Microwave Spectrum, Conformational Equilibrium, Intramolecular Hydrogen Bonding, Tunneling, and Quantum Chemical Calculations for 1-Ethenylcyclopropan-1-ol

The microwave spectra of 1-ethenylcyclopropan-1-ol, (CH₂)₂C(OH)CCH₂, and one deuterated species, (CH₂)₂C(OD)CCH₂, have been investigated in the 11.0−60.0 GHz region. The (ac,ap)- and (ac,sc1)-conformers denoted Syn 1 and Skew 1 were assigned. Each of these two forms is stabilized with an intramolecular hydrogen bond formed between the hydrogen atom of the hydroxyl group and the π electrons of the double bond. In the Syn 1 rotamer the CC−C−O chain of atoms takes a syn conformation (dihedral angle = −2.6°) and the H−O−C−C link of atoms is gauche (dihedral angle = −67.2° from syn). The CC−C−O link of atoms takes a skew conformation (dihedral angle = 132.1° from syn) in the Skew 1 rotamer, while the H−O−C−C= dihedral angle is −67.1°. Syn 1 is preferred by 4.9(6) kJ mol^-1 relative to Skew 1. Syn 1 is virtually a hybrid of the most stable conformer of unsubstituted ethenylcyclopropane, and unsubstituted cyclopropanol, Skew 1, is the corresponding hybrid of the second rotamer of ethenylcyclopropane and the most stable one of cyclopropanol. The spectrum of Syn 1 is perturbed by tunneling of the hydroxyl group. An analysis yielded 2280.184(60) MHz for the tunneling frequency and 39.82(19) MHz for the Coriolis coupling term μ_ca for the normal species. The corresponding values were 72.401(27) and 5.2(10) MHz, respectively, for the deuterated species. A potential function for the tunneling motion consisting of three cosine terms was found to have the following potential constants:  V₁ = −918.2, V₂ = −900.0, and V₃ = 418.0 cm^-1. This double-minimum function yields a barrier of 16.6(50) kJ mol^-1 at the anti position and 10.6(30) kJ mol^-1 at syn. The microwave work has been assisted by ab initio computations at the MP2/cc-pVTZ level of theory as well as density functional theory calculations at the B3LYP/6-31G* level. These calculations indicate that there are only three stable rotameric forms of the molecule. The gas-phase IR spectrum in the O−H stretching region revealed a broad and complex band red-shifted by roughly 50 cm^-1 presumably as a result of internal hydrogen bonding.