FT microwave spectra have been observed and analyzed for the S (in-plane) and A (out-of-plane) conformers of propene-3-d₁ in the 10-22 GHz region. Both conformers display splittings due to deuterium quadrupole coupling; for the latter one only, a 19 MHz splitting due to internal rotation of the partially deuterated methyl group has been observed. In addition to rotational constants, the analysis yielded quadrupole coupling constants and parameters describing the tunneling splitting and its rotational dependence. Improved rotational constants for parent propene and the three ¹³C₁ species are recently available.^a Use of vibration-rotation interaction constants computed at the MP2(FC)/cc-pVTZ level gave equilibrium rotational constants for these six species and for fourteen more deuterium isotopologues with diminished accuracy from early literature data. A semiexperimental equilibrium structure, r_e^SE, has been determined for propene by fitting fourteen structural parameters to the equilibrium rotational constants. The new r_e^SE structure compares well with an ab initio equilibrium structure computed with the all-electron CCSD(T)/cc-pV(Q,T)Z model and with a structure obtained using the mixed regression method with predicates and equilibrium rotational constants.

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Footnotes:

^a N. C. Craig, P. Groner, A. R. Conrad, R. Gurusinghe, M. J. Tubergen J. Mol. Spectrosc. 248, 1-6 (2016).

Alkanes of the form CH₃(CH₂)_nCH₃ generally favor ground state geometries that have co-planar carbon atoms. In this study, we have looked at a long chain hydrocarbon with a terminal carbon-carbon triple bond, viz., 1-octyne. Guided by the results of the 1-hexyne studies,Atticks, K.; Bohn, R. K.; Michaels H. H. Int’l J. of Quantum Chem. 2001, 85, 514-519; Utzat, K.; Bohn, R. K.; Michaels H. H. J. Mol. Struct. 2007, 841, 22-27hree possible low energy conformers were studied which we reference as anti-anti (AA, straight chain), anti-gauche (AG, terminal methyl group is gauche), and gauche-anti (GA, ethyl group is gauche). An initial broadband chirp-pulse was performed between 7-13 GHz and a total of sixty-eight transitions were fit. Additional measurements on a Balle Flygare cavity instrument yielded an additional seventy-three lines belonging to three of the conformers. Transitions for all 8 of the singly substituted ¹³C isotopologues, in natural abundance, have also been observed for the AA conformer. Ab-initio optimizations at the MP2/6-311++g(2d,2p) level of theory and basis set for these three conformers will be compared to experimental rotational constants. Structure determinations of the AA conformer will also be discussed. Atticks, K.; Bohn, R. K.; Michaels H. H. Int’l J. of Quantum Chem. 2001, 85, 514-519; Utzat, K.; Bohn, R. K.; Michaels H. H. J. Mol. Struct. 2007, 841, 22-27t

To develop synthetic molecular motors and machinery that can mimic their biological counterparts has become a stimulating quest in modern synthetic chemistry. Gas phase studies of these simpler synthetic model systems provide the necessary isolated conditions that facilitate the elucidation of their structural intricacies. We report the first high-resolution rotational study of a synthetic molecular rotary motor based on chiral overcrowded alkenesVicario et al. Chem. Commun., 5910-5912 (2005)C₂₇H₂₀) using chirp-pulsed Fourier transform microwave spectroscopyBrown et al. Rev. Sci. Instrum., 79, 053103 (2008) Rotational constants and quartic centrifugal distortion constants were determined based on a fit using more than two hundred rotational transitions spanning 5 ≤ J ≤ 21 in the 2−4 GHz frequency range. Despite the lack of polar groups, the rotor’s asymmetry produces strong a− and b−type rotational transitions arising from a single predominant conformer. Evidence for fragmentation of the rotor allows for unambiguous identification of the isolated rotor components. The experimental spectroscopic parameters of the rotor are compared and discussed against current high-level ab initio and density functional theory methods.

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Footnotes:

Vicario et al. Chem. Commun., 5910-5912 (2005)( Brown et al. Rev. Sci. Instrum., 79, 053103 (2008).

The pure rotational spectra of 2- and 3-fluorobenzaldehyde have been investigated using a chirped pulse Fourier transform microwave (FTMW) spectrometer in the range of 8-18 GHz and a Balle-Flygare FTMW spectrometer in the range of 4-26 GHz. As in a previous study of monofluorobenzaldehydes,José L. Alonso and Rosa M. Villamañán, J. Chem. Soc., Faraday Trans. 2, 1989, 85(2), 137-149nly transitions due to a single planar conformer were observed for 2-fluorobenzaldehyde (O-trans) whereas two planar conformers (O-trans and O-cis) of 3-fluorobenzaldehydes were confirmed. Transitions due to the seven unique ¹³C isotopologues of each of the three molecules have been observed for the first time. Their rotational constants were used to derive the effective ground state (r₀) and substitution (r_s) structures. The results compare favourably with the equilibrium (r_e) geometries which were determined following geometry optimization at the MP2/aug-cc-pVTZ level of theory.

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Footnotes:

José L. Alonso and Rosa M. Villamañán, J. Chem. Soc., Faraday Trans. 2, 1989, 85(2), 137-149o

The pure rotational spectrum of the 2,3-difluoropyridine-CO₂ complex was measured on a chirped-pulsed Fourier transform microwave (CP-FTMW) spectrometer in the 3 – 18 GHz frequency range. The spectrum was analyzed to find the spectroscopic constants of the complex, including the quadrupole coupling constants and the centrifugal distortion constants. The spectrum of the 2,3-difluoropyridine-¹³CO₂ complex was also measured and analyzed. Experimental constants were compared to the results of ab initio calculations.

Previously studied complexes of hydrogen chloride with fluoroethylenes demonstrate that the secondary interaction between the chlorine atom of the HCl and a hydrogen of the ethylene occurs with the sterically accessible cis H-atom in vinyl fluoride rather than the electrostatically favorable geminal hydrogen. However, with 1,1,2-trifluoroethylene the opposite occurs and electrostatics is favored over sterics. The two hydrogen atoms in trans-1,2-difluoroethylene are electrostatically equivalent and each offer the possibility of interacting in a geminal or in a cis fashion. Thus, the observed structure of trans-1,2-difluoroethylene-HCl, with a secondary interaction to the cis H-atom, is consistent with the favorable steric interactions associated with this configuration. On the other hand, in cis-1,2-difluoroethylene only electrostatically-equivalent geminal H-atoms are present. We find that rather than adopting the sterically unfavorable arrangement found in 1,1,2-trifluoroethylene-HCl, the hydrogen chloride in this complex instead forms a bifurcated hydrogen bond with the two F-atoms, and there is no secondary interaction involving the chlorine atom.

The determination of the structures of heterodimers of haloethylenes with protic acids has provided a wealth of information and a few surprises concerning intermolecular forces and the sometimes cooperative and sometimes competing effects of electrostatic, steric, and dispersion forces. In seeking to apply this knowledge to larger systems with a wider variety of possible interactions and binding sites, we extend the carbon chain by one atom via the addition of a trifluoromethyl moeity. As a first step the microwave rotational spectra of the halopropene monomers, 2,3,3,3-tetrafluoropropene, 2-chloro-3,3,3-trifluoropropene, (E)-1-chloro-3,3,3-trifluoropropene, and (Z)-1-chloro-3,3,3-trifluoropropene, and their complexes with the argon atom are obtained and analyzed to obtain molecular structures.

The preferred equilibrium structure of germanium dicarbide (GeC₂) has been an open question for decades: while high-level quantum chemical calculations predict an L-shaped ground state structure, the very flat potential energy surface of the species prevents a T-shaped structure from being entirely ruled out¹. By recording for the first time the rotational spectrum of GeC₂ using sensitive microwave and millimeter techniques, we establish that the molecule adopts a vibrationally-averaged T-shaped structure in the ground state. From isotopic substitution of 14 isotopologues, a precise r₀ structure has been derived. This structural work should serve as an important benchmark for future calculations. ¹ Sari et al., J. Chem. Phys. 117 10008 (2002)

Following the recent discovery of T-shaped GeC₂ by chirped-pulse FT microwave spectroscopy, evidence has been found for two longer carbon chains, GeC₄ and GeC₅, guided by high-level quantum chemical calculations of their molecular structure. Like their isovalent Si-bearing counterparts, those with an even number of carbon atoms are predicted to possess ¹Σ ground states, while odd-numbered carbon chains have low-lying ³Σ linear isomers; all are predicted to be highly polar. With the exception of ⁷³Ge, rotational lines of the other four Ge isotopic species have been observed between 6 and 18 GHz. From these measurements, the Ge-C bond length has been determined to high precision, and can be compared to that found in other Ge species, such as GeC [1] and GeC₃Ge [2] studied previously at rotational resolution. Somewhat surprisingly, the spectrum of GeC₅ very closely resembles that of ¹Σ molecule, presumably owing to the very large spin-orbit constant of atomic Ge, which is manifest as an equally large spin-spin constant in the chain. A comparison between the production of SiC_n and GeC_n chains by laser ablation, including the absence of those with n=3, will be given. [1] C. R. Brazier and J. I. Ruiz, J. Mol. Spectrosc., 270, 26-32 (2011). [2] S. Thorwirth et al., J. Phys. Chem. A, 120, 254-259 (2016).

Complexes of argon and water with silver iodide have been formed in the gas phase by laser ablation of a silver iodide rod and studied using a chirped-pulse Fourier transform microwave spectrometer. Ar···AgI was characterized by its rotational spectrum and ab initio calculations carried out at the CCSD(T)(F12c)/cc-pVTZ-F12 explicitly correlated level of theory. The molecule was shown to be linear in the ground state, with atoms in the order shown. The Ar···Ag and Ag-I bond lengths, r₀(Ar···Ag) = 2.6759 Å and r₀(Ag-I) = 2.5356 Å, were determined. Other factors such as the dissociation energy, the intermolecular quadratic stretching force constant and the change in ionicity of AgI upon forming the complex were also determined and will be discussed with comparison to the series Ar···AgX (X = F, Cl, Br and I). Data of the H₂O···AgI complex will also be presented with isotopic studies ongoing.