Abstract
Ab initio SCF were performed on the trans and gauche rotamers of succinonitrile (SN) and on the monohydrated rotamers to determine the most probable hydration site(s) of water with SN and the relative strengths of these interactions and to examine the effect of one water molecule on the relative stability of the rotamers. MOPAC was used to determine the location of water on the rotamers of SN; the geometries of the hydrated rotamers were then optimized at the HF/4–31G level followed by single‐point calculations at the HF/6–31G and HF/6–31G* levels. The results show that trans SN is more stable than is gauche SN by 1.2 kcal/mol (at the HF/4–31G level with zero‐point energy corrections) and 0.8 kcal/mol (at the HF/6–31G* level, excluding zero‐point energy corrections). The hydrogen bond formed between the methylene hydrogen of trans SN and the oxygen of water is more stable than is the hydrogen bond formed between the nitrogen of SN~t~ and the hydrogen of water by 0.63 kcal/mol. The most stable hydrogen‐bonding interaction for the gauche rotamer occurs for the methylene—OH~2~ hydrogen‐bonding interaction, which also has the largest hydration energy (Δ__E__ = −4.39 kcal/mol), but it is less stable than is the most stable trans hydrogen‐bonding interaction (SN~t~—OH~2~) by 0.63 kcal/mol. Our results suggest that the trans rotamer of SN should associate with more water molecules than should the gauche. The relative rotamer stability between the trans and gauche rotamers of SN decreases by 0.2 kcal/mol (at the HF/6–31G* level) when water is hydrogen bonded to the methylene group of SN. A mechanism is also proposed to explain the phase‐separation behavior of this system.