Microwave and far-infrared spectroscopy of water-containing, hydrogen-bonded dimers : vibrational, rotational, and tunneling dynamics

Abstract

Microwave and far-infrared high-resolution spectroscopies have been used to investigate the vibrational, rotational, and tunneling dynamics of water-containing, hydrogenbonded dimers, towards the goal of converting the recorded spectra into intermolecular potential energy surfaces (IPSs). Such intermolecular interactions are the basis for much of biological structure and function, as well as reaction and solution dynamics, and a comprehensive set of key IPSs can form the basis set for molecular modeling of these complex phenomena. Specifically studied in this work are the following dimers and some of their isotopomers: N_2•••H_2O, OC•••H_2O, H_3N•••H_2O, CH_3OH•••H_2O. These dimers constitute a series of increasing binding energy, dynamical complexity, and chemical and biological importance. The dimers of water with isoelectronic N_2 and CO are treated jointly because of their similar properties and level of study. The b-type K = 0 → 1 rotational spectra for N_2•••H_2O and OC•••H_2O, as well as isotopomers containing HOD, D_2O, and ^(13)CO, were recorded between 325 and 661 GHz and measurements were also extended for the a-type spectra of the N_2•••H_2O and OC•••H_2O isotopomers. Each rotational transition was split by the effects of large-amplitude tunneling of the hydrogen and nitrogen nuclei, and the number of tunneling components corresponded with that predicted from PI theory. After the effects of tunneling had been removed from the rotational constants, the structures of the two different complexes were fit to the rotational moments of inertia. These structures correspond to a nearly linear hydrogen bond and alignment of heavy atoms. The tunneling selection rules for OC•••H_2O were confirmed to be top-to-bottom, bottom-to-top, and the tunneling splittings were obtained from the difference in the A_(eff) rotational constants. Using two- and three-term Fourier expansions, the one-dimensional tunneling coordinate was fit to the tunneling splittings for OC•••H_2O and OC•••D_2O. Ab initio surfaces are used both to visualize the tunneling modes and as a basis for normal mode analyses. A reasonable and justified normal mode separation is presented, with the five intermolecular degrees of freedom separated into geared and anti-geared in-plane and out-of-plane motions of the the subunits as well as the intermolecular stretch. Finally, partial VRT spectra for both complexes recorded between 42 and 53 cm^(-1) are presented and preliminarily assigned to the in-plane geared motion. Microwave and far-infrared spectra of the H_3N•••H_2O dimer were recorded from 36 to 86 GHz and 520 to 800 GHz. 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, were 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. Non-linear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V_3 = 10.5 ±5.0 cm^(-1) for the barrier height to internal rotation for the NH3 monomer. A non-linear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos^(-1) [〈λ_z〉] determined from 〈λ_z〉, the projection of the ammonia's angular momentum onto the framework. 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^(-1) was derived for the water tunneling motion about its c axis. Finally, microwave spectra of CH_3OH•••H-2O and CH_3OD•••D_2O were recorded between 20 and 60 GHz, along with data from our collaborators at the National Institute for Science and Technology for CH_3OH•••H_2O, ^(13)CH_3OH•••H_2O, CH_3OH•••DOH, CD_3OH•••H_2O, and CH_3OD•••D_2O between 7 and 24 GHz. Because CH_3OH and H_2O are both capable of accepting and donating hydrogen bonds, there existed some question as to which donor-acceptor pairing of the molecules was the lowest energy form. This question is further emphasized by the ambiguity and variety present in previous experimental and computational results. Transitions from the methyl torsional A state were assigned for the various isotopomers. The fit of the structure to the rotational constants gave unambiguous confirmation that the only conformation observed in the supersonically cooled molecular beams corresponded to a water-donor, methanol-acceptor complex.