**4. Conclusions**

Reactions of amino alcohols (3-amino-1-propanol, 2-amino-1-butanol, or 2-amino-2-methyl-1-propanol) and quinaldinic acid have produced salts, which consist of protonated amino alcohol and deprotonated quinaldinic acid. The obtained products obey the ΔpKa rule. Of the three products, (3a1pOHH)quin (**1**), (2a1bOHH)quin (**2**), and (2a2m1pOHH)quin (**3**), the last two are polymorphic. A structural survey has revealed all

five possible heterosynthons in their crystal structures. The supramolecular structures of all are built of the NH3 <sup>+</sup>···−OOC synthon in combination with one to up to three other heterosynthons. Interestingly, the OH···N(quin−) synthon occurs only in one phase. The **2a**/**2b** and **3a**/**3b** polymorphic pairs differ both in the types of hydrogen bonds and in *π*···*π* stacking interactions. Due to the former, they are hydrogen bond isomers of the same compound. The presented series is yet another demonstration of polymorphism among molecular solids.

**Supplementary Materials:** The following supporting information can be downloaded, PXRD patterns (Figures S1–S3), packing diagrams (Figures S4–S6), IR spectra for 1–3b (Figures S7–S11), 1H NMR spectra for 1–3 (Figures S12–S14), and TG/DSC curves (Figures S15–S17).

**Author Contributions:** Conceptualization, B.M. and N.P.; validation, B.M. and N.P.; writing review and editing, B.M. and N.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Slovenian Research Agency (Junior Researcher Grant for N.P. and the Program Grant P1-0134).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** The authors thank Romana Cerc Korošec for running the TG/DSC experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Obtained from the authors upon request.
