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

Abstract

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.

On the Choice of Optimal Methodology for Calculation of 13C and 1H NMR Isotropic Chemical Shifts in Cagelike Systems. Case Studies of Adamantane, 2-Adamantanone, and 2,4-Methano-2,4-dehydroadamantane

“…Continuing our work , on this subject, in the present paper we report a HF SCF and density functional study of isotropic 13 C and 1 H chemical shifts for three cagelike systems: adamantane and 2-adamantanone characterized by highly stiffened structures and a 2,4-methano-2,4-dehydroadamantane (a [3.1.1] propellane) system with “inverted” carbon atoms. , We have used the optimized geometries at gradient-corrected density functional levels of theory with a DZP quality basis set for both the systems of interest and the standard for calculation of the isotropic chemical shifts (TMS). The performances of various density functional methodologies and the conventional HF SCF procedure in predicting both the 13 C and 1 H isotropic chemical shifts and the relative shifts with respect to the most deshielded center within a given molecule were tested.…”