Quantum-Induced Symmetry-Breaking in the Deuterated Dihydroanthracenyl Radical

The hydrogen-atom adduct with anthracene, 9-dihydroanthracenyl radical (C<sub>14</sub>H<sub>11</sub>), and its deuterated analogue, have been identified by laser spectroscopy coupled to time-of-flight mass spectrometry, supported by time-dependent density functional theory calculations. The electronic spectrum of 9-dihydroanthracenyl radical exhibits an origin band at 19115 cm<sup>-1</sup> and its ionization energy was determined to be 6.346(1) eV. The spectra reveal a low-frequency vibrational progression corresponding to a mode described by a butterfly-inversion. In the deuterated analogue, a zero-point-energy imbalance along this coordinate is found to lead to a doubling of the observed spectral lines in the progression. This is attributed to quantum-induced symmetry breaking as previously observed in isotopologues of CH<sub>5</sub><sup>+</sup>.