Microwave Spectra, Dipole Moments, and Torsional Potential Constants of cis- and trans-Cyclopropanecarboxaldehyde

Microwave spectra have been assigned for the ground state and three excited torsional states of species of cyclopropanecarboxaldehyde with the oxygen atom cis and trans to the cyclopropane ring. Dipole moments obtained by analysis of the Stark effect of the rotational transitions are as follows: cis species: ${\mu }_a\text{\hspace{0.17em}=\hspace{0.17em}2.02\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{D}{\mathrm{,\; \mu }}_b\text{\hspace{0.17em}=\hspace{0.17em}1.86\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{D}{\mathrm{,\; \mu }}_c\text{\hspace{0.17em}=\hspace{0.17em}0, μ\hspace{0.17em}=\hspace{0.17em}2.74\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{D}$ ; trans species: ${\mu }_a\text{\hspace{0.17em}=\hspace{0.17em}3.22\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{D}{\mathrm{,\; \mu }}_b{\text{\hspace{0.17em}=\hspace{0.17em}0, μ}}_c\text{\hspace{0.17em}=\hspace{0.17em}0.49\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{D}\text{, μ\hspace{0.17em}=\hspace{0.17em}3.26\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{D}$ . From relative intensity measurements estimates of the torsional excitation energies of the two species and the cis–trans energy difference were obtained. By analysis of these energy values the first three coefficients in a Fourier series expansion of the torsional potential were deduced to be $V_1\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}0.23\hspace{0.17em}±\hspace{0.17em}0.12}\mathrm{kcal}/\mathrm{mole}{\mathrm{,\; V}}_2\text{\hspace{0.17em}=\hspace{0.17em}4.39\hspace{0.17em}±\hspace{0.17em}0.40}\mathrm{kcal}/\mathrm{mole}$ , and $V_3\text{\hspace{0.17em}=\hspace{0.17em}0.28\hspace{0.17em}±\hspace{0.17em}0.10}\mathrm{kcal}/\mathrm{mole}$ .