands. The vibration involves large amplitude motion of the hydrogen bond acceptor and is assigned as the ν8 acceptor wag. The spectra indicate strong coupling of both the donor–acceptor interconversion and donor tunneling motions to the excited vibrational coordinate. This measurement provides a benchmark for future efforts toward the determination of an accurate potential energy surface for the water dimer.

Notes: The first characterization of the bending potential of the C7 cluster is reported via the observation of the v=11 and v=20 levels of the ν11(πu) bend as hot bands associated with the ν4(σu) antisymmetric stretch fundamental. The lower state hot band rotational constants are measured to be 1004.4(1.3) and 1123.6(9.0) MHz, constituting a 9.3% and 22% increase over the ground state rotational constant [918.89(41) MHz]. These large increases are strong evidence for extremely large amplitude, anharmonic bending modes in this cluster. In addition, quartic and sextic centrifugal distortion constants determined for the ground and ν4=1 states are found to be anomalously large and negative, evidencing strong perturbations between stretching and bending modes.

Notes: Over 150 lines in six tunneling subbands of an intermolecular vibration located near 25 cm−1 have been measured with partial hyperfine resolution and assigned to (NH3)2. The transitions sample all three types of tunneling states (A, G, E) and are consistent with the following assumptions: (1) G36 is the appropriate molecular symmetry group; (2) the equilibrium structure contains a plane of symmetry; (3) interchange tunneling of inequivalent monomers occurs via a trans path; (4) the 2C3+I limit of hydrogen exchange tunneling is appropriate; (5) tunneling and rotational motions are separable. A qualitative vibration–rotation tunneling energy level diagram is presented. Strong perturbations are observed among the states of E symmetry. This work supports the conclusions of Nelson et al. [J. Chem. Phys. 87, 6365 (1987)].

Notes: The first far infrared intermolecular vibration–rotation spectrum of the ternary van der Waals cluster has been measured near 39.5 cm−1 and assigned to an a-type ∑ bending vibration of Ar2HCl. Spectra of both chlorine isotopes were observed and nuclear quadrupole hyperfine structure was resolved. Values of the fitted constants (rotational constants, hyperfine projections) evidence large amplitude out-of-plane motion, and demonstrate the sensitivity of spectroscopic observables to the three body forces operative in the Ar2HCl system. Spectroscopic predictions calculated by Hutson et al. from pairwise-additive and "three-body'' corrected potential energy surfaces [J. Chem. Phys. 90, 1337 (1989)] are compared to experimental results.

Notes: The technique of direct laser absorption spectroscopy in fast ion beams has been employed for the determination of absolute integrated band intensities (S0v) for the ν3 fundamental bands of H3O+ and NH+4. In addition, the absolute band intensities for the ν1 fundamental bands of HN+2 and HCO+ have been remeasured. The values obtained in units of cm−2 atm−1 at STP are 1880(290) and 580(90) for the ν1 fundamentals of HN+2 and HCO+, respectively; and 4000(800) and 1220(190) for the ν3 fundamentals of H3O+ and NH+4, respectively. Comparisons with ab initio results are presented.

Notes: In order to address the well-known problem that the nearly cyclic structure of (NH3)2 deduced from microwave spectra differs greatly from the hydrogen-bonded equilibrium structure obtained from ab initio calculations, we have calculated the vibration–rotation–tunneling (VRT) states of this complex, and explicitly studied the effects of vibrational averaging. The potential used is a spherical expansion of a site–site potential which was extracted from ab initio data. The six-dimensional VRT wave functions for all the lowest states with J=0 and J=1 were expanded in products of radial (van der Waals stretch) functions and free-rotor states for the internal and overall rotations, which were first adapted to the complete nuclear permutation inversion group G36. Although the (expanded) potential is too approximate to expect quantitative agreement with the observed microwave and far-infrared spectra, we do find several interesting features: The 14N quadrupole splittings and the dipole moment of the complex, which are indicative of the orientational distributions of the NH3 monomers, are substantially affected by vibrational averaging. The interchange tunneling of the two monomers is not quenched. In the ortho–ortho and para–para states, of A and E symmetry, this tunneling manifests itself in a very different manner than in the ortho–para states of G symmetry. In contrast with the interpretation of Nelson et al. [J. Chem. Phys. 87, 6364 (1987)], we believe that the Gα and Gβ states observed by these authors correspond to a single VRT state which is split by (hindered) NH3 monomer inversion.