Model Description of Some Molecular Properties by the Modified-Atom-in-Molecule (MAM) Approach

Conclusive evidence is presented whlch shows that the concept of modified atoms in molecule (MAM) is a viable model for a good description of numerous molecular properties. Atomic modification . can be decomposed to isotropic and anisotropic components. The isotropic change caused by molecular formation is given by the electric monopoles of atoms. It is a consequence of the charge drift accompanying chemical bonding. Atomic monopoles reproduce diamagneticshielding of the nuclei rJAd, diamagnetic susceptibility xd and ESCA shifts with an intriguing success. The atomic monopole model is easily extended to include higher local multipoles (i. e. anisotropic contribution), thus yielding satisfactory total molecular multipoles and extramolecular electrostatic potentials. Salient directional properties of covalent bonds are well described by the use of polarized atomic orbitals. It was shown that hybridization is the underlying concept which explains interrelations between steric features and local bond properties. Hybridization rationalizes in a natural and simple way the electron pair (Lewis) bond which is one of the corner stones of chemistry being particularly important for the first row atoms. It was concluded that the high information content of hybrid AOs can be ascribed to the fact that they conform to the local symmetry of the immediate molecular environment. Thus the HAOs are local wavefunctions of the zeroth order which describe atomic angular distortions. Although atoms can not be uniquely defined within molecules, the MAM model has high interpretative power yielding reasonable results. Special attention deserves a picture of charged atoms immersed in the »sea« of mixed electron density, because it is free of any arbitrariness in the slicing of molecular volume of partitioning of overlap charge. Finally, the definition of pseudo-observables is given. It was concluded that atomic monopoles and hybridization indices are pseudo-observables par exceHence. A.pparently there is colour, apparently sweetness, apparently bitterness; actuaUy there are only atoms and the void. Democritus, 420 B. C

Model Description of Some Molecular Properties by the Modified-Atom-in-Molecule (MAM) Approach

Conclusive evidence is presented whlch shows that the concept of modified atoms in molecule (MAM) is a viable model for a good description of numerous molecular properties. Atomic modification . can be decomposed to isotropic and anisotropic components. The isotropic change caused by molecular formation is given by the electric monopoles of atoms. It is a consequence of the charge drift accompanying chemical bonding. Atomic monopoles reproduce diamagneticshielding of the nuclei rJAd, diamagnetic susceptibility xd and ESCA shifts with an intriguing success. The atomic monopole model is easily extended to include higher local multipoles (i. e. anisotropic contribution), thus yielding satisfactory total molecular multipoles and extramolecular electrostatic potentials. Salient directional properties of covalent bonds are well described by the use of polarized atomic orbitals. It was shown that hybridization is the underlying concept which explains interrelations between steric features and local bond properties. Hybridization rationalizes in a natural and simple way the electron pair (Lewis) bond which is one of the corner stones of chemistry being particularly important for the first row atoms. It was concluded that the high information content of hybrid AOs can be ascribed to the fact that they conform to the local symmetry of the immediate molecular environment. Thus the HAOs are local wavefunctions of the zeroth order which describe atomic angular distortions. Although atoms can not be uniquely defined within molecules, the MAM model has high interpretative power yielding reasonable results. Special attention deserves a picture of charged atoms immersed in the »sea« of mixed electron density, because it is free of any arbitrariness in the slicing of molecular volume of partitioning of overlap charge. Finally, the definition of pseudo-observables is given. It was concluded that atomic monopoles and hybridization indices are pseudo-observables par exceHence. A.pparently there is colour, apparently sweetness, apparently bitterness; actuaUy there are only atoms and the void. Democritus, 420 B. C

Gas-phase dissociation reactions of protonated saxitoxin and neosaxitoxin

The aim of this study was to investigate the behavior of the protonated paralytic shellfish poisons saxitoxin (STX) and neosaxitoxin (NEO) in the gas-phase after ion activation using different tandem mass spectrometry techniques. STX and NEO belong to a group of neurotoxins produced by several strains of marine dinoflagellates. Their chemical structures are based on a tetrahydropurine skeleton to which a 5-membered ring is fused. STX and NEO only vary in their substituent at N-1, with STX carrying hydrogen and NEO having a hydroxyl group at this position. The collision-induced dissociation (CID) spectra exhibited an unusually rich variety and abundance of species due to the large number of functional groups within the small skeletal structures. Starting with triple-quadrupole CID spectra as templates, linked ion-trap MSn data were added to provide tentative dissociation schemes. Subsequent high-resolution FTICR experiments gave exact mass data for product ions formed via infrared multiphoton dissociation (IRMPD) from which elemental formulas were derived. Calculations of proton affinities of STX and NEO suggested that protonation took place at the guanidinium group in the pyrimidine ring for both molecules. Most of the observed parallel and consecutive fragmentations could be rationalized through neutral losses of H2O, NH3, CO, CO2, CH2O and different isocyanate, ketenimine and diimine species, many of which were similar for STX and NEO. Several exceptions, however, were noted and differences could be readily correlated with reactions involving NEO's additional hydroxyl group. A few interesting variations between CID and IRMPD spectra are also highlighted in this paper.Peer reviewed: YesNRC publication: Ye

Gas-phase dissociation reactions of protonated saxitoxin and neosaxitoxin

The aim of this study was to investigate the behavior of the protonated paralytic shellfish poisons saxitoxin (STX) and neosaxitoxin (NEO) in the gas-phase after ion activation using different tandem mass spectrometry techniques. STX and NEO belong to a group of neurotoxins produced by several strains of marine dinoflagellates. Their chemical structures are based on a tetrahydropurine skeleton to which a 5-membered ring is fused. STX and NEO only vary in their substituent at N-1, with STX carrying hydrogen and NEO having a hydroxyl group at this position. The collision-induced dissociation (CID) spectra exhibited an unusually rich variety and abundance of species due to the large number of functional groups within the small skeletal structures. Starting with triple-quadrupole CID spectra as templates, linked ion-trap MSn data were added to provide tentative dissociation schemes. Subsequent high-resolution FTICR experiments gave exact mass data for product ions formed via infrared multiphoton dissociation (IRMPD) from which elemental formulas were derived. Calculations of proton affinities of STX and NEO suggested that protonation took place at the guanidinium group in the pyrimidine ring for both molecules. Most of the observed parallel and consecutive fragmentations could be rationalized through neutral losses of H2O, NH3, CO, CO2, CH2O and different isocyanate, ketenimine and diimine species, many of which were similar for STX and NEO. Several exceptions, however, were noted and differences could be readily correlated with reactions involving NEO's additional hydroxyl group. A few interesting variations between CID and IRMPD spectra are also highlighted in this paper.Peer reviewed: YesNRC publication: Ye

Superbasicity of a Bis-guanidino Compound with a Flexible Linker: A Theoretical and Experimental Study

The bis-guanidino compound H2C{hpp}(2) (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H](+) and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "-N-H center dot center dot center dot N=" hydrogen bonding to form an eight-membered C3N4H heterocycle. Solid-state CPMAS N-15 NMR spectroscopy confirms protonation at one of the imine nitrogens, although line broadening is consistent with solid-state proton transfer between guanidine functionalities. X-ray diffraction data have been recorded over the temperature range 50-273 K. Examination of the carbon-nitrogen bond lengths suggests a degree of "partial protonation" of the neutral guanidino group at higher temperatures, with greater localization of the proton at one nitrogen position as the temperature is lowered. Difference electron density maps generated from high-resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system. Computational analysis of I and its conjugate acid [I-H](+) indicate strong cationic resonance stabilization of the guanidinium group, with the nonprotonated group also stabilized, albeit to a lesser extent. The maximum barrier to proton transfer calculated using the Boese-Martin for kinetics method was 2.8 kcal mol(-1), with hydrogen-bond compression evident in the transition state; addition of zero-point vibrational energy values leads to the conclusion that the proton transfer is barrierless, implying that the proton shuttles freely between the two nitrogen atoms. Calculations determining the gas-phase proton affinity and the pK(a) in acetonitrile both indicate that compound I should behave as a superbase. This has been confirmed by spectrophotometric titrations in MeCN using polyphosphazene references, which give an average pK(a) of 28.98 +/- 0.05. Triadic analysis indicates that the dominant term causing the high basicity is the relaxation energy