Hydrogen‐bonding Structures and Energetics of Acrylamide Isomers, Tautomers, and Dimers: An ab initio Study and Spectral Analysis

Abstract

Hydrogen‐bonding patterns and energetic profiles of acrylamide isomers (syn‐ and skew‐), tautomers (amide and imidic acid forms) and 13 stable dimers have been studied using the second‐order Møller–Plesset perturbation theory with basis sets up to aug‐cc‐pVTZ. Syn‐acrylamide is the most stable monomer with a reaction barrier of 4.15 kcal/mol for the syn–skew isomerization reaction. The direct amide–imidic acid tautomerization reaction is separated by too high a barrier to surpass. The most stable dimer corresponds to the planar double‐hydrogen‐bonded configuration, indicating its crucial role in determining the stability of the formed complex. Moreover, hydrogen bonds have significant effects on the infrared spectral features, which can be consistently explained solely based on the acrylamide dimeric structures and energetics without monomeric and dimeric tautomer forms. The results are useful for studying the stability of the acrylamide clusters in condensed‐phase samples such as those in food chemistry studies.

Hydrogen Bonding Aggregation in Acrylamide: Theory and Experiment

“…Based on the combination of AICc and r i 2 for the fits of c(x) and ln K i (1/T), we can conclude that models 6s (and the equivalent model 10s), 8s, and 10g are good. All of these models give the value of the bond energy in a dimer as about 6−8 kcal/mol, which corresponds much more closely to the values calculated by DFT 12,13 discussed earlier. Another important property of these models is that the values they give for both C 1 and C 2 are smaller than those found for one-constant models.…”

“…It can be seen that the absolute values of the predicted bond energies are close to 1 kcal/mol for all models. However, according to DFT calculations, the absolute value of the bond energy in an acrylamide dimer in a vacuum can be estimated as 7−9 kcal/mol, 12,13 which is much larger than the values obtained for one-parameter models.…”

“…In addition to the strengths of the association model listed above, we also believe that it will yield insights into hydrogen-bonded acrylamide aggregates that would be difficult, or even impossible, to obtain by other techniques, such as density functional theory (DFT) 12,13 and molecular dynamics simulations 14 . DFT is a powerful tool that gives many valuable insights into the physics of hydrogen bonding, such as the effect of the conformation and relative positions of the molecules on the energy of the hydrogen bonding interaction.…”

“…From both experimental studies (for example, NMR spectroscopy) and DFT simulations it is known that tautomerism is not present in acrylamide because imidic acid has an energy approximately 11 kcal/mol higher than the ground state energy of the syn-isomer 13 , so we do not consider the possibility of tautomerism in our models. Isomerism is also not considered, because 95% of the molecules are in the syn-isomer state at room temperature due to the large difference in ground state energy between the two isomers 22 .…”

“…These give better correspondence to the bond energies in acrylamide dimers predicted by DFT. 12,13 At the end of the paper, we take a first step toward the application of the association model approach to the description of hydrogen bonding in block copolymers by calculating the structure factor of a disordered melt of block copolymer with one hydrogen bonding self-associating block and one non-hydrogen bonding block in the random phase approximation (RPA). Next, the models and parameters determined for acrylamide are used to estimate how the Flory−Huggins parameter that appears in the RPA formula for the structure factor of the diblock copolymer with a polyacrylamide block is shifted by the presence of hydrogen bonds in these systems.…”