r0pt Figure In a talk at the 74th Annual ISMS, the structure that had determined through traditional rotational spectroscopy utilizing CP-FTMW techniques for the title molecule was presented. Based on questions received at this presentation, it was decided that the structure needed to be better refined to resolve issues with the bond lengths found using Kraitchman's Single Substitution method. In order to achieve this goal, H.D. Rudolph's Double Substitution method was utilized. This presentation will describe what went wrong when using Kraitchman's substitution, and how Rudolph's method could be adapted for future structure determinations involving complex coordinates.

We here present the first microwave study of acetohydroxamic acid (CH₃CONHOH), a glycine isomer, using a broadband chirped pulse FTMW spectrometer equipped with a laser ablation vaporization system (LA-CP-FTMW)L. Kolesniková, I. León, E. R. Alonso, S. Mata, J. L. Alonso, J. Phys. Chem. Lett., 10 (2019), 1325-1330. We have characterized two distinct structures (Z-amide and E-amide) of acetohydroxamic acid in the supersonic expansion. Accurate experimental values of the ¹⁴N nuclear quadrupole coupling constants together with the rotational constants and also the barrier to the internal rotation (V₃) of the methyl group have been provided for both conformers. The precise spectroscopic information provided here could be relevant to guide readioastronomical searches of acetohydroxamic acid in the ISM.M. Sanz-Novo acknowledges funding from the Spanish “Ministerio de Ciencia, Innovación y Universidades” under predoctoral FPU Grant (FPU17/02987).html:<hr /><h3>Footnotes: L. Kolesniková, I. León, E. R. Alonso, S. Mata, J. L. Alonso, J. Phys. Chem. Lett., 10 (2019), 1325-1330.. M. Sanz-Novo acknowledges funding from the Spanish “Ministerio de Ciencia, Innovación y Universidades” under predoctoral FPU Grant (FPU17/02987).

For most people, the name 17β-Hydroxyandrost-4-en-3-one probably means nothing. When its common name “testosterone” is used instead, one of the most important steroids comes to one’s mind. We have vaporized the solid testosterone by laser ablation, and one conformer has been characterized using chirped-pulse Fourier transform microwave LA-CP-FTMW spectroscopy. The detection of such a large biomolecule highlights the actual sensitivity of rotational spectroscopy combined with laser ablation to tackle large molecules with high melting points. To confirm such an improvement, we will also report the results of 1-(4-tert-Butyl)phenyl)tricyclo[3.3.1.0^2,8]nona-3,6-dien-9-one, a large fluxional molecule that is central to the understanding of the phenomena of valence tautomerism.

Pure rotational spectra of the ³⁷ClZnCH₃, ClZnCD₃, and ClZn¹³CH₃ isotopologues of monomeric ClZnCH₃ (~X¹A₁) have been recorded using millimeter-wave direct absorption techniques in the frequency range 263 – 303 GHz. These species were synthesized in the gas phase in a DC discharge by the reaction of zinc vapor, produced in a Broida-type oven, with ³⁷ClCH₃ (in natural chlorine abundance), ClCD₃, or Cl¹³CH₃. The data for each isotopologue were analyzed with a symmetric top Hamiltonian and rotational and centrifugal constants determined. In combination with previous measurements of Cl⁶⁴ZnCH₃, Cl⁶⁶ZnCH₃, and Cl⁶⁸ZnCH₃, an r_m⁽²⁾ structure was determined for this organozinc compound. The bond lengths were calculated to be r_Cl−Zn = 2.0831(1) Å, r_Zn−C = 1.9085(1) Å, and r_C−H = 1.1806(5) Å. The H-C-H bond angle was found to be 110.5^° – slightly larger than that in methane. These data serve to benchmark future structure calculations of organozinc compounds, which are widely used in organic synthesis.

The solid AlaAla dipeptide (m.p. 285ºC) has been placed in the gas phase by laser ablation and studied using a chirped-pulse Fourier-transform microwave (LA-CP-FTMW) and molecular-beam Fourier-transform micro-wave (LA-MB-FTMW) spectroscopies. Two rotamers have been identified through their rotational and nuclear quadrupole coupling constants. The two conformers are stabilized by a different type of intramolecular interactions forming closing five- and seven-membered ring configurations. Additionally, conformer interconversion gives rise to missing conformers in the conformational landscape.

There is speculation about the role that polycyclic aromatic hydrocarbons (PAHs) play in the generation of molecular hydrogen in the interstellar medium (ISM). As gas phase reactions cannot explain the rate of formation of H₂ in the ISM, the community is looking to grain surface interactions. On these surfaces, H atoms would interact with PAHs through either physisorption or chemisorption. Upon contact with another H atom, molecular hydrogen would be formed and released. We aim to study the interaction H₂ would have with a PAH by using rotational spectroscopy to investigate the structure of these complexes. Two PAHs, acenaphthene and fluorene, have been complexed with H₂, D₂, and HD in order to accurately determine the structure of the van der Waals complexes formed.

1H-1,2,3-triazole (C_s; κ = 0.94; μ_a = 4.1 D, μ_b = 1.6 D) and 2H-1,2,3-triazole (C_2v; κ = 0.82; μ_b = 0.52 D) are tautomers (C₂H₃N₃), the former of which is formally the product of a 1,3-dipolar cycloaddition of two explosive compounds, hydrazoic acid and acetylene. We have analyzed the rotational spectra of these tautomers in the 130 – 360 GHz frequency range and least-squares fit the observed transitions to sextic Hamiltonians with low error (σ < 50 kHz). While ¹³C and ¹⁵N isotopologues have been observed at natural abundance and similarly analyzed, several synthetic techniques had to be employed to access deuterium-substituted isotopologues. The rotational constants of the numerous isotopologues, along with vibration-rotation interaction and electron mass corrections predicted using coupled-cluster and density functional theories, have been used to determine highly precise semi-experimental equilibrium structures (r_e^SE) of the 1,2,3-triazoles. Equilibrium structures calculated at the above-mentioned levels of theory, as well as a computational prediction using basis set extrapolation and additional corrections for the 2H structure, are compared to the semi-experimental structures.

As has recently been demonstrated with pyrimidine, using multiple isotopic substitution combined with coupled-cluster calculations treating the vibration-rotation interaction and the electron mass, it is possible to determine highly precise semi-experimental equilibrium structures (r_e^SE). The CCSD(T)/cc-pCV5Z equilibrium structure of pyrimidine agrees with the r_e^SE structure within the 2σ uncertainties for all parameters. With the intent of improving the structure determinations of thiophene (8 – 360 GHz) and thiazole (130 -360 GHz), spectra have been collected. For each molecule, the main isotopologue vibrational ground state spectrum has been fit to a sextic Hamiltonian with low error (σ < 35 kHz). All heavy atom isotopologues (³⁴S, ¹³C, ¹⁵N, ³³S) were detectable at natural abundance and similarly fit to sextic Hamiltonians. To obtain many more isotopologues, several deuteration strategies were employed: 1) acid-catalyzed H/D exchange, 2) base-catalyzed H/D exchange, and 3) lithium-halogen exchange using brominated forms of these species with subsequent addition of D₂O. These samples have resulted in a total of over two dozen isotopologues for each of the sulfur-containing heterocycles, including multiple substitutions of each atom. The resulting r_e^SE structures will be discussed, including an exploration of the number of isotopologues to sufficiently determine all parameters, computational treatment of the rotational constants, and comparison to theoretical r_e structures containing sulfur. With pyrimidine, these structure determinations set a new standard of accuracy and precision for semi-experimental equilibrium structures (r_e^SE).

This is a continued discussion regarding the substitution effectcts on the three-dimensional shape and electron distribution of three methylamine substituted pyridines, 2-, 3-, and 4-picolylamine. The microwave spectra of all three molecules were collected over the frequency range of 4-18 GHz. In addition, the ¹³C and ¹⁵N isotopologues, along with the deuterated amine groups, were collected for each molecule in natural abundance. From these spectra, exact structures were determined which allowed for accurate rotation of the quadruple constants from the molecular frame into an internal axis. Next, an extended Townes and Dailey analysis was used to determine the lone pair density around each nitrogen and then compared to those of benzylamine and pyridine. Results of this analysis and how it explains the configuration of the methylamine group in each of the picolylamines will be discussed.

Recent algorithmsN.A. Seifert et. al., Journal of Molecular Spectroscopy 312, 13 (2015). L. Yeh, L. Satterthwaite, and D. Patterson, The Journal of Chemical Physics 150, 204122 (2019) have demonstrated context-free assignment of rotational constants-needing no knowledge of the chemical species other than the rotational spectrum. Efforts to date to subsequently determine molecular structure require further information including assignment of singly-substituted isotopologuesJ. Yan and D. Patterson. Submitted to the 75th International Symposium on Molecular Spectroscopy (2020). K. Mayer et. al., Proceedings of the 74th International Symposium on Molecular Spectroscopy (2019)., mass spectroscopy^d, and data mining^dM. Muckle, A. Mikhonin, D. McDaniel, and/or J. Neill, Proceedings of the 74th International Symposium on Molecular Spectroscopy (2019). We investigate two methodologies to resolve sign ambiguities of Kraitchman’s substitution coordinates. The first methodology requires candidate rotational constants of doubly-substituted isotopologues. Given many such candidates, we have worked out how to determine 1) which candidates for singly- and doubly-substituted isotopologues are most probable, and 2) doubly-substituted atoms’ relative position octant. This is realizable given resolution of doubly-substituted species in natural abundance, which is 10-100x order of magnitude above our instrument's present signal to noise.
The second methodology requires precision measurement of the electric dipole moments and magnetic g-factors of both the parent and singly-substituted isotopologues. The magnetic g-factor is measured via application of a large magnetic field to our microwave spectrometer, as done by Flygare et. al. (1969). For a 6 carbon molecule, this could be realized given the ability to resolve a ≈ 10⁻⁵ percent difference between magnetic g-factors of the parent and singly-substituted isotopolog species, as well as resolve the electric dipole moment to 4 or 5 significant figures. Improving the capabilities of microwave spectrometers to within these thresholds would therefore enable context-free molecular structure determination.

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Footnotes:

N.A. Seifert et. al., Journal of Molecular Spectroscopy 312, 13 (2015).

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Footnotes:

J. Yan and D. Patterson. Submitted to the 75th International Symposium on Molecular Spectroscopy (2020).

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M. Muckle, A. Mikhonin, D. McDaniel, and/or J. Neill, Proceedings of the 74th International Symposium on Molecular Spectroscopy (2019)..

In preparation for our FTMW study of the complex between hydrogen and silver bromide, H₂ AgBr, we have investigated the microwave spectra of our source of bromine, dibromodifluoromethane, CBr₂F₂. Previous sources of bromine to produce AgBr proved either too damaging to the instrumentation in the long term (Br₂), or did not produce enough AgBr to be useful (CHBr₃). In addition, silver bromide, originally studied by Hoeft, Lovas, Tiemann, and Torring in 1971,J. Hoeft, F. J. Lovas, E. Tiemann, T. Torring, Z. Naturforsch. 25a, 35 (1970).nd remeasured by Evans and Gerry in 2000,C. J. Evans, M. C. L. Gerry, J. Chem. Phys. 112, 1321 (2000).as remeasured once more. We plan to produce H₂ AgBr, using laser ablation of a silver rod within a supersonic expansion of an argon carrier gas containing H₂ and CBr₂F₂. This is part of our ongoing study of hydrogen complexed with metal halides which include H₂ CuF,D. J. Frohman, G. S. Grubbs II, Z. Yu, S. E. Novick, Inorg. Chem. 52, 816 (2013).₂ AgCl,G. S. Grubbs II, D. A. Obenchain, H. M. Pickett, S. E. Novick, J. Chem. Phys. 141, 114306 (2014).nd H₂ AuCl.D. A. Obenchain, G. S. Grubbs II, H. M. Pickett, S. E. Novick, J. Chem. Phys. 146, 204302 (2017).hese molecules are “models” of the molecular hydrogen storage in the cavities of metal organic frameworks (MOFs).

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Footnotes:

J. Hoeft, F. J. Lovas, E. Tiemann, T. Torring, Z. Naturforsch. 25a, 35 (1970).a C. J. Evans, M. C. L. Gerry, J. Chem. Phys. 112, 1321 (2000).w D. J. Frohman, G. S. Grubbs II, Z. Yu, S. E. Novick, Inorg. Chem. 52, 816 (2013).H G. S. Grubbs II, D. A. Obenchain, H. M. Pickett, S. E. Novick, J. Chem. Phys. 141, 114306 (2014).a D. A. Obenchain, G. S. Grubbs II, H. M. Pickett, S. E. Novick, J. Chem. Phys. 146, 204302 (2017).T