Interstellar Anions: The Role of Quantum Chemistry - The Journal of

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Interstellar Anions: The Role of Quantum Chemistry Ryan C. Fortenberry* Georgia Southern University, Department of Chemistry, Statesboro, Georgia 30460, United States ABSTRACT: Six anions have been conclusively detected in the interstellar medium (ISM). They all arrived within a five-year window ending five years ago. Why have no new anions been detected? It is likely a lack of laboratory data for novel anions. This work reviews the role that valence and dipole-bound excited states may play in the formation, detection, and lifetime of anions that may yet be observed in the ISM and how quantum chemistry enhances this understanding. The list of interstellar anions has certainly not been exhausted by any means, but electronic, spectroscopic, and structural data must be provided to aid in any future detections. Quantum chemistry has the flexibility and completeness to provide a full picture of these systems and has shown exceptional accuracies of late. The work reviewed herein gives an overview of what quantum chemical computations have produced and will continue to provide related to anions and how this will enhance both laboratory experiment and astronomical observation.

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Fermi and Teller20 provide the original values for the minimum, ideal dipole moment necessary to bind an additional electron in a diffuse s-type orbital: 1.625 D. It is now accepted that a dipole moment of at least 2.0 D is required to bind an excess electron in a dipole-bound state. Most systems may require even up to 2.5 D due to non-Born−Oppenheimer and centrifugal effects, but, in any case, a significant amount of charge separation is required to keep the excess electron bound.18,21−30 Indeed, potentially every neutral radical with a dipole moment of greater than 2.5 D can support a corresponding dipole-bound anion.31,32 Even molecular clusters are known to bind excess electrons in a dipole-bound fashion.16,18,19,28,31−39 A problem in studying dipole-bound anions experimentally is that dipole-bound anionic states are understandably weakly bound and subsequently experimentally transient in many cases. Some dipole-bound states have been measured at less than 10 meV.34,35,40 Consequently, the dipolebound state exists right at the threshold of stability. For a second dipole-bound state to exist, the dipole moment must be in excess of 9.6 D.21 Hence, a near-zero set of small anions will support any states beyond the first dipole-bound one, and few excited states will be accessible except as metastable states at best. In contrast to dipole-bound ground-state anions, if the ground state of the anion possesses all of its electrons in the valence electron cloud, the dipole-bound state can be accessed as an excited state through valence to dipole-bound excitation. The notable examples of this behavior are CH2CN− and CH2CHO−, characterized both experimentally and computa-

INTRODUCTION It was long believed by astronomers that anions could not exist in space. In counterpoint, the cyanate anion (OCN−) was detected in 1979 toward the W33 gas cloud,1 but this anion is believed to be trapped in astronomical ices and is a product of ice grain mantle chemistry.2 Subsequently, OCN− has not been detected in the interstellar medium (ISM) in the gas phase, indicating that the formation of any gas-phase anions proceeds through a different pathway. Over a quarter of a century later, C6H− was finally detected in the ISM in 2006,3 the first truly interstellar anion. In the time since then, five other anions have been detected: C4H−, C8H−, CN−, C3N−, and C5N−, with the last coming in 2010.4−10 Additionally, a mass continuum of anions from single electrons and diatomics to very large systems was also unexpectedly detected in Titan’s upper atmosphere by the Cassini mission.11 As a result, anions are present in various astronomical environments, but it is still unclear how or why they form. There are many suggestions and hypotheses, but more clear conclusions can only be drawn with more data. What appears to be the most likely pathway for the creation of closed-shell, gas-phase interstellar anions is through the dipole-bound excited-state pathway.12−15 For such a process, the neutral radical is actually the key to the molecular physics. The polarization of the electrons present in a neutral molecule induces a charge separation such that an electron can be attracted to the partial positive portion of the molecular dipole.16 However, a dipole−monopole interaction scales as 1/ r2, where r is the distance between the electron and center of positive charge. A typical monopole−monopole interaction scales as 1/r. As a result, the dipole-bound electron is loosely associated with the molecule in a highly diffuse orbital, even more diffuse than typical Rydberg orbitals.16−19 © XXXX American Chemical Society

Received: May 27, 2015 Revised: July 28, 2015

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DOI: 10.1021/acs.jpca.5b05056 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A tionally.26,41−44 Both the nitrile (CH2CN−) and enolate (CH2CHO−) have valence ground states brought about by spin-pairing the additional electron. The closed-shell nature of these anions induces notable stabilization and is easily constructed by dehydrogenating closed-shell neutrals into the corresponding radicals. As a result, the singly occupied orbital in the neutral radical readily accepts an additional electron or, equivalently, retains the extra electron after deprotonation, depending on dynamical formation. Regardless, the valence occupation of the additional electron lowers the electron binding energies (eBEs) by a factor of more than 2 orders of magnitude to the whole electronvolt level instead of the millielectronvolt level as is the case for dipole-bound states. Even though the eBE is relatively large, the dipole-bound excitations of the nitrile and enolate anions of interest are still in the nearinfrared (NIR).26,41−44 Since these low-energy excitations will likely be the only electronically excited states present for these systems, the astrochemistry community has taken notice of anions and their lone dipole-bound excited states. In fact, CH2CN− has been proposed as a diffuse interstellar band (DIB) carrier for the line at 803.7 nm.45−48 The DIBs are a series of visible to NIR absorption peaks detected with significant uniformity toward most celestial objects.49−53 The assumed rotational substructure observed in these electronic bands indicates they are molecular in provenance, but the carriers of the DIBs has been called “the longest standing unsolved problem in spectroscopy.”54 It has even been said, “... there is no better way to lose a scientific reputation than to speculate on the carrier of the diffuse bands.”55 However, CH2CC: has been proposed as a DIB carrier,56 in addition to CH2CN−,45,46 but not all of the experimentally observed electronic features of propadienylidene are present in the DIBs. It was only very recently that two lines were conclusively linked with a specific molecule, C60+, nearly a century after the first observation of the DIBs.57 The detected interstellar anions have all been found in valence ground states, and their neutral radicals have been known in space for some time.58 With the exception of CN and C4H, the ground states of the other corresponding neutral radicals, C6H, C8H, C3N, and C5N, are known to be strongly dipolar. Therefore, the possibility exists that these known anions can support dipole-bound states as excited states. As a result, the dipole-bound formation hypothesis appears to be a highly likely pathway for their creation. The strongest support for this mechanism actually comes from C4H− and the inconclusive detection of C2H−. The C2H and C4H radicals are now known to be 2Σ+ ground states, while the larger C2nH radicals are 2Π.59−66 The crossover in ground-state terms for the C2nH radical family takes place at C4H, where the energy difference between these two states has been approximated to be 100 wavenumbers with the 2 Π state being higher. The most significant piece of evidence supporting the dipole-bound formation hypothesis is the dipole moment for these states of interest. The dipole value shifts from small (∼0.8 D/ < 2.0 D) in the 2Σ+ states of the C2nHs to large (∼4.0 D/ > 2.0 D) in the 2Π C2nHs whether they are ground or excited states.12,13,67 Hence, the 2Σ+ radicals cannot support a dipole-bound state, but the 2Π can. Even though C4H is much more abundant than C6H in the ISM, C6H− is significantly more abundant than C4H−,13 and, again, C2H− has yet to be conclusively detected. C6H− can form from a dipole-bound excited state and relax to the valence ground state. C4H must first excite into the 2Π state before it can bind the excess

electron and subsequently relax to the valence anion ground state. Even though the excitation energy between 2Σ+ and 2Π is small, the cool and diffuse ISM inhibits this process. As a result, the dipole-bound formation mechanism elegantly explains the observations.13 Differently, CN does not possess a large enough dipole moment to utilize this formation pathway. CN− is likely created through molecular collisional processes since CN is so abundant (even more so than C2H) and is a common reaction product.15,68,69 In many of these anionic studies, quantum chemistry has been instrumental in the analysis of the anions in question. CH2CHO−, for instance, was first analyzed using configuration interaction theory. The electronic and geometric structures of the anion in its valence ground and dipole-bound excited states provided insight into what the photoelectronic spectra of this system should look like in comparison to puzzling experimental results.41 Another example is the detection of C5N− in the ISM, which relied heavily on quantum chemical coupled cluster computations for comparison with the observed and derived rotational constants.8,70 There has been some discussion as to the validity of this claim since no conclusive experimental data have been produced, but most of the other modern astronomical detections have had corroborating quantum chemical computations of some kind in addition to experimental findings.14 As a result, it is still largely accepted that C5N− is in fact present in the cirumstellar envelope of the carbon-rich star IRC+10216 as stated by the initial reports. Quantum chemistry is one of the best available means of providing more data and information about negatively charged systems. Modern theoretical approaches and recent advances in computational hardware71,72 promise highly accurate data in relatively short order. In this article, recent work from our group is highlighted related to the use of quantum chemistry as a tool for providing more information for anions. The discussion of electronically excited states and rovibrational spectra of negatively charged molecules is presented along with some discussion as to the implications of these findings for the astrochemical community. The flexibility that quantum chemical computation can provide, naturally, is highlighted, and its strengths and accuracies are part of the discussion as well. Computation provides insights that experiment cannot in some cases and is absolutely essential to further our understanding of the electronic properties of anions, especially for application to the ISM and even planetary atmospheres like that of Titan.

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COMPUTATIONAL METHODOLOGY The basic method for the determination of whether an excited state of an anion can, first, exist or, second, be determined is actually quite simple. Dipole moments are easily computed with modern methods, and an artifact of the construction of basis sets determines the excited-state type. Straightforward geometry optimization of the neutral radical with subsequent computation of the dipole moment provides for the necessary means of determining whether the corresponding closed-shell anion can support the dipole-bound excited state. After the geometry optimization for the ground-state anion is performed, vertical excitations with increasing levels of even-tempered diffuseness are performed followed by vertical computations of the eBE. If the excitation energy for a certain state decreases with the increase in diffuseness, the state is a candidate for classification as dipole-bound provided that the dipole moment is sufficiently large in the corresponding neutral radical. As the B


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The Journal of Physical Chemistry A

systems has been shown to be quite accurate for state-of-the-art potential energy surfaces (PESs) defined from quartic force fields (QFFs).98−102 QFFs are fourth-order Taylor series approximations to potential of the nuclear Hamiltonian and are constructed as

diffuseness of the basis set is increased, the probability of describing the complete space in which the electron in a dipolebound state may be found also increases. From the particle-ina-box (PIB) model system, it is known that as the length of the box increases, the energy for a given level decreases. The closer the probability is to 1 for describing the possible space described by the dipole-bound electron, the lower the computed excitation energy will be. In most studies,73−77 the geometries are optimized with coupled cluster theory78,79 at the singles, doubles, and perturbative triples [CCSD(T)] level,80 the “gold standard” of quantum chemistry.81,82 The aug-cc-pVTZ basis set,83,84 which has been shown to be adequate for computation of the ground states of anions with proper diffuse functions,85 is also employed. The open-shell computations utilize the unrestricted Hartree−Fock (UHF) reference wave function,86,87 while the closed-shell anions utilize restricted Hartree−Fock (RHF).88 For larger systems such as polycyclic aromatic hydrocarbons (PAHs) and their derivatives, coupled cluster computations are quite time-intensive. It has been shown that B3LYP/6-31+G** computations89−91 are sufficient for computing optimized geometries of PAHs.92 Regardless of the procedure utilized, once the ground-state anion geometry is determined, the electronically excited states are computed via equation-of-motion (EOM)93,94 CCSD again with an increasingly diffuse grouping of Huzinaga/Dunningstyle cc-pVXZ, aug-cc-pVXZ, d-aug-cc-pVXZ, and t-aug-ccpVXZ basis sets83,84 with X = D and/or T, typically. While a dipole-bound orbital is easily determined by analysis of the molecular orbitals (MOs) involved, it is essential that the energy for the excitation be converged as a function of basis set diffuseness necessitating at least some basis set comparison. The basis sets are abbreviated from here on as pVDZ, apVDZ, dapVDZ, and tapVDZ, respectively, and similarly for the tripleζ sets, as well. Another EOM formalism with one less creation operator in the variational portion of the wave function expansion is called equation-of-motion ionization potential (EOMIP).95 When utilized with anions, EOMIP computations give the vertical eBE values. Since no electrons are sufficiently diffuse in the radical, the apVXZ basis set is perfectly adequate for the determination of the eBEs, but the X level must be conserved between the excitation energy and eBE computations. Alternatively, the EOM electron attachment can also be utilized, but it has fewer terms for the same level of wave function truncation leading to faster computations that can be less descriptive.96 As a result, EOMIP is used in the presented work. The vertical computations minimize the number of variables involved in comparing excited-state energies and eBEs to 1: the position (or lack thereof) of the excess electron in the wave function. Adiabatic computations are also useful. The energy difference between the RHF-CCSD(T)/aug-cc-pVTZ optimized geometry of a given anion and the UHF-CCSD(T)/augcc-pVTZ optimized geometry of the corresponding neutral radical should give more experimentally understandable results. However, the adiabatic computations can be problematic since exceptionally high accuracies are necessary to classify the eBEs and excitation energies,16 and this process is further complicated by allowing a wave function response to change in the nuclear positions. The detection of most interstellar molecules relies on radioastronomy and rotational molecular spectroscopy.97 The computation of rovibrational properties of various molecular

V=

1 2

∑ FijΔiΔj + ij

1 6

∑ FijkΔiΔjΔk + ijk

1 24

∑ FijklΔiΔjΔk Δl ijkl

(1)

with displacements in terms of Δi (0.005 Å for bond lengths and 0.005 radians for angles) and the force constants as Fij.... Highly accurate CCSD(T)/aug-cc-pV5Z geometry optimizations corrected for core-correlation from the Martin−Taylor core-correlating basis sets103 serving as the reference geometry for the QFF. A composite energy approach based on aug-ccpVTZ, aug-cc-pVQZ, and aug-cc-pV5Z complete basis set limit extrapolated104 energies corrected once more for core correlation and often for scalar relativity105 as well as higherorder electron correlation effects define the actual surface. Accuracies compared to experiment are often as good as 1 cm−1 and have been produced for hydride stretches,100,106−111 giving significant spectroscopic insight into systems, such as anions, where experiment can be hampered by the very nature of the systems involved.112,113 However, QFFs can have their failings in the prediction of rovibrational spectroscopic data, especially for molecules that have shallow PES minima. We have experienced issues with a few of these systems, mostly protonated structures (l-C2H3 and OCHCO+, refs 111 and 114) and noble gas compounds (ArH2+ and ArH3+, refs 112 and 113). In both cases, QFFs can provide useful data such as high-energy fundamental vibrational frequencies or rotational constants, but larger PESs that explicity describe more of the system are often needed to fully characterize these systems.114 Anions can exhibit such behavior since the additional electron destabilizes the electron cloud reducing many of the bond strengths. Hence, larger PESs coupled to diffusion Monte Carlo methods or vibrational configuration interaction computations115,116 can overcome these limitations to provide highly accurate data,114 but either choice represents a significant increase in computing time due to the larger PES as compared to a QFF. As a result, QFFs are a preferred choice by many groups as long as the nuclei within the molecule chosen are not loosely bound.

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REVIEW AND DISCUSSION Electronic States. CH2CN− and CH2CHO− are experimentally known to possess dipole-bound excited states.42−46 Comparison of the experimental data for these anions to the theoretical data reveals that the theoretical methodology laid out above is adequate and, largely, accurate.73 First, the adiabatic eBE of the dipole-bound excited state is computed to be 0.010 eV, in line with previous benchmarks of other dipolebound anion states.34,35,40 As the diffuseness of the pVDZ basis set is increased, the EOM-CCSD 2 1A′ state energy of CH2CN− decreases from 5.22 to 2.17 eV for apVDZ to 1.63 eV for dapVDZ to 1.54 eV for tapVDZ, nearly the 1.543 eV excitation/electron binding energy observed experimentally by Lykke and co-workers42 and corroborated by Cordiner and Sarre.46 The triple-ζ basis sets provided comparable results as do the EOM-CC3 computations, which make use of the expensieve CC3 iterative-triples, O(N7), coupled cluster excited-state method.117,118 The enolate anion also shows clear convergence in the EOM-CCSD vertical excitation energy C


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Table 1. Vertical Excitation Energies and apVDZ eBEsa for the Benchmarked CH2CN− and CH2CHO− Anions from Ref 73 molecule

a

transition

method

pVDZ

apVDZ

dapVDZ

tapVDZ

pVTZ

apVTZ

dapVTZ

eBE

CH2CN−

2 1A′ ←1 1A′ 1 1A″←1 1A′

5.22 4.68 5.62 5.55

2.17 2.09 2.52 2.48

1.63 1.55 1.89 1.82

1.54 1.46 1.78 1.75

4.56 4.56 5.06 4.94

2.20 2.09 2.50 2.43

1.79 1.68 2.03 1.96

1.52

CH2CHO−

CCSD CC3 CCSD CC3

1.76

All energies given in electronvolts.

Additionally, open-shell anions like C3− and C5− are much more likely to have valence excited states if they are valence in their ground states as well.124 However, the current literature suggests that open-shell anions are generally more likely to be dipole-bound in their ground states and not support excited states of any kind due to their exceptionally low eBEs. Finally, all of the known interstellar anions are closed-shell even though most of the experimental data for valence ground state anions is for open-shell anions. Consequently, our discussion of quantum chemical analysis for anions will be limited to closed-shell anions. Utilizing the above methodology, valence excited states are clearly distinguished from their dipole-bound brethren in vertical excitations. As the spatial extent of the orbitals increases, the excitation energy for valence excited states must change little. The rationale comes from the fact that probability density for a valence state will be localized to a significantly closer range to the molecular core than the dipolebound states. Valence excited states exist independently of dipole strength in the corresponding neutral opening up the photophysics of anions significantly. As a result, the aug-ccpVXZ vertical excitations are adequate for the energies of these excited-state transitions since the orbital characters show valence virtual orbitals involved. Valence Excited State-Containing Anions. The first small anion for which a valence excited state was computed using the vertical excitation methodology was the silicon analogue of CH2CN−, CH2SiN−.73 The 1 1B1 ← 1 1A1 vertical excitation out of the 3b1 HOMO into a valence a1 orbital with 2.13 eV in energy gives indication that this excited state is not dipolebound but valence. The CH2CN− a′ HOMO is given in Figure 1a for comparison to the CH2SiN− b1 HOMO (Figure 1b). The CH2SiN− valence a1 LUMO is shown in Figure 1c. The shift in excitation energy from pVDZ to apVDZ is nonnegligible, but the shift from apVDZ to dapVDZ is small with the shift from dapVDZ to tapVDZ being insignificant. This is shown in Table 2. The dipole-bound state (2 1B1) with excitation once more out of the 3b1 HOMO shows significant decrease in the excitation energy from 6.44 to 2.39 eV for the double-ζ basis set. This energy differs by 0.21 eV between dapVDZ and tapVDZ as opposed to 0.00 eV for the valence 1 1 B1 excited state. Additionally, the vertical eBE is 2.34 eV, enough above the 1 1B1 state for it to be classified as valence, while the 2 1B1 state is 0.05 eV above the eBE indicating that it is close enough to be considered, at least, a candidate dipolebound excited state. Since several anions containing period-two and period-three atoms were tested in this initial study73 and only CH2SiN− demonstrated a propensity for the possession of a valence excited state, it was hypothesized that something unique to silicon may need to be present for small anions to possess valence excited states. As a result, silicon analogues of the simple C3N− chain were tested as well as the smaller SiCN− and CSiN− chains.74 Indeed the stable silicon analogues of

computations of its 2 1A′ state from 5.62 eV with pVDZ to 1.78 eV, with tapVDZ very close to the experimental 1.76 eV excitation energy.43,44 The triple-ζ and CC3 computations add further credence that this approach is valid. These data are given in Table 1 collected from ref 73. Additionally, the vertical tapVDZ excited-state energies are close to experiment as well, indicating that these states are well-described as dipole-bound with this methodology. The adiabatic computations, given in Table 1 of ref 73, similarly show close correlation between experiment and the computed dipole-bound excited state and the eBE, as well as ab initio computation of the dipole moment for the neutral radical. From the nitrile and enolate benchmarking anions, clearly the more expensive CC3 computations are not necessary for the determination of the dipole-bound excited states. Since these states are dominated by single excitations out of the highest occupied molecular orbitals (HOMOs) into a highly diffuse orbital, the double-excitation character is minimal limiting the need for triples-inclusion.59 Additionally, the accepting virtual orbital is a diffuse s-type orbital20,21 meaning that the excited-state term will be a singlet state (as we have spin-restricted them to be) most often of the totally symmetric irreducible representation (irrep). The ground state must always be a singlet of the totally symmetric irrep in any closedshell species, whether charged or neutral. Hence, dipole-bound excited states are rarely variationally accessible and must rely on excited-state methods. Granted, CH2CHO− is an exception to this rule due to its 2a″ HOMO. This unique property contributed to its early demonstration as a carrier of a dipolebound excited state.41 Conversely, closed-shell anions often reduce symmetry due to the added electron repulsion giving a higher probability that the HOMO will be totally symmetric.73−76 The valence excitation of an additional electron, however, makes the photophysics of any anion substantially richer.119 In a valence arrangement, the additional electron is bound to the molecule independent of the dipolar strength and fills an exceptionally stabilized MO. Valence excited states have been discovered or computed in medium-sized molecules with strong electron-withdrawing groups like 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane and 7,7,8,8-tetracyanoquinodimethane,120,121 as well as a few other systems,31,32,119,122,123 but this phenomenon has not routinely appeared with small anions. Some exceptional experimental evidence from Maier and co-workers12,122,123 clearly shows valence excited states in the analysis of pure carbon chains and those that are hydrogenor nitrogen-terminated. Besides C3− and C5−,124,125 all of the examined anions with valence excited states have eight or more heavy atoms. These analyses require tremendous care and detail in the experimental exploration limiting the volume of anions that can be analyzed in the laboratory. Consequently, the flexibility of quantum chemistry makes it useful in the exploration of anions where such valence excited states may be found. D


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Figure 2. SiCCN− HOMO (a) and PIB LUMO (b).

(Figure 2b). This can be viewed as a PIB π → π* transition between the n = 2 HOMO and n = 3 lowest unoccupied molecular orbital (LUMO). The PIB orbital labeling and the molecular orbital labeling number are off due to the 2p core orbitals in the silicon atom creating the 1 π orbital. The 1 1Π state of CCSiN− is valence, as well, 0.50 eV below another valence excited state (2 1Σ+) at 4.21 eV. The upper valence state is the analogous excitation as the same state in SiCCN−, while the lower state results from excitation out of the 11σ into the n = 3 PIB π* LUMO. The 3 π HOMO and 4 π* PIB LUMO are shown in Figure 3a,b, respectively. Hence, CCSiN− can support two valence excited states. Furthermore, the π → π* excitation for the 1 1Π state is relatively bright with a 5 × 10−2 oscillator strength.

Figure 1. CH2CN− HOMO (a), the CH2SiN− HOMO (b), and the CH2SiN− valence virtual orbital (c).

C3N− all support valence excited states,74 as shown in Table 2; CSiCN− is not a stable structure. SiCCN− exhibits a valence 1 1 + Σ excited state 0.09 eV below the 1 1Π dipole-bound state and 0.06 eV below the eBE at 3.33 eV. The 3 π HOMO (Figure 2a) is responsible for all of these electronic dynamics. The valence excitation has the electron accepted by the 4 π* orbital

Table 2. EOM-CCSD Vertical Excitation Energies,a Vertical EOMIP-CCSD eBEs,a,b and Oscillator Strengthsc from GroundState CCSD(T)/aug-cc-pVTZ Geometries for Increasingly Diffuse Basis Sets from Refs 73−76 and This Work molecule CH2SiN− SiCCN− CCSiN−

C3H− CCSiH− C3OH− C3NC− C4N− C5H− HBCN− C3N− C5 N −

1 2 2 1 1 2 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 2 2 3

transition

pVDZ

apVDZ

dapVDZ

tapVDZ

B1← 1 1A1 1 A1← 1 1A1 1 + Σ ← 1 1Σ+ 1 Π ← 1 1Σ+ 1 Π ← 1 1Σ+ 1 + Σ ← 1 1Σ+ 1 Π ← 1 1Σ+ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 + Σ ← 1 1Σ+ 1 + Σ ←1 1Σ+ 1 + Σ ←1 1Σ+

2.90 6.44 3.58 7.17 4.15 4.35 6.20 1.21 6.38 2.09 6.40 2.52 5.60 1.47 6.35 0.90 6.03 0.83 6.56 1.18 7.10 11.32 4.07 11.22

2.24 3.38 3.30 4.50 3.75 4.22 5.71 1.15 3.01 2.05 4.02 2.48 3.26 1.45 4.67 0.89 4.38 0.81 3.34 1.12 2.60 5.92 3.99 5.75

2.12 2.59 3.27 3.50 3.71 4.21 4.82 1.14 2.46 2.05 3.32 2.48 2.91 1.45 3.75 0.89 3.55 0.81 2.91 1.12 1.68 4.75 3.99 4.91

2.12 2.39 3.27 3.36 3.71 4.21 4.60 1.14 2.37 2.05 3.26 2.48 2.88 1.45 3.57 0.89 3.41 0.81 2.86 1.12 1.55 4.57 3.99 4.65

1

f

eBE 2.34 3.33

4.51 2.34 3.27 2.93 3.53 3.36 2.89 − 1.53 4.51 − 4.55

3 4 1 1 5 1 2 2 3 1 2 6 2 2 1 9 1 5 9 1 3 6 2 7

× × × × × × × × × × × × × × × × × × × × × × × ×

10−3 10−3 10−5 10−2 10−2 10−7 10−5 10−3 10−3 10−2 10−2 10−3 10−2 10−3 10−3 10−4 10−3 10−4 10−3 10−2 10−3 10−2 10−6 10−2

pVTZ

apVTZ

dapVTZ

2.69 5.87 3.41 6.45 3.89 4.23 5.99 1.17 5.68 2.11 6.00 2.49 5.26 1.46 6.11 0.88 6.30 0.82 6.02 1.14 5.61

2.29 3.33 3.24 4.36 3.71 4.17 5.68 1.14 3.03 2.07 3.92 2.45 3.33 1.45 4.58 0.88 4.33 0.81 3.37 1.11 2.42

2.23 2.73 3.23 3.62 3.70 4.16 4.93 1.13 2.61 2.07 3.45 2.45 3.07 1.45 3.90 0.88 3.73 0.81 3.07 1.11 1.78

a

In electronvolts. bComputed with EOMIP-CCSD/t-aug-cc-pVDZ. cOscillator strengths ( f values) reported are for EOM-CCSD/dapVTZ except C3N− and C5N−, which are EOM-CCSD/dapVDZ. E


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even though they are isoelectronic to other anions that do. Removing the quasi-degenerate occupied and virtual former π pair of orbitals as a result of cyclization (HBNC− and cC3C2H−) or a symmetry increase (H2BCC−) present in these anions increases the orbital energy for any accepting virtual orbital to the point where valence excited states cannot be accessed below the eBE.76 Certainly other anions may possess valence excited states besides those associated with symmetry breaking to create the closed-shell anion. It was originally asserted that C3N− does not possess even a dipole-bound excited state from the adiabatic computations.73 However, revisiting the vertical double-ζ computations clearly shows a dipole-bound excited state. The energy converges to 4.57 eV for the 2 1Σ+ dipole-bound excited state, and the vertical eBE is 4.51 eV shown in Table 2, within computational accuracy for this state to exist. Additionally, C3N has a dipole moment of 2.9 D73 large enough for a dipolebound state to be quite likely.31,32 C5N− gives all appearances of having a valence excited state as given in Table 2. The proscribed excited-state anion methodology produces a 2Σ+ ground state of C5N with a dipole moment of 3.45 D where the 2Π state lies only a couple of hundred wavenumbers higher. The 3π HOMO of the anion can excite one electron into the n = 4 PIB π* LUMO creating a valence excited-state computed (EOM-CCSD/tapVDZ) to be 3.99 eV or exciting into the s-type σ actual LUMO at 4.65 eV to create the dipole-bound state. The latter 1Σ+ state is clearly dipole-bound after the energy decreases from 11.22 eV for the pVDZ level to the aforementioned 4.65 eV tapVDZ level and since the vertical eBE is computed to be 4.55 eV close to the experimental 4.45 eV value.127 The excitation for the dipolebound state is out of the HOMO−2 13σ orbital, which would create the 2Σ+ C5N radical upon removal. Since C5N− has a different orbital arrangement than C7N− and the longer chains of this family, C5N−’s valence excitation does not follow the pattern observed by Maier and co-workers in their seminal study for this family123 or even the isoelectronic CnH− family where C8H− and higher have produced valence excited states.12,123 The presence of a dipole-bound state for C5N− and not for the longer chains further clouds its electronic spectrum. All of these factors should explain the difficulty in previous experimental characterization for the electronic spectrum of C5N−. Larger Systems. As the length of the chain anion increases, the likelihood of valence excited states appears to increase. This appears to be a direct result of a PIB-like excitation process. When the box length increases, the excitation energy decreases. This simple, semiquantitative understanding can be extended to higher-dimensional π systems such as those related to the particle-on-a-surface or the particle-on-a-ring. Several deprotonated PAH neutral radicals possess notable dipole moments,92 but few are large enough to support a dipole-bound state. Neutral, closed-shell PAHs themselves rarely possess dipole moments of any magnitude due to their symmetry and homogeneous atomic composition. Alternatively, PAHs containing at least one nitrogen heteroatom (PANHs) can have dipole moments greater than 2.0 D.128,129 Exploration of deprotonated but closed-shell PANH derivative anions was undertaken for electronically excited states of quinoline-, acridine-, and pyrenidine-based structures.77 Because of the size of these systems and the number of isomers necessary to analyze for their relative energies, modifications to the standard anion characterization procedure

Figure 3. CCSiN− HOMO (a) and PIB LUMO (b).

Silicon, thus, appeared to be a meaningful constituent in creating anions with valence excited states. To test this hypothesis, related anions containing phosphorus, sulfur, and even aluminum were analyzed75 in three classes: those structurally related to CH2CN− or CH2SiN−, carbideterminated molecules like CCSiN−, and dative-bonded structures and analogues of BH3NH2− since dative-bonded structures are known to produce large dipole moments.126 It is unquestioned that the presence of the larger atoms increases the valence contribution to the excited-state character, but no anions in the set analyzed containing phosphorus, sulfur, or aluminum produce pure, valence excited states. However, silicon is not present in one anion where a singlet valence excited state is shown, C3H−. The evidence for this is collected in Table 2, where clear valence excited-state character is present. In fact, the energy shift from pVDZ to tapVDZ for the 1 1A″ state is only 0.07 eV and only 0.01 eV from apVDZ to tapVDZ converging to 1.14 eV. The eBE is 2.34 eV, which is nearly coincident with the 2 1A′ dipole-bound state excitation energy at 2.37 eV. Its decrease from pVDZ to tapVDZ is 4.01 eV with a 6.38 eV excitation for the localized pVDZ basis set. Similar behavior is present in the silicon analogue, CCSiH−, where the carbide moiety is conserved. The two excited states of C3H− are lower in energy than those in CCSiH− since the eBE is lower giving less energetic room for the excitation to develop.75 The valence excitations of C3H− and CCSiH− are the result of spin-pairing the valence π HOMO electrons. The π orbital (and molecular symmetry) breaks into two parts where there once was one: the a′ and a″. As a result, low-energy excitations between the two pieces are possible and largely responsible for the valence excited state. To test the robustness of this mechanism, several new anions of the :CC−C̈ −R− family were examined.76 In the case of C3H−, R = H. Replacement of R with −OH, −NC, −CN, and −C2H all produce valence excited states,76 as shown in Table 2. The isoelectronic HBCN− also produces a double-ζ valence excited state at 1.12 eV, 0.02 eV below that of C3H−. Hence, it appears that the fine electronic structure and not necessarily the atomic composition itself affects the existence of valence excited states. Since C3H− and the longer C5H− anions both possess valence and dipolebound excited states, it is assumed that adding an additional carbide group to any of these :CC−C̈ −R− molecules will create other anions with valence excited states. If the CC− C̈ −R− composition is altered, the splitting of the former π orbital is no longer available for valence excitations. HBNC−, H2BCC−, and c-C3C2H− do not possess valence excited states F


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Table 3. Double Zeta Deprotonateda PANH Derivative Anion Vertical Excitation Energies (eV), eBEs (eV), and Oscillator Strengths ( f from apVDZ+4s) for a Sample of Those Found in Ref 77 molecule

transition

2-quinoline− (N1H2)a 6-isoquinoline− (N2H6)a −

5-acridine (N1H5)

a

3−2-azaanthracene− (N3H3)a 8-pyrenidine− (N1H8)a 3−2-azapyrenene− (N3H3)a

1 2 1 2 1 2 1 2 1 2 1 2

A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 B1 ← 1 1A1 1 A1 ← 1 1A1 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1 A″ ← 1 1A′ 1 A′ ← 1 1A′ 1

pVDZ

apVDZ

apVDZ+4s

2.26 5.65 2.66 4.31 2.25 6.22 2.26 6.08 2.26 5.90 2.10 5.73

2.31 2.94 2.67 2.65 2.28 3.23 2.31 3.06 2.33 2.93 2.22 2.78

2.31b 2.03 2.67b 2.08 2.28 2.49 2.31 2.47 2.33 2.44 2.22 2.19

f

eBE

3 × 10−5

2.03

10−5 10−3 10−6 10−3 10−6 10−7 10−5 10−4 10−5

2.08

2 2 7 1 3 1 5 5 3

× × × × × × × × ×

2.49 2.47 2.44 2.19

a

Deprotonation takes place at the hydrogen noted with the number given in the molecular name. The parenthetical name is that from the shorthand of ref 77. bThis value was not computed explicitly but is inferred from the pattern of the expanding basis sets observed.

were implemented to produce results within a finite amount of time. As mentioned previously, B3LYP/6-31+G** has been shown to characterize well the geometries of deprotonated PAH anions.92 Additionally, instead of the linear combination of atomic orbitals creating the diffuse s-type orbitals for the dipole-bound states, four increasingly diffuse s-type orbitals are provided directly from the basis set allowing for immediate access to those states at a fraction of the computational cost. The so-called “apVDZ+4s” basis set created from the aug-ccpVDZ basis with the four extra functions (+4s) has been shown to be effective for small molecules.130 For this study on larger anions,77 none of the 14 quinoline derivative anions possess valence excited states. However, those with large enough dipole moments give indication that they can support dipole-bound excited states. This changes for the larger acridine and pyrenidine classes of deprotonated anions. Acridine and pyrenidine both have 23 deprotonated derivative isomers per each class. All of the acridine anions possess valence excited states, and those with large enough neutral dipole moments also may possess dipole-bound states. The 5acridine and 3−2-azaanthracene anion derivatives highlight this in Table 3 with the orbitals involved in the valence excitation of the 3−2-azaanthracene anion derivative shown in Figure 4a,b. For pyrenidine, which is the largest of the three system classes analyzed, most of anions possess valence excited states as is the

case for the exemplary 8-pyrenidine deprotonated anion derivative. However, several pyrenidine-class anion derivatives do not possess valence excited states. All are deprotonated 2azapyrenidine derivatives, which is a C2v molecule in its standard form. The 3−2-azapyrene derivative demonstrates such behavior in Table 3, where the valence 1 1A″ state excitation energy is 2.22 eV and the dipole-bound 2 1A′ excitation energy and eBE are 2.19 eV.77 The valence excited states of the acridine and pyrenidine derivative anions all correspond to excitations out of the HOMO/HOMO−1 where the probability density is isolated on the resulting carbene carbon after deprotonation. The accepting virtual valence orbital is, once more, the π* PIB-like LUMO. This is not the actual LUMO, which is the dipolebound orbital. The PIB or particle-on-a-ring or particle-on-asurface (whichever, as they are largely equivalent depending upon the chosen mathematical basis) model π system appears to be disrupted by the nitrogen heteroatom at the most symmetric position in 2-azapyrenidine. As such, the π* PIB LUMO+1 is more favorable for accepting the electron, raising the valence excitation energy above the dipole-bound excited state and the eBE. Differently, an interesting finding for these sets of PANH anions is that no matter the size of the anion or the type of excitation, the energies for the entire set of anions all fall within 1.5 to 2.5 eV, indicating a strong correlation in spectra for deprotonated PANHs.77 Rovibrational/Rovibronic Analysis. The detection of any anion in the ISM is most easily accomplished through rotational spectroscopy due to the ladder of lines producing self-consistent results. As such, the rotational and spectroscopic constants of any candidate astromolecule are important for their potential observation. Additionally, vibrationally excited rotational constants are also of use since these states of known interstellar molecules have also been observed.8 In either case, the rovibrational spectroscopic data for molecules of interstellar interest, including anions, have been produced based on the earlier-discussed methodology. Furthermore, a nice review by Barone and co-workers97 has recently been published on the quantum chemical rovibrational computation of astrochemically relevant prebiotic molecules. One belief as to why no new anions have been detected in the ISM in over five years lies in the relative dearth of rovibrational reference data for the anions of interest. For instance, even though CH2CN has been known in the ISM for

Figure 4. 3−2-Azaanthracene− (N3H3) a1 HOMO (a) and b1* PIBlike LUMO (b). G


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The Journal of Physical Chemistry A many years,131 the rotational spectrum of the anion has not been explored experimentally for the proper Cs geometry. Since all known interstellar anions have been observed in their valence ground states, a QFF for CH2CN− has been produced to assist in its potential interstellar detection.47 If this anion is to play a role in the DIBs, its electronic ground state must be observable in the ISM, and the data provided previously serve to give reference for the necessary spectroscopic constants. C 3 H − became an object of controversy within the astrochemistry community when rotational lines observed in the Horsehead nebula photodissociation region/photon-dominated region (PDR) were originally attributed to l-C3H+.132 The QFF-determined quartic-centrifugal distortion constant for l-C3H+ differed from that observed in the Horsehead nebula PDR by over 40%.133 C3H− was suggested as an alternative14 because it has a more favorable D-type constant and a Bef f that was close to both l-C3H+ and the observed lines. It was suggested that the additional valence excited state could function as a “doorway”12 to stability creating a steady-state of material in this UV photon-rich environment.14 In other words, the anion forms via the dipole-bound excited state, but its destruction is hindered due to the presence of the valence excited state creating enough material for detection at any given moment. Ultimately, it was determined that l-C3H+ is the carrier of the Horsehead nebula PDR lines in question134 and that its D-type constant is significantly affected by vibrational averaging.135 Hence, some rotational constants for linear molecules can be affected in previously unexpected ways, but the suggestion for C3H− and its new molecular physics gave the astrochemistry community a viable alternative for consideration in astronomical searches.136 Both the linear and cyclic C3H radicals are known to exist in the ISM137,138 making their anions also likely to be present. Even though l-C3H− was not detected in the Horsehead nebula, it may be present elsewhere in the ISM, and the cyclic anion isomer may as well. Rovibrational spectroscopic data have also been produced for 1A1 c-C3H−.139 The work on cyclic C3H− also improves the literature for this molecule where previous results140 appear to have questionable values for some of the fundamental vibrational frequencies. CCOH− has been shown likely to possess a dipole-bound excited state,73 and its valence ground state rovibrational spectroscopic constants and frequencies have also been provided to the community.141 Additionally, the phosphorus analogue of the known C3N− system, C3P−, has recently been shown to possess a potentially stabilizing valence excited state as well as a candidate dipolebound excited state. The spectroscopic data for the valence ground state of C3P− have also been produced from a QFF.142 While the ground electronic state rovibrational data are highly useful for the detection of anions in the ISM, the clear inidicator for the dipole-bound anion formation hypothesis would be to detect an anion in its dipole-bound excited state. The most straightforward way to do this would be to detect the rotational or rovibrational spectrum of the electronically excited dipole-bound state, but this requires rovibronic capabilities, an exceptionally difficult quantum chemical formulation.143,144 The rotational spectrum of C3 for its A 1Πu state has been observed toward various astronomical objects145 highlighting that such rovibronic data are of significance for astrochemistry, especially for those molecules that have a zero dipole moment and/or a rotationally brighter excited state. Recent work in our group has shown that the wellestablished composite approach used for the analysis of the

aforementioned anions and other species can be extended to excited-state electronic structure methods producing electronically excited rovibrational (rovibronic) spectroscopic data.146,147 Since dipole-bound excited states are often variationally inaccessible, excited-state methodologies are necessary for the rovibrational analysis of such states. Many of the points necessary for the excited-state QFF can also become variationally inaccessible if the symmetry of the system changes for a bending mode, as an example. Such is the case for many points on the tested C2H radical in its excited Ã 2Π state.147 While the linear Ã 2Π geometries are variationally accessible, bending the molecule leads to variational collapse due to a lowering of symmetry. Additionally, the need for the highly diffuse functions dipole-bound states is intractable with the t-aug-cc-pV5Z basis set, but the +4s orbitals have shown promise in overcoming this obstacle.130 Work is currently ongoing in this area, but clearly rovibronic reference data are a valuable next step in the analysis of anions in the ISM.

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ASTROPHYSICAL IMPLICATIONS OF ANION EXCITED STATES The Formation of Interstellar Anions. Various theories exist as to the creation of anions. Suggestions include photo-, associative, and collisional detachment in addition to grain chemistry; a nice review of these processes has also been published lately.69 Various mechanisms have various benefits,15,148 but, thus far, only the dipole-bound formation mechanism discussed in this work explains the relatively low astronomically observed C4H− to C4H ratio so elegantly.13 Additionally, the chemical dynamics of this process are not well understood, especially in astrochemical contexts. The eBE for the neutral radical in regard to the dipole-bound state is, again, exceptionally small, in the centi- to millielectronvolt range. In any case, attachment will be barrierless and with a large reaction cross-section. The current understanding is that any neutral molecule with a sufficient dipole moment will bind an extra electron almost by construction.31 It is currently unclear if the incident interstellar electron’s kinetic energy can be overcome by the minimum required strength of the dipole−monople attraction itself or where the new dipolar strength threshold for kinetic capture may be. Larger dipole moments almost certainly enhance capture probabilities. As a result, the rate of relaxation from the dipole-bound state to a valence electronic state may be more influential in the retention of the captured electron than many of the dipole-bound state’s properties. The relaxation rate is likely dependent upon the valence state’s eBE, total molecular Franck−Condon factors, and vibrational progressions among many emerging considerations. To provide more insight into this process, work is also ongoing in this area to establish the electron attachment rates for anions through dipole-bound excited states. In the search for new anions in the ISM, it is unlikely that the two known families, C2nH− and C2n−1N−, will add any new members. Detection of C2H− is not anticipated since the radical must be in an electronically excited state, much like C4H, to bind the additional electron. However, this state is higher in energy in the shorter chain making its progression more difficult even though C2H is so abundant. Regardless that the longer chains are strongly dipolar in their 2Π ground states, they are also not expected to be detected due to likely recombination into PAHs and fullerenes.149,150 The cyanoterminated chains behave differently but are not likely to increase their interstellar anionic populations, either. The H


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Vertical and adiabatic excited-state computations are producing clear markers of valence excited states and possibilities for dipole-bound excited states in many new anions. From the current set of data, it appears that predictable valence excited states in closed-shell anions occur in a few sets of conditions. Closed-shell anions that are long carbon chains, are larger deprotonated PAH/PANH anion derivatives, have period-three atoms, or break symmetry have all shown instances of valence excited states. Furthermore, once a molecule has a valence excited state, larger molecules of the same class will as well. Many anions with one or more of these characteristics, however, do not possess valence excited states below the eBE or coincident dipole-bound excited-state transition energy. Regardless, the increase in the number of anions shown quantum chemically to possess excited states reviewed here should improve the understanding and application of this chemical phenomenon. Additionally, the growth of rovibrational spectroscopic data for anions from quantum chemistry will no doubt increase the likelihood of detecting more negatively charged species in the ISM and possibly also in planetary atmospheres. Further advances, especially in electronically excited-state QFFs, will give even more data for comparison to astronomical observations. While laboratory experiment certainly is necessary in the analysis of anions, as well, quantum chemical computations can provide a complete chemical physics picture for a large number of candidate anions. Certainly, the theories can be incomplete and are not perfectly accurate, but the volume of candidates that can be analyzed and the increasing accuracy of the computations make quantum chemistry a necessary tool in the analysis of anions. The direct application of quantum chemistry to terrestrially transient molecules is natural to better understand low-density and low-temperature regions of the ISM. Quantum chemistry can guide or interpret experimental results and will enhance astrochemical insights, making it an invaluable tool for the astrochemist and molecular astrophysicist.

corresponding radicals invert their dipole moments going from relatively large in the 2Σ+ C3N and C5N radicals to small for the longer 2Π C2n−1N radicals.151 C7N is where the shift in groundstate term takes place for this radical family, and the photophysics of the corresponding anions is more complex than that of the isoelectronic hydrocarbons,123 as discussed previously. If the dipole-bound formation mechanism is correct, the longer C2n−1N− anions will probably not be detected in the ISM. C4H− has to excite into the strongly dipolar 2Π state to create the anion, but it has a very high abundance.13,152,153 C7N has yet to be detected in the ISM indicating that its abundance is quite small. C7N− will be significantly less abundant, by analogy with C4H−, than C7N. Consequently, it is unlikely that any more of the C2n−1N− family of anions will be detected in the ISM with the current generation of telescopes. It is also of note that the eBEs and dipole-bound excited states for C3N− and C5N− are nearly coincident with one another. As such, it may be difficult to distinguish them experimentally, but C3N− should be brighter since its dipole-bound state oscillator strength is more than an order of magnitude greater than the longer C5N−. Additionally, the ketenyl radical has been detected in the ISM, and it is known to be strongly dipolar.73,154 HCCO is also quite common in the ISM with a population only an order of magnitude under that of HCO. Hence, if the HCCO− anion is also detected, more evidence is again present for dipole-bound excited-state gas-phase creation of anions from their neutral radicals. Detection of the aforementioned C3P− anion is also a possibility since its valence excited state will likely enhance its interstellar stability. The anion form is also likely more abundant than the radical in a similar fashion as C6H− is more common than C6H. Furthermore, C3P− has a larger dipole moment than the radical making this an excellent anion for interstellar searches. Anions and the Diffuse Interstellar Bands. Most anions do not possess electronically excited states. Those that do possess few. Additionally, if an anion has multiple electronically excited states, the excitation profiles of the valence and dipolebound states will appear to be very different from one another potentially producing different line profiles. Additionally, anionic excited states are often found toward the red and NIR regions of the electromagnetic spectrum. These signs all bode well for anions as potential carriers of the DIBs, whose electronic features are in the same range and have few correlations with one another.52,54 Additionally, larger systems, such as PAHs and PANHs, will have even further red-shifted excited states. None of these anionic excitations appear to be exceptionally bright, but they could still help to explain the myriad of weak DIBs observed in so many interstellar sightlines without needing to overwhelm the molecular, or even carbon, budget of their respective regions.77 Lastly, the computation of rovibronic spectra for anions will give more conclusive evidence for DIB correlation since the rotational substructure of the absorption peaks can be modeled.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography

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CONCLUSIONS The terrestrial reactivity of anions makes them a natural target for quantum chemical study. Anions are known to exist in the ISM, but it is unclear why they form and what their subsequent chemistry is. More data are necessary to open these avenues of research, and computation can provide this.

Ryan C. Fortenberry earned his bachelor’s of science in mathematics and master’s of science in communication from Mississippi College in 2006 and 2007, respectively, while engaging in computational I


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(7) Thaddeus, P.; Gottlieb, C. A.; Gupta, H.; Brünken, S.; McCarthy, M. C.; Agùndez, M.; Guèlin, M.; Cernicharo, J. Laboratory and Astronomical Detection of the Negative Molecular Ion C3N−. Astrophys. J. 2008, 677, 1132−1139. (8) Cernicharo, J.; Guèlin, M.; Agundez, M.; McCarthy, M. C.; Thaddeus, P. Detection of C5N− and Vibrationally Excited C6H in IRC+10216. Astrophys. J. 2008, 688, L83−L86. (9) Agùndez, M.; Cernicharo, J.; Guèlin, M.; Kahane, C.; Roueff, E.; Klos, J.; Aoiz, F. J.; Lique, F.; Marcelino, N.; Goicoechea, J. R.; Gonzàlez Garcia, M.; Gottlieb, C. A.; McCarthy, M. C.; Thaddeus, P. Astronomical Identification of CN−, the Smallest Observed Molecular Anion. Astron. Astrophys. 2010, 517, L2. (10) Cordiner, M. A.; Charnley, S. B.; Buckle, J. V.; Walsh, C.; Millar, T. J. Discovery of Interstellar Anions in Cepheus and Auriga. Astrophys. J., Lett. 2011, 730, L18. (11) Coates, A. J.; Crary, F. J.; Lewis, G. R.; Young, D. T.; Waite, J. H., Jr.; Sittler, E. C., Jr. Discovery of Heavy Negative Ions in Titan’s Ionosphere. Geophys. Res. Lett. 2007, 34, L22103. (12) Pino, T.; Tulej, M.; Güthe, F.; Pachkov, M.; Maier, J. P. Photodetachment Spectroscopy of the C2nH− (n = 2−4) Anions in the Vicinity of Their Electron Detachment Threshold. J. Chem. Phys. 2002, 116, 6126−6131. (13) Agúndez, M.; Cernicharo, J.; Guélin, M.; Gerin, M.; McCarthy, M. C.; Thaddeus, P. Search for Anions in Molecular Sources: C4H− Detection in L1527. Astron. Astrophys. 2008, 478, L19−L22. (14) Fortenberry, R. C.; Huang, X.; Crawford, T. D.; Lee, T. J. HighAccuracy Quartic Force Field Calculations for the Spectroscopic Constants and Vibrational Frequencies of 1 1A′ l-C3H−: A Possible Link to Lines Observed in the Horsehead Nebula PDR. Astrophys. J. 2013, 772, 39. (15) Carelli, F.; Gianturco, F. A.; Wester, R.; Satta, M. Formation of Cyanopolyyne Anions in the Interstellar Medium: The Possible Role of Permanent Dipoles. J. Chem. Phys. 2014, 141, 054302. (16) Simons, J. Molecular Anions. J. Phys. Chem. A 2008, 112, 6401− 6511. (17) Simons, J. Theoretical Study of Negative Molecular Anions. Annu. Rev. Phys. Chem. 2011, 62, 107−128. (18) Jordan, K. D.; Wang, F. Theory of Dipole-Bound Anions. Annu. Rev. Phys. Chem. 2003, 54, 367−396. (19) Smith, B. H.; Buonaugurio, A.; Chen, J.; Collins, E.; Bowen, K. H.; Compton, R. N.; Sommerfeld, T. Negative Ions of p-Nitroaniline: Photodetachment, Collisions, and Ab Initio Calculations. J. Chem. Phys. 2013, 138, 234304. (20) Fermi, E.; Teller, E. The Capture of Negative Mesotrons in Matter. Phys. Rev. 1947, 72, 399−408. (21) Coulson, C. A.; Walmsley, M. The Minimum Dipole Moment Required to Bind an Electron to a Finite Electric Dipole. Proc. Phys. Soc., London 1967, 91, 31−32. (22) Crawford, O. H.; Dalgarno, A. Bound States of an Electron in a Dipole Field. Chem. Phys. Lett. 1967, 1, 23. (23) Jordan, K. D.; Luken, W. Theoretical Study of the Binding of an Electron to a Molecular Dipole: LiCl−. J. Chem. Phys. 1976, 64, 2760. (24) Turner, J. E. Minimum Dipole Moment Required to Bind an Electron: Molecular Theorists Rediscover Phenomenon Mentioned in Fermi-Teller Paper Twenty Years Earlier. Am. J. Phys. 1977, 45, 758. (25) Crawford, O. H.; Garrett, W. R. Electron Affinities of Polar Molecules. J. Chem. Phys. 1977, 66, 4968. (26) Gutsev, G.; Adamowicz, A. The Valence and Dipole-Bound States of the Cyanomethide Ion, CH2CN−. Chem. Phys. Lett. 1995, 246, 245−250. (27) Gutsev, G.; Adamowicz, A. Relationship between the Dipole Moments and the Electron Affinities for Some Polar Organic Molecules. Chem. Phys. Lett. 1995, 235, 377−381. (28) Gutowski, M.; Skurski, P.; Boldyrev, A. I.; Simons, J.; Jordan, K. D. The Contribution of Electron Correlation to the Stability of Dipole-Bound Anionic States. Phys. Rev. A: At., Mol., Opt. Phys. 1996, 54, 1906. (29) Desfrançois, C.; Abdoul-Carime, H.; Schermann, J.-P. GroundState Dipole-Bound Anions. Int. J. Mod. Phys. B 1996, 10, 1339−1397.

chemistry research with Prof. David H. Magers. He completed his Ph.D. at Virginia Tech in Theoretical Chemistry working under Prof. T. Daniel Crawford in the spring of 2012 before beginning his NASA Postdoctoral Program Fellowship at the NASA Ames Research Center in Mountain View, CA, under the supervision of Dr. Timothy J. Lee. Ryan has been at his current position at Georgia Southern University since the fall of 2013. He has over 40 publications in press or in submission mostly related to the ways in which quantum chemistry can be applied to problems within astrophysics. Ryan is active in the ACS Astrochemistry community, most notably, having been one of the chief organizers for the “Bringing Astrochemicals Back to Earth Symposium” as part of the 250th ACS National Meeting in Boston, MA, in the fall of 2015. Ryan relishes his work at the intersection of chemistry and astrophysics. Additionally, he enjoys science communication, soccer, reading, the outdoors, traveling, and (most especially) time with his family.

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ACKNOWLEDGMENTS R.C.F. would like to thank Georgia Southern Univ. for providing the necessary start-up funds. Additionally, he would like to thank his research group at Georgia Southern for their efforts and enthusiasm in these and other projects: W. J. Morgan (especially for generating the orbital probability plots), M. L. Theis, R. A. Theis, M. Kitchens, J. Lukemire, J. D. Enyard, and M. E. Berlyoung. Additionally, he would like to thank those mentors who have invested significantly in his work: T. J. Lee (NASA Ames), T. D. Crawford (Virginia Tech), D. H. Magers (Mississippi Collge), X. Huang (SETI Institute), and J. S. Francisco (Purdue/Nebraska). D. Hanstorp of the Univ. of Gothenburg is also acknowledged for being willing to discuss the experimental possibilities of spectroscopically analyzing anions in the laboratory. A. Candian of the Leiden Observatory is also acknowledged for driving much of the interest and effort in the PAH/PANH work. The PSI4Education team is also acknowledged for their contributions to computational chemistry in the undergraduate classroom. Quite importantly, the author would like to thank F. A. Bischoff of HumboldtUniv. zu Berlin for providing technical and copy comments for this manuscript and Profs. J. LoBue of Georgia Southern Univ. and C. Fortenberry of Mississippi College for help in editing the manuscript. L. F. Fortenberry of Georgia Southern Univ. is also acknowledged for her continual support.

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