Ring planarity problem of 2-oxazoline revisited using microwave spectroscopy and quantum chemical calculations

In a previous infrared, Raman, and microwave spectroscopic work,1 it was claimed that 2-oxazoline has a planar ring equilibrium conformation, and the ring-puckering potential function V(z) = 22.2(z(4) + 1.31z(2)) cm(-1), where z is a dimensionless reduced coordinate, was derived. This function poorly reproduces the rotational constants of the lowest and most important puckering states. The microwave spectrum has been reinvestigated and largely extended to include more than 4600 transitions of the ground state and six excited states of the ring-puckering vibration allowing accurate centrifugal distortion constants to be obtained for the first time. A new potential function V(z) = 38.8(z(4) - 0.65z(2)) cm(-1) has been determined. This function yields much better agreement between calculated and observed rotational constants, especially for the lowest puckering states, than the previous function and predicts a nonplanar ring equilibrium conformation. The barrier to ring planarity is determined to be 49(8) J/mol. The ground-state energy level is 35 cm(-1) above the barrier maximum. Theory predicts that three of the five Watson centrifugal distortion constants, ΔJK, ΔK, and δK, should vary with the puckering state, whereas ΔJ and δJ should be unaffected. It was found that ΔJK and ΔK indeed behave in the expected manner, while deviations were seen for the three other centrifugal distortion constants. The ab initio methods HF, MP2, CCSD, CCSD(T), and CCSD(T)-F12 with large basis sets as well as several DFT methods were used in an attempt to reproduce the low experimental barrier to the planar ring. Only the MP2 method yielded a satisfactory prediction of the barrier. The CCSD and the CCSD(T) calculations predict a planar ring, whereas the energy differences between a planar and a nonplanar ring obtained in the CCSD(T)-F12 computations are so small that a definite conclusion cannot be drawn.