Mass spectrometry (MS) is a powerful and widespread technique but is inherently uncapable to differentiate species with same m/z ratio. For this reason, many classical approaches were developed based on differential fragmentation patterns in tandem MS. More recently, new techniques for isomer identification in MS were developed, and we highlight the photodissociation techniques that will be discussed in this review. This photodissociation techniques explore the selective activation of isomers to induce isomer specific dissociations in the gas-phase that depend on the spectroscopic features of the isomers. If tunable radiation sources are available, this selective activation may be monitored as a function of the radiation wavelength, allowing the acquisition of vibrational or UV-Vis spectra of the isolated ions, making it possible to evaluate different isomers, protomers and even conformers by MS. These photodissociation techniques are even more powerful when coupled to ion mobility techniques, that will be briefly discussed. Because these techniques enhance the fields and analytical applications where MS can be used, this review will discuss the classical MS methods for isomer differentiation that lead to the fundamentals of ion spectroscopy and selective ion activation, with emphasis on the vibrational ion spectroscopy.

A reatividade em fase gasosa das alcoxissilanas do tipo Me4-nSi(OEt)n (n = 1-3) foi investigada por ressonância ciclotrônica de íons por transformada de Fourier (FT-ICR) com o intuito de se caracterizar os mecanismos de ataque nucleofílico nesses importantes precursores de novos materiais. Nucleófilos como F-, MeO- e EtO- reagem rapidamente e de forma preferencial atacando o átomo de Si, formando a espécie pentacoordenada de Si. Esta em seguida sofre processos de eliminação iniciados, ou por um íon metídio nascente Me-, ou um íon etoxido EtO-, que por sua vez, podem abstrair um próton gerando carbânions ou silóxidos. Carbânions do tipo X(Me)3-nSi(OEt)nCH2 (X = F, MeO, EtO e n = 1-3) e silóxidos do tipo X(Me)4-nSi(OEt)n-1(X = F, MeO, EtO e n = 2-3) podem ser facilmente dissociados por excitação multifotônica no infravermelho induzida por um laser de CO2 gerando uma grande variedade de silóxidos simples e silicatos análogos à espécies intermediárias encontradas nas primeiras etapas de processos sol-gel. Para as alcoxissilanas que contêm mais grupos etóxido, uma reação competitiva análoga a uma eliminação E2 é observada, onde o nucleófilo abstrai um hidrogênio β de um grupo etoxila, eliminando etileno. A cinética, distribuição relativa de produtos e termoquímica dessas reações também são apresentadas para alguns casos específicos.

The gas-phase reactivity of the Me4-nSi(OEt)n (n = 1-3) alkoxysilanes was investigated by Fourier transform ion cyclotron resonance (FT-ICR) technique in order to characterize the fundamental mechanisms associated with nucleophilic attack on these important precursors of new materials. Typical nucleophiles such as F-, MeO- and EtO- react readily and preferentially by attack at the silicon center with formation of a pentacoordinated siliconate that undergoes elimination processes initiated by either a nascent methide ion, Me, or an ethoxide ion, EtO, that can abstract a proton -to yield either a carbanion or a siloxide-type anion. Carbanions of the type X(Me)3-nSi(OEt)nCH2 (X = F, MeO, EtO and n = 1-3) and siloxide ion of the type X(Me)4-nSi(OEt)n-1O- (X = F, MeO, EtO and n = 2-3) can be easily dissociated by infrared multiphoton excitation with a CO2 laser to give rise to a large variety of simple siloxide and silicate-type anions that are reminiscent of intermediate species in the early stages of sol-gel processes. For the higher-ethoxy containing substrates, a competing reaction is observed that is analogous to an E2 elimination reaction in which the nucleophile abstracts a β-proton from the ethoxy group leading to elimination of ethylene. The kinetics, relative product distribution and thermochemistry of these reactions are also reported for some specific cases.