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2018 Team New Publication

The Anharmonic Quartic Force Field Infrared Spectra of Decorated PAHs

C. J. Mackie, A. Candian, X. Huang, E. Maltseva, A. Petrignani, J. Oomens, W. J. Buma, T. J. Lee, and A. G. G. M. Tielens

Abstract:
Polycyclic aromatic hydrocarbons (PAHs) have been shown to be ubiquitous in a large variety of distinct astrophysical environments and are therefore of great interest to astronomers. The majority of these findings are based on theoretically predicted spectra, which make use of scaled DFT harmonic frequencies for band positions and the double harmonic approximation for intensities. However, these approximations have been shown to fail at predicting high-resolution gas-phase infrared spectra accurately, especially in the CH-stretching region (2950–3150 cm1, 3 mm). This is particularly worrying for the subset of hydrogenated or methylated PAHs to which astronomers attribute the observed non-aromatic features that appear in the CH-stretching region of spectral observations of the interstellar medium (ISM). In our previous work, we presented the anharmonic theoretical spectra of three linear PAHs and five non-linear PAHs, demonstrating the importance of including anharmonicities into theoretical calculations. In this work we extend these techniques to two methylated PAHs (9-methylanthracene, and 9,10-dimethylanthracene) and four hydrogenated PAHs (9,10-dihydroanthracene, 9,10-dihydrophenanthrene, 1,2,3,4- tetrahydronaphthalene, and 1,2,3,6,7,8-hexahydropyrene) in order to better understand the aliphatic IR features of substituted PAHs. The theoretical spectra are compared with the spectra obtained under matrix isolation low-temperature conditions for the full vibrational fundamental range and under highresolution, low-temperature gas-phase conditions for the CH-stretching region. Excellent agreement is observed between the theoretical and high-resolution experimental spectra with a deviation of 0.00% 0.17%, and changes to the spectra of PAHs upon methylation and hydrogenated are tracked accurately and explained. DOI: 10.1039/c7cp06546a

Click here for a full version pdf

(Received 25th September 2017, Accepted 28th November 2017)


Team New Publication

The Failure of Correlation to Describe Out-of-Plane Carbon=Carbon Bending

R. C. Fortenberry, T. J. Lee, and J. P. Layfield

Abstract:
Carbon-carbon multiply bonded systems are improperly described with standard, wave function-based correlation methods and Gaussian one-particle basis sets implying that thermochemical, spectroscopic, and potential energy surface computations are consistently erroneous. For computations of vibrational modes, the out-of-plane bends can be reported as imaginary at worst or simply too low at best. Utilizing the simplest of aromatic structures (cyclopropenylidene) and various levels of theory, this work diagnoses this known behavior as a combined one-particle and n-particle basis set effect for the first time. In essence, standard carbon basis sets do not describe equally well sp, sp2, and sp3 hybridized orbitals, and this effect is exacerbated post-Hartree-Fock by correlation methods. The latter allow for occupation of the π and π∗ orbitals in the expanded wave function that combine with the hydrogen s orbitals. As a result, the improperly described space is non-physically stabilized by post-Hartree-Fock correlation. This represents a fundamental problem in wavefunction theory for describing carbon. Published by AIP Publishing. https://doi.org/10.1063/1.5013026

Click here for a full version pdf

(Received 9 November 2017; accepted 3 December 2017; published online 11 December 2017)


Team New Publication

Photochemistry of Coronene in Cosmic Water Ice Analogs at Different Concentrations

A. L. F. de Barros, A. L. Mattioda, A. Ricca, G.A. Cruz-Diaz, and L. J. Allamandola

Abstract:
This work presents the photochemistry of ultraviolet (UV) irradiated coronene in water ices at 15 K studied using mid-infrared Fourier transform (FTIR) spectroscopy for C24H12:H2O at concentrations of (1:50), (1:150), (1:200), (1:300), and (1:400). Previous UV irradiation studies of anthracene:H2O, pyrene:H2O, and benzo[ghi]perylene:H2O ices at 15 K have shown that aromatic alcohols and ketones, as well as CO2 and H2CO, are formed at very low temperatures. Likewise, here, in addition to the coronene cation, hydroxy-, keto-,and protonated coronene (coronene H+) are formed. The rate constants for the decay of neutral coronene and for the formation of photoproducts have been derived. It is shown that Polycyclic Aromatic Hydrocarbons (PAHs) and their UV induced PAH:H2O photoproducts have mid-infrared spectroscopic signatures in the 5–8 μm region that can contribute to the interstellar ice components described by Boogert et al. as C1–C5. Our results suggest that oxygenated and hydrogenated PAHs could be in UV-irradiated regions of the interstellar medium where water-rich ices are important. Key words: astrochemistry – ISM: clouds – ISM: kinematics and dynamics – ISM: molecules – protoplanetary disks – techniques: spectroscopic

Click here for a full version pdf

(October 20, 2017)


Registration for 2018 Summer Internships Begins October 18, 2017

Each year we interview candidates for potential intern opportunities assisting our team members with the three modules of our research. NASA Ames Research Center has announced the OSSI (One-Stop Shopping Initiative) registration for Summer Internships in 2018 starting October 18, 2017. Our three opportunities listed below are not currently listed but will be available by November 1, 2017 to select during the registration process. Meanwhile, the NAI ARC team has posted our team Intern Application Template (in PowerPoint for download) to qualify applicants for our projects. Let us take you to the registration requirements and templates.

Our Three Opportunities

Team Bullet  Laboratory Studies of Chemical Processing in Astrophysical Ices

Team Bullet  Computer Modeling of Protoplanetary Disks

Team Bullet  Computational Quantum Chemistry Studies of Astrophysical Ices and Gases

(October 17, 2017)


NASA Ames NAI CAN 7 team's support of the Choctaw and Chickasaw STEM camps presented to the SMD American Indian/Alaskan Native Working Group

POC: Andrew Mattioda

On Thursday, October 19, 2017, Dr. Andrew Mattioda presented a talk covering the Ames' NAI (NASA Astrobiology Institute) CAN 7 Team's work supporting the Choctaw and Chickasaw Nations' STEM camps (Science, Technology, Engineering and Mathematics). The talk was conducted for NASA's Science Mission Directorate American Indian/Alaskan Native Working Group. The presentation discussed the history of the Ames' CAN 7 teams Native American outreach efforts, the development of hands-on science activities for the high school aged youth as well as the support for the Choctaw and Chickasaw STEM camps. Since 2015, members of the Ames NAI CAN 7 team have travel to Oklahoma to participate in the Chickasaw and Choctaw STEM camps for Native American youth. At the camps, the team members discuss their backgrounds and how they arrived at their NASA careers, providing the students a glimpse of the diverse backgrounds of the team members. Team members also conduct hands-on science activities with the youth demonstrating the scientific concepts behind their astrobiology research. Dr. Mattioda presented the talk alongside representatives from the Choctaw and Chickasaw nations.

During the presentations, Mr. Luke Kerr, program manager for the Chickasaw Nation STEM Academy, had this to say about their partnership with NASA Ames' NAI CAN 7 team "Oklahoma isn't, especially the rural parts that we work with, generally considered to be a hub for STEM. However, we're doing our small part to change that! Without which most of the students in our program wouldn't have access to this type of exposure. That's a huge reason why partnerships like those with the NASA Ames Research Center are priceless. It may seem trivial to some, but I can assure you that when our students hear that scientists from NASA Are making a special trip, for them, their eyes like up."

(October 19, 2017)


Team New Publication

The Formation of Nucleobases from the Ultraviolet Photoirradiation of Purine in Simple Astrophysical Ice Analogues

Christopher K. Materese, Michel Nuevo, and Scott A. Sandford

Abstract:
Nucleobases are the informational subunits of RNA and DNA and are essential to all known forms of life. The nucleobases can be divided into two groups of molecules: the pyrimidine-based compounds that include uracil, cytosine, and thymine, and the purine-based compounds that include adenine and guanine. Previous work in our laboratory has demonstrated that uracil, cytosine, thymine, and other nonbiological, less common nucleobases can form abiotically from the UV photoirradiation of pyrimidine in simple astrophysical ice analogues containing combinations of H2O, NH3, and CH4. In this work, we focused on the UV photoirradiation of purine mixed with combinations of H2O and NH3 ices to determine whether or not the full complement of biological nucleobases can be formed abiotically under astrophysical conditions. Room-temperature analyses of the resulting photoproducts resulted in the detection of adenine, guanine, and numerous other functionalized purine derivatives. Key Words: Pyrimidine—Nucleobases—Interstellar; Ices—Cometary; Ices—Molecular processes— Prebiotic chemistry. Astrobiology 17, 761–770.

click here for a full version pdf

Team New Publication

Mechanisms of the Formation of Adenine, Guanine, and Their Analogues in UV-Irradiated Mixed NH3:H2O Molecular Ices Containing Purine

Partha P. Bera,Tamar Stein, Martin Head-Gordon, and Timothy J. Lee

Abstract:
We investigated the formation mechanisms of the nucleobases adenine and guanine and the nucleobase analogues hypoxanthine, xanthine, isoguanine, and 2,6-diaminopurine in a UV-irradiated mixed 10:1 H2O:NH3 ice seeded with precursor purine by using ab initio and density functional theory computations. Our quantum chemical investigations suggest that a multistep reaction mechanism involving purine cation, hydroxyl and amino radicals, together with water and ammonia, explains the experimentally obtained products in an independent study. The relative abundances of these products appear to largely follow from relative thermodynamic stabilities. The key role of the purine cation is likely to be the reason why purine is not functionalized in pure ammonia ice, where cations are promptly neutralized by free electrons from NH3 ionization. Amine group addition to purine is slightly favored over hydroxyl group attachment based on energetics, but hydroxyl is much more abundant due to higher abundance of H2O. The amino group is preferentially attached to the 6 position, giving 6-aminopurine, that is, adenine, while the hydroxyl group is preferentially attached to the 2 position, leading to 2-hydroxypurine. A second substitution by hydroxyl or amino group occurs at either the 6 or the 2 position depending on the first substitution. Given that H2O is far more abundant than NH3 in the experimentally studied ices (as well as based on interstellar abundances), xanthine and isoguanine are expected to be the most abundant bi-substituted photoproducts. Key Words: Astrophysical ice—Abiotic organic synthesis— Nucleic acids—Origin of life—RNA world. Astrobiology 17, 771–785.

click here for a full version pdf

2016 NAI Annual Science Report is now available

Dr. Cruz-Diaz presenting at Lassen Volcanic National Park

Sections for each of the NAI's 12 research teams not only highlight the scientific achievements for the year—with details on research, links to papers, and photos from the field—but also reflect the individual personalities that make up the diverse group of scientists and specialists that is the NAI.

Click here for a full view

An international symposium titled: “Molecules in Space: Linking the Interstellar Medium to the (exo)-planets” was organized by Dr. Partha P Bera (BAERI/SSA) and Alexander Tielens (Leiden)

The symposium was held at the American Chemical Society’s fall national meeting at Washington DC, between 20-24th August, 2017. The symposium was very highly attended with average session attendance of over 70 people. It covered, over nine sessions, a wide breadth of subjects related to astrochemistry, that included organic inventory of the gas phase, the chemistry of the dark clouds, interplay of gas and dust, hot-cores and corinos, proto-planetary disks, high-resolution spectroscopy, the diffuse interstellar bands, the chemistry of atmospheres of stars and planets, and present and future opportunities such as ALMA, SOFIA, and JWST. Each session began with an overview talk by an eminent scholar in the field, followed by talks on observation, experiment, quantum chemistry calculations, and modeling. A total of 66 (including invited and contributed talks) talks and 12 posters were presented at the symposium by participants from all over the world. A special total solar eclipse viewing and discussion session was organized and was very well attended by the attendees of the conference.

(August 20, 2017)


Dr. Scott Sanford gave a talk on the OSIRIS-REx mission on August 10th at NASA Ames Research Center

The OSIRIS-REx Asteroid Sample Return Mission - Bringing samples from Asteroid 101955 Bennu to Earth.

Abstract: These OSIRIS-REx spacecraft was launch in September 2016 and is now on its way to rendezvous with the asteroid 101955 Bennu. Bennuis classified as a Type B asteroid, suggesting that it is likely to contain organics and other primitive materials dating from the origin of the Solar System. Bennuis also an Earth-crossing asteroid and currently constitutes one of the highest terrestrial impact hazards of any known asteroid. During its rendezvous with Bennu, the OSIRIS-REx spacecraft will use a number of onboard instruments to characterize various properties of the asteroid before descending to the asteroid's surface to capture samples. The collected samples will then be returned to Earth in September 2023, after which they will be available for study using state-of-the-art analytical techniques in terrestrial laboratories.



Lassen Volcanic Dark Sky Festival 2017

The Lassen Dark Sky Festival was held on Saturday August 11 and Sunday 12, 2017. One of our NAI ARC team member, Gustavo Cruz Diaz provided a public presentation at the annual event. Lassen Dark Sky Festival is a four-day long event organized every year by the Lassen Volcanic National park to observe, celebrate, and preserve the Dark Sky.

Dr. Gustavo Cruz presented a talk in "Astrochemistry is the chemistry of the stars". Astrochemistry studies the chemistry that can take place in extraterrestrial and potential life-related environments such as Mars and it is concerned with the production, distribution, and fate of the chemical precursors of life. Astrochemistry is devoted to determine the chemical inventory available for the origin of life, provide a record of chemical processes over the history of the Universe, and establish markers to search for in exploration of extraterrestrial environments.

After the talk Dr. Gustavo Cruz introduced an activity, it was the hand held spectroscopy. Using 3 kinds of light sources, He explained them how spectroscopy works and why it is so important to astronomers. There were about 50 people or more, it was full.

Dr. Cruz-Diaz presenting at Lassen Volcanic National Park

Dr. Cruz-Diaz presented his talk.

Dr. Cruz-Diaz presenting at Lassen Volcanic National Park

Dr. Cruz-Diaz talked about Astrochemistry.

Dr. Cruz-Diaz presenting at Lassen Volcanic National Park

Dr. Cruz-Diaz introduced the spectroscopy to the audience .

Dr. Cruz-Diaz presenting at Lassen Volcanic National Park

Engaged audience around Dr. Cruz-Diaz.

Learn more about Lassen Volcanic National Park, click here.




The NASA Ames NAI team participates in the Choctaw Nation Summer STEM (Science, Technology, Engineering and Mathematics) Camp

Drs. Christopher Materese and Andrew Mattioda, of the NASA Ames NAI (NASA Astrobiology Institute) CAN 7 team, participated in the Choctaw Nation Summer STEM Camp, held in Hartshorne, OK.

Click here for a complete summary and more photographs of this year's Choctaw STEM Camp 2017.

Choctaw STEM Camp


Timothy Lee presented an invited talk entitled "Computational Study of the Formation of Prebiotic Molecules in Astrophysical Environments: Mixed Ices and Gas-Phase Conditions"

At the 5th International Conference on Chemical Bonding held at 22-26 June, 2017. He presented the latest work on the formation of ringed, aromatic compounds in the gas-phase starting from small C2 organic molecules, and formation of the nucleobases in irradiated mixed ices seeded with pyrimidine or purine. Both studies were compared with analogous experimental studies carried out at NASA Ames (for the gas-phase work, Salama's group, and for the mixed ices work, Sandford's group).



The NASA Ames NAI Team participates in the Chickasaw Nation Aeronautics and Space Academy (CNASA) 2017

Drs. Melissa Kirven-Brooks & Michel Nuevo traveled to Ada, Oklahoma to inspire students of the Chickawaw Nation to learn about Astrobiology.

Click here for a complete summary and more photographs of this year's Chickasaw STEM Camp 2017.

Chickasaw Academy

Chickasaw Nation Academy in Ada, Oklahoma.

Photograph of the group of participants at Chickasaw STEM Camp 2017

Group picture with most of the students.

(June 6, 2017)


 

Welcome our new Summer Interns 2017!

To learn more about our summer interns please follow this link.

Click here to meet our Summer Interns for 2017!


New Publication: "Ab initio dynamics and photoionization mass spectrometry reveal ion–molecule pathways from ionized acetylene clusters to benzene cation"

Tamar Stein, Biswajit Bandyopadhyaya,Tyler P. Troya , Yigang Fanga , Oleg Kostkoa , Musahid Ahmeda, and Martin Head-Gordon.

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; and Department of Chemistry, University of California, Berkeley, CA 94720 Contributed by Martin Head-Gordon, April 14, 2017 (sent for review October 18, 2016; reviewed by Michal Farnik and Krishnan Raghavachari)

The growth mechanism of hydrocarbons in ionizing environments, such as the interstellar medium (ISM), and some combustion conditions remains incompletely understood. Ab initio molecular dynamics (AIMD) simulations and molecular beam vacuum-UV (VUV) photoionization mass spectrometry experiments were performed to understand the ion–molecule growth mechanism of small acetylene clusters (up to hexamers). A dramatic dependence of product distribution on the ionization conditions is demonstrated experimentally and understood from simulations. The products change from reactive fragmentation products in a higher temperature, higher density gas regime toward a very cold collision-free cluster regime that is dominated by products whose empirical formula is (C2H2)n +, just like ionized acetylene clusters. The fragmentation products result from reactive ion– molecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C4H4 + and C6H6 + structures solvated with one or more neutral acetylene molecules. Such species contain large amounts (>2 eV) of excess internal energy. The role of the solvent acetylene molecules is to affect the barrier crossing dynamics in the potential energy surface (PES) between (C2H2)n + isomers and provide evaporative cooling to dissipate the excess internal energy and stabilize products including the aromatic ring of the benzene cation. Formation of the benzene cation is demonstrated in AIMD simulations of acetylene clusters with n > 3, as well as other metastable C6H6 + isomers. These results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.



Team member Dr Partha P Bera and collaborator Prof. Xander Tielens will be organizing a 5-day long symposium in the 254th National Meeting of the American Chemical Society to be held in Washington DC in August 2017



Team member Dr. Partha P Bera presented a 45-minute long talk at the 253rd National Meeting of the American Chemical Society

on 4th of April 2017 in San Francisco, CA.


Team member Dr. Partha P Bera represented the Ames Team in the executive council meet for the international partners at Abscicon

Abstract:

Identification of nucleobases in extraterrestrial carbonaceous chondrites, such as Murchison, implies their formation in an abiotic condition, and supports their prebiotic role in early Earth. Physicochemical processes by which these complex molecules are synthesized in icy grains are not well understood. The products of UV photo-irradiation of purine and pyrimidine in H2O, NH3 and CH4 ices have been explored using fancy new density functional theory (DFT) methods (wB97M–V) along with large correlation consistent basis sets, and compared against laboratory experimental results. Mechanisms studied include those starting with neutral pyrimidine and purine, and their cationic counterparts, and then reacting with neutrals and radicals generated by radiation. Reaction mechanisms that involved cations on the purine or pyrimidine proved to be the ones that are most important. The calculations reveal that the formation of nucleobases is energetically and kinetically favorable. The gas phase mechanism of their formation proved ineffective, and the presence of one or several water molecules is necessary in order for the final products to form. Explicit solvent calculations using a polarized continuum model (PCM) established the effect of the ice matrix and product formation preferences. Uracil forms rather easily as oxidation is rather easy in pure H2O ices. The scope of thymine formation in H2O:CH4 mixed molecular ices, however, is limited due to the inefficiency of the methylation of pyrimidine, and its oxidized derivatives. Thymine is a minor component of the products in the experimental samples. Amine group addition to purine leading to adenine and guanine in mixed NH3 and H2O ices is facile. Although adenine is the most likely monosubstituted photoproduct in mixed H2O:NH3 ice, isoguanine and xanthine are the bi-substituted products. Many of the photoproducts in UV-irradiated H2O and pyrimidine ice mixtures are found in an experimental study. The results support the scenario in which prebiotic molecules, such as the nucleobase uracil, can be formed under abiotic processes in astrophysically relevant interstellar environments, and on surfaces of icy grains before being delivered to telluric planets such as Earth. But, it constrains the formation of thymine as well as its role in the origin of life.



New Publication: "Planetesimal Formation by the Streaming Instability in a Photoevaporating Disk"

, and Published 2017 April 10. The Astrophysical Journal. Volume 839 Number 1

Abstract:

Recent years have seen growing interest in the streaming instability as a candidate mechanism to produce planetesimals. However, these investigations have been limited to small-scale simulations. We now present the results of a global protoplanetary disk evolution model that incorporates planetesimal formation by the streaming instability, along with viscous accretion, photoevaporation by EUV, FUV, and X-ray photons, dust evolution, the water ice line, and stratified turbulence. Our simulations produce massive (60–130 M ⊕) planetesimal belts beyond 100 au and up to ~20 M ⊕ of planetesimals in the middle regions (3–100 au). Our most comprehensive model forms 8 M ⊕ of planetesimals inside 3 au, where they can give rise to terrestrial planets. The planetesimal mass formed in the inner disk depends critically on the timing of the formation of an inner cavity in the disk by high-energy photons. Our results show that the combination of photoevaporation and the streaming instability are efficient at converting the solid component of protoplanetary disks into planetesimals. Our model, however, does not form enough early planetesimals in the inner and middle regions of the disk to give rise to giant planets and super-Earths with gaseous envelopes. Additional processes such as particle pileups and mass loss driven by MHD winds may be needed to drive the formation of early planetesimal generations in the planet-forming regions of protoplanetary disks.


(April 10, 2017)


Team member Partha P Bera gave an Invited talk at the Spring National Meeting of the American Chemical Society in San Francisco, CA on April 4, 2017.

SESSION: Expanding the Frontiers in Condensed Phase Astrochemistry: Electron Transfer Processes in Ices & Catalysis on Interstellar Grains, “Nucleobase synthesis via UV-induced oxidation of their precursors in astrophysical ices: A quantum chemical perspective (final paper number: PHYS 235)”

(April 13, 2017)


New NAI-funded Publication

matrix

Article title: Infrared Spectroscopy of Matrix-Isolated Neutral Polycyclic Aromatic Nitrogen Heterocycles: The Acridine Series. It details infrared spectroscopic changes as the PAH grows, adding bay areas, and incorporation of nitrogen into the PAH framework, mostly for the acridine based PANHs.

Article reference: SAA15023 Journal title: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy Corresponding author: Dr. Andrew L. Mattioda First author: Dr. Andrew L. Mattioda Final version published online: 07-Apr-2017 DOI information: 10.1016/j.saa.2017.03.044

Link to full article: https://authors.elsevier.com/a/1Ur584xB1qMXEw

Abstract:

The matrix-isolated, mid-infrared spectra of seven acridine-based polycyclic aromatic nitrogen heterocycles (PANHs) have been measured and compared to their non-nitrogen containing parent molecule. The acridine species investigated include acridine, benz[a]acridine, benz[c]acridine, dibenz[a,j]acridine, dibenz[c,h]acridine, dibenz[a,h]acridine and dibenz[a,c]acridine. The previously reported results for 1 and 2-azabenz[a]anthracenes are included for comparison. The experimentally determined band frequencies and intensities are compared with their B3LYP/6–31G(d) values. The overall agreement between experimental and theoretical values is good and in line with our previous investigations. Shifts, typically to the blue, are noted for the C–H out-of-plane (CHoop) motions upon insertion of a nitrogen atom. The formation of a bay region upon addition of additional benzene rings to the anthracene/acridine structure splits the solo hydrogen motions into a bay region solo and an external solo hydrogen, with the bay region solo hydrogen coupling to the quartet hydrogen motions and the external solo hydrogen coupling with the duo hydrogen motions resulting in an extreme decrease in intensity for the CHoop solo hydrogen band when the external hydrogen is replaced by a nitrogen atom. The C–C and C–H in-plane region of this acridine series exhibits the characteristic two fold increase in intensity, noted previously for PANHs. The strong ≈1400 cm −1 band, which was identified in the previous PANH study, is noted in several molecular species as well as another strong PANH feature between 1480 and 1515 cm −1 for several molecules. The presence of these strong bands appear to be primarily responsible for the two-fold increase in the C-H in-plane region's (1100–1600 cm −1) intensity. The C–H stretching region can be characterized by contributions from the solo (bay or external), duo and quartet hydrogens, similar to what was observed in the dibenzopolyacene compounds.

(April 13, 2017)


Registration for 2017 Summer Internships Begins November 1, 2016

Each year we interview candidates for potential intern opportunities assisting our team members with the three modules of our research. NASA Ames Research Center has announced the OSSI (One-Stop Shopping Initiative) registration for Summer Internships in 2017 starting October 18, 2016. Our three opportunities listed below are not currently listed but will be available by November 1, 2016 to select during the registration process. Meanwhile, the NAI ARC team has posted our team Intern Application Template (in PowerPoint for download) to qualify applicants for our projects. Let us take you to the registration requirements and templates.

Our Three Opportunities

Team Bullet  Laboratory Studies of Chemical Processing in Astrophysical Ices

Team Bullet  Computer Modeling of Protoplanetary Disks

Team Bullet  Computational Quantum Chemistry Studies of Astrophysical Ices and Gases

(October 25, 2016)


Announcing Two New NAI-Funded Publications

Fortenberry, R.C., Moore, M.M., and Lee, T.J., “Excited State Trends in Bidirectionally Expanded Closed-Shell PAH and PANH Anions,” 2 September 2016, J. Phys. Chem. A, 120, 7327
doi: 10.1021/acs.jpca.6b06654

Abstract:

Proton-bound complexes produce exceptionally bright vibrational modes for stretches involving the hydrogen atom. Binding a proton between various arrangements of N2 and carbon monoxide molecules is known to produce such behavior, and there are four distinct structures involving N2, CO, and a proton. The problem arises in that all four have the same mass and are, consequently, extremely difficult, if not impossible, to resolve experimentally. Fortunately, quantum chemical predictions have produced accurate descriptions of this bright mode and other spectral features for OCHCO+, NNHNN++, and NN−HCO+. The last of this family to be analyzed is CO−HNN+, which is done here. Utilizing high-level coupled cluster computations and quartic force fields, the bright vibrational mode of CO−HNN+ is shown to shift to the red, and the C−O bond is destabilized in this arrangement as opposed to the lowerenergy NN−HCO+ isomer studied previously. Furthermore, the 1.87 D center-of-mass dipole moment, spectroscopic constants, and other anharmonic fundamental frequencies and intensities are produced for CO−HNN+ to assist in definitive experimental and even astrochemical classification of this and the other three related mass-57 proton-bound complexes.

Fortenberry, R.C., Lee, T.J., and Francisco, J.S., “Quantum Chemical Analysis of the CO-HNN+ Proton-Bound Complex,” 19 September 2016, J. Phys. Chem. A, 102, 7745,
doi: 10.1021/acs.jpca.6b07515

Abstract:

Some anions are known to exhibit excited states independent of external forces such as dipole moments and induced polarizabilities. Such states exist simply as a result of the stabilization of valence accepting orbitals whereby the binding energy of the extra electron is greater than the valence excitation energy. Closed-shell anions are interesting candidates for such transitions since their ground-state, spin-paired nature makes the anions more stable from the beginning. Consequently, this work shows the point beyond which deprotonated, closed-shell polycyclic aromatic hydrocarbons (PAHs) and those PAHs containing nitrogen heteroatoms (PANHs) will exhibit valence excited states. This behavior has already been demonstrated in some PANHs and for anistropically extended PAHs. This work establishes a general trend for PAHs/PANHs of arbitrary size and directional extension, whether in one dimension or two. Once seven six-membered rings make up a PAH/ PANH, valence excited states are present. For most classes of PAHs/PANHs, this number is closer to four. Even though most of these excited states are weak absorbers, the sheer number of PAHs present in various astronomical environments should make them significant contributors to astronomical spectra.

(October 25, 2016)


NAI ARC Team Member Wins Science Innovation Fund Award Dr. Andrew L. Mattioda

NASA Ames Research Center announced the winners of the
Science Innovation Fund (SIF) Awards for FY 2017.
Dr. Andrew L. Mattioda is one of the recipients for the ICEE (In-situ Carbon Exposure Experiment) Proof of Concept. The cause of the size disparity between PAHs found in the ISM and Solar System environments as well as their influence on Solar System organics
is not yet understood.

Innovation:
Presently PAH radiation experiments are conducted utilizing only UV photons, and, in limited instances, protons. ICEE PoC will demonstrate that PAH chemistry is dependent upon the radiation type and physical environment, opening a new area of research.  ICEE PoC experiments will provide data to determine whether the development of the ICEE laboratory is justified. ICEE

PoC investigates factors impacting the survival and chemical evolution of large PAHs irradiated under conditions similar to the proto-solar nebula. Infrared spectra of the PAHs will be measured before and after irradiation providing a unique data set revealing whether these carbon-bearing species would have been able to (1) survive harsh proto-solar conditions, and (2) evolve to form the prebiotic inventory currently found in Solar System materials. 

(October 18, 2016)


Welcome Ana de Barros, Visiting Research Scientist from Brazil Ana de Barros

She is a visiting research scientist from Federal Center for
Technological Education (CEFET/RJ), Rio de Janeiro, Brazil.

Ana is here on a one year fellowship sponsored by the
Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES). NASA and the Brazilian Space Agency (AEB)
recently signed (2015) an agreement to enhance collaborations
and educational opportunities between the agencies. 

(October 18, 2016)


New NAI-funded Publication

Peverati, P., Bera, P. P., Head-Gordon, M., and Lee, T.J., “Insights into Hydrocarbon Chain and Ring Formation in the Interstellar Medium: Computational Study of the Isomers of C4H3+, C6H3+ and C6H5+ and their Formation Pathways.” 20 October 2016, Astrophysical Journal, 830, 128, doi:10.3847/0004-637X/830/2/128

Abstract:

Small hydrocarbons such as acetylene is present in circumstellar envelopes of carbon-rich stars, but the processes that yield larger molecules, and eventually polycyclic aromatic hydrocarbons (PAHs), remain poorly understood. To gain additional insight into the early steps of such processes, electronic structure calculations were performed on the potential energy surfaces of C4H3+, C6H3+ and C6H5+. The results establish reactive pathways from acetylene and its ion to formation of the first aromatic ring. We characterize the stable isomers, their spectroscopic properties, and many of the transition structures that represent barriers to isomerization. The pathways to stabilized C4H3+ and C6H3+ are most likely to arise from unimolecular decomposition of hot
C4H4+ and C6H4+ by H atom elimination. By contrast, we found an ion-molecule pathway to C6H5+ to be very stable to fragmentation and elimination reactions even without collisional stabilization. This aromatic species is a good nucleation center for the growth of larger PAHs in interstellar conditions.

(October 20, 2016)


NAI-funded Publication

Mackie, C. J., Candian, A., Huang, X., Maltseva, E., Petrignani, A., Oomens, J., Mattioda, A. L., Buma, W. J., Lee, T. J., and Tielens, A. G. G. M., “The anharmonic quartic force field infrared spectra of five non-linear Polycyclic Aromatic Hydrocarbons: benz[a]anthracene, chrysene, phenanthrene, pyrene, and triphenylene”, August 2016, J. Chem. Phys. 145, 084313

Abstract:

The study of interstellar polycyclic aromatic hydrocarbons (PAHs) relies heavily on theoretically predicted infrared spectra. Most earlier studies use scaled harmonic frequencies for band positions and the double harmonic approximation for intensities. However, recent high-resolution gas-phase experimental spectroscopic studies have shown that the harmonic approximation is not sufficient to reproduce experimental results. In our previous work, we presented the anharmonic theoretical spectra of three linear PAHs, showing the importance of including anharmonicities into the theoretical calculations. In this paper, we continue this work by extending the study to include five non-linear PAHs (benz[a]anthracene, chrysene, phenanthrene, pyrene, and triphenylene), thereby allowing us to make a full assessment of how edge structure, symmetry, and size influence the effects of anharmonicities. The theoretical anharmonic spectra are compared to spectra obtained under matrix isolation low-temperature conditions, low-resolution, high-temperature gas-phase conditions, and high-resolution, low-temperature gas-phase conditions. Overall, excellent agreement is observed between the theoretical and experimental spectra although the experimental spectra show subtle but significant differences.

(August 31, 2016)


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