Guest and host deuterium effects of the large ZFS parameters D of dimethylbenzaldehydes in durene single crystals

Abstract

The ZFS parameters D of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1 h 1 and -1 d 1 guests in perhydrogenated and perdeuterated durene single crystals are determined by comparing the experimental and calculated resonance curves. It is found that the deuterium substitution of the guest aldehydic group in a given host leads to the decrease of the D values and to the increase of the energy gaps Δ E T between the zero-point levels of the 3nπ* and 3ππ* states of the guests. On the other hand, the perdeuteration of the host results in the decrease of Δ E T with a corresponding increase of the D value of a given guest. The D value of 1 cm −1 determined for 2,5-dimethylbenzaldehyde-1 h 1 in perdeuterated durene is the lategest ever found for an aromatic carbonyl compound. Correlations between D and Δ E T indicate that the ZFS parameters D of the guests are determined by contributions from both spin-spin and spin-orbit interactions between the 3nπ* and 3ππ* states. The large guest and host deuterium effects observed on the D values are attributed to the changes of the gaps Δ E T of the guests.