THE WATER–CARBON MONOXIDE DIMER: NEW INFRARED SPECTRA, AB INITIO ENERGY LEVEL CALCULATIONS, AND A CURIOUS INTERMOLECULAR MODE

Weakly-bound H$_{2}$O-CO has a planar structure with approximately co-linear heavy atoms (O, C, O) and a hydrogen bond between the water and the carbon of the CO. Proton tunneling (H atom interchange) gives rise to two states corresponding to distinct nuclear spin modifications. The magnitude of the splitting in the ground rotational state is about 0.8 \wn for H$_{2}$O-CO and 0.04 \wn for D$_{2}$O-CO. Due to the almost linear heavy atom configuration, H$_{2}$O-CO has a large A rotational constant, equal to about 19 \wn (12 \wn for D$_{2}$O-CO), so the K quantum number is highly significant. Water-CO was first studied in the microwave and millimeter regions. Infrared spectra have been observed in the regions of the C-O stretch, the O-H stretch, the D$_{2}$O bend, and the H$_{2}$O bend. Here we study the O-D stretch region (3.6 $\mu$m) for the first time, observing D$_{2}$O-CO, HOD-CO, and DOH-CO. We also extend the C-O stretch region results to include the K = 1 $\leftarrow$ 0 subbands, thus determining A rotational constants for the v(CO) = 1 excited state. But more significantly, we also observe additional K = 1 $\leftarrow$ 0 combination bands in both regions which involve the lowest intermolecular vibration of water-CO. This mode, which lies at 43 – 49 \wn depending on isotopologue, can be identified as the in-plane CO bend. It is observed for H$_{2}$O-CO, D$_{2}$O-CO, and HOD-CO, and exhibits anomalous isotope shifts: even though their A-values are quite different, the D$_{2}$O-CO mode is only slightly lower in energy than that of H$_{2}$O-CO. Detailed rotational energy level calculations, based on a recent high-level ab initio potential energy surface \footnote {Y. N. Kalugina, A. Faure, A. van der Avoird, K. Walker, and F. Lique, Phys. Chem. Chem. Phys. 20, 5469 (2018).}, are in good agreement with experiment, including the newly observed intermolecular mode. As well, the calculations show that the unobserved K = 0 level of this mode lies above the observed K = 1 level, thus explaining the anomalous isotope shifts

THE CO–(D2O)2 AND CO–(D2O)3 COMPLEXES: INFRARED SPECTRA AND STRUCTURAL CALCULATIONS

The weakly-bound CO–(D$_2$O)$_2$ and CO–(D$_2$O)$_3$ complexes have been studied in the C-O stretching fundamental of the CO monomer. The van der Waals complexes are generated in a supersonic slit-jet apparatus and probed using a quantum cascade laser. One band was observed and analysed for each complex. The trimer, CO–(D$_2$O)$_2$, band is composed of a/b-type transitions establishing that the CO monomer lies nearly in the a-b inertial plane. The observed rotational constants lead to a small value of the inertial defect indicating that the heavy atoms in the trimer are co-planar. We observe no evidence of tunneling splitting and conclude that the large amplitude tunneling that exists in the free D$_2$O dimer is quenched by the presence of the CO monomer. The CO–(D$_2$O)$_3$ band is also composed of a/b-type transitions establishing that the CO monomer lies nearly in the a-b inertial plane.\\ Theoretical calculations were performed to find minima on the potential energy surfaces for both complexes at B2PLYP-D3BJ level of theory and applying counterpoise correction for the basis set superposition error. Further optimisations were then carried out at different coupled cluster levels of theory and extrapolating to the complete basis set limit. The rotational parameters at CCSD(T*)-F12c level of theory give results in very good agreement with those obtained from the observed spectra. In both complexes, the experimental structure corresponds to the lowest energy isomer.\\ The corresponding bands for CO–(H$_2$O)$_2$ and CO–(H$_2$O)$_3$ are significantly predissociated which hampers their detailed rovibrational analysis

A hamiltonian to obtain a global frequency analysis of all the vibrational bands of ethane

The interest in laboratory spectroscopy of ethane stems from the desire to understand the methane cycle in the atmospheres of planets and their moons and from the importance of ethane as a trace species in the terrestrial atmosphere. Solar decomposition of methane in the upper part of these atmospheres followed by a series of reactions leads to a variety of hydrocarbon compounds among which ethane is often the second most abundant species. Because of its high abundance, ethane spectra have been measured by Voyager and Cassini in the regions around 30, 12, 7, and 3 $\mu$m. Therefore, a complete knowledge of line parameters of ethane is crucial for spectroscopic remote sensing of planetary atmospheres. Experimental characterization of torsion-vibration states of ethane lying below 1400 cm$^{-1}$ have been made previously \footnote{N. Moazzen-Ahmadi and J. Norooz Oliaee, J. Quant. Spectrosc. Radiat. Transfer, submitted.}, but extension of the Hamiltonian model for treatment of the strongly perturbed \nub{8} fundamental and the complex band system of ethane in the 3 micron region requires careful examination of the operators for many new torsionally mediated vibration-rotation interactions. Following the procedures outlined by Hougen \footnote{J.T. Hougen, Can. J. Phys., 42, 1920 (1964)}{$^,$} \footnote{J. T. Hougen, Can. J. Phys., 43, 935 (1965)}, we have re-examined the transformation properties of the total angular momentum, the translational and vibrational coordinates and momenta of ethane, and for vibration-torsion-rotation interaction terms constructed by taking products of these basic operators. It is found that for certain choices of phase, the doubly degenerate vibrational coordinates with and symmetry can be made to transform under the group elements in such a way as to yield real matrix elements for the torsion-vibration-rotation couplings whereas other choices of phase may require complex algebra. In this talk, I will discuss the construction of a very general torsion-vibration-rotation Hamiltonian for ethane, as well as the prospect for using such a Hamiltonian to obtain a global frequency analysis (based in large part on an extension of earlier programs and ethane fits$^a$ from our laboratory) of all the vibrational bands of ethane at or below the 3-micron region

Infrared spectra of C2H4 dimer and trimer

Spectra of ethylene dimers and trimers are studied in the nu11 and (for the dimer) nu9 fundamental band regions of C2H4 (~2990 and 3100 cm-1) using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The deuterated trimer has been observed previously, but this represents the first rotationally resolved spectrum of (C2H4)3. The results support the previously determined cross-shaped (D2d) dimer and barrel-shaped (C3h or C3) trimer structures. However, the dimer spectrum in the nu9 fundamental region of C2H4 is apparently very perturbed and a previous rotational analysis is not well verified.Comment: 21 pages, 4 figure

Three new infrared bands of the He-OCS complex

Three new infrared bands of the weakly-bound He-OCS complex are studied, using tunable lasers to probe a pulsed supersonic slit jet expansion. They correspond to the (0400) <-- (0000), (1001)<-- (0000), and (0401) <-- (0000) transitions of OCS at 2105, 2918, and 2937 cm-1, respectively. The latter band is about 7900 times weaker than the previously studied OCS nu1 fundamental. Vibrational shifts relative to the free OCS monomer are found to be additive. Since carbonyl sulfide has previously been shown to be a valuable probe of superfluid quantum solvation effects in helium clusters and droplets, the present results could be useful for future studies of vibrational effects in such systems.Comment: 16 pages, 1 figure, 4 table

SPECTRA OF C6H6-Rgn (n=1,2) IN THE 3 MIRCON INFRARED BAND SYSTEM OF BENZENE

Benzene-noble gas complexes were one of the earliest topics of interest in spectroscopic investigation of van der Waals (vdW) complexes. Smalley et al.{\footnote{S. M. Beck, M. G. Liverman, D. L. Monts and R. E. Smalley, J. Chem. Phys. 70, 232 (1979).}} observed C$_6$H$_6$-(He)$_{1,2}$ vdW complexes in the late 1970s by means of electronic spectroscopy. A recent study on the same species was done by Hayashi and Oshima{\footnote{M. Hayashi and Y. Ohshima , Chem. Phys. 419, 131 (2013).}} at higher resolution (250 MHz). Here, we present an extensive infrared observation of C$_6$H$_6$-Rg$_n$ (n=1,2) with the rare gas being He, Ne, or Ar, in the 3 micron region. The spectra were observed using a tunable optical parametric oscillator to probe a pulsed supersonic-jet expansion from a slit nozzle.\\ Benzene monomer is known to have a complex band system in this region.{\footnote{J. Pliva and A.S. Pine, J. Mol. Spectrosc. 126, 82 (1987).}} The strongest band, centered around 3047.91 \wn, belongs mainly to the C-H stretching fundamental $\nu_{12}$ of symmetry E$_{1u}$. Other strong perpendicular bands occurring just above the main band as a result of intensity borrowing via anharmonic resonances between the fundamental $\nu_{12}$ and the combinations are $\nu_{2}+\nu_{13}+\nu_{18}$, occurring near 3079 \wn, and $\nu_{13}+\nu_{16}$ and $\nu_{3}+\nu_{10}+\nu_{18}$, both occurring near 3100 \wn. The latter two bands are separated by merely 1.45 \wn. Although data analysis and observation are presently ongoing, we observe analogous bands for C$_6$H$_6$-Rg$_n$ (n=1,2). Spectra were assigned to a symmetric top with C$_{6v}$ symmetry with the rare gas atom being located on the C$_6$ symmetry axis. Spectra of the C$_6$H$_6$-Rg$_2$ trimers are in agreement with a D$_{6h}$ symmetry structure, where the rare gas atoms are positioned above and below the plane of the Benzene monomer. Although jet conditions have resulted in excellent signal to noise for the dimer and trimer spectra, we have not been able to identify any lines which might be due to tetramers or larger clusters. We intend to pursue the search for large clusters using a cooled nozzle