13C chemical-shift anisotropy of alkyl-substituted aromatic carbon in anthracene derivatives

The (13)C chemical-shift anisotropy in anthracene derivatives (9,10-dimethylanthracene, 9,10-dihydroanthracene, dianthracene, and triptycene) has been measured by the 2D FIREMAT timed pulse sequence and the corresponding set of principal values has been determined by the TIGER processing method. These molecules expand the data base of (13)C CSA measurements of fused aromatic rings some bridged by sp(3) carbon resulting in an unusual bonding configuration, which leads to distinctive aromatic (13)C CSA values. Crystal lattice distortions to the CSA were observed to change the isotropic shift by 2.5 to 3.3 ppm and changes as large as 8.3 ppm in principal components. Modeling of the CSA data by GIPAW DFT (GGA-PBE/ultrafine) shielding calculations resulted in an rms chemical-shift distance of 2.8 ppm after lattice including geometry optimization of the diffraction structures by the GIPAW method at GGA-PBE/ultrafine level. Attention is given to the substituted aromatic carbon in the phenyl groups (here forth referred to as the α-carbon) with respect to CSA modeling with electronic methods. The (13)C CSA of this position is accurately determined due to its spectral isolation of the isotropic shift that limits overlap in the FIREMAT spectrum. In cases where the bridging ring is sp(3) carbon, the current density is reduced from extending beyond the peripheral phenyl groups; this plays a significant role in the magnetic shielding of the α-position. Nuclear independent chemical-shift calculations based on GIAO DFT (B3LYP/6-31G(d)) shielding calculations were used to model the intramolecular π-interactions in dianthracene and triptycene. These NICS results estimate the isotropic shift of the α-position in dianthracene to be insignificantly affected by the presence of the neighboring aromatic rings. However, a notable change in isotropic shielding, Δσ(iso)=-2.1 ppm, is predicted for the α- position of triptycene. Experimentally, the δ22 principal component at the α-position for both dianthracene and triptycene increases by at least 12 ppm compared to 9,10-dihydroanthracene. To rationalize this change, shielding calculations in idealized structures are explored. The spatial position of the bicyclic scaffolding of the bridging ring plays a key role in the large increase in δ22 for the α-carbon.