Stretching our understanding of C3: Experimental and theoretical spectroscopy of highly excited nν1 + mν3 states (n ≤ 7 and m ≤ 3).

We present the high resolution infrared detection of fifteen highly vibrationally excited nν1 + mν3 combination bands (n ≤ 7 and m ≤ 3) of C3 produced in a supersonically expanding propyne plasma, of which fourteen are reported for the first time. The fully resolved spectrum, around 3 µm, is recorded using continuous wave cavity ring-down spectroscopy. A detailed analysis of the resulting spectra is provided by ro-vibrational calculations based on an accurate local ab initio potential energy surface for C3 (X̃1Σg+). The experimental results not only offer a significant extension of the available data set, extending the observed number of quanta v1 to 7 and v3 to 3, but also a vital test to the fundamental understanding of this benchmark molecule. The present variational calculations give remarkable agreement compared to experimental values with typical accuracies of ∼0.01% for the vibrational frequencies and ∼0.001% for the rotational parameters, even for high energy levels around 10 000 cm-1.

High-level theoretical rovibrational spectroscopy beyond fc-CCSD(T): The C3 molecule.

An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C3 molecule in its electronic ground state (X̃(1)Σg (+)). Special care has been taken in the convergence of the potential relative to high-order correlation effects, core-valence correlation, basis set size, and scalar relativity. Based on the aforementioned PES, several rovibrational states of all (12)C and (13)C substituted isotopologues have been investigated, and spectroscopic parameters based on term energies up to J = 30 have been calculated. Available experimental vibrational term energies are reproduced to better than 1 cm(-1) and rotational constants show relative errors of not more than 0.01%. The equilibrium bond length has been determined in a mixed experimental/theoretical approach to be 1.294 07(10) Å in excellent agreement with the ab initio composite value of 1.293 97 Å. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 1(1), 0)←(0, 0(0), 0) combination band might be observable by high-resolution spectroscopy.

Rovibrational states of N3- and CO2 up to high J: a theoretical study beyond fc-CCSD(T).

An accurate near-equilibrium potential energy surface (PES) has been constructed for the azide ion (N(3)(-)) on the basis of coupled cluster calculations up to CCSDTQ (Kállay, M.; Surján, P. R. J. Chem. Phys. 2001, 115, 2945.), with contributions from inner-shell correlation and special relativity being taken into account as well. A larger number of rovibrational states has been investigated by variational calculations with Watson's isomorphic Hamiltonian for linear molecules. Analogous calculations for CO2 demonstrate the high quality of this type of calculations. The G(v) values of the symmetric stretching and bending vibration of 14N(3)(-) are predicted to be ν1 = 1307.9 cm(-1) and ν2 = 629.3 cm(-1), with an uncertainty of ca. 1 cm(-1). Fermi resonance is less pronounced for the lower polyads of 14N(3)(-) compared with 12C16O2 but is as strong as in CO2 for the lowest diad of isotopologue 15-14-15. The band origin of the antisymmetric stretching vibration of 14N(3)(-) is calculated to be ν3 = 1986.4 cm(-1), only 0.1 cm(-1) lower than the experimental value. The corresponding vibrational transition dipole moment is predicted to be as large as µ = 0.476 D, 46% higher than calculated for CO2. The perturbed combination tone (01(1)1), which was accessible through diode laser IR spectroscopy, undergoes anharmonic interaction with at least two other vibrational states.

Rovibrational states of ClHCl- isotopologues up to high J: a joint theoretical and spectroscopic investigation.

Explicitly correlated coupled cluster theory at the CCSD(T*)-F12b level (T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys., 2007, 127, 221106) and two precise spectroscopic parameters (K. Kawaguchi, J. Chem. Phys., 1988, 88, 4186) were used to construct an accurate near-equilibrium analytical potential energy function (PEF) for the highly anharmonic centrosymmetric hydrogen-bonded complex ClHCl(-) (Re = 3.1153 Å). From variational calculations with that PEF, a large number of rovibrational energies of different isotopologues up to high values of the rotational quantum number J was obtained. Theory helped with the assignment of lines observed by IR diode laser spectroscopy in the ν1 + ν3 combination band of (35)ClH(35)Cl(-) and (37)ClH(35)Cl(-) and enabled us to elucidate rather subtle patterns of rovibrational interactions. Furthermore, transition dipole moments were predicted and analysed as well as unusual isotopic effects.

FHF- isotopologues: highly anharmonic hydrogen-bonded systems with strong Coriolis interaction.

Explicitly correlated coupled cluster theory at the CCSD(T*)-F12b level in conjunction with the aug-cc-pV5Z basis set has been used in the calculation of three-dimensional potential energy and dipole moment surfaces for the bifluoride ion (FHF(-)). An empirically corrected analytical potential energy function (PEF) was obtained by fit to four pieces of accurate spectroscopic information. That PEF was used in variational calculations of energies and wave functions for a variety of rovibrational states of the isotopologues FHF(-), FDF(-), and FTF(-). Excellent agreement with available data from IR laser diode spectroscopy is observed and many predictions are being made. Unusual isotope effects among the spectroscopic constants and unusual features of the calculated line spectra are discussed.

Accurate potential energy surface and calculated spectroscopic properties for CdH2 isotopomers.

Ab initio calculations employing the coupled cluster method CCSD(T), in conjunction with a small-core pseudopotential for the cadmium atom, have been employed to construct a near-equilibrium potential energy function (PEF) and an electric dipole moment function (EDMF) for CdH(2). The significance of the spin-orbit interaction was checked and found to be of minor importance. Making use of two pieces of experimental information for the most abundant isotopomer (114)CdH(2), we obtained a refined PEF, which, within variational calculations of rovibrational states and wave functions, reproduces all available experimental data (S. Yu, A.Shayesteh, and P. F. Bernath, J. Chem. Phys. 2005, 122, 194301) very well. In addition, numerous predictions are made. In particular, the nu(2) band origins for (114)CdH(2) and (114)CdD(2) are predicted at 605.9 and 436.9 cm(-1), respectively, and the state perturbing the e parity levels of the (0,0(0),1) state of (114)CdH(2) at J = 12-17 is identified as the (0,3(3),0) state. Assignments for further perturbations found in the emission spectra are given as well.