THE PURE ROTATIONAL SPECTRUM OF PERFLUOROOCTANONITRILE C 7

THE PURE ROTATIONAL SPECTRUM OF PERFLUOROOCTANONITRILE, C 7 F 15 CN, STUDIED USING CAVITY- AND CHIRPED-PULSED FOURIER TRANSFORM MICROWAVE SPECTROSCOPIES. W. C. Bailey, R. Bohn, C. T. Dewberry, G. S. Grubbs II and S. A. Cooke WH 13

1938 – Dr. Roy Plunkett Discovers Polytetrafluoroethene (PTFE)

26 CF 2’s produce a full 360 o turn, but a full period occurs every 13 carbons.

Perfluorooctanonitrile 395 amu, 24 atoms, 190 e- Initial calculations: Starting geometries generated with OMEGA MP 2/6 -311 G** At this level of theory: Calculated Rotational Constants (MHz) A = 627. 50 B = 131. 78 C = 130. 54

MP 2/6 -311 G**

Predicted spectrum

The Search Accelerated Correct Intensity FTMW Spectrometer “A Search Accelerated Correct Intensity Fourier Transform Microwave Spectrometer with Pulsed Laser Ablation Source. ” G. S. Grubbs II, C. T. Dewberry, K. C. Etchison, K. Kerr and S. A. Cooke Rev. Sci. Instrum. 78, 096106 (2007).

Circuits RH 03 Thurs 2. 04 pm Acknowledgements: Brooks Pate Jens-Uwe Grabow

Sample of observed spectrum

Spectroscopic constants Calculated Rotational Constants (MHz) A = 627. 50 B = 131. 78 C = 130. 54

Calculated Rotational Constants (MHz) A = 627. 50 B = 131. 78 C = 130. 54

New calculation at the PBE 0/aug-cc-p. VTZ level.

Effective structure

Effective structure

Second moments Pcc / u Å2 Pcc per CF 2/CF 3 unit / u Å2 CF 3 -CF 2 -CF 3 134. 338 44. 78 CF 3 -(CF 2)3 -CF 3 229. 616 45. 92 CF 3 -(CF 2)4 -CF 3 281. 88 46. 98 CF 3 -(CF 2)5 -CF 2 CN 340. 58 pseudo CF 3 -(CF 2)4 -CF 3 284. 12 47. 35 pseudo CF 3 -(CF 2)3 -CF 3 231. 99 46. 40

In 7 carbons the helix has coiled ~ 104 o cf 97 o from x-ray

Conclusions 1. First rotational spectroscopic study of perfluorooctanonitrile 2. The size of the molecule is consistent with it being treated as a small cyanide terminated oligomer of PTFE. 3. Analysis yields insight into the perfluoro alkyl helix. Seven carbons completing a full quarter turn.

Acknowledgements • Laboratory Funding from the NSF

• Fourier transform rotational spectroscopy has been used to collect the spectrum of perfluooctanonitrile. The spectrum was weak and only one conformer was observed. The assigned spectrum currently consists of both a- and b-type transitions spanning J = 8 to 40. The rotational constants are small, A = 681. 37155(18) MHz, B = 126. 116097(48) MHz, and C = 124. 284824(49) MHz. The spectroscopic constants together with quantum chemical calculations have been used to identify the structure of the observed conformer. Notably the helical nature of the perfluoro alkyl chain is fully in evidence. Further calculations confirm that the nitrogen quadrupole coupling tensor is such that nitrogen hyperfine splitting will not be observable at the high J transitions recorded in our experiments. Spectroscopic constants and a discussion of the molecular structure will be presented.