22 research outputs found
On the origin of the 11.3 micron unidentified infrared emission feature
The 11.3 m emission feature is a prominent member of the family of
unidentified infrared emission (UIE) bands and is frequently attributed to
out-of-plane bending modes of polycyclic aromatic hydrocarbon (PAH) molecules.
We have performed quantum mechanical calculations of 60 neutral PAH molecules
and found that it is difficult to reconcile the observed astronomical feature
with any or a mix of these PAH molecules. We have further analyzed the fitting
of spectra of several astronomical objects by the NASA PAH database program and
found that reasonable fittings to the observed spectra are only possible by
including significant contributions from oxygen and/or magnesium containing
molecules in the mix. A mixed of pure PAH molecules, even including units of
different sizes, geometry and charged states, is unable to fit the astronomical
spectra. Preliminary theoretical results on the vibrational spectra of simple
molecules with mixed aromatic/aliphatic structures show that these structures
have consistent bundles of vibrational modes and could be viable carriers of
the UIE bands.Comment: 28 pages, 11 figures, accepted for publication in Ap
A Theoretical Study on the Vibrational Spectra of PAH Molecules with Aliphatic Sidegroups
The role of aliphatic side groups on the formation of astronomical
unidentified infrared emission (UIE) features is investigated by applying the
density functional theory (DFT) to a series of molecules with mixed
aliphatic-aromatic structures. The effects of introducing various aliphatic
groups to a fixed polycyclic aromatic hydrocarbon (PAH) core (ovalene) are
studied. Simulated spectra for each molecule are produced by applying a Drude
profile at =500 K while the molecule is kept at its electronic ground state.
The vibrational normal modes are classified using a semi-quantitative method.
This allows us to separate the aromatic and aliphatic vibrations and therefore
provide clues to what types of vibrations are responsible for the emissions
bands at different wavelengths. We find that many of the UIE bands are not pure
aromatic vibrational bands but may represent coupled vibrational modes. The
effects of aliphatic groups on the formation of the 8 m plateau are qua
ntitatively determined. The vibrational motions of methyl (CH) and
methyl ene (CH) groups can cause the merging of the vibrational bands
of the pa rent PAH and the forming of broad features. These results suggest
that aliphatic structures can play an important role in th e UIE phenomenon.Comment: 29 pages, 13 figures, Accepted for publication in Ap
On the Origin of the 3.3 Micron Unidentified Infrared Emission Feature
The 3.3 m unidentified infrared emission feature is commonly attributed
to C-H stretching band of aromatic molecules. Astronomical observations have
shown that this feature is composed of two separate bands at 3.28 and 3.30
m and the origin of these two bands is unclear. In this paper, we perform
vibrational analyses based on quantum mechanical calculations of 153 organic
molecules, including both pure aromatic molecules and molecules with mixed
aromatic/olefinic/aliphatic hydridizations. We find that many of the C-H
stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) molecules
are coupled. Even considering the un-coupled modes only, the correlation
between the band intensity ratios and the structure of the PAH molecule is not
observed and the 3.28 and 3.30 m features cannot be directly interpreted
in the PAH model. Based on these results, the possible aromatic, olefinic and
aliphatic origins of the 3.3 m feature are discussed. We suggest that the
3.28 m feature is assigned to aromatic C-H stretch whereas the 3.30 m
feature is olefinic. From the ratio of these two features, the relative
olefinic to aromatic content of the carrier can be determined.Comment: 33 pages, 14 figures. Accepted for publication in Ap
The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis
Understanding the molecular vibrations underlying each of the unknown
infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9,
7.7, 11.3, 15.8, 16.4, 18.9 mm) observed in or towards astronomical objects is
a vital link to uncover the molecular identity of their carriers. This is
usually done by customary classifications of normal mode frequencies such as
stretching, deformation, rocking, wagging, skeletal mode, etc. A large
literature on this subject exists and since 1952 ambiguities in classifications
of normal modes via this empirical approach were pointed out by Morino and
Kuchitsu [1]. New ways of interpretation and analyzing vibrational spectra were
sought within the theoretical framework of quantum chemistry [2,3]. Many of
these methods cannot easily be applied [3] to the large, complex molecular
systems which are one of the key research interests of astrochemistry. In
considering this demand, a simple and new method of analyzing and classifying
the normal mode vibrational motions of molecular systems was introduced [4].
This approach is a fully quantitative method of analysis of normal mode
displacement vector matrices and classification of the characteristic
frequencies (fundamentals) underlying the observed IR bands. Outcomes of
applying such an approach show some overlap with customary empirical
classifications, usually at short wavelengths. It provides a quantitative
breakdown of a complex vibration (at longer wavelengths) into the contributed
fragments like their aromatic or aliphatic components. In addition, in
molecular systems outside the classical models of chemical bonds and structures
where the empirical approach cannot be applied, this quantitative method
enables an interpretation of vibrational motion(s) underlying the IR bands
On the presence of metallofullerenes in fullerene-rich circumstellar envelopes
The presence of neutral C60 fullerenes in circumstellar environments has been firmly established by astronomical observations as well as laboratory experiments and quantum-chemistry calculations. However, the large variations observed in the C60 17.4 μm/18.9 μm band ratios indicate that either additional emitters should contribute to the astronomical infrared (IR) spectra or unknown physical processes exist besides thermal and UV excitation. Fullerene-based molecules such as metallofullerenes and fullerene-adducts are natural candidate species as potential additional emitters, but no specific specie has been identified to date. Here we report a model based on quantum-chemistry calculations and IR spectra simulation of neutral and charged endo(exo)hedral metallofullerenes, showing that they have a significant contribution to the four strongest IR bands commonly attributed to neutral C60. These simulations may explain the large range of 17.4 μm/18.9 μm band ratios observed in very different fullerene-rich circumstellar environments like those around planetary nebulae and chemically peculiar R Coronae Borealis stars. Our proposed model also reveals that the 17.4 μm/18.9 μm band ratio in the metallofullerenes simulated IR spectra mainly depends on the metal abundances, ionization level, and endo/ exoconcentration in the circumstellar envelopes. We conclude that metallofullerenes are potential emitters contributing to the observed IR spectra in fullerene-rich circumstellar envelopes. Our simulated IR spectra indicate also that the James Webb Space Telescope has the potential to confirm or refute the presence of metallofullerenes (or even other fullerene-based species) in circumstellar environment
Ab initio relativistic-consistent calculations and charge density and experimental mass-spectroscopic analysis of mono and poly-nuclearclusters of group 11 and 12 transition metals and metal chlorides: ySeyedabdolreza Sadjadi.
The electron density function of molecular systems supplies a package of information. Quantum mechanical methods of producing and analyzing this function have been significantly improved during the past few years. The advent of accurate pseudopotentials and corresponding basis sets for Kohn-Sham density functional and for post-Hartree-Fock electron-correlated approaches have enabled the inclusion of scalar relativistic and spin-orbit coupling effects as well as electron correlation effects into the electron density function. The unpacking of the information embedded in such a function via the quantum theory of atoms in molecules (QTAIM) became possible by utilizing the very new subshell fitting method of reconstructing the density distribution of core electrons that had been replaced by the pseudopotentials.
These theoretical advances were applied in this thesis to characterize and explore the topological features of metal-metal bonding as one of the fundamental types of bonds formed between two elements. Group 11 and 12 transition metals which include gold and mercury as the most relativistic elements were the main focus of this work. Mono and poly-nuclear compounds (with up to 4 metal atoms) in both pure metal clusters and chloro-complexes were studied by ab initio MØller-Plesset perturbation calculations followed by QTAIM analysis on the relaxed density. Some of these chloro-complexes of copper, gold, zinc and cadmium metals were identified in the gas phase by mass spectrometric experiments. The general formulas of the set of molecules studied in group 11 were : M2, MCl, MCl+, MCl2, MCl2+, M2Cl+, M2Cl2^(s+), M2Cl3+, M3Cl2+, M3Cl3+, M3Cl5+, M4Cl5+ and M4Cl7+ and in group 12 were : M2, MCl, MCl+, MCl2, M2Cl3+, M3Cl5+, M4Cl7+ and M2^(s+). The topological features of metal-metal bonding were calculated along with atomic properties for each individual local minimum isomer found. The comparison of
the metal-metal bonding within the complexes and with the dimers revealed new
features of metal-metal bonding in 3d, 4d and 5d transition metal elements of
groups 11 and 12. With the aid of strong correlation between bond dissociation
energy and electron density at the location of the bond critical points found in the
case of dimers, the strength of the metal-metal bonding in the complexes was
estimated. The electron density’s basin properties calculated accurately for all the
clusters and their isomers in this thesis provided more insight also into the nature
of M-Cl bondings in the group 11 and 12 chloride clusters. Ultimately the bonding
information was used to predict the viability of these clusters in the gas phase.published_or_final_versionChemistryDoctoralDoctor of Philosoph
The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis
Understanding the molecular vibrations underlying each of the unknown infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9, 7.7, 11.3, 15.8, 16.4, 18.9 μ m) observed in or towards astronomical objects is a vital link to uncover the molecular identity of their carriers. This is usually done by customary classifications of normal-mode frequencies such as stretching, deformation, rocking, wagging, skeletal mode, etc. A large literature on this subject exists and since 1952 ambiguities in classifications of normal modes via this empirical approach were pointed out by Morino and Kuchitsu New ways of interpretation and analyzing vibrational spectra were sought within the theoretical framework of quantum chemistry. Many of these methods cannot easily be applied to the large, complex molecular systems which are one of the key research interests of astrochemistry. In considering this demand, a simple and new method of analyzing and classifying the normal mode vibrational motions of molecular systems was introduced. This approach is a fully quantitative method of analysis of normal-mode displacement vector matrices and classification of the characteristic frequencies (fundamentals) underlying the observed IR bands. Outcomes of applying such an approach show some overlap with customary empirical classifications, usually at short wavelengths. It provides a quantitative breakdown of a complex vibration (at longer wavelengths) into the contributed fragments such as their aromatic or aliphatic components. In addition, in molecular systems outside the classical models of chemical bonds and structures where the empirical approach cannot be applied, this quantitative method enables an interpretation of vibrational motion(s) underlying the IR bands. As a result, further modifications in the structures (modeling) and the generation of the IR spectra (simulating) of the UIE carriers, initiated by proposing a PAH model, can be implemented in an efficient way. Here fresh results on the vibrational origin of the spectacular UIE bands based on astrochemistry molecular models, explored through the lens of the quantitative method applied to thousands of different vibrational motion matrices are discussed. These results are important in the context of protoplanetary nebulae and planetary nebulae where various molecular species have been uncovered despite their harsh environments