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.