NOPR Collection:

Acceleration of self-consistent field convergence in semiempirical molecular orbital methods

Sun, 01 Sep 1991 00:00:00 GMT

Title: Acceleration of self-consistent field convergence in semiempirical molecular orbital methods Authors: Duncan, Richard H Abstract: The self-consistent field (SCF) procedure, even in semiempirical MO theory, can often be fairly tedious for large molecules. Moreover, it is often noticed that in some molecules with heteroatoms, the SCF process for the CNDO/2 and INDO MO methods is prolonged due to oscillatory behaviour. Some simple methods are presented for accelerating SCF convergence by dampening these oscillations and by reducing computational labour on each SCF iteration. Use of only the diagonal density matrix elements provides a sensitive and facile alternative method for estimating convergence. The effectiveness of these methods for saving computer time without loss of essential accuracy is also demonstrated. Page(s): 739-744

New block copolymers: Part VII- Morphology of (A - B)n-type block copolymers

Sun, 01 Sep 1991 00:00:00 GMT

Title: New block copolymers: Part VII- Morphology of (A - B)n-type block copolymers Authors: Mahato, Benimadhab; Maiti, Sukumar Abstract: Detailed morphological behaviour of poly(pentamethylene terephthalate)-b-(acrylonitrile-butadiene rubber) copolymer (5GT-b-ATBN) films annealed, benzene etched and iodine stained has been investigated by scanning electron microscopy. The morphology of the copolymer samples depends on the percentage of polyester (hard) segment and on the method of film preparation. Besides hard and soft block phase, a mixed phase has also been detected. Page(s): 745-750

Thermodynamics of solution of some nonelectrolyte solutes at infinite dilution in N-methylpyrrolidone stationary phase by gas liquid partition chromatography

Sun, 01 Sep 1991 00:00:00 GMT

Title: Thermodynamics of solution of some nonelectrolyte solutes at infinite dilution in N-methylpyrrolidone stationary phase by gas liquid partition chromatography Authors: Rao, B Rajeshwar; Bhagat, S D Abstract: Thermodynamic properties of solution (∆G_s, ∆H_{s ,}∆S_{s ,}∆G_e, ∆H_eand ∆S_e) at infinite dilution of seventeen nonelectrolytes in N-methylpyrrolidone stationary phase have been determined using gas-liquid partition chromatography at 37, 45 and 55°C. The results are discussed in terms of thermodynamic interactions between the components of the binary nonelectrolyte solutions and the nature of pure components at molecular level. Page(s): 751-755

Dynamics of multiphasic glass powder catalyzed autoxidation of sulphur dioxide in bulk aqueous phase

Sun, 01 Sep 1991 00:00:00 GMT

Title: Dynamics of multiphasic glass powder catalyzed autoxidation of sulphur dioxide in bulk aqueous phase Authors: Rani, Ashu; Prasad, D S N; Jain, Usha; Gupta, K S Abstract: Kinetics of autoxidation of sulphite has been studied in unbuffered as well as ac tate-buffered aqueous suspensions of glass powder. The buffer concentration has an inhibitory effect n the reaction and the nature of the rate law depends on the buffer concentration. At[CH₃COONa] = 0.14 mol dm^-3 and varying [CH₃COONa], the kinetics in the two pH regimes, 5 < pH 5.9 and 4.2 < pH < 5.0, are different. In the first pH regime, with increasing [glass], the rate profile passes through a maximum. For the region having linear dependence on [glass],the initial rate, R_obs,is defined by the rate law (A)at atmospheric O₂ pressure:
R_obs= R_uncat + (K₄+ K₅[H+]^-1)[glass][S(IV)]² ... (A)
where R_uncat, is the rate of uncatalyzed reaction. The rate has an almost inverse dep ndence on particle size. On the other hand, in the regime 4.2 < pH< 5.0, an inducation period is observed which decreases on decreasing buffer concentration and on increasing pH and [S(IV)]. It also decreases on passing oxygen. When the buffer concentration is lower, viz., [CH₃COONa] = 0.07 mol dm^-3 and [CH₃COONa] is varied, the kinetics are in conformity with the rate law (B): R_obs=( K₆+ K₇[H⁺]^-1)[glass][S(IV)] ... (B)
In unbuffered solutions, an initial rapid drop in [S(IV)]in the high pH region (6.2-6.8 ) and an induction period in the low pH region (3.49-4.44) have been noted. In the middle pH region (4.8-5.5), the kinetics obey the rate law (C): R_obs= K₈[S(IV)] [glass] ... (C)
The suggested autoxidation mechanism is of Langmuir-Hinshelwood type which envisions the adsorption of both dioxygen and S(IV) on glass surface. Page(s): 756-764