Results and Discussion
The lowest energy all trans conformer of n-hexane (C2h) lacks a dipole moment and is therefore spectroscopically unobservable. The lowest energy all trans structure of perfluoroalkanes, beginning with C4F10, is helical. A figure of the helical structure of C6F14, similar to Figure 3, given in reference 14 sparked our interest as this structure clearly has a dipole moment. The helicity of perfluoroalkanes results in structures with overall C2 symmetry. For an odd numbered chain, like C5F12,15 the C2 axis corresponds to the b-axis and b-type transitions are observed. For an even numbered chain the C2 axis becomes the c-axis and, thus, c-type transitions are expected for C6F14. The observation and assignment of the microwave spectrum of C6F14 to c-type transitions (see Table 1 and Supplementary Table 1) unambiguously shows that the lowest energy structure of C6F14 is helical. The observation of c-type transitions requires that the molecule have a non-zero dipole moment along the c axis. If the molecule were non-helical, the structure would have C2h symmetry and no dipole moment along any principal axis.
Table 1. Spectroscopic constants of C6F14
C6F14
A/MHz
824.9001(9)
B/MHz
202.2195(8)
C/MHz
198.3355(10)
DJ/kHz
0.00165(13)
Paa/uÅ2
2217.304
Pbb/uÅ2
330.798
Pcc/uÅ2
281.857
Kappa
-0.9876022
No. Lines
46
Std. Dev./kHz
1.8
The structure of C3F8 was recently determined to be non-helical (C2v).15 The Pcc second moment is 134.338 u Å2, or 44.78 u Å2 per CF2/CF3 group. Assuming transferability, C6F14 would have a Pcc of 6 x 44.78 or 268.68 u Å2 if it exhibited C2h symmetry. The observed Pcc is 281.857 u Å2, or 46.98 u Å2 per CF2/CF3 group. This increase in Pcc also confirms the helical structure of C6F14 because C atoms as well as F atoms lie outside the ab plane and contribute to Pcc. The Pcc second moment increases steadily as the C chain becomes longer. The Pcc per CF2/CF3 group for C3F815 is 44.78 u Å2 and is predicted to be 45.16 u Å2 for C4F10 (from PBE0/VTZ calculations). This value increases to 45.92 u Å2 for C5F1215 and to 46.98 u Å2 for C6F14. The trend demonstrates that as the C chain length increases more mass is displaced outside the ab plane and contributes to Pcc due to the helicity of the structures.
The microwave data alone do not allow for an exact determination of the CCCC dihedral angles. To analyze the structural parameters of C6F14, the principal coordinates of the computed
