Theoretical ab Initio Study of the Series of N 2 X + Cations with X = F, Cl, Br, and I. New Insights on the “Unusual” N 2 F + Species Aristotle Papakondylis* Department of Chemistry, Laboratory of Physical Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, Athens 157 71, Greece ABSTRACT: The series of cations N 2 X + (X ̃ 1 Σ + ), with X = F, Cl, Br, I, has been theoretically studied by variational multireference CI and coupled-cluster techniques in conjunction with basis sets of quintuple-ζ quality. We report electronic and geometric structure data and harmonic frequencies as well as binding energies and potential energy curves. A new rationalization is provided for the bonding mode in N 2 F + , which provides an explanation for the unusually short N-F bond. 1. INTRODUCTION The fluorodiazonium cation, N 2 F + , became famous as an important precursor in the synthesis of N 5 + by Christe et al. 1 It was first prepared by Moy and Young II 2 some 50 years ago. These researchers used the reaction of cis-difluorodiazine with arsenic pentafluoride to produce a white solid identified as the N 2 F + AsF 6 - ionic compound. The presence of N 2 F + in this solid was further confirmed a few years later by Shamir and Bineboym 3 and Christe et al. 4 through Raman and infrared spectroscopy. These workers concluded that N 2 F + is linear and asymmetric and that the N-N bond has a triple bond character. The first ab initio study on N 2 F + was reported in 1975 by Pulay et al. 5 who performed Hartree-Fock calculations to predict the r N-F = 1.28 Å and r N-N = 1.10 Å bond distances and also the vibrational force constants. In 1978, Peters 6 employed the MP2/6-31G* method to find r N-F = 1.256 Å, r N-N = 1.138 Å and a heat of formation ΔH f 0 = 329 kcal/mol. The first crystallographic study of N 2 F + AsF 6 - salt was carried out in 1991 by Christe et al. 7 They found a total r N-F + r N-N = 2.316 Å distance which was empirically partitioned as r N-F = 1.217 Å and r N-N = 1.099 Å. In the same work theoretical calculations at the LDF level gave r N-F = 1.225 Å and r N-N = 1.106 Å and vibrational frequencies in good agreement with experiment. It was pointed out by these authors that the N-F bond length in N 2 F + is the shortest observed between the two elements. As an explanation to this fact the sp-hybridization of the terminal N atom was invoked. A subsequent millimeter-wave spectroscopic investigation of the gas-phase structure of N 2 F + by Botschwina et al. 8 yielded r N-F = 1.2461 Å and r N-N = 1.1034 Å. These authors also performed CEPA calculations with relatively small basis sets to find r N-F = 1.2612 Å, somewhat larger than their experimental number. From these values it is clear that the N- F bond length is much smaller than its typical value of ∼1.35 Å but also that the N 2 moiety preserves its triply bonded N-N distance. In 1995 Cacace et al. 9 reported production of gaseous N 2 F + from the ionization of NF 3 /HN 3 mixtures. The mechanism of formation of fluorodiazonium ions was investigated by MIKE and FT-ICR spectrometries combined with post-SCF ab initio calculations. The problem of the short N-F bond in N 2 F + was revisited in 2002 by Bickelhaupt et al. 10 These authors performed density functional calculations at the BP86/TZ2P level to obtain r N-F = 1.245 Å and r N-N = 1.112 Å as well as a binding energy D e = 102.5 kcal/mol with respect to the N 2 + (X 2 Σ g + ) + F( 2 P) dissociation channel. They also claimed that reduced steric and Pauli repulsion effects are responsible for the short N-F bond, a conclusion that was at variance with the Christe at al. 7 assumption about sp-hybridization of the N atom. A different rationalization was presented by Harcourt 11 on the basis of valence bond (VB)/STO-3G calculations. He proposed that the dominant Lewis-type VB structures are those involving a single positive charge on either of the N atoms. Moreover he suggested that N-F π-bonding is one of the factors responsible for the shortening of the N-F bond of N 2 F + relative to that of NF 4 + . In a recent paper Christe at al. 12 were able for the first time to obtain the individual N-N and N-F distances in the solid state; through crystallographic analysis of the newly prepared N 2 F + Sb 2 F 11 - solid they found r N-F = 1.257 Å and r N-N = 1.089 Å which are among the shortest experimentally observed N-F and N-N bonds. They also used the CCSD(T)/aug-cc-pVTZ methodology to calculate r N-F = 1.2357 Å and a heat of formation ΔH f (0 K) = 291.2 kcal/mol. From the foregoing discussion it becomes quite clear that one has to deal with a rather “unusual” cation. N 2 F + has a ground state of X ̃ 1 Σ + symmetry. Its potential energy surface correlates adiabatically to the ground state fragments N 2 + (X 2 Σ g + ) + F( 2 P) since the ionization potentials IP(N 2 )< IP(F). However, as we will see later the particular electronic structure of N 2 + (X 2 Σ g + ) (vide infra) cannot favor a strong covalent N-F bond as is thought so far. In the present study we attempt to show that the intrinsic electronic configuration of N 2 F + corresponds to a higher asymptotic channel, namely N 2 (X 1 Σ g + )+F + ( 1 D), explaining the peculiar character of the cation. Although lying higher than the N 2 + (X 2 Σ g + ) + F( 2 P) asymptote, by ∼4.4 eV, it will be shown that it can be significantly stabilized when involved in the bonding. Received: October 17, 2016 Revised: November 14, 2016 Published: November 14, 2016 Article pubs.acs.org/JPCA © 2016 American Chemical Society 9660 DOI: 10.1021/acs.jpca.6b10471 J. Phys. Chem. A 2016, 120, 9660-9666