Mid-IR spectra of different conformers of phenylalanine in the gas phase.

The experimental mid- and far-IR spectra of six conformers of phenylalanine in the gas phase are presented. The experimental spectra are compared to spectra calculated at the B3LYP and at the MP2 level. The differences between B3LYP and MP2 IR spectra are found to be small. The agreement between experiment and theory is generally found to be very good, however strong discrepancies exist when -NH2 out-of-plane vibrations are involved. The relative energies of the minima as well as of some transition states connecting the minima are explored at the CCSD(T) level. Most transition states are found to be less than 2000 cm(-1) above the lowest energy structure. A simple model to describe the observed conformer abundances based on quasi-equilibria near the barriers is presented and it appears to describe the experimental observation reasonably well. In addition, the vibrations of one of the conformers are investigated using the correlation-corrected vibrational self-consistent field method.

Structure determination of small vanadium clusters by density-functional theory in comparison with experimental far-infrared spectra.

The far-infrared vibrational spectra for charged vanadium clusters with sizes of 3-15 atoms have been measured using infrared multiple photon dissociation of Vn+Ar-->Vn(+)+Ar. Using density-functional theory calculations, we calculated the ground state energy and vibrational spectra for a large number of stable and metastable geometries of such clusters. Comparison of the calculated vibrational spectra with those obtained in the experiment allows us to deduce the cluster size specific atomic structures. In several cases, a unique atomic structure can be identified, while in other cases our calculations suggest the presence of multiple isomers.

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.

Vibrational spectroscopy of gas-phase neutral and cationic phenanthrene in their electronic groundstates.

Various experimental methods are applied to retrieve the vibrational structure of phenanthrene in its neutral and cationic groundstates. The linear infrared (IR) absorption spectra in the 400-1650 cm(-1) range of jet-cooled phenanthrene and its cation, both clustered with either an argon or a neon atom, are obtained via photo-induced cluster dissociation spectroscopy. The spectra observed are in good agreement with calculated spectra of the bare species. However, the observed spectrum of cationic phenanthrene shows more lines and lines with different intensities in the 900-1400 cm(-1) range than expected from calculations. Additional spectra of the perdeuterated phenanthrene Ar cation, and the warm (T approximately > room temperature) bare phenanthrene cation are recorded. Also the mass-analyzed threshold ionization spectra of bare phenanthrene and phenanthrene-Ar are recorded and compared with each other. Comparison of the spectral data recorded to calculated spectra of bare neutral, cationic and cationic perdeuterated phenanthrene, as well as to IR spectra recorded in matrix-isolation experiments, explicitly demonstrates that cluster dissociation spectroscopy is a valid and powerful method to obtain IR spectroscopic information of bare neutral and cationic jet-cooled poly-aromatic hydrocarbons.

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.

Excitation of C60 using a chirped free electron laser.

Gas phase C60 is resonantly excited using picosecond infrared (IR) pulses from a free electron laser. The excitation can be very high, reaching levels where the thermal emission of electrons from C60 is observed. The excitation is much more efficient when the IR radiation is chirped to lower frequencies during the excitation process. The excitation process is modeled and the results are compared to the experiment.

Conformation of macromolecules in the gas phase: use of matrix-assisted laser desorption methods in ion chromatography.

Conformational data for macromolecules in the gas phase have been obtained by the coupling of a matrix-assisted laser desorption ion source to an ion chromatograph. A series of polyethylene glycol (PEG) polymers "cationized" (converted to a cation) by sodium ions (Na(+)PEG9 to Na(+)PEG19) and a protonated neurotransmitter protein, bradykinin, were studied. Mobilities of Na(+)PEG9 to Na(+)PEG19 are reported. Detailed modeling of Na(+)PEG9 with molecular mechanics methods indicates that the lowest energy structure has the Na(+) ion "solvated" by the polymer chain with seven oxygen atoms as nearest neighbors. The agreement between the model and experiment is within 1 percent for Na(+)PEG9, Na(+)PEG13, and Na(+)PEG17, giving strong support to both the method and the deduced structures. Similar agreement was obtained in initial studies that modeled experimental data for arginine-protonated bradykinin.

Evidence from ion chromatography experiments that met-cars are hollow cage clusters.

Ion chromatography studies were performed to assess various models proposed for the structure of M(8)C(12) species, the met-cars. A laser desorption source was used to make a sequence of titanium-carbon clusters centered around Ti(8)C(12)(+). The Ti(8)C(12)(+) was determined to be a hollow cage cluster, with the dodechadron structure originally propposed termined to be a hollow cage cluster, with the dodecahedron structure originally proposed giving the best fit to experiment; cubic structures could be excluded. Collisional breakup of Ti(8)C(12)(+) yielded only Ti(7)C(12)(+) under the experimental conditions described herein, and modeling indicated that the cage structure was retained. Both Ti(8)C(11)(+) and Ti(8)C(13)(+) were made by the cluster source, and again, dodecahedral-type cage structures were consistent with experiment. The extra carbon atom in Ti(8)C(13)(+) was attached exohedrally to a single titanium atom. No evidence for an endohedral species was found.

Gas-phase ion chromatography: transition metal state selection and carbon cluster formation.

Gas-phase ion chromatography can separate ions that have the same mass but differ in isomeric structure or electronic configuration. The main features of this technique are briefly outlined, and applications to a series of problems in transition metal chemistry and carbon cluster chemistry are described. Examples in transition metal chemistry include state-selective reactivity, excited state deactivation, and state-selective ligand binding energies. For clusters, ion chromatography was used to determine the structure of pure carbon cluster ions as a function of size from C(4) to C(84). The results indicate that carbon grows first in linear chains, transforms to monocyclic planar rings at about C(10), and forms new families of planar bi-, tri-, and tetracyclic rings at C(20), C(30), and C(40), respectively. Fullerenes, which mysteriously appear at C(30) and dominate by C(50), are generated by heating the planar ring systems above an isomerization barrier rather than by growth of graphite precursors.

Isomers of Small Carbon Cluster Anions: Linear Chains with up to 20 Atoms.

The structure of small carbon cluster anions, Cn(-) (4

Influence of body position on jugular venous oxygen saturation, intracranial pressure and cerebral perfusion pressure.

Elevation of the head as a common practice to reduce raised intracranial pressure (ICP) has been discussed controversially of late. Some investigators were able to show that besides lowering ICP head elevation may also reduce cerebral perfusion pressure (CPP). For a new evaluation of optimal head position in neurosurgical care it would be of importance to know the influence of body position on cerebral perfusion. We therefore employed continuous jugular venous oximetry, monitoring cerebral oxygenation, to study the effect of 0 degrees, 15 degrees, 30 degrees, and 45 degrees head elevation on ICP, CPP and jugular venous oxygen saturation (SJVO2) in 25 comatose patients with reduced intracranial compliance. As expected, head elevation significantly reduced ICP from 19.8 +/- 1.3 mmHg at 0 degrees to 10.2 +/- 1.2 mmHg at 45 degrees. Already at 30 degrees 92% of the possible effect on ICP was detected. There was no statistically significant change in CPP and SJVO2 associated with varying head position. Individual reactions of CPP to changes in head position, however, were quite unpredictable. The data suggest that an individual approach to head elevation is to be preferred. A moderate head elevation between 15 degrees and 30 degrees significantly reduces ICP and, in general, does not impair cerebral perfusion. Jugular venous oximetry may be used to optimize ICP, CPP and cerebral oxygenation.