Photon activation of peptides in the VUV

Ultraviolet Photoactivation Using Synchrotron Radiation for Tandem Mass Spectrometry of Polysiloxanes

Gas-phase decompositions of polymer ions play an important role in mass spectrometry to obtain accurate structural information. In this work, UV photoactivation experiments were performed from two poly(dimethylsiloxane)s bearing different end groups (two trimethylsilyl, or α-sec-butyl and ω- trimethylsilyl). Precursor ions, such as [Polysiloxane+Cation]⁺ produced by an electrospray source, were stored in a linear ion trap and then submitted to synchrotron UV irradiation during different activation times and over a range of wavelengths (52 to 248 nm) from extreme UV (XUV) to deep UV. Upon photoactivation of a precursor ion from poly(dimethylsiloxane) (PDMS; with two trimethylsilyl end groups, [PDMS₂₅+Na]⁺), important fragmentations were observed, including the loss of a methyl radical followed by various heterolytic cleavages along the polymer backbone, for photon energies typically >9.5-10 eV (ionization threshold of the neutral oligomer). This report focuses on different aspects: (i) the identification of the UV photodissociation (UV-PD) products of PDMS, (ii) the influence of the irradiation time for two photon energies (10 or 20 eV), (iii) the influence of the energy of the photon for two activation times (100 or 5000 ms), (iv) the influence of the nature of the cation, and (v) the influence of the end groups of PDMS. Synchrotron UV irradiation with a tunable wavelength was a great opportunity to study the effect of the photon energy and to probe the original mechanisms of ion decomposition from poly(dimethylsiloxane).

Spectroscopy of N,N-dimethylformamide in the VUV and IR regions: Experimental and computational studies

The electronic absorption spectrum of N,N-dimethylformamide (DMF) is studied in the 45 000-80 000 cm^-1 (5.6-9.9 eV) region using synchrotron radiation. The vacuum ultraviolet (VUV) spectrum comprises mostly of Rydberg series of ns, np, and nd types converging to the first two ionization potentials (IPs). Quantum defect values obtained are consistent with excitation of an electron from the highest occupied molecular orbitals localized on nitrogen (4a″) and oxygen (16a'); in addition, the 3s Rydberg transition converging to the third IP (3a″) is observed at 8.95 eV. A reinvestigation of the infrared spectrum of DMF in the 500-4000 cm^-1 region with the help of density functional theory (DFT) calculations establishes the planarity of the ground state and leads to revision of several vibrational assignments. Vertical excited state energies and their valence/Rydberg character are predicted using time dependent DFT calculations; excellent correlation is achieved between theoretical results and experimentally observed spectral features. Potential energy curves of the first few excited states give additional insights into the nature of the excited states and their role in photodissociation dynamics. The absorption spectrum of DMF in the region >63 400 cm^-1 (7.85 eV) as well as a complete set of spectral assignments in the VUV region (45 000-80 000 cm^-1) is reported for the first time. This work represents a comprehensive study of the absorption spectra of DMF in the VUV and infrared regions.