PAH IR

# AIBs

$$\begin{array}{cc} \text{Band (}\mu m\text{)} & \text{Wavenumber (cm}^{-1}\text{)} & \text{Assignment} & \text{Charge state} & \text{Label} \\ \hline \color{red}3.3 & \color{red}3030 & \color{red}\text{aromatic C–H stretching mode} & \color{red}\text{neutral} & \color{red} \bullet \\ 3.4 & 2941 & \text{aliphatic CH stretching mode in methyl groups} & \text{neutral} & \color{green} \bullet \\ & & \text{CH stretching mode in hydrogenated PAHs} & \text{neutral} & \color{green} \bullet \\ 5.2 & 1923 & \text{ combination mode, CH out of plane bend } & \text{neutral} \\ 5.65 & 1770 & \text{ combination mode, CH out of plane bend } & \text{neutral} \\ 6.0 & 1667 & \text{ CO stretching mode (?) } & \text{cation} \\ \color{red}6.2 & \color{red}1613 & \color{red}\text{ aromatic CC stretching mode } & \color{red}\text{cations} & \color{red}\blacksquare \\ 6.9 & 1449 & \text{ aliphatic CH bending modes } & \\ \color{red}7.6 & \color{red}1316 & \color{red}\text{ CC stretching and CH in-plane bending modes } & \color{red}\text{cation} & \color{red}\blacktriangle \\ \color{red}7.8 & \color{red}1282 & \color{red}\text{ CC stretching and CH in-plane bending modes } & \color{red}\text{cation} & \color{red}\blacktriangle \\ \color{red}8.6 & \color{red}1163 & \color{red}\text{ CH in-plane bending modes } & \color{red}\text{cation} \\ 11.0 & 909 & \text{ CH out-of-plane bending modes, solo } & \text{cation} \\ \color{red}11.2 & \color{red}893 & \color{red}\text{ CH out-of-plane bending modes, solo } & \color{red}\text{neutral} & \color{red} \bigstar \\ 12.0 & 833 & \text{ CH out-of-plane bending modes, duo } & \\ 12.7 & 787 & \text{ CH out-of-plane bending modes, duo } & \text{cation (?)} \\ 13.6 & 735 & \text{ CH out-of-plane bending modes, quartet } & \\ 14.2 & 704 & \text{ CH out-of-plane bending modes, quartet } & \\ 15.8 & 633 & \text{ in-plane and out-of-plane C–C–C bending modes } & \text{neutral} \\ 16.4 & 610 & \text{ in-plane and out-of-plane C–C–C bending modes (pendant ring ?) } & \text{cation (?)} \\ 17.4 & 575 & \text{ in-plane and out-of-plane C–C–C bending modes } & \text{cation} \\ 17.8 & 562 & \text{ in-plane and out-of-plane C–C–C bending modes } & \text{neutral \& cation} \\ \hline \text{Plateaus (}\mu m\text{)} \\ 3.2-3.6 & 3125-2778 & \text{overtone/combination modes, CC stretch} \\ 6.0-9.0 & 1667-1111 & \text{blend of many CC stretch/CH in-plane bend modes (in PAH clusters)} \\ 11.0-14.0 & 909-714 & \text{blend of CH out of plane bending modes (in PAH clusters)} \\ 15.0-19.0 & 667-526 & \text{blended in-plane and out-of-plane C–C–C bending modes} & \text{neutral}\\ \end{array}$$

# Papers

• [2021 - A&A - S. Banhatti et.al. Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands]
• IRPD and VPT2 IR spectra of naphthalene$^{2+}$, anthracene$^{2+}$ and phenanthrene$^{2+}$ in 500 - 1700 cm$^{-1}$
• Singlet is more stable than triplet for these three PAH dications.
• IRPD which only absorbs a single photon can record IR at 4K by tagging PAHs with rare-gas atoms. The rare-gas atoms has little impacts on harmonic IR for naphthalene$^{2+}$.
• Anharmonic of singlet and triplet for the same dication are very different.
• anthracene$^{2+}$ and phenanthrene$^{2+}$ are isomers under the same stoichiometry. It is important to tell whether isomerization/contamination plays a role in IR.
• Q: another isomer for naphthalene$^{2+}$?
• Differences in the relative integrated intensities in these spectral ranges serve as identifiers for the charge state of PAHs in the ISM
• [2018 - PCCP - Cameron Mackie et.al. The anharmonic quartic force field infrared spectra of hydrogenated and methylated PAHs]
• The hydrogenated and methylated PAHs contribute to the non-aromatic features in CH stretching region where anharmonic treatments are important. They are considered to be responsible for addition features at 3.40, 3.46, 3.51, 3.56 $\mu$m (2941, 2890, 2849, and 2809 cm$^{-1}$, minor features) and 6.9 $\mu$m.
• No experimental bands were found to explain the 3.56 $\mu$m feature.
• The quartic force field of PAHs is not stable when using B97-1 functional and TZ2P basis set. B3LYP/N07D has better agreement with experiments (30 cm$^{-1}$ to 3 cm$^{-1}$ in the CH stretching region).
• The rotation of methyl groups often has low barrier and becomes an issue in static anharmnic treatment.
• CH bending mode has strong dependencies on the number of adjacent hydrogens (solo, duo, trio). Significant changes in 1000-600 cm$^{-1}$ are thought to be related with the loss of solo, duo, trio, quarto edges. These losses are not unique to hydrogenation, which means if oxygen occupies carbon atoms it may have similar changes.
• This paper compares pristine and hydrogenated for 3 different PAHs. Only one degree of hydrogenation is considered for each PAH. The most dramatic change occurs in the CH stretching region. It would be interesting to focus on one PAH and vary the degree of hydrogenation.
• It just occurs to me that a group of people working on static anharmonic treatment for PAHs because they have better assignments of vibrational modes which can be used as tracers for different types structures (hydrogenated, nitrogen-functionalised, ... PAHs). The goal is not to just predict accurate IR, but also use the abundance of different IR features to trace local chemical and physical environments in ISM.
• [2015 - JCP - Cameron Mackie et.al. The anharmonic quartic force field infrared spectra of three polycyclic aromatic hydrocarbons: Naphthalene, anthracene, and tetracene]
• Gaussian 09 can do VPT2-based anharmonic calculations, but cannot reproduce IR features in CH stretching region of PAHs. If not considering resonances or polyads, anharmonic combination or overtone band intensities computed by Gaussian 09 are reliable for PAH systems.
• Harmonic approximation failed to explain the large number of CH stretching bands of naphthalene, anthracene, and tetracene due to resonances in this region. All of the fundamental CH–stretching modes for each of the PAHs studied are in resonance with many combination bands.
• Type-two Fermi resonances ($\nu_i \approx \nu_j + \nu_k$) with the CH stretching mode around 3000 cm$^{-1}$ and CC stretching modes around 1500 cm$^{-1}$
• 5-5.5 $\mu$m (2000-1820 cm$^{-1}$) region where combination bands are important is poorly described (no bands) by harmonic approximation
• The current "VPT2 + resonance" gives good prediction for bands in CH stretching region for anthracene, and tetracene, but the performance is worse in naphthalene possibly due to more complex resonance polyads.
• First anharmonic IR for anthracene and tetracene
• [2014 - A&A - Rosenberg et.al. Random mixtures of polycyclic aromatic hydrocarbon spectra match interstellar infrared emission]
• An emission model is needed to compare theoretical adsorption spectra with astronomical AIB emission spectra
• [2013 - JPCL - Infrared Spectra of Protonated Pyrene and Its Neutral Counterpart in Solid para-Hydrogen]
• IRPD has improved resolutions than IRMPD, but is difficult to tag large protonated PAHs (the largest working example is protonated naphthalene) with Ar because of their large internal energy.
• Experimental studies of protonated PAHs (from benzene to coronene) in the gas phase.
• IRPD has high resolution and sharp transition with width ~ 5cm$^{-1}$ because of the low temperature (<50K) and the single photon dissociation process.
• IRMPD has lower resolution (~ 30 cm$^{-1}$) due to the finite bandwidth of free electron laser.
• The highest-frequency CC stretch mode at around 1600 cm$^{-1}$ approaches the 6.2 μm UIR feature with increasing #ring.
• The intense, lower-frequency CC stretch modes at 1450 cm$^{-1}$ are characteristic of all linear catacondensed H+PAH and correlate with the 6.9 μm UIR feature, which is however only weakly observed in the astronomical spectrum.
• IRMPD shows typical redshift and broadening of bands than in the cold Ar matrix. (coronene$^+$ is used as an example)
• Redshifts and broadening are less pronounced for coroneneH$^+$ due to its lower dissociation (meaning that it cannot reach temperatures as high as coronene$^+$).
• The RPMD approach suffers from serious resonance problems at high frequencies and low temperatures
• [2004 - ApJ - The Mid-Infrared Laboratory Spectra of Naphthalene (C10H8) in Solid H2O]