Truncated, correlated, wave function methods either produce imaginary frequencies (in the extreme case) or nonphysically low frequencies in out-of-plane motions for carbon and adjacent atoms when the carbon atoms engage in π bonding. Cyclopropenylidene is viewed as the simplest aromatic hydrocarbon, and the present as well as previous theoretical studies have shown that this simple molecule exhibits this behavior in the two out-of-plane bends (OPBs). This nonphysical behavior has been treated by removing nearly linear dependent basis functions according to eigenvalues of the overlap matrix, by employing basis sets where the spd space saturatation is balanced with higher angular momentum functions, by including basis set superposition/incompleteness error (BSSE/BSIE) corrections, or by combining standard correlation methods with explicitly correlated methods to produce hybrid potential surfaces. However, this work supports the recently described hypothesis that the OPB problem is both a method and a basis set effect. The correlated wave function's largest higher-order substitution term comes from a π → π* excitation where constructive interference of both orbitals artificially stabilizes the OPB. By employing schema to overcome this issue, the symmetric OPB ν is the predicted to be the second-brightest transition, and it will be observed very close to 775 cm. However, more work from the community is required to formulate better how carbon atoms interact with their adjacent atoms in π-bonded systems. Such bonds are ubiquitous in all of chemistry and beyond.

The search for possible, natural, noble gas molecules has led to quantum chemical, spectroscopic analysis of NeCCH, ArCCH, and ArCN. Each of these systems has been previously shown to be a stable minimum on its respective potential energy surface. However, no spectroscopic data are available for laboratory detection or interstellar observation of these species, and the interstellar medium may be the most likely place in nature where these noble gas cations are found. The bent shape of NeCCH is confirmed here with a fairly large dipole moment and a bright C-H stretching frequency at 3101.9 cm. Even if this molecule is somewhat unstable, it is likely observable now that the spectral ranges of where to look have been established. ArCCH is much more stable but has dim double harmonic intensities for the vibrational fundamentals and a dipole moment below 0.5 D making its rotational transitions likely buried in the astronomical weeds. Even so, ArCCH cannot be excluded as a possibility in laboratory experiments of hydrocarbons in argon-rich environments. ArCN, on the other hand, has a dipole moment of greater than 3.5 D, an observable C-N stretching fundamental at 2189.6 cm (4.567 microns), and a viable formation pathway through HCN, a highly-abundant interstellar molecule. Consequently, these molecules containing noble gas atoms are spectroscopically classified at high-level for the first time and may be present in observable regions of outer space.

No abstract