Evolution of the helium droplet beam mass spectrum as the pyrolysis source is heated to decompose the di-vinyl sulfone (DVS) precursor. Pyrolytic decomposition of DVS results in a significant increase in ion signal for mass channel m/z = 26 u (C2H2)+.
Schematic energy level diagram and allowed transitions for the vinyl radical. Symmetry labeling is based on the C 2v (M) molecular symmetry group and assumes a feasible interchange of the two H atom nuclei in the CH2 group, which proceeds via the tunneling of the lone α-CH bond from one C s equilibrium geometry to the other through a C 2v transition state. At 0.4 K, the two nuclear spin isomer populations are cooled completely to either the 000 + level [ortho (Γns = A 1)] or the level [para (Γns = B 2)]. The splitting between the lower (+) and upper (−) tunneling levels is exaggerated. The spin weight ratios will be 4:4 if there is a mechanism for the feasible exchange of all three H atoms (see text).
Survey scan in the CH stretch region. Ion signal depletion is measured in mass channel m/z = 26 u. Arrows indicate peaks assigned to the vinyl radical. Almost all of the remaining intensity is attributed to helium solvated ethylene. 31 An intense precursor (DVS) peak at 3033.62 cm−1 is completely absent in this spectrum, indicating both the extent of DVS decomposition and the selectivity associated with the m/z = 26 u mass channel.
Difference mass spectrum (Laser OFF−ON) obtained with the laser frequency fixed to the peak of the ν3 transition at 2904.02 cm−1. A single mass channel (m/z = 26 u) carries almost all of the signal associated with the laser-induced depletion of droplets containing single vinyl radicals.
Spectrum of the ν3 CH2 symmetric stretching band, showing both a-type and b-type components. The four observed transitions are assigned to those shown schematically on the left-hand side of Fig. 2 . The a-type transitions are shown as the inset. The red curve is a simulation based on the constants reported in Table III .
Spectrum of the ν2 CH2 antisymmetric stretching band, showing both a-type and b-type components. The observed transitions are assigned to those shown schematically on the right-hand side of Fig. 2 . The red curve is a simulation based on the best fit parameters reported in Table IV (see Table IV caption for details).
Survey scan in the lone α-CH stretching region (mass channel 26 u). Vertical arrows indicate peaks assigned to the vinyl radical. The peak marked by an asterisk is due to an unidentified impurity. The remaining depletion signal is attributed to helium solvated ethylene. 31
Spectrum assigned to the ν1 lone α-CH stretch band, showing the a-type and b-type lines. The bottom red simulation assumes no change in tunneling splitting upon vibrational excitation and an A′+B′ constant of 5.61 cm−1. The single a-type transition is shown as the inset, along with simulations of the predicted spectrum assuming either a v = 1 tunneling splitting of 0.44 cm−1 (red) or the value predicted by Nesbitt and Dong 7 (blue, 0.31 cm−1). The peak marked by an asterisk is due to an unidentified impurity.
Tunneling splitting versus effective reduced mass, assuming a 1D potential along the in-plane α-CH rock coordinate. The CCSD(T)/aug-cc-pVTZ potential is scaled to reproduce the empirically determined 1602 cm−1 barrier height. A ≈5% increase in effective reduced mass reproduces the observed ≈20% decrease in tunneling splitting.
C2H3 vibrational frequencies (cm−1) and relative intensities (in parentheses).
Transitions observed in the ν3 band (CH2 symmetric stretch). a
Tunneling splittings and rotational constants (cm−1) obtained from the ν3 CH2 symmetric stretch band.
Transitions observed in the ν2 band (CH2 antisymmetric stretch). a
Transitions observed in the ν1 band (lone α-CH stretch). a
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