3. Computational Details
Isotopically substituted and unsubstituted molecular structures were first geometry-optimized to an energy minimum. The 6-311(2d,3p) basis set was selected and was used consistently in the geometry optimizations and all subsequent single-point calculations. Isotopic effects were included by performing the geometry optimization using a finite-difference approach using energies alone, calculated with the mass-dependent diagonal Born-Oppenheimer energy correction (DBOC)47,48 applied. This was carried out at the CCSD level of theory using the CFOUR code49 , with the following tight convergence criteria (in atomic units): Maximum coupled-cluster amplitude change > 10-10, RMS energy gradient < 10-5, smallest linear equation (DIIS) residual < 10-10 and integrals tolerance 10-15.
Torsional scans (using constrained geometry optimization) were then made based on the converged structures from the previous step, with the C1 atom position coordinates and the C1-(H,D,T)3 and C1-(H,D,T)10 bond lengths constrained to their previously converged energy minimum values, using the B3LYP DFT functional, very tight geometry convergence criteria and an ’ultrafine’ grid for the DFT integrals as implemented in the G09 vE.0150 code. The full optimization Hessian was recalculated at every optimization step. Finally, single-point wavefunction calculations were run using G09, the same DFT functional, basis set and integral parameters, with the SCF convergence criteria < 10-10 RMS change in the density matrix, and the corresponding wavefunctions analysed using AIMAll51. Next generation QTAIM analyses for Uσ-space trajectories and derived quantities were calculated from the resulting molecular graphs and wavefunctions using our in-house developed software package QuantVec (formerly AIMPAC2-Suite)52.