Computational and Theoretical Chemistry 1007 (2013) 9–14 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc À=0 À=0 First-principles investigations on C5O5 and C6O6 oxocarbons ⇑ ⇑ Jin-Chang Guo a,b, Hai-Gang Lu a, , Si-Dian Li a, a Institute of Molecular Science, Shanxi University, Taiyuan 030001, Shanxi, People’s Republic of China b Institute of Materials Science and Department of Chemistry, Xinzhou Teachers’ University, Xinzhou 034000, Shanxi, People’s Republic of China article info abstract Article history: À=0 Comprehensive first-principles calculations on the geometrical and electronic properties of C5O5 and Received 3 October 2012 À=0 À C6O6 oxocarbons have been performed in this work. Both C5O5 monoanion and C5O5 neutral are found Received in revised form 5 December 2012 À 2 00 to possess perfect planar pentagonal structures at their ground states D5h C5O5 ð A2Þ (1) and D5h C5O5 Accepted 5 December 2012 1 0 À À 2 ð A2Þ (2), while C6O6 and C6O6 appear to favor the slightly distorted quasi-planar C2v C6O6 ( A1) (3) Available online 20 December 2012 1 and C2v C6O6 ( A1) (4), respectively. Adaptive natural density partitioning (AdNDP) analyses clearly reveal 2À=À=0 the r- and p-bonding patterns of the monocyclic CnO oxocarbons (n = 5 and 6). The adiabatic Keywords: n detachment energies (ADEs) and low-lying vertical detachment energies (VDEs) of the monoanions have Oxocarbons been calculated at the coupled cluster level with triple excitations (CCSD(T)), with ADE = 3.723 and First-principles calculations À À Geometrical structures VDE = 3.776 eV for C5O5 and ADE = 3.629 eV and VDE = 3.679 eV for C6O6 CCSD(T)//MP2. The first excited 3 Electronic structures states of the neutrals are predicted to have the approximate term values of T1 = 1.08 eV ( A2) for C5O5 and 3 Electron detachment energies T1 = 1.22 eV ( B1) for C6O6. The predictions made in this work may help facilitate experimental character- À À izations of C5O5 and C6O6 monoanions. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. À À 1. Introduction croconate in 1973 [12]. Trace amounts of C5O5 and C6O6 monoa- nions in gas phases were observed in mass spectrum in 1999 À Oxocarbons CnOn (n = 3–6), the cyclic polymers of carbon mon- [41].C6O6 monoanion was also recently produced from oligomer- oxide, have attracted considerable attention in the past several ization of CO on molybdenum anions in 2006 [42]. As a powerful 2À decades [1–42]. The well-known CnOn dianions (n = 3–6) have experimental technique, PES can be used to directly probe the been extensively studied in chemistry due to their p-aromaticity, electronic states of both monoanions and their neutrals. But only capacity as either bridging ligands or building blocks in interesting very accurate theoretical calculations at coupled cluster levels materials and usefulness in analytic applications [27–42]. In con- are expected to provide reliable theoretical predictions to assign À trast, much less attention has been paid to CnOn neutrals and the PES spectra of CnOn monoanions, as noticed in the case of À À CnOn monoanions which started to arouse the curiosity of chemists C4O4 [32]. The ground states of CnOn (n = 5, 6) neutrals are known À in the past decade. The main theoretical investigations in this as- to be singlet [17,33]. However, the ground states of CnOn (n =5,6) pect are focused on the unusual electronic structure of C4O4 which monoanions and the exited states of their neutrals still remain was predicted to have a triplet ground state at coupled cluster level unknown to date. In this work, we present comprehensive first- with triple excitations (CCSD(T)) [27–31]. This prediction was con- principles investigations on the geometrical and electronic struc- À=0 À=0 firmed in 2012 by photoelectron spectroscopy (PES) measurements tures of C5O5 and C6O6 and predict the low-lying one-electron À À À coupled with an electrospray ionization (ESI) source of C4O4 clus- detachment energies of C5O5 and C6O6 to facilitate their ters generated from squaric acid solution of C4O4H2, combined experimental characterizations. We also provide detailed 2À=À=0 with CCSD(T) calculations [32]. Shortly after this report, a compre- molecular orbital analyses for CnOn clusters (n = 5, 6) to help hensive molecular orbital (MO) analysis for the whole oxocarbon understand their r- and p-bonding patterns and probe the À neutral series (CO)n (n = 2–6) appeared very recently [33]. one-electron detachment channels from CnOn monoanions to their At the best of our knowledge, limited investigations have been CnOn neutrals. À reported for CnOn monoanions and their neutrals CnOn (n =5,6) [28–42]. Experimentally, C OÀ radical was firstly observed in ESR 5 5 2. Computational methodology measurements by oxidation of bis(tripheny1phosphine) iminium À The geometries of CnOn neutrals and CnOn monoanions (n =5,6) ⇑ Corresponding authors. were fully optimized at both B3LYP [43–44] and MP2 [45–46] lev- E-mail addresses: [email protected] (H.-G. Lu), [email protected] els with the Dunning’s correlation consistent basis sets (aug-cc- (S.-D. Li). pVTZ) [47] for both carbon and oxygen. Vibrational analyses at 2210-271X/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2012.12.003 10 J.-C. Guo et al. / Computational and Theoretical Chemistry 1007 (2013) 9–14 B3LYP level were carried out to ensure that all of the optimized of the monoanions are systematically shorter than the correspond- geometries were true minima on potential energy surfaces of the ing values of their neutrals (by about 0.037 Å in average) due to the À systems. Natural bond orbital (NBO) analyses were performed to fact that the extra electron on CnOn occupies the totally delocalized gain insight into the bonding characters of these species. In addi- bonding p-HOMO of the systems over the oxocarbon rings (see 2À=1À=0 2À=1À=0 tion, chemical bonding analyses of C5O5 and C6O6 were Fig. 3). It is this delocalized multi-center-one-electron p-HOMO À performed utilizing the recently proposed adaptive natural density that makes CnOn monoanions thermodynamically more stable 2À partitioning (AdNDP) method [48–50] with the 6-31G basis set. It than their CnOn neutrals. The highly stable CnOn dianions are pro- À has been shown that the AdNDP results are not sensitive to the duced with one more extra electron in the p-HOMO of CnOn .In level of theory or basis set used. Molecular visualization was addition, it is expected that the symmetric ring breathing modes performed using Molekel 5.4 [51]. Adiabatic detachment energies of the neutrals to be excited upon detaching one electron from (ADEs) were calculated as the energy differences between the the monoanions. The calculated symmetrical breathing modes anions and the corresponding neutrals at their ground-state (a1)ofC5O5 (2) and C6O6 (4) have the vibrational frequencies of structures, while the vertical detachment energies (VDEs) as the 1754 cmÀ1 and 1742 cmÀ1 (after the 0.97 scaling) at B3LYP/aug- energy differences between the anions and their neutrals at the cc-pVTZ level, respectively, which appear to be 70–80 cmÀ1 lower À1 ground-state structures of the anions. CCSD(T) [52–54] single- than the ring breathing frequency of 1824 cm calculated for C4O4 point energy calculations were performed at both B3LYP and at the same theoretical level [32]. MP2 geometries. One-electron detachment energies of the monoa- As demonstrated in Table 1, NBO analysis indicates that C and O nions were also approximated with the outer valence Green func- atoms in oxocarbons well follow the octet rule with the total tion approach (OVGF) [55]. All the calculations in this work were Wiberg atomic bond indices of WBIC = 3.87–3.89 and WBIO = 1.93– performed using the Gaussian 03 software package [56]. 2.17. The C–C Wiberg bond indices of WBIC–C = 0.89–0.97 calculated for (1–4) clearly indicate that C–C bonds are typical covalent single bond. We notice the C–C bond indexes (WBIC– 3. Results and discussion C 0.97) of the monoanions are clearly higher than their neutrals with WBIC–C 0.89–0.90, in consistent with the shorter C–C bond 3.1. Structures and stabilities lengths in the monoanions than in their neutrals discussed above. The C–O Wiberg bond indices with the bond orders of WBIC– À The optimized structures of CnO monoanions (n = 5,6) and n O = 1.71–1.92 for (1–4) show that the C–O bonds are typical double their neutrals are shown in Fig. 1, with the C–C and C–O bond bonds in these species. Given the fact that oxygen is more electro- À lengths at both B3LYP and MP2 levels indicated. Both C5O5 (1), negative than carbon, it is easy to understand that O atoms in oxo- monoanion and C5O5 (2), neutral are found to take perfect planar carbons carry negative net atomic charges (qO = À0.37|e| À D5h structures, while C6O (3) and C6O6 (4) possess the slightly dis- 6 À0.53|e|), while C atoms are positively charged with qC = À torted quasi-planar C2v geometries. The C2v conformation of C6O6 +0.33|e| +0.38|e|. (3) is a true minimum on the potential energy surface of the mono- The singlet ground states of CnOn neutrals (n = 5, 6) have been anion without imaginary frequencies (see Table 1), while other iso- predicted to be unstable with respect to dissociation to n CO mol- mers such as D6h,D3d,D2,C2, and Cs turn out to be transition states ecules [17,29,37].