Rotational Isomerism in 1-Pentyne from the Microwave Spectrum

The gas‐phase microwave spectrum of 1‐pentyne has been investigated in the region 8–40 GHz. The spectrum can be attributed to two rotational isomers in which the methyl group is, respectively, trans and gauche to the acetylene substituent. The rotational constants of the trans form are $\text{A = 23\hspace{0.17em}340 ± 200}$ , $\text{B = 2230.566 ± 0.020}$ , and $\text{C = 2116.385 ± 0.020}\mathrm{MHz}$ , and those for the gauche form are $\text{A = 9921.112 ± 0.030}$ , $\text{B = 3172.780 ± 0.010}$ , and $\text{C = 2634.025 ± 0.010}\mathrm{MHz}$ . From these data a plausible structure has been derived which gives the gauche dihedral angle as $\text{115° ± 3°}$ from the trans position. For each form several vibrationally excited states have been assigned to the torsional motion around the central carbon—carbon bond, the methyl internal rotation, and the $\mathrm{C}–\mathrm{C}\equiv \mathrm{C}$ bending motion. The torsional vibration frequency is $\text{106 ± 10}\mathrm{cm}\text{−1}$ in the trans form and $\text{114 ± 10}\mathrm{cm}\text{−1}$ in the gauche. Stark effect measurements yield ${\mu }_{\mathrm{trans}}\text{= 0.842 ± 0.010}\mathrm{D}$ and ${\mu }_{\mathrm{gauche}}\text{= 0.769 ± 0.028}$ D. The gauche‐form ground state is $\text{27 ± 36}\mathrm{cm}\text{−1}$ below the trans ground state. Four coefficients in the Fourier expansion of the potential energy for rotation about the central $\mathrm{C}–\mathrm{C}$ bond have been derived: V₁=0.940, V₂=−1.045, V₃=3.518, and V₄=0.071 kcal/mole. It is suggested that the slightly greater stability of the gauche form may be due to an attractive dipole—dipole interaction.