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    Nearly two decades since the detection of cyclopropenone (c-C3H2O) in the interstellar medium (ISM), the understanding of how this molecule comes to be remains incomplete. Many hypotheses place the ubiquitous hydrocarbon c-C3H2 at the centre of such discussions. However, insights into c-C3H2 chemistry are further complicated by the recent detection of ethynyl cyclopropenylidene (c-C3HC2H) and the observation of a radio line possibly belonging to methylenecyclopropene (c-C3H2CH2). In a necessary reconciliation of past and current work on the chemical capabilities of c-C3H2 in interstellar environments, the formation pathways of several functionalized cyclopropenes from c-C3H2 and a hydrogenated radical are explored. Chemically accurate CCSD(T)-F12/cc-pVTZ-F12 calculations are used to evaluate the energies of reaction and generate structures along the reaction pathway for formation products deemed chemically plausible. Potential energy scans are used to include or rule out certain paths to product formation based on conformation to the necessary requirements of cold interstellar chemistry. Four functionalized cyclopropenes in addition to c-C3H2O have net exothermic reactions when forming from c-C3H2 (c-C3H2CC, c-C3H2S, c-C3H2NH, and c-C3H2CH2). The former three are found to have reaction profiles favourable for formation in the cold ISM, while c-C3H2CH2 can only form by passage through an association barrier that must be mitigated by an energy source of some kind. c-C3H2S and c-C3H2NH are the best candidates for new spectroscopic searches. A complete detection of c-C3H2CH2 is necessary to fully understand cyclopropenylidene chemistry in the ISM.

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  2. Deprotonated azabenzene anions require dipole moments in their corresponding neutral radicals of more than 3.5 D in order to exhibit dipole-bound excited states (DBXSs). This is notably larger than the typical 2.0–2.5 D associated with such behavior. Similar computational analysis on deprotonated purine derivatives also conducted herein only requires the more traditional 2.5 D dipole moment, implying that the single six-membered azabenzene rings have additional factors at play in binding diffuse electrons. The present study also shows that the use of coupled cluster singles and doubles with a double-zeta correlation consistent basis set and additional diffuse functions originating from the center-of-charge for all aspects of the computations decreases the error in predicting DBXSs to less than 0.006 eV at worst and likely less than 0.003 eV for most cases. These results can influence the modeling of molecular spectra beyond fundamental chemical curiosity with application to astrochemistry, solar energy harvesting, and combustion chemistry among others.

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  3. New high-level ab initio quartic force field (QFF) methods are explored which provide spectroscopic data for the electronically excited states of the carbon monoxide, water, and formaldehyde cations, sentinel species for expanded, recent cometary spectral analysis. QFFs based on equation-of-motion ionization potential (EOM-IP) with a complete basis set extrapolation and core correlation corrections provide assignment for the fundamental vibrational frequencies of the A˜2B1 and B˜2A1 states of the formaldehyde cation; only three of these frequencies have experimental assignment available. Rotational constants corresponding to these vibrational excitations are also provided for the first time for all electronically excited states of both of these molecules. EOM-IP-CCSDT/CcC computations support tentative re-assignment of the ν1 and ν3 frequencies of the B˜2B2 state of the water cation to approximately 2409.3 cm−1 and 1785.7 cm−1, respectively, due to significant disagreement between experimental assignment and all levels of theory computed herein, as well as work by previous authors. The EOM-IP-CCSDT/CcC QFF achieves agreement to within 12 cm−1 for the fundamental vibrational frequencies of the electronic ground state of the water cation compared to experimental values and to the high-level theoretical benchmarks for variationally-accessible states. Less costly EOM-IP based approaches are also explored using approximate triples coupled cluster methods, as well as electronically excited state QFFs based on EOM-CC3 and the previous (T)+EOM approach. The novel data, including vibrationally corrected rotational constants for all states studied herein, provided by these computations should be useful in clarifying comet evolution or other remote sensing applications in addition to fundamental spectroscopy. 
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    Free, publicly-accessible full text available February 1, 2024
  4. Free, publicly-accessible full text available January 19, 2024
  5. Although methanediamine (CH 2 (NH 2 ) 2 ) has historically been the subject of theoretical scrutiny, it has never been isolated to date. Here, we report the preparation of methanediamine (CH 2 (NH 2 ) 2 )—the simplest diamine. Low-temperature interstellar analog ices composed of ammonia and methylamine were exposed to energetic electrons which act as proxies for secondary electrons produced in the track of galactic cosmic rays. These experimental conditions, which simulate the conditions within cold molecular clouds, result in radical formation and initiate aminomethyl (ĊH 2 NH 2 ) and amino ( N . H 2 ) radical chemistry. Exploiting tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) to make isomer-specific assignments, methanediamine was identified in the gas phase upon sublimation, while its isomer methylhydrazine (CH 3 NHNH 2 ) was not observed. The molecular formula was confirmed to be CH 6 N 2 through the use of isotopically labeled reactants. Methanediamine is the simplest molecule to contain the NCN moiety and could be a vital intermediate in the abiotic formation of heterocyclic and aromatic systems such as nucleobases, which all contain the NCN moiety. 
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    Free, publicly-accessible full text available December 20, 2023
  6. Abstract

    Five substituted cyclopropenylidene derivatives (c-C3HX, X=CN, OH, F, NH2), all currently undetected in the interstellar medium (ISM), are found herein to have mechanistically viable, gas-phase formation pathways through neutral–neutral additions of ·X ontoc-C3H2. The detection and predicted formation mechanism ofc-C3HC2H introduces a need for the chemistry ofc-C3H2and any possible derivatives to be more fully explored. Chemically accurate CCSD(T)-F12/cc-pVTZ-F12 calculations provide exothermicities of additions of various radical species toc-C3H2, alongside energies of submerged intermediates that are crossed to result in product formation. Of the novel reaction mechanisms proposed, the addition of the cyano radical is the most exothermic at -16.10 kcal mol−1. All five products are found to or are expected to have at least one means of associating barrierlessly to form a submerged intermediate, a requirement for the cold chemistry of the ISM. The energetically allowed additions arise as a result of the strong electrophilicity of the radical species as well as the product stability gained through substituent-ring conjugation.

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  7. High-level rovibrational characterization of methanediol, the simplest geminal diol, using state-of-the-art, purely ab initio techniques unequivocally confirms previously reported gas phase preparation of this simplest geminal diol in its C 2 conformation. The F12-TZ-cCR and F12-DZ-cCR quartic force fields (QFFs) utilized in this work are among the largest coupled cluster-based anharmonic frequencies computed to date, and they match the experimental band origins of the spectral features in the 980–1100 cm −1 range to within 3 cm −1 , representing a significant improvement over previous studies. The simulated spectrum also matches the experimental spectrum in the strong Q branch feature and qualitative shape of the 980–1100 cm −1 region. Additionally, the full set of rotational constants, anharmonic vibrational frequencies, and quartic and sextic distortion constants are provided for both the lowest energy C 2 conformer as well as the slightly higher C s conformer. Several vibrational modes have intensities of 60 km mol −1 or higher, facilitating potential astronomical or atmospheric detection of methanediol or further identification in laboratory work especially now that gas phase synthesis of this molecule has been established. 
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  8. The potential formation of halogen bonded complexes between a donor, heptafluoro-2-iodopropane (HFP), and the three acceptor heterocyclic azines (azabenzenes: pyridine, pyrimidine, and pyridazine) is investigated herein through normal mode analysis via Raman spectroscopy, density functional theory, and natural electron configuration analysis. Theoretical Raman spectra of the halogen bonded complexes are in good agreement with experimental data providing insight into the Raman spectra of these complexes. The exhibited shifts in vibrational frequency of as high as 8 cm −1 for each complex demonstrate, in conjunction with NEC analysis, significant evidence of charge transfer from the halogen bond acceptor to donor. Here, an interesting charge flow mechanism is proposed involving the donated nitrogen lone pair electrons pushing the dissociated fluorine atoms back to their respective atoms. This mechanism provides further insight into the formation and fundamental nature of halogen bonding and its effects on neighboring atoms. The present findings provide novel and deeper characterization of halogen bonding with applications in supramolecular and organometallic chemistry. 
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