Microwave spectrum and internal rotation of 2-butyne-1, 1, 1-d3 (dimethylacetylene), CH3C≡CCD3

Abstract

The rotational transitions of CH3C≡CCD3 have been observed for J=13←12, 15←14, 18←17, 19←18, 20←19, 22←21, 23←22, and 26←25 using a source-frequency modulation microwave spectrometer with a 3.7 m long free space absorption cell maintained at −50 to −60 °C. The observed spectrum clearly shows the effect of internal rotation with a small potential barrier. The expression for the rotational transition frequency derived by Kirchhoff and Lide [J. Chem. Phys. 43, 2303 (1965)] using a second-order perturbation theory for the contributions of the internal rotation was employed in analyzing the observed spectrum, and the least-squares analysis has yielded the rotational constant B=2982.4980±0.0011 MHz and a few centrifugal distortion constants. The barrier height to internal rotation has been estimated to be 5.62±0.16 cm−1.

Fast motion of molecular rotors in metal–organic framework struts at very low temperatures

“…The energy barrier of in-plane conformation is accounted for 50 cal mol -1 , while in crossed arrangement it was found to be lower than a few calories per mole and comparable to the thermal agitation at 0.5 K (Figure 2E), showing the extremely-high torsional flexibility of the rotator, comparable to methyl rotation flexibility. [32][33][34][35] The in-plane and crossed arrangements of the two carboxylate groups in the ligand are characteristic of the crystal structures with Zr and Zn metal nodes, respectively, thus, it is expected that the molecular wheels in Zn-FTR will exhibit extremely high mobility. Rotor minimum conformations for A) crossed and B) in-plane arrangements as in Zn-FTR and Zr-FTR, respectively.…”

“…13,24 The impressive dynamics here shown is comparable to the fastest conceivable rotation speed among organic moieties at low temperature, such as methyl groups, in the solid and even in the gas phase. [31][32][33][34][35] The strikingly low barrier suggests that not only does the rotor symmetry frustration ensure fullfledged barrierless 'mechanical' flexibility, but also that the framework was engineered as light, yet robust, to support rotors that behave as isolated entities.…”

Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

“…Though dimethylacetylene cannot be investigated by microwave spectroscopy due to the lack of a permanent dipole moment, the basic concepts of this chemical bonding suggest that a methyl group connected to an acetylene fragment CH3−C≡C−R (called the propynyl methyl group) features an extremely low torsional barrier (V3 < 10 cm -1 ). This assumption was confirmed by dimethylacetylene-d3 (molecule (1) in Figure 11) [212] and methylsilylacetylene (2) [213], two molecules related to dimethylacetylene possessing a barrier to methyl internal rotation of 5.62( 16) and 3.77(70) cm -1 , respectively. Very low V3 potentials of 1.00900 (42) and 2.20(12) cm -1 were observed for 2-butynoic acid (9) (when R = COOH) [214] and tetrolyl fluoride (8) (R = COF) [215], respectively.…”

Conformational effect on the almost free internal rotation in 4-hexyn-3-ol studied by microwave spectroscopy and quantum chemistry

“…rotation might arise by separating the methyl rotor from the rest of the molecule by the acetylenic group with cylindrical symmetry. Only a very limited number of molecules of the type CH 3 − −C≡≡C− −R have been studied by microwave spectroscopy, among them are dimethylacetylene-d 3 (R = CD 3 ), 14 1,1,1-trifluoro-2-butyne (R = CF 3 ), 15 methylsilylacetylene (R = SiH 3 ), 16 tetrolyl fluoride (R = COF), 17 2-butynoic acid (R = COOH), 18 1-chloro-2-butyne (R = CH 2 Cl), 19 2-butynol (R = CH 2 OH), 20 and 3-pentyn-1-ol (R = CH 2 CH 2 OH), 21 which is the most recent investigation. The three-fold barrier to internal rotation of the CH 3 − −C≡≡C− − methyl group (called the propynyl methyl group) is lower than 10 cm −1 in all cases.…”