Proximity effects in the electron ionisation mass spectra of substituted cinnamamides.

The electron ionisation mass spectra of an extensive set of 53 ionised monosubstituted and disubstituted cinnamamides [XC6H4CH=CHCONH2, X = H, F, Cl, Br, I, CH3, CH3O, CF3, NO2, CH3CH2, (CH3)2CH and (CH3)3C; and XYC6H3CH=CHCONH2, X = Y = Cl; and X, Y = F, Cl or Br] are reported and discussed. Particular attention is paid to the significance of loss of the substituent, X, from the 2-position, via a rearrangement that is sometimes known as a proximity effect, which has been reported for a range of radical-cations, but is shown in this work to be especially important for ionised cinnamamides. When X is in the 2-position of the aromatic ring, [M - X]+ is formed to a far greater extent than [M - H]+; in contrast, when X is in the 3-position or 4-position, [M - H]+ is generally much more important than [M - X]+. Parallel trends are found in the spectra of XYC6H3CH=CHCONH2: the signal for [M - X]+ dominates that for [M - Y]+ when X is in the 2-position and Y in the 4-position or 5-position, irrespective of the nature of X and Y. Further insight is obtained by studying the competition between expulsion of X· and alternative fragmentations that may be described as simple cleavages. Loss of ·NH2 results in the formation of a substituted cinnamoyl cation, [XC6H4CH=CHCO]+ or [XYC6H3CH=CHCO]+; this process competes far less effectively with the proximity effect when X is in the 2-position than when it is in the 3-position or 4-position. Additional information has been obtained by investigating the competition between formation of [M - H]+ by the proximity effect and loss of CH3· by cleavage of a 4-alkyl group to give a benzylic cation, [R1R2CC6H4CH=CHCONH2]+ (R1, R2 = H, CH3).

Experimental and computational evidence for C=O π-bonding in [CH2OH]+ and related oxonium ions.

The question of whether [CH2OH]+ should be described as the hydroxymethyl cation, +CH2OH, or protonated formaldehyde, CH2=OH+, is reconsidered in the light of experimental information and new computational evidence. Previous arguments that the charge distribution in [CH2OH]+ may be probed by considering the incremental stabilisation of [CH2OH]+ induced by homologation on carbon (to give [CH3CHOH]+) or oxygen (to produce [CH2OCH3]+) are critically examined. Cation stabilisation energies are shown to be better indicators of the nature of these oxonium ions. Further insight into the structure of larger CnH2n+1O+ oxonium ions is obtained by considering the site of protonation of enol ethers and related species. Computational information, including AIM (Atoms and Molecules) and NBA (Natural Bond Analysis) charges on the carbon and oxygen atoms in [CH2OH]+ and related species, is considered critically. Particular attention is focused on the calculated bond lengths and barriers to rotation about the C-O bond(s) in [CH2OH]+, [CH3CHOH]+, [(CH3)2COH]+, CH3OH and [CH2OCH3]+ and the C-N bond in [CH2NH2]+. Trends in these data are consistent with appreciable π-bonding only in the C-O connections which correspond to the C=O bond in the parent aldehyde or ketone from which the oxonium ion may be considered to be derived by protonation or alkyl cationation.

Structure reactivity relationship in the accelerated formation of 2,3-diarylquinoxalines in the microdroplets of a nebuliser.

Competition experiments in which 1,2-phenylenediamine, C6H4(NH2)2, condenses with equimolar quantities of benzil, (C6H5CO)2, and a 3,3'- or 4,4'-disubstituted benzil (XC6H4CO)2 (X = F, Cl, Br, CH3 or CH3O) to form a mixture of 2,3-diphenylquinoxaline and the corresponding 2,3-diarylquinoxaline (Ar = XC6H4) in the microdroplets produced in a nebuliser allow a Hammett relationship with a ρ value of 1.85 to be developed for this accelerated condensation in the nebuliser. This structure reactivity relationship reveals that an appreciable amount of negative charge builds up on the carbon of the carbonyl group of the benzil during the rate-limiting step of the reaction, thus confirming that this process involves nucleophilic addition of the 1,2-phenylenediamine to the benzil. In general, the presence of an electron donating substituent, particularly in the 4 and 4' positions, in the benzil retards the reaction, whereas an electron attracting substituent, especially in the 3 and 3' position, accelerates it.

Accelerated generation of (protonated) imines and quinoxalines by formation of C=N bonds in the microdroplets of a nebuliser.

Ions corresponding to protonated imines appear in the positive ion electrospray mass spectra of mixtures of the parent aromatic aldehyde and arylamine. The formation of these imine products occurs readily in the electrospray source nebuliser, even without the application of a spray potential. This accelerated formation of C=N bonds in the nebuliser has been extended to encompass the preparation of quinoxalines from a range of substituted phenylenediamines and benzils. The condensation may be induced either under conventional positive ion electrospray conditions (to give the protonated quinoxalines) or when the nebuliser is disconnected from the mass spectrometer (to give the neutral quinoxaline). Ions corresponding to intermediate adducts formed by condensation of the phenylenediamine component with the protonated benzil are observed in many cases when the condensation occurs in the mass spectrometer. This finding supports an interpretation based on nucleophilic addition in droplets generated by the nebuliser.