Ming Wah Wong

The structure and stability of the O2+2 dication: a dramatic failure of Møller—Plesset perturbation theory

Abstract By comparison with results of full configuration-interaction and large-scale multireference configuration-interaction calculations, it is shown that the widely used RMP2 and RMP4 methods fail to describe even qualitatively the structure and kinetic stability of the O 2+ 2 dication, an isoelectronic analogue of molecular nitrogen. A barrier to dissociation of more than 200 kJ mol −1 completely disappears. In contrast, much better results are obtained from other methods based on a single reference function, such as quadratic configuration interaction and the Brueckner doubles procedure.

Isoelectronic analogs of molecular nitrogen: Tightly bound multiply charged species

The structures and stabilities of N2 and its 15 possible first‐row isoelectronic analogs (CO, BF, BeNe, NO+, CF+, BNe+, O2+2, NF2+, CNe2+, OF3+, NNe3+, ONe4+, F4+2, FNe5+, and Ne6+2) have been examined using ab initio molecular orbital theory. Equilibrium structures have been obtained at a variety of levels of theory including MP3/6‐311G(d) and ST4CCD/6‐311+G(2df ) and dissociation energies determined at the MP4/6‐311+G(3d2f ) level. Full potential energy curves for dissociation, including dissociation barriers, have been obtained at the CASSCF/6‐311G(d) level. Spectroscopic constants have also been determined at this level. For the neutral and monocation analogs of N2, the calculated equilibrium geometries, dissociation energies, and spectroscopic constants are in good agreement with the experimental values. The dication analogs of N2, namely O2+2, NF2+, and CNe2+, are all found to be kinetically stable species lying in deep potential wells. In particular, the hitherto unobserved NF2+ dication is predicted to have a short equilibrium bond length (1.102 A) and a large barrier (445 kJ mol−1) to dissociation to N++F+. Thus NF2+ should be experimentally accessible in the gas phase. The (experimentally known) O2+2 dication is predicted to contain the shortest bond between any two heavy atoms, our best estimate of the bond length being 1.052 A. The first excited state (A 3Σ+u) of O2+2 is predicted to be unbound, and observed metastable decomposition processes are reinterpreted in terms of the ground‐state (X 1Σ+g) potential surface. In agreement with previous theoretical studies, we find that CNe2+ is a kinetically stable species, albeit with a relatively long C–Ne bond length. The OF3+ trication is calculated to have a relatively short bond but lies in a well of depth only 23 kJ mol−1. The potential energy curves of the other highly charged species are found to be purely repulsive.The structures and stabilities of N2 and its 15 possible first‐row isoelectronic analogs (CO, BF, BeNe, NO+, CF+, BNe+, O2+2, NF2+, CNe2+, OF3+, NNe3+, ONe4+, F4+2, FNe5+, and Ne6+2) have been examined using ab initio molecular orbital theory. Equilibrium structures have been obtained at a variety of levels of theory including MP3/6‐311G(d) and ST4CCD/6‐311+G(2df ) and dissociation energies determined at the MP4/6‐311+G(3d2f ) level. Full potential energy curves for dissociation, including dissociation barriers, have been obtained at the CASSCF/6‐311G(d) level. Spectroscopic constants have also been determined at this level. For the neutral and monocation analogs of N2, the calculated equilibrium geometries, dissociation energies, and spectroscopic constants are in good agreement with the experimental values. The dication analogs of N2, namely O2+2, NF2+, and CNe2+, are all found to be kinetically stable species lying in deep potential wells. In particular, the hitherto unobserved NF2+ dication is predict...

Helides of carbon and silicon: an ab initio study of their geometric and electronic structures

Abstract The electronic and geometric structures of carbon and silicon helide ions (CHem+n and SiHenm+; n = 1–4 and m = 1–4) were examined using ab initio molecular orbital theory. Equilibrium geometries were obtained at the MP2/6-31G∗ and QCISD(T)/6-311G(MC)∗* levels. Consistent with previous results, the ground states of the mono- and di-cations are found to have long and weak bonds, whereas the excited states are characterized by shorter and stronger bonds. The more highly charged tri-cations and tetra-cations all exhibit stronger bonds to helium than their singly- and doubly-charged counterparts. These and other interesting structural features of the helide ions have been rationalized using a simple molecular orbital (MO) model. In particular, the trends in C-He and Si-He bond lengths can readily be understood in terms primarily of the number of electrons occupying antibonding orbitals. Electrostatic repulsion and the modification of the nature of the antibonding orbitais with increasing molecular charge also play an important role in determining the equilibrium bond distances in the helide ions. The SiHenm+ systems are found to be better able to accommodate positive charge than the CHem+n ions. Reference values of C-He and Si-He bond lengths, for systems with no occupied antibonding orbitals and in which electrostatic repulsion is not significant, are assigned as approximately 1.05 and 1.49 A, respectively. The ground states of helide ions with a particular charge show nearly constant bond lengths, independent of the number of helium atoms; this result may be attributed to a constant occupancy of antibonding orbitals. For example, there is a clustering of Si-He distances around the value of 1.57 A in the quadruply-charged ions SiHe4+, SiHe24+, SiHe34+ and SiHe44+, in each of which the antibonding orbitals are empty.

Neutralization-reionization of CH4+•: At which stage does fragmentation occur?

Abstract Experimental (collision-induced dissociative ionization (CIDI) and neutralization-reionization mass spectrometry (NRMS)) and theoretical (ab initio molecular orbital) procedures were used to examine the origin of the fragmentation observed in NRMS experiments on the methane radical cation. Both theory and experiment indicate that in NRMS experiments involving neutralization by species of high ionization energy (e.g. Hg or Xe), the fragmentation occurs largely at the reionization rather than the neutralization stage. The reverse is true for the strongly exothermic neutralization with Na. Corresponding experiments [C 2 H 6 ] +• ions showed similar behaviour.

How well can RMP4 theory treat homolytic fragmentations

The observation that fourth-order restricted Moller-Plesset perturbation theory (RMP4) gives a satisfactory description of the He:+ potential curve up to the transition structure for dissociation has led to a general examination of the applicability of RMP4 theory to the study of homolytic fragmentation in dications. It appears that, in many cases, RMP4 energies do indeed provide a useful estimate of the barrier height impeding such fragmentations and that the barrier may be estimated economically by calculating RMP4 single-point energies on the UMP2 geometries of the equilibrium and transition structures. We propose the use of a new quantity, the A parameter, as an approximate measure of the rate of convergence of the RMP perturbation series and hence of the reliability of RMP4 energies.

The [HCS]+ and [H2CS]2+ potential energy surfaces: Predictions of bridged equilibrium structures

Abstract Ab initio molecular orbital calculations with moderately large basis sets and incorporating electron correlation are carried out for carbon monosulfide (CS) and its mono- and di-protonated forms. The global minimum on the [HCS]+ potential energy surface is the linear thioformyl cation (HCS+). The isomeric CSH+ cation has an unusual bridged structure. It lies in a potential well, 298 kJ mol−1 above HCS+, but is separated from HCS+ by only a very small barrier (10 kJ mol−1). The remarkable feature about the [H2CS]2+ potential surface is that in this case a bridged structure (HCSH2+) represents the lowest-energy isomer. The thioformaldehyde dication (H2CS2+) is predicted to have little or no barrier towards rearrangement to HCSH2+.

The structure of the C2H2+4 dication

Abstract High-level ab initio molecular-orbital calculations indicate that the only C 2 H 2+ 4 isomer likely to be observable in the gas phase is the perpendicular ethylene dication ( 1 ). Other C 2 H 2+ 4 isomers lie higher in energy with extremely small barriers for collapse to 1 .

A theoretical study of the C3H42+ potential energy surface

Abstract Ab initio molecular orbital theory using basis sets up to 6-311G**, with electron correlation incorporated at the fourth-order Moller-Plesset perturbation level, has been used to examine the C 3 H 4 2+ potential energy surface. The allene dication, CH 2 CCH 2 2+ ( 1 ), is predicted to be the most stable C 3 H 4 2+ ion. It has a planar D 2h geometry, in contrast to the perpendicular D 2d arrangement of neutral allene. The vinyl methylene dication, CH 2 CHCH 2+ ( 2 ), has an unusual bridged structure at the MP2/6-31G* level, but higher-level calculations suggest that it may collapse without a barrier to 1 . The cyclopropene dication, CHCH 2 C H 2+ ( 3 ), is found to prefer a geometry with a planar tetracoordinate carbon. It lies 47 kJ mol −1 above 1 , with a calculated barrier to ring opening of 28 kJ mol −1 . It is a potentially observable C 3 H 4 2+ isomer. The propyne (CH 3 CCH 2+ ) and cyclopropylidene ( CH 2 CC H 2+ ) dications are predicted not to be stable species. Several possible triplet C 3 H 4 2+ isomers were examined but were all found to be considerably higher in energy than the singlets. Despite its large estimated heat of formation (2679 kJ mol −1 ), the allene dication is predicted to be thermodynamically stable towards deprotonation and with respect to CC cleavage into CH 2 C +· and CH 2 +· . The predicted stability of the allene dication is consistent with the experimental observation of C 3 H 4 2+ in the gas phase. Our calculated ionization energies for the processes C 3 H 4 +· → C 3 H 4 2+ and C 3 H 4 → C 3 H 4 2+ are somewhat lower than available experimental values.

The vinyl dication (CH2CH2+: classical or bridged?

Abstract Ab initio molecular orbital calculations at moderately high levels of theory have been carried out for classical ( 1 ) and bridged ( 2 ) forms of the vinyl dication (C 2 H 2+ 3 ). The highest level calculations indicate that the classical structure ( 1 ) is preferred by 16 kJ mol −1 and that the bridged form ( 2 ) represents a transition structure for 1,2-hydrogen migration. Fragmentation of 1 to C 2 H 2+ 2 + H + or CH + 2 + CH + is highly exothermic but is impeded by large barriers. The CH 3 C 2+ dication ( 3 ) is a high-energy species with only a small barrier to rearrangement to 1 . Vertical and adiabatic ionization energies have been calculated for the process C 2 H + 3 → C 2 H 2+ 3 and compared with experimental values.

Structure and stability of the tetrahydridoselenonium dication, SeH42+

Ab initio molecular orbital theory with triple-zeta-valence plus polarization basis sets and with electron correlation incorporated at the fourth-order Møller-Plesset level has been used to study the tetrahydridoonium dications, OH42+, SH42+, and SeH42+. The tetrahydridoselenonium dication SeH42+ is predicted to have a tetrahedral (Td)structure, similar to OH42+ and SH42+, with short bonds to hydrogen (1.483 Å). Although deprotonation of SeH42+ is thermodynamically favored Cby 104 kJ mol−1), such a reaction is inhibited by a large barrier (240kJmol−1]. Thus, SeH42+ lies in a deep potential well and as an isolated species should have a long lifetime in the gas phase. The estimated heat of formation, ΔH°f, for SeH42+ is very high (2483 kJ mol−1], as is the case for OH42+ and SH42+. Of the group IV onium dications (OH42+, SH42+, and SeH42+), SeH42+ displays the greatest kinetic and thermodynamic stability toward proton loss. Substantial solvent stabilization is required in order to generate SeH42+ in solution.