Intermolecular clamping by hydrogen bonds: 2-pyridone⋅NH3

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• The 2-pyridone (2PY) molecule exhibits interesting photophysical properties, including non-radiative decay pathways influenced by its structure.
• Hydrogen bonding plays a crucial role in modifying molecular properties and intermolecular interactions.
• Understanding excited-state dynamics is essential for controlling photochemical processes.

Purpose of the Study:

• To investigate the effect of a doubly hydrogen-bonded complex with ammonia (NH3) on the excited-state properties of 2-pyridone (2PY).
• To elucidate the structural and dynamical changes in the S(1) state of the 2PY·NH3 complex.
• To compare the spectroscopic and theoretical findings with previous studies.

Main Methods:

• Combined experimental techniques: mass-resolved UV vibronic spectroscopy of jet-cooled 2PY·NH3 and its isotopomers using two-color resonant two-photon ionization.
• Ab initio theoretical calculations: Density Functional Theory (B3LYP) and correlated methods (MP2, CC2) to determine equilibrium structures, frequencies, and vibronic assignments.
• Spectroscopic analysis of the S(1)←S(0) spectrum up to ≈1200 cm(-1) above the 0(0) band.

Main Results:

• The S(1) state non-radiative decay in the 2PY·NH3 complex is significantly slowed for vibrations above ≈300 cm(-1) compared to bare 2PY.
• Overtone bands associated with out-of-plane vibrations in bare 2PY are weaker or absent in the complex, indicating a clamped planar geometry in the S(1) state.
• Calculations predict a doubly H-bonded bridged structure of C(S) symmetry, with asymmetric bridge expansion in the excited state primarily due to weakening of the NH3-keto interaction.

Conclusions:

• The doubly hydrogen-bonded bridge in 2PY·NH3 effectively clamps the 2PY moiety into a planar geometry in the S(1) state, altering its photophysics.
• Ammonia acts as a strong hydrogen bond donor in this bridged system, influencing the excited-state dynamics and structure.
• The excited-state electronic excitation leads to an asymmetric expansion of the hydrogen-bonded bridge, predominantly driven by the NH3-keto hydrogen bond.