THEORETICAL-STUDY OF ROTATIONAL-ISOMERISM IN ETHYL PSEUDOHALIDES

The structure and conformational stability of ethyl pseudohalides CH3C H2-XCN (X = O, S, Se) were investigated using ab initio calculations a t the MP2 level of theory with a triple-zeta basis set augmented with polarization and diffusion functions. Full optimization was performed on the minimum energy structures as well as on the transition state fo rms. The relative stabilities of rotational conformers were calculated at the MP4 level using MP2 optimized reference geometries. The nature of all considered stationary points was verified by calculation of th e harmonic vibrational frequencies. The calculated bond lengths, bond angles, dipole moments, and rotational constants of optimized global m inima structures agree very well with the corresponding experimental d ata obtained from microwave spectroscopic studies. Also, available exp erimental frequencies are in good accord with the theoretical values. For ethyl cyanate CH3CH2-OCN, the antiperiplanar (trans) form is predi cted to be more stable than the synclinal (gauche) form, and the synpe riplanar (cis) form corresponds to the transition state. For both ethy l thiocyanate CH3CH2-SCN and ethyl selenocyanate CH3CH2-SeCN, the gauc he form is the global minimum while the trans-conformer is a local min imum and the cis-form is a transition state.