Photodissociation dynamics of 2,5-dihydroxyacetophenone

Area of Science:

• Physical Chemistry
• Photochemistry
• Spectroscopy

Background:

• 2,5-dihydroxyacetophenone (DHAP) is a crucial matrix compound in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry.
• Understanding the photodissociation dynamics of DHAP is essential for optimizing MALDI applications and interpreting experimental results.
• Previous studies have explored the photodissociation of related aromatic ketones, providing a basis for comparison.

Purpose of the Study:

• To investigate the photodissociation pathways of 2,5-dihydroxyacetophenone (DHAP) at 193 nm.
• To determine the primary and minor dissociation channels and analyze the photofragment translational energy distributions.
• To elucidate the electronic states involved in the photodissociation process and compare the dynamics with acetophenone and phenol derivatives.

Main Methods:

• Experiments were conducted using a molecular beam setup.
• Multimass ion imaging techniques were employed to study the photodissociation.
• Comparison with Rice-Ramsperger-Kassel-Marcus (RRKM) calculations was performed.

Main Results:

• Two major dissociation channels were identified: C(6)H(3)(OH)(2)COCH(3) --> OC(6)H(3)(OH)COCH(3) + H and C(6)H(3)(OH)(2)COCH(3) --> C(6)H(3)(OH)(2) + COCH(3).
• Minor channels involving the loss of CH(3) or CO + CH(3) were also observed.
• Analysis of translational energy distributions suggests that the H-loss channel originates from an excited state with a repulsive O-H potential.

Conclusions:

• The photodissociation of DHAP at 193 nm proceeds through distinct pathways, with H-atom elimination being a significant channel.
• The observed dynamics for the second major channel are inconsistent with dissociation occurring on the ground or triplet states following RRKM theory.
• The study provides valuable insights into the photochemistry of DHAP, relevant for its use in MALDI and for fundamental understanding of aromatic ketone photodissociation.