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Femtosecond midinfrared study of the photoinduced Wolff rearrangement of diazonaphthoquinone
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10.1063/1.2971037
/content/aip/journal/jcp/129/9/10.1063/1.2971037
http://aip.metastore.ingenta.com/content/aip/journal/jcp/129/9/10.1063/1.2971037

Figures

Image of FIG. 1.
FIG. 1.

Scheme of the photoreaction of 2-diazo-1-naphthoquinone (DNQ) according to Vleggaar et al. (Ref. 12). After diazo separation the still disputed carbene intermediate is formed. The existence of the resulting ketene, on the other hand, is a well established fact. Further reactions with the solvent ROH (e.g., water or methanol) on a microsecond time scale are not shown in this scheme.

Image of FIG. 2.
FIG. 2.

Experimentally measured midinfrared steady state absorption spectra of DNQ dissolved in (a) methanol and (b) water. In the gray regions strong solvent absorption prevents an analysis of the spectra. (c) Normal modes of DNQ as obtained from DFT calculations using the B3LYP correlation functional with the basis set.

Image of FIG. 3.
FIG. 3.

DFT calculation of the normal modes of possible intermediate species of the Wolff rearrangement reaction of DNQ: (a) Carbene and (b) ketene. All calculations were performed under the assumption of isolated molecules. The normal mode spectra were convoluted with Lorentzian peak functions with FWHM.

Image of FIG. 4.
FIG. 4.

Transient infrared spectra of DNQ in methanol recorded at different delay times from 0.5 to 50 ps in the region around , showing the bleached vibrational bands of the reactant DNQ at 1541, 1565, 1600, and . The small positive change around in absorbance can additionally be seen in the transients in Fig. 5.

Image of FIG. 5.
FIG. 5.

Transients recorded between 1590 and . The transient at only shows bleach of the DNQ ground state vibration, while in the other transients an additional positive absorbance contribution is visible.

Image of FIG. 6.
FIG. 6.

Two-dimensional contour plot of the spectrally and time-resolved transient absorption signals of DNQ dissolved in methanol. The data show bleaching (negative changes in optical density) of the ground state vibrational modes of DNQ after photoexcitation with 400 nm femtosecond pulses. A new band appears in the lower wavenumber region and seems to shift to within about 10 ps. Note the nonlinear color scale in the graph.

Image of FIG. 7.
FIG. 7.

Two-dimensional contour plot of the spectrally and time-resolved transient absorption signals of DNQ dissolved in water. The data show bleaching (negative changes in optical density) of the ground state vibrational modes of DNQ after photoexcitation with 400 nm femtosecond pulses. A new band appears in the lower wavenumber region and seems to shift to within about 3 ps. Note the nonlinear color scale in the graph.

Image of FIG. 8.
FIG. 8.

Bleach-corrected data corresponding to Fig. 6 (methanol). Based on the experimental FTIR spectrum the time and frequency dependences of the bleach features were modeled and subtracted from the data.

Image of FIG. 9.
FIG. 9.

Bleach-corrected data corresponding to Fig. 7 (water). Based on the experimental FTIR spectrum the time and frequency dependences of the bleach features were modeled and subtracted from the data.

Image of FIG. 10.
FIG. 10.

Transient infrared spectra of DNQ in methanol recorded at different delay times from 0.4 to 50 ps, showing the bleached vibrational bands of the reactant DNQ and the rising and shifting positive absorption from lower to higher wavenumbers. Steady state infrared spectra are displayed at the bottom. Note that the dip in the positive absorption at is due to the weaker bleach contribution of the stretching vibration of DNQ at this position (also clearly visible in Fig. 6).

Image of FIG. 11.
FIG. 11.

Transient infrared spectra of DNQ in water recorded at different delay times from 0.4 to 30 ps, showing the bleached vibrational bands of the reactant DNQ and the rising and shifting positive absorption from lower to higher wavenumbers. Steady state infrared spectra are displayed at the bottom.

Image of FIG. 12.
FIG. 12.

Transients in the ketene formation region at early delay times, recorded at in methanol (circles) and at in water (squares) showing immediate rise of the ketene absorption band within the time resolution of the experiment. The oscillatory features at negative delay times are due to perturbed free induction decay.

Image of FIG. 13.
FIG. 13.

Model for the concerted ketene formation channel of the photoinduced Wolff rearrangement of DNQ in methanol on the basis of the collected transient absorption data. is related to the excited state population, the fraction of the population returning to a hot ground state, and the population that has refilled the vibrational ground state of DNQ. , , and are the populations in the ketene hot ground states , , and the vibrational ground state , respectively.

Image of FIG. 14.
FIG. 14.

Temporal evolution of the absorption changes at fixed spectral positions for DNQ in methanol. Bleach contributions in all transients are related to the DNQ reactant bands in the ground state. The new absorption band and its shift to higher wavenumbers can be observed in the transients from 2084 to . The dashed line represents the DNQ bleach contribution, the dotted line the ketene contribution, and the solid line the sum of both.

Tables

Generic image for table
Table I.

Spectral positions and assignment of calculated (DNQ in vacuum) and by FTIR absorption spectroscopy experimentally measured [DNQ in methanol, see Fig. 2(a); DNQ in water, see Fig. 2(b)] most prominent vibrational modes in units of . denotes the stretching mode of the respective group and the numbers indicate the naphthalene ring modes according to Ref. 22.

Generic image for table
Table II.

Quantitative results of the data fit of the measured transients for DNQ in methanol (MeOH) displayed in Fig. 14 and of a corresponding measurement in water .

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/content/aip/journal/jcp/129/9/10.1063/1.2971037
2008-09-03
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Femtosecond midinfrared study of the photoinduced Wolff rearrangement of diazonaphthoquinone
http://aip.metastore.ingenta.com/content/aip/journal/jcp/129/9/10.1063/1.2971037
10.1063/1.2971037
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