High accuracy ab initio quantum chemistry on P, Si, and C-containing molecules
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The ground states and low-lying triplet states of P2H2 isomers and the associated isomerization transition states have been investigated systematically and the global minimum has been confirmed to be planar trans-HPPH diphosphene, lying 3.2 kcal mol-1 below cis-HPPH with the aug-cc-pVQZ CCSD(T) method upon inclusion of zero point vibrational energy corrections. This transition state for the trans-cis isomerization reaction exhibits multireference character and the reaction barrier is predicted to be 35.2 kcal mol-1 using the cc-pVQZ Mukherjee multireference coupled cluster (Mk-MRCCSD) (2e/2MO) method. The energy crossing of the singlet and triplet adiabatic potential energy surface is studied using Mk-MRCCSD method with the cc-pVQZ basis set, which predicts that the 3B skewed HPPH is 1.4 kcal mol-1 lower in energy than corresponding 1A skewed HPPH at the 3B skewed HPPH optimized geometry. An accurate quartic force field for disilacyclopropenylidene-1S has been determined employing ab initio coupled-cluster (CC) with single and double excitations and perturbative treatment for triple excitations [CCSD(T)] method in combination with the correlation consistent core-valence quadruple zeta (cc-pCVQZ) basis set. The vibration-rotation coupling constants, equilibrium and zero-point vibration corrected rotational constants, centrifugal distortion constants, harmonic and fundamental vibrational frequencies for six isotopologues of disilacyclopropenylidene-1S are reported using vibrational second-order perturbation theory (VPT2). The anharmonic corrections for the vibrational motions involving the H bridged bonds are found to be more than 5% with respect to the corresponding harmonic vibrational frequencies.