Nature of Bonding in [3.1.1]Propellane. Vibrational Spectra and Normal Coordinate Analysis of 2,4-Methano-2,4-didehydroadamantane, 2,4-(Dimethylmethano)-2,4-didehydroadamantane, and Their Dihydro Congeners

The properties of the propellane bond in 2,4-methano-2,4-didehydroadamantane (1) and 2,4-(dimethylmethano)-2,4-didehydroadamantane (2) have been determined on the basis of the spectroscopic and chemical evidence. A monitoring of reaction of propellane 2 with dimethyl disulfide by Raman spectroscopy has indicated the cleavage of the central bond between inverted carbon atoms. Raman and infrared spectra of 1 and 2 as well as of their dihydro congeners 2,4-methanoadamantane (3) and 2,4-(dimethylmethano)adamantane (4) have been recorded. To describe the vibrational and molecular orbital properties of these compounds (harmonic frequencies, heats of formations, bond orders, and charge distributions), AM1 semiempirical calculations were performed. The calculated properties are found to be in fair agreement with the observed ones. By combining the results of spectroscopic investigations and molecular orbital calculations, the central bond between inverted carbon atoms is associated with the highest occupied molecular orbitals in propellanes 1 and 2. Assuming the transferability of the adamantane scaling factors for force constants within the studied group of molecules, an “a priori” assignment of the observed bands has been obtained.