Conformationally flexible molecules pose a serious challenge to typical quantum chemistry workflows for rotational spectroscopy. These molecules can have a very large number of conformers whose relative energies differ by only a few hundred wavenumbers, and subtle effects can significantly reorder these relative energies, making it difficult to begin the process of assigning their spectra. Further, different conformer search tools do not produce the same results, and it is very difficult to be certain that all relevant conformers have been found as molecular complexity increases. Once possible conformers have been identified, questions then remain about appropriate theoretical methods for subsequent geometry optimizations to use to best match to experiment. The ROT25 benchmark setGrimme, S. and Steinmetz, M., Phys. Chem. Chem. Phys., 2013, 15(38), 16031-16042 (2013).s limited in molecular size and diversity, and so we are actively working to expand this set to include these larger, more flexible molecules, many of which are from the flavors and perfume industries. In this talk, we will discuss this benchmarking work in more detail, which includes identifying molecules where our typical computational workflows predict rotational constants that differ by more than a few percent from experimental values. We will also discuss comparisons between experimental and calculated structures for four closely-related compounds - geranial (E-isomer of citral), geraniol (alcohol form of geranial), neral (Z-isomer of citral), and nerol (alcohol form of neral).

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Grimme, S. and Steinmetz, M., Phys. Chem. Chem. Phys., 2013, 15(38), 16031-16042 (2013).i

For any spectroscopic technique, the isospectrality problem of determining whether a captured spectrum uniquely defines a molecule is critically important. Rotational spectroscopy provides unique conformational and structural insights, and it has long been considered that a rotational spectrum, when assigned, represents a unique "fingerprint" of a molecule.E. B. Wilson, Microwave Spectroscopy in Chemistry, Science 162, 59-66 (1968)ecent efforts on solving the inverse problem determining the molecular geometry from a set of rotational constants or the rotational spectrum itself led us to revisit the question of the uniqueness of the solution to that problem. We use a funnel-based technique for identifying possible isospectral examples from large datasets of molecular geometries including QM9, QM7x, GEOM, and a subset of PubChem. We present examples where calculated rotational constants of chemically distinct molecules are identical to within the uncertainty of those calculations. The number of twin molecules falls rapidly with increasing number of parameters to be matched, such as rotational constants and dipole moment projections, and with tightening the allowed uncertainties in these parameters. Although the experimental spectra of these twins are most likely distinguishable, their chemical identity cannot be established with nowadays calculations alone, and the inverse problem is ill-posed. These collisions must be resolved with better theoretical methods or additional isotopic substitution or nutation experiments.

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E. B. Wilson, Microwave Spectroscopy in Chemistry, Science 162, 59-66 (1968)R

Broadband rotational spectroscopy is an isomer-, conformer-, and quantum state-specific quantitative technique that is now being used is the gas-phase chemical reaction dynamics and kinetics studies. It is also gaining popularity as an analytical chemistry tool in industrial settings. Richness of data in high-resolution broadband rotational spectra allows for unambiguous assignment of multiple species but requires thorough analysis. Here we are developing an active learning-based approach to assign rotational spectra while optimizing its acquisition. Rotational spectroscopy can require aggregating numerous measurements of a sample gas, often spanning many hours across a wide range of microwave frequencies. The time and quantity required to identify the contents of a sample is an important factor for chemists when choosing a spectroscopic technique. While the standard practice in rotational spectroscopy is to intensively measure microwave band regions in a sequential order, we propose a Bayesian active learning strategy to iteratively jump between band region measurements, leading to a significant time and sample savings for identifying unknown molecules. We suggest that this active learning strategy may be more generally applicable for other kinds of spectroscopies where acquisition time is a significant bottleneck.

Rotational spectroscopy is an incredibly powerful technique for measuring the quantized rotational states of molecules. While rotational spectroscopy can reveal a tremendous amount of molecular information that few spectroscopies can emulate, it is not commonly utilized for general analytical chemistry. But the greatest barriers to wider utilization are not experimental; rather, they are computational. We outline these computational challenges within two classes: those used for the forward and inverse spectral mapping. For these mappings, we introduce four tools: RotoCalc, RotoFit, RotoSearch, and RotoMC. Each of these open-source tools yields state-of-the-art performance for their respective sub-tasks as compared to similar software packages. RotoCalc computes spectral frequencies, intensities, and transition states given rotational constants and dipoles. We demonstrate that, without sacrificing accuracy, RotoCalc shows a roughly four-fold speed increase over both SPCAT and ASROT. We show that RotoFit is robust to peak misassignment and perturbations, RotoMC can recover rotational constants for a given spectrum without spectral labelling, and RotoSearch can rapidly identify a molecule within a database from a given spectrum.

Combining cryogenic ion vibrational spectroscopy and computational chemistry has proven to be a useful approach for determining the molecular structure in a variety of systems. Two key bottlenecks in this analytical method is the generation of a large number of computed vibrational spectra and their comparison to the experimental data. For large and floppy molecules or clusters with many components, there is a wide conformational space to sample for potential structures and performing these calculations at high levels of theory can quickly become computationally expensive. Additionally, since the comparison with the experimental spectrum is done manually, it is not yet well-suited for large datasets of potential structures. Moreover, there is a lack of quantitative metrics to evaluate the agreement between the computed and experimental spectra. I have developed a new automated workflow to define important criteria in the spectra and then use them in an automated spectral comparison program. In this talk, I will demonstrate how well this program works for a system of acetaminophen isomers. Since these isomers have an identical chemical formula and similar functional groups, this system is a good test case to show both some of its strengths and its limitations when working with very similar structures. In sum, this automated workflow will make this technique more suitable for more complicated systems that require an intensive amount of data analysis to make a spectral assignment.

We have investigated the Huggins band photodissociation dynamics of ozone via spin-forbidden channels using imaging of all three spin-orbit states of ³P_J oxygen. Our studies herein provide branching fractions of the spin allowed O₂(X³Σ_g⁻), and spin forbidden O₂(a¹∆_g) and O₂(b¹Σ_g⁺) state channels. Our branching fractions differ from previous works but can be partially attributed to the absence of O₂(b¹Σ_g⁺) state products in the O(³P₁) channel. Calculations have suggested strong J-dependence in the spin-orbit coupling of the spin-forbidden channels to the B state of ozone, and we observe clear experimental evidence of this dependence. The vibrational distributions of the three channels have also been extracted from the data by using rotational information from recent studies. The vibrational distributions peak around v=0, in agreement with spin-allowed calculations. Slight deviations are observed with increasing wavelength, which is suspected to be due to features on the potential energy surfaces of the spin-forbidden states which should motivate future dynamical calculations. Angular distributions from the data have shown state dependent anisotropy parameters of β=0.89|0.54|0.53 for the three product channels, which have been analyzed to determine possible differences in critical geometry using the non-axial recoil model.

New experimental apparatus and techniques enable rotational spectra to be collected at unprecedented speeds. This increase in data throughput has not come with a corresponding growth in analysis throughput. There are a number of programs which currently seek to automate the assignment of rotational spectra in the microwave regime, but as yet there are none designed to work in the millimeter/submillimeter regime. Rebelfit (Rotational Extended Broadband Experimental Line FITter) uses spectral stacking in a global fit to improve mm/submm rotational constants from ab initio or a priori constants. The design of the Rebelfit program and the results of benchmarking tests will be presented.

New millimeter/submillimeter spectra of magnesium chloride (MgCl) have been recorded in two electronic excited states that have not been observed before experimentally. In addition to the X ²Σ⁺ ground state, well-studies for decades, two ²Σ rotational patterns have been recorded with equilibrium rotational constants of B_e = 5522.03(62) MHz and B_e = 6369.99(28) MHz, respectively, derived from the observation of multiple vibrational satellite lines. In contrast, the ground state has B_e near 7350 MHz. Given the ∆S = 0 selection rule, it is interesting that these two states are observed at all with direct absorption methods. Furthermore, the state with the smaller rotational constant vanishes above N=30 – 29, while the latter only appears at N=26 – 25 and above. The derived equilibrium bond lengths are r_e = 2.536 Å and 2.361 Å, which agrees roughly with theoretical bond lengths from past work for the outer minimum of the (3) ²Σ state, a double well potential, and the (4) ²Σ state (Abu el kher et al. 2019). In an effort to further understand the origin of these states, highly accurate MRCISD+Q/aug-cc-pV5Z-DK computations have been conducted. Potential energy curves were obtained for the ground electronic state and 14 doublet excited states. Spin-orbit coupled energy curves were also computed via the Breit-Pauli formalism. While most aspects of the energy profile match calculations reported by other groups, some interesting differences in the avoided crossings of Ω components are noted. Discrete Variable Representation calculations incorporating vibronic coupling are necessary to resolve the experimentally observed break in rotational patterns and are currently in progress.

Understanding molecular structure and favoured conformations or stereochemistry is pivotal in many fields of chemistry. In this talk, the synergy between theory and experiment is highlighted as it provides new insights into the conformation of acetic anhydride, CH₃COOCOCH₃. The microwave spectrum of the parent and D6 isotopologues, as well as a series of preliminary calculations were previously reported at this conference. Two observations distinguish this system from related anhydrides [N. Love et al. J. Mol. Spectrosc., 397, 111844 (2023)]. First, the spectrum unambiguously indicates a nonplanar trans conformation. This is in sharp contrast with related systems which adopt a nonplanar cis geometry. Second, the agreement between experimental and calculated rotational constants and internal rotation barriers is of lower quality than that obtained for other systems. Calculations at the M06-2X/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory confirm and extend previous calculations suggesting that the observed conformation is driven by an interaction between a methyl hydrogen on one side of the molecule and the carbonyl oxygen on the other side, forming an intramolecular six-membered ring. Moreover, there is a highly irregular dependence of the total electronic energy on the internal rotation coordinate of the ring methyl group (α). Thus, functional form of V(α) is distinctly different from the usual (1/2)(1-cosα) representation, which may give rise to errors embedded in the fitted barrier. Close examination of the calculated structure further reveals a flexing of the heavy atom backbone and an abrupt switching of the hydrogen involved in the ring as the CH₃ proceeds along its internal rotation coordinate. This causes up to 8% changes in the rotational constants, an effect that is absent for cis conformers, for which both methyl groups are on the periphery of the molecule. Such changes likely exacerbate the expected differences between equilibrium and vibrationally averaged values. Interestingly, the discrepancies between theoretical and calculated barrier are found to much smaller for the fully deuterated derivative. Finally, the calculations do find a cis conformer for this system, but the energy difference between the cis and trans forms is too small to be reliably determined from theory. However, their interconversion should be facile and thus, the observation of the trans form at the low temperature of a supersonic jet strongly suggests that it represents the global energy minimum.

Room temperature microwave spectra of 1,1-difluorobutene in a low-pressure static cell have been recorded and analyzed in the 18−26 GHz range, leading to a congested data set containing transitions of multiple states for this flexible molecule. Transitions belonging to at least 4 different states have been identified, with assignment and fitting in progress. While fitted rotational constants indicate that all assigned spectra belong to a single conformer of the butene backbone, methyl group internal rotation and large amplitude inversion motions are both possible within the most stable conformer. In addition to large amplitude motions, other low energy vibrational modes may be excited with sufficient population to observe in the room temperature sample. Extensive ab initio investigations at the MP2/cc-pVTZ level of theory/basis set, have been performed to help unravel the complex potential energy landscape and identify the carriers of the multiple rotational spectra that have been assigned for 1,1-difluorobutene. Both anharmonic frequency calculations and estimates of barriers to large amplitude motions have proven crucial to developing a full understanding of this flexible molecule.