Benzene is of central importance in combustion chemistry because formation of the first aromatic ring is the rate-limiting step in polycyclic aromatic hydrocarbon (PAH) growth. For these reasons, the fragmentation and isomerization of benzene are highly topical, as are intermediates formed in route to larger rings. Using a combination of broadband and cavity Fourier-transform microwave spectroscopies and newly developed analysis and assignment tools, the discharge products of benzene have been extensively studied in the 2-19 GHz frequency range. In addition to rotational transitions from 30 known species, evidence has been found for eight entirely new hydrocarbon molecules; these species include both branched and chain fragments of benzene, high energy isomers, and larger molecules such as phenyldiacetylene and isomers of fulvenallene; together they account for nearly 1/3 of the observed transitions, and about 1/2 of the spectral intensity. Benzene fragmentation and isomerization, as well pathways that may lead to larger aromatic rings will be discussed in the context of the newly discovered molecules.

With the advent of broadband microwave spectroscopy, rotational spectra can now be routinely acquired of many gigahertz of frequency bandwidth. When applied to chemical mixtures of unknown composition, however, spectral analysis often becomes tedious and time consuming. Electrical discharges are examples of complex mixtures with rich rotational spectra, owing to fragmentation of stable molecules and rapid chemical reactions that subsequently take place in the energetic plasma. In this talk, we describe a workflow - which we have developed in Python - for analyzing the products in a benzene discharge. The workflow is designed to be reproducible, automated, and open-source, and can be applied to help assign both laboratory and astronomical spectral line surveys. As part of this workflow, we will discuss how entirely new molecules - those that give rise to strong rotational lines but whose stoichometry and structure are unknown - can be analyzed and identified with minimal chemical intuition.

Silicon-nitrogen compounds are an important class of molecules, with implications in fields ranging from molecular astrophysics as refractory species in evolved stars, and in terrestrial applications such as chemical vapor deposition. In this talk, we present the gas-phase detection and microwave rotational spectroscopy of two new silicon-nitrogen molecules: silyl isocyanide (SiH₃NC, X̃ ¹A₁) and aminosilane (H₂NSi, X̃ ²B₂). Both species are readily produced in an electrical discharge, combining silane (SiH₄) with either methyl cyanide (CH₃CN) or ammonia (NH₃) to produce the species of interest. Using Fourier-transform and double resonance microwave spectroscopy, we were able to measure the three lowest rotational transitions (at 10, 20, 30 GHz) for SiH₃NC, and for H₂NSi, the two lowest transitions at 30 and 60 GHz. By substituting the precursors for rare-isotope enriched ones (e.g. ¹⁵NH₃), we were able to extend the measurements to several isotopologues: SiH₃¹⁵NC and SiH₃N¹³C for silyl isocyanide, and H₂ ¹⁵NSi, D₂NSi for aminosilane. The experiments are supplemented by high level quantum chemical calculations, which provided predictions of rotational constants, multipole moments, and in the case of aminosilane, the spin-rotation interaction tensor elements.

The recent advent of chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy has dramatically increased the amount of data available to microwave spectroscopists. However, this information is only useful when it can be translated into molecular information, and ultimately chemical knowledge. With such large volumes of information, new, more sophisticated tools are required to fully interpret the data without large increases in research hours required. Several methods, both computational and experimental, have been developed to simplify and automate the assignment of microwave spectra. Such tools can also make microwave spectroscopy far more approachable and usable for non-experts. To manage the expanding data volumes and use cases of microwave spectroscopy, increasingly computationally efficient spectral assignment methods are desirable to enable rapid and complete spectral assignment. We have recently developed a numerically efficient, high-speed program for the prediction and fitting of rotational spectra. This program is built on a simple framework that is applicable to a wide variety of molecules. We will present this program, discuss its performance, and demonstrate its use in fitting CP-FTMW spectra.

Microwave spectroscopy is a widely applied tool for determining molecular structures, studying molecular astrophysics, and in more recent times decomposing complex mixtures. All of these applications are made possible due to the tight connection between the observable rotational transitions and a molecule's rotational constants, which in turn depend on its principal moments of inertia. Electronic structure calculations are often used to provide predictions of rotational constants, although much like other molecular properties, the accuracy depends heavily on the method and basis used. Accurate estimates based on "proper" quantum chemistry require highly correlated methods such as coupled-cluster theory, along with large basis sets that incorporate effects such as core-valence electron correlation and scalar relativity. For larger molecules, this approach remains intractable due to the excessive computational cost, thus empirical scaling of low-cost theoretical constants is highly desirable. In this talk, we will present a large systematic benchmark study comprising 11 commonly used low-cost electronic structure theories, 7 basis sets, and 78 closed-shell species of varying elemental composition. By comparing with experimentally determined values, our analysis ranks the performance of each method/basis combinations in our set, highlighting the best combinations and revealing potential shortcomings and weaknesses of select methods. Finally, we determine empirical scaling constants for each method/basis combination that can be used to account for the effect zero-point vibration on equilibrium rotational constants.

l0pt Figure In Springer's Handbook for Atomic, Molecular, and Optical Physics, William Harter gives a semiclassical theory of Hamiltonian Rotational Energy Surfaces. Therein accurate spectral estimates follow from classical precession integrals, real and complex. An analogy between the phase plane and the phase sphere suggests that integrals along a rotational energy surface can be known to the same precision as familiar standards such as the complete elliptic integral of the first kind. Newly developing integral-differential algorithms allow us to refine calculations on both domains. We will showcase a few results with high symmetry, and explain how they fit into a fully Riemannian perspective, which also improves upon the picture of tunneling phenomena.

Penta-3,4-dienenitrile, (P34DN, H₂C=C=CHCH₂CN), is an intriguing target for detection in the interstellar medium and the atmosphere of Titan because of its structural similarity to known interstellar molecules and because it is a constitutional isomer of the simple aromatic compound, pyridine (c-C₅H₅N). P34DN has recently been synthesized in our group and its millimeter-wave spectrum has been collected from  ∼ 130-375 GHz. Computational studies reveal that P34DN has two stable conformers linked by the internal rotation of the −CH₂CN group and separated by \textless1 kcal/mol. The internal rotation is the lowest fundamental, ν₂₇, in both conformers. Over 800 distinct rotational transitions of the ground vibrational state of the syn conformer (C_s, μ_a=1.7 D, μ_b=3.1 D, CCSD(T)/cc-pVTZ) have been least-squares fit to a single-state centrifugally distorted rotor model using Kisiel’s ASFIT, and analysis of this ground state is ongoing. The anti-clinal conformer of P34DN has C₁ symmetry (μ_a=3.4 D, μ_b=1.9 D, μ_c=0.6 D, CCSD(T)/cc-pVTZ). Rotational transitions for the ground state and the vibrational states ν₂₇, 2ν₂₇, 3ν₂₇, and 4ν₂₇ have been identified. Clear evidence of strong Coriolis coupling between the ground state and ν₂₇ ( ∼ 52 cm⁻¹) has been observed, primarily between K=18 of the ground state and K=14 of the excited state. Coupling is also observed between the v = 1 and v = 2 of ν₂₇. This presentation will discuss the current analysis of the coupling between the ground state and ν₂₇ v = 1 and the current status of their coupled least-squares fit using Pickett’s SPFIT program.

In this study, we collected the 135-375 GHz rotational spectrum of 2-cyanopyridine, a N-heteroatom analog of the interstellar molecule, benzonitrile. 2-Cyanopyridine's strong dipole moment (μ_a = 5.5 D, μ_b = 1.9 D) and the fact that it is a cyano substituted aromatic molecule make it another attractive species for detection by radioastronomy. The ground state of 2-cyanopyridine was fit to a centrifugally distorted single state model using Kisiel’s ASFIT (N_lines  ∼ 6500, σ = 0.043) and primarily includes ^bR_{−1, 1}, ^aR_{0, 1}, and ^bR_{1, 1} type lines. The two lowest fundamentals, ν₃₀ and ν₂₁, display effects of strong Coriolis interactions and require treatment via a two-state model. Discreet local resonances with ∆K_a = 3 perturbation have been seen along with the effects of a strong a-type global perturbation. Currently, using Pickett's SPFIT, around 16,000 distinct rotational transitions for these states have been measured, from K_a = 0 to 49 and J" = 11 to 146, leading to an experimental energy difference of ∆E_30,21=793379.9 MHz ( ∼ 26.5 cm⁻¹, compared to a 30.6 cm⁻¹ B3LYP/6-311+G(2d,p) anharmonic frequency prediction). Six perturbation terms, including G_a, G_b, F_bc and the higher order terms, G_a^J, G_b^J, G_b^K, are currently being treated; and those predicted agree to within 10% of the prediction. This presentation will expand on the progress of the two state least squares fit and full results of the millimeter-wave analysis of 2-cyanopyridine.

The rotational spectrum of trifluoroacetic anhydride (TFAA, CF₃COOCOCF₃) has been observed by chirped-pulse and cavity Fourier transform microwave spectroscopy. Collection and processing of chirped-pulse spectra were expedited through the use of a recently developed automation program, which will be briefly described. Over 250 transitions were recorded between 6 and 18 GHz ranging from J” = 2 to 24. Spectra were readily fit to a Watson A-reduced Hamiltonian with no evidence of internal rotation of the CF₃ groups. The experimental rotational constants and the observation of exclusively b-type spectra are consistent with a staggered TFAA configuration predicted to be the global minimum by calculations at the M06-2X/6-311++G(3df,3pd) level of theory.