Two small thermodynamically stable organic dications Ayse Yumak a , Ilhan Yavuz a , Zikri Altun a , Carl Trindle b, * a Marmara University, Department of Physics, Göztepe Kampus 34722 Kadiköy/Istanbul, Turkey b University of Virginia, Department of Chemistry, USA article info Article history: Received 7 August 2009 Received in revised form 21 September 2009 Accepted 29 September 2009 Available online 7 October 2009 Keywords: Organic cations Interstellar molecules Density functional calculations Coupled cluster calculations abstract Small multiply charged species may be long-lived without the benefit of stabilization by a medium, pri- marily owing to the Coulomb barrier that impedes both formation and dissociation of the systems. We extend the category of small organic dicationic structures that may possess thermodynamic stability to include derivatives of cyanoacetylene. The linear dication HCCCNH(2+) and its isomer HCCNCH(+2) are characterized in detail; according to CCSD(T)/6-31G(d,p)//MCSCF(7,13)/6-31G(d,p) estimates, HCCCNH(2+) is thermodynamically stable by 2.8 eV and the barrier to colinear dissociation is ca 4.1 eV. The isomer HCCNCH(2+) is thermodynamically stable by about 2.1 eV and the barrier to colinear dissociation is ca 4.5 eV. Preparation of these systems by charge stripping of the predecessor H 2 C 3 N mon- ocation may be feasible. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The counterintuitive notion that polycations may be long- lived despite their considerable destabilization from Coulomb repulsion was dramatically presented by Pauling [1]. He pro- posed that rare gases may form kinetically stable (metastable) dicationic diatomic molecules. He 2 2+ illustrates this remarkable behavior; the He–He distance is 0.704 Å and its barrier to disso- ciation is 1.2 eV [2]. The principle of stabilization is that the Coulomb repulsion pen- alty can be offset if the removal of the electrons enhances covalent bonding. This accounts for the (meta) stability of a number of dia- tomic dications such as O 2þ 2 , which is isoelectronic with N 2 . In fact defining a dimensionless potential energy and interatomic distance for these diatomics and subtracting the simplest estimate of Cou- lomb repulsion from the dication curve allows the graphs of bind- ing energy for CH and (unstable) OH 2+ to be almost perfectly superimposed [3]. A large number of dications involving metal atoms are known [4] and long-lived dications in organic systems are also well recog- nized [5]. The most common examples involve S+AS+ bonds; in contrast, small dications with first row atoms are less well known. Landmark syntheses of hydrazinium and diazenium dications in- voked the stabilizing influence of tricyclic and bicyclic frames [6,7]. Only low temperature and superacidic medium allow observa- tion of the diazenium dication [8]. Carbon-based dications derived by removal of electrons from a CC pi bond must be stabilized by substituents [9]. Dications can be generated in the gas phase. C 2 H 2þ 6 has been detected in mass spectrometry; calculations suggest the structure CH 2 ACH 4 ; that is, something like a CH 1þ 2 substitution on a CH 5 cation [10]. Superacidic media permit extend the lifetime of highly charged species and permit study of heteronium dications in which the struc- tural features RAC+ = O+AR 0 and RC+ = N+R 0 R 00 appear [11]. Schröder et al. [12] have observed that signals for C 4 H 2þ 3 are common in mass spectrometric electron ionization studies of hydrocarbons. By B3LYP/cc-pVTZ computations they attempted to elucidate the dication’s structure. According to their B3LYP/cc- pVTZ calculations, a variety of monocationic structures lie within 2 eV of the most stable linear form of the singlet protonated diacet- ylene, H 2 CCCCH 1+ (Chart 1). This is the energy reference point in Ta- ble 3 of Ref. [12]. Protonation of diacetylene liberates 8.00 eV; ionization of this system requires 16.38 eV. The resulting doublet dication is more stable than the doublet monocation of diacetylene by 1.46 eV (Table 1). Remarkably, the ‘‘cyclobutadienyl” dication (species 2 C 2+ in Table 1) is also energetically available; 0.27 eV is liberated as it is formed upon protonation of the diacetylene monocation. There is some hint that the exo-methylene cyclopropenyl dication (species 2 B 2+ in Table 1) is also stable, with energy very near that of the diacetylene monocation. These dicationic systems are of interest partly because of their long lives, partly because the charges derive from a pi manifold, and partly because the systems are open shell. Open shell systems are not common among the dications studied to date, although examples are known (the triplet HCl dication [13] is a notable a few). 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.09.052 * Corresponding author. Address: University of Virginia, Department of Chemis- try, 2’9 McCormick Road, Charlottesville, VA 22904, USA. Tel.: +1 434 924 2168; fax: +1 434 924 3710. E-mail address: cot@virginia.edu (C. Trindle). Journal of Molecular Structure: THEOCHEM 940 (2010) 6–12 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem