We describe a new interactive program (DAPPERS: Data Analysis Package for Productive and Enthusiastic Rotational Spectroscopists) for the rapid processing of dense and complex rotational spectra of asymmetric rotors. DAPPERS is based on an intelligent algorithm that quickly and accurately locates spectral progressions with user-identified quantum number identities and then seamlessly integrates with Pickett’s SPCAT and SPFIT programs to allow complete assignment and fitting. The program has been shown to be proficient across the full range of asymmetry parameters and can produce final fits containing hundreds of transitions of any type (a, b, or c) or branch (P, Q, or R) in just a few minutes. The software interacts with the user through a simple graphical user interface, and includes a peak-finder with adjustable baseline drift compensation, as well as a number of visualization features. It is designed for easy installation and use, with special attention given to the design of a single executable file platform. It is available for download, together with extensive documentation, at www.chem.umn.edu/groups/kleopold.

Microwave spectra of pivalic anhydride ((CH₃)₃CCOOCOC(CH₃)₃) and pivalic – trifluoroacetic anhydride ((CH₃)₃CCOOCOCF₃) have been observed by chirped-pulse and cavity Fourier transform microwave spectroscopy. For pivalic anhydride, eight isotopologues corresponding to substitution on each atom in the heavy atom frame have been observed. For the pivalic – trifluoroacetic anhydride, ten isotopologues have been analyzed, and Kraitchman analyses provide accurate structural information about the heavy atom frame for both systems. The results indicate that the carbonyl groups are not coplanar, but rather, form dihedral angles of 53.9(39)^° in pivalic anhydride and 46.5(16)^° in pivalic – trifluoroacetic anhydride. These results are in good agreement with M06-2X/6-311++G(3df,3pd) calculations. Calculations at the same level indicate that the barriers to the carbonyl groups twisting through a planar configuration are 1.7 kcal/mol and 0.7 kcal/mol for pivalic and pivalic - trifluoroacetic anhydrides, respectively. Spectra for both forms of the parent species were assigned and fit in under five minutes using the new rotational spectral fitting program DAPPERS, and a brief synopsis of the process will be presented. A straightforward synthesis for the mixed pivalic – trifluoroacetic anhydride, which should be applicable to other anhydrides, will also be described.

Microwave studies of hydrated acid anhydrides (RCOOCOR-H₂O) provide an interesting venue for characterizing local minima on the potential energy landscape of reactive systems. This talk reports the microwave spectra of the monohydrates of pivalic anhydride (R = R' = C(CH₃)₃), trifluoracetic anhydride (R = R' = CF₃), and the mixed pivalic-trifluoroacetic anhydride (R = C(CH₃)₃, R' = CF₃). While the ultimate fate of these species in bulk aqueous solution is hydrolysis to form the parent acids, this work captures the precursor complexes and investigates the effect of R and R' on the geometry of the monohydrate. Microwave spectra of the parent species and their D₂O and DOH isotopologues are reported, as is an in-depth computational analysis at different levels of theory. Calculated structures show varying degrees of hydrogen bonding and electrophilic-nucleophilic interaction, depending on the particular combination of R and R' groups.

Buckminsterfullerene, C₆₀, is the largest molecule for which quantum state resolved spectra have been observed, marking an important step towards quantum control of complex polyatomic systems. The first high resolution experiments, made possible by a combination of cavity-enhanced direct frequency comb spectroscopy and buffer-gas coolingP. B. Changala, M. L. Weichman, K. F. Lee, M. E. Fermann, and J. Ye, Science 363, 49 (2019). revealed detailed insights into the rovibrational structure of C₆₀, while also posing several outstanding spectroscopic questions. To address these, we have constructed a new spectrometer targeting the 8.5 μm vibrational band based on a continuous-wave quantum cascade laser (QCL). Linewidth narrowing via optical feedback stabilization is used to efficiently couple QCL light into a high-finesse optical cavity, providing high absorption detection sensitivity and a 100-fold improvement over the previous comb measurements. This talk will focus on new observations of low-J transitions, rovibrational perturbations, and saturated absorption effects. We will also discuss progress towards high resolution measurements of electronically excited C₆₀. The extraordinarily precise spectroscopic information revealed by such experiments presents new challenges for modern quantum chemistry and high accuracy ab initio spectroscopy of truly many-electron systems.

---

Footnotes:

P. B. Changala, M. L. Weichman, K. F. Lee, M. E. Fermann, and J. Ye, Science 363, 49 (2019).,

Porphyrins are ubiquitous macrocycles in biology, where they perform a variety of functions ranging from behaving as chromophores in proteins to molecular/electron shuttles. Porphyrins often incorporate a metal center and have substituents on the macrocycle periphery to allow this versatility. These species have been studied for decades, using many techniques primarily focused on solution and solid phase environments. While such techniques yield much useful information, the chemical environment and the temperatures involved in such studies usually lead to solvatochromic shifts, broadening of spectral lines, and spectral congestion. Even spectra obtained in cryogenic matrices exhibit shifts whose magnitude and even direction are hard to predict. As a consequence, the intrinsic photophysical properties of these molecules, i.e., in the absence of effects caused by chemical environments, have been elusive. We are able to circumvent these difficulties by studying cryogenically prepared, mass selected ions. Here we report the electronic and infrared spectra of metal free protoporphyrin mono- and dianions. We interpret the experimental data in the framework of quantum chemical calculations.

The Green Fluorescent Protein (GFP) is one of the most widely used fluorescent markers in bioimaging, and much work has been dedicated to understanding the electronic structure of its anionic chromophore in solution, protein, and in vacuo. A model for this chromophore (deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone, HBDI , see Figure 1) contains the complete conjugated system, and replaces the linkers to the protein structure with methyl groups. Previous work by many groups has established that the initial photophysics of the chromophore upon excitation is likely to be similar in the protein and in vacuo, but quite different from aqueous solution.S. Brøndsted Nielsen, A. Lapierre, J. U. Andersen, U. V. Pedersen, S. Tomita, L.H. Andersen, Phys. Rev. Lett. 87 (2001) 228102 W. Zagorec-Marks, M. M. Foreman, J. R. R. Verlet, J. M. Weber, J. Phys. Chem. Lett. 10 (2019) 7817-7822 All previous spectroscopy in vacuo has been performed at room temperature, which introduces spectral congestion through hot bands. By studying this chromophore as a cryogenically prepared, mass-selected ion, we are able to provide unprecedented resolution in the electronic band origin region of the S₁ ← S₀ electronic transition. Figure Figure 1: Structure of HBDI⁻.

---

Footnotes:

S. Brøndsted Nielsen, A. Lapierre, J. U. Andersen, U. V. Pedersen, S. Tomita, L.H. Andersen, Phys. Rev. Lett. 87 (2001) 228102

---

Footnotes:

Solvation plays an important role in the function of fluorescent proteins. Many of these proteins contain a functional water molecule in the chromophore pocket, which can influence the electronic properties of the chromophore. In the case of the Green Fluorescent Protein (GFP), a single water molecule is coordinated to the phenolate group of the chromophore, raising questions about the effect this has on the electronic spectrum of GFP. Here, we present the electronic and infrared spectra of a model system for the GFP chromophore in complexes with up to two water molecules.

Catalytic reduction of CO₂ to produce chemical fuels is a viable strategy for meeting the growing demand for renewable energy, but better catalysts are needed to perform this chemistry on an industrial scale. The first step in designing improved catalysts is characterizing the structure of bare catalyst species. Nickel cyclam has been shown to act as an efficient and selective molecular catalyst for CO₂ reduction

Large molecules, molecules with about 15 or more heavy atoms, have high angular momentum quantum states populated even at the low temperatures produced by seeded pulsed jet expansion. The rotational spectroscopy of these molecules is often more easily described using semiclassical energy expressions and selection rules. One prominent feature of the rotational spectra of molecules in the semiclassical limit is a set of strong, equally spaced transitions that correlate to the collapse of the well-known quartets in the rotational spectra of asymmetric top molecules. These quartets occur for classically stable rotation about the principal axis with both the smallest and largest moments-of-inertia. Over several measurements of the broadband, chirped-pulse Fourier transform microwave spectra of large molecules with semiclassical limit spectra, we have noticed that the quartets for motion around the a-principal axis (prolate quartets) frequently show very fast decay of the free-induction decay (FID) relative to the oblate quartet transitions and the “normal” asymmetric top rotational transitions. To further explore this effect, we have performed Hahn echo experiments that show the rapid decay is caused by dephasing instead of population relaxation. It has also been observed that these transitions do not show significantly faster FID dynamics in a coaxial nozzle cavity FTMW instrument. A summary of results over a series of molecules and possible physical origins of the short dephasing times of the prolate quartets will be presented.

Microwave spectra have been observed for the dihydrate and trihydrate of triflic acid (CF₃SO₃H-(H₂O)₂ and CF₃SO₃H-(H₂O)₃). For the dihydrate, spectra for CF₃³⁴SO₃H-(H₂O)₂ and CF₃SO₃H-(D₂O)₂ were also identified. The fitted rotational constants for both the triflic acid dihydrate and trihydrate are in good agreement with those predicted from their respective global minimum structures at the M06-2X/6-311++G(3df,3pd) level of theory. The two water molecules in the dihydrate bind to triflic acid in a cyclic arrangement to form an 8-membered ring, resembling an insertion of a second water molecule into the weak hydrogen bond of the triflic acid monohydrate. For the triflic acid trihydrate, however, rather than forming a 10-membered ring from the insertion of a third water molecule into the dihydrate geometry, the global minimum structure represents a hydronium triflate ion pair solvated by two water molecules ((CF₃SO₃⁻H₃O⁺)-(H₂O)₂). A summary of the sequential hydration of triflic acid leading to complete proton transfer will be discussed.

Gas phase trimethylammonium triflate, (CH₃)₃NH⁺ - ⁻OSO₂CF₃, has been observed by microwave spectroscopy. The ion pair was produced by on-the-fly mixing of trimethylamine and the superacid triflic acid in a supersonic jet. An initial fit with unresolved ¹⁴N hyperfine structure was obtained within several minutes from the chirped-pulse spectrum using a new fitting program for dense spectral, DAPPERS, and the nitrogen hyperfine structure was then measured by cavity FTMW spectroscopy. Rotational constants are in good agreement with those calculated at the M06-2X/6-311++G(3df,3pd) level. The good agreement, together with the small observed ¹⁴N quadrupole coupling constants, indicate essentially complete proton transfer from the triflic acid to the trimethylamine. Ion pair formation in un-solvated acid – base complexes, while not unprecedented, is unusual and likely related to the superacidity of triflic acid.