Mid-IR Spectra of Quinoline and Phenanthridine in H₂O and Ar

Figure 1. The 3800-550 cm^-1 (2.63-18.18 microns) IR spectra of Quinoline (C₉H₇N) isolated in an argon matrix (Ar/C₉H₇N > 1000; above) and in solid H₂O (H₂O/C₉H₇N = 20; below). The large broad absorptions in the (lower) spectrum centered near 3250, 2200, 1600, and 750 cm^-1 (3.08, 4.5, 6.25 and 13.3 microns) are caused by the amorphous solid H₂O at 15 K. Because H₂O absorptions dominate the spectrum some of the quinoline features are obscured. An * below a band in the argon matrix spectrum indicates the presence of a group of peaks caused by, or with a large contribution from, matrix isolated H₂O.

Figure 2. The 3150-2900 cm^-1 (3.175-3.448 microns) IR spectra, from bottom to top, of (A) quinoline in solid H2O at 15 K (H₂O/C₉H₇N = 20) (B) quinoline in H2O minus 75% of a comparable spectrum of pure H₂O, and (C) quinoline isolated in an argon matrix (Ar/C₉H₇N > 1000) at 15 K. Although the weak aromatic C-H absorptions in trace A are obscured by the long wavelength wing of the 3 micron H2O, the partial subtraction of pure H2O reveals the C-H stretches of quinoline in H₂O as broad absorptions in trace B spanning the range from ~3090 to ~3025 cm^-1 (3.24-3.3 microns), as opposed to the distinct sharp absorptions of quinoline isolated in argon (trace C). Such broad absorptions are to be expected since H₂O tends to interact strongly with neighboring molecules.

The positions and profiles of these absorptions are broadly consistent with past astronomical observations. However, they are not all the same. It is interesting to note that there is a difference in the position of the aromatic C-H stretches between young stellar objects (YSOs) where it is consistently 3.25 microns (Brooke et al. 1999) and a galactic center line of sight (GCS 3) where it was 3.28 microns (Chiar et al. 2000). If the C-Hs of quinoline in H₂O appeared in an astronomical spectrum blended together as a single broad feature it would be centered at ~3.28 microns essentially the same position as that observed by Chiar et al (2000), towards GCS 3, but with a FWHM twice that of the astronomical absorption. On the other hand, if the stronger shorter wavelength feature is considered separately, its center is very close (~0.01 microns) to those of YSOs (Brooke et al. 1999).

Figure 3. The 1750-1550 cm^-1 (5.71-6.45 micron) IR spectra of quinoline (A) isolated in an argon matrix (Ar/C₉H₇N > 1000) and (B) in solid H₂O (H₂O/C₉H₇N = 20) at 15 K. The ensemble of peaks in the upper spectrum contains major contributions from matrix isolated H₂O, making it unclear if any quinoline peaks are present. However, in the lower spectrum the H₂O has blended into the broad underlying absorption feature, revealing quinoline peaks as shoulders between 1630 and 1570 cm^-1 (6.14-6.37 micron).

Figure 4. The 1490-900 cm^-1 (6.71- 11.1 micron) IR spectra of quinoline isolated in an argon matrix (Ar/C₉H₇N > 1000; above) and in solid H₂O (H₂O/C₉H₇N = 20; below) at 15 K. The vertical hatched lines help to show that the positions of many of the peaks of quinoline in H₂O are shifted to higher frequency relative to those in Ar matrix. An arrow indicates the most extreme example of this, a feature in H₂O that presumably corresponds to the peak near 1445 cm^-1 (6.92 micron) in the argon spectrum.

Figure 5. The 850-600 cm^-1 (11.76-16.67 micron) IR spectra of quinoline isolated in an argon matrix (Ar/C₉H₇N > 1000; above) and in solid H₂O (H₂O/C₉H₇N = 20; below) at 15 K. The vertical hatched lines highlight shifts to higher frequency in the positions of absorptions corresponding to C-H out-of-plane bending motions of quinoline in the presence of H₂O.

Figure 6. The 840-740 cm^-1 (11.9-13.5 micron) IR spectrum of quinoline in solid H₂O (H₂O/C₉H₇N = 20) at six temperatures between 15 and 125 K. The positions of the C-H out-of-plane bending motions of quinoline depend slightly on temperature over this >100 K range.

Figure 7. The 1700-550 cm^-1 (5.88-18 micron) IR spectra of phenanthridine (C₁₃H₉N) isolated in an argon matrix (Ar/C₁₃H₉N > 1000; above) and in solid H₂O (H₂O/C₁₃H₉N > 10; below). The large broad absorptions in the (lower) spectrum centered near 1600, and 750 cm^-1 (6.25 and 13.3 micron) are caused by the amorphous solid H₂O at 15 K. The positions of the phenanthridine peaks are essentially unchanged. As was the case with the spectrum of matrix isolated quinoline (Figure 3) the ensemble of peaks near 1600 cm^-1 (6.25 micron) region of the upper spectrum contain significant contributions from matrix isolated H₂O.