PNNL

Abstract

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 ?12 and ?13 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 ?12 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 ?12 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 ?13 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 ?13(strong) = 1240.34858(4) cm-1, B' = 0.0354192(7) cm-1 and B? = 0.0354355(7) cm-1 and ?13(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-? quality, developed by Dunning and coworkers, yield best estimates for the vibrationally averaged structural parameters r(C-C)=1.568 Å, r(C-F)a=1.340 Å, r(C-F)ß=1.329 Å, a(F-C-F)=110.3°, ?z(C-C-C)=89.1° and d(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