1-Germavinylidene (GeCH₂), Germyne (HGeCH), and 2-Germavinylidene (H₂GeC) Molecules and Isomerization Reactions among Them: Anharmonic Rovibrational Analyses

Theoretical investigations of three equilibrium structures and two associated isomerization reactions of the GeCH₂ - HGeCH - H₂GeC system have been systematically carried out. This research employed ab initio self-consistent-field (SCF), coupled cluster (CC) with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD­(T)] wave functions and a wide variety of correlation-consistent polarized valence cc-pVXZ and cc-pVXZ-DK (where X = D, T, Q) basis sets. For each structure, the total energy, geometry, dipole moment, harmonic vibrational frequencies, and infrared intensities are predicted. Complete active space SCF (CASSCF) wave functions are used to analyze the effects of correlation on physical properties and energetics. For each of the equilibrium structures, vibrational second-order perturbation theory (VPT2) has been utilized to obtain the zero-point vibration corrected rotational constants, centrifugal distortion constants, and fundamental vibrational frequencies. The predicted rotational constants and anharmonic vibrational frequencies for 1-germavinylidene are in good agreement with available experimental observations. Extensive focal point analyses, including CCSDT and CCSDT­(Q) energies and basis sets up to quintuple zeta, are used to obtain complete basis set (CBS) limit energies. At all levels of theory employed in this study, the global minimum of the GeCH₂ potential energy surface (PES) is confirmed to be 1-germavinylidene (GeCH₂, 1). The second isomer, germyne (HGeCH, 2) is predicted to lie 40.4(41.1) ± 0.3 kcal mol^–1 above the global minimum, while the third isomer, 2-germavinylidene (H₂GeC, 3) is located 92.3(92.7) ± 0.3 kcal mol^–1 above the global minimum; the values in parentheses indicate core–valence and zero-point vibration energy (ZPVE) corrected energy differences. The barriers for the forward (1→2) and reverse (2→1) isomerization reactions between isomers 1 and 2 are 48.3(47.7) ± 0.3 kcal mol^–1 and 7.9(6.6) ± 0.3 kcal mol^–1, respectively. On the other hand, the barriers of the forward (2→3) and reverse (3→2) isomerization reactions between isomers 2 and 3 are predicted to be 55.2(53.2) ± 0.3 kcal mol^–1 and 3.3(1.6) ± 0.3 kcal mol^–1, respectively.