The Structure and Ring Puckering Barrier of Cyclobutane

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 1

Objectives 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. 2

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

Ha Hb Ha H H C 1 Hb C 3 Hb Ha C 2 H C 4 C C C Hb H C H H Ha D 2 d D 4 h 4

CH 2 Rocking Angle b b 5

Calculated D 2 d Structural Parameters 6

Calculated D 4 h Structural Parameters 7

Cyclobutane Barrier Height 8

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Barrier Height (DE) Extrapolation to CBS Limit Method “a”, exponential extrapolation gives 505 cm -1 for CCSD(T)/cc-p. Vn. Z and 498 cm-1 for CCSD(T)/aug-cc-p. Vn. Z. Method “b”, difference method, D(DE) (MP 2 – l; n) = DE (MP 2 ; n) – DE (l ; n), gives 503 cm-1 for CCSD(T)/cc-p. Vn. Z and 495 cm-1 for CCSD(T)/aug-cc-p. Vn. Z. Experimental value: DE = 510(3) cm-1 [Egawa, et al. , JCP 86 6018 (1987)] and 514. 8 cm-1 (± 4. 4) [T. B. Malloy, W. J. Lafferty, JMS 54 20 (1975)] assuming coupling between ring puckering and CH 2 rocking. 10

Spectroscopic Constants for D 2 d C 4 H 8 11

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

E. D. Glendening, A. M. Halpern, J. Phys. Chem. A 109 635 (2005). D 2 d structure CCSD(T)/cc-p. VQZ: l l l C–C = 1. 550 Å, Ha–C = 1. 091 Å, Hb–C = 1. 091 Å H–C–H = 109. 07°, C–C–C = 88. 07° q = 29. 59°, b = 5. 67°. Extrapolated barrier height (exponential) V 0 = 482 cm-1 [CCSD(T)/cc-p. Vn. Z, n = D, T, Q]. 13

Calculated C 4 F 8 Structural Parameters 14

Perfluorocyclobutane Barrier Height 15

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C 4 F 8 Ring Puckering Energy Levels Harris, et al. J. Molec. Spectrosc. 62 149 (1976) m = 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 = -8. 80 for V 0 = 131 cm-1, q = 17. 4, m = 1528 amu calculated 1 2 = 40 cm-1 sideband transition. 17

The End 18