Probing different spin states in xylyl radicals and ions

Abstract

Resonant one-color two-photon ionization spectroscopy and mass-selected threshold photoelectron spectroscopy were applied to study the electronic doublet states of the three xylyl (methyl-benzyl) radicals above 3.9 eV as well as the singlet and triplet states of the cations up to 10.5 eV. The experiments are complemented by quantum chemical calculations and Franck-Condon simulations to characterize the transitions and to identify the origin bands, allowing a precise determination of singlet-triplet splittings in the cations. Torsional motions of the methyl group notably affect the D → D transition of m-xylyl. All other investigated transitions either lead to electronic states with very low rotational barriers or suffer from spectral broadening in excess of methyl torsional energy levels. The methyl internal rotational potential is faithfully reproduced with the most basic ab initio methods, yet hyperconjugation could not be identified as a significant force shaping them. Time-dependent density functional theory describes the excited electronic states better than wave function theory approaches, notably EOM-CCSD.

Low-Energy Photoelectron Spectrum and Dissociative Photoionization of the Smallest Amides: Formamide and Acetamide

“…The modeled multiplet fine structure of the first band is characteristic of the methyl internal rotation upon ionization and has recently been observed in the high-resolution electronic spectrum of the oxylyl radical as well. 69 In the neutral minimum geometry at the B3LYP/6-311++G(d,p) level of theory, one methyl hydrogen is antiperiplanar with the oxygen atom [2b]. In the ground state cation, a methyl hydrogen is synperiplanar with the oxygen atom (see below), [2a] in Figure 2, which involves a 30°rotation of the methyl group.…”

“…In addition to the NH 2 bend and NCO asymmetric stretch combinations at 1620 and 1690 cm –1 , the coarse structure of the X̃ + (A′) state spectrum of acetamide is also characterized by the 370 cm –1 CH 3 rocking mode. The modeled multiplet fine structure of the first band is characteristic of the methyl internal rotation upon ionization and has recently been observed in the high-resolution electronic spectrum of the o -xylyl radical as well . In the neutral minimum geometry at the B3LYP/6-311++G­(d,p) level of theory, one methyl hydrogen is antiperiplanar with the oxygen atom [ 2b ].…”

The Gas-Phase Infrared Spectra of Xylyl Radicals

“…This shows that the isomers can be distinguished by IR spectroscopy. This is particularly relevant for meta-xylyl, because the D 3 ← D 0 transition is broader and potentially overlaps with the other isomers. , However, the excellent agreement with computed spectra shows that meta-xylyl does not isomerize in the pyrolysis reactor under the conditions employed in the present study, which is relevant for future studies on the kinetics and dynamics of the radical. Such an isomerization to ortho- and para-isomer has been observed in previous work. , In the present work, a nitrite precursor was employed (see Scheme ), which yields xylyl radicals under milder conditions than for the meta-xylyl bromide precursor used before .…”

“…The dye laser was scanned around 310 nm and for each xylyl isomer, the UV wavelength was set to the respective absorption maximum (see figure labels). In the wavelength region around 310 nm, the D 3 ← D 0 transition in xylyl is excited. , …”

“…Research on xylyl is impeded by the existence of three isomers, differing in the relative position of the methyl- and methylene-group. The low-lying excited electronic states have been characterized by emission spectroscopy, − including a determination of the torsional parameters. ,, Furthermore, the higher-lying C-state has only recently been investigated by multiphoton ionization, , and the ionization energies (IE) have been determined by photoelectron-photoion coincidence spectroscopy. , The excited-state dynamics was investigated by velocity map imaging and by femtosecond time-resolved spectroscopy . Reilly et al detected the C 8 H 9 isomer methyltropyl in a xylene discharge, examined the isomerization between xylyl and methyltropyl computationally, and reported accurate numbers for stationary points on the reaction coordinate between the isomers.…”