Microwave Spectrum of Bromocyclobutane

Abstract

Rotational transitions of four isotopic species of bromocyclobutane have been observed. For C₄H₇Br⁷⁹, [Complex chemical formula] or [Complex chemical formula] the rotational constants are A = 10 003.4±13 Mc, B = 1629.41±0.03 Mc, C = 1488.48±0.03 Mc; for C₄H₇Br⁸¹, A = 10 002.6±13 Mc, B = 1615.14±0.03 Mc, and C = 1476.50±0.03 Mc. The values for the α‐deuterated compound are, for C₃H₆CDBr⁷⁹, A = 9534.7±13 Mc, B = 1613.67±0.03 Mc, and C = 1486.24±0.03 Mc; for C₃H₆CDBr⁸¹, A = 9533.4±13 Mc, B = 1599.55±0.03 Mc, and C = 1474.24±0.03 Mc. A set of structural parameters which reproduce these constants within 1.8 Mc were obtained with an electronic computer: bond distances C_α–C_β = 1.540±0.003 Å, C_β–C_γ = 1.548±0.003 Å, C–Br = 1.939 Å, C–H = 1.096 Å, and C—D = 1.087 Å; bond angles C_βC_γC_β = 88° 06′±08′, C_βC_αC_β = 88°41′±08′, HC_γH = 110°44′, HC_βH = 108°44′, HC_αBr = 111°, angle of C_βC_αC_β plane with C–Br = 131°00′±08′, and dihedral angle = 29°22′±08′ (the dihedral angle is the angle made by the normals of the C_βC_αC_β and C_βC_γC_β planes). The quadrupole‐coupling constant of C₄H₇Br⁷⁹ along the C–Br bond direction is 512.2±5.0 Mc and the asymmetry parameter η_bond is —0.002±0.014; these figures yield an upper limit of about 1% double‐bond character due to conjugation of the Br atom with the C ring and an ionicity of the halogen bonding orbital of 25% assuming 8.6% s character. From frequency and intensity measurements of rotational satellites a low‐lying vibrational mode was identified. Its effect is a displacement of all atoms in the molecule (out‐of‐plane bending). Its first and second excited states are 120±25 and 251±56 cm^‐1, respectively, above ground level. The three lowest levels of the vibrational mode were shown to be large‐amplitude vibrations of the molecule about a dynamical equilibrium dihedral angle of 25° to 29°. Highly excited vibrational states, which cause increasing planarity of the carbon ring, are expected to lead to tunneling through the potential barrier to the less stable ``axial'' isomer of bromocyclobutane. The existence of the axial isomer by direct observation could not be established due to instrumental difficulties.