Ab initio calculation of zero-field splitting and spin-orbit coupling in ground and excited triplets of m-xylylene

We report CASSCF(6,6)/cc-pVDZ optimized geometries, energies (also single-p oint CASPT2(6,6)/cc-pVDZ), electron spin-spin dipolar interaction (D,E) ten sor, and spin-orbit coupling (SOC) for m-xylylene in the lowest triplet T-1 (1(3)B(2)), in the next tripler 1(3)A(1), and in the slightly higher 2(3)B (2). The zero-field splitting (zfs) parameters computed for T-1 (D/hc = 0.0 13 cm(-1), E/hc = -0.003 cm(-1)) agree well with the observed values \D/hc\ = 0.011 cm(-1), \E/hc\ < 0.001 cm(-1). If (3)A(1) is the T-2 state as calc ulated, its computed D/hc (-0.040 cm(-1)) and E/hc (0.001 cm(-1)) agree wit h the value \D/hc\ = 0.04 +/- 0.01 cm(-1) deduced from experiment assuming E = 0. If 2(3)B(2) is the T-2 slate, the experimental data need to be reeva luated, since its computed E/hc value (-0.012 cm(-1)) is not negligible rel ative to D/hc (0.038 cm(-1)). The SOC matrix elements of T-1-T-2 with the l owest and the pi pi* excited singlets are small (similar to 0.01-0.1 cm(-1) ), while those with representative (1)sigma pi* states are large (similar t o 10 cm(-1)). The former lack one-center terms and therefore are much small er than expected from the standard one-electron approximation. Computed SOC affects D and E slightly, and supports the proposed vibronic mechanism of intersystem crossing from T-2.