High-resolution infrared spectroscopy of jet cooled cyclobutyl in the α-CH stretch region: large-amplitude puckering dynamics in a 4-membered ring radical

Abstract

Gas-phase cyclobutyl radical (c-C₄H₇) is generated at a rotational temperature of T_rot = 26(1) K in a slit-jet discharge mixture of 70% Ne/30% He and 0.5–0.6% cyclobromobutane (c-C₄H₇Br). A fully rovibrationally resolved absorption spectrum of the α-CH stretch fundamental band between 3062.9 cm⁻¹ to 3075.7 cm⁻¹ is obtained and analyzed, yielding first precision structural and dynamical information for this novel radical species. The α-CH stretch band origin is determined to be 3068.7887(4) cm⁻¹, which implies only a modest (≈0.8 cm⁻¹) blue shift from rotationally unresolved infrared spectroscopic studies of cyclobutyl radicals in liquid He droplets [A. R. Brown, P. R. Franke and G. E. Douberly, J. Phys. Chem. A, 2017, 121, 7576–7587]. Of particular dynamical interest, a one-dimensional potential energy surface with respect to the ring puckering coordinate is computed at CCSD(T)/ANO2 level of theory and reveals a double minimum C_s puckered geometry, separated by an exceedingly shallow planar C_2v transition state barrier (E_barr ≈ 1 cm⁻¹). Numerical solutions on this double minimum potential yield a zero-point energy for the ground state (E_zero-point ≈ 27 cm⁻¹) greatly in excess of the interconversion barrier. This is indicative of highly delocalized, large amplitude motion of the four-membered ring structure, for which proper vibrationally averaging of the moment of inertia tensor reproduces the experimentally determined inertial defect remarkably well. Finally, intensity alternation in the experimental spectrum due to nuclear spin statistics upon exchange of three indistinguishable H atom pairs (I_H = ½) matches K_a + K_c = even : odd = 36 : 28 predictions, implying that the unpaired electron in the radical center lies in an out-of-plane p_π orbital. Thus, high-resolution infrared spectroscopy provides first experimental confirmation of a shallow double minimum ring puckering potential with a highly delocalized ground state wave function peaked at a planar C_2v transition state geometry consistent with a cyclobutyl π radical.