Hydrogen bond strengths in microhydrated clusters of HNO and HONO: energetic insights via a molecular tailoring approach

Abstract

The strengths of various types of individual hydrogen bonds (HBs) present in microhydrated clusters of HNO and HONO, i.e., HNO(H₂O)_n and HONO(H₂O)_n (n = 1–4), were examined using a molecular tailoring approach-based (MTA-based) method at the MP2/aug-cc-pVTZ//MP2/6-311++G(d,p) level of theory. The calculated binding energies (E_BE) fall in a broader range for the HONO(H₂O)_n complexes (from −2.32 to −38.52 kcal mol⁻¹) than for the HNO(H₂O)_n complexes (from −2.93 to −34.49 kcal mol⁻¹). The HBs involving HONO as the proton donor (HB energy falls between 7.88 and 15.08 kcal mol⁻¹) are stronger than that involving HNO as the proton donor (HB energy falls between 3.11 and 7.24 kcal mol⁻¹). This may be attributed to the stronger acidity of the proton present in HONO. Among various HNO(H₂O)_n and HONO(H₂O)_n complexes, the strongest HB (energy = 15.08 kcal mol⁻¹) was found in one of the isomers of HONO(H₂O)₄. The most distinctive feature of HBs of HONO compared to that of HNO is their greater energy and cooperativity contribution when it interacts with water as the HB donor. On the other hand, the –NO moiety of HNO is a stronger HB acceptor than that of HONO in their monohydrate complexes, but this trend is not observed in the larger clusters because of the greater cooperative contribution found in HONO. Furthermore, in HNO(H₂O)_n complexes, the self-associative interaction between water molecules is preferred in larger clusters (n = 4), whereas cross-associative interactions between HONO and water are dominant along with the water⋯water interactions for all HONO(H₂O)_n=1–4 complexes, leading to the formation of the most stable cyclic cluster.