B. Computational methodology

The ground-state optimized geometries and energies of neutral N,N-DMF and the various fragmentation products were obtained at the MP2/aug-cc-pVTZ level using the Gaussian09 software package.¹⁹ In accord with the earlier studies discussed in Sec. I, products corresponding to the N–CO “peptide” bond fragmentation, CH₃ loss, and H-loss are all deduced to be energetically accessible at this wavelength.

Further calculations were performed using the MOLPRO Version 2010.1 computational package.²⁰ Using the MP2 optimised parent molecule geometry, vertical excitation energies and transition dipole moments for various neutral states of N,N-DMF were calculated using a state-averaged CASSCF method, employing the same aug-cc-pVTZ basis set as the initial geometry optimisation. The calculated transition dipole moments for vertical excitation to the 1¹A″, 2¹A′, and 3¹A′ states are superimposed on the molecular structure of N,N-DMF shown in Fig. ​Fig.11.

The choice of the active space was a balance between attempting to describe all significant static correlation effects in the ground and excited electronic states in as even-handed a way as possible across the potential energy surface, and constraining the calculation to a manageable size. The active space comprises 12 electrons arranged in nine orbitals, namely, three σ and two σ^* orbitals, the in-plane oxygen p_y orbital, π and π^* orbitals centered around the C=O bond, and the nitrogen p_x orbital. Individual potential energy curves (PECs) along the N–CO “peptide,” methylamino N–CH₃, and aldehydic C–H stretching coordinates were calculated at the CASPT2 level for the ground and various excited electronic states of the N,N-DMF neutral molecule. In each case, the remainder of the nuclear framework was frozen at the ground-state optimised geometry as the active coordinate was varied. A small imaginary level shift of 0.5 a.u. was applied to encourage convergence and to circumvent the presence of intruder states. Vertical excitation energies obtained from the CASPT2 calculations are in good agreement with the available experimental data.^6,7

In addition to the potential energy surface calculations outlined above, vertical ionization energies for the various relevant molecular fragments were calculated at the HF/aug-cc-pVTZ level with the P3 electron propagator theory correlation correction. These are shown in Table ​TableI,I, and will be used to rationalise the presence or absence of signals from the various possible photolysis products following irradiation with the 193 nm pump and 118 nm (10.48 eV) probe radiation.

Vertical ionization energies for the photolysis products of N,N-dimethylformamide, calculated at the HF/aug-cc-pVTZ level with the P3 electron propagator theory correlation correction.