Steen Hammerum

Mass spectrometry in structural and stereochemical problems: CCXLV. The electron impact induced fragmentation reactions of 17-oxygenated progesterones

The mass spectral fragmentation of a number of 17alpha-hydroxy-, 17alpha-acetoxy-, and 17alpha-methoxyprogesterones have been examined. Unlike other steroidal delta4-3-ketones, fragmentation reactions associated with the alpha,beta-unsaturated ketonic function are not particularly significant; rather, abundant ions are formed by decomposition processes occurring in and around ring D. Reactions of diagnostic significance include complete or partial loss of ring D, and elimination of the C-17 side chain (CH3CO), followed by loss of the C-17 oxygen function together with a hydrogen atom (H2O, CH3COOH, CH3OH).

Investigating the α-Effect in Gas-Phase SN2 Reactions of Microsolvated Anions

The α-effect-enhanced reactivity of nucleophiles with a lone-pair adjacent to the attacking center-was recently demonstrated for gas-phase S(N)2 reactions of HOO(-), supporting an intrinsic component of the α-effect. In the present work we explore the gas-phase reactivity of microsolvated nucleophiles in order to investigate in detail how the α-effect is influenced by solvent. We compare the gas-phase reactivity of the microsolvated α-nucleophile HOO(-)(H2O) to that of microsolvated normal alkoxy nucleophiles, RO(-)(H2O), in reaction with CH3Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO(-)(H2O) and clearly demonstrate the presence of an α-effect for the microsolvated α-nucleophile. The association of the nucleophile with a single water molecule results in a larger Brønsted βnuc value than is the case for the unsolvated nucleophiles. Accordingly, the reactions of the microsolvated nucleophiles proceed through later transition states in which bond formation has progressed further. Calculations show a significant difference in solvent interaction for HOO(-) relative to the normal nucleophiles at the transition states, indicating that differential solvation may well contribute to the α-effect. The reactions of the microsolvated anions with CH3Cl can lead to formation of either the bare Cl(-) anion or the Cl(-)(H2O) cluster. The product distributions show preferential formation of the Cl(-) anion even though the formation of Cl(-)(H2O) would be favored thermodynamically. Although the structure of the HOO(-)(H2O) cluster resembles HO(-)(HOOH), we demonstrate that HOO(-) is the active nucleophile when the cluster reacts.

Heats of formation and proton affinities by the G3 method

Abstract Proton affinities of 29 simple organic molecules calculated with the G3 method agree well with experimental results from the literature as well as with results obtained with the G2(MP2) and CBS-Q methods. The choice of empirical correction in the G3 method introduces a small systematic error on either G3 heats of formation or G3 proton affinities.

Alkyl Radicals as Hydrogen Bond Acceptors: Computational Evidence

Spectroscopic, energetic and structural information obtained by DFT and G3-type computational studies demonstrates that charged proton donors can form moderately strong hydrogen bonds to simple alkyl radicals. The presence of these bonds stabilizes the adducts and modifies their structure, and gives rise to pronounced shifts of IR stretching frequencies and to increased absorption intensities. The hydrogen bond acceptor properties of alkyl radicals equal those of many conventional acceptors, e.g., the bond length changes and IR red-shifts suggest that tert-butyl radicals are slightly better acceptors than formaldehyde molecules, while propyl radicals are as good as H(2)O. The hydrogen bond strength appears to depend on the proton affinity of the proton donor and on the ionization energy of the acceptor alkyl radical, not on the donor-acceptor proton affinity difference, reflecting that the charge-transfer aspects of hydrogen bonding are particularly conspicuous when the acceptor polarity and basicity is low.

Slow alkyl, alkene, and alkenyl loss from primary alkylamines: Isomerization of the low-energy molecular ions prior to fragmentation in the μsec timeframe

Abstract Isomerization of the molecular ion precedes alkyl radical elimination from many primary aliphatic amines. Rearrangement of the carbon skeleton is initiated by 1,5-hydrogen abstraction by the -NH 2 +· and converts primary-alkyl amines to sec - and tert -alkyl amines; fragmentation is then by α-cleavage of the rearranged molecular ions. Isomerization is also encountered for amines branched at the α-carbon atom. The elimination of propene from n -pentylamine and of a butenyl radical from neo -pentylamine is discussed. Remote (δ or e) cleavage does not contribute to the low-energy reactions.

Isomers of Amine Molecular Ions; The Structures of C2H7N+. and related radical cations

[CH3CHNH3]+. and [CH2CH2NH3]+. ions exist as distinct, stable species in the gas-phase. These ions are formed from a variety of precursors, and they can be characterized by their unimolecular and collision-induced reactions. The properties of deuterium labeled analogs confirm the proposed structures. Evidence that stable C3H9N+. ions with unconventional structures also exist is presented; these and other amine ion isomers are formed from inter alia, alkylamine molecular ions in the ion source.

Alkane loss from the molecular ions of alcohols, ketones, and amines in the μsec timeframe

Abstract Neutral alkane molecules are eliminated in the low-energy reactions of many low molecular weight secondary alcohols, ketones and sec-alkyl primary amines. The alkane molecule consists of one α-alkyl group plus a hydrogen atom from the first carbon of the ‘other’ α-alkyl (formally a simple 1,2-elimination). Alkane loss competes inefficiently with other intramolecular hydrogen abstraction reactions (e.g., the McLafferty rearrangement), and is only observed for compounds with relatively short alkyl groups. When an α-substituent is methyl exceptional behavior is sometimes encountered: alkane loss is not important for methyl ketones (except acetone), and methane loss from α-methyl amines occurs only for isopropylamine. Tertiary alcohols have not been observed to eliminate alkane molecules, and alkane loss does not occur for secondary and tertiary amines.

Experimental verification of the intermediacy and interconversion of ion–neutral complexes as radical cations dissociate

Specific hydrogen exchange provides experimental verification of the intermediacy of ion–neutral complexes in the reactions of simple alcohol and ether radical cations; interconversion of ion–neutral pairs can occur when loss of alkyl radicals and loss of alkane molecules have similar energy requirements.

Hydrogen Bonding to Alkanes: Computational Evidence

The structural, vibrational, and energetic properties of adducts of alkanes and strong cationic proton donors were studied with composite ab initio calculations. Hydrogen bonding in D-H(+)...H-alkyl adducts contributes to a significant degree to the interactions between the two components, which is substantiated by NBO and AIM results. The hydrogen bonds manifest themselves in the same manner as conventional hydrogen bonds, D-H bond elongation, D-H vibrational stretching frequency red shift and intensity increase, and adduct stabilization. The alkane adducts also exhibit elongation of the C-H bonds involved and a concurrent red shift, which is rationalized in terms of charge-transfer interactions that cause simultaneous weakening of both the O-H and C-H bonds. Like other dihydrogen-bonded adducts, the adducts possess a bent structure and asymmetric bifurcated hydrogen bonds. The hydrogen bonds are stronger in adducts of isobutane and in adducts of stronger acids. Intramolecular hydrogen bonding in protonated long-chain alcohols manifests itself in the same manner as intermolecular hydrogen bonding and can be equally strong.

Formation and stabilization of intermediate ion–neutral complexes in radical cation dissociation reactions

Intermediate ion–neutral complexes arising when radical cations dissociate will be only moderately stabilized by polarization forces, inasmuch as the distance between the components of the complex cannot be less than the sum of their van der Waals radii; reactions via such complexes are primarily important for systems where the critical energy for simple cleavage is low.