Structure and dynamics of methacrylamide, a computational and free-jet rotational spectroscopic study

• Published in: Journal of molecular structure Vol. 1248; p. 131391
• Main Authors: Maris, Assimo, Melandri, Sonia, Evangelisti, Luca, Vigorito, Annalisa, Sigismondi, Silvia, Calabrese, Camilla, Usabiaga, Imanol
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.01.2022
• Subjects: Amide; Gas-phase structure; Large amplitude motions; Nuclear quadrupole hyperfine structure; Quantum mechanical calculations; Rotational spectroscopy; Gas-phase structure; Amide; Large amplitude motions; Nuclear quadrupole hyperfine structure; Quantum mechanical calculations; Rotational spectroscopy
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2021.131391

[Display omitted] The conformational space of methacrylamide was explored by quantum mechanical modeling and surveyed in the 59.6–104.0 GHz frequency range using a millimeter-wave Stark-modulated free-jet absorption spectrometer. According to the relative orientation of the two unsaturated bonds, two conformers were observed, namely s-trans (A=5234.360(1), B=3364.9717(8) and C=2173.099(1) MHz) and s-cis (A=5207.292(1), B=3470.930(1) and C=2113.496(1) MHz). The s-trans conformation is the global minimum, with relative energy 4(2) kJ mol−1 and calculated isomerization barrier 15 kJ mol−1. Except for the methyl hydrogen atoms, s-cis-methacrylamide is planar and its methyl internal rotation barrier is 10.2(1) kJ mol−1. In s-trans-methacrylamide the allyl and amino frames form a dihedral angle of about 30° and the methyl internal rotation barrier is 7.4 kJ mol−1. This different behaviour is explained in terms of attractive and repulsive intramolecular interactions between groups: CH2/CO and CH3/NH2 for s-cis, CH2/NH2 and CH3/CO for s-trans. The tunneling splitting related to the double-well potential describing the interconversion between the two equivalent s-trans forms is 837.97(2) MHz and was reproduced by a one-dimensional flexible model using a 3.6 kJ mol−1 interconversion barrier.