Paul J.A. Ruttink

Aspects of the CH5N2 potential energy surface: ions CH3NHNH+, CH3NNH2+ and CH2NHNH2+ and radicals CH2NHNH2 studied by theory and experiment

Abstract The CH 5 N + 2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G** level of theory and by mass spectrometric experiments. The calculations confirm earlier experimental observations that the diazapropylium ions CH 3 NHNH + , 1 + , CH 3 NNH + 2 , 2 + and CH 2 NHNH + 2 , 3 + and the hydrazonium ion CH 2 NNH + 3 , 4 + , are stable species. Theory predicts 1 + and 2 + to be higher in energy than 3 + , by 7–8 kcal mol −1 , causing a serious discrepancy with existing experimental values, which indicate that 1 + and 2 + are considerably more stable than 3 + . The theoretical values are insensitive to inclusion of electron correlation in the geometry determinations. From a critical evaluation of existing energetic data for N 2 H + 3 , CH 5 N + 2 and C 2 H 7 N + ions, and collision experiments on deuterium labelled species, it is concluded that theory is correct and that several reported appearance energy (AE) measurements on hydrazines are probably in error owing to interferences from traces of amines. From AE measurements not affected by these interferences, Δ H f ( 3 + ) is proposed to be 204 ± 5 kcal mol −1 from which theory leads us to recommend Δ H f values of 211 ± 5 kcal mol for 1 + and 2 + . Ab initio calculated proton affinities for HNNH, CH 3 NNH and CH 3 -NNCH 3 lead to proposed enthalpies for 1 + and 2 + which are consistent with these values. Theory further predicts the ring-closed form of 3 + to be a remarkably stable species (16.7 kcal mol −1 above 3 + ) but the hydrogen bridged entity CH 2 = N H 2H 2 + previously proposed to be responsible for the facile interconversion between 3 + and 4 + , is not a minimum on the potential energy surface. In fact, large energy barriers (42–63 kcal mol −1 ) prohibit interconversion among ions 1 + , 2 + , 3 + and 4 + , via 1,2-H shifts. Metastable CH 5 N + 2 ions dissociate to HCN + NH + 4 and to HCNH + + NH 3 and in agreement with experiment, the reacting configuration for HCN formation is the ion 4 + . Formation of HCN from 4 + is exothermic but the reverse barrier is large (84 kcal mol −1 ) thus accounting for the persistence of 4 + in the gas phase and in neutral solvents. The small kinetic energy release (KER) accompanying this reaction is rationalized in terms of ion/dipole attraction in the dissociating [HCN⋯NH 4 ] + complex.

Isomeric distonic and H-bridged [C2H6O]+ radical cations

Abstract Ab initio molecular-orbital calculations (SD Cl/6-31G★★ confirm that the most stable [C2H6O]+ isomer is the distonic ion [CH2CH2OH2]+. Mass spectrometric measurements give ΔH0f, 298 = 732 ± 5 kJ mol−1. This ion interconverts with a yet-unidentified isomer, [CH2CH⋯H⋯OH2]+, a hydrogen-bridged water/ethene radical cation complex. The latter ion lies in a shallow well, 38 kJ mol−1 above [CH2CH2OH2]+ and 41 kJ mol−1 below the dissociation into [C2H4]+ and H2O. Above the interconversion barrier the isomers behave like ethene-ion/water-dipole complexes in which the dipole can move all around the ion. This behaviour may relate to the very small kinetic energy release in the dissociation.

Self-catalysis in the gas-phase: enolization of the acetone radical cation

Abstract Because of a prohibitively large barrier, the solitary acetone radical cation, CH3C(O)CH3 + (1 +) does not rearrange, neither spontaneously nor by activation, to its more stable enol isomer, CH2C(OH)CH3 + (1a +). However, this isomerization occurs smoothly by an ion–molecule interaction with neutral acetone itself. The dimer radical cation, [ 1 + ⋯ 1 ], generated under conditions of chemical ionization dissociates to m/z 58 and collision-induced dissociation (CID) experiments show that these ions have the enol structure 1a +. Labeling experiments indicate that the reaction can be viewed as a simple 1,3-hydrogen shift within the acetone radical cation of the complex. Ab initio calculations at the CBS-Q/DZP level of theory indicate that this isomerization is best described as a proton transport catalysis rather than as a spectator model. Our calculations show that the incipient radical formed during the proton abstraction is not CH3C(O)CH2 , but rather the less stable configuration CH3C(O )CH2 stabilized by CH3C(OH)CH3+. This behaviour can be rationalized by arguments based on ion-dipole interactions. The incipient radical CH3C(O )CH2 is transformed to its more stable configuration CH3C(O)CH2 via surface crossing. However, this process does not occur via the usual “minimum to minimum crossing” but rather by the novel process of “transition state to minimum crossing”. The abstracted proton is then donated back to the oxygen atom of CH3C(O)CH2 to yield the hydrogen-bridged radical cation [ 1a + ⋯ 1 ]. The observed tautomerization of the acetone radical cation by acetone itself can be viewed as “self-catalysis”.

Ab initio calculations on the three lowest states of HO+2

Abstract Ab initio calculations were performed for the three lowest lying states of HO+2. The ground state was found to be a bend 3A″ state. The first excited 1A′ state cannot appropriately be described by a single determinant, therefore a MC SCF calculation was employed.

The optimization of MCSCF functions by application of the generalized brillouin theorem: The LiH2 potential energy surface

The generalized Brillouin theorem is used to construct an optimization procedure for MCSCF functions by iterative contracted CI calculations. Special attention is paid to the MO transformation step in each iteration. In this method the MCSCF calculation may easily be augmented by a restricted CI calculation involving a configuration set which is uniquely determined by the trial function. An application to the calculation of the potential energy surface for linear LiH2 in the reaction LiH + H⇆Li + H2 leads to the conclusion that this restricted CI is necessary, in order to obtain satisfactory results for the potential energy barrier in this reaction.

Benzonitrile assisted enolization of the acetone and acetamide radical cations: proton-transport catalysis versus an intermolecular H+/· transfer mechanism

Abstract The acetamide radical cation, CH 3 C(O)NH 2 ·+ , can be induced to rearrange into its more stable enol isomer, CH 2 C(OH)NH 2 ·+ , by an ion–molecule interaction with benzonitrile, C 6 H 5 CN, under conditions of chemical ionization. (This enolization does not occur unassisted because of a prohibitively high energy barrier: 26 kcal/mol, from a CBS-QB3 calculation.) The initially formed [C 6 H 5 CN ⋯ acetamide] ·+ adduct ion isomerizes to a stable hydrogen bridged radical cation [C 6 H 5 CN ⋯ HOC(NH 2 )CH 2 ] ·+ en route to its dissociation into the enol ion. Multiple collision and deuterium labeling experiments on the acetamide/benzonitrile and the previously reported acetone/benzonitrile systems, indicate that the acetone ion enolizes by way of a base-catalyzed 1,3-proton shift (“proton-transport catalysis”) but that a different mechanism must be operative in the acetamide system. Ab initio and density functional theory calculations at the PMP3//RHF/D95∗∗ and PMP3//B3LYP/D95∗∗ level of theory support a mechanism which can be described as a consecutive H + /H · transfer between the partners of the [C 6 H 5 CN ·+ ⋯ acetamide] encounter complex. The calculations provide a rationale for the observed isotope effects and lead to a tentative explanation for the differences in interaction of the title ions with benzonitrile.

The isomeric ions produced by the gas phase protonation of HNCO and HCNO

Abstract Ab initio molecular orbital theory calculations combined with mass spectrometric experiments show that the gas phase protonation of HNCO yields [H 2 NCO] + (Δ H f 0 =672 kJ mol −1 ), whereas HCNO produces [HCNOH] + (Δ H f 0 =990 kJ mol −1 ). The proton affinity of fulminic acid, HCNO, is estimated to be 758 kJ mol −1 .

Exact size consistency of multireference Møller–Plesset perturbation theory

Single-reference closed-shell Moller)Plesset perturbation theory is well known for its size consistency, a quality that is essential for consistent comparisons of calculations on molecules of different size. However, it is far from obvious whether this quality can be retained in the multireference case. In this work it is shown that an exactly size consistently generalization to multireference perturbation theory can be constructed. The central result is that the zeroth-order Hamiltonian should be constructed using separate projection operators for each excitation level, i.e., it should contain no couplings between different excitation levels. Q 1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 549)558, 1999

Lowering large 1,2-H shift barriers by proton-transport catalysis: the challenging case of the pyridine radical cation

Under conditions of chemical ionization, the pyridine radical cation rearranges to its more stable α-ylide isomer by an ion–molecule reaction with a suitable reagent, such as 2-cyanopyridine. The initially formed [pyridine•+•••2-cyanopyridine] adduct isomerizes, by way of proton-transport catalysis, to a stable complex of 2-cyanopyridine with the α-ylide ion which may dissociate. Multiple collision experiments on deuterium labelled pyridines indicate that a further isomerization may occur: about half of the (metastable) complex ions undergo a cyano-transfer leading to a very stable distonic ion. A subsequent 1,4-hydrogen shift may lead to the di-2-pyridyl ketimine ion which could account for the observed survivor signal in the neutralization–reionization mass spectrum of the complex. Ab initio calculations at the RHF/6-31G(d) level of theory fully support these findings and provide a rationale for the observed cyano-transfer reaction.

On the Evaluation of CI Matrix Elements for a Canonically Ordered Basis

Using the unitary group approach it is shown that the amount of storage needed for the construction of symbolic CI matrix element lists for N-electron basis functions with large numbers of open shells and arbitrary multiplicities may substantially be reduced compared to methods currently available in the literature.