Quantum mechanical computations at DFT level were carried out on the processes involved in the thermal reaction SP reversible arrow ME, where SP is the nitro-substituted spirobenzopyran (1', 3-dihydro-1', 3', 3'-trimethyl-6-nitro-spiro[2H-1-benzopyran-2,2'-[2H]indole]) in the closed form and ME is the corresponding open form. A detailed theoretical description of the overall reaction is reported along with the thermodynamic parameters for all intermediates and transition states. The obtained activation energy value is ill agreement with the available experimental data in solution. (C) 2004 Elsevier B.V. All rights reserved.