Low barrier methyl rotation in 3-pentyn-1-ol as observed by microwave spectroscopy

Abstract

The rotational spectrum of 3-pentyn-1-ol, CH 3 −C≡C−CH 2 CH 2 OH, was measured using a molecular beam Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. A two-dimensional potential energy surface was calculated at the MP2/6-311++G(d,p) level of theory for a conformational analysis, yielding five conformers. The most stable conformer exhibits C 1 symmetry and was assigned in the spectrum by comparison with the results from quantum chemical calculations. The barrier to internal rotation of the propynyl methyl group CH 3 −C≡C− was found to be only 9.4552(94) cm −1 . Molecular parameters and internal rotation parameters could be accurately determined using the program xiam and belgi-C 1 . The internal rotation barrier was compared with those of other molecules containing a propynyl methyl group.

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan‐2‐one

“…For example, the methoxy methyl barrier of methyl butyrate is 420.155(71) cm À 1 for the (a,a) and 419.447(59) cm À 1 for the (g � ,a) conformer, [16] clearly lower than the value of 429.324(23) cm À 1 found for methyl propionate. [15] Similarly, the barrier of the propionyl methyl group of 9.46 cm À 1 found for 3pentyn-1-ol [17] is higher than that of 7.16 cm À 1 found for 4hexyn-3-ol. [18] In the C s class, the torsional barriers of pentan-2-one (188.3843(50) cm À 1 ), [2] hexan-2-one (186.9198(50) cm À 1 ), [3] heptan-2-one (185.469(16) cm À 1 ), [4] and octan-2-one (185.1442(16) cm À 1 ) (see Table 2) strongly support this assump-tion: The longer the alkyl chain, the lower the barrier.…”

“…For example, the methoxy methyl barrier of methyl butyrate is 420.155(71) cm −1 for the (a,a) and 419.447(59) cm −1 for the (g±,a) conformer, clearly lower than the value of 429.324(23) cm −1 found for methyl propionate . Similarly, the barrier of the propionyl methyl group of 9.46 cm −1 found for 3‐pentyn‐1‐ol is higher than that of 7.16 cm −1 found for 4‐hexyn‐3‐ol…”

“…In methyl alkynoates the two examples are methyl propionate (429.324(23) cm À 1 ) [15] and methyl butyrate (420.155(71) cm À 1 for the (a,a) and 419.447(59) cm À 1 for the (g � ,a) conformer). [16] For alkynyl alcohols, there are the examples of 3-pentyn-1-ol (9.46 cm À 1 ) [17] and 4-hexyn-3-ol (7.16 cm À 1 ). [18] 2.…”

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

“…The robustness of the double anchorage firmly supports the axel to the framework, unlike a methyl rotor, which is mono-dentate and is thus pivoted only on one side. 34,43 The cumulative angular displacement of the rotors at room temperature, shown in number of turns, is depicted in Figure 4B for three selected rotators in Zn-FTR. Clockwise or counter-clockwise rotation can be differentiated by positive or negative angular displacement.…”

Interplay Between Microwave Spectroscopy and X‐ray Diffraction: The Molecular Structure and Large Amplitude Motions of 2,3‐Dimethylanisole

“…We first identified the (01) species of some a-type transitions with small splittings, and gradually increased the number of (01) lines in the fit until finally, 188 lines were assigned and included in Fit 00/01 also given in Table 2. [30] In our recent study on two conformers of MMA, the results of the fits obtained by XIAM were compared with those obtained by a newly developed program, aixPAM, where further parameters in addition to those fitted in XIAM were included. This observation has been found previously in other low barrier internal rotation problems treated by XIAM, such as ethyl acetate (101.606(23) cm À1 , 85.3 kHz), [27] allyl acetate (98.093(12) cm À1 , 54.0 kHz), [28] vinyl acetate (151.492(34) cm À1 , 92.3 kHz), [29] and 3-pentyn-1-ol (9.4552(94) cm À1 , 20.7 kHz).…”