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Martin Saunders

Publications and source records attributed to Martin Saunders.

17 recordsLinked to original sources
PubMed2006

Fine tuning the production of nanosized beta-carotene particles using spinning disk processing.

Nanoparticles of trans-beta-carotene are accessible using spinning disk processing (SDP), by varying the reaction conditions and the choice of surfactant, macrocyclic amphiphiles, sulfonato-calix[4,5,6,8]arenes, and alpha,beta-cyclodextrins. SDP ensures rapid mixing and fast kinetics, and nanoparticles of the carotene formed in the presence of the calixarenes are stable with respect to extraction of the carotene into an organic solvent, unlike in the presence of the cyclodextrins. Insight into the supramolecular structure of the carotene nanoparticles has also been established. The mean particle sizes (dynamic light scattering, DLS) have been optimized at 40(2) and 56(1) nm and 71.4(6) and 82(1) nm, respectively, for each sulfonato-calix[5,6 and 4,8]arene, whereas the cyclodextrins form nanoparticles with a mean diameter of 71(1) and 68.5(6) nm, respectively. Zeta-potential studies show stability of all the colloidal dispersions at pH > 4 with values below -30 mV. UV-visible spectroscopy shows a blue shift indicative of H-aggregates of the carotene within the nanoparticles. The surface area derived from BET studies is 39.12 m(2)/g corresponding to particles of 76.7(5) nm in diameter, in agreement with sizes obtained from DLS and TEM measurements.

Antioxidants↗
PubMed2006

Mindless chemistry.

Applications of an automated stochastic search procedure for locating all possible minima with a given composition are illustrated by the pentatomic molecules BCNOS, CAlSiPS, C(4)B(-), C(4)Al(-), and CBe(4)(2-), as well as by C(6)Be, the C(6)Be(2-) dianion, and C(6)H(2). All previously identified minima were reproduced, and many new structures, often with nonintuitive geometries, were found.

Journal Article↗
PubMed2006

Kinetic energy release of C70(+) and its endohedral cation N@C70(+): activation energy for N extrusion.

Unimolecular decomposition of C70(+) and its endohedral cation N@C70(+) were studied by high-resolution mass-analyzed ion kinetic energy (MIKE) spectrometry. Information on the energetics and dynamics of these reactions was extracted. C70(+) dissociates unimolecularly by loss of a C2 unit, whereas N@C70(+) expels the endohedral N atom. Kinetic energy release distributions (KERDs) in these reactions were measured. By use of finite heat bath theory (FHBT), the binding energy for C2 emission from C70(+) and the activation energy for N elimination from N@C70(+) were deduced from KERDs in the light of a recent finding that fragmentation of fullerene cations proceeds via a very loose transition state. The activation energy measured for N extrusion from N@C70(+) was found to be lower than that for C2 evaporation, higher than the value from its neutral molecule N@C70 obtained on the basis of thermal stability measurements, and coincident with the theoretical value. The results provide confirmation that the proposed extrusion mechanism in which the N atom escapes from the cage via formation of an aza-bridged intermediate is correct.

Journal Article↗
PubMed2005

Unimolecular dissociations of C70+ and its noble gas endohedral cations Ne@C70+ and Ar@C70+: cage-binding energies for C2 loss.

The energetics and dynamics of unimolecular decompositions of C70+ and its noble gas endohedral cations, Ne@C70+ and Ar@C70+, have been studied using tandem mass spectrometry techniques. The high-resolution mass-analyzed ion kinetic energy (HR-MIKE) spectra for the unimolecular reactions of C70+, Ne@CC70+, and Ar@C70+ were recorded by scanning the electrostatic analyzer and using single-ion counting that was achieved by combination of an electron multiplier, amplifier/discriminator, and multichannel analyzer. These cations dissociate unimolecularly via loss of a C2 unit, and no endohedral atom is observed as fragment. The activation energies for C2 evaporation from Ne@C70+ and Ar@C70+ are lower than those for elimination of the endohedral noble gas atoms. The kinetic energy release distributions (KERDs) for the C2 evaporation have been measured and, by use of the finite heat bath theory (FHBT), the binding energies for the C2 emission have been deduced from the KERDs. The C2 evaporation energies increase in the order DeltaEvap(C70+) < DeltaEvap(Ne@C70+) < DeltaEvap(Ar@C70+), but no big difference in the cage binding was observed for C70+, Ne@C70+, and Ar@C70+, indicating incorporations of the Ne and Ar atoms into C70 contribute a little to the stability of C70 toward C2 loss, which is in good agreement with theoretical calculations but contrasts with the findings in their C60 analogues and in metallofullerenes that the decay energies of the filled fullerenes are much higher than those of the corresponding empty cages.

Journal Article↗
PubMed2005

Transmutation of fullerenes.

Fullerenes were pyrolyzed by subliming them into a stream of flowing argon gas and then passing them through an oven heated to approximately 1000 degrees C. C(76), C(78), and C(84) all readily lost carbons to form smaller fullerenes. In the case of C(78), some isomerization was seen. Pyrolysis of (3)He@C(76) showed that all or most of the (3)He was lost during the decomposition. C(60) passes through the apparatus with no decomposition and no loss of helium.

Journal Article↗
PubMed2005

Helium entry and escape through a chemically opened window in a fullerene.

(3)He has been inserted into the cavity of an open-cage fullerene derivative close to room temperature, reaching an incorporation fraction of 0.1%. The rate of escape of (3)He from this fullerene was monitored by (3)He NMR to yield the activation barrier and to compare the size of the orifice to those of other open-cage fullerenes. The equilibrium constant was also measured.

Journal Article↗
PubMed2004

A new rearrangement process in tert-amyl cation.

13C NMR spectroscopy of the 2-methyl-2-butyl-1-13C cation (13C-labeled tert-amyl cation) indicates that interchange of the inside and outside carbons occurs via a barrier of 19.5 +/- 2.0 kcal/mol. A plausible mechanism involves hydride migration in the proposed 2-pentyl cation 4 to form 3-pentyl cation 5. Via the protonated cyclopropane intermediate 6, which undergoes degenerate corner-to-corner hydride shift, the secondary 3-pentyl cation 5' with the label shifted to the central carbon atom is formed. The tert-amyl cation obtained from 5' in the reverse process has the 13C label on an inside carbon atom. All intermediates and transition structures were located on the PES theoretically at the MP2/6-31G(d,p) level of theory. The rearrangement rate of the doubly labeled tert-amyl cation (methyl-13C-butyl-1-13C cation), followed by means of 13C NMR, revealed that the process that interchanges inside and outside carbons has the highest barrier. Comparison of the initial rates revealed that isotopomer 1e arises considerably more slowly than other isotopomers, indicating that in the overall rearrangement process transition structure 5-TS has the highest energy.

Journal Article↗
PubMed2004

Isotopic scrambling in Di-13C-labeled 2-butyl cation: evidence for a protonated cyclopropane intermediate.

The (13)C NMR spectrum of 2-butyl-1,2-(13)C(2) cation (1) is unchanged on heating the sample to -78 degrees C, indicating no isomerization to another isotopomer. In contrast, the spectrum of 2-butyl-2,3-(13)C(2) cation (2) shows rapid formation of all of the other isotopomers except 1. These results are consistent with a protonated cyclopropane intermediate in the rearrangement process. In this mechanism, either C(1) and C(2) or C(3) and C(4) interchange. Only the bond between C(2) and C(3) breaks.

Journal Article↗
PubMed2004

Stochastic search for isomers on a quantum mechanical surface.

In studying molecules with unusual bonding and structures, it is desirable to be able to find all the isomers that are minima on the energy surface. A stochastic search procedure is described for seeking all the isomers on a surface defined by quantum mechanical calculations involving random kicks followed by optimization. It has been applied to searching for singlet structures for C6 using the restricted Hartree-Fock/6-311G basis set. In addition to the linear chain and ring previously investigated, 11 additional structures (A-K) were located at this level. These provide a basis for discussing qualitative bonding motifs for this carbon cluster. The application of a similar idea to searching for transition states is discussed.

Journal Article↗
PubMed2003

Rearrangement pathways of five-membered ring enlargement in carbocations: quantum chemical calculations and deuterium kinetic isotope effects.

Three plausible routes for the five-membered ring expansion in the equilibrating 2-cyclopentyl-2-propyl and 1-(2-propyl)cyclopentyl cations 1A/1B were located on the PES, all calculated at the MP4/6-31G(d)//MP2/6-31G(d) level of theory. In pathway I, the six-membered transition structure (TS-I) connects the less stable cyclopentyl cation 1A and the 1,2-dimethylcyclohexyl carbocation (2) via a barrier of 16.4 kcal/mol. In pathway II, which has a barrier of 16.3 kcal/mol, the methyl migration occurs first in the more stable 1B via transition structure TS-II. Pathway III involves the uphill hydride shift and formation of the secondary cation 3, which undergoes Wagner-Meerwein 1,2-isopropyl shift via a transition structure TS-III and the protonated carbocation intermediate 4. The barrier pathway III is for 17.0 kcal/mol. Experimental secondary deuterium isotope effects of the rearrangement were measured for the hexadeuterated 1A-d6/1B-d6 (kH/kD = 2.40) and tetradeuterated 1A-d4/1B-d4 (kH/kD = 0.18) cations by means of 1H NMR. Comparison of the experimental data with the theoretical values (kH/kD = 2.40 for 1B-d6 and kH/kD = 0.24 for 1B-d4, respectively) obtained with QUIVER revealed that pathway II is a major reaction route.

Journal Article↗
PubMed2002

Two helium atoms inside fullerenes: probing the internal magnetic field in C60(6-) and C70(6-).

A 3He NMR resonance of C606- containing He is assigned to He2@C606-, thus showing that C60 can also accommodate two helium atoms. The ratio of the di-helium compound relative to the mono- is 1:200, 10 times lower than the equivalent counterpart of C70. The 3He NMR chemical shift of He2@C606- is 0.093 ppm downfield from the already known resonance of He@C606-. In the reduced endohedral mono- and di-helium C70, the 3He NMR chemical shift of He2@C706- is 0.154 ppm upfield from the peak of He@C706-.

Journal Article↗
PubMed2002

129Xe NMR spectrum of xenon inside C(60).

Xenon was inserted into C(60) by heating C(60) in 3000 atm of xenon gas at 650 degrees C. The Xe@C(60) was separated from the empty C(60) by using HPLC. The (13)C resonance for Xe@C(60) is shifted downfield by 0.95 ppm (192 Hz). (129)Xe NMR showed a line 179.2 ppm downfield from xenon gas.

Journal Article↗
PubMed2002

Direct detection and quantitation of He@C60 by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry.

In this paper, we report negative ion microelectrospray Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry of C60 samples containing approximately 1% 3He@C60 or 4He@C60. Resolving He@C60- and 4He@C60- from C60 containing 3 or 4 13C instead of 12C atoms is technically challenging, because the target species are present in low relative abundance and are very close in mass. Nevertheless, we achieve baseline resolution of 3He@C60- from 13C3(12C57-) and 4He@C60- from 13C4(12C56-) in single-scan mass spectra obtained in broadband mode without preisolation of the ions of interest. The results constitute the first direct mass spectrometric observation of endohedral helium in a fullerene sample at this (low) level of incorporation. The results also demonstrate the feasibility of determining the extent of He incorporation from the FT-ICR mass spectral peak heights. The present measurements are in agreement with those obtained by the pyrolysis method [1-3]. Although limited in sensitivity, the mass spectral method is faster and easier than pyrolysis.

Journal Article↗