HOMO and LUMO energy levels (eV) of IPr# calculated at the B3LYP 6-311++g(d,p) level^a.

NHC	HOMO [eV]	LUMO [eV]
IPr#	−6.16	−0.96
BIAN-IPr#	−6.16	−0.95
Np#	−6.15b	−0.97b
IPr*	−6.12	−0.90
IPr	−6.01	−0.48
IMes	−5.90	−0.33

^aSee ESI for details.

^brac-Np# (C₂-symmetric), meso-Np# (C_S-symmetric), −6.13 eV and −0.94 eV. rac-Np# is more stable than meso-Np# by 0.56 kcal mol⁻¹ calculated at the B3LYP 6-311++g(d,p) level.