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Biomedical subjects

M A Duncan

Publications and source records attributed to M A Duncan.

At least 19 recordsLinked to original sources
PubMed2006

Photodissociation of metal-silicon clusters: encapsulated versus surface-bound metal.

Metal-silicon cluster cations of the form MSi(n)+ (M = Cu, Ag, Cr) are produced in a molecular beam with pulsed laser vaporization. These species are mass-selected in a reflectron time-of-flight spectrometer and studied with laser photodissociation at 532 and 355 nm. For the noble metals copper and silver, photodissociation of the n = 7 and 10 clusters proceeds primarily by the loss of metal atoms, indicating that the metal is not located within the interior of silicon cages, and that metal-silicon bonding is weaker than silicon-silicon bonding. Chromium-silicon clusters for n = 7 also lose primarily the metal atom, but at n = 15 and 16 these dissociate via the loss of silicon, producing smaller metal-silicon species. This behavior is consistent with stronger metal-silicon bonding and encapsulated metal structures, as suggested previously by theory. MSi6(+) cations are produced efficiently in all of these photodissociation processes, indicating that these species have enhanced stability compared to other small clusters. Improved values are obtained for the ionization potentials of Si7 and Si10.

Letter↗
PubMed2006

IR spectroscopy of M+(Acetone) complexes (M = Mg, Al, Ca): cation-carbonyl binding interactions.

M(+)(acetone) ion-molecule complexes (M = Mg, Al, Ca) are produced in a pulsed molecular beam by laser vaporization and studied with infrared photodissociation spectroscopy in the carbonyl stretch region. All of the spectra exhibit carbonyl stretches that are shifted significantly to lower frequencies than the free-molecule value, consistent with metal cation binding on the oxygen of the carbonyl. Density functional theory is employed to elucidate the shifts and patterns in these spectra. Doublet features are measured for the carbonyl region of Mg(+) and Ca(+) complexes, and these are assigned to Fermi resonances between the symmetric carbonyl stretch and the overtone of the symmetric carbon stretch. The carbonyl stretch red shift is greater for Al(+) than it is for the Mg(+) and Ca(+) complexes. This is attributed to the smaller size of the closed-shell Al(+), which enhances its ability to polarize the carbonyl electrons. Density functional theory correctly predicts the direction of the carbonyl stretch shift and the relative trend for the three metals.

Acetone↗
PubMed2005

Photodissociation of vanadium, niobium, and tantalum oxide cluster cations.

Transition-metal oxide clusters of the form M(n)O(m) (+)(M=V,Nb,Ta) are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. Consistent with earlier work, cluster oxides for each value of n produce only a limited number of stoichiometries, where m>n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic of a Nd:YAG (yttrium aluminum garnet) laser. All of these clusters require multiphoton conditions for dissociation, consistent with their expected strong bonding. Dissociation occurs by either elimination of oxygen or by fission, repeatedly producing clusters having the same specific stoichiometries. In oxygen elimination, vanadium species tend to lose units of O(2), whereas niobium and tantalum lose O atoms. For each metal increment n, oxygen elimination proceeds until a terminal stoichiometry is reached. Clusters having this stoichiometry do not eliminate more oxygen, but rather undergo fission, producing smaller M(n)O(m) (+) species. The smaller clusters produced as fission products represent the corresponding terminal stoichiometries for those smaller n values. The terminal stoichiometries identified are the same for V, Nb, and Ta oxide cluster cations. This behavior suggests that these clusters have stable bonding networks at their core, but additional excess oxygen at their periphery. These combined results determine that M(2)O(4) (+), M(3)O(7) (+), M(4)O(9) (+), M(5)O(12) (+), M(6)O(14) (+), and M(7)O(17) (+) have the greatest stability for V, Nb, and Ta oxide clusters.

Journal Article↗
PubMed2005

Infrared photodissociation spectroscopy of Mg(+)(H2O)Ar(n) complexes: isomers in progressive microsolvation.

Ion-molecule complexes of the form Mg(H2O)Ar(n)+ (n = 1-8) are produced by laser vaporization in a pulsed-nozzle cluster source. These complexes are mass-selected and studied with infrared photodissociation spectroscopy in the O-H stretch region. The spectra are interpreted with the aid of ab initio calculations on the n = 1-5 complexes, including examination of various isomeric structures. The combined spectroscopic and theoretical studies reveal the presence of multiple isomeric structures at each cluster size, as the argon atoms assemble around the Mg(+)(H2O) unit. Distinct infrared resonances are measured for argon-on-metal, argon-on-OH and argon-on-two-OH isomers.

Journal Article↗
PubMed2005

IR spectroscopy and density functional theory of small V+(N2)n complexes.

V+(N2)n clusters are generated in a pulsed nozzle laser vaporization source. Clusters in the size range of n = 3-7 are mass selected and investigated via infrared photodissociation spectroscopy in the N-N stretch region. The IR forbidden N-N stretch of free nitrogen becomes strongly IR active when the molecule is bound to the metal ion. Photodissociation proceeds through the elimination of intact N2 molecules for all cluster sizes, and the fragmentation patterns reveal the coordination number of V+ to be six. The dissociation process is enhanced on vibrational resonances and the IR spectrum is obtained by monitoring the fragmentation yield as a function of wavelength. Vibrational bands are red-shifted with respect to the free nitrogen N-N stretch, in the same way seen for the C-O stretch in transition metal carbonyls. Comparisons between the measured IR spectra and the predictions of density functional theory provide new insight into the structure and bonding of these metal ion complexes.

Journal Article↗
PubMed2005

Vibrational spectroscopy of Ni+ (benzene)n complexes in the gas phase.

Ni+ (benzene)n (n = 1-6) and Ni+ (benzene)n Ar(1,2) (n = 1,2) are produced by laser vaporization in a pulsed nozzle cluster source. The clusters are mass selected and studied by infrared laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer. The excitation laser is an OPO/OPA system that produces tunable IR in the C-H stretching region of benzene. Photodissociation of Ni+ (benzene)n complexes occurs by the elimination of intact neutral benzene molecules, while Ni+ (benzene)n Ar(1,2) complexes lose Ar. This process is enhanced on resonances, and the vibrational spectrum is obtained by monitoring the fragment yield versus the infrared wavelength. Vibrational bands in the 2700-3300 cm(-1) region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. A dramatic change in the IR spectrum is seen at n = 3 and is attributed to the presence of external benzene molecules acting as solvent molecules in the cluster. The results of previous theoretical calculations are employed to investigate the structures, energetics, and vibrational frequencies of these complexes. The mono-benzene complex is found to have a C2v structure, with benzene distorted by the metal pi-bonding. The di-benzene complex is found to have a D2h structure, with both benzenes distorted. The comparison between experiment and theory provides intriguing new insight into the bonding in these prototypical pi-bonded organometallic complexes.

Journal Article↗
PubMed2005

Ultraviolet and infrared photodissociation of Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar clusters.

Ion-molecule complexes of the form Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar are produced by laser vaporization in a pulsed nozzle cluster source. These clusters are mass-selected and studied with ultraviolet (355 nm) photodissociation and resonance-enhanced infrared photodissociation spectroscopy in the C-H stretch region of benzene. In the UV, Si(+)(C6H6)n clusters (n = 1-5) fragment to produce the Si(+)(C6H6)n mono-ligand species, suggesting that this ion has enhanced relative stability. IR photodissociation of Si(+)(C6H6)n complexes occurs by the elimination of benzene, while Si(+)(C6H6)(n)Ar complexes lose Ar. Resonances reveal C-H vibrational bands in the 2900-3300 cm(-1) region characteristic of the benzene ligand with shifts caused by the silicon cation bonding. The IR spectra confirm that the major component of the Si(+)(C6H6)n ions studied have the pi-complex structure rather than the isomeric insertion products suggested previously.

Journal Article↗
PubMed2004

Growth dynamics and intracluster reactions in Ni+(CO2)n complexes via infrared spectroscopy.

Ni(+)(CO(2))(n), Ni(+)(CO(2))(n)Ar, Ni(+)(CO(2))(n)Ne, and Ni(+)(O(2))(CO(2))(n) complexes are generated by laser vaporization in a pulsed supersonic expansion. The complexes are mass-selected in a reflectron time-of-flight mass spectrometer and studied by infrared resonance-enhanced photodissociation (IR-REPD) spectroscopy. Photofragmentation proceeds exclusively through the loss of intact CO(2) molecules from Ni(+)(CO(2))(n) and Ni(+)(O(2))(CO(2))(n) complexes, and by elimination of the noble gas atom from Ni(+)(CO(2))(n)Ar and Ni(+)(CO(2))(n)Ne. Vibrational resonances are identified and assigned in the region of the asymmetric stretch of CO(2). Small complexes have resonances that are blueshifted from the asymmetric stretch of free CO(2), consistent with structures having linear Ni(+)-O=C=O configurations. Fragmentation of larger Ni(+)(CO(2))(n) clusters terminates at the size of n=4, and new vibrational bands assigned to external ligands are observed for n> or =5. These combined observations indicate that the coordination number for CO(2) molecules around Ni(+) is exactly four. Trends in the loss channels and spectra of Ni(+)(O(2))(CO(2))(n) clusters suggest that each oxygen atom occupies a different coordination site around a four-coordinate metal ion in these complexes. The spectra of larger Ni(+)(CO(2))(n) clusters provide evidence for an intracluster insertion reaction assisted by solvation, producing a metal oxide-carbonyl species as the reaction product.

Journal Article↗
PubMed2004

Infrared photodissociation spectroscopy of V+(CO2)n and V+(CO2)nAr complexes.

V+(CO2)n and V+(CO2)nAr complexes are generated by laser vaporization in a pulsed supersonic expansion. The complexes are mass-selected within a reflectron time-of-flight mass spectrometer and studied by infrared resonance-enhanced (IR-REPD) photodissociation spectroscopy. Photofragmentation proceeds exclusively through loss of intact CO2 molecules from V+(CO2)n complexes or by elimination of Ar from V+(CO2)nAr mixed complexes. Vibrational resonances are identified and assigned in the region of the asymmetric stretch of free CO2 at 2349 cm(-1). A linear geometry is confirmed for V+(CO2). Small complexes have resonances that are blueshifted from the asymmetric stretch of free CO2, consistent with structures in which all ligands are bound directly to the metal ion. Fragmentation of the larger clusters terminates at the size of n=4, and a new vibrational band at 2350 cm(-1) assigned to external ligands is observed for V+(CO2)5 and larger cluster sizes. These combined observations indicate that the coordination number for CO2 molecules around V+ is exactly four. Fourfold coordination contrasts with that seen in condensed phase complexes, where a coordination number of six is typical for V+. The spectra of larger complexes provide evidence for an intracluster insertion reaction that produces a metal oxide-carbonyl species.

Journal Article↗
PubMed2004

Infrared signature of structures associated with the H+(H2O)n (n = 6 to 27) clusters.

We report the OH stretching vibrational spectra of size-selected H+(H2O)n clusters through the region of the pronounced "magic number" at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6 </= n </= 27. A multiplet of such bands appears at small cluster sizes. This pattern simplifies to a doublet at n = 11, with the doublet persisting up to n = 20, but then collapsing to a single line in the n = 21 and n = 22 clusters and reemerging at n = 23. This spectral simplification provides direct evidence that, for the magic number cluster, all the dangling OH groups arise from water molecules in similar binding sites.

Journal Article↗
PubMed2002

Infrared spectra of gas-phase V(+)-(benzene) and V(+)-(benzene)(2) complexes.

The organometallic ions V+-(benzene) and V+-(benzene)2 are produced by laser vaporization in a pulsed nozzle source. They are trapped and mass selected in an ion-trap/time-of-flight mass spectrometer, and their infrared spectra are measured with resonance-enhanced multiphoton photodissociation (IR-REMPD) spectroscopy with a tunable free-electron laser. Vibrational bands in the 600-1800 cm-1 region are characteristic of the benzene molecular moiety perturbed by the metal cation bonding. Experimental data are compared to the IR spectra derived from density functional calculations. Vibrational patterns in V+-(C6H6) indicate that the metal is bound in an eta6 pi-bonding configuration, while V+-(C6H6)2 is a sandwich. Trapped-ion IR-REMPD is a general method to access the vibrational spectroscopy of organometallic ions and their clusters.

Benzene Derivatives↗
PubMed2001

Acute chorea due to ondansetron in an obstetric patient.

This case describes a 34-year-old female who developed severe chorea following intravenous administration of ondansetron after elective caesarean section under spinal anaesthesia. A course of benztropine led to a decrease in severity of the reaction which resolved on the fifth postoperative day. Ondansetron is believed to cause extrapyramidal reactions through actions on 5HT 3 receptors in the basal ganglia. Whereas isolated case reports of extrapyramidal reactions to ondansetron do exist, they have typically been associated with higher doses and have resolved within hours. We report the lengthiest extrapyramidal reaction to ondansetron yet described.

Journal Article↗
PubMed2000

Titanium carbide nanocrystals in circumstellar environments.

Meteorites contain micrometer-sized graphite grains with embedded titanium carbide grains. Although isotopic analysis identifies asymptotic giant branch stars as the birth sites of these grains, there is no direct observational identification of these grains in astronomical sources. We report that infrared wavelength spectra of gas-phase titanium carbide nanocrystals derived in the laboratory show a prominent feature at a wavelength of 20.1 micrometers, which compares well to a similar feature in observed spectra of postasymptotic giant branch stars. It is concluded that titanium carbide forms during a short (approximately 100 years) phase of catastrophic mass loss (>0.001 solar masses per year) in dying, low-mass stars.

Astronomical Phenomena↗
PubMed2000

Traumatic bilateral internal carotid artery dissection following airbag deployment in a patient with fibromuscular dysplasia.

This case describes a 39-yr-old male, presenting with left hemiplegia after a road traffic accident involving frontal deceleration and airbag deployment. Brain computerized tomography (CT) scan revealed a right parietal lobe infarct. Contrast angiography demonstrated bilateral internal carotid artery dissection and fibromuscular dysplasia. The patient was treated with systemic heparinization. Neurological improvement, evidenced by full return of touch sensation, proprioception and nociception began 10 days after the injury. To our knowledge, this is the first case report of carotid artery dissection associated with airbag deployment. Forced neck extension in such settings may result in carotid artery dissection because of shear force injury at the junction of the extracranial and intrapetrous segments of the vessel. Clinicians should consider carotid artery injury when deterioration in neurological status occurs after airbag deployment. We propose that the risk of carotid artery dissection was increased by the presence of fibromuscular dysplasia.

Accidents, Traffic↗
PubMed1999

Sickle cell disease and aortic valve replacement: use of cardiopulmonary bypass, partial exchange transfusion, platelet sequestration, and continuous hemofiltration.

Sickle cell disease in patients undergoing open heart procedures presents a multitude of challenges to the medical staff. With improved techniques of cardiopulmonary bypass, surgery, and anesthesia for treating patients with sickle cell disease, perfusionists will likely encounter patients with this genetic disorder on a more frequent basis. A 40-year-old black woman was admitted to our institution with recurrent Staphylococcus epidermidis and sepsis. She underwent transesophageal echocardiography and cardiac catheterization and was subsequently diagnosed with severe aortic insufficiency. The aortic valve was replaced. Herein, we report our experience in the preoperative, perioperative, and postoperative management of this patient. We present a concise update on the current literature and techniques used by others in similar cases, and we provide a brief section on future considerations to assist fellow practitioners in recognizing this disease and meeting the accompanying challenges.

Adult↗
PubMed1997

Spectroscopy of metal ion complexes: gas-phase models for solvation.

Weakly bound metal ion complexes are produced in molecular beams and studied with mass-selected laser photodissociation spectroscopy. The metal ions Mg+ and Ca+ are the focus of these studies because they have a single valence electron and strong atomic resonance lines in convenient wavelength regions. Weakly bound complexes of these ions with rare-gas atoms and small molecules are prepared with laser vaporization in a pulsed nozzle cluster source. The vibrationally and rotationally resolved electronic spectra obtained for these complexes help to determine the complexes' structures and bonding energetics. Observations from these studies have provided many new insights into the fundamental interactions in electrostatic bonding.

Journal Article↗
PubMed1994

Performance of nonpsychotic relatives of schizophrenic patients on cognitive tests.

We tested 54 nonpsychotic first degree relatives of 23 schizophrenic probands and 18 control subjects matched for age and education on several neuropsychological tests. The tests were selected to assess overall intellectual ability or because previous work indicated that they are particularly sensitive measures of cognitive dysfunction in schizophrenic patients. The relatives of schizophrenic patients performed significantly worse than the control subjects on tests of verbal fluency and on Trailmaking, part B. Each of these tests contributed unique variance to the discrimination between groups. The groups did not differ significantly on the number of perseverative errors on the Wisconsin Card Sorting Test, Wechsler Adult Intelligence Scale-Revised block design or vocabulary, or Trailmaking, part A. Eight relatives who met DSM-III-R criteria for schizotypal personality disorder were more impaired than the remaining 46 relatives on letter fluency, but otherwise their performance was similar to that of nonschizotypal relatives. These data suggest that close relatives of schizophrenic patients may have subtle neuropsychological impairments that are not necessarily associated with clinical symptoms of schizophrenia spectrum disorders.

Adult↗