IR spectra of naphthalene (C₁₀H₈) in frozen in solid H₂O

Figure 1: The 4000-500 cm^-1 (2.5-20 µm) IR spectra of C₁₀H₈ in (a) the gas phase at 273 K (from Cane et al. 1996), (b) an argon matrix at 12 K (Ar/C₁₀H₈ > 1000; from Hudgins & Sandford 1998a), (c) in pure form at 15 K, and (d) in an H₂O matrix (H₂O/C₁₀H₈ ~8) at 15 K. The pure C₁₀H₈ spectrum was obtained by directly vacuum depositing C₁₀H₈ vapor onto a CsI window. Note how the strong 3300 cm^-1 (3.0 µm) O-H stretching band of H₂O reduces the spectral contrast of the aromatic C-H stretching bands between 3150 and 2900 cm^-1 (3.18-3.45 µm).

Figure 2:The 3150-2900 cm^-1 (3.17-3.45 µm) IR spectra of C₁₀H₈ (a) in the gas phase at 273 K (from Cane et al. 1996), (b) in an argon matrix at 12 K (Ar/C₁₀H₈ > 1000; from Hudgins & Sandford 1998a), (c) in pure form at 15 K, and (d) in an H₂O matrix (H₂O/C₁₀H₈ ~ 8) at 15 K. The characteristic profiles of the P, Q, R branches of the gas phase C₁₀H₈ are not obvious due to extensive overlapping of multiple C-H stretching modes.

Figure 3:The 1300-1100 cm^-1 (7.7-9.1 µm) IR spectra of C₁₀H₈ (a) in the gas phase at 273 K (from Cane et al. 1996), (b) in an argon matrix at 12 K (Ar/C₁₀H₈ > 1000; from Hudgins & Sandford 1998a), (c) in pure form at 15 K, and (d) in an H₂O matrix (H₂O/C₁₀H₈ ~ 8) at 15 K. Solid state interactions result in bands broadening, blending, shifting, and, in some cases, changing their relative strengths.

Figure 4: The 1800-1000 cm^-1 (5.56-10.0 µm) IR spectra of H₂O-C₁₀H₈ ices as a function of C₁₀H₈ concentration. Naphthalene bands in this region are largely due to C-C stretching and C-H in-plane bending modes.The spectra are from (a) pure C₁₀H₈, (b) H₂O/C₁₀H₈ ~ 8, (c) H₂O/C₁₀H₈ ~ 15, (d) H₂O/C₁₀H₈ ~ 40, (e) H₂O/C₁₀H₈ ~ 80, and (f) pure H₂O. The very broad band centered near 1660 cm^-1 (6.0 µm) is due to H-O-H bending modes of the H₂O. All spectra in this figure were taken from samples deposited and maintained at 15 K.

Figure 5: The 1000-500 cm^-1 (10.0-20.0 µm) IR spectra of H₂O-C₁₀H₈ ices as a function of C₁₀H₈ concentration. Naphthalene bands in this region are largely due to C-H out-of-plane bending modes. he spectra are from (a) pure C₁₀H₈, (b) H₂O/C₁₀H₈ ~ 8, (c) H₂O/C₁₀H₈ ~ 15, (d) H₂O/C₁₀H₈ ~ 40, (e) H₂O/C₁₀H₈ ~ 80, and (f) pure H₂O. The very broad band centered near 750 cm^-1 (13.3 µm) is due to librational modes of the H₂O. All spectra in this figure were taken from samples deposited and maintained at 15 K.

Figure 6: The 1800-1000 cm^-1 (5.56-10.0 µm) spectra of an H₂O/C₁₀H₈ ~ 15 ice as a function of ice temperature. Spectra were taken from the same sample after it was deposited at 15 K and subsequently warmed in steps at 2 K/minute to temperatures of 25, 50, 75, 100, 125, 150, and 175 K. The very broad band centered near 1660 cm^-1 (6.0 µm) is due to H-O-H bending vibrations of the H₂O. Note the band splitting that occurs at temperatures above 125 K as the original amorphous H₂O ice matrix is transformed into its cubic phase.

Figure 7: The 1000-500 cm^-1 (10.0-20.0 µm) spectra of an H₂O/C₁₀H₈ ~ 15 ice as a function of ice temperature. Spectra were taken from the same sample after it was deposited at 15 K and subsequently warmed in steps at 2 K/minute to temperatures of 25, 50, 75, 100, 125, 150, and 175 K. The very broad band centered near 750 cm^-1 (13.3 µm) is due to librational modes of the H₂O. Note the band splitting that occurs at temperatures above 125 K as the original amorphous H₂O ice matrix is transformed into its cubic phase.

Figure 8: The 3150-2950 cm^-1 (3.17-3.39 µm) IR spectra of H₂O-C₁₀H₈ ices as a function of C₁₀H₈ concentration. The spectra are from (a) pure C₁₀H₈, (b) H₂O/C₁₀H₈ ~ 8, (c) H₂O/C₁₀H₈ ~ 15, (d) H₂O/C₁₀H₈ ~ 40, (e) H₂O/C₁₀H₈ ~ 80, and (f) pure H₂O. The steep drop off to higher frequencies is due to the shoulder of the very strong O-H stretching band of H₂O ice centered near 3300 cm^-1 (3 µm). All spectra in this figure were taken from samples deposited and maintained at 15 K.

Figure 9: The profile of the O-H stretching feature of a pure H₂O ice and H₂O-C₁₀H₈ samples having various C₁₀H₈ concentrations at 15 K. Increasing concentrations of C₁₀H₈ cause the featureÕs centroid to move to higher frequencies, produce a dangling O-H feature near 3600 cm^-1 (2.8 µm), and alters the low frequency wing of the band. The lower trace shows the residuals that remain after 75% of the 3.0 µm H₂O feature is removed from the spectrum of the H₂O-C₁₀H₈ ~ 15 sample using the spectrum of the pure H₂O ice. Such corrections improve the apparent spectral contrast of the aromatic C-H stretching feature but cannot be taken to completion because of the H₂O-C₁₀H₈ interactions.

Figure 10: The 3150-2950 cm^-1 (3.17-3.39 µm) spectra of an H₂O/C₁₀H₈ ~ 15 ice as a function of ice temperature. Spectra were taken from an ice deposited at 15 K that was subsequently warmed to 25, 50, 75, 100, 125, 150, and 175 K.

This data is from Sandford, S.A., Bernstein, M.P., and Allamandola, L. J. The mid-infrared laboratory spectra of naphthalene (C₁₀H₈) in solid H₂O ApJ, 607, 346-360, (2004). This paper (and others) can be downloaded as a PDF file at: http://www.astrochem.org/currentpubs.htm