Donor-acceptor Interchange Research Articles

Measurement of Donor-Acceptor Interchange Tunnelling in Ar(H2O)2 using Rotational Spectroscopy and a Re-look at Its Structure and Bonding

In this work, we have measured the donor-acceptor interchange tunnelling splitting in the ground vibrational state of Ar(H2O)2. In the previous investigations, the donor-acceptor tunnelling splitting in fully deuterated species, Ar(D2O)2, was measured to be about 106 MHz. It could not be measured for Ar(H2O)2, as the splitting was expected to be several GHz. Besides, for (D2O)2, both A1 and B1 states have non-zero statistical weight appearing on both sides of the E1 state, having a semi-rigid rotor type spectrum making their observation easier, compared to that of (H2O)2, which has zero statistical weight for the B1 state. With the help of a fourfold periodic potential, we have accurately predicted the fingerprints of b-dipole A1+↔A1- transitions and observed them using a pulsed nozzle Fourier transform microwave spectrometer. Measurement of these transitions enabled us to determine the donor-acceptor tunnelling splitting of 4257.41(4) MHz in Ar(H2O)2. More detailed structural parameters of Ar(H2O)2 have been evaluated in this work and critically compared with (H2O)2. Atoms in Molecules (AIM), non-covalent interactions (NCI) index and natural bond orbital (NBO) analysis have also been carried out for both (H2O)2 and Ar(H2O)2, and these calculations confirm that the presence of argon (Ar) does not affect the structure and bonding in (H2O)2.

Spectra of the D2O dimer in the O-D fundamental stretch region: Vibrational dependence of tunneling splittings and lifetimes.

The fundamental O-D stretch region (2600-2800 cm-1) of the fully deuterated water dimer (D2O)2 is studied using a pulsed supersonic slit jet source and a tunable optical parametric oscillator source. Relatively high spectral resolution (0.002 cm-1) enables all six dimer tunneling components to be observed, in most cases, for the acceptor asymmetric O-D stretch, the donor free O-D stretch, and the donor bound O-D stretch vibrations. The dominant acceptor switching tunneling splittings are observed to decrease moderately in the excited O-D stretch states, to roughly 75% of their ground state values, whereas the smaller donor-acceptor interchange splittings show more dramatic and irregular decreases. Excited state predissociation lifetimes, as determined from the observed line broadening, show large variations (0.2 ≤ τ ≤ 5 ns) depending on the vibrational state, K-value, and tunneling symmetry. Another very weak band is tentatively assigned to a combination mode involving an intramolecular O-D stretch plus an intermolecular twist overtone. Asymmetric O-D stretch bands of the mixed isotopologue dimers D2O-DOH and D2O-HOD are also observed and analyzed.

Spectra of water dimer from a new ab initio potential with flexible monomers

We report the definition and testing of a new ab initio 12-dimensional potential for the water dimer with flexible monomers. Using our recent accurate CCpol-8s rigid water pair potential [W. Cencek, K. Szalewicz, C. Leforestier, R. van Harrevelt, and A. van der Avoird, Phys. Chem. Chem. Phys. 10, 4716 (2008)] as a reference for the undistorted monomers' geometries, a distortion correction has been added, which was taken from a former flexible-monomer ab initio potential. This correction allows us to retrieve the correct binding energy D(e)=21.0 kJ mol(-1), and leads to an equilibrium geometry in close agreement with the one obtained from benchmark calculations. The kinetic energy operator describing the flexible-monomer water dimer has been expressed in terms of Radau coordinates for each monomer and a recent general cluster polyspherical formulation describing their relative motions. Within this formulation, an adiabatic scheme has been invoked in order to decouple fast (intramolecular) modes and slow (intermolecular) ones. Different levels of approximation were tested, which differ in the way in which the residual potential coupling between the intramolecular modes located on different monomers and the dependence of the monomer rotational constants on the dimer geometry are handled. Accurate calculations of the vibration-rotation-tunneling levels of (H(2)O)(2) and (D(2)O)(2) were performed, which show the best agreement with experiments achieved so far for any water potential. Intramolecular excitations of the two monomers were calculated within two limiting cases, to account for the lack of non-adiabatic coupling between intramolecular modes due to the intermolecular motion. In the first model, the excitation was assumed to stay either on the donor or the acceptor molecule, and to hop between the two moieties upon donor-acceptor interchange. In the second model, the excitation remains on the same molecule whatever is the dimer geometry. Marginal frequency differences, less than 2 cm(-1), were obtained for all modes, and the resulting infrared shifts are in good agreement with experiments.

Vibration−Rotation−Tunneling Levels of the Water Dimer from an ab Initio Potential Surface with Flexible Monomers

The 12-dimensional ab initio potential for the water dimer with flexible monomers from Huang et al. (J. Chem. Phys. 2008, 128, 034312) was used in accurate calculations of the vibration-rotation-tunneling (VRT) levels of (H2O)2 and (D2O)2 involving the intermolecular rovibrational and tunneling states as well as the intramolecular vibrations. For the intermolecular VRT levels we used a 6 + 6d model in which the fast intramolecular vibrations are adiabatically separated from the much slower intermolecular vibrations, tunneling motions, and overall rotations. We also tested two six-dimensional (6d) rigid monomer models in which the monomers were frozen either at their equilibrium geometry or at their ground state vibrationally averaged geometry. All the results from the 6 + 6d model agree well with the large amount of detailed experimental data available from high-resolution spectroscopy. For most of the parameters characterizing the spectra the results of the two 6d rigid monomer models do not significantly differ from the 6 + 6d results. An exception is the relatively large acceptor tunneling splitting, which was the only quantity for which the 6d model with the monomers frozen at their equilibrium geometry was not in good agreement with the experimental data. The 6d model with monomers at their vibrationally averaged geometry performs considerably better, and the full 6 + 6d results agree with the measurements also for this quantity. For the excited intramolecular vibrations we tested two 6 + 6d models. In the first model the excitation was assumed to be either on the donor in the hydrogen bond or on the acceptor, and to hop from one monomer to the other upon donor-acceptor interchange. In the second model the monomer excitation remains localized on a given monomer for all dimer geometries. Almost the same frequencies of the intramolecular vibrations were found for the two models. The calculations show considerable variations in the frequencies of the intramolecular modes for transitions involving different tunneling levels and different values of the rotational quantum number K. For K = 0 --> 0 transitions these variations largely cancel, however. A comparison with experimental data is difficult, except for the acceptor asymmetric stretch mode observed in high-resolution spectra, because it is not clear how much the different transitions contribute to the (unresolved) peaks in most of the experimental spectra. The large red shift of the donor bound OH stretch mode is correctly predicted, but the value calculated for this red shift is too small by more than 20%. Also in the smaller shifts of the other modes we find relatively large errors. It is useful, however, that our detailed calculations including all ground and excited state tunneling levels provide an explanation for the splitting of the acceptor asymmetric stretch band observed in He nanodroplet spectra, as well as for the fact that the other bands in these spectra show much smaller or no splittings.

An ab initio study of tunneling splittings in the water dimer.

Tunneling splittings in the water dimer have been determined by the semiclassical WKB method, based on pathways characterized at the computational level of second-order Møller-Plesset (MP2) theory with basis sets of aug-cc-pVTZ quality. This calculation takes into account all three acceptor tunneling, donor-acceptor interchange, and bifurcation tunneling rearrangements of the water dimer. The tunneling splittings were evaluated as 7.73 cm(-1) (large splitting) and 0.42 cm(-1) (small splitting), which are in good agreement with the corresponding experimental values of 11.18 cm(-1) and 0.70 cm(-1), respectively.

Quantum path-integral molecular dynamics calculations of the dipole-bound state of the water dimer anion

The equilibrium structure of the negatively charged water dimer (H 2O) 2 − has been studied using the path-integral molecular dynamics simulation. All the atomic motions as well as the excess electron were treated quantum mechanically, employing a semi-empirical model combining a water–water interatomic potential with an electron–water pseudopotential. It is demonstrated that the molecular structure of (H 2O) 2 − is more flexible than that of (H 2O) 2; both the donor switching and donor–acceptor interchange can more effectively occur in (H 2O) 2 − than in (H 2O) 2. We conclude that this floppy character is a result of the breakdown of the adiabatic Born–Oppenheimer picture.

Theoretical study of rearrangements in water dimer and trimer

The global minimum and transition states for the acceptor-tunnelling, donor-acceptor interchange and bifurcation tunnelling rearrangements of the water dimer, and the single-flip, bifurcation and concerted proton transfer processes in the water trimer have been reinvestigated. Our analysis of the tunnelling splittings and spectroscopy is based on ab initio calculations at the computational level of second-order Møller-Plesset (MP2) theory with basis sets of aug-cc-pVXZ quality (X = D, T, Q for the dimer; X = D, T for the trimer). In both water dimer and trimer, the binding energy, barrier heights, intermonomer distances, and harmonic frequencies converge smoothly as the size of the basis set increases. In the water dimer, the binding energy was evaluated as 5.09kcal mol−1, while the activation energies are 0.52 (acceptor-tunnelling) 0.79 (donor-acceptor interchange), and 1.94 kcal mol−1 (bifurcation tunnelling) at the MP2/aug-cc-pVQZ level. In the water trimer, the binding energy was evaluated as 16.29 kcal mol−1, while the activation energies are 0.28 (single-flip), 2.34 (bifurcation), and 26.36 (proton transfer) kcal mol−1 at the MP2/aug-cc-pVTZ level.

Rotational spectra, structures, and dynamics of small Arm–(H2O)n clusters: The Ar–(H2O)2 trimer

Rotational-tunneling spectra for Ar–(H2O)2 and Ar–(D2O)2 have been observed with the Balle–Flygare Fourier transform microwave spectrometer. The tunneling levels of the trimer appear to correlate with those of the water dimer. The “a” dipole transitions from the A1+ and E+ states of Ar–(H2O)2 and A1+, B1+, and E+ states of Ar–(D2O)2 could be fit to a semirigid rotor Watson Hamiltonian. However, only the E+ states give “b” dipole transitions near rigid rotor predictions. The “b” dipole transitions for A1+ and B1+ are rotational-tunneling spectra. For Ar–(D2O)2, these transitions were observed and the donor–acceptor interchange tunneling splitting is determined as 106.3 MHz, compared to about 1100 MHz in the free (D2O)2. From this splitting, the barrier for interchange tunneling is calculated to be 642 cm−1. This splitting for Ar–(H2O)2 is estimated as 4–5 GHz. This and the spin statistical weight of 0 for the B1+ state have made it difficult to observe the “b” dipole rotational tunneling spectra for Ar–(H2O)2. From the rotational constants for (H2 18O) containing trimers, the O–O distance in the trimer is estimated as 2.945 Å. This is significantly (0.035 Å) shorter than the O–O distance reported for water dimer. The Ar is located on the “b” axis of the water dimer. Assuming the water to be a structureless sphere in the trimer, leads to Ar-c.m.(H2O) distance of 3.637 Å, very close to the same value in the Ar–H2O dimer.

Ab initio study of (NH3)2: Accurate structure and energetics

The equilibrium and saddle point structures of the ammonia dimer were investigated employing a series of correlation-consistent basis sets, cc-pVXZ and aug-cc-pVXZ(X=D,T,Q) at the Hartree–Fock (HF), second order and fourth order Møller–Plesset perturbation theory (MP2, MP4) levels. It is shown that the energies and geometries of the stationary structures are sensitive to the basis set and electron correlation, and the proper combination of electron correlation treatment (MP2 or better) and basis set requirement (aug-cc-pVTZ or better) is necessary to determine the equilibrium structure and ordering of the stationary structures. The equilibrium structure of the ammonia dimer is found to be an asymmetric cyclic (eclipsed) structure with Cs symmetry. Although this is generally consistent with the equilibrium structure predicted from the model potential by Olthof et al. [J. Chem. Phys. 101, 8430 (1994)], it appears to be more shifted toward the linear structure than the centrosymmetric cyclic structure, which signifies a certain degree of hydrogen bonding in this dimer. The MP2 barrier height to proton donor–acceptor interchange at MP2/aug-cc-pVQZ//MP2/aug-cc-pVTZ is 7.6 cm−1, which is in excellent agreement with the value predicted from the model potential by Olthof et al. While the barrier height to rotation of one monomer (hydrogen-bond acceptor) about its C3 axis is relatively low, ∼20 cm−1, the corresponding value of the other monomer (hydrogen-bond donor) is shown to be rather high, probably more than 450 cm−1 at the equilibrium structure. The binding energies of the ammonia dimer are computed with and without counterpoise correction for basis set superposition error. The MP2 complete basis set (CBS) limit electronic binding energy of (NH3)2 is estimated to be within 13.2±0.3 kJ/mol.

Spectroscopic determination of the water pair potential.

A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D2O)2 spectra through a rigorous calculation of the water dimer eigenstates. It accurately reproduces most ground state vibration-rotation-tunneling spectra and yields excellent second viral coefficients. The calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations. The dimer binding energy and acceptor switching and donor-acceptor interchange tunneling barriers are in excellent agreement with recent ab initio theory, as are cyclic water trimer and tetramer structures and binding energies.

The solvation of sodium ions in water clusters: intermolecular potentials for Na+-H2O and H2O-H2O

New model potentials are constructed for Na+-H2O and H2O-H2O, using quantum mechanical calculations of the monomer wavefunctions. One parameter in each model potential is fitted to experimental dimer properties. The resulting Na+-H2O potential is consistent with published thermodynamic and ab initio data, and the H2O-H2O potential reproduces the structure and energy of the water dimer at equilibrium, and the second virial coefficient of steam, within experimental uncertainties. The donor-acceptor interchange tunnelling pathway on the water dimer potential energy surface has a lower energy barrier than the acceptor-acceptor interchange, in agreement with recent spectroscopic studies. When a simple non-additive induction potential is included, calculated thermodynamic properties of solvated sodium ions are in agreement with experimental data. For small clusters in the gas phase, the first solvation shell of the sodium ion is predicted to contain four water molecules.

Determination of the intermolecular potential energy surface for (HCl)2 from vibration--rotation--tunneling spectra.

An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)2 has been determined by a direct nonlinear least-squares fit to 33 microwave, far-infrared and near-infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (l99l)] and a rigorous four-dimensional dynamical method (described in the accompanying paper). The global minimum (De= -692 cm-1) is located near the hydrogen-bonded L-shaped geometry (R=3.746 angstroms, theta1=9 degrees, theta2=89.8 degrees, and phi=180 degrees). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor-acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm-1) process involves a contraction of 0.1 angstroms in the center of mass distance (R) at the C2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)].

Donor–acceptor interchange tunneling in HDO–DOH and the higher energy HDO–HOD isotopomer

The microwave and submillimeter spectra of the a-type K=0←0 and K=1←1, c-type K=1←0, and isotopically allowed b-type K=1←0 bands of the O–D bonded HDO–DOH water dimer isotopomer and the higher energy O–H bonded HDO–HOD isotopomer have been measured using molecular-beam electric resonance optothermal and pulsed-nozzle Fourier-transform microwave spectrometers. The present results obtained in He and He/Ne seeded molecular beams give the first evidence for the presence of the higher energy O–H bonded mixed protonated-deuterated water dimers. These species were not reported previously in studies using seeded Ar molecular beams. The donor–acceptor interchange tunneling splittings are found to be 1322.1019(43) and 5004.059(20) MHz for the HDO–DOH and the metastable HDO–HOD dimers, respectively. For both isotopomers, the donor-accepter interchange tunneling-state selections rules for the b- and c-type bands are consistent with tunneling pathways corresponding to geared partial internal rotation of the two subunits in double-minima potentials. The larger tunneling splitting in HDO–HOD is primarily the consequence of the smaller effective reduced mass for tunneling in this system compared to that in HDO–DOH. The presence of both b- and c-type K=1←0 bands allows the direct measurement of the largest tunneling splitting, that associated with the internal rotation about the O–H–O or O–D–O bond of the nonbonded proton/deuteron on the proton donating unit. For the K=0 state of HDO–DOH this splitting is 214 208.38(23) MHz, while for the K=0 state of HDO–HOD it is 117 440.97(17) MHz. A strong b-type Coriolis interaction is observed between the upper K=0 and lower K=1 states in HDO–DOH, similar to that observed previously in (H2O)2.

Ab initio search for the equilibrium structure of the ammonia dimer

The equilibrium structure of the NH3 dimer is investigated using large, efficient basis sets, 6–311+G(3d,2p) and [7s5p3d,4s1p] extended with bond functions, at the second-order Mo/ller–Plesset perturbation approximation (MP2) and higher levels. Intermolecular energies and optimized dimer structures are obtained with the full counterpoise correction for the basis set superposition error. The stabilities of two possible equilibrium structures, one containing a nearly linear hydrogen bond with Cs symmetry and the other a cyclic configuration with C2h symmetry, are examined. In a basis without bond functions, the Cs structure is found more stable. As bond functions are added, however, the C2h structure becomes more stable. This establishes the importance of the dispersion energy which is disproportionally underestimated for the C2h structure in a purely nucleus-centered basis. The stability of the C2h structure relative to the Cs is retained at the higher levels up to the complete forth order (MP4SDTQ). The minimum energy path connecting the two equivalent Cs structures via the C2h structure is calculated. The resulting potential curves are extraordinarily flat in a broad region around the C2h structure but rise steeply upon approaching the Cs structure containing a nearly linear hydrogen bond, indicating that the donor–acceptor interchange barrier is absent in the NH3 dimer. The equilibrium structure for the NH3 dimer found in the present study probably has the cyclic form with C2h symmetry.