Hydrogen-bond donors and acceptors, straight from the σ-profile

Hydrogen bonding is the interaction that most often decides whether a molecule dissolves, partitions, or co-crystallizes. One of the quiet strengths of the σ-profile is that hydrogen-bond capacity falls out of it directly — no separate empirical donor/acceptor count required.

Where H-bonding lives in the profile

Recall the profile's wings. Strongly positive-polarized surface (the σ region around polar hydrogens) is donor character. Strongly negative-polarized surface (around lone pairs on O, N, F) is acceptor character. The area under each wing beyond a threshold is a continuous, physical measure of donor and acceptor capacity — richer than an integer count of donors and acceptors, because it reflects strength and accessibility, not just presence.

Computing the split

A donor/acceptor decomposition partitions the σ-profile into nonpolar, donor, and acceptor regions using σ thresholds, then integrates the area in each. The result is a small set of numbers — donor area, acceptor area, and their balance — that feed directly into COSMO-RS-style hydrogen-bonding terms and into property models.

Why a continuous measure beats counting

• It captures strength: a strongly acidic O–H and a weakly acidic one are different donors, and the area-based measure sees that.
• It captures accessibility: a buried acceptor contributes less surface than an exposed one.
• It is consistent with the thermodynamics: the same surface that drives COSMO-RS H-bond energy is the surface you are integrating.
• It avoids brittle SMARTS rules for what 'counts' as a donor or acceptor.

Using it

Donor/acceptor areas are excellent features for solubility, permeability, and partitioning models, and they are interpretable: when a model leans on acceptor area, you can point to the part of the molecule responsible. mfsig.com returns the hydrogen-bond donor/acceptor decomposition alongside the σ-profile and σ-moments, all from the same validated surface.