Johan K. Terlouw

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.

GENERATION OF NEUTRAL AND CATIONIC HYDROGEN SHIFT ISOMERS OF PYRIDINE : A COMBINED EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION

Abstract Azacyclohexatriene-3-ylidene (3) , an isomer of pyridine (2) and its previously generated Hammick intermediate (1) , is accessible in the gas phase by one-electron reduction of the corresponding radical cation in neutralization-reionization mass spectrometric experiments. The experimental finding that this carbene is stable on the microsecond time scale is in agreement with results of quantum chemical calculations, which indicate that all these C 5 H 5 N and C 5 H 5 N ·+ isomers, including also the isomers of azacyclohexatriene-4-ylidene 4 and 4 ·+ , correspond to minima that are separated by significant barriers, thus preventing facile unimolecular isomerization. Although neutral pyridine is much more stable than the ylidic carbenes 1 , 3 , and 4 , for the radical cations the C 5 H 5 N ·+ isomers are nearly isoenergetic, with 4 ·+ actually being lowest in energy.

The imidic acids HNC(H)OH and CH3NC(H)OH and their tautomeric carbenes H2N7z.sbnd;C̈OH and CH3N(H)C̈OH: stable species in the gas phase formed by one-electron reduction of their cations

Abstract The prototype imidic acid, formimidic acid, HNC(H)OH, 2 , as well as aminohydroxy-carbene, 3 , both isomeric with formamide, 1 , have been generated in the gas phase by one-electron reduction of the corresponding radical cations. The ionic precursors 2 + and 3 + were produced by appropriate dissociative ionizations, whereas 1 + was formed by direct ionization of formamide. From a detailed analysis of collision induced dissociation (CID) mass spectra, in particular MS/MS/MS data from D- and 15 N-labelled isotopomers, it was possible to assign structures to ions 1 + , 2 + and 3 + and to ascertain that, for each, ion beams could be obtained which were free from the other two isomers. Reduction of these ions in neutralization-reionization experiments produced intense “survivor” signals. By combining the information contained in CID mass spectra of the “survivor” ions with that from labelling experiments, the existence of 2 and 3 as stable molecules in the gas phase could be established. Among the vibrationally excited molecules we find evidence for the simplest amide-iminol tautomerism, i.e. 1 ↔ 2 (a reaction which because of the high barrier is not observed in solution). At higher internal energies decarbonylation occurs. The experimental findings on ions and neutrals 1-3 concur with previously published ab initio MO studies which were supplemented by our own results at the UMP3/6-31G * //4-31G (+ZPVE) level of theory. Using the same experimental methodology, the existence of the imidic acid CH 3 NC(H)OH and the carbene CH 3 N(H)COH, the methyl homologues of 2 and 3 , was also established.

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”.

The thiazole ylide: a frequently invoked intermediate is a stable species in the gas phase.

The 1, 2-hydrogen shift isomers of neutral (singlet and triplet) thiazole (1) and its radical cation have been investigated by a combination of mass spectro-metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3 H3 NS and C3 H3 NS(.+) isomers and transition state structures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcalmol(-1) , it is expected to be capable of independent existence, since the 1, 2-hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol(-1) . The other 1, 2-hydrogen shift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2-ylidene structure (3), which is apparently unstable, but rather to the ring-opened species HSCHCHNC (4), which is 34.5 kcalmol(-1) higher in energy than 1. The barrier in this case is lower but still large (54.9 kcalmol(-1) ). The triplet ground states of 1, 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol(-1) ) relative to their singlet states. Interestingly, in addition to 2(.+) and 4(.+) , the cyclic radical cation 3(.+) is predicted to be stable although it is substantially higher in energy than ionised thiazole 1(.+) (by 53.9 kcalmol(-1) ), whereas 2(.+) and 4(.+) are much closer in energy (only 10.2 and 27.0 kcalmol(-1) higher, respectively). Dissuading 2(.+) and 3(.+) from isomerising to 1(.+) are energy barriers of 52.6 and 15.3 kcalmol(-1) , respectively. Experimentally, dissociative ionisation of 2-acetylthiazole enabled the generation of 2(.+) , which could be differentiated from 1(.+) by collisional activation mass spectrometry. Reduction of the ylide ion 2(.+) in neutralisation-reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.

Characterization of ginseng saponins using electrospray mass spectrometry and collision-induced dissociation experiments of metal-attachment ions

Electrospray mass spectrometry (ESMS) and collision-induced dissociation (CID) methodologies have been developed for the structural characterization of ginseng saponins (ginsenosides). Ginsenosides are terpene glycosides containing a triterpene core to which one to four sugars may be attached. They are neutral molecules which readily form molecular metal-attachment ions in positive ion ESMS experiments. In the presence of ammonium hydroxide intense deprotonated ions are generated. Both positive and negative ion ESMS experiments were found to be useful for molecular mass and structure determination of ten ginsenoside standards. Negative ion experiments made possible the determination of the molecular mass of each ginsenoside standard, the mass of the triterpene core and the masses and sequences of the sugar residues. Positive ion ESMS experiments with the alkali metal cations Li+ or Na+ and the transition metal cations Co2+, Ni2+ and Zn2+ were also useful in determining molecular masses. These alkali and transition metal cations form strongly bonded attachment ions with the ginsenosides. As a result, the CID mass spectra of the metal attachment ions show a variety of (structure characteristic) fragmentations. These experiments can be used to determine the identity of the triterpene core, the types and attachment points of sugars to the core and the nature of the O-glycosidic linkages in the appended disaccharides. Combining the results from the negative and positive ion experiments provides a promising approach to the structure analysis of this class of natural products.

The gas phase chemistry of the methyl carbamate radical cation H2NCOOCH+3: isomerization into distonic ions, hydrogen-bridged radical cations and ion—dipole complexes

Abstract The unimolecular chemistry of the methyl carbamate radical cation, H2NCOOCH+3, 1, has been investigated by a combination of mass spectrometry based experiments (metastable ion (MI), collisional activation (CA), colision-induced dissociative ionization (CIDI), neutralization—reionization (NR) spectrometry, 2H, 13C and 18O isotopic labeling, appearance energy (AE) measurements), and ab initio molecular orbital calculations, executed at the SDCI/G—31G**//4—31G level of theory and corrected for zero-point energies. These calculations indicate that besides ionized methyl carbamate there are at least seven other equilibrium structures inlcuding distonic ions and hydrogen-bridged radical cations. The most stable isomer is the hydrogen-bridged species [H2NCHO ⋯ H ⋯ OCH]+ which is best viewed as the carbenium ion H2NCHOH+ interacting with the formly dipole. The related species [H2NCO ⋯ H ⋯ OCH2]+ in which the hydroxyaminocarbene ion H2NCOH+ interacts with the formaldehyde dipole is also a stable species. This hydrogen-bridged radical cation is the key intermediate in the spontaneous unimolecular dissociations of methyl carbamate ions. Experimentally, the metastable molecular ions form two sets of products, namely. H2NCHOH+ + HCO (the components of the most stable isomer) and [CH2O ⋯ H ⋯ NH2]+ + CO. The minimum energy requirement paths have been located by ab initio calculations and the reactions follow multistep isomerizations. In the first step, H2NCOOCH+3 , 1, isomerizes via a 1,4-hydrogen shift to the distonic ion H2NC(OH)OCH+2 , 2, which then rearranges to the hydrogen-bridged radical ion [H2NCO ⋯ H ⋯ OCH2]+ . The incipient formaldehyde molecule can then donate a hydrogen to the C atom of H2NCOH, followed by loss of HCO or it can accept the hydroxyl hydrogen to form a CH2OH radical; this radical then migrates within the electrostatic field of the H2 N + CO ion towards the N atom to form the complex [H2CO ⋯ H ⋯ NH2CO+ . This latter species, which can be viewed as a formaldehyde and a CO molecule interacting with NH+3 lies in a shallow potential well only and sheds CO to produce [CH2O ⋯ H ⋯ NH2] , as observed experimentally. It is stressed that only with the aid of high level ab initio calculations could the above mechanisms be elucidated.

Dihydroxycarbene HO-C̈-OH: Its formation in the gas-phase by electron transfer to its radical cation

Abstract Dihydroxycarbene, HO-C-OH, has been generated and identified in the gas-phase by electron capture of the corresponding radical cations in neutralization—reionization experiments. The precursor ions were made by dissociative ionization of dihydroxyfumaric acid and their structure was established on the basis of collision experiments. The experimental results are consistent with previously published ab initio calculations.

Thiofulminic acid (HC→S) and nitrile sulfides (RCN→S) in the gas phase

Thiofulminic acid (HCNS) and its derivatives have been identified in the gas phase by neutralization-reionization mass spectrometry, and benzonitrile sulfide also by matrix isolation IR spectroscopy following flash vacuum pyrolysis.

On the existence of novel nitrides and oxides of copper; CuN, CuO2 and CuNO

Abstract Electron impact ionization of anhydrous copper(II) nitrate yields the copper-containing cations CuN + , CuO + , CuNO + 2 . Collisional activation is used to structurally characterize the ions which can also be transformed to their corresponding neutral analogues by using neutralization-reionization mass spectrometry.