Structural and conformational properties and intramolecular hydrogen bonding of (methylenecyclopropyl)methanol, as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectra of (methylenecyclopropyl)methanol (H 2 C=C 3 H 3 CH 2 OH) and one deuterated species (H 2 C=C 3 H 3 CH 2 OD) have been investigated in the 20-80 GHz spectral range. Accurate spectral measurements have been performed in the 40-80 GHz spectral interval. The spectra of two rotameric forms, denoted conformer I and conformer IX, have been assigned. Both these rotamers are stabilized by intramolecular hydrogen bonds formed between the hydrogen atom of the hydroxyl group and the pseudo-π electrons on the outside of the cyclopropyl ring, the so-called "banana bonds". The carbon-carbon bond lengths in the ring are rather different. The bonds adjacent to the methylene group (H 2 C=) are approximately 7 pm shorter that the carbon-carbon bond opposite to this group. It is found from relative intensity measurements of microwave transitions that conformer IX, in which the hydrogen bond is formed with the banana bonds of the long carbon-carbon bond, is 0.4(3) kJ/mol more stable than conformer I, where the hydrogen bond is formed with the pseudo-π electrons belonging to the shortest carbon-carbon bond of the ring. The microwave study has been augmented by quantum chemical calculations at the MP2/6-311++G**, G3 and B3LYP/6-311++G** levels of theory.