Electronic structures and rovibronically averaged geometries of the X 6Ai' and A 6Ai" states of FeOH.

We have recently reported a theoretical prediction of the rovibronic spectra of the FeOH molecule. These spectra have not been observed experimentally. In the present work, we complement the previously published information by reporting the details of the electronic structure of FeOH together with rovibrationally averaged structural parameters. The electronic ground state is X (6)A(i)', which is Renner-degenerate with the A (6)A(i)" state; the two states correlate with a (6)Delta state at linearity. We have calculated the three-dimensional potential energy surfaces (PESs) of the X and A states, which are close in energy over the range of geometries studied, at the MR-SDCI+Q+E(rel)/[Roos ANO (Fe), aug-cc-pVQZ (O, H)] level of theory. The equilibrium structure of the X state is bent with r(e)(Fe-O)=1.806 A, r(e)(O-H)=0.952 A, and angle(e)(Fe-O-H)=134.2 degrees. The barrier to linearity is 273 (266) cm(-1) in the X (A) state so that FeOH is quasilinear in the X and A states. The Fe-O bonds in both states are ionic and the bending potentials are shallow, resulting in large amplitude bending motion. The rovibrationally averaged structures of the X (6)A' and A (6)A" electronic states have been calculated for the average of the X and A PESs by the variational MORBID method as expectation values in terms of rotation-vibration wave functions. FeOH is said to be quasilinear, but the rovibrationally averaged structure is bent with (0)=1.805 A, (0)=0.967 A, and (0)=141(14) degrees (where the quantity in parentheses is the quantum mechanical uncertainty), which is close to the equilibrium structure. We demonstrate that by means of the Yamada-Winnewisser quasilinearity parameter we can distinguish linear and quasilinear molecules.

The rotational spectrum of H(32)SOH and H(34)SOH above 1 THz.

Accurate spectral data of H(32)SOH and H(34)SOH at 1.3 THz were recorded using a synthesizer based multiplier spectrometer. The spectra were analyzed together with data from an earlier study which contain measurements at 1.9 THz. The combination of both data sets allows to determine experimentally the tunneling splitting of energy levels with K(a) = 4 and 5 for the first time. The obtained results are essential to test a novel model on torsional tunneling splitting in HSOH. Transitions with K(a) = 1 <-- 0, K(a) = 2 <-- 1, and K(a) = 3 <-- 2 all exhibit strong c-type and somewhat weaker b-type transitions. In contrary, transitions with K(a) = 4 <-- 3 display only c-type but no b-type transitions. The absence of b-type transitions is completely unexpected and yet not well understood. For the H(34)SOH isotopolog the data set has been substantially extended by the new measurements of (r)Q(3)-branch transitions at 1.3 THz. Based on the new data the accuracy of the H(34)SOH molecular parameters has been significantly improved.

Higher energy states in the CO dimer: millimeter-wave spectra and rovibrational calculations.

New extensive millimeter-wave measurements of the 12C16O dimer have been made, and more than 300 new spectral transitions have been observed in the frequency range 81-135 GHz. A joint analysis of these and previous millimeter-wave data yielded the precise location of 33 new energy levels of A+ symmetry and 20 levels of A- symmetry. These energy levels are located at 8-18 cm(-1) above the zero-point level. Some of them belong to already known stacks, and others make up 9 new stacks of the dimer. Newly determined stacks have K=0, 1, and, for the first time, 2, where K is the projection of the total angular momentum on the intermolecular axis. The energy levels from accompanying rovibrational calculations with the use of a recently developed hybrid CCSD(T)/DFT-SAPT potential are in very good agreement with experiment. Analysis of the calculated wave functions revealed that two new stacks of A+ symmetry with K=2 correspond to overall rotation of the dimer while the other newly observed stacks belong to the geared bend overtone modes. The ground vibrational states of the two "isomers" found are more or less localized at the two minima in the potential surface, whereas all the geared bend excited states show a considerable amount of delocalization.

Gas-phase detection of HSOH: synthesis by flash vacuum pyrolysis of di-tert-butyl sulfoxide and rotational-torsional spectrum.

Gas-phase oxadisulfane (HSOH), the missing link between the well-known molecules hydrogen peroxide (HOOH) and disulfane (HSSH), was synthesized by flash vacuum pyrolysis of di-tert-butyl sulfoxide. Using mass spectrometry, the pyrolysis conditions have been optimized towards formation of HSOH. Microwave spectroscopic investigation of the pyrolysis products allowed-assisted by high-level quantum-chemical calculations--the first measurement of the rotational-torsional spectrum of HSOH. In total, we have measured approximately 600 lines of the rotational-torsional spectrum in the frequency range from 64 GHz to 1.9 THz and assigned some 470 of these to the rotational-torsional spectrum of HSOH in its ground torsional state. Some 120 out of the 600 lines arise from the isotopomer H(34)SOH. The HSOH molecule displays strong c-type and somewhat weaker b-type transitions, indicating a nonplanar skew chain structure, similar to the analogous molecules HOOH and HSSH. The rotational constants (MHz) of the main isotopomer (A=202 069, B=15 282, C=14 840), determined by applying a least-squares analysis to the presently available data set, are in excellent agreement with those predicted by quantum-chemical calculations (A=202 136, B=15 279, C=14 840). Our theoretical treatment also derived the following barrier heights against internal rotation in HSOH (when in the cis and trans configurations) to be V(cis) approximately equal to 2216 cm(-1) and V(trans) approximately equal to 1579 cm(-1). The internal rotational motion results in detectable torsional splittings that are dependent on the angular momentum quantum numbers J and K(a).