Rovibrational Spectroscopy of Trans and Cis Conformers of 2-Furfural from High-Resolution Fourier Transform and QCL Infrared Measurements

Abstract

The ortho-isomer 2-furfural (2-FF), which is a primary atmospheric pollutant produced from biomass combustion, is also involved in oxidation processes leading to the formation of secondary organic aerosols. Its contribution to radiative forcing remains poorly understood. Thus, monitoring 2-FF directly in the atmosphere or in atmospheric simulation chambers to characterize its reactivity is merited. The present study reports an extensive jet-cooled rovibrational study of trans and cis conformers of 2-FF in the mid-IR region using two complementary setups: a continuous supersonic jet coupled to a high-resolution Fourier transform spectrometer on the IR beamline of the SOLEIL synchrotron (JET-AILES), and a pulsed jet coupled to a mid-IR tunable quantum cascade laser spectrometer (SPIRALES). Firstly, jet-cooled spectra recorded at rotational temperatures ranging between 20 and 50 K were exploited to derive reliable excited-state molecular parameters of trans- and cis-2-FF vibrational bands in the fingerprint region. The parameters were obtained from global fits of 11,376 and 3355 lines distributed over eight and three vibrational states (including the ground state), respectively, with a root mean square of 12 MHz. In a second step, the middle resolution spectrum of 2-FF recorded at 298.15 K and available in the HITRAN database was reconstructed by extrapolating the data derived from our low-temperature high-resolution analyses to determine the cross sections of each vibrational band of both 2-FF conformers in the 700-1800 cm^-1 region. Finally, we clearly demonstrated that the contribution of hot bands observed in the room temperature 2-FF spectrum, estimated between 40 and 63% of the fundamental band, must be imperatively introduced in our simulation to correctly reproduce the HITRAN vibrational cross sections of 2-FF with a deviation smaller than 10%.

Keywords: QCL source; furfural; jet-cooling; rovibrational spectroscopy; synchrotron-based FTIR spectroscopy; vibrational cross section.

Figure 1

Calculated equilibrium geometry (hybrid/CBS) of the trans- and cis-2-FF conformers. The $X,Y,Z$ axes correspond to the $a,b,c$ principal axes, respectively. The large arrow indicates the orientation of the permanent dipole moment.

Figure 2

Jet−AILES FT-mid-IR spectrum of 2-FF measured at $0.5$ $\mathrm{c}$$\mathrm{m}$⁻¹ resolution displayed in the 730–1820 $\mathrm{c}$$\mathrm{m}$⁻¹ region. Two insets are zoomed-in regions on the most intense bands. Vibrational assignments reported on the figure are based on comparison with hybrid/CBS anharmonic calculations. “t” and “c” in subscript indicate the “trans” and the “cis” conformers, respectively.

Figure 3

Overall view of ${\nu }_{17}$ and ${\nu }_{23}$ bands of both trans and cis conformers of 2-FF. In black, the Jet−AILES spectrum recorded at $0.001$ $\mathrm{c}$$\mathrm{m}$⁻¹ resolution. The intensity ratios of Q-branches assigned to these four bands correctly agree with the conformational energy difference determined by Durig et al. [7] for T${}_{vib}$ = $180\left(30\right)$$\mathrm{K}$. In red, the two trans conformer bands simulated at T${}_{rot}$ = 50 $\mathrm{K}$. An expanded view in the P-branch of the ${\nu }_{17}$ band displays the good match between experimental and simulated spectra.

Figure 4

Overall view of the ${\nu }_7$ and ${\nu }_6$ band of trans-2-FF: in black the Jet−AILES spectrum recorded at $0.002$ $\mathrm{c}$$\mathrm{m}$⁻¹ resolution; in red, both bands simulated at T${}_{rot}$ = 50 $\mathrm{K}$. The two structured Q-branches observed at 1480.3 and 1481 $\mathrm{c}$$\mathrm{m}$⁻¹ are possibly involved in the perturbation of the ${\nu }_7$ band.

Figure 5

Overall view of the ${\nu }_{14}$ band of 2−FF: in black, the SPIRALES spectrum; in red, the ${\nu }_{14}$ band of trans and cis conformers simulated at T${}_{rot}$ = 30 $\mathrm{K}$. Two expanded views of observed and calculated trans spectra in the P-Branch and the Q-branch are shown.

Figure 6

Overall view of the ${\nu }_{17}$ + ${\nu }_{15}$ combination band of trans−2−FF: in black, the SPIRALES spectrum; in red, the simulated spectrum at T${}_{rot}$ = 20 $\mathrm{K}$. An expanded view of observed and calculated spectra in the P-branch is displayed.

Figure 7

Overall view of the ${\nu }_5$ band of trans- and cis-2−FF: (top, black trace) the Jet-AILES spectrum; (middle, black trace) the SPIRALES spectrum; in blue, the simulated spectrum of both conformers at T${}_{rot}$ = 50 $\mathrm{K}$ with the Jet-AILES setup; in red, the simulated spectrum of both conformers at T${}_{rot}$ = 20 $\mathrm{K}$ with the SPIRALES setup. A relative abundance trans/cis equal to 3, similar to room temperature conditions, was assumed in the simulation. An expanded view of Jet-AILES and SPIRALES versus calculated spectra in the P- and R-Branches, respectively, is shown.

Figure 8

Top: Comparison of 2-FF room temperature integrated cross sections from the HITRAN database (blue line) and band contour PGOPHER simulations using jet-cooled high-resolution data (red line). The simulated band contours were constructed with the fitted parameters summarized in Table 2 and Table 3 from the nine rovibrational bands analyzed at high resolution. Five additional bands (marked with a star) were added with molecular parameters scaled from the fitted ones of the trans conformer $B_{vib}^{cis}=\frac{B_{GS}^{cis}\times B_{vib}^{trans}}{B_{GS}^{trans}}$, except for the $({\nu }_{18}+{\nu }_{10})$ combination band which was not analyzed at high resolution for the trans conformer. The addition of hot bands was required to reproduce the cross sections measured at room temperature (black line). Bottom: Expanded view of the HITRAN cross sections of both ${\nu }_{17}$ and ${\nu }_{23}$ bands (blue line) compared to simulations of fundamental and hot band contours of trans- and cis-2-FF conformers (black line for the full simulation; red, green, orange, and magenta for the ${\left({\nu }_{17}\right)}_t$, ${\left({\nu }_{17}\right)}_c$, ${\left({\nu }_{23}\right)}_t$, and ${\left({\nu }_{23}\right)}_c$ individual contributions, respectively).

Figure 8

Top: Comparison of 2-FF room temperature integrated cross sections from the HITRAN database (blue line) and band contour PGOPHER simulations using jet-cooled high-resolution data (red line). The simulated band contours were constructed with the fitted parameters summarized in Table 2 and Table 3 from the nine rovibrational bands analyzed at high resolution. Five additional bands (marked with a star) were added with molecular parameters scaled from the fitted ones of the trans conformer $B_{vib}^{cis}=\frac{B_{GS}^{cis}\times B_{vib}^{trans}}{B_{GS}^{trans}}$, except for the $({\nu }_{18}+{\nu }_{10})$ combination band which was not analyzed at high resolution for the trans conformer. The addition of hot bands was required to reproduce the cross sections measured at room temperature (black line). Bottom: Expanded view of the HITRAN cross sections of both ${\nu }_{17}$ and ${\nu }_{23}$ bands (blue line) compared to simulations of fundamental and hot band contours of trans- and cis-2-FF conformers (black line for the full simulation; red, green, orange, and magenta for the ${\left({\nu }_{17}\right)}_t$, ${\left({\nu }_{17}\right)}_c$, ${\left({\nu }_{23}\right)}_t$, and ${\left({\nu }_{23}\right)}_c$ individual contributions, respectively).