Abstract
Structures, thermochemical properties, bond energies, and internal rotation potentials of acetic acid hydrazide (CH3CONHNH2), acetamide (CH3CONH2), and N-methyl acetamide (CH3CONHCH3), and their radicals corresponding to the loss of hydrogen atom, have been studied. Gas-phase standard enthalpies of formation and bond energies were calculated using the DFT methods B3LYP/6-31G(d,p), B3LYP/6-31G(2d,2p) and the composite CBS-QB3 methods employing a series of work reactions further to improve the accuracy of the ΔHf°(298 K). Molecular structures, vibration frequencies, and internal rotor potentials were calculated at the DFT level. The parent molecules’ standard formation enthalpies of CH3–C=ONHNH2, CH3–C=ONH2, and CH3–C=ONHCH3 were evaluated as −27.08, −57.40, and −56.48 kcal mol−1, respectively, from the CBS–QB3 calculations. Structures, internal rotor potentials, and C–H and N–H bond dissociation energies are reported. The DFT and the CBS-QB3 enthalpy values show close agreement, and this accord is attributed to the use of isodesmic work reactions for the analysis. The agreement also suggests this combination of the B3LYP/work reaction approach is acceptable for larger molecules. Internal rotor potentials for the amides are high, ranging from 16 to 22 kcal mol−1.
Highlights
Thermochemical and spectroscopic investigation of N-methyl Acetamide (NMA)and other substituted amides are considered as model compounds for the peptide bonds in proteins; understanding their thermochemistry can provide information about the secondary structure of proteins in the gas phase as well as inferences in solution and helpful information toward understanding kinetics
B3LYP/work reaction approach is acceptable for larger molecules
Spectroscopic features is assumed as the fundamental basis for spectroscopic methods to monitor protein structure and dynamics [1]. Both of these properties are of interest for future applications, only if both structures and spectroscopic properties of NMA are observed in the natural environment of the biological system(s)
Summary
Thermochemical and spectroscopic investigation of N-methyl Acetamide (NMA)and other substituted amides are considered as model compounds for the peptide bonds in proteins; understanding their thermochemistry can provide information about the secondary structure of proteins in the gas phase as well as inferences in solution and helpful information toward understanding kinetics. There are several reasons for interest in the structure and chemistry of these amide systems These include: (i) clear understanding of the NMA structure is considered as the basis for understanding the geometric constraints imposed by the peptide linkages that determine, at least partly, the protein structure; (ii) detailed understanding of NMA spectroscopic features is assumed as the fundamental basis for spectroscopic methods to monitor protein structure and dynamics [1]. Both of these properties are of interest for future applications, only if both structures and spectroscopic properties of NMA are observed in the natural environment of the biological system(s).
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