Ab Initio Study of the Solvent Effects on the Singlet−Triplet Gap of Nitrenium Ions and Carbenes

Abstract

Highly correlated ab initio molecular orbital methodologies have been used in the study of substituent effects on the singlet−triplet gaps of a series of nitrenium ions and their corresponding isoelectronic carbenes. Calculations in solution were carried out with the isodensity polarizable continuum models, IPCM. The results show a net stabilization of the singlet species as a result of electronic density donation of the substituents to a vacant p orbital located in the central atom. It is shown that this “π-donor−π-acceptor” dependence of the singlet−triplet gap is more significant in the case of the nitrenium ions, due to the presence of a positive charge. In addition, it was found that the singlet−triplet gaps decrease with the polarity of the solvent, most likely due to stabilizing electrostatic interactions between the solvent and the charge distribution of the singlet.

Electronic Properties of Nitrenium Ions

“…The effect of halogen substitution was examined by Gobbi and Frenking 17 as well as Gonzales et al 18 The halogens act as p donors and s acceptors and thus they act to raise the energy of the p orbital relative to the s ( PhCH 2 CH 2 Ph FIGURE 6.6. Generation of NH 2 þ through photolysis of an N-aminopyridinum ion.…”

A Computational Investigation of HCN2+ Isomeric Structures: Implications for the Chemistry of Titan's Atmosphere

“…[9,10] At all of these levels, this ion was found to be planar but not linear, with a CÀN1ÀH bond angle of 120.48 at the B3LYP/6-311 + + G(d,p), 116.98 at the MP2/6-311 + + G(d,p), and 119.98 at the QCISD/6-311 + + G(d,p) level of theory. In addition, QCISD gave CÀN1 and CÀN2 bond lengths of 1.245 and 1.233 , respectively, suggesting that the CÀN1 bond is not purely single but has significant double bond character.…”

“…[8,9,10] This species is actually a diradical, whose eight active electrons are distributed in the five active orbitals shown in Figure 2 (1) and that the eight active electrons are distributed in these five orbitals according to the occupancy c 1 (1.97)c 2 (1.93)c 3 (1.93)-c 4 (1.00)c 5 (1.00). Therefore this species should be reasonably well described using single-reference theoretical methods and in fact, at the MP2/6-311 + + G(d,p) level of theory, [9] it was found to be linear with a CÀN1 bond length of 1. Similar to the ions of HÀNÀCÀN + connectivity, we have located three distinct diazomethyl cations HÀCÀNÀN + at the CASSCF level of theory, namely the singlet isomers 2S and 2S', and the triplet isomer 2T.…”

“…However, the theoretical level employed in that investigation was not high enough to support definitive conclusion on the structure and stability of the isomeric structures of HCN 2 + . In addition, although the singlet and triplet open-chain HÀNÀCÀ N + and HÀCÀNÀN + isomers have been recently revisited [9][10][11] using density functional (DFT) and high-level of theory ab initio calculations such as quadratic configuration interaction [QCISD and QCISD(T)] and complete basis set (CBS) methods, the obtained results are not directly comparable. Furthermore, the extension to other conceivable isomers of HCN 2 + would also be desirable.…”

A Complete Active Space Self-Consistent Field (CASSCF) Study of the Reaction Mechanism of theα-Alkynone Rearrangement

“…This energy gap is significantly larger than S-T gaps of substituted carbenes and most nitrenes [18] and the extensively studied C 7 H 6 [19] [20]. The mentioned size of the S-T gap in studied vinylidenes justifies, therefore, the original calculation [2] [3] of singlet reaction paths only.…”