High Resolution Infrared Spectroscopy in the 1200--1300 cm-1 Region and Accurate Theoretical Estimates for the Structure and Ring-Puckering Barrier of Perfluorocyclobutane
We present experimental infrared (IR) spectra and theoretical electronic structure results for the geometry, anharmonic vibrational frequencies and accurate estimates of the magnitude and the origin of the ring puckering barrier in C4F8. High-resolution (0.0015 cm-1) spectra of the ν12 and ν13 parallel bands of perfluorocyclobutane (c-C4F8) were recorded for the fist time by expanding a 10% c-C4F8 in helium mixture in a supersonic jet. Both bands are observed to be rotationally resolved in a jet with a rotational temperature of 15 K. The ν12 mode has b2 symmetry under D2d that correlates to a2u symmetry under D4h and consequently has ± ← ± ring puckering selection rules. A rigid rotor fit of the ν12 band yields the origin at 1292.56031(2) cm-1 with B' = 0.0354137(3) cm-1 and B" = 0.0354363(3) cm-1. The ν13 mode is of b2 symmetry under D2d that correlates to b2g under D4h and in this case the ring puckering selection rules are ± ! m. Rotational transitions from the ground and first excited torsional states will be separated by the torsional splitting in the ground and excited vibrational states and indeed we observe a splitting of each transition into strong and weak intensity components with a separation of approximately 0.0018 cm-1. The strong and weak sets of transitions were fit separately again using a rigid rotor model to give ν13(strong) = 1240.34858(4) cm-1, B' = 0.0354192(7) cm-1 and B" = 0.0354355(7) cm-1 and ν13(weak) = 1240.34674(5) cm-1, B' = 0.0354188(9) cm-1 and B" = 0.0354360(7) cm-1. High level electronic structure calculations at the MP2 and CCSD(T) levels of theory with the family of correlation consistent basis sets of quadruple-ζ quality, developed by Dunning and coworkers, yield best estimates for the vibrationally averaged structural parameters r(C-C)=1.568 Å, r(C-F)α=1.340 Å, r(C-F)β=1.329 Å, α(F-C-F)=110.3°, θz(C-C-C)=89.1° and δ(C-C-CC)=14.6° and rotational constants of A=B=0.03543 cm-1, C=0.02898 cm-1, the latter within 0.00001 cm-1 from the experimentally determined values. Anharmonic vibrational frequencies computed using higher energy derivatives at the MP2 level of theory are all within < 27 cm-1 (in most cases < 5 cm-1) from the experimentally measured fundamentals. Our best estimate for the ring puckering barrier at the CCSD(T)/CBS (Complete Basis Set) limit is 132 cm-1. Analysis of the C4F8 electron density suggests that the puckering barrier arises principally from the σCC→σCF * hyperconjugative interactions that are more strongly stabilizing in the puckered than in the planar form. These interactions are, however, somewhat weaker in C4F8 than in C4H8, a fact that is consistent with the smaller barrier in the former (132 cm-1) with respect to the latter (498 cm-1). Acknowledgements: Part of this work was performed in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) under the auspices of the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy under Contract DE-AC06-76RLO 1830 with Battelle Memorial Institute, which operates the Pacific Northwest National Laboratory. The EMSL is a national user facility funded by the Office of Biological and Environmental Research in the U.S. Department of Energy. Computer resources were provided by the Division of Chemical Sciences, US Department of Energy. EDG acknowledges the Indiana State University Office of Information Technology for the use of their computer resources.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 920513
- Report Number(s):
- PNNL-SA-54711; KC0301020; TRN: US200818%%612
- Journal Information:
- Journal of Physical Chemistry A, 111(44):11328-11341, Vol. 111, Issue 44
- Country of Publication:
- United States
- Language:
- English
Similar Records
Structure, Vibrational Spectra and Ring Puckering Barrier of Cyclobutane
Synchrotron-based far infrared study of the rotation-vibration-inversion spectrum of silacyclobutane below 500 cm{sup −1}: The ν{sub 29} and ν{sub 30} bands