Microwave and tunable far-infrared laser spectroscopy of the ammonia–water dimer

Abstract

Microwave and far-infrared spectra of the H, N-HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a-type pure rotational microwave data extend the previous m = 0, K = 0 A symmetry manifold measurements of Herbine and Dyke [J. Chem. Phys. 83, 3768 ( 1980) ] to higher frequency and also provide an additional set of microwave transitions in the mK = + 1 E symmetry manifold. Two sets of five b-type rotation-tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V, = 10.5 f 5.0 cm-' for the barrier height to internal rotation for the NH, monomer. A nonlinear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos-'[(A,)] d e t ermined from (A,), the projection of the ammonia's angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of -700 cm -' is derived for the water tunneling motion about its c axis.

Microwave rotation-tunneling spectroscopy of the water–methanol dimer: Direct structural proof for the strongest bound conformation

“…The FT cavity can also scan only from 7 GHz to about 26 GHz, and even then the antenna must be manually adjusted to optimize the sensitivity from the lowest to the highest frequency. On the other hand, the Caltech planar supersonic jet/direct absorption microwave spectrometer, also described previously, 37,10 begins coverage at about 18 GHz and can reach up to 80 GHz. This machine has a much lower resolution of a few hundred kHz, but can scan quickly, up to 20 MHz/min or 1.2 GHz/h, allowing a broad, automated collection of data.…”

“…The water-methanol dimer is the latest, and one of the most chemically complex, in a series of water-containing, hydrogen-bonded dimers studied in our laboratories by microwave and far-infrared spectroscopy, for the purpose of fitting the observed spectra to intermolecular potential energy surfaces ͑IPSs͒. [1][2][3][4][5][6][7][8][9][10][11] The complex formed between methanol and water is particularly interesting because both hydrophobic and hydrophilic forces contribute strongly to the IPS of this relatively small dimer, creating a high degree of anisotropy. In addition, the watermethanol dimer will provide a test of the adaptability of the IPSs of simpler dimers to larger, multi-functional interactions.…”

Reaction Dynamics of H2O+ (D2O+) + NH3 Studied with Crossed Molecular Beams and Density Functional Theory Calculations

“…The calculations did not produce evidence for a transition state directly connecting 1 and 2: all attempts to calculate TS 12 collapse to TS 13 . This study also found that the transition state TS 11 corresponding to internal rotation of the NH 4 + unit of isomer 1 lies 13 kJ/mol above structure 1. On the basis of the error estimates arising from calculation of the exothermicities of reactions 1 and 2, the absolute error limits of these calculations were estimated to be (10 kJ/mol.…”

“…The first pathway involves isomerization within intermediate complex 1, DO‚‚‚DNH 3 + . In this case, internal rotation of the NH 3 D + unit in 1 exchanges the bridging hydrogen atom through TS 11 , lying 13 kJ/mol above the energy of 1. The second pathway proceeds through intermediate complex 2, D 2 O‚‚‚HNH 2 + , in which an internal rotation of the D 2 HO + unit to form DHO‚‚‚DNH 2 + precedes decay to products.…”

“…[1][2][3][4][5][6][7][8][9] Water and ammonia are both important small molecules that can form a stable hydrogen-bonded adduct, either as a neutral or ionic complex. [10][11][12][13][14][15][16][17][18][19] Chemical reactions between these species or occurring within complexes of these species include electron transfer and particle exchange and therefore provide paradigms for the most elementary reactions in systems amenable to theoretical treatments via ab initio quantum chemistry. In a recent paper, 20 we reported the crossed beam experimental results of the proton transfer reaction between H 3 O + and NH 3 .…”