Barrier to Methyl Internal Rotation of 1-Methylvinoxy Radical in the X̃(²A‘ ‘) and B̃(²A‘ ‘) States:  Experiment and Theory

Abstract

The jet-cooled laser induced fluorescence spectrum of the B̃ ← X̃ electronic transition of the 1-methylvinoxy radical is assigned, including both hot and cold bands. The barrier to methyl internal rotation in both X̃ and B̃ states is determined by fitting pure torsional transitions to a one-dimensional hindered-rotor model. The resulting 3-fold torsional barrier parameters are V 3‘ = −740 ± 30 cm-1 for the B̃ state (minimum-energy conformation with one methyl CH bond cis to the frame CO bond) and V 3‘ ‘ = +130 ± 30 cm-1 for the X̃ state (methyl CH bond trans to CO). The intensity pattern clearly indicates a change in the preferred methyl conformation upon excitation, while ab initio calculations provide the absolute conformations in each state. A variety of ab initio methods including CASSCF, multireference CI, and coupled-cluster techniques were applied to both the X̃ and the B̃ states of 1-methylvinoxy. Only the largest coupled-cluster calculations yield a B̃-state barrier in good quantitative agreement with experiment. In unsubstituted vinoxy, a B̃-state geometry adjusted earlier to fit experimental rotational constants (ref ) is evidently in error.

Theoretical spectroscopic study of acetyl (CH3CO), vinoxy (CH2CHO), and 1-methylvinoxy (CH3COCH2) radicals. Barrierless formation processes of acetone in the gas phase

“…Nevertheless, using MRCI/CASSCF/AVTZ theory, we have identified the lowest excited electronic states to assure a clean enough ground electronic state to apply the used rovibrational models. Results are shown in Table 4 and were compared with previous experimental data [22][23][24][25][26]29,31,[33][34][35][36][37][38] or data from theoretical works 39,40 .…”

“…For the CH 2 CHO isomer, the A(A")←X(A") and B(A")←X(A") transitions has been centered at 1.0 eV 26,33,34 and at 3.6 eV [22][23][24][25]33,35,36 using different experimental techniques. The B(A")←X(A") transition of CH 3 COCH 2 was found at 3.4 eV by Williams et al 37,38 who also provided internal rotation barriers. To our knowledge, all the previous experimental data concerned doublet states.…”

Electronic spectra of the jet-cooled 1-methylvinylthio radical

“…8 is similar to those of the methylvinoxy radicals, which mainly consist of the progressions of methyl torsion levels. 20,21 Therefore, the band system starting at 21 354 cm −1 could be tentatively assigned to those originating from one or both of the 2-MVT radicals. A detailed analysis for the band system is in progress.…”

“…11,12 It is interesting that the methyl substitution effect of CH 2 CHS on theB −X origin position is compared with that of the oxygen analog, the vinoxy radical. The origin band positions of the vinoxy and its methyl-substituted species have been experimentally determined, [19][20][21] as summarized in Table VIII. It is seen that the origin band of the 1-methylvinoxy radical shifts from that of the vinoxy radical by −1503 cm −1 , while the shifts of 2-methylvinoxy radicals are much smaller.…”

“…20 The in-plane C-H bond of the CH 3 group of the 1-methylvinoxy radical is trans to the conjugated C-C bond in the ground state, while it is cis to the C-C bond in the excited state. The methyl group rotation upon the electronic excitation causes an extensive progression associated with the methyl torsional mode in the vibrational structure of the electronic spectrum.…”