1ST AND 2ND ENERGY DERIVATIVE ANALYSES OF THE VINYLIDENE AND ACETYLENE TRIPLET-STATE POTENTIAL-ENERGY HYPERSURFACES

First and second derivatives of the orbital, electronic, nuclear, and total energies for the self-consistent-field (SCF) wave function have been used to study the triplet state of the CCH2 and HCCH molecules. T he diagonal elements of the Lagrangian matrix for the general open-she ll SCF wave function are used as the ''orbital'' energies. The first a nd second derivatives of the orbital energies in terms of the normal c oordinates are determined by the finite difference method, while the c orresponding derivatives of the electronic, nuclear, and total SCF ene rgies are obtained via analytic derivative techniques. It is demonstra ted that the derivatives of the energies with respect to the normal co ordinates provide useful chemical information for the triplet state po tential energy hypersurfaces of the CCH2 and HCCH molecules. Specifica lly, the isomerization reactions from a B-3(2) vinylidene to b 3B(u) a cetylene and from a B-3(2) acetylene to b 3B(u) acetylene are found to be electronically stable processes and their negative force constants (or imaginary vibrational frequencies) are seen to be controlled by t he negative nuclear contributions. Energy derivative analysis may give strong quantitative support for arguments based on the conventional q ualitative molecular orbital (MO) theory.