Effects of O18 isotopic substitution on the rotational spectra and potential splitting in the OH–OH2 complex: Improved measurements for O16H–O16H2 and O18H–O18H2, new measurements for the mixed isotopic forms, and ab initio calculations of the A2′-A2″ energy separation

Abstract

Rotational spectra have been observed for (16)OH-(16)OH(2), (16)OH-(18)OH(2), (18)OH-(16)OH(2), and (18)OH-(18)OH(2) with complete resolution of the nuclear magnetic hyperfine structure from the OH and water protons. Transition frequencies have been analyzed for each isotopic form using the model of Marshall and Lester [J. Chem. Phys. 121, 3019 (2004)], which accounts for partial quenching of the OH orbital angular momentum and the decoupling of the electronic spin from the OH molecular axis. The analysis accounts for both the ground ((2)A(')) and first electronically excited ((2)A(")) states of the system, which correspond roughly to occupancy by the odd electron in the p(y) and p(x) orbitals, respectively (where p(y) is in the mirror plane of the complex and p(x) is perpendicular to p(y) and the OH bond axis). The spectroscopic measurements yield a parameter, rho, which is equal to the vibrationally averaged (2)A(')-(2)A(") energy separation that would be obtained if spin-orbit coupling and rotation were absent. For the parent species, rho = -146.560 27(9) cm(-1). (18)O substitution on the water increases /rho/ by 0.105 29(10) cm(-1), while substitution on the OH decreases /rho/ by 0.068 64(11) cm(-1). In the OH-OH(2) complex, the observed value of rho implies an energy spacing between the rotationless levels of the (2)A(') and (2)A(") states of 203.76 cm(-1). Ab initio calculations have been performed with quadratic configuration interaction with single and double excitations (QCISD), as well as multireference configuration interaction (MRCI), both with and without the inclusion of spin-orbit coupling. The MRCI calculations with spin-orbit coupling perform the best, giving a value of 171 cm(-1) for the (2)A(')-(2)A(") energy spacing at the equilibrium geometry. Calculations along the large-amplitude bending coordinates of the OH and OH(2) moieties within the complex are presented and are shown to be consistent with a vibrational averaging effect as the main cause of the observed isotopic sensitivity of rho.

Microwave spectrum of the OD–OH2 complex: A strong deuterium isotope effect on angular momentum quenching in the hydroxyl moiety

“…OH-OH 2 has a C s equilibrium geometry, with the OH bond directed in the vicinity of a lone electron pair on the water, but large amplitude motion of the water through a very small barrier in the C 2v configuration renders the complex effectively planar. Spectroscopically, OH-OH 2 has been observed in matrix infrared experiments [5][6][7] and by microwave spectroscopy in the gas phase [8][9][10]. A theoretical framework applicable to the analysis of its rotational spectrum has also been described [24,25].…”

“…In our laboratory, we have recently completed a set of studies on the microwave spectrum of 16 OH-16 OH 2 and its 18 O isotopologues [8,10]. A central feature of that work was the determination of a spectroscopic parameter, q, which describes the quenching of the OH orbital angular momentum due to loss of cylindrical symmetry upon complexation.…”

“…In the absence of rotation, the energy difference between the 2 A 0 and 2 A 00 states is approximately given by [10,24] DE Eð 2 A 0 Þ À Eð 2 A 00 Þ % À ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi…”

“…The OH-OH 2 complex, in particular, has attracted considerable attention, and indeed a variety of experimental [5][6][7][8][9][10] and computational [6,[10][11][12][13][14][15][16][17][18][19][20][21][22][23] studies have explored the structure, energetics, and reactivity of the adduct. OH-OH 2 has a C s equilibrium geometry, with the OH bond directed in the vicinity of a lone electron pair on the water, but large amplitude motion of the water through a very small barrier in the C 2v configuration renders the complex effectively planar.…”

A diabatic parameterization of the twofold ground state potential energy surface of the H2O-OH molecular complex

“…9,10 Experiments were also performed to analyze finely the shape of the coupled PES around the main minimum. 11 The transition state associated with hydrogen atom transfer from H 2 O to OH was also identified. 10,[12][13][14] All these studies show that the transition gap is located relatively far above the two minima.…”