Next Article in Journal
Assessment of Chemical Composition and Anti-Penicillium Activity of Vapours of Essential Oils from Abies Alba and Two Melaleuca Species in Food Model Systems
Next Article in Special Issue
Minimal Active Space for Diradicals Using Multistate Density Functional Theory
Previous Article in Journal
The Signaling Pathways and Targets of Natural Compounds from Traditional Chinese Medicine in Treating Ischemic Stroke
Previous Article in Special Issue
Martini 3 Model of Cellulose Microfibrils: On the Route to Capture Large Conformational Changes of Polysaccharides
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Molecules
Volume 27
Issue 10
10.3390/molecules27103100
molecules-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Communication

HCnH− Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation

by
Ioan Bâldea
Theoretical Chemistry, Heidelberg University, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany
Molecules 2022, 27(10), 3100; https://doi.org/10.3390/molecules27103100
Submission received: 12 April 2022 / Revised: 30 April 2022 / Accepted: 9 May 2022 / Published: 12 May 2022
(This article belongs to the Special Issue A Commemorative Special Issue Honoring Professor Donald Truhlar)
Browse Figures
Versions Notes

Abstract

:
To be detectable in space via radio astronomy, molecules should have a permanent dipole moment. This is the plausible reason why HCnH chains are underproportionally represented in the interstellar medium in comparison with the isoelectronically equivalent HCnN chain family, which is the most numerous homologous series astronomically observed so far. In this communication, we present results of quantum chemical calculations for the HCnH family at several levels of theory: density functional theory (DFT/B3LYP), coupled-cluster expansions (ROCCSD(T)), and G4 composite model. Contradicting previous studies, we report here that linear HCnH− anion chains with sizes of astrochemical interest are unstable (i.e., not all calculated frequencies are real). Nonlinear cis and trans HCnH− anion chains turn out to be stable both against molecular vibrations (i.e., all vibrational frequencies are real) and against electron detachment (i.e., positive electroaffinity). The fact that the cis anion conformers possess permanent dipole is the main encouraging message that this study is aiming at conveying to the astrochemical community, as this makes them observable by means of radio astronomy.

Graphical Abstract

1. Introduction

Although only representing a small fraction of the extraterrestrial matter, astronomical molecules are very interesting for space sciences because they provide valuable information on the physical and chemical conditions as well as the time evolution of the environments where they are detected. Searching for and confirming the presence of new molecular species plays a role of paramount importance in deepening understanding of astrochemical evolution in the interstellar and circumstellar medium [1].
According to the 2018 census [2], 204 molecules were astronomically detected. Out of them, carbon-based chains represent an important class. With seven members astronomically observed (HCN [3], HC2N [4], HC3N [5], HC4N [6], HC5N [7], HC7N [8], HC9N [9]), the HCnN chains form the most numerous homologous series detected so far. This is in contrast to the case of the isoelectronically equivalent HCnH chains [10,11,12,13,14,15,16], out of which only three members ( n = 2 , 4 , 6 ) were astronomically detected: acetylene H−CC−H [17,18], diacetylene H−CC−CC−H [19], and triacetylene H−CC−CC−CC−H [19].
However, based on chemical intuition and substantiated below (see discussion related to Figure 1), it would be completely implausible to claim that members of the HCnN family are more numerous in nonterrestrial environments than members of the HCnH family. Rather, this underproportional representation of the HCnH chains found so far in space relative to the HCnN chains should be related to the complete different difficulty facing HCnH detection versus HCnN detection in space.
Possessing permanent dipole, linear HCnN chains can be detected by radio astronomy, which is par excellence the method to observe extraterrestrial molecules that marked the boom in reporting new molecules in space since the early 1960s [2]. According to existing studies—comprising not only neutral HCnH0 [10,20] but also cation HCnH+ [21,22] and anion HCnH− species [23,24,25]—HCnH chains are linear. If they are linear (more precisely, centrosymmetric), they have zero dipole moments. Such chains cannot be detected via radio astronomy.
Still, are all HCnH− chains with molecular sizes of astrochemical interest really linear and centrosymmetric? This was the fundamental question that triggered the investigation whose results will be presented below, and emphasizing anions is part of our recent [26,27,28,29,30] and ongoing effort to understand their role in astrochemistry, which is claimed to even compete with that of the parent neutrals [29,31,32,33].
The prediction of nonlinear HCnH− anion chains stable against both molecular vibrations (i.e., computed vibrational frequencies are all real) and (excepting n = 4 ) electron detachment (i.e., positive electroaffinity E A > 0 ) and possessing permanent dipole moments ( μ ≠ 0 ) is the main finding reported here. This is the encouraging new message that we aim at conveying to the astrochemical community. To better emphasize it, a series of technical details will be skipped here and deferred to a longer write-up that follows.

2. Methods

All quantum chemical calculations in conjunction with this study were done using the GAUSSIAN 16 [34] suite of programs on the bwHPC platform [35].
The enthalpies of formation Δ f H 0 0 and cis-trans splitting (see Section 3.3) were computed by means of the G4 composite model [36,37]. Recall that in contrast to “simple” models wherein the total electronic energy at frozen geometry—often obtained from optimization at another/lower level of theory (e.g., DFT with smaller basis sets)—is computed by means of a given method (e.g., CCSD(T) and larger basis sets), to achieve high (“chemical”) accuracy, “composite” models (also referred to as compound model chemistries [38]) combine several results obtained via ab initio high-level methods with smaller basis sets with lower-level (DFT) theories using larger basis sets. Within G4, optimization and vibrational frequency calculations are done at the DFT/B3LYP/GTBas3 [34] level. The pertaining zero point energy corrected using an adequate scaling factor as well as thermal corrections to enthalpy and Gibbs free energy are added to the electronic energy estimated at frozen geometry by combining various ab initio methods and basis sets (GAUSSIAN keywords [34] CCSD(T), E4T, FrzG4)/GTBas1, MP4 = FrzG4/GTBas2, MP4 = FrzG4/GTBas3, MP2 = Full/GTLargeXP, HF/GFHFB1, HF/GFHFB2) to obtain values of the total energy, enthalpy, and Gibbs free energy. These estimates turn out to be more accurate than the most elaborate and computationally demanding “simple” ab initio methods (including coupled-cluster (CC) and quadratic configuration interaction (QCI) expansions with singles, doubles and triples corrections (CCSD(T) and QCISD(T), respectively) [36,37].
The values of the adiabatic electron attachment energy EA including corrections due to zero point energy (ZPE) adjusted by means of adequate scaling factors, as standard in compound model chemistries [34,38], were estimated as energies of reaction HCnH0 + e− → HCnH− at zero temperature, which obviates issues related to the so-called “ion convention” or “electron convention” for the charged species [39,40].
For consistency with previous and ongoing work on related systems [20,27,29,41,42,43,44,45,46] and in order to handle shorter and longer molecules on the same footing, all single-point quantum chemical calculations were carried out at the ROCCSD(T) level of theory, wherein restricted open-shell coupled-cluster expansions include single and double excitations as well as perturbative corrections due to triple excitations [47]. All molecular geometries utilized in these single-point calculations were optimized by means of the three parameter B3LYP hybrid DFT/HF exchange correlation functional [48,49,50,51] and 6-311++G(3df,3pd) basis sets [52,53]; more precisely, restricted RB3LYP for closed shell and unrestricted UB3LYP for open shell species. See Appendix B for further details.

3. Results and Discussion

3.1. Enthalpies of Formation: HCnH versus HCnN

In vein with those noted in the Introduction, let us start by comparing the values of the enthalpies of formation of the HCnH chains with those of the HCnN chains. Numerical results obtained using the G4 composite model are collected in Table 1 and depicted in Figure 1. As visible in Figure 1, by and large, neutral HCnN and HCnH chains possess comparable enthalpies of formation Δ f H 0 0 . Importantly, some astronomically detected members of the HCnN family have values of Δ f H 0 0 larger than values for shorter members of the HCnH family not yet detected in space. With the grain of salt that formation mechanisms and kinetics are more important for the interstellar synthesis than in laboratory synthesis, the trend seen in Figure 1—corroborated with the important fact that, after all, the HCnN synthesis requires the presence of extra nitrogen atoms—does by no means substantiate any claim on HCnN members more numerous in space than HCnH members. HCnH’s unfavorable balance in space should not be sought in the production mechanism but rather in the lack of a dipole moment.

3.2. Stable HCnH− Anion Chains with Astrochemically Interesting Sizes Are Nonlinear

Insight gained in conjunction with our recent investigations of astrochemically relevant carbon chain anions [26,27,28,29] made us skeptical that shorter HCnH− anion chains possess a stable linear geometry, as claimed earlier [23]. Our extensive attempts to optimize HCnH− anions imposing strict linear conformation confirmed previous results reported for sizes n ≥ 9 [24,25]; we also found that irrespective whether n is odd or even, such sufficiently long HCnH− anions, linear and invariant under spatial inversion, are stable against molecular vibrations, i.e., all calculated frequencies were real.
However, our calculations disagreed with previous work [23] claiming that HC4H−, HC6H−, and HC8H− are linear and possess a 2 Π u , 2 Π g , and 2 Π u ground state, respectively. Whether even ( n = 4 k , n = 4 k + 2 ) or odd ( n = 2 k + 1 ), we found that strictly linear structures at n ≤ 8 are unstable. Optimization of these anions constrained to be linear invariably ended with molecular conformations having exactly two imaginary frequencies. These two imaginary frequencies correspond to the in-phase and out-of-phase superposition of two vibrational modes, namely the two H−C−C bending modes of the chain ends. In view of this state of affairs, it was not at all surprising to find out that genuine anions’ local energy minima (i.e., all vibrational frequencies real) correspond to cis and trans conformers wherein the two chain ends are bent, as visualized in Figure 2. Full information on the optimized cis and trans anions is presented in Table A2, Table A3, Table A4, Table A5 and Table A6 of Appendix B.
With regard to the specific cases considered in ref. [23], let us mention that at the UB3LYP/6-311++G(3df,3pd) level of theory, we found that the (unstable) linear HC4H−, HC6H−, and HC8H− conformers lie at 520 meV, 229 meV, and 60 meV above the stable nonlinear conformers. These values are much larger that the cis-trans energy splittings Δ c i s − t r a n s presented in Table 3.
The foregoing analysis made it clear that nonlinear cis and trans anions are “stable” in the sense that they correspond to local energy minima. Equally important for the anions’s “stability” is whether they are also stable against electron detachment, i.e., whether their electroaffinity EA (difference between the total energy of the neutral and the total energy of the anion) is positive. Inspection of Table 2 reveals that with one exception, all computed values of EA computed by us are positive. The exception in question is HC4H; this is not surprisingly for small closed-shell molecular species whose anions are rarely stable. Still, given the fact that diacetylene (HC4H) was already detected in space [19], HC4H−’s instability against electron detachment is not so “dramatic” from an astrochemical perspective.

3.3. Relevant Properties of Cis and Trans Anions

To obtain the cis-trans energy splitting, we estimated Δ c i s − t r a n s ≡ Δ f H 0 , c i s 0 HC n H − − Δ f H 0 , t r a n s 0 HC n H − via the G4 composite model. Inspection of the values thus obtained, which are presented in Table 3 and Figure 3, reveals that pragmatically speaking, none of the cis-trans energy splitting significantly differs from zero; all values listed in Table 3 are definitely smaller than the “chemical accuracy” of ∼1 kcal/mol. Consequently, it is reasonable to assume that if present, cis and trans conformers of HCnH− anion chains coexist in extraterrestrial environments.
Putting it better, one can rephrase as follows: cis HCnH− anion conformers can be present in the interstellar medium even if they are slightly higher in energy than their trans counterparts. We said “better” because from the present standpoint, cis anions have a paramount advantage. While the (nearly) centrosymmetric trans anions have (nearly) zero dipole moments, dipole moments of cis HCnH− anions are substantial; see Table 4 and Figure 4. Above, we wrote “nearly” because the (inherently finite) numerical accuracy prevents us to say whether—in contrast with the well-resolved C 2 v symmetry of the cis anions—the trans anions are strictly C 2 h symmetric or only approximately.
We do not want to end this section before mentioning that although not very well separated in energy, cis and trans anion isomers have properties sufficiently different from each other enabling experiments to distinguish between them. As illustration, infrared spectra of cis isomers are depicted along with those of trans isomers in Figure A2 of Appendix C. To understand that choosing above infrared spectra as a specific example was not coincidental, let us note that the detection of HC4H in ISO observations of CRL 618 [19] relied on laboratory information on the bending mode ν 8 = 627.89423 ( 10 ) cm − 1 [56].
Parenthetically, the difference between the aforementioned value of ν 8 with so many digits after the comma and its counterpart at the B3LYP/6-311++G(3df,3pd) level of theory computed by us is ν 8 = 624.650 cm − 1 , which may give a (non-astro-)chemist who is not up with astrophysical ways a flavor that only a perfect match between laboratory spectra and observed lines can give a reliable astronomical identification.

4. Conclusions

Contrary to previous literature reports [23], we demonstrated that stable HCnH− anion chains with astrochemically sizes ( n ≤ 8 ) not too large to be accessible via chemical synthesis in extraterrestrial environments are nonlinear. They can be astronomically observed via radio astronomy because they possess sufficiently large permanent dipoles (cf. Table 4) and electron detachment energies (cf. Table 2).
We do hope that this finding will stimulate laboratory experiments aiming at the accurate characterization of HCnH− anions as a necessary prerequisite for the proper assignment of extraterrestrial signals associated with rovibrational lines. Because any calculation, even obtained with the most sophisticated quantum chemical methods, is unable to give a rovibrational spectrum precise enough to generate a detection in space, it can only help (though it is a lot!) with the laboratory interpretation of an experimental spectrum, which, then, can be used for astronomical observations.

Funding

In the initial stage, this research was funded by the German Research Foundation (DFG grant BA 1799/3-2). Computational support from the state of Baden-Württemberg through bwHPC and the German Research Foundation through Grant No. INST 40/575-1 FUGG (bwUniCluster 2.0, bwForCluster/MLS&WISO 2.0, and JUSTUS 2.0 cluster) is gratefully acknowledged.

Data Availability Statement

The data that support the findings of this study are available from the author upon reasonable request.

Conflicts of Interest

No conflict of interest to declare.

Appendix A

Molecules 27 03100 g0a1 550
Figure A1. Enthalpies of formation Δ f H 0 0 presented in Figure 1 are redrawn here to allow direct comparison between the isoelectronic chains HCn+1H and HCnN.
Figure A1. Enthalpies of formation Δ f H 0 0 presented in Figure 1 are redrawn here to allow direct comparison between the isoelectronic chains HCn+1H and HCnN.
Molecules 27 03100 g0a1
Table
Table A1. Enthalpies of formation of the isoelectronic HCn+H and HCnN chain species computed using the G4 composite model. All values are in kJ/mol.
Table A1. Enthalpies of formation of the isoelectronic HCn+H and HCnN chain species computed using the G4 composite model. All values are in kJ/mol.
nHCn+1HHCnN
0128.668
1479.383389.195
2370.720228.807
3659.255544.779
4597.481457.818
5863.361729.829
6822.640679.914
71080.580938.613
81043.330905.162
91298.2201153.130
101266.5601124.610
111525.3201375.680
12 1347.420

Appendix B

Except for HC4H−—which is less interesting in view of its instability against electron detachment ( EA < 0 , cf. Table 2)—we report below the Cartesian coordinates of the presently considered cis and trans anions; see Table A2, Table A3, Table A4, Table A5 and Table A6.
For open shell optimization, we carried out unrestricted calculations UB3LYP because, according to our experience [26], spin contamination has a negligible impact on the DFT estimates. This obviates the need for restricted open shell ROB3LYP calculations. This sharply contrasts the coupled-cluster (CC) estimates, for which we employed restricted open shell ROCCSD(T) methods; similar to previous studies of related species [26], the impact of spin contamination turned out again to be important, which is a fact that makes less computationally demanding unrestricted UCCSD(T) methods inadequate.
Table
Table A2. Cartesian coordinates in Å of the cis and trans HC3H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
Table A2. Cartesian coordinates in Å of the cis and trans HC3H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
HC3H− cis Trans
S 2 before 0.7692 0.7692
S 2 after 0.7502 0.7502
AtomXYZXYZ
H0.0000000.566638−2.0293600.000000−0.431664−2.096631
C0.000000−0.226564−1.2938250.0000000.267660−1.271822
C0.000000−0.080148−0.0000000.000000−0.0000000.000000
C0.000000−0.2265641.2938250.000000−0.2676601.271822
H0.0000000.5666382.0293600.0000000.4316642.096631
Table
Table A3. Cartesian coordinates in Å of the cis and trans HC5H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
Table A3. Cartesian coordinates in Å of the cis and trans HC5H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
HC5H− cis Trans
S 2 before 0.7791 0.7794
S 2 after 0.7505 0.7505
AtomXYZXYZ
H0.0000000.325138−3.4636830.000000−0.389640−3.424168
C0.000000−0.217856−2.5383090.0000000.241108−2.553977
C0.0000000.034756−1.2950560.0000000.047397−1.297268
C0.0000000.1543780.0000000.000000−0.000000−0.000000
C0.0000000.0347561.2950560.000000−0.0473971.297268
C0.000000−0.2178562.5383090.000000−0.2411082.553977
H0.0000000.3251383.4636830.0000000.3896403.424168
Table
Table A4. Cartesian coordinates in Å of the cis and trans HC6H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
Table A4. Cartesian coordinates in Å of the cis and trans HC6H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
HC6H− cis Trans
S 2 before 0.7660 0.7660
S 2 after 0.7502 0.7501
AtomXYZXYZ
H0.0000000.509362−4.0684710.000000−0.429635−4.080898
C0.000000−0.122855−3.1973560.0000000.179751−3.193660
C0.0000000.007350−1.9480750.0000000.017416−1.948151
C0.000000−0.019817−0.6271390.0000000.011855−0.626980
C0.000000−0.0198170.6271390.000000−0.0118550.626980
C0.0000000.0073501.9480750.000000−0.0174161.948151
C0.000000−0.1228553.1973560.000000−0.1797513.193660
H0.0000000.5093624.0684710.0000000.4296354.080898
Table
Table A5. Cartesian coordinates in Å of the cis and trans HC7H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
Table A5. Cartesian coordinates in Å of the cis and trans HC7H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
HC7H− cis Trans
S 2 before 0.7870 0.7871
S 2 after 0.7509 0.7509
AtomXYZXYZ
H0.4316600.000000−0.9218600.000000−0.330843−4.787382
C−0.0172040.0000000.0472440.0000000.144397−3.832244
C0.1607260.0000001.2835170.0000000.020735−2.589456
C0.2263670.0000002.5969120.0000000.016697−1.275568
C0.2912940.0000003.8703460.000000−0.000000−0.000000
C0.3676710.0000005.1462210.000000−0.0166971.275568
C0.4459560.0000006.4556980.000000−0.0207352.589456
C0.3951780.0000007.7082390.000000−0.1443973.832244
H0.9976050.0000008.5933550.0000000.3308434.787382
Table
Table A6. Cartesian coordinates in Å of the cis and trans HC8H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
Table A6. Cartesian coordinates in Å of the cis and trans HC8H− anion conformers optimized at the UB3LYP/6-311++G(3df,3pd) level of theory. Subscripts “before” and “after” label the values of the total spin before and after annihilation of the first spin contaminant.
HC8H− cis trans
S 2 before 0.7695 0.7695
S 2 after 0.7502 0.7502
AtomXYZXYZ
H0.0000000.395448−5.421548−0.5700410.000000−0.903775
C0.000000−0.095135−4.4722750.0000000.0000000.000000
C0.0000000.012485−3.2339410.0000000.0000001.242922
C0.000000−0.007051−1.9087700.1340720.0000002.561466
C0.000000−0.010887−0.6566940.2475510.0000003.808382
C0.000000−0.0108870.6566940.3641660.0000005.116584
C0.000000−0.0070511.9087700.4775170.0000006.363543
C0.0000000.0124853.2339410.6116710.0000007.681980
C0.000000−0.0951354.4722750.6113240.0000008.925066
H0.0000000.3954485.4215481.1835950.0000009.827551

Appendix C

Molecules 27 03100 g0a2 550
Figure A2. Differences between infrared spectra of cis and trans HCnH− anion conformers visible here make it possible to experimentally differentiate between them. Curves obtained by using a Lorentzian convolution of halfwidth of 5 c m − 1 . (a) HC 3 H − ; (b) HC 5 H − ; (c) HC 6 H − ; (d) HC 7 H − .
Figure A2. Differences between infrared spectra of cis and trans HCnH− anion conformers visible here make it possible to experimentally differentiate between them. Curves obtained by using a Lorentzian convolution of halfwidth of 5 c m − 1 . (a) HC 3 H − ; (b) HC 5 H − ; (c) HC 6 H − ; (d) HC 7 H − .
Molecules 27 03100 g0a2

References

  1. Herbst, E.; van Dishoeck, E.F. Complex Organic Interstellar Molecules. Annu. Rev. Astron. Astrophys. 2009, 47, 427–480. [Google Scholar] [CrossRef]
  2. McGuire, B.A. 2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules. Astrophys. J. Suppl. Ser. 2018, 239, 17. [Google Scholar] [CrossRef]
  3. Snyder, L.E.; Buhl, D. Observations of Radio Emission from Interstellar Hydrogen Cyanide. Astrophys. J. Lett. 1971, 163, L47. [Google Scholar] [CrossRef]
  4. Guélin, M.; Cernicharo, J. Astronomical Detection of the HCCN Radical—Toward a New Family of Carbon-Chain Molecules? Astron. Astrophys. 1991, 244, L21–L24. [Google Scholar]
  5. Turner, B.E. Detection of Interstellar Cyanoacetylene. Astrophys. J. Lett. 1971, 163, L35. [Google Scholar] [CrossRef]
  6. Cernicharo, J.; Guélin, M.; Pardo, J.R. Detection of the Linear Radical HC4N in IRC +10216. Astrophys. J. Lett. 2004, 615, L145. [Google Scholar] [CrossRef] [Green Version]
  7. Avery, L.W.; Broten, N.W.; MacLeod, J.M.; Oka, T.; Kroto, H.W. Detection of the Heavy Interstellar Molecule Cyanodiacetylene. Astrophys. J. Lett. 1976, 205, L173–L175. [Google Scholar] [CrossRef]
  8. Kroto, H.W.; Kirby, C.; Walton, D.R.M.; Avery, L.W.; Broten, N.W.; MacLeod, J.M.; Oka, T. The Detection of Cyanohexatriyne, H(C≡ C)3CN, in Heile’s Cloud 2. Astrophys. J. Lett. 1978, 219, L133–L137. [Google Scholar] [CrossRef]
  9. Broten, N.W.; Oka, T.; Avery, L.W.; MacLeod, J.M.; Kroto, H.W. The Detection of HC9N in Interstellar Space. Astrophys. J. Lett. 1978, 223, L105–L107. [Google Scholar] [CrossRef]
  10. Fan, Q.; Pfeiffer, G.V. Theoretical Study of Linear Cn (n = 6–10) and HCnH (n = 2–10) Molecules. Chem. Phys. Lett. 1989, 162, 472–478. [Google Scholar] [CrossRef]
  11. Fulara, J.; Freivogel, P.; Forney, D.; Maier, J.P. Electronic Absorption Spectra of Linear Carbon Chains in Neon Matrices. III. HC2n+1H. J. Chem. Phys. 1995, 103, 8805–8810. [Google Scholar] [CrossRef]
  12. Seburg, R.A.; DePinto, J.T.; Patterson, E.V.; McMahon, R.J. Structure of Triplet Propynylidene. J. Am. Chem. Soc. 1995, 117, 835–836. [Google Scholar] [CrossRef]
  13. Maier, J.P. Electronic Spectroscopy of Carbon Chains. J. Phys. Chem. A 1998, 102, 3462–3469. [Google Scholar] [CrossRef]
  14. Pino, T.; Ding, H.; Güthe, F.; Maier, J.P. Electronic Spectra of the Chains HC2nH (n = 8–13) in the Gas Phase. J. Chem. Phys. 2001, 114, 2208–2212. [Google Scholar] [CrossRef]
  15. Ding, H.; Schmidt, T.W.; Pino, T.; Boguslavskiy, A.E.; Güthe, F.; Maier, J.P. Gas Phase Electronic Spectra of the Linear Carbon Chains HC2n+1H (n = 3–6,9). J. Chem. Phys. 2003, 119, 814–819. [Google Scholar] [CrossRef] [Green Version]
  16. Woods, P.M.; Millar, T.J.; Zijlstra, A.A.; Herbst, E. The Chemistry of CRL 618. In Planetary Nebulae: Their Evolution and Role in the Universe, Proceedings of the 209th Symposium of the International Astronomical Union, Canberra, Australia, 19–23 November 2001; Kwok, S., Dopita, M., Sutherland, R., Eds.; Astronomical Society of the Pacific: San Francisco, CA, USA, 2003; Volume 209, p. 279. [Google Scholar]
  17. Ridgway, S.T.; Hall, D.N.B.; Kleinmann, S.G.; Weinberger, D.A.; Wojslaw, R.S. Circumstellar acetylene in the infrared spectrum of IRC+10 216. Nature 1976, 264, 345–346. [Google Scholar] [CrossRef]
  18. Lacy, J.H.; Evans, N.J.; Achtermann, J.M.; Bruce, D.E.; Arens, J.F.; Carr, J.S. Discovery of Interstellar Acetylene. Astrophys. J. Lett. 1989, 342, L43. [Google Scholar] [CrossRef]
  19. Cernicharo, J.; Heras, A.M.; Tielens, A.G.G.M.; Pardo, J.R.; Herpin, F.; Guélin, M.; Waters, L.B.F.M. Infrared Space Observatory’s Discovery of C4H2, C6H2, and Benzene in CRL 618. Astrophys. J. Lett. 2001, 546, L123–L126. [Google Scholar] [CrossRef] [Green Version]
  20. Bâldea, I. Alternation of Singlet and Triplet States in Carbon-Based Chain Molecules and Its Astrochemical Implications: Results of an Extensive Theoretical Study. Adv. Theor. Simul. 2019, 2, 1900084. [Google Scholar] [CrossRef]
  21. Kaiser, R.I.; Sun, B.J.; Lin, H.M.; Chang, A.H.H.; Mebel, A.M.; Kostko, O.; Ahmed, M. An Experimental and Theoretical Study on the Ionization Energies of Polyynes H–(C≡C)n–H; n = 1–9). Astrophys. J. 2010, 719, 1884. [Google Scholar] [CrossRef] [Green Version]
  22. Lam, C.S.; Lau, K.C. Thermochemical Trends in Carbon Chain Molecules HC2kH/HC2k-1H (k = 1–6) Studied by Explicitly Correlated CCSD(T)-F12b Composite Methods. J. Phys. Chem. A 2021, 125, 5385–5396. [Google Scholar] [CrossRef] [PubMed]
  23. Carelli, F.; Satta, M.; Grassi, T.; Gianturco, F.A. Carbon-Rich Molecular Chains in Protoplanetary and Planetary Atmospheres: Quantum Mechanisms and Electron Attachment Rates for Anion Formation. Astrophys. J. 2013, 774, 97. [Google Scholar] [CrossRef] [Green Version]
  24. Horny, L.; Petraco, N.D.K.; Pak, C.; Schaefer, H.F. What Is the Nature of Polyacetylene Neutral and Anionic Chains HC2nH and HC2nH− (n = 6–12) That Have Recently Been Observed? J. Am. Chem. Soc. 2002, 124, 5861–5864. [Google Scholar] [CrossRef] [PubMed]
  25. Horny, L.; Petraco, N.D.K.; Schaefer, H.F. Odd Carbon Long Linear Chains HC2n+1H (n = 4–11): Properties of the Neutrals and Radical Anions. J. Am. Chem. Soc. 2002, 124, 14716–14720. [Google Scholar] [CrossRef] [PubMed]
  26. Bâldea, I. Long Carbon-Based Chains of Interstellar Medium Can Have a Triplet Ground State. Why Is This Important for Astrochemistry? ACS Earth Space Chem. 2019, 3, 863–872. [Google Scholar] [CrossRef]
  27. Bâldea, I. Profiling C4N Radicals of Astrophysical Interest. Mon. Not. R. Astron. Soc. 2020, 493, 2506–2510. [Google Scholar] [CrossRef]
  28. Bâldea, I. Extensive Quantum Chemistry Study of Neutral and Charged C4N Chains: An Attempt to Aid Astronomical Observations. ACS Earth Space Chem. 2020, 4, 434–448. [Google Scholar] [CrossRef]
  29. Bâldea, I. Profiling Astrophysically Relevant MgC4H Chains. An Attempt to Aid Astronomical Observations. Mon. Not. R. Astron. Soc. 2020, 498, 4316–4326. [Google Scholar] [CrossRef]
  30. Bâldea, I. Alternation of Singlet and Triplet States in Carbon-Based Chain Molecules and Its Astrochemical Implications: Results of an Extensive Theoretical Study. Adv. Theor. Simul. 2022, 5, 2100480. [Google Scholar] [CrossRef]
  31. McCarthy, M.C.; Gottlieb, C.A.; Gupta, H.; Thaddeus, P. Laboratory and Astronomical Identification of the Negative Molecular Ion C6H−. Astrophys. J. Lett. 2006, 652, L141. [Google Scholar] [CrossRef]
  32. Vogelhuber, K.M.; Wren, S.W.; Shaffer, C.J.; McMahon, R.J.; McCoy, A.B.; Lineberger, W.C. Photoelectron Spectroscopy of HC4N−. J. Chem. Phys. 2011, 135, 204307. [Google Scholar] [CrossRef] [PubMed]
  33. Millar, T.J.; Walsh, C.; Field, T.A. Negative Ions in Space. Chem. Rev. 2017, 117, 1765–1795. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  34. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Petersson, G.A.; Nakatsuji, H.; et al. Gaussian 16; Revision B.01; Gaussian, Inc.: Wallingford, CT, USA, 2016. [Google Scholar]
  35. BwForCluster JUSTUS 2. Available online: https://wiki.bwhpc.de/e/Category:BwForClusterJUSTUS2 (accessed on 12 April 2022).
  36. Curtiss, L.A.; Redfern, P.C.; Raghavachari, K. Gaussian-4 Theory. J. Chem. Phys. 2007, 126, 084108. [Google Scholar] [CrossRef] [PubMed]
  37. Curtiss, L.A.; Redfern, P.C.; Raghavachari, K. Gaussian-4 Theory Using Reduced Order Perturbation Theory. J. Chem. Phys. 2007, 127, 124105. [Google Scholar] [CrossRef]
  38. Ochterski, J.W. Thermochemistry in Gaussian; Gaussian Inc.: Pittsburg, PA, USA, 2000; Available online: https://gaussian.com/wp-content/uploads/dl/thermo.pdf (accessed on 12 April 2022).
  39. Bartmess, J.E. Thermodynamics of the Electron and the Proton. J. Phys. Chem. 1994, 98, 6420–6424. [Google Scholar] [CrossRef]
  40. Kelly, C.P.; Cramer, C.J.; Truhlar, D.G. Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide. J. Phys. Chem. B 2007, 111, 408–422. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  41. Bâldea, I. Extending the Newns-Anderson Model to Allow Nanotransport Studies Through Molecules with Floppy Degrees of Freedom. Europhys. Lett. 2012, 99, 47002. [Google Scholar] [CrossRef] [Green Version]
  42. Bâldea, I.; Köppel, H.; Wenzel, W. (4,4’)-Bipyridine in Vacuo and in Solvents: A Quantum Chemical Study of a Prototypical Floppy Molecule From a Molecular Transport Perspective. Phys. Chem. Chem. Phys. 2013, 15, 1918–1928. [Google Scholar] [CrossRef] [Green Version]
  43. Bâldea, I. A Quantum Chemical Study from a Molecular Transport Perspective: Ionization and Electron Attachment Energies for Species Often Used to Fabricate Single-Molecule Junctions. Faraday Discuss. 2014, 174, 37–56. [Google Scholar] [CrossRef] [Green Version]
  44. Xie, Z.; Bâldea, I.; Frisbie, C.D. Why One Can Expect Large Rectification in Molecular Junctions Based on Alkane Monothiols and Why Rectification Is So Modest. Chem. Sci. 2018, 9, 4456–4467. [Google Scholar] [CrossRef] [Green Version]
  45. Xie, Z.; Bâldea, I.; Haugstad, G.; Frisbie, C.D. Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions. J. Am. Chem. Soc. 2019, 141, 497–504. [Google Scholar] [CrossRef] [PubMed]
  46. Xie, Z.; Bâldea, I.; Frisbie, C.D. Energy Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Alkyl Thiols and Dithiols on Ag, Au, and Pt Electrodes. J. Am. Chem. Soc. 2019, 141, 18182–18192. [Google Scholar] [CrossRef] [PubMed]
  47. Watts, J.D.; Gauss, J.; Bartlett, R.J. Coupled-Cluster Methods with Noniterative Triple Excitations for Restricted Open-Shell Hartree-Fock and Other General Single Determinant Reference Functions. Energies and Analytical Gradients. J. Chem. Phys. 1993, 98, 8718–8733. [Google Scholar] [CrossRef]
  48. Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785–789. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  49. Becke, A.D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098–3100. [Google Scholar] [CrossRef] [PubMed]
  50. Becke, A.D. A New Mixing of Hartree-Fock and Local Density-Functional Theories. J. Chem. Phys. 1993, 98, 1372–1377. [Google Scholar] [CrossRef]
  51. Stephens, P.J.; Devlin, J.F.; Chabalowski, C.F.; Frisch, M.J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623–11627. [Google Scholar] [CrossRef]
  52. Petersson, G.A.; Bennett, A.; Tensfeldt, T.G.; Al-Laham, M.A.; Shirley, W.A.; Mantzaris, J. A Complete Basis Set Model Chemistry. I. The Total Energies of Closed-Shell Atoms and Hydrides of the First-Row Elements. J. Chem. Phys. 1988, 89, 2193–2218. [Google Scholar] [CrossRef]
  53. Petersson, G.A.; Al-Laham, M.A. A Complete Basis Set Model Chemistry. II. Open-Shell Systems and the Total Energies of the First-Row Atoms. J. Chem. Phys. 1991, 94, 6081–6090. [Google Scholar] [CrossRef]
  54. Blanksby, S.J.; Dua, S.; Bowie, J.H.; Schröder, D.; Schwarz, H. Gas-Phase Syntheses of Three Isomeric C5H2 Radical Anions and Their Elusive Neutrals. A Joint Experimental and Theoretical Study. J. Phys. Chem. A 1998, 102, 9949–9956. [Google Scholar] [CrossRef]
  55. Dua, S.; Blanksby, S.J.; Bowie, J.H. Formation of Neutral C7H2 Isomers from Four Isomeric C7H2 Radical Anion Precursors in the Gas Phase. J. Phys. Chem. A 2000, 104, 77–85. [Google Scholar] [CrossRef]
  56. Arié, E.; Johns, J. The bending energy levels of C4H2. J. Mol. Spectrosc. 1992, 155, 195–204. [Google Scholar] [CrossRef]
Molecules 27 03100 g001 550
Figure 1. Enthalpies of formation Δ f H 0 0 of some astronomically observed HCnN chains are larger than enthalpies of formation of shorterHCnH chains not yet detected in space. On this basis, there is no reason to assume that HCnH species are less numerous in space than HCnN species, although, as visible in this figure, HCnN molecules already astronomically observed are much more numerous than HCnH molecules.
Figure 1. Enthalpies of formation Δ f H 0 0 of some astronomically observed HCnN chains are larger than enthalpies of formation of shorterHCnH chains not yet detected in space. On this basis, there is no reason to assume that HCnH species are less numerous in space than HCnN species, although, as visible in this figure, HCnN molecules already astronomically observed are much more numerous than HCnH molecules.
Molecules 27 03100 g001
Molecules 27 03100 g002 550
Figure 2. In contrast to the linear conformers, which are unstable against H−C−C bending vibrations at the two molecular ends, cis and trans HCnH− anion isomers with n ≤ 8 correspond to local energy minima.
Figure 2. In contrast to the linear conformers, which are unstable against H−C−C bending vibrations at the two molecular ends, cis and trans HCnH− anion isomers with n ≤ 8 correspond to local energy minima.
Molecules 27 03100 g002
Molecules 27 03100 g003 550
Figure 3. Because the cis-trans energy splitting Δ c i s − t r a n s ≡ Δ f H 0 , c i s 0 HC n H − − Δ f H 0 , t r a n s 0 HC n H − computed via G4 as enthalpy of isomerization at zero temperature is very small, cis and trans anion conformers are expected to coexist in the interstellar medium.
Figure 3. Because the cis-trans energy splitting Δ c i s − t r a n s ≡ Δ f H 0 , c i s 0 HC n H − − Δ f H 0 , t r a n s 0 HC n H − computed via G4 as enthalpy of isomerization at zero temperature is very small, cis and trans anion conformers are expected to coexist in the interstellar medium.
Molecules 27 03100 g003
Molecules 27 03100 g004 550
Figure 4. Cis HCnH− anions with n ≤ 8 possess reasonably large permanent dipole moments, and this can make them observable via rovibrational spectroscopy.
Figure 4. Cis HCnH− anions with n ≤ 8 possess reasonably large permanent dipole moments, and this can make them observable via rovibrational spectroscopy.
Molecules 27 03100 g004
Table
Table 1. Enthalpies of formation of the HCnH and HCnN chain families computed using the G4 composite model. All values are in kJ/mol.
Table 1. Enthalpies of formation of the HCnH and HCnN chain families computed using the G4 composite model. All values are in kJ/mol.
Number of Carbon AtomsHCnHHCnN
1128.668389.195
2479.383228.807
3370.720544.779
4659.255457.818
5597.481729.829
6863.361679.914
7822.640938.613
81080.580905.162
91043.3301153.130
101298.2201124.610
111266.5601375.680
121525.3201347.420
Table
Table 2. Adiabatic electron attachment energies EA computed using the G4 composite model and via ROCCSD(T) at the B3LYP/6-311++G(3df,3pd) minima. Values in eV. Notice that except for HC4H−, all the other HCnH− listed possess positive EAs and are therefore stable against electron detachment.
Table 2. Adiabatic electron attachment energies EA computed using the G4 composite model and via ROCCSD(T) at the B3LYP/6-311++G(3df,3pd) minima. Values in eV. Notice that except for HC4H−, all the other HCnH− listed possess positive EAs and are therefore stable against electron detachment.
MoleculeG4ROCCSD(T)
HC3H1.1851.047
HC4H−0.355−0.736
HC5H 11.5311.420
HC6H0.2980.195
HC7H 21.9352.029
HC8H0.8050.667
1 Ref. [54] reported EA = 1.51 eV at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G level of theory. 2 Ref. [55] reported EA = 1.86 eV at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G level of theory.
Table
Table 3. Cis-trans anion energy splitting Δ c i s − t r a n s estimated within the G4 composite model. Because all these values are smaller than the “chemical accuracy” of ∼1 kcal/mol, one can expect that cis and trans HCnH anions coexist in space.
Table 3. Cis-trans anion energy splitting Δ c i s − t r a n s estimated within the G4 composite model. Because all these values are smaller than the “chemical accuracy” of ∼1 kcal/mol, one can expect that cis and trans HCnH anions coexist in space.
Anionkcal/molmeV
HC3H−0.50521.9
HC4H−0.1787.7
HC5H−−0.260−11.3
HC6H−0.1888.2
HC7H−0.49421.4
HC8H−−0.217−9.4
HC9H−−0.66829.0
Table
Table 4. Dipole moment μ of anion’s cis isomers computed via single-point ROCCSD(T) calculations at the geometry optimized via UB3LYP/6-311++G(3df,3pd).
Table 4. Dipole moment μ of anion’s cis isomers computed via single-point ROCCSD(T) calculations at the geometry optimized via UB3LYP/6-311++G(3df,3pd).
Cis AnionDipole Moment (Debye)
HC3H−3.061
HC4H−2.385
HC5H−2.313
HC6H−2.310
HC7H−1.801 1
HC8H−1.752
1 Ref. [55] reported µ = 1.63 D at the B3LYP/aug-cc-pVDZ//B3LYP/6-31G level of theory.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Bâldea, I. HCnH− Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation. Molecules 2022, 27, 3100. https://doi.org/10.3390/molecules27103100

AMA Style

Bâldea I. HCnH− Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation. Molecules. 2022; 27(10):3100. https://doi.org/10.3390/molecules27103100

Chicago/Turabian Style

Bâldea, Ioan. 2022. "HCnH− Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation" Molecules 27, no. 10: 3100. https://doi.org/10.3390/molecules27103100

Article Metrics

Back to TopTop