Potential energy scans and vibrational assignments of cyclopropanecarboxylic acid and cyclopropanecarboxamide.

The structural stability and internal rotations in cyclopropanecarboxylic acid and cyclopropanecarboxamide were investigated by the DFT-B3LYP and the ab initio MP2 calculations using 6-311G** and 6-311+G** basis sets. The computations were extended to the MP4//MP2/6-311G** and CCSD(T)//MP2/6-311G** single-point calculations. From the calculations the molecules were predicted to exist predominantly in the cis (C=O group eclipses the cyclopropane ring) with a cis-trans barrier of about 4-6kcal/mol. The OCOH torsional barrier in the acid was estimated to be about 12-13kcal/mol while the corresponding OCNH torsional barrier in the amide was calculated to be about 20kcal/mol. The equilibrium constant k for the cis<-->trans interconversion in cyclopropanecarboxylic acid was calculated to be 0.1729 at 298.15K that corresponds to an equilibrium mixture of about 85% cis and 15% trans. The vibrational frequencies were computed at the DFT-B3LYP level. Normal coordinate calculations were carried out and potential energy distributions were calculated for the low energy cis conformer of the molecules. Complete vibrational assignments were made on the basis of normal coordinate calculations and comparison with experimental data of the molecules.

Rotational barriers in monomeric CH2=CX-COOH and CH2=CX-CONH2 (X is H or CH3) and vibrational analysis of methacrylic acid and methacrylamide.

The internal rotations in acrylic and methacrylic acids CH2=CX-COOH and their amides CH2=CX-CONH2 (X is H or CH3) were investigated by DFT-B3LYP calculations with 6-311+G** basis set. The potential energy curves were consistent with two minima that correspond to planar cis and trans conformation in the case of the acids (or cis and near-trans forms in the case of the amides). Acrylic acid and acrylamide were predicted to have the cis form as the low and predominant conformation of the molecules. In the case of the methacrylic acid and methacrylamide, the conformational relative stability was predicted to reverse as going from the acrylic to the metha compounds. The trans conformer in methacrylic acid or the near-trans in methacrylamide were predicted to be thermodynamically low energy structures of the molecules. The CCCO rotational barrier was calculated to vary from 4 to 6kcal/mol in the four molecules. The OCOH and OCNH torsional barriers were calculated to be about 13 and 22kcal/mol in the acids and the amides, respectively. The vibrational frequencies of methacrylic acid and methacrylamide were computed at the DFT-B3LYP/6-311+G** level and reliable vibrational assignments were made on the basis of normal coordinate analyses and comparison with experimental data of both molecules in their low energy conformations.