Quantum chemical description of the molecular vibrational spectra treatment of the Four-membered N-heterocycles

Abstract EN

M1NDO/3-FORCES and (6-31G*) ab initio calculations are reported for the vibration frequencies of cyclobutadiene, azete, 1,3-diazete and azetine.

Similar sequences and comparable frequencies were obtained through both methods for all four molecules.

Assignment of all vibrational modes was possible.

The localization of each mode at the atoms in the molecule is discussed.

For azete, the CH stretch vibrations are localized at one atom for each mode.

For 1,3-diazetine pure C-N and C=N vibrations are described.

For 1-azetine mixing of the sym.

CH2 stretch vibrations wih the =C-H stretch is observed.

The calculated C=N vibration is 1570 cm’1 compared with the experimental frequency of 1693 cm'1.

For the same molecule, mixing is detected for the ring puckering vibration with that of CH2 twist and rock.

For all molecules the ring out of plane vibrations show the highest IR absorption intensity.