A theoretical study of the rearrangement processes of energized CCCB and CCCAl

The rearrangement reactions of energized CCCB and CCCAl have been studied by means of quantum mechanical electronic structure calculations. Potential barriers were determined at UCCSD(T)/aug-cc-pVTZ with optimized molecular geometries and harmonic vibrational frequencies determined at the UB3LYP/6-311+G(3df) level. Two planar fully cyclized isomers are key intermediates in both systems. One of these is the “rhombic” structure, (analogous to rhombic C4) which is called the “kite” isomer. The other fully cyclized structure is called the “fan” structure. The quartets of CCCB and CCBC are the ground states of these structures [by 49.8 and 7.9 kJ mol−1 (E values), respectively], whereas the ground state of kite C3B is the doublet (by 131.8 kJ mol−1). The rearrangement of doublet CCCB is more energetically favourable than that of the quartet, with a maximum barrier of +68.6 kJ mol−1 together with the formation of fan C3B (−60.7 kJ mol−1), and then CCBC (+40.6 kJ mol−1). Quartet CCCB rearranges through fan C3B (+31.4 kJ mol−1) to give CCBC (+82.8 kJ mol−1) over a maximum barrier of +184.9 kJ mol−1. The C3Al system is different from C3B in a number of ways. Doublet CCCAl is the ground state (by 116.3 kJ mol−1) and rearrangement to fan C3Al requires only 21.8 kJ mol−1 of excess energy. Fan C3Al (+18.8 kJ mol−1) then converts to the kite isomer (−12.1 kJ mol−1) over a barrier of 50.2 kJ mol−1. Conversion to CCAlC is energetically unfavourable requiring some 371 kJ mol−1 of excess energy [at the UCCSD(T)/aug-cc-pVTZ//UB3LYP/6-311+G(3df) level of theory]. Rearrangement of quartet CCCAl is more complex, but again, the cyclic kite and fan forms are in equilibrium and ring opening to CCAlC is unfavourable.