Theoretical and Experimental Photoelectron Spectroscopy Characterization of the Ground State of Thymine Cation

Abstract

We report on the vibronic structure of the ground state X̃(2)A″ of the thymine cation, which has been measured using a threshold photoelectron photoion coincidence technique and vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum, recorded over ∼0.7 eV above the ionization potential (i.e., covering the whole ground state of the cation) shows rich vibrational structure that has been assigned with the help of calculated anharmonic modes of the ground electronic cation state at the PBE0/aug-cc-pVDZ level of theory. The adiabatic ionization energy has been experimentally determined as AIE = 8.913 ± 0.005 eV, in very good agreement with previous high resolution results. The corresponding theoretical value of AIE = 8.917 eV has been calculated in this work with the explicitly correlated method/basis set (R)CCSD(T)-F12/cc-pVTZ-F12, which validates the theoretical approach and benchmarks its accuracy for future studies of medium-sized biological molecules.

Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

“…[16] The threshold photoelectrons pectrum (TPES) of thymine is showni nF igure 2u pt oap hoton energy of 11.5 eV,t hat is, past the third excited electronic state band, recorded by both bulk evaporation and aerosol flash vaporization. Aside from a poorer signal-to-noise ratio than observed in the bulk evaporation TPES, the vaporizationt echnique does not have am ajor impact on theT PES fine structure, which implies as imilar rotational temperature in both cases.T he rotational envelope is broader than that observed in the TPES of Majdi et al and Ho- for theà þ state correspondstoaconvolution with a20meV FWHMGaussian to show fine structure in the 10.15-10.25 eV range.T he notation used to denote the vibrational modes is from Majdi et al [18] Chem.E ur.J. 2019, 25,14192 -14204 www.chemeurj.org chlaf et al, [18,19] who resolved somewhat more features in the photoelectron spectrumr ecordedi ns upersonic expansion owing to rotational cooling.…”

“…[17] Their computational approachw as justifiedb yc omparison with the high-resolution spectrum acquired by Choi et al [16] Thef irst vibrationally resolved slow photoelectron spectrum of thymine over aw ider energy range was measured at Synchrotron SOLEIL by Majdi et al and Hochlaf et al up to ap hoton energy of 10.3 eV. [18,19] They explored numerous ab initio approaches to find the most appropriate way to calculate ionization energies. Additionally,t hey assigned vibrational transitions in the ground state, relying also on the Franck-Condon (FC) calculations of Bravaya et al [17] However,i ns ome cases, anharmonicf requency calculations at the CASSCF/aug-cc-pVDZ level of theory led to different assignmentsf rom the ones proposedb yB ravayae tal.…”

Photoionization Spectroscopy of Nucleobases and Analogues in the Gas Phase Using Synchrotron Radiation as Excitation Light Source

“…We note that experimental PES band positions are satisfactorily reproduced by the differentiated PIE spectrum (dPIE/dE) also presented in [43]; however, the relative spectral intensities of the experimental PES bands from [23] are not reproduced by the dPIE/dE curve. Very recently, Majdi et al [136] have recorded a thymine SPES spectrum at the SOLEIL synchrotron, under similar conditions to those depicted in [11] for adenine and cytosine. This spectrum is shown in Fig.…”

“…The full analysis of these bands is under investigation. The experimental work of Majdi et al [136] reveals that the experimental capabilities at the DESIRS beamline of the SOLEIL synchrotron are powerful enough to get insight into the NABs cationic structure. It confirms our [136] analysis for adenine and cytosine [11] where the congestion of the spectra resides in fact in the intrinsic complex electronic, tautomeric, and vibronic structure of adenine and cytosine (see also below).…”

VUV photoionization and dissociative photoionization spectroscopy of the interstellar molecule aminoacetonitrile: Theory and experiment

“…This points out again the outstanding capabilities of the explicitly correlated coupled clusters approaches for the deduction of accurate energetics for medium sized molecular species, as has been demonstrated before for other molecules (see Refs. [10,[27][28][29][30]). …”