The structure and helicity of perfluorooctanonitrile, CF 3 –(CF 2 ) 6 –CN W.C. Bailey a , R.K. Bohn b , C.T. Dewberry c , G.S. Grubbs II c , S.A. Cooke c,⇑ a Chemistry–Physics Department, Kean University, Union, NJ 07083, USA b Departments of Chemistry and Physics, University of Connecticut, Storrs, CT 06269-3060, USA c Department of Chemistry, University of North Texas, 1155 Union Circle, #305070 Denton, TX 76203-5017, USA article info Article history: Received 18 August 2011 In revised form 31 August 2011 Available online 10 September 2011 Keywords: Perfluorooctanonitrile Microwave Spectra Helical Structure abstract New molecular structural data is presented for a cyanide terminated oligomer of polytetrafluoroethene. The target molecule, CF 3 –(CF 2 ) 6 –CN, has been seeded within a pulsed supersonic expansion of argon. The result of this action is to cool the species to rotational temperatures below 4 K. Within this state, the pure rotational spectrum of the oligomer has been recorded using two types of Fourier transform microwave spectroscopy. A total of 111 transitions have been identified involving rotational J levels between 6 and 40. Only a- and b-type transitions were observed. The spectrum has been analyzed using a Hamiltonian containing all three rotational constants and one centrifugal distortion constant, D J . The experimental spectroscopic constants have been used to develop an effective molecular structure by scaling the quan- tum chemical calculated structure. The data shows that the seven carbon perfluorinated chain for the isolated oligomer twists 104°. This compares well to the C 7 F 13 -twist of 97° anticipated from the X-ray structure of phase II polytetrafluoroethene. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction A famous, fortuitous accident lead to the discovery of polytetra- fluoroethene, PTFE, by Plunkett in 1938 [1]. In 1954, Bunn and Howells [2] recorded X-ray diffraction photographs of drawn fibers of PTFE below 19 °C. From these photographs a chain repeat- distance was determined to be 16.8 Å. The photographs further revealed a reflection from a plane perpendicular to the chain axis with a Bragg spacing of 1.294 Å, one thirteenth of the chain repeat. This finding is explained by a plane zig-zag structure which twists 180° in 13 carbon atoms. A full 360° twist of the chain occurs in 26 carbon atoms, but the actual period is one half this because the zig-zag structure consists of two rows of carbon atoms and one 180° twist brings the fourteenth carbon in the second row to be just above the first carbon in the first row. A recent review by Yashima [3] discusses the importance of helical polymer synthesis in regards to enantiomeric separations and asymmetric catalysis, and goes on to discuss the difficulties associated with structural determinations. Powerful structural tools discussed in that review include single crystal and liquid crystal X-ray diffraction, circular dichroism, and also atomic force microscopy. These tools focus on unit cell or ‘‘generic’’ structural parameters, rather than focusing on molecular structural details. Implicit in a recent study by Fournier et al. [4] is a novel, ‘‘bot- tom up’’ approach to polymer structure determinations. In this approach, high resolution rotational spectroscopy augmented with accurate quantum chemical calculations is used to provide detailed structural information on proto-type oligomeric units. In this paper we further demonstrate the use of rotational spec- troscopy in regards to the structural determinations of oligomers. A fundamental study of the perfluoro helix in PTFE is the subject of this work. Several important ‘‘bottom up’’ carbon chain lengths are impor- tant in the structural evolution of PTFE. The first question concerns the chain length required for helicity to manifest. Fournier et al. [4] have answered this question using experimental data from the C 3 and C 5 systems. A four carbon chain is required for a helical struc- ture. Further chain lengths of interest include the 7, 13, and 26 carbon species as these lengths should be sufficient to exhibit one quarter, one half, and one full helical twist, respectively. Following Fournier et al. we have employed a ‘‘bottom up’’ rota- tional spectroscopy approach to characterize an oligomer contain- ing a perfluorinated eight carbon chain. Rotational spectroscopy requires that the target species possess a dipole moment. In order to increase the dipole moment of C 7 F 15 – we have studied a species with a terminal –CN group. This group may be considered a ‘‘polar- o-phore’’, in analogy to the addition of a chromophore in ultra-vio- let/visible spectroscopy. 0022-2852/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2011.09.001 ⇑ Corresponding author. Present address: Purchase College SUNY, 735 Anderson Hill Road, Purchase, NY 10577, USA. E-mail address: stephen.cooke@purchase.edu (S.A. Cooke). URL: http://openscholar.purchase.edu/cooke/ (S.A. Cooke) Journal of Molecular Spectroscopy 270 (2011) 61–65 Contents lists available at SciVerse ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms