Biomedical subjects

Stephan Irle

Publications and source records attributed to Stephan Irle.

11 recordsLinked to original sources

PubMed2006

Modeling carbon nanostructures with the self-consistent charge density-functional tight-binding method: vibrational spectra and electronic structure of C(28), C(60), and C(70).

The self-consistent charge density-functional tight-binding (SCC-DFTB) method is employed for studying various molecular properties of small fullerenes: C(28), C(60), and C(70). The computed bond distances, vibrational infrared and Raman spectra, vibrational densities of states, and electronic densities of states are compared with experiment (where available) and density-functional theory (DFT) calculations using various basis sets. The presented DFT benchmark calculations using the correlation-consistent polarized valence triple zeta basis set are at present the most extensive calculations on harmonic frequencies of these species. Possible limitations of the SCC-DFTB method for the prediction of molecular vibrational and optical properties are discussed. The presented results suggest that SCC-DFTB is a computationally feasible and reliable method for predicting vibrational and electronic properties of such carbon nanostructures comparable in accuracy with small to medium size basis set DFT calculations at the computational cost of standard semiempirical methods.

Journal Article↗

PubMed2006

Origin of the linear relationship between CH2/NH/O-SWNT reaction energies and sidewall curvature: armchair nanotubes.

The origin of the linear relationship between the reaction energy of the CH2/NH/O exo and endo additions to armchair (n, n) single-walled carbon nanotubes (SWNTs) and the inverse tube diameter (1/d) measuring sidewall curvature was elucidated using density functional theory and density functional tight binding methods for finite-size SWNT models with n = 3, 4, ..., 13. A nearly perfect linear relationship between DeltaE and 1/d all through exohedral (positive curvature) and endohedral (negative curvature) additions is due to cancellation between the quadratic contributions of the SWNT deformation energy and the interaction energy (INT) between the deformed SWNT and CH2/NH/O adducts. Energy decomposition analysis shows that the quadratic contributions in electrostatic, exchange, and orbital terms mostly cancel each other, making INT weakly quadratic, and that the linear 1/d dependence of INT, and therefore of DeltaE, is a reflection of the 1/d dependence of the back-donative orbital interaction of b1 symmetry from the occupied CH2/NH/O p pi orbital to the vacant C=C pi* LUMO of the SWNT. We also discuss the origin of the two isomers (open and three-membered ring) of the exohedral addition product and explain the behavior of their associated minima on the C-C potential energy surfaces with changing d.

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PubMed2006

The C60 formation puzzle "solved": QM/MD simulations reveal the shrinking hot giant road of the dynamic fullerene self-assembly mechanism.

The dynamic self-assembly mechanism of fullerene molecules is an irreversible process emerging naturally under the nonequilibrium conditions of hot carbon vapor and is a consequence of the interplay between the dynamics and chemistry of polyyne chains, pi-conjugation and corresponding stabilization, and the dynamics of hot giant fullerene cages. In this feature article we briefly present an overview of experimental findings and past attempts to explain fullerene formation and show in detail how our recent quantum chemical molecular dynamics simulations of the dynamics of carbon vapor far from thermodynamic equilibrium have assisted in the discovery of the combined size-up/size-down "shrinking hot giant" road that leads to the formation of buckminsterfullerene C60, C70, and larger fullerenes. This formation mechanism is the first reported case of order created out of chaos where a distinct covalent bond network of an entire molecule is spontaneously self-assembled to a highly symmetric structure and fully explains the fullerene formation process consistently with all available experimental observations a priori. Experimental evidence suggests that it applies universally to all fullerene formation processes irrespective of the carbon source.

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PubMed2006

Theory and experiment agree: single-walled carbon nanotube caps grow catalyst-free with chirality preference on a SiC surface.

High-temperature quantum chemical molecular dynamics simulations have been performed on model systems of thin SiC crystal surfaces with two graphene sheets placed on top of either C or Si face. In agreement with experiment, we find that (a) the C-face-attached graphene layer warps readily to form small diameter, stable nanocaps, suitable for further perpendicular growth of nanotubes, (b) the Si-face-attached graphene sheet does not readily wrap and forms more volatile Si-graphene bonds, and (c) C face nanocaps appear to anneal to dome-shape structures with zigzag chirality.

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PubMed2006

A study of the reaction of N+ with O2: experimental quantification of NO+(a 3Sigma+) production (298-500 K) and computational study of the overall reaction pathways.

The product branching ratios for NO+(X 1Sigma+) and NO+(a 3Sigma+) produced from the reaction of N+ with O2 have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO+(a) at both temperatures, despite the fact that the reaction to form NO+(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N+ + O2 reaction show that the reaction from N+(3P) + O2(3Sigma(g)) reactants starts with an efficient early stage charge transfer to the N(2D) + O2+(X 2Pi) channel, which gives rise to the O2+(X 2Pi) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO+ and O+ products. Pathways to produce NO+(a 3Sigma+) are found to be less favorable than pathways leading to the major product NO+(X 1Sigma+). Production of N(2D) has implications for the concentration of NO in the mesosphere.

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PubMed2006

Fe/C interactions during SWNT growth with C2 feedstock molecules: A quantum chemical molecular dynamics study.

We are presenting the first quantum chemical molecular dynamics (QM/MD) model simulations for iron catalyzed single-walled carbon nanotube (SWNT) growth based on the density functional tight binding (DFTB) quantum chemical potential. As model systems, open-ended (10,10) armchair tube fragments were selected with 0, 10, and 20 Fe atoms attached in 1,4-positions on the open rims, and ensembles of randomly oriented C2 molecules were included to simulate carbon plasma feedstock molecules. Isokinetic trajectories at 1500 K to 3000 K show that divalent Fe increases the number of coordination partners with carbon and/or Fe, depending on the Fe concentration. Fe/C interactions weaken the tube sidewall due to electron transfer from Fe into antibonding carbon orbitals, and C2 addition occurs mainly in an Fe-C2-Fe bridge addition mechanism, while growth of polyyne chains characteristic for high-temperature carbon systems is suppressed in the presence of Fe on the rims of the growing SWNT. Our findings are the first quantum chemical evidence for the importance of intermetallic interactions during SWNT growth.

Carbon↗

PubMed2005

Sensitivity of ammonia interaction with single-walled carbon nanotube bundles to the presence of defect sites and functionalities.

Ammonia adsorption on single-walled carbon nanotubes (SWNTs) was studied by means of infrared spectroscopy at both cryogenic (approximately 94 K) and room (approximately 300 K) temperatures. At 94 K, vacuum-annealed SWNTs showed no detectable ammonia uptake. However, the ammonia adsorption was found to be sensitive to the functionalities and defects on the nanotube surfaces. NH3 adsorption was detected on HNO3-treated nanotubes, characterized by significant functionalities and defects, prior to vacuum annealing. NH3 desorbed from those nanotubes above 140 K, indicating a weak adsorbate-nanotube interaction (approximately 30 kJ/mol). Exposure of annealed samples to ambient air, which possibly regenerated functionalities and defects on nanotube surfaces, restored partially the ammonia uptake capacity. No ammonia adsorption on SWNTs was observed by infrared spectroscopy at room temperature with up to 80 Torr dosing pressure. This work suggests the influence of functionalities and/or defect densities on the sensitivity of SWNT chemical gas sensors. Our theoretical studies on NH3 adsorption on pristine and defective tubes, as well as oxidized tubes, corroborate these findings.

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PubMed2005

Towards formation of buckminsterfullerene C60 in quantum chemical molecular dynamics.

We present follow-up studies on the formation mechanism of fullerene molecules from random ensembles of C2 molecules using quantum chemical molecular dynamics. Two possible roadmaps are investigated as to how buckminsterfullerene C60 and higher fullerenes could be formed. In a "size-up" scenario, fullerenes of the cage size of C72-C96 were found to form directly from high concentrations of C2 molecules at 2000 K with periodic supply of batches of additional C2's. In a "size-down" approach, smaller fullerenes are sometimes formed by losing carbon fragments in "fall-off" or "pop-out" annealing processes under prolonged heating of giant fullerenes, which were self-assembled at initial stages from C2's with lower concentrations. Both roadmaps are found to provide explanations for the appearance of C60 and larger fullerenes in combustion and carbon arc experiments.

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PubMed2004

Analytical second-order geometrical derivatives of energy for the self-consistent-charge density-functional tight-binding method.

Analytical formulation of the second-order geometrical derivatives of energy for the self-consistent-charge density-functional tight-binding (SCC-DFTB) method is presented. To test its quality and numerical performance, the derived formalism has been coded and applied for calculation of harmonic vibrational frequencies for a set of 17 small and medium size molecules. For this set, the average absolute deviation from experiment is 99 cm(-1) for SCC-DFTB vs 62 cm(-1) for the Møller-Plesset second-order perturbation theory with the cc-pVDZ basis set (MP2/cc-pVDZ) and 32 cm(-1) for the B3LYP density functional method with the same basis set (B3LYP/cc-pVDZ), while the maximal deviation is 465 cm(-1) vs 1,741 cm(-1) for MP2/cc-pVDZ and 112 cm(-1) for B3LYP/cc-pVDZ. The SCC-DFTB results are in reasonable agreement with experiments as well as with ab initio and density-functional results, and are better than other semiempirical methods. The SCC-DFTB method allows for considerable computational time saving when compared to other methods while retaining similar overall accuracy. Data for a series of conjugated polyenes show that an analytical formulation of SCC-DFTB is noticeably faster than its numerical formulation.

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PubMed2003

[Tetracaine drops versus lidocaine gel for topical anaesthesia in cataract surgery].

BACKGROUND: in addition to intracameral anaesthesia, drop anaesthesia with tetracaine drops has been established for topical anaesthesia in cataract surgery. Anaesthesia with lidocaine gel is less established. We compared pain sensitivity of patients undergoing surgery with drop anaesthesia to patients who underwent surgery with gel anaesthesia. PATIENTS AND METHODS: In a questionnaire-based survey in which patients assessed their sensitivity to pain during cataract surgery, we compared the sensitivity to pain of 49 patients during surgery with gel anaesthesia to 62 patients who consecutively underwent surgery with drop anaesthesia. Patients assessed pain intensity postoperatively by checking off a 10-graduation pain scale. Lidocaine gel (2 %) was applied once while tetracaine drops (1 %) were applied three times. All interventions were carried out by two surgeons as phako emulsifications with implantation of a posterior chamber lens via posterior-limbal access. In addition to pain sensitivity we also considered the patients' age, duration of surgery and surgical complications. The data for the patients' age, the duration of surgery and the assessment of intensity were not normally distributed so that a Mann-Whitney-rank-sum test was performed. RESULTS: No differences were observed between tho two groups, neither regarding age (P = 0.63) nor the duration of the surgical intervention (p = 0.43). However, in patients treated with tetracaine drops pain sensitivity was reported as being significantly less (mean 1,77) than in patients treated with lidocaine gel (mean 2.22; P < 0.043). 73.5 % of patients with gel anaesthesia and 83.9 % of those with drop anaesthesia reported "negligible" pains. There were no intra-operative complications. CONCLUSION: Both gel anaesthesia and drop anaesthesia are suitable methods for cataract surgery under local anaesthesia. According to patients' assessment, drop anaesthesia with tetracaine is better suited for pain elimination.

Anesthesia, Local↗

PubMed1996

Electronic Structure and Properties of Trihalogen Cations X(3)(+) and XY(2)(+) (X, Y = F, Cl, Br, I).

Electronic structures, charge distributions, geometries, valence force constants, and vibrational frequencies of the homoatomic clusters F(3)(+), Cl(3)(+), Br(3)(+), and I(3)(+) and of the heteroatomic clusters ClF(2)(+), BrF(2)(+), IF(2)(+), BrCl(2)(+), ICl(2)(+), and IBr(2)(+) were determined. The self-consistent field approach extended by MP2-correlation energy or density-functional corrections was applied using various basis sets. It was found that d- and f-type polarization functions play a crucial role as in some other halogen compounds. The MP2 approach yields the most satisfactory results. The effect of the crystalline environment surrounding the Cl(3)(+), Br(3)(+), and I(3)(+) species is successfully simulated by a Madelung potential. Frequencies calculated in the crystal field are in reasonable agreement with the more reliable ones among the experimental results. Coupling force constants were determined. They are not consistent with some empirical rules. Bonding and charge distributions of the formally mixed-valence systems X(+)Y(2)(0) are discussed. X(+) behaves like a divalent chalcogen with high electronegativity. Each of the X(+)-Y bonds in XY(2)(+) is very similar to the bond in X-Y. We predict the experimentally unknown F(3)(+) to be stable in vacuum but not in the solid state. Structures and frequencies of XY(2)(+) species, which are as yet unknown, are also predicted.

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