Proton tunnelling and nitro-group torsion in 2-nitromalonaldehyde

Abstract

The rotational spectra of the normal and O-monodeuterated species of 2-nitromalonaldehyde have been observed in the two lowest vibrational states (0⁺0^–) due to the proton tunnelling and in several excited states due to the nitro-group torsion.

No evidence for interaction between the two vibrational modes has been observed.

The analysis of each vibrational state in isolation has shown a remarkable difference in the centrifugal distortion constants between the 0⁺ and 0^– states of the proton tunneling for both isotopic species due to Coriolis coupling.

The use of a coupled Hamiltonian for the 0⁺ and 0^– states of the OD species has allowed the determination of the energy separation between these states (ΔE₀₁= 3.00 ± 0.02 cm^–1), while for the H species a rough value of ΔE₀₁= 35 ± 15 cm^–1 has been obtained from relative intensity measurements.

Some considerations are given concerning the barrier to proton tunnelling.

From the variation of the inertial defect with torsional quantum number the value v_tor= 81 ± 2 cm^–1 has been calculated for nitro-group torsion. It agrees with the value v_tor= 80 ± 20 cm^–1 obtained from intensity measurements.