Vibrational spectra and anharmonic coupling of the O–H stretch in 10-hydroxybenzo[h]quinoline: Insights from constrained nuclear–electronic orbital theory

Hydrogen bonds can exhibit pronounced nuclear quantum effects (NQEs) that strongly influence proton-transfer dynamics. 10-hydroxybenzo[h]quinoline (HBQ) is a prototypical excited-state intramolecular proton transfer (ESIPT) system with a strong intramolecular hydrogen bond, and its vibrational spectra and anharmonic mode couplings have recently attracted considerable interest. Here, we use constrained nuclear-electronic orbital density functional theory (CNEO-DFT) and molecular dynamics (CNEO-MD), which account for NQEs, particularly nuclear quantum delocalization effects, to investigate the O-H stretching vibration (νOH) of HBQ and its anharmonic coupling with other modes in the ground state. The νOH frequency calculated by CNEO-DFT agrees closely with the experiment, and CNEO-MD reproduces the characteristic finger-like structure of the infrared spectrum. Normal mode frequency shift analysis and CNEO-MD simulations with resonant νOH excitation consistently identify several low-frequency in-plane backbone bending modes that are strongly coupled to the νOH. Taken together, this work establishes a detailed understanding of the ground-state vibrational spectrum and νOH mode couplings in HBQ, providing a basis for further investigation of their roles in ESIPT dynamics.