Biomedical subjects

A D McLean

Publications and source records attributed to A D McLean.

At least 37 records · Page 2Linked to original sources

PubMed1992

Influence of caster diameter on the static and dynamic forward stability of occupied wheelchairs.

The hypothesis that the static and dynamic forward stability of an occupied wheelchair would increase as a function of the caster diameter was tested in 20 able-bodied subjects. A device was attached to the wheelchair frame so that casters with different diameters could be used. With platform testing of static stability, the occupied wheelchair equipped with casters with diameters of 10.2, 20.3, 25.4, and 33.0cm, tipped at mean (+/- 1SD) angles of 23.8 degrees (+/- 1.3 degrees), 25.2 degrees (+/- 1.4 degrees), 26.1 degrees (+/- 1.4 degrees), and 28.2 degrees (+/- 1.9 degrees), respectively. The relationship between static stability (y, in degrees) and caster diameter (x, in cm) can be expressed by the equation y = 23.5-.0196 chi + .00484 x2 (p = .0001). Dynamic stability was tested by having subjects descend a 5 degree ramp, by gravity alone, from progressively farther up the ramp until a full forward tip occurred (footrests contacted the floor) when the wheelchair struck a 5-cm-high obstruction with sufficient speed. The mean tipping speeds for the dynamic tests were .85 (+/- .08), .85 (+/- .08), .89 (+/-.07), and 1.04 (+/- .14) m/sec for the wheelchair fitted with the caster diameters ranging from smallest to largest, respectively. The relationship between dynamic stability (y, tipping speed in m/sec) and caster diameter (chi, in cm) can be expressed by the equation y = .788 + .0139 chi-.00110 chi 2 + .0000276 chi 3 (p = .0001). The relationship between caster diameter and forward wheelchair stability should be considered in wheelchair design and prescription.

Equipment Design↗

PubMed1991

Calculations on the rate of the ion-molecule reaction between NH3+ and H2.

The rate coefficient for the ion-molecule reaction NH3(+) + H2 --> NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He --> NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.

Ammonia↗

PubMed1991

Is interstellar detection of higher members of the linear radicals CnCH and CnN feasible?

Rotational constants and dipole moments for linear-chain radicals CnCH and CnN are estimated using a combination of ab initio molecular orbital calculations and observed data on the starting members of the series. CnCH with n = 0-5 have been observed by radioastronomy in carbon-rich interstellar clouds; higher members of the series have 2 pi ground states with large dipole moments and are strong candidates for observation. CN and C3N have also been observed by radioastronomy; higher members of the series, with the possible exception of C5N, have 2 pi ground states with near-zero dipole moments making their interstellar detection hopeless under present observational conditions. C5N can be a strong candidate only if it has a 2 sigma ground state, and our best computations so far indicate that this is not the case.

Acetylene↗

PubMed1991

Calculations of H2O microwave line broadening in collisions with He atoms: sensitivity to potential energy surfaces.

Broadening parameters for three microwave lines of water at 22.2, 183.3, and 380.2 GHz, in a bath of helium atoms, are calculated using accurate molecular scattering S matrices obtained from two theoretical potentials presented by Palma et al., J. Chem. Phys. 89, 1401 (1988). For the 22 GHz line results are in substantial agreement with values presented in that work, indicating the accuracy of approximate methods used there. The present work improves the potential energy surfaces, computed from perturbation theory (MP4) and variational interacting correlated fragments (ICF1) wave functions, by correcting them for basis set superposition error (BSSE), and recomputes the line broadening using a different procedure for fitting computed energy points. In addition, the entire set of calculations are repeated with a quite different basis set for orbital expansion to establish the reliability of the potential energy surface. We show that adjustments for superposition error are essential, and that broadening cross sections computed from the new surfaces are changed 10%-30% from the old, significantly improving agreement with experiment. The MP4 BSSE adjusted surface appears to be the most accurate, giving room temperature broadenings of 8.9, 11.8, and 10.0 angstroms2 compared with experimental determinations of 12.2 +/- 1.2, 11.9, and 11.2 angstroms2 for the 22, 183, and 380 GHz lines, respectively. Thus, computed line to line variation is larger than observed. The ICF1 BSSE adjusted results for pressure broadening cross section parallel those from the MP4 BSSE calculations but are about 10% smaller. We believe our computed results are stable with respect to basis set for orbital expansion and that the scattering calculations are accurate. Any theoretical inadequacy has been pinpointed to too few points on the potential energy surface resulting in an inadequate description of the angle dependence. It is not clear whether the present discrepancy between computation and experiment stems from this or from errors in the experimental values, although we show some indication that additional information on the surface might decrease the computed broadenings, worsening agreement with experiment.

Computer Simulation↗

PubMed1990

Preventing occupied wheelchairs from falling down stairs.

The hypothesis was tested that wheelchairs could be prevented from accidentally falling down stairs. A rigid post was attached to the wheelchair frame immediately behind the caster (clearance of 13 mm) such that it would strike the floor just after the casters dropped off an edge. The device was tested by means of a 3-degree ramp at the lower end of which was a level surface that ended with a 11-cm vertical drop. Twenty able-bodied subjects descended the ramp, by gravity alone, from progressively greater distances up the ramp, to determine the threshold at which the speed of the occupied wheelchair (with and without the device in place) was sufficient to induce a forward tip down the step. Forward tips occurred at a mean (+/- 1 SD) threshold speed of 0.77 (+/- 0.06) m/s with the device and at 0.38 (+/- 0.04) m/s without it, a mean difference of 0.39 (+/- 0.07) m/s (p less than 0.0005). This preliminary study suggests that such a wheelchair feature might improve the safety of wheelchairs in conditions involving inadvertent loss of caster support, as when they drop off a stair or ledge.

Accidental Falls↗

PubMed1989

Ab initio determination of mode coupling in HSSH: the torsional splitting in the first excited S-S stretching state.

A mechanism for the enhanced splitting detected in the millimeter-wave rotational spectra of the first excited S-S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S-S stretching state and excited torsional states, has been investigated in part by the use of ab initio theory. Based on an ab initio potential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second-order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.

Hydrogen↗

PubMed1989

A priori predictions of the rotational constants for HC13N, HC15N, C5O.

Ab initio molecular orbital theory is used to estimate the rotational constant for several carbon-chain molecules that are candidates for discovery in interstellar space. These estimated rotational constants can be used in laboratory or astronomical searches for the molecules. The rotational constant for HC13N is estimated to be 0.1073 +/- 0.0002 GHz and its dipole moment 5.4 D. The rotational constant for HC15N is estimated to be 0.0724 GHz, with a somewhat larger uncertainty. The rotational constant of C5O is estimated to be 1.360 +/- 2% GHz and its dipole moment 4.4. D.

Carbon↗

PubMed1988

Intermolecular potential for thermal H2O-He collisions.

Theoretical potentials for rotational excitation of H2O by He were constructed via several methods, all of which start with a large basis set SCF interaction. The semiempirical Hartree-Fock with damped dispersion (HFD) model adds a damped long-range attraction with parameters adjusted to fit experimental total differential cross sections. Purely ab initio potentials add correlation energies obtained via perturbation theory (MP2 and MP4) or a variational method (ICF1). Scattering calculations were performed on all surfaces to compare with available beam scattering and pressure broadening data and to assess sensitivity of state-to-state rates to uncertainties in the potential. From comparison with the limited experimental data, the ICF1 surface appears to be marginally better than the MP4 surface. Thermal rates calculated from this surface should be accurate to better than 50%, at least for the larger, more important rates.

Chemical Phenomena↗

PubMed1988

Structural isomers of C2N+: a selected-ion flow tube study.

Reactivities of the structural isomers CCN+ and CNC+ were examined in a selected-ion flow tube at 300 +/- 5 K. The less reactive CNC+ isomer was identified as the product of the reactions of C(+) + HCN and C(+) + C2N2; in these reactions only CNC+ can be produced because of energy constraints. Rate coefficients and branching ratios are reported for the reactions of each isomer with H2, CH4, NH3, H2O, C2H2, HCN, N2, O2, N2O, and CO2. Ab initio calculations are presented for CCN+ and CNC+; a saddle point for the reaction CCN+ --> CNC+ is calculated to be 195 kJ mol-1 above the CNC+. The results provide evidence that the more reactive CCN+ isomer is unlikely to be present in measurable densities in interstellar clouds.

Ammonia↗

PubMed1988

Collisional excitation of interstellar water.

Rates for rotational excitation of water molecules in collisions with He atoms have been obtained from a new, accurate theoretical interaction potential. Rates among the lowest 40 ortho levels are given for kinetic temperatures to 1400 K and among the lowest 29 para levels for kinetic temperatures to 800 K.

Energy Transfer↗

PubMed1988

The sensitivity of gas-phase models of dense interstellar clouds to changes in dissociative recombination branching ratios.

The approach of Bates to the determination of neutral product branching ratios in ion-electron dissociative recombination reactions has been utilised in conjunction with quantum chemical techniques to redetermine branching ratios for a wide variety of important reactions of this class in dense interstellar clouds. The branching ratios have then been used in a pseudo time-dependent model calculation of the gas phase chemistry of a dark cloud resembling TMC-1 and the results compared with an analogous model containing previously used branching ratios. In general, the changes in branching ratios lead to stronger effects on calculated molecular abundances at steady state than at earlier times and often lead to reductions in the calculated abundances of complex molecules. However, at the so-called "early time" when complex molecule synthesis is most efficient, the abundances of complex molecules are hardly affected by the newly used branching ratios.

Ammonia↗

PubMed1987

A detailed investigation of proposed gas-phase syntheses of ammonia in dense interstellar clouds.

The initial reactions in two proposed gas-phase synthesis of interstellar ammonia have been studied in detail. The rate of the slightly endothermic reaction N+(H2) --> NH+(H) under interstellar conditions has been reinvestigated under thermal and nonthermal (translational excitation) conditions based on low-temperature laboratory data. In the thermal case, the importance or lack of importance of this reaction in synthesizing ammonia is determined by how the existing laboratory data at low temperature are interpreted. In particular, it is unclear to what degree the greater rotational energy of laboratory H2 as compared with that of interstellar H2 affects the rate of reaction. In the nonthermal case, whether or not this reaction is important in synthesizing ammonia depends, in addition, upon which of two experimental investigations involving this reaction at low temperature is correct. The exothermic reaction N(H3+) --> NH2+(H) has been studied via ab initio quantum chemical methods and found to possess a large activation energy barrier. Consequently, this latter reaction does not take place appreciably in interstellar clouds and cannot be used to synthesize ammonia.

Ammonia↗

PubMed1987

A high-precision ab initio determination of the equilibrium geometry and force field of HOC+.

Ab initio molecular orbital theory is used to determine if the molecular ion HOC+ is linear (as are the isoelectronic species HCN, HNC, HCO+, etc.), or if it is quasilinear. Near the Hartree-Fock limit the molecule is either linear, or very close to linear. Electron correlation favors the linear geometry, leading to the unequivocal prediction of a linear molecule. Detailed comparisons between HOC+ and isoelectronic HNC show an apparent lack of convergence in the bending potential for the former which is remedied by the addition of f functions to the basis set. The HOC+ potential energy surface is computed in the bending and bend-stretch coordinates and fit to an analytical function. Use of this function to compute the rotation-vibration energies results in improved agreement with experiment relative to previous potentials by nearly two orders of magnitude, as documented in the accompanying paper.

Astronomical Phenomena↗

PubMed1987

Collisional excitation of interstellar cyclopropenylidene.

Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation.

Astronomical Phenomena↗

PubMed1986

A priori predictions of the rotational constants for protonated formaldehyde and protonated methanol.

Protonated formaldehyde and protonated methanol are candidate interstellar molecules and models for classes of protonated oxygen compounds. Ab initio molecular orbital theory has been used to compute rotational constants to guide spectroscopic searches both in the laboratory and in space. The ab initio results are empirically corrected to account for systematic deficiencies in the theory and zero-point vibrational effects; they are expected to be accurate to approximately +/-2%. For H2COH+ the resultant constants are (in GHz) A = 194.3, B = 34.28, and C = 29.14; for H3COH2+ A = 103.7, B = 21.18, and C = 20.30.

Astronomical Phenomena↗

PubMed1986

Is N-protonated hydrogen isocyanide, H2NC+, an observable interstellar species?

Ab initio molecular orbital theory is used to examine the singlet and triplet potential energy surfaces for the CH2N+ system. The results confirm those of earlier studies which suggested that the singlet H2NC+ isomer could be formed via the corresponding triplet isomer. Also, it is shown that the reaction HCN+ + H2 might lead to this metastable isomer without invoking the triplet species. The best test of the hypothesis that this molecule can be formed by gas phase, ion molecule reactions and may be an important precursor in the interstellar synthesis of HCN and HNC is to search for it in space. To this end, theoretical predictions are made of its rotational frequencies and its vibrational frequencies and intensities to serve as a guide to laboratory spectroscopists and radioastronomers.

Astronomical Phenomena↗

PubMed1986

Ab initio molecular orbital studies of low-energy, metastable isomers of the ubiquitous cyclopropenylidene.

The discovery of cyclopropenylidene in space suggests that other C3H2 isomers may be present and a tentative detection of one such isomer, propargylene (HCCCH), has been reported. Ab initio molecular orbital theory has been used to characterize five low-lying, metastable isomers of cyclopropenylidene. Four are found to be stable minima and their vibrational frequencies are reported, although the stable form of triplet propargylene is not firmly established. Extended calculations including the electron correlation energy, show that the lowest in energy is singlet propadienylidene, followed by propargylene; the singlet and triplet of the latter are too close in energy to allow an assignment of the ground state; triplet propadienylidene is at a significantly higher energy. Rotational frequencies computed to an expected accuracy of +/- 1%-2% do not confirm the tentative detection of propargylene in space, although the discrepancy between theory and the observation is not so great as to unequivocally rule out this possibility.

Astronomy↗

PubMed1986

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds.

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.

Astronomical Phenomena↗