Microwave Spectrum of Bromocyclobutane

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 Cα–Cβ = 1.540±0.003 Å, Cβ–Cγ = 1.548±0.003 Å, C–Br = 1.939 Å, C–H = 1.096 Å, and C—D = 1.087 Å; bond angles CβCγCβ = 88° 06′±08′, CβCαCβ = 88°41′±08′, HCγH = 110°44′, HCβH = 108°44′, HCαBr = 111°, angle of CβCαCβ 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 CβCαCβ and CβCγCβ planes). The quadrupole-coupling constant of C4H7Br79 along the C–Br bond direction is 512.2±5.0 Mc and the asymmetry parameter ηbond 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.