Electron-Diffraction and Theoretical Investigation of the Molecular Structure of Octafluorobicyclo[2.2.0]hex-1(4)-ene in the Gas Phase: Another Example of a Molecule with an Unusually Long C(sp 3 )-C(sp 3 ) Single Bond Alan D. Richardson, † Kenneth Hedberg,* ,† Christopher P. Junk, ‡ and David M. Lemal ‡ Department of Chemistry, Oregon State UniVersity, CorVallis, Oregon 97331-4003 and Department of Chemistry, Dartmouth College, HanoVer, New Hampshire 03755 ReceiVed: December 3, 2002; In Final Form: January 31, 2003 The molecular structure of octafluorobicyclo[2.2.0]hex-1(4)-ene has been measured in the gas phase by electron diffraction at room-temperature aided by results from molecular orbital theory. The results are consistent with D 2h symmetry for the molecule. The bond distances (r g /Å) and bond angles (∠ R /deg) with estimates of 2σ uncertainties are r(C 1 dC 4 ) ) 1.376(14), r(C 1 sC 2 ) ) 1.530(3), r(C 2 sC 3 ) ) 1.627(5), r(CsF) ) 1.336(2), and ∠(FsCsF) ) 108.6(3). The planes of the -CF 2 groups are tipped away from the bisector of the Cs CsC angle toward each other by about 2.8°. The measured length of the C 2 sC 3 bond is even greater than the lengths of the structurally similar bonds in hexafluorocyclobutene and 1,2-dichloro-3,3,4,4-tetrafluoro- cyclobutene and agrees with prediction from quantum-mechanical structure optimizations at the B3LYP, B3PW91, and MP2 levels of theory. This and other features of the structure are discussed in terms of the bond-altering effects of electrostatic repulsions and rehybridization arising from electronegativity differences. Introduction The structures of certain substituted cyclobutenes have a very interesting feature. When all the hydrogen atoms of the C 3 -C 4 bond (the unique single bond) are replaced by fluorines, this bond appears to be unusually long. For example, an early gas- phase electron-diffraction (GED) investigation of the structure of hexafluorocyclobutene led to r a (C 3 -C 4 ) ) 1.595(16) Å, 1 some 0.05 Å greater than the “normal” value for nominal sp 3 - sp 3 carbon-carbon single bonds. This unexpected circumstance indicated that a check was in order and led us to reinvestigate the structure of the molecule. The value from our study (r a ) 1.582(11) Å) 2 was a bit smaller than the original one, but nevertheless had to be reckoned as supporting the notion of C 3 - C 4 as very long. A subsequent study of the structurally similar 1,2-dichloro-3,3,4,4-tetrafluorocyclobutene also found this bond to be very long (r a ) 1.598(10) Å). 3 Although the picture from these GED studies seems clear, it is clouded by results from microwave (MW) spectroscopy, which suggest that the bond in question is considerably shorters 1.552(6) Å (r s + r 0 ) in hexafluorocyclobutene 4 and 1.551(15) Å in 1,2-dichloro-3,3,4,4-tetrafluorocyclobutene, 5 each a bit less than in cyclobutene itself (r s ) 1.566(3) Å). 6 The GED and MW results for these two molecules taken separately are obviously inconsistent and raise the question as to which is the more nearly correct. There are two reasons we favor those from GED. First, it is expected that an F 2 C-CF 2 bond will be longer than an H 2 C-CH 2 , as is found in perfluoroethane (r g ) 1.545- (8) Å) 7 vs that in ethane (r g ) 1.533(2) Å). 8 Second, we have shown 9 that analysis of the hexafluorocyclobutene structure based on a combination of our GED data with the rotational constants from the MW work leads to a structure, including an r g value for C 3 -C 4 equal to 1.585(8) Å, which is close to that obtained from the GED data alone. Moreover, this structure fits the measured rotational constants to within 0.3 MHz. The reason for the difference between the GED and MW results for these molecules remains unclear. Nor do molecular orbital calculations resolve the question: for hexafluorocy- clobutene optimizations yield r(C 3 -C 4 ) values ranging from 1.544 Å (HF/6-31G(d)) to 1.584 Å (LDA+BP/TZP), 10 and for 1,2-dichloro-3,3,4,4-tetrafluorocyclobutene from 1.543 Å (HF/ 6-31G(d)) to 1.571 Å (B3LYP/cc-pVDZ). It does appear that higher level theoretical calculations predict larger values for the bond in question, and given that the r g (thermal average) type of distance from GED is expected to be greater than the r e type from theory, these calculations lend support to the GED values. Nevertheless, the question is still not settled, so to add further data on the experimental side we have undertaken a GED investigation of octafluorobicyclo[2.2.0]hex-1(4)-ene, hereafter OFBH. As is seen in Figure 1, this molecule consists of two fused four-member rings sharing a carbon-carbon double bond and resembles the halocyclobutenes mentioned above in that the environment of bonds C 2 -C 3 and C 5 -C 6 is similar to that of bond C 3 -C 4 in the cyclobutenes. Experimental Section The synthesis of OFBH is outlined in Scheme 1 and was carried out at Dartmouth College; 11 full details are to be pub- lished later. Samples of OFBH (estimated >98% pure) were sent to Ore- gon State University in sealed glass ampules. To prepare for the diffraction experiments, an ampule was placed in a nitrogen glovebag and frozen to 77 K. The top of the ampule was re- † Oregon State University. ‡ Dartmouth College. SCHEME 1 3064 J. Phys. Chem. A 2003, 107, 3064-3068 10.1021/jp022518q CCC: $25.00 © 2003 American Chemical Society Published on Web 04/05/2003