The molecular structure and conformation of bicyclo[3.3.1]nonan-9-one: ab initio and DFT calculations Jaebum Choo a , Sunghwan Kim b , Hyun Joo b , Younghi Kwon b, * a Department of Chemistry, Hanyang University, Ansan 425-791, South Korea b Department of Chemistry, Hanyang University, Seoul 133-791, South Korea Received 20 February 2002; accepted 5 August 2002 Abstract Ab initio and density functional theory methods have been applied to investigate the molecular structures and 13 C chemical shifts for bicyclo[3.3.1]nonan-9-one (BNO), and energy differences and barrier potentials between its conformers. The calculated free energy difference ðDG 1 Þ between twin-chair (CC) and boat-chair (BC) conformations is about 1 kcal/mol, while the inversion barrier ðDG ‡ 1 Þ between them is about 6 kcal/mol. Because of this small energy difference, BNO might have orientationally disordered crystal structure with the CC and BC conformers. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Bicyclo[3.3.1]nonan-9-one; Bicyclo[3.3.1]nonane; Ab initio method; Density functional theory method 1. Introduction Molecules in approximately globular shape, such as adamantane and camphor, often undergo rapid overall molecular reorientation immediately below their melting points [1–4]. Such molecular solids are known as plastic crystals or orientationally disordered solids. The molecular motions in these disordered states are depicted as a rapid tumble from one orientation to another [5,6], with correlation times in the range of 10 210 –10 212 s [7]. In addition to the rapid reorientation, the molecules also execute slower translational motion and sometimes coupled molecu- lar rotation and transition [8]. On cooling, these solids slowly lose their mobility, and eventually undergo one or several transitions until they reach a fully ordered arrangement. Because of their potential conformational mobility, bicyclo[3.3.1]nonane (BN) and bicyclo[3.3.1]nonan- 9-one (BNO) have been of great interest. These compounds have three possible conformational minima, corresponding to the twin-chair (CC), boat- chair (BC), and twisted twin-boat (BB) conformers. The electron diffraction study on gaseous BN revealed that the BN molecule exists predominantly in the CC form. However, as the temperature rises, the portion of the BC form increases: 5% at 65 8C and 25% at 400 8C [9]. According to this study, the free energy difference DG between CC and BC forms is 2.3 – 2.5 kcal/mol. As expected from these results, there are no X-ray studies on BN and its derivatives at room temperature providing the evidence of CC $ BC conformational disorder. On the contrary, 0166-1280/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0166-1280(02)00565-1 Journal of Molecular Structure (Theochem) 619 (2002) 113–120 www.elsevier.com/locate/theochem * Corresponding author. Fax: þ 82-2-2299-0762. E-mail address: yhkwon@hanyang.ac.kr (Y. Kwon).