Simulation of volume polarization for the influence of solvation on chemical shielding

At an electron ionization of 70 eV, all cationic fragments for N,N-dimethylformamide (DMF) and N-methylformamide (NMF) were detected with a quadrupole mass spectrometer and confirmed using isotopically-labelled molecules. Ion efficiency curves near the threshold region (5–20 eV) for all of the major cationic fragments were measured and fit with a Wannier-type function to determine their appearance energies (AEs). Possible reaction pathways for the formation of each observed cation were proposed, and the corresponding enthalpies (ΔH⁰s) of reaction were evaluated from quantum chemical calculations using the Gaussian09 computational quantum chemistry package. Our results suggest that hydrogen loss for both molecules occurs primarily from the carbonyl carbon. We confirm that hydrogen transfer from the carbonyl site to the nitrogen site occurs during amide bond cleavage. The resultant cations, C2H6N+ and CH4N+, and their further fragments, formed by losing an H atom, H2, or C2H2, were first identified in mass spectra and were further probed by comparing the AEs with calculated ΔH⁰s of all possible formation pathways. Additionally, we suggest that hydrogen transfer also occurs when the N-CH3 bond breaks in DMF. This detailed study of the simplest molecule with the peptide linkage contributes to our understanding of electron impact on peptide-like molecule