Understanding the Exceptional Properties of Nitroacetamides in Water: A Computational Model Including the Solvent

• Main Authors: Giovanni La Penna, Fabrizio Machetti
• Format: Article
• Language: English
• Published: MDPI AG 2018-12-01
• Series: Molecules
• Subjects: amides; carbanions; C–H acidity; nitro-aci tautomerism; molecular dynamics; density-functional theory
• Online Access: https://www.mdpi.com/1420-3049/23/12/3308

Proton transfer in water involving C&#8315;H bonds is a challenge and nitro compounds have been studied for many years as good examples. The effect of substituents on acidity of protons geminal to the nitro group is exploited here with new p<inline-formula> <math display="inline"> <semantics> <msub> <mi>K</mi> <mi>a</mi> </msub> </semantics> </math> </inline-formula> measurements and electronic structure models, the latter including explicit water environment. Substituents with the amide moiety display an exceptional combination of acidity and solubility in water. In order to find a rationale for the unexpected p<inline-formula> <math display="inline"> <semantics> <msub> <mi>K</mi> <mi>a</mi> </msub> </semantics> </math> </inline-formula> changes in the (ZZ<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>&#8242;</mo> </msup> </semantics> </math> </inline-formula>)NCO- substituents, we measured and modeled the p<inline-formula> <math display="inline"> <semantics> <msub> <mi>K</mi> <mi>a</mi> </msub> </semantics> </math> </inline-formula> with Z=Z<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>&#8242;</mo> </msup> </semantics> </math> </inline-formula>=H and Z=Z<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>&#8242;</mo> </msup> </semantics> </math> </inline-formula>=methyl. The dominant contribution to the observed p<inline-formula> <math display="inline"> <semantics> <msub> <mi>K</mi> <mi>a</mi> </msub> </semantics> </math> </inline-formula> can be understood with advanced computational experiments, where the geminal proton is smoothly moved to the solvent bath. These models, mostly based on density-functional theory (DFT), include the explicit solvent (water) and statistical thermal fluctuations. As a first approximation, the change of p<inline-formula> <math display="inline"> <semantics> <msub> <mi>K</mi> <mi>a</mi> </msub> </semantics> </math> </inline-formula> can be correlated with the average energy difference between the two tautomeric forms (<i>aci</i> and <i>nitro</i>, respectively). The contribution of the solvent molecules interacting with the solute to the proton transfer mechanism is made evident.