Ultraviolet Photodissociation of the N, N-Dimethylformamide Cation

Abstract

N,N-Dimethylformamide (DMF) provides a useful small-molecule model for studying features of the peptide bond that forms the backbone of proteins. We report results from a comprehensive multimass velocity-map imaging study into the ultraviolet (UV) photolysis of the N,N-dimethylformamide cation (DMF⁺) at wavelengths of 225, 245, and 280 nm. Electronic structure calculations on DMF and DMF⁺ were employed to help interpret the experimental results. DMF⁺ ions are generated by 118 nm single-photon ionization of neutral DMF. Subsequent UV photolysis is found to lead to selective cleavage of the N-CO amide bond. This yields HCO + NC₂H₆⁺ as major products, with virtually all of the excess energy released into internal modes of the fragments. The data also indicate a small branching ratio into the HCO⁺ + NC₂H₆ product pair, which can be accessed from the 3²A' electronic state of DMF⁺. N-CO bond dissociation can also be accompanied by simultaneous intramolecular hydrogen transfer from the oxygen to the nitrogen end of the amide bond, in which case NCH₄⁺ can be formed efficiently at all three wavelengths. The primary NC₂H₆⁺ product is relatively long-lived, but the high degree of internal excitation often results in secondary fragmentation via a variety of pathways to form CH₃⁺, NH₄⁺, NCH₂⁺, and NC₂H₄⁺, with secondary dissociation more likely at higher photon energies. The isotropic velocity-map images recorded for the various fragments attest to the long lifetime of NC₂H₆⁺ and also imply that dissociation most probably occurs from the same set of electronic states at all wavelengths studied; these are thought to be the 1²A' ground state and 2²A' first excited state of the DMF⁺ cation.

Figure 1

Equilibrium geometries of (a) DMF and (b) DMF⁺ optimized in UMP2/aug-cc-pVTZ calculations. Carbon atoms are shown in black, oxygen atoms in red, nitrogen atoms in green, and hydrogen atoms in blue. The main bond lengths and angles are highlighted in the figure. A full list of internal coordinates is provided in the Supporting Information.

Figure 2

Excitation energies to the first ten electronic states of DMF⁺ at the ground-state geometries of neutral DMF and of the DMF⁺ cation. The red arrows indicate the photoexcitation for ionization (118 nm) and photolysis (225, 245, and 280 nm), and the red-shaded regions show the range of energies that may be accessed depending on the extent of vibrational relaxation of the nascent ions. On the right are the appearance energies for various product channels taken from Li et al.

Figure 3

ToF spectrum for the products formed following 118 nm ionization of DMF.

Figure 4

Background-subtracted ToF mass spectra for the photolysis products of DMF⁺ at wavelengths of 225 nm (top panel), 245 nm (center panel), and 280 nm (bottom panel).

Figure 5

Abel-inverted velocity-map images for the various ionic products formed following photolysis of DMF⁺ at 225, 245, and 280 nm, together with the corresponding kinetic energy distributions (normalized to unit area under the curve) for each ion.