Layer resolved H-bond cooperativity in cluster model of boric acid: a combined IR, Raman, NCI, and QTAIM study

Boric acid (BA) is a classical H-bonded molecular solid in which intricate cooperative H-bonded interactions govern structural stability, electronic structures and vibrational properties. Despite extensive studies, a clear molecular-level understanding of how H-bond cooperativity influences vibrational stratification and interlayer interactions is limited. Vibrational analyses from this study reveal clear stratification of O-H stretching modes due to distinct H-bonding environments, with strongly cooperative inner-ring H-bonds providing a pronounced red-shifted frequency (~ 2932 cm⁻¹), while comparatively weaker outer-ring interactions lead to a blue shift ~ 3318 cm⁻¹. Non-covalent interaction (NCI) and quantum theory of atoms in molecules (QTAIM) analyses confirm that intralayer stabilization is dominated by strong H-bonding, whereas interlayer interactions are significantly weaker and primarily van der Waals in nature. The calculated H-bonding binding energies further quantify this disparity, with intra-layer interactions (~ - 47 kJ/mol) greatly exceeding interlayer interactions (~ -3 kJ/mol). Further, the frontier molecular orbital analysis indicates large energy gaps for both monolayer and bilayer systems, with slight reduction upon stacking due to enhanced interlayer delocalization. Overall, the results provide direct evidence of H-bond cooperativity and its role in governing vibrational and electronic properties in layered BA systems.

Initial geometries were derived from experimentally validated crystal structures and modelled as finite hexameric clusters representing boric-acid rosette motifs. Geometry optimizations were carried out using the Perdew-Burke-Ernzerhof (PBE) functional with a plane-wave basis set and ultrasoft pseudopotentials as implemented in the Quantum ESPRESSO package. Subsequent single-point calculations were performed using the M05-2X/6-31G(d,p) basis set using Gaussian 09 to obtain accurate wave functions for non-covalent interaction analysis. NCI analyses were conducted using the Multiwfn program, while QTAIM analysis was used to characterize bond critical points and interaction strengths. Vibrational frequencies (IR and Raman) were computed at the same level of theory to probe H-bonding environments. Molecular visualization was carried out using VMD software.