Semi-experimental structures (r_e^SE) are derived from experimental ground-state rotational constants combined with theoretical vibrational corrections. They permit a meaningful comparison with equilibrium structures based on high-level ab initio computations. Typically, the vibrational corrections are evaluated by second-order vibrational perturbation theory (VPT2). The amount of error introduced by this approximation is generally thought to be small; however, it has not been thoroughly quantified. Herein, we assess the accuracy of the theoretical vibrational corrections by extending the treatment to fourth-order (VPT4) for a series of small, linear molecules. When possible, comparisons to exact variational results are also made. Typical corrections to bond distances are on the order of 10⁻⁵ Å. Treatment of vibrational effects beyond VPT2 will thus be important when one wishes to know bond distances confidently to four decimal places. More substantial corrections (10⁻⁴ Å) are seen for HNC, CNCN, and NCCN. Certain molecules with shallow bending potentials, e.g., HOC⁺, are not amenable to a VPT2 description and are not improved by VPT4.

Molecular structures help determine spectroscopic parameters that allow molecular identification and reveal qualitative information about bonding, energetics and other things. Today, the most satisfactory method for determining very high-accuracy structures is a mixed experimental-theoretical approach that uses ground state rotational constant data from microwave and/or high-res vibrational spectroscopy and vibrational corrections obtained from quantum-chemical calculations (r_e^SE). Here we consider two different ab initio composite schemes for obtaining equilibrium structures (energy scheme vs geometry scheme) and compare them to the semi-experimental equilibrium structure. The comparison is performed for a test set of ten molecules which includes ome diatomics, linear triatomics and a few polyatomics. The ab initio calculations were performed using three levels of composite chemical recipes. The results showed that as the overall rigor of calculation is increased, the semi-experimental and the ab initio numbers agree with each other within the desirable level of accuracy ( < 0.0003 Å ) for all molecules in the test set. The composite recipe based on correcting the PES (energy scheme) and the one dependent on correcting the geometry directly (geometry scheme) also show excellent agreement with each other.

We report the characterization of thiobenzoic acid (C₆H₅COSH) by broadband and cavity microwave spectroscopy. Assignment of the chirped-pulse spectrum was carried out with the DAPPERS software. Using cavity spectroscopy, isotopologue spectra were observed for all carbons (¹³C), the oxygen (¹⁸O), and the sulfur (³⁴S and ³³S). Hyperfine structure for ³³S was observed and quadrupole coupling constants were determined. Spectra of the deuterated species were obtained via H/D exchange in a mixture of thiobenzoic acid and D₂O. A Kraitchman analysis was performed using all heavy-atom isotopic data, and yielded excellent agreement with M06-2X 6-311++G(d,p) calculations. No conformers or tautomers were observed.

para-substituted benzoic acids, such as para-nitrobenzoic acid, para-aminobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid, play important roles in atmospheric chemistry and are used as precursors for a wide range industrial products. Surprisingly, there are no high-resolution spectroscopic studies of these compounds, and we describe here their rotational spectroscopic investigations combined with electronic structure calculations. The rotational spectra were recorded using a 2 to 6 GHz chirped-pulse Fourier transform microwave spectrometer, which is based on the design by Pate et al.¹ Experimentally, all four substituted benzoic acids were found to exist in the cis-configuration, the global minimum energy configuration, of the carboxylic acid group. In all instances, except para-aminobenzoic acid, the global minimum structure is planar. Nudged elastic band calculations² for the wagging motion of the amino group in para-aminobenzoic acid suggest that it is a barrierless large amplitude motion and indeed, no tunnelling splittings were observed in the spectra. For para-chlorobenzoic acid, the nuclear quadrupole hyperfine structures of the ³⁵Cl and ³⁷Cl isotopologues were measured and analyzed. In the case of para-hydroxybenzoic acid two conformers were experimentally and theoretically identified. We noticed in the theoretical structures of the trans-conformers that the carboxylic acid group is pushed slightly out of plane, depending on the nature of the para-substituent. We were able to correlate the out-of-plane angle with the corresponding Hammett constant³ via the molecular electrostatic potential.^{4 1}Pérez, C., et al., Chem. Phys. Lett. 2013, 571, 1–15. ²Ásgeirsson, V., et al., J. Chem. Theory Comput. 2021, 17 (8), 4929–4945. ³Hansch, C., et al., Chem. Rev. 1991, 91 (2), 165–195. ⁴Sayyed, F. B., et al., New J. Chem. 2009, 33 (12), 2465–2471.

The millimeter-wave spectrum of ketene has been collected and analyzed from 130 GHz to 750 GHz and provided highly precise spectroscopic constants from a sextic S-reduced Hamiltonian. The synthesis of deuteriated samples enabled the spectroscopic measurements of five previously unreported ketene isotopologues. Combined with previous work, this resulted in a new highly precise and accurate semi-experimental (r_e^SE) structure for ketene from 32 independent moments of inertia. This r_e^SE structure was determined with the experimental rotational constants from all available isotopologues, together with computed vibration-rotation interaction and electron-mass distribution corrections from coupled-cluster singles, doubles, and perturbative triple calculations [CCSD(T)/cc-pCVTZ]. The 2σ uncertainties of the parameters of the r_e^SE are ≤ 0.007 Å and 0.014^° for the bond distances and independent angle, respectively. Only S-reduced spectroscopic constants were used in the structure determination, due to a breakdown in the A reduction of the Hamiltonian for the most prolate ketene species. All four structural parameters are in agreement with the “best theoretical estimate” (BTE) calculated from the CCSD(T)/cc-pCV6Z r_e structure with corrections for extrapolation to the complete basis set, the incomplete treatment of electron correlation, the diagonal Born-Oppenheimer breakdown, and relativistic effects. The discrepancies between the current r_e^SE and previously reported r_e^SE structures will be discussed.

The rotational spectra of many isotopologues of chlorobenzene (C₆H₅Cl) have been collected over portions of the 2 to 360 GHz frequency region. Several deuterium-enriched samples were generated via Grignard reaction of bromochlorobenzenes with a D₂O quench. The measured transitions of 22 isotopologues were least-squares fit to A- and S-reduced, sextic distorted-rotor Hamiltonians. The resultant rotational constants of all available isotopologues, with CCSD(T)/cc-pCVTZ corrections for vibration-rotation interaction and electron-mass distribution, were used to determine a highly precise semi-experimental equilibrium (r_e^SE) structure of chlorobenzene. The preliminary r_e^SE structure proved critical for finding ¹³C-atom isotopologues of the deuterium-substituted species in the available rotational spectra by enabling more accurate predictions of the rotational constants for these very low-abundance species. Transitions for several more isotopologues are expected to be available in the collected data. The highly accurate and precise r_e^SE structure will be compared to a CCSD(T)/cc-pCV5Z equilibrium (r_e) and r_e^SE structures of other molecules determined with the same methodology.

Molecular rotational spectroscopy is a powerful for determining the structures of molecular clusters. A key feature of the technique is the high spectral resolution that makes it possible to record the spectra of the large number of isomers generated in the pulsed jet expansion without significant spectral overlap. However, when many clusters are produced the spectral line density can make it difficult to identify individual spectra. This work explores the potential to assign a cluster size for each transition in the spectrum of water clusters using isotopic dilution. Previous work on the structures of water clusters used ¹⁸O water “spiking” to obtain the oxygen atom framework geometry from the analysis of the set of isotopomers coming from single H₂¹⁸O incorporation into the water cluster. That work showed that, to a good approximation, the H₂¹⁸O substitutes into the geometry according to simple statistics. This observation suggests that the water cluster size for each transition in the spectrum can be determined by the reduction in the signal intensity when a known percentage of H₂¹⁸O is spiked into the water sample. The success of this isotope dilution approach to generating seize-selected cluster spectra is illustrated using the known assignments of water cluster spectra. The analysis method is then used to identify a weak spectrum for the water heptamer that was previously unassigned. On the large water cluster size limit, the method led to the successful identification of four hydrogen bond network isomers of (H₂O)₁₄ and a polar isomer of (H₂O)₁₆. Many transitions are left unassigned including many in the N=8-10 size range based on their isotope dilution behavior.

(E)-1,2,3,3,3-pentafluoropene results from the formal replacement of one of the two geminal fluorine atoms in 1,1,2-trifluoroethylene (the one located cis to the fluorine on the singly halogenated carbon) with a trifluoromethyl group. As determined from the analysis of the rotational spectra of the respective species, the structure of (E)-1,2,3,3,3-pentafluoropene-argon is consistent with the often-observed preference for maximizing the number of argon-heavy atom interactions, with argon locating away from the olefinic place and in the FC=CCF cavity, which is in many ways similar to the structure of 1,1,2-trifluoroethylene. However, the replacement of the fluorine atom with the trifluoromethyl group has a significant effect on the structure of the heterodimer with acetylene. Whereas the acetylene interacts with the geminal fluorine-carbon pair in the ethylene, when binding to the propene it does so via the cis fluorine-carbon pair.

(Z)-1,2,3,3,3-pentafluoropene results from the formal replacement of one of the two geminal fluorine atoms in 1,1,2-trifluoroethylene (the one located trans to the fluorine on the singly halogenated carbon) with a trifluoromethyl group. Structures for the gas-phase heterodimers of (Z)-1,2,3,3,3-pentafluoropene with argon and with acetylene are obtained from the analysis of the microwave spectra of each complex. While the binding of argon to the two olefins is similar in both cases, with argon locating in the FC=CF cavity, but away from the olefinic plane, the predicted structures of the corresponding heterodimers with acetylene show the possibility of distinct differences.

New techniques for chiral analysis have recently emerged from the field of rotational spectroscopy. Traditional approaches to chiroptical spectroscopy, like circular dichroism, produce chiral signatures that are too weak for practical applications in rotational spectroscopy. The introduction of microwave three-wave mixing spectroscopy by Patterson, Schnell, and Doyle in 2013 renewed interest in chiral analysis by rotational spectroscopy. Work in the field has included the development of a chiral derivatization method that uses noncovalent interactions to generate complexes between the analyte and a small, chiral “tag” molecule in the pulsed jet expansion used in many Fourier transform microwave spectroscopy instruments. Recently, there has been increasing interest in using deuterium incorporation to improve the safety and efficacy of active pharmaceutical ingredients (APIs). The most chemically specific modification of the API needed to achieve improved drug performance can potentially involve the incorporation of a single deuterium atom at a prochiral R₁-CH₂-R₂ position. Applications of rotational spectroscopy approaches to chiral analysis will be discussed in the context of aiding the development of catalytic methods to add a single deuterium at the enzyme targeted benzylic CH₂ position. The relative strengths and weaknesses of three-wave mixing and chiral tag rotational spectroscopy in meeting the needs of rapid analysis of the enantioisotopomers produced using this new chemistry will be discussed.

As part of our efforts in evaluating substituted oxiranes for use as potential chiral tags for the conversion of enantiomeric molecules into spectroscopically distinct diastereomeric complexes for chiral analysis, we examine the gas-phase heterodimers formed between two such species using quantum chemistry and microwave spectroscopy. The lowest energy isomers of the various diastereomers of 3,3,3-trifluoro-1,2-epoxypropane–trans-1,3,3,3-tetrafluoro-1,2-epoxypropane are predicted to share structural features with the previously characterized 3,3,3-trifluoro-1,2-epoxypropane homodimers. Like the homodimers, the diastereomeric heterodimers have significantly different rotational constants, and thus, easily distinguishable microwave spectra. The spectrum of (S)-3,3,3-trifluoro-1,2-epoxypropane–(1R, 2S)-trans-1,3,3,3-tetrafluoro-1,2-epoxypropane and its enantiomer has been obtained and analyzed to be consistent with the quantum chemistry predictions. Despite being predicted at a lower energy, the search for the spectrum of (S)-3,3,3-trifluoro-1,2-epoxypropane–(1S, 2R)-trans-1,3,3,3-tetrafluoro-1,2-epoxypropane and its enantiomer is on-going.