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Lowest triplet (n, π*) electronic state of acrolein: Determination of structural parameters by cavity ringdown spectroscopy and quantum-chemical methods
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Image of FIG. 1.
FIG. 1.

Schematic illustration of potential-energy curves for twisting of the CH2 group in the lowest triplet excited states of acrolein. The energy scale is based on calculations of Reguero et al. 7

Image of FIG. 2.
FIG. 2.

Cavity ringdown absorption spectrum of the band of the T 1(n, π*) ← S 0 transition of acrolein. The spectrum was recorded using a 1-m cell at room temperature. The pressure of the gaseous acrolein sample was 10 Torr.

Image of FIG. 3.
FIG. 3.

Comparison of room-temperature and jet-cooled CRD spectra of acrolein. Vibronic hot-band assignments are those of Osborne and Ramsay. 15

Image of FIG. 4.
FIG. 4.

Observed room-temperature CRD spectrum of acrolein (black) along with a simulation (red) produced by using the constants in Table I . The fitting procedure included eight of the ten sub-band heads assigned as shown with K a values; excluded from the fit were the low-intensity K a = 8, ΔN = +1, and K a = 4, ΔN = +2 sub-band heads.

Image of FIG. 5.
FIG. 5.

Observed (black) and simulated (red) spectra of acrolein as in Fig. 4 , but shown on a wider horizontal scale. The simulated spectrum was scaled vertically so that its primary maximum has the same intensity as that of the observed spectrum. The simulation model included only the band of the T 1(n, π*) ← S 0 transition and does not account for the presence of vibronic hot bands, such as the sequence assigned previously. 15

Image of FIG. 6.
FIG. 6.

Black trace: jet-cooled CRD spectrum of acrolein in the region of the band of the T 1(n, π*) ← S 0 transition; red trace: simulation using the parameters in Table I and a temperature of 63 K; grey trace: baseline CRD scan recorded with the sample absent, showing noise level. The stick spectrum at the bottom of the figure shows the relative intensities of all calculated rotational lines before applying a Voigt lineshape and co-adding to obtain the red simulation trace. The blue and pink curves on the stick spectrum highlight the |ΔN| = 2 branches of the K a = 1 and K a = 5 sub-bands, respectively.

Image of FIG. 7.
FIG. 7.

Room-temperature CRD spectrum (black), along with simulated spectra using UCCSD(T) (green) and CASPT2 (blue) inertial constants and fixed molecular constants listed in Table I .

Image of FIG. 8.
FIG. 8.

Top two traces: room-temperature CRD spectrum, as in Fig. 4 , recorded at a dye-laser resolution of 0.1 cm−1, and spectrum recorded at higher resolution (0.03 cm−1) by using the dye laser's intracavity etalon accessory. Lower six traces: spectra simulated at 0.03-cm−1 resolution. The simulations employed the fixed constants in Table I , along with fitted values of A and . For each simulation, the quantity (B C ) was constructed by using the indicated value of Δ0. The inertial defects, in units of amu-Å2, are those of S 0 malonaldehyde (+0.10), S 1 s-trans-glyoxal (+0.028), T 1 acrolein (+0.01, as determined in this work), S 0 acrolein (−0.021), S 0 s-trans-glyoxal (−0.066), and S 0 s-cis-glyoxal (−0.13). The inertial defects of S 0 malonaldehyde (a species with restricted out-of-plane vibration) and S 0 s-trans- and s-cis-glyoxal (species with exaggerated out-of-plane vibration) are physically implausible for T 1 acrolein.


Generic image for table
Table I.

Molecular constants (cm−1) used in the fit of the T 1S 0 rotational contour of acrolein at room temperature.

Generic image for table
Table II.

Computed bond lengths (Å) and angles (deg), along with corresponding inertial constants (cm−1), for acrolein in its T 1(n, π*) state.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Lowest triplet (n, π*) electronic state of acrolein: Determination of structural parameters by cavity ringdown spectroscopy and quantum-chemical methods