Conformational behavior of cyanoacetic acid: A combined matrix isolation fourier transform infrared spectroscopy and theoretical study

This work reports the first experimental study of the cyanoacetic acid (CAA) monomer. Samples of CAA were isolated in low-temperature argon, krypton, and xenon matrixes and characterized using FTIR spectroscopy. Annealing experiments revealed the presence of different conformers in the matrixes. The direct interconversion of the gauche-cis (gc) into the cis-cis (cc) conformer was observed upon annealing of matrixes before formation of aggregates. The use of different matrix host gases enabled separation of the observed conformational effects from the site-splitting of IR absorptions. The assignment of the experimental spectra was based on both the annealing results and the comparison of the experimental data with results of theoretical calculations performed at the B3LYP/aug-cc-pVTZ level of theory. The relative energies of the observed conformers, as well as the transition states separating these forms, were calculated at the MP2, MP4, QCISD, and CCSD levels of theory with the aug-cc-pVxZ (x = D, T, and Q) basis sets. The planar cc conformer was found to be the lowest energy form of CAA. The second conformer, with a nonplanar backbone, gc, is doubly degenerated by symmetry and was predicted to be 1.3 kJ mol^-1 (ZPVE corrected CCSD/aug-cc-VTZ energies) less stable than the cc form. In matrixes a fraction of the gc conformer undergoes geometrical distortion toward the planar trans-cis (tc) structure, which in the gaseous phase corresponds to the saddle point between the two mirror gc forms.