Maurizio Speranza

Gas-phase enantioselectivity

Abstract Determination of the intrinsic noncovalent interactions governing chiral recognition in diastereomeric complexes constitutes the basis for understanding information transfer between molecules in living systems as well as in synthetic supramolecular structures. The most important experimental methodologies so far employed for this task are illustrated in the present review. Emphasis is put on the principles and the applications of techniques, such as radiolysis, Fourier transform ion cyclotron resonance (FTICR) and collision-induced dissociation (CID) mass spectrometry, and resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) spectroscopy, that allow measurement of the relative stability of diastereomeric ion/molecule and molecule/molecule complexes and quantification of the short-range forces controlling their enantioselective evolution to products.

ENERGETICS OF MOLECULAR COMPLEXES IN A SUPERSONIC BEAM : A NOVEL SPECTROSCOPIC TOOL FOR ENANTIOMERIC DISCRIMINATION

A subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces is responsible for the different stabilities and spectral features of supersonically expanded van der Waals complexes between (R)-(+)-1-phenyl-1-propanol (R) and (R)-(−)- (r) or (S)-(+)-2-butanol (s). The homochiral Rr complex is more stable than the heterochiral Rs diastereomer in both the ground and excited states (see drawing). This was determined by the first direct measurement of the binding energies of the diastereomeric complexes by two color resonance two photon ionization/time-of-flight mass spectrometry.

Chiral aggregates of indan-1-ol with secondary alcohols and water: Laser spectroscopy in supersonic beams

One color, mass selected resonant two-photon ionization (1cR2PI) spectra of supersonically expanded bare (R)-(−)indan-1-ol (IR) and its complexes with chiral and achiral molecules (solv) have been investigated. The excitation spectrum of bare IR has been analyzed and discussed on the basis of theoretical predictions at the B3LYP/6-31G** level of theory. The excitation spectra of its diastereomeric complexes with (R)-(−)-and (S)-(+)hexan-2-ol (XR or XS, respectively) and water (W) are characterized by significant shifts of their S0 ← S1 band origin relative to that of bare IR. The extent and the direction of these shifts are found to depend upon the structure and the configuration of solv and are attributed to different short-range interactions in the ground and excited [IR·solv] complexes. In particular the [IR·W]n complexes display band origins blue-shifted relative to that of bare IR, attributed to the presence of an O–H⋯π electrostatic interaction between IR and W in [IR·W]n. The [IR·XR] and [IR·XS] equilibrium structures have been calculated by a molecular dynamical (MM3) sampling and PM3 semiempirical local optimization.

Chirality and intermolecular forces: studies using R2PI experiments in supersonic beams

One- and two-color, mass-selected resonant two-photon ionization (R2PI) spectra of the S1←S0 transitions in the bare (R)-(+)-1-phenylethan-1-ol (ER) and its complexes with a solvent molecule (solv: (S)-(+)butan-2-ol (BS), (R)-(−)butan-2-ol (BR), or water (W)) have been recorded after a supersonic molecular beam expansion. The excitation spectrum of bare ER conforms to theoretical predictions at the B3LYP/6-31G** level of theory by pointing to the formation of a single conformer. The one-color R2PI excitation spectra of the diastereomeric complexes [ER–solv] (solv: BS or BR) are characterized by significant shifts of their band origin relative to that of bare ER. The extent and the direction of these spectral shifts are found to depend upon the structure and the configuration of solv and are attributed to different short-range interactions in the ground and excited [ER–solv] complexes. In analogy with strictly related diastereomeric complexes, the phenomenological binding energy of the homochiral [ER–BR] is found to be greater that of the heterochiral one [ER–BS]. The one-color R2PI excitation spectra of the [ER–W] complex displays two signals blue shifted by 54 and 73 cm−1, relative to the S1←S0 band origin of bare ER, which indicate the presence of a O–H···π electrostatic interaction between ER and W.

Short-range interactions within molecular complexes formed in supersonic beams: structural effects and chiral discrimination

One- and two-color, mass-selected R2PI spectra of the S1<--S0 transitions in the bare chiral chromophore R-(+)-1-phenyl-1-propanol (R) and its complexes with a variety of alcoholic solvent molecules (solv), namely methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, S-(+)-2-butanol, R-(-)-2-butanol, 1-pentanol, S-(+)-2-pentanol, R-(-)-2-pentanol, and 3-pentanol, were recorded after a supersonic molecular beam expansion. Spectral analysis, coupled with theoretical calculations, indicate that several hydrogen-bonded [R.solv] conformers are present in the beam. The R2PI excitation spectra of [R.solv] are characterized by significant shifts of their band origin relative to that of bare R. The extent and direction of these spectral shifts depend on the structure and configuration of solv and are attributed to different short-range interactions in the ground and excited [R.solv] complexes. Measurement of the binding energies of [R.solv] in their neutral and ionic states points to a subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces, which control the spectral features of the complexes and allow enantiomeric discrimination of chiral solv molecules.

Gas phase ion/molecule reactions in monogermane-hydrocarbon mixtures: a comparative Fourier transform mass spectrometry and chemical ionization mass spectrometry study

Abstract The gas phase ion/molecule reactions of GeH 4 with some simple saturated (CH 4 , C 2 H 6 ) and unsaturated hydrocarbons (C 2 H 2 , C 3 H 4 , C 3 H 6 ) have been studied by high pressure mass spectrometry and Fourier transform mass spectrometry. The effects of the nature of the hydrocarbon and of the total pressure and the relative concentrations of the reagent gases on the formation of GeC containing ions are reported. Saturated hydrocarbons give only limited amounts of such species, whereas a variety of GeC containing products are efficiently produced when alkenes or alkynes are added to germane. In these latter cases, GeCH + 3 and GeCH + 5 are among the most abundant products, invariably accompanied by ions containing germanium, carbon and hydrogen in relative yields increasing with the hydrocarbon partial pressure. For all systems, the reaction pattern is presented and discussed in relation to the preparation of amorphous GeC containing materials for photovoltaic applications.

Energetics of monohydrated chiral R(+)-1-phenyl-1-propanol: supersonic beam experiments and density functional calculations

Abstract Isolated hydrated clusters of chiral R(+)-1-phenyl-1-propanol have been studied in a supersonic molecular beam by means of one- and two-color resonant two-photon ionization mass-resolved spectroscopy. Excitation and threshold ionization spectra have been determined for monohydrated clusters together with the electronic ground state binding energy. Cluster spectra have been analyzed with the aid of ab initio molecular orbital calculations conducted at the B3LYP/6-31G ∗∗ level of the theory. The analysis gives information on the cluster structures and the binding energies.

Spectroscopic enantiodifferentiation of chiral molecules in the gas phase

Enantiodiscrimination of several chiral molecules, i.e. S-(+)- (s) vs R-(−)-2-butanol (r) and S-(+)- (s′) vs R-(−)-2-butylamine (r′), is obtained by two-color resonance-enhanced two-photon ionization time-of-flight (2cR2PI-TOF) spectroscopy of their supersonically expanded complexes with a suitable chromophore, i.e. R-(+)-1-phenyl-1-propanol (R). Analysis of the 2cR2PI-TOF excitation spectra indicates that these complexes are formed in the supersonic beam in several isomeric hydrogen-bonded structures. The different batochromic shifts of the band origin regions of the molecular diastereomeric complexes relative to that of bare R provide a viable means for spectroscopically discriminating the enantiomers. A further tool for their differentiation is provided by the very reproducible differences in the mass spectral fragmentation patterns of the jet-cooled diastereomeric aggregates. Chirality 11:376–380, 1999.

Ionic Lewis superacids in the gas phase. Part 1. Ionic intermediates from the attack of gaseous SiF+3 on n-bases

The reactivity of the SiF+3 cation towards oxygen bases (H2O, CH3OH, and CH3CH2OH) and nitrogen bases (NH3 and CH3NH2) has been studied using Fourier-transform ion cyclotron resonance mass spectrometry. The SiF+3 ion exclusively attacks the n-centre of the selected bases, yielding excited onium intermediates that undergo fragmentation by elimination of either an alkyl cation (CH3OH, CH3CH2OH, and CH3NH2) or an HF molecule (H2O, NH3, and CH3NH2). In H2O, various protonated fluorosilicic and silicic acids are formed which can be readily converted into their esters and amides by reaction with alcohols and ammonia, respectively. The structure and the reactivity of several such species have been investigated by mass-analyzed ion kinetic energy-collision induced dissociation spectroscopy and ab initio calculations. The extreme affinity of SiF+3 toward n-type electrons ranks it as a powerful gaseous “Lewis superacid”, suitable for generating long-lived, highly reactive ions, e.g. CH+3, in “non nucleophilic” gaseous media, such as, for instance, SiF4.

Gas‐phase enantioselective reactions in noncovalent ion‐molecule complexes

Noncovalent diastereomeric ion-molecule complexes are produced in the gas phase and are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects. This review article describes the state-of-art in this field with special emphasis on the most recent mass spectrometric studies of the structure, dynamics, and reactivity of diastereomeric ion/molecule aggregates.