Electrostatics and Dispersion in X–H···Y (X = C, N, O; Y = N, O) Hydrogen Bonds and Their Role in X–H Vibrational Frequency Shifts

Abstract

The frequency shifts of donor stretching vibration in X–H···Y (X = C, N, O; Y = N, O) hydrogen-bonded complexes of phenylacetylene, indole, and phenol are linearly correlated with the electrostatic component of the interaction energy. This linear correlation suggests that the electrostatic component, which is the first-order perturbative correction to the stabilization energy, is essentially localized on the X–H group. The linear correlation suggests that the electrostatic tuning rate, which is a measure of the X–H oscillator to undergo shifts upon hydrogen bonding per unit increase in the electrostatic component of the stabilization energy, was found to be in the order of O–H > N–H > C–H. Interestingly, for each of the donor groups, viz., C–H, N–H, and O–H, the vibrational frequency shifts were inversely correlated to the dipole moment of the acceptor separately, which is counterintuitive vis-à-vis the electrostatic component. This implies that extrapolation to zero dipole moment of the acceptor will yield very large shifts in the hydrogen-bonded X–H stretching frequencies. The trends in the variation of the dispersion and exchange-repulsion components and the total interaction energy vis-à-vis frequency shifts of donor stretching vibration are similar for hydrogen-bonded complexes of phenylacetylene, indole, and phenol. Furthermore, it was observed that the vibrational frequency shifts of all of the complexes are linearly correlated with the charge transfer from the filled orbital of the hydrogen acceptor to the vacant antibonding (σ*) orbital of the X–H donor group on the basis of natural bonding orbital calculations.

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“…Nevertheless, these levels of impact on the photometric and astrometric RMS estimates are still acceptable, especially for the astrometric RMS estimates. For statistical constraints on the off-nucleus and dual AGN population (e.g., Shen et al 2019), these systematics will not cause significant issues because a ∼ 10 mas systematic uncertainty in the astrometric RMS still implies a stringent upper limit of the inferred pair separation. For individual cases, however, we caution that a large astro-metric RMS signal may be due to extended host morphology.…”

“…Confirming (i.e., resolving) them at < 100 mas imaging resolution may be challenging. But at the very least, these astrometric RMS estimates place stringent limits on the off-nucleus distance of these small-scale pairs (e.g., Shen et al 2019).…”

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