Preoptimised VB: a fast method for the ground and excited states of ionic clusters I. Localised preoptimisation for (ArCO)+, (ArN2)+ and N4+

Photodissociation of [Ar-N2]+ induced by near-IR femtosecond laser fields by ion-trap time-of-flight mass spectrometry

Photodissociation of [Ar-N₂]⁺ induced by a near-IR (800 nm) femtosecond laser pulse is investigated using ion-trap time-of-flight mass spectrometry. The intra-complex charge transfer proceeding in the course of the decomposition of the electronically excited Ar⁺(²P_3/2)⋯N₂(X¹Σ_g ⁺), prepared by the photoexcitation of the electronic ground Ar(¹S₀)⋯N₂ ⁺(X²Σ_g ⁺), is probed by the ion yields of Ar⁺ and N₂ ⁺. The yield ratio γ of N₂ ⁺ with respect to the sum of the yields of Ar⁺ and N₂ ⁺ is determined to be γ = 0.62, which is much larger than γ ∼ 0.2 determined before when the photodissociation is induced by a nano-second laser pulse in the shorter wavelength region between 270 and 650 nm. This enhancement of γ at 800 nm and the dependence of γ on the excitation wavelength are interpreted by numerical simulations, in which the adiabatic population transfer from Ar⁺(²P_3/2)⋯N₂(X¹Σ_g ⁺) to Ar(¹S₀)⋯N₂ ⁺(X²Σ_g ⁺) at the avoided crossings is accompanied by the vibrational excitation in the N₂ ⁺(X²Σ_g ⁺) moiety followed by the intra-complex vibrational energy transfer from the N₂ ⁺(X²Σ_g ⁺) moiety to the intra-complex vibrational mode leading to the dissociation.

Confirmation of the “long-lived” tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations

Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.

Rotationally resolved infrared spectrum of the charge transfer complex [Ar-N2]+

Difficulties in preparing cluster ions for spectroscopic studies have limited our understanding of intermolecular forces in charged complexes that are typical of many reactive intermediates. Here, the infrared spectrum of the charge transfer complex [Ar-N2]+, recorded in a supersonic planar plasma with a tunable diode laser spectrometer, is presented. More than 70 adjacent rovibrational transitions were measured near 2272 wave numbers and assigned to the molecular nitrogen stretching fundamental in the 2Sigma+ ground state of [Ar-N2]+. The accurate structural parameters that were determined confirm a linear structure and show that the major part of the charge is located at the argon atom. The latter result is surprising and implies a charge switch of the cationic center upon complexation.

Experimental detection of tetranitrogen

Tetranitrogen (N4), which has been the subject of great theoretical interest, has been prepared from the N4+ cation and positively detected as a gaseous metastable molecule with a lifetime exceeding 1 microsecond in experiments based on neutralization-reionization mass spectrometry. An examination of the geometry of N4+ and the fragmentation pattern of the (14)N2(15)N2 neutral molecule has revealed that the latter is characterized by an open-chain geometry with two distinct, closely bound N2 units joined by a longer weaker bond.