Time-dependent density functional theory (TDDFT) offers a tractable means to predict electronic excitation spectra of ices if sufficient care is taken in selecting the DFT functional, basis sets, and the number of excited states. In this work 48 functionals in conjunction with various correlation consistent basis sets were benchmarked for predicting electronic spectra. A training set of important astromolecules was chosen: carbon monoxide, water, hydrogen cyanide, ammonia, methanol, and the hydroxyl radical. Vertical excitation energies between the ground and first excited state and some higher excited states were calculated using multireference configuration interaction calculations to serve as benchmarks for gas phase TDDFT calculations. When used in conjunction with aug-cc-pVDZ quality correlation consistent basis sets, two functionals, MPW1K and ?B97, emerged as very good choices to model electronic excitations for most of the molecules in the training set. Cluster calculation predictions for the UV spectra of amorphous ices of water and ammonia were performed at the TDDFT-MPW1K/AVDZ and TDDFT-?B97/AVDZ levels and then compared against experimental data. The calculations qualitatively reproduce the shapes of the spectra for the most part and quantitatively predict the position of the critical first absorption peak. [GRAPHICS]

ABSTRACT Quantum chemical cluster calculations employing density functional theory and correlation consistent basis sets reveal the following pathways by which hydroxide anions (OH–) may form in amorphous astrophysical ices: (1) hydroxyl radicals (OH), which may arise in ice via ultraviolet photolysis, can capture electrons; (2) adsorbed hydrogen atoms can capture electrons to form H–, which reacts with water to yield H2 and OH–; (3) NaOH deposited on ice dissociates into Na+ and OH–; (4) NaH deposited on ice dissociates into Na+ and H–; H– then reacts with water to yield H2 and OH– as above. The IR spectrum of ice-bound OH– is presented, based on nine clusters containing up to 31H2O and 1–2 OH– anions. The interaction of OH– in ice with cations is also explored. Prior work shows that when HCO+ is deposited on pure amorphous water clusters, it reacts with H2O to form formic acid (HCOOH) and the hydronium (H3O+). When HCO+ is deposited on a cluster containing OH–, the reaction proceeds in almost the same manner, but the H3O+ and OH– charge centres migrate through the water network toward each other and tend to neutralize one another by forming H2O. This occurred in all but one of seven cases considered; migration occurred even when the oxygen atom attacked by HCO+ is over 10 Å from the oxygen atom in OH–. Cations and anions can interact in ice via pathways not present in the gas phase or incorporated in current models.

Sodium-bearing species such as NaCl in the gas phase have been observed in an assortment of carbon-rich and oxygen-rich stellar atmospheres and interstellar environments such as the high-mass protostellar disc surrounding Orion Src1 and the protobinary system, IRAS 16547 -4247. Their detection in relatively low-temperature regions is yet to be made. In this paper, we consider the synthesis of sodium-bearing species with an emphasis on NaCl, via both gas-phase and grain-surface chemistry under assorted interstellar conditions. We also consider the chemistry leading to the gas-phase species NaH and NaOH. Two classes of numerical simulations were run: models under isothermal conditions at temperatures from 10 to 800 K with varied intervals, and three-phase warm-up models that consist of an initial isothermal collapse at 10 K, followed by a warm-up phase in which temperature rises linearly to 200 K, and finally a hot core phase. We have included reactive desorption for both models to produce gaseous NaCl, NaH, and NaOH. We found that for isothermal models over a broad parameter space, the fractional abundances of gaseous NaCl and NaOH can reach above 2 x10-10 and approx. 1 x10-10, respectively, are in the detection range of observational facilities such as Atacama Large Millimeter/Submillimeter Array and JWST. For warm-up models, we found that if we consider molecules to be co-desorbed with water, gaseous NaCl can have a sufficiently large abundance for detection. We then conclude that both gaseous NaCl and NaOH can be detected; however, more experiments and quantum mechanical calculations are needed to constrain the relevant reaction rates better.

One of a small number of known magnesium-containing astromolecules, magnesium isocyanide (MgNC) was first detected in 1986. MgNC is an intriguing reactant to consider: it is an open-shell radical in which its metal atom forms a bond with CN that is a mixture of ionic and covalent character. While its gas phase astrochemistry has received prior attention, the grain surface chemistry of MgNC has never been studied. Because of its ionic character, MgNC is found to interact far more strongly with an ice surface than molecules with a greater degree of covalency. As a radical, it may react with closed-shell molecules deposited from the gas phase. In this work, cluster calculations treated with density functional theory and correlation consistent basis sets were used to model the deposition of MgNC on clusters containing 17 and 24 water molecules, which were then allowed to react with acetylene (HCCH) and hydrogen cyanide (HCN) as well as with H atoms. The addition of H to MgNC-nH2O yields hydromagnesium isocyanide (HMgNC), a known astromolecule that may be ejected into the gas phase. HCCH and HCN bind to MgNC-nH2O to form intermediate radical compounds that may then also react with H atoms. There is enough reaction energy from H addition to eject fragments of the intermediates into the gas phase: the vinyl radical (C2H3) for HCCH and the methaniminyl radical (H2CN) for HCN. That leaves MgNC-nH2O to perform further catalytic activity. Alternatively, various hydrogenated divalent Mg compounds may also be stabilized and frozen into the ice or potentially ejected into the gas phase. Benchmark coupled cluster theory calculations in limited systems were used to characterize the submerged reaction barriers present when HCCH or HCN add to MgNC in the gas phase.

ConspectusInterstellar clouds and the outer reaches of protostellar and protoplanetary systems are very cold environments where chemistry is limited to processes that have little or no reaction barrier (in the absence of external energy input). This account reviews what is known about cation-ice reactions, which are not currently incorporated in astrochemical network models. Quantum chemical cluster calculations using density functional theory have shown that barrierless reactions can occur when gas phase cations such as HCO+, OH+, CH3+, and C+ are deposited on an icy grain mantle with energies commensurate with other gas phase species. When cations react with molecules on ice surfaces, the pathways and products often differ significantly from gas phase chemistry due to the involvement of water and other molecules in the ice. The reactions studied to date have found pathways to abundant and important astromolecules such as methanol, formic acid, and carbon dioxide that are very favorable and may be more efficient pathways than gas phase processes. Other products that can be produced include glycolonitrile, its precursors, and related isocyanide compounds. This account describes for the first time ice surface reactions between the carbon cation, C+, and two common astromolecules, methanol (CH3OH) and formic acid (HCOOH), which can yield precursors to glyoxal, hydroxyketene, vinyl alcohol, and acetaldehyde. The quantum chemical methodology used to explore reaction surfaces is also used to predict both vibrational and electronic spectra of reactant and product ices, which offers guidance for possible experimental studies of these reactions. While theoretical calculations indicate that cation-ice reactions are efficient and offer novel pathways to important astrochemical compounds, experimental confirmation would be very welcome. Cations and ice-covered grain mantles are certainly present in cold astrophysical environments. The account concludes with a discussion of how cation-ice reactions could be incorporated into reaction network models of the formation and destruction of molecules in interstellar clouds and protoplanetary systems. Further studies will involve characterizing additional rcactions and more extensive treatment of the most important cation-ice reactions to better ascertain reaction branching outcomes.

Quantum chemical cluster calculations show that reactions of C+ with HCN or HNC embedded in the surface of an icy grain mantle can account for the formation of a recently detected molecule, glycolonitrile, which is considered to be an important precursor to ribonucleic compounds. Reactions of cations deposited on ice mantles with minimal kinetic energy have been found theoretically to result in previously unknown pathways to significant organic compounds in protostellar systems and the interstellar medium. In density functional theory cluster calculations involving up to 24H(2)O, C+ reacts consistently with HCN embedded in ice to yield the neutral HOCHNC radical with no barrier, along with H3O+ as a byproduct. If HOCHNC then reacts with H, three species can be formed: HOCH2NC (isocyanomethanol), HOCH2CN (glycolonitrile), and HOCHNCH. For the C++ HNC reaction on ice, the HOCHCN and H2OCCN radicals form as intermediates, the first of which is another direct precursor to glycolonitrile via H addition. In addition to characterizing reaction pathways, predictions are provided of the vibrational and electronic spectra of the HCN and HNC starting clusters and the HOCHNC ice-bound intermediate.

Accurate and thorough characterization of the chemistry of compounds containing the third-row elements sulfur and chlorine is critical for modeling the composition of the atmosphere of Venus. We have used a combination of ab initio quantum chemistry and kinetic theory to characterize a group of nine exothermic reactions that involve the exotic sulfur-chlorine species SCl, SCl2, and HSCl, which are thought to be present in trace quantities in the atmosphere of Venus and are included to various degrees in the published atmospheric models. Reaction pathways were characterized with coupled cluster theory at the RCCSD(T) level with triple zeta quality correlation consistent basis sets. For reactions with barriers that lie above the reactant asymptote, the barrier height was extrapolated to the RCCSD(T) complete basis set level via single-point calculations with quadruple and quintuple zeta quality sets. Rate coefficients were predicted with capture theory and transition state theory as appropriate. We have found that in some cases addition-elimination reactions can compete with abstraction reactions due to the tendency of sulfur to form hypervalent compounds and intermediates via recoupled pair bonding.

Spectroscopy is a fundamental component of physical chemistry courses.To encourage students to take more interest in the critical contributions of spectroscopic properties to the subject, I assign different molecules to the students.In the second semester of a two-semester course that covers statistical and classical thermodynamics, the students draw molecules from a hat on the first day of class.They look up spectroscopic properties for their molecules on the web and use the values in various homework, quiz, and exam problems.In a one semester "principles" course, they build a closed-shell molecule from a limited set of atoms.I then provide the students with calculated values of spectroscopic properties, which they use in a term project about their molecules that covers structure, spectra, partition functions, and properties derived from partition functions.

ADVERTISEMENT RETURN TO ISSUEPREVViewpointNEXTViewpoint on ACS PHYS Division Sponsored Virtual SeminarsHeather Abbott-LyonHeather Abbott-LyonKennesaw State University, Chair-Elect, Astrochemistry subdivisionMore by Heather Abbott-Lyonhttp://orcid.org/0000-0001-6844-2487, Carlos R. BaizCarlos R. BaizUniversity of Texas at Austin, Chair-Elect, Biophysical subdivisionMore by Carlos R. Baizhttp://orcid.org/0000-0003-0699-8468, Partha P. BeraPartha P. BeraNASA-Ames Research Center, Chair, Astrochemistry subdivisionMore by Partha P. Berahttp://orcid.org/0000-0003-0843-3209, Kyle CrabtreeKyle CrabtreeUniversity of California, Davis, Vice-Chair, Astrochemistry subdivisionMore by Kyle Crabtreehttp://orcid.org/0000-0001-5629-5192, Qiang CuiQiang CuiBoston University, Secretary/Treasurer, PHYSMore by Qiang Cuihttp://orcid.org/0000-0001-6214-5211, Ryan C. FortenberryRyan C. FortenberryUniversity of Mississippi, Past-Chair, Astrochemistry subdivisionMore by Ryan C. Fortenberryhttp://orcid.org/0000-0003-4716-8225, Christy F. LandesChristy F. LandesRice University, Vice-Chair, PHYSMore by Christy F. Landeshttp://orcid.org/0000-0003-4163-6497, Anne B. McCoy*Anne B. McCoyUniversity of Washington, Past-Chair, PHYSMore by Anne B. McCoyhttp://orcid.org/0000-0001-6851-6634, Rodrigo NoriegaRodrigo NoriegaUniversity of Utah, PHYSMore by Rodrigo Noriegahttp://orcid.org/0000-0003-1199-0866, and David E. WoonDavid E. WoonUIUC, Secretary, Astrochemistry subdivisionMore by David E. Woonhttp://orcid.org/0000-0003-3831-5078Cite this: J. Phys. Chem. A 2021, 125, 8, 1680Publication Date (Web):January 28, 2021Publication History Published online28 January 2021Published inissue 4 March 2021https://pubs.acs.org/doi/10.1021/acs.jpca.1c00302https://doi.org/10.1021/acs.jpca.1c00302editorialACS PublicationsCopyright © Published 2021 by American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views906Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (660 KB) Get e-AlertscloseSUBJECTS:Physical chemistry,Space chemistry Get e-Alerts

Density functional theory cluster calculations indicate that the intermediate HOCHNC readily forms when C + reacts with HCN embedded in the surface of an icy grain mantle.Subsequent H addition to HOCHNC yields the iscyano compound HOCH 2 NC.There is enough energy from the H addition for HOCH 2 NC to isomerize to HOCH 2 CN (glycolonitrile), an important prebiotic molecule that was recently detected with ALMA observations toward the solar-type protostellar source IRAS 16293-2422 B by Zeng et al. [MNRAS 2019, 484, L43].It was found that H can also add to HOCHNC to form HOCHNCH without a barrier.The analogous reactions of C + with HNC in ice will also be discussed.Vibrational spectra of the various ice-bound reactants, intermediates, and products will be presented.The calculations were performed with B3LYP using aug-cc-pVDZ sets on C, N, and O and cc-pVDZ sets on H.

Possible formation routes of interstellar amino acids have been proposed where the first step consists in the transformation of organic nitriles (RCH2 - C N) into ketenimines (RCH = C = NH). Such a transformation shows high-energy barriers and needs energetic processing to occur. We show in this study that the CN bond could be activated through H-addition reactions to form ketenimines when starting from organic alpha, beta-unsaturated nitriles (C N). Consequently, both methyl ketenimine CH3CH = C = NH and ethylcyanide CH3CH2C N might have their origin from H2C=CH - C N + 2H reaction occurring on the interstellar icy grains. During the hydrogenation of H2C = CH - C N ice under interstellar conditions, CH3CH2C N has been formed with an abundance 10 times higher than that of CH3CH = C = NH. Our measurements lead us to conclude that such a distribution would be directly linked to the pi-electrons delocalization H3C - C degrees H - C N <-> H3C -CH = C = N degrees occurring during the reduction of the CC double bond of acrylonitrile.

In this paper we complete our studies of the bonding in the ClFFn+.0.- series, characterizing the structures and energetics of the ground and low-lying excited states of CIFn- (n = 1-6) via coupled cluster calculations with large correlation consistent basis sets and interpreting the results of the calculations using concepts from generalized valence bond (GVB) theory. The chlorine anion is isoelectronic with the argon atom, possessing three 3p lone pairs. However, the electrons in Cl- are much less tightly bound than those of Ar, and it is thus possible for the fluorine atom to form recoupled pair bonds and recoupled pair bond dyads with the chlorine anion even if this is not possible with the argon atom. The calculated dissociation energies, D-0(Fn-1Cl-F), for the anions are 28.9, 72.3, 12.5, 59.9, 4.5, and 38.8 kcal/mol for n = 1-6, and the adiabatic electron affinities, EA(0)(CIFn), are 2.25, 4.78, 3.29, 5.47, 3.80, and 5.43 eV, respectively [CCSD(T)/RCCSD(T)/AVQ(+)Z]. Unlike the cation series, the ClFn- anions have competing dissociation channels, ClFn- -> ClFn-1 + F- and CIFn- -> ClFn-1- + F, because of the similar electron affinities of the fluorine atom and some of the CIFn-1 species. The CIF2- and CIF4- anions dissociate to F- and (CIF, CIF3), while the remaining members of the family dissociate to F and CIFn-1-. However, the electronic structure of the ClF2- species at its equilibrium geometry is consistent with the addition of a fluorine atom to CIF-. In addition to the ground states of ClFn-, three previously unknown excited states were found for ClF2-, ClF3-, and CIF4- with excitation energies of Delta E-0 = 2.24, 2.02 and 2.11 eV, respectively. In contrast to the ground states, which possess the maximum number of recoupled pair bond dyads, the excited state species have one fewer dyad and two additional recoupled pair bonds. (C) 2017 Elsevier B.V. All rights reserved.

The valence of an element often exceeds the number of singly occupied orbitals in the electronic configuration of the ground state of the atom. In the early main group elements, the increase in valence is attributed to the formation of sp(n) hybrid orbitals and in the late main group elements beyond the first row to the formation of 3-center, 4-electron (3c-4e) bonds. Our studies have shown that a single new concept-recoupled pair bonding-underlies the increases in valence in both groups of elements. In this report, we describe recent studies of the CFn and SFn molecules that illustrate the nature of recoupled pair bonds and recoupled pair bond dyads and compare and contrast the recoupled pair bonds formed with the electrons in ns lone pairs (early main group elements) and np lone pairs (late main group elements beyond the first row). Recoupled pair bonding also accounts for many of the other differences in the chemistry of the elements in the first and subsequent rows of the Periodic Table, which is known as the first row anomaly.

at Urbana-Champaign offers two courses in astrochemistry, one lecture (Chem 450) and one laboratory (Chem 451).Both courses present the opportunity for advanced undergraduate and graduate students to learn about various spectroscopic concepts as they are applied toward an exotic subject, astrochemistry.In the lecture course, each student devotes a substantial fraction of the course work to one of the known astromolecules, building a wiki page for it during the semester, presenting a brief oral description about it in class, and then finally writing a paper about it.The course covers electronic, vibrational, and rotational spectroscopy, along with Einstein coefficients, line widths, and the interpretation of actual astronomical spectra.It also covers relevant reactions and reaction networks.Students learn to use pgopher for modeling rotational spectra.The lab course focuses on the methylidyne radical (CH).It begins with its chemistry and spectroscopy and then moves on to laboratory study of its electronic spectrum as observed in a butane flame and then collected with the university's 12" f/15 Brashear refracting telescope in the campus observatory built in 1896.Students learn to use IGOR to reduce CCD data.

As a chemistry undergraduate at the Missouri University of Science and Technology, which was then called the University of Missouri at Rolla, Thom was fascinated by the ability of organic chemists to rationalize the course of a broad range of chemical reactions by shuttling electrons around in the molecules.However, he was also somewhat skeptical-this seemed too good to be true.The following year he took a course in quantum mechanics in the Physics Department and realized that quantum mechanics was the means for fact-checking the explanations in organic chemistry.Thus began his passion for chemical theory.In the quantum mechanics course, he encountered the differential equation for the states and wavefunctions of the harmonic oscillator.Since he had just had a course in numerical analysis and the university was touting its new, "very powerful" computer that it had just installed (an IBM 1620!), Thom decided to solve the harmonic oscillator equation on the computer.After teaching himself the new programming language developed by IBM-FORTRAN-and coding the problem, he was amazed to find that his numerical solution agreed perfectly with that obtained analytically.Thus began his passion for computing.

Icy grain mantles that accrete on refractory dust particles in the very cold interstellar medium or beyond the snow line in protoplanetary disks serve as minute incubators for heterogeneous chemistry. Ice mantle chemistry can differ significantly from the gas phase chemistry that occurs in these environments and is often richer. Modeling ices and their chemistry is a challenging task for quantum theoretical methods, but theory promises insight into these systems that is difficult to attain with experiments. Density functional theory (DFT) is predominately employed for modeling reactions in icy grain mantles due to its favorable scalability, but DFT has limitations that risk undercutting its reliability for this task. In this work, basic protocols are proposed for identifying the degree to which DFT methods are able to reproduce experimental or higher level theoretical results for the fundamental interactions upon which ice mantle chemistry depends, including both reactive interactions and non-reactive scaffolding interactions. The exemplar of this study is the reaction of C(+) with H2O, where substantial methodological differences are found in the prediction of gas phase relative energetics for stationary points (about 10 kcal mol(-1) for the C-O bond energy of the H2OC(+) intermediate), which in turn casts doubt about employing it to treat the C(+) + H2O reaction on an ice surface. However, careful explorations demonstrate that B3LYP with small correlation consistent basis sets performs in a sufficiently reliable manner to justify using it to identify plausible chemical pathways, where the dominant products were found to be neutral HOC and the CO(-) anion plus one and two H3O(+) cations, respectively. Predicted vibrational and electronic spectra are presented that would serve to verify or disconfirm the pathways; the latter were computed with time-dependent DFT. Conclusions are compared with those of a recent similar study by McBride and coworkers (J. Phys. Chem. A, 2014, 118, 6991).

Hypervalence occurs when an element forms more bonds than expected based upon the number of unpaired electrons in the ground atomic state. Thus sulfur is hypervalent because compounds like SF4 and SF6 are stable, but oxygen is not hypervalent because the analogous compounds are not stable. In the recoupled pair bonding model, hypervalence occurs when lone-pair electrons such as the 3p and 3s pairs of ground state sulfur atoms can favorably participate in chemical bonds. Using sulfur fluoride species SFn (n = 1–6) as an example, we show that this conditional form of bonding depends on the stability of the lone-pair in question and the ability of a given ligand to break up the pair. Recoupled pair bonding involves two steps, the initial formation of a bond that decouples a lone-pair and recouples one of the electrons in the lone-pair orbital with a electron in a singly occupied ligand orbital (recoupled pair bond), and the formation of a second bond as another ligand is coupled to the electron left over from recoupling (recoupled pair bond dyad). The first of these bonds is weaker than a covalent bond and its bond length is larger because of the adverse influence of the third electron. The second bond is usually stronger than a covalent bond and the two bond lengths are much closer to the bond lengths of the corresponding covalent bonds. The recoupled pair bond dyad has properties that distinguish it from a pair of covalent bonds. In this chapter, we use orbitals and other information from both multi-configurational self-consistent field (MCSCF) and approximate generalized valence bond (GVB) wavefunctions to demonstrate the bonding character of recoupled pair bonds and bond dyads. We also compare them to other models of hypervalency, such as the Rundle–Pimentel three-center, four-electron model.

Sulfur and fluorine can participate in a variety of bonding motifs, lending significant diversity to their chemistry. Prior work has identified three distinct minima for disulfur tetrafluoride (S2F4) compounds: two FSSF3 isomers and one SSF4 species. We used a combination of sophisticated explicitly correlated coupled cluster calculations and generalized valence bond (GVB) theory to characterize the electronic structure of these species as well as additional stationary points on the potential energy surface with F2SSF2 connectivity. On the singlet surface, the two stationary points considered in this work with an F2SSF2 structure are first- or second-order saddle points and not minima. However, on the triplet surface, we discovered a novel C2 symmetric F2SSF2 minimum that was anticipated from the structure of an excited state ((3)B1) of SF2. Analysis using the GVB wave function in conjunction with the recoupled pair bonding model developed by our group provides a straightforward explanation of the bonding in all of the S2F4 structures considered here. In addition, the model predicted the existence of the F2SSF2((3)B) minimum.