1 The Structure and Ring Puckering Barrier of Cyclobutane: A Theoretical Study Sotiris S. Xantheas, Thomas A. Blake Environmental Molecular Sciences Laboratory Pacific Northwest National Laboratory Richland, Washington The Structure and Ring Puckering Barrier of Cyclobutane: A Theoretical Study Sotiris S. Xantheas, Thomas A. Blake Environmental Molecular Sciences Laboratory Pacific Northwest National Laboratory Richland, Washington 20 June 2006

2 ObjectivesObjectives Calculate the ring puckering barrier of perfluorocyclobutane, C 4 F 8. As a check, calculate the ring puckering barrier of cyclobutane, C 4 H 8. Calculate structural parameters and anharmonic force field of C 4 H 8 and C 4 F 8, as well.

3 Theoretical Approach Optimal geometries were calculated for the D 2d (minimum) and D 4h (transition) states of C 4 H 8 using HF, MP2, MP4 and CCSD(T) levels of theory. (ecce/NWChem at PNNL.) Basis sets used were cc-pVnZ and aug-cc-pVnZ with n = D, T, Q, 5. Anharmonic force field was calculated by numerical differentiation along normal modes at the MP2 level of theory. (Gaussian 98, 03, MOLPRO at NERSC/LBNL) Basis sets used were cc-pVTZ and aug-cc-pVDZ.

4 D 2d D 4h H H H H H H H H C C C C HH HH HH HH HH HH HH C4C4 C3C3 C2C2 C1C1 HH 

5 CH 2 Rocking Angle  

6 Calculated D 2d Structural Parameters

7 Calculated D 4h Structural Parameters

8 Cyclobutane Barrier Height

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10 Barrier Height (  E) Extrapolation to CBS Limit Method “a”, exponential extrapolation gives 505 cm -1 for CCSD(T)/cc-pVnZ and 498 cm -1 for CCSD(T)/aug-cc-pVnZ. Method “b”, difference method,  (  E) (MP2 – ; n) =  E (MP2 ; n) –  E ( ; n), gives 503 cm -1 for CCSD(T)/cc-pVnZ and 495 cm -1 for CCSD(T)/aug-cc-pVnZ. Experimental value:  E = 510(3) cm -1 [Egawa, et al., JCP (1987)] and cm -1 (± 4.4) [T.B. Malloy, W.J. Lafferty, JMS (1975)] assuming coupling between ring puckering and CH 2 rocking.

11 Spectroscopic Constants for D 2d C 4 H 8

12 Vib. Modes for D 2d C 4 H 8 : MP2/aug-cc-VDZ

13 E.D. Glendening, A.M. Halpern, J. Phys. Chem. A (2005). D 2d structure CCSD(T)/cc-pVQZ: C–C = Å, H  –C = Å, H  –C = Å H–C–H = °, C–C–C = 88.07°  = 29.59°,  = 5.67°. Extrapolated barrier height (exponential) V 0 = 482 cm -1 [CCSD(T)/cc-pVnZ, n = D, T, Q].

14 Calculated C 4 F 8 Structural Parameters

15 Perfluorocyclobutane Barrier Height

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17 C 4 F 8 Ring Puckering Energy Levels Harris, et al. J. Molec. Spectrosc (1976)  = 1501 amu, V 0 = 124 cm -1 based on measured 1  2 = 30 cm -1 sideband transition. Ring puckering potential: V = A(z 4 + Bz 2 ) A = 6.76, B = for V 0 = 131 cm -1,  = 17.4,  = 1528 amu calculated 1  2 = 40 cm -1 sideband transition.

18 The End