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  1. Abstract

    A dust nucleating agent may be present in interstellar or circumstellar media that has gone seemingly undetected and unstudied for decades. Some analyses of the Murchison CM2 meteorite suggest that at least some of the aluminum present within condensed as aluminum nitrides instead of the long-studied, but heretofore undetected suite of aluminum oxides. The present theoretical study utilizes explicitly correlated coupled cluster theory and density functional theory to provide a formation pathway from alane (AlH3) and ammonia to the cyclic structure Al2N2H4, which has the proper Al/N ratio expected of bulk aluminum nitrides. Novel rovibrational spectroscopic constants are computed for alane and the first two formed structures, AlNH6and AlNH4, along the reaction pathway for use as reference in possible laboratory or observational studies. Theν8bending frequency for AlNH6at 755.7 cm−1(13.23μm) presents a vibrational transition intensity of 515 km mol−1, more intense than the antisymmetric C−O stretch of carbon dioxide, and contains a dipole moment of 5.40 D, which is ∼3× larger than that of water. Thus, the present reaction pathway and rovibrational spectroscopic analysis may potentially assist in the astrophysical detection of novel, inorganic species which may be indicative of larger dust grain nucleation.

     
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  2. Abstract

    With the recent radioastronomical detection ofcis-trans-carbonic acid (H2CO3) in a molecular cloud toward the Galactic center, the more stable but currently unobservedcis-cisconformer is shown here to have strong IR features. While the higher-energycis-trans-carbonic acid was detected at millimeter and centimeter wavelengths, owing to its larger dipole moment, the vibrational structure ofcis-cis-carbonic acid is more amenable to its observation at micron wavelengths. Even so, both conformers have relatively large IR intensities, and some of these fall in regions not dominated by polycyclic aromatic hydrocarbons. Water features may inhibit observation near the 2.75μm hydride stretches, but other vibrational fundamentals and even overtones in the 5.5–6.0μm range may be discernible with JWST data. This work has employed high-level, accurately benchmarked quantum chemical anharmonic procedures to compute exceptionally accurate rotational spectroscopic data compared to experiment. Such performance implies that the IR absorption and even cascade emission spectral features computed in this work should be accurate and will provide the needed reference for observation of either carbonic acid conformer in various astronomical environments.

     
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  3. ABSTRACT

    The recent synthesis of diaminomethane ((NH2)2CH2) under simulated interstellar conditions implies that this molecule is likely present in the interstellar medium (ISM), and this work provides the highly accurate quantum chemically computed rotational constants and IR frequencies for comparison to experiment that may aid in its potential future detection. This simplest geminal diamine of prebiotic note has a transition for its ν18 fundamental frequency at 702.5 cm−1 (14.23 μm) with an intensity on level with that of the antisymmetric stretch in carbon dioxide. Additionally, its 1.72 D dipole moment for the most stable C2v conformer should make it detectable with radiotelescopes as well. The C2 conformer may also be detectable in similar regions where the C2v form is present, but only warmer or higher-energy regions will allow for possible observation of the C1 conformer. In any case, rotational constants for these other two conformers are also provided in addition to the full set of fundamental vibrational frequencies for the C2v conformer. These data will assist with further laboratory classification and possible interstellar detection of this prebiotic molecule.

     
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  4. New computational and experimental studies have been carried out for the MgCCH radical in its X2Σ+ state. Coupled cluster theory with single, double, and perturbative triples, CCSD(T), was used in conjunction with post-CCSD(T) and scalar relativistic additive corrections to compute vibrational quartic force fields for this molecule. From the quartic force fields, higher-order spectroscopic properties, including rotational constants, were obtained. In tandem, the five lowest energy rotational transitions for MgCCH, N = 1→0 through N = 5→4, were measured for the first time using Fourier transform microwave/millimeter wave methods in the frequency range 9 -50 GHz. The radical was created in the Discharge Assisted Laser Ablation Source (DALAS) developed in the Ziurys group. A combined fit of these data with previous millimeter direct absorption measurements have yielded the most accurate rotational constants for MgCCH to date. The computed principle rotational constant lies within 1.51-1.65 MHz of the experimental one, validating the computational approach. High-level theory was then applied to produce accurate rovibrational spectroscopic constants for MgCCH+, including a rotational constant of B0 = 5354.5–5359.5 MHz.. These new predictions will further the experimental study of MgCCH+, and aid in the low-temperature characterization of MgCCH, detected towards the circumstellar shell of IRC+10216, a carbon-rich star. 
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    Free, publicly-accessible full text available October 24, 2024
  5. ABSTRACT

    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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  6. 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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  7. 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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  8. Abstract

    A hybrid quartic force field approach produces the same accuracies as non‐hybrid methods but for less than one quarter of the computational time. This method utilizes explicitly correlated coupled cluster theory at the singles and doubles level inclusive of perturbative triples (CCSD(T)‐F12b) in conjunction with a triple‐ basis set, core electron correlation, and scalar relativity for the harmonic terms and CCSD(T)‐F12b with a valence double‐ basis set for the cubic and quartic terms. There is no sacrifice in the prediction of fundamental anharmonic vibrational frequencies or vibrationally‐averaged rotational constants as compared to experiment, but the time saved is notable. Other hybrid methods are examined involving different sizes of basis sets and composite terms included or excluded. Not one is more accurate; only one is faster. F12 (also called F12c) is tested as well, but it has an increase in computational time for no increase in accuracy. As such, this work reports a hybrid and composite approach (F12‐TcCR+DZ) in the computation of rovibrational spectral data which can be applied to the observation of novel molecules in the gas phase in the laboratory and potentially even in astrophysical environments.

     
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