Excited state relaxation mechanisms of paracetamol and acetanilide

The photochemical pathways of acetanilide and paracetamol were investigated using the XMS-CASPT2 quantum chemical method and the cc-pVDZ (correlation consistent polarized valence double- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ζ</mml:mi></mml:mrow> </mml:math> ) basis set. In both compounds, the bright state is the second excited state, designated as a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mrow></mml:mrow> <mml:mn>1</mml:mn></mml:msup> <mml:mo>(</mml:mo> <mml:msup><mml:mi>ππ</mml:mi> <mml:mo>*</mml:mo></mml:msup> </mml:mrow> </mml:math> L_a) state. Through a detailed exploration of the potential energy profile and the conical intersection structure between the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mrow></mml:mrow> <mml:mn>1</mml:mn></mml:msup> <mml:mo>(</mml:mo> <mml:msup><mml:mi>ππ</mml:mi> <mml:mo>*</mml:mo></mml:msup> </mml:mrow> </mml:math> L_a) and ground states, we gained a better understanding of how cleavage might occur in both molecules upon photoexcitation. Other potential relaxation mechanisms, including crossings with the dark <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mrow></mml:mrow> <mml:mn>1</mml:mn></mml:msup> <mml:mfenced><mml:mrow><mml:mi>n</mml:mi> <mml:msup><mml:mi>π</mml:mi> <mml:mo>*</mml:mo></mml:msup> </mml:mrow> </mml:mfenced> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mrow></mml:mrow> <mml:mn>1</mml:mn></mml:msup> <mml:mo>(</mml:mo> <mml:msup><mml:mi>ππ</mml:mi> <mml:mo>*</mml:mo></mml:msup> </mml:mrow> </mml:math> L_a) states, are also discussed in detail.