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.