Spectroscopic Constants and Vibrational Frequencies for l-C3H+ and Isotopologues from Highly-Accurate Quartic Force Fields: The Detection of l-C3H+ in the Horsehead Nebula PDR Questioned

Very recently, molecular rotational transitions observed in the photon-dominated region of the Horsehead nebula have been attributed to l-C3H+. In an effort to corroborate this finding, we employed state-of-the art and proven high-accuracy quantum chemical techniques to compute spectroscopic constants for this cation and its isotopologues. Even though the B rotational constant from the fit of the observed spectrum and our computations agree to within 20 MHz, a typical level of accuracy, the D rotational constant differs by more than 40%, while the H rotational constant differs by three orders of magnitude. With the likely errors in the rotational transition energies resulting from this difference in D on the order of 1 MHz for the lowest observed transition (J = 4 <- 3) and growing as J increases, the assignment of the observed rotational lines from the Horsehead nebula to l-C3H+ is questionable

CL8. Can We Find Interstellar Nitrogen in Exotic Molecules?

The nitrogen molecule is one of the most strongly bound systems known. It also makes up a vast majority of the Earth’s atmosphere. However, the origin of this molecule in the atmosphere is still largely unknown. Is the nitrogen primordial to the Earth or was it formed through biological or geological processes? One way to find out is through observation of stellar systems similar to the early solar system. The chief issue is that N2 is not observable through vibrational or rotational means due to a lack of a permanent or induced dipole moment, and electronic observation is difficult to quantify. As a result, molecular tracers must be observed, instead. Dinitrogen bonds are notoriously unstable making laboratory synthesis and observation is difficult for such species. However, quantum chemical techniques are not hampered in such ways making them ideal for spectral prediction and structural analysis. This work will discuss the structure, binding, and relative energetics of the NNHNN+, NN-HCO+, and CO-HNN+ proton-bound complexes where the proton-shuttle motion is exceptionally bright. Additionally, the rovibrational spectra and relative energies of HNNS radicals will be described where previous work has predicted these species to be intermediates in the formation of N2 and the recently observed SH radical

The Possible Interstellar Anion CH2CN-: Spectroscopic Constants, Vibrational Frequencies, and Other Considerations

It is hypothesized that the A ^1B_1 <- X ^1A' excitation into the dipole-bound state of the cyanomethyl anion (CH2CN-) is proposed as the carrier for one diffuse interstellar band. However, this particular molecular system has not been detected in the interstellar medium even though the related cyanomethyl radical and the isoelectronic ketenimine molecule have been found. In this study we are employing the use of proven quartic force fields and second-order vibrational perturbation theory to compute accurate spectroscopic constants and fundamental vibrational frequencies for ^1A' CH2CN- in order to assist in laboratory studies and astronomical observations

The Failure of Correlation to Describe Out-of-Plane Carbon=Carbon Bending

Carbon-carbon multiply bonded systems are improperly described with standard correlation methods and basis sets. For computations of vibrational modes, the out-of- plane bends can be reported as imaginary at worst or simply too low at best. Utilizing the simplest of aromatic structures (cyclopropenylidene) and various levels of theory, this work diagnoses this known behavior for the first time. A combined 1-particle and n-particle basis set effect conspire to produce these non-physical results. When moving from sp2 to sp3 hybridization in the carbon atoms, the larger number of basis functions overcorrects the energy. This is exacerbated by correlation methods. These allow for occupation of the and orbitals in the expanded wave function that combine with the hydrogen s orbitals. As a result, the improperly described space can be further and non-physically stabilized by post-Hartree-Fock correlation. This represents a fundamental problem with at least Hartree-Fock based methods of all flavors in describing carbon. Beyond being a flaw in quantum chemical theory, other repercussions will be present in computations regarding spectroscopy as well as energy and environmental studies where highly-accurate hydrocabon vibrational transitions or thermochemical data are needed

Nolan to Wesley, 28 June 1957

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