Willem J. Bouma

Distonic radical cations : Guidelines for the assessment of their stability

Abstract Ab initio molecular orbital calculations on the distonic radical cations CH2(CH2)nN+H3 and their conventional isomers CH3(CH2)nNH2+ (n = 0,1, 2 and 3) indicate a preference in each case for the distonic isomer. The energy difference appears to converge with increasing n towards a limit which is close to the energy difference between the component systems CH3·H2+CH3+NH3 (representing the distonic isomer) and CH3CH3+CH3NH2+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH3·Y+CH3X+H and CH3YH+CH3X+. (or CH3YH+. + CH3X) to predict the relative energies of the distonic ions ·Y(CH2)nX+H and their conventional isomers HY(CH2)nX+. (X = NH2, OH, F, PH2, SH, Cl; Y = CH2, NH, O) and testing the predictions through explicit calculations for systems with n = 0,1 and 2. Although the predictions based on component systems are often close to the results of direct calculations, there are substantial discrepancies in a number of cases; the reasons for such discrepancies are discussed. Caution must be exercised in applying this and related predictive schemes. For the systems examined in the present study, the conventional radical cation is predicted in most cases to lie lower in energy than its distonic isomer. It is found that the more important factors contributing to a preference for distonic over conventional radical cations are the presence in the system of a group(X) with high proton affinity and the absence of a group (X, Y or perturbed (C—C) with low ionization energy.

The additivity of polarization function and electron correlation effects in ab initio molecular-orbital calculations

Abstract Several examples are preseted to show that estimated third-order Moller-Plesset (MP3) relative energies obtained from schemes which assume additivity of correlation and polarization function effects are likely to provide the most reliable energy comparisons in cases where full MP3 calculations with polarization basis sets are not feasible.

Unusual low-energy isomers for simple radical cations

Abstract The radical cations of formaldimine, methylamine, formaldehyde, methanol, diazene, hydrazine, nitroxyl, hydroxylamine and hydrogen peroxide, and of isomers derived formally from these systems by means of a 1,2-hydrogen shift have been studied using ab initio molecular orbital theory, including electron correlation. For the ions of formaldimine, methylamine and methanol, evidence is presented that the 1,2-hydrogen-shifted species lie lower in energy than the conventional isomers.

The structure of vinyl alcohol

Abstract Theoretical structures for vinyl alcohol are obtained using ab initio molecular orbital theory and the minimal STO-3G and split-valence 4-31G basis sets. The preferred conformation has HOCC syn . Correction for the systematic deficiencies of both basis sets and use of experimental rotational constants leads to a prediction of the complete r o structure for vinyl alcohol.

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...

The ionization of ethylene oxide

High-level ab initio molecular orbital calculations have been used to investigate the ionization of ethylene oxide. Vertical ionization to the (ring-closed) 2B1 state leads to an ion separated from the more stable C…C ring-opened form by a barrier of just 10 kJ mol−1. The 2A1 ring-closed cation is predicted to rearrange with little or no barrier to the 2B1 form.

The nature of the C…C ring-opened form of the ethylene oxide radical cation

CAS SCF and Moller-Plesset perturbation calculations with polarized basis sets have been used to investigate the nature of the C…C ring-opened isomer of the ethylene oxide radical cation. The best calculations predict a planar C 2V structure for CH 2 OCH 2 + , lying 82 kJ mol −1 below the ring-closed ethylene oxide radical cation. However, the possibility of a slightly distorted structure, lying in a very shallow potential well, cannot be completely ruled out.

An ab initio molecular orbital study of the CH2O+− isomers: The stability of the hydroxymethylene radical cation

Abstract Ab initio molecular orbital theory with the 4-31G and 6-31G* basis sets has been used to examine three CH 2 O +• isomers. The formaldehyde radical cation (CH 2 O +• ) is predicted to be the most stable isomer, with the hydroxymethylene radical cation (HCOH +• ) lying about 41 kJ mol −1 higher in energy. The third isomer, an oxonium ion (COH 2su+• ), is quite high in energy, lying 257 kJ mol −1 above the formaldehyde radical cation. The barrier to intramolecular rearrangement of the hydroxymethylene radical cation to the more stable formaldehyde radical cation is found to be substantial (ca. 248 kJ mol −1 ).

On the structures and relative energies of CH3F⨥ isomers

Ab initio calculations have identified two isomers in the CH3F⨥ potential energy surface, one corresponding to ionized fluoromethane (CH3L⨥ and the other to the methylenefluoronium radical cation (CH2TH⨥. The latter is predicted to lie lower in energy to 46 kJ mol−1. Examination of rearrangement and fragmentation pathways leads to the conclusion that both ions should be stable, observable species. The importantce of electron correlation in satisfactorily describing the structure of CH3l⨥ is demonstrated.

Unusual gas-phase isomers of simple organic radical cations

Abstract Molecular orbital calculations are well-suited to the study of gas-phase ion chemistry. Recent calculations have led to the discovery of a number of hitherto unsuspected and undetected small organic radical cations.