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51.
52.
Michael J. S. Dewar Marye Anne Fox Kay A. Campbell Chia-Chung Chen James E. Friedheim Mary K. Holloway Shi Choon Kim Phillip B. Liescheski Ali M. Pakiari Tze-Pei Tien Eve G. Zoebisch 《Journal of computational chemistry》1984,5(5):480-485
The energies of the lowest singlet (S1) and triplet (T1) states of 28 molecules have been calculated by the “half-electron” (MNDO -HE ) and spin-unrestricted (UMNDO ) versions of MNDO . While most of the calculated values are too negative, because of overestimation of the correlation energy in MNDO -HE and UMNDO , the errors are systematic and depend in an understandable way on the nature of the molecular orbitals (MO S) involved. When appropriate corrections are applied, the calculated energies agree with experiment almost as well as they do for ground states. This justifies the use of MNDO -HE or UMNDO for studies of excited state processes. 相似文献
53.
A new approach is presented to evaluate the depth-dependent quenching of the fluorescence of membrane-bound probes and integral proteins. By utilizing at least three quenchers of known and distinctly different depths, the following parameters can be recovered: most probable depth of the probe; dispersion of the depth distribution, which will depend on the size of probe and fluctuations in its position; and quenching efficiency, which is related to the exposure of a particular fluorophore to the lipid phase. The exposure of tryptophan residues in integral proteins can be quantitatively determined with respect to the model compound (tryptophan octyl ester). The proposed method was applied to the investigation of membrane complexes of the bee venom melittin and cytochrome b5. 相似文献
54.
Sanfelix PC Al-Halabi A Darling GR Holloway S Kroes GJ 《Journal of the American Chemical Society》2005,127(11):3944-3951
We present results of classical trajectory (CT) calculations on the sticking of protons to the basal plane (0001) face of crystalline ice, for normal incidence at a surface temperature (Ts) of 80 K. The calculations were performed for moderately low incidence energies (Ei) ranging from 0.05 to 4.0 eV. Surprisingly, significant reflection is predicted at low values of Ei (< or = 0.2 eV) due to repulsive electrostatic interactions between the incident proton and the surface water molecules with one of their H-atoms pointing upward toward the gas phase. The sticking probability increases with Ei and converges to unity for Ei > or = 0.8 eV. In the case of sticking, the proton is trapped in the ice forming a Zundel complex (H5O2+), with an average binding energy of 9.9 eV with a standard deviation of 0.5 eV, independent of the value of Ei. In nearly all sticking trajectories, the proton is implanted into the ice surface, with a penetration depth that increases with Ei. The strong interaction with the neighboring water molecules leads to a local rupture of the hydrogen bonding network, resulting in collision induced desorption of water (puffing), a process that occurs with significant probability even at the lowest Ei considered. The probability of water desorption increases with Ei. In nearly all trajectories in which water desorption occurs, a single three-coordinated water molecule is desorbed from the topmost monolayer. 相似文献
55.
56.
The technique of ferromagnetic resonance at 23 GHz has been used to determine the first three anisotropy constants of pure Ni down to 4.2K. A temperature and orientation dependent linewidth has also been observed. 相似文献
57.
George C. Levy Clive E. Holloway Richard C. Rosanske James M. Hewitt Craig H. Bradley 《Magnetic resonance in chemistry : MRC》1976,8(12):643-647
The small negative magnetogyric ratio (γ) of the 15N nucleus decreases the efficiency of 15N? 1H dipole-dipole relaxation to about 25% of that for an analogous 13C nucleus. This may lead to greater competition from other relaxation mechanisms in 15N n.m.r. and consequent partial or total quenching of the negative nuclear Overhauser effect (NOE). In unfavorable circumstances nulling of the 15N resonance can occur. Previous 15N relaxation studies have examined isotopically enriched, low molecular weight compounds. The present study examines several small to intermediate size organic compounds containing nitrogen at natural isotopic abundance. In contrast to some of the earlier studies, 15N? 1H dipolar relaxation was found to be dominant for protonated nitrogen atoms, even for two tertiary nitrogens (the tertiary amine nitrogen in 1,2,3,4,6,7,12,12b-octahydroindolo[2,3-a] quinolizine and the oxime nitrogen in 3-methyl-2-pentanone ketoxime). The magnitude of the NOE and the moderate value of T1 indicate effective dipolar relaxation from neighboring but not directly bonded protons in these cases. Nitro groups were found, as expected, to have predominant contributions from non-dipolar mechanisms, and in one case (2-methyl-2-nitro-1, 3-propanediol) signal nulling (NOE of η = ?1) was observed. The effect of paramagnetic impurities was demonstrated for ethanolamine, which contains a basic nitrogen. In this case T1DD(15N? 1H) = 4·3 s; added Ni(acac)2 at 1 × 10?4 Molar reduced the 15N T1 to 0·065 s and consequently the NOE to η = 0. 相似文献
58.
An earlier short communication on this topic reports some incorrect 11B? 13C coupling parameters. The correct data are given together with some 10B coupling and isomer shift data. 相似文献
59.
60.
P.H Holloway 《Surface science》1976,54(2):506-508
The chemisorption of nitric oxide on (110) nickel has been investigated by Auger electron spectroscopy, LEED and thermal desorption. The NO adsorbed irreversibly at 300 K and a faint (2 × 3) structure was observed. At 500 K this pattern intensified, the nitrogen Auger signal increased and the oxygen signal decreased. This is interpreted as the dissociation of NO which had been bound via nitrogen to the surface. By measuring the rate of the decomposition as a function of temperature the dissociation energy is calculated at 125 kJ mol?1. At ~860 K nitrogen desorbs. The rate of this desorption has been measured by AES and by quantitative thermal desorption. It is shown that the desorption of N2 is first order and that the binding energy is 213 kJ mol?1. The small increase in desorption temperature with increasing coverage is interpreted as due to an attractive interaction between adsorbed molecules of ~14 kJ mol?1 for a monolayer. The (2 × 3) LEED pattern which persists from 500–800 K is shown to be associated with nitrogen only. The same pattern is obtained on a carbon contaminated crystal from which oxygen has desorbed as CO and CO2. The (2 × 3) pattern has spots split along the (0.1) direction as and . This is interpreted as domains of (2 × 3) structures separated by boundaries which give phase differences of and π. The split spots coalesce as the nitrogen starts to desorb. A (2 × 1) pattern due to adsorbed oxygen was then observed to 1100 K when the oxygen dissolved in the crystal leaving the nickel (110) pattern. 相似文献