The side-on spatial arrangement of C-Br and C=O were crucial for the heavy-atom effect because of better mutual alignment of the in-plane Br and O p orbitals. This resulted in large HONTO coefficients in the T1 state (Figure ). However, the arrangement is energetically unfavorable because there was Coulomb repulsion, although Br and O in the TP geometry of 1 were within the sum of their van der Waals radii according to the X-ray structure (Figure ). 15a,27 Indeed, DFT calculations for the aldehyde 2a indicated that the side-on conformation was more unstable (by 3 kcal mol -1 ) for both S0 and T1 states than the conformation with oxygen pointing away from the Br (Figure ). Interestingly, the TP geometry of 1 involved two-fold intramolecular S…O noncovalent chalcogen bonding interactions, 28 with interaction energies as large as 7.74 kcal mol -1 , according to the natural bonding orbital analysis of 1a. 15a The chalcogen bonds stabilized the TP conformer and increased kp by forcibly fixing Br in the vicinity of carbonyl oxygens.