Sung Ho Suck

Mo studies of polymers. I. Use of MNDO to calculate geometries, vibrational frequencies and the electronic band structures of polymers; formalism and application to polyethylene

Abstract A general treatment of linear polymers is developed using MNDO and the tight binding approximation. Analytical expressions are derived for first derivatives of the energy. These are used to calculate geometries vibration frequencies and elastic moduli. Application of this treatment to polyethylene gave results in good agreement with experiment and also a reasonable account of its electronic band structure.

MNDO calculations of molecular electric polarizabilities, hyperpolarizabilities, and nonlinear optical coefficients

Abstract Molecular electric polarizabilities, hyperpolarizabilities, and nonlinear optical coefficients have been calculated by the MNDO SCF MO method with reasonably satisfactory results.

Role of various water clusters in IR absorption in the 8–14-μm window region

A controversy exists over the cause of the IR absorption in the 8-14-microm wavelength window region. One explanation for the continuum absorption is to consider the accumulation of the far wings of strong water monomer absorption lines. On the other hand, other researchers believe that the IR absorption originates from the water dimer complex. Some other researchers claim that either water clusters of larger size or hydrated aerosols are responsible for the continuum absorption. Here we explore the hitherto less-understood systems of water clusters, both homomolecular and heteromolecular, including the water dimer, to enhance our knowledge of the cause of the IR absorption. The present study indicates that homomolecular water clusters larger than the dimer are not responsible for the continuum absorption.

Water cluster interpretation of IR absorption spectra in the 8–14-μm wavelength region

Recently various investigators have suggested that the anomalous atmospheric absorption spectra in the window wavelength region of 8-14 microm is due to the presence of the water dimer or other water clusters. This suggestion has been made on the grounds that the IR absorptivity has a dependency on the squared value of the partial water vapor pressure or nonlinear vapor pressure and negative temperature dependence, and that the effect of foreign broadening is negligible compared with that of self-broadening. Here we present a theoretical study of the IR continuous absorption by the water dimer in the path containing pure water vapor. In this paper we report our computed results of the intermolecular normal vibrational frequencies, concentrations, and absorption coefficients of the water dimer in the gas phase, focusing our primary attention to the IR absorption in the 8-14-microm wavelength region. Our analysis indicates that the dimer may be an important contributor to the IR absorption in the above mentioned spectral range, and that this region corresponds to the wing of the vibrational transition band associated with an intermolecular librational motion of the water dimer.

Structures and properties of fluorinated pyridines; assignment of the two homo's of pyridine

Abstract Structures and properties of fluorinated pyridines have been studied by microwave spectroscopy and theoretically by the MINDO/3 and MNDO methods. While both MINDO/3 and MNDO reproduce the geometries of fluorinated pyridines within their experimental limits of accuracy, MINDO/3 does better in predicting the small changes in geomeyry due to substitution of hydrogen by fluorine in pyridine. Both MINDO/3 and MNDO give rotational constants in quite good agreement with experiment, but the errors in MINDO/3 are somewhat smaller. Both methods give acceptable estimates of heats of formation of pyridines. MNDO gives better estimates of ionization energies and dipole moments, the results agreeing quite well with experiment. Whereas 14 N nuclear quadrupole coupling constants are calculated satisfactory by both methods. MNDO does better in predicting the changes in constants caused by fluorine substitution.

Study of the electronic energy band structure of polyethylene using MINDO/3

Abstract The electronic structure of polyethylene has been calculated using crystal orbital theory and the latest version (MINDO/3) of the MINDO semi-empirical SCF MO method. The results are in better agreement with the observed photoelectron spectrum than those given by an analogous calculation using MINDO/2, which in turn has been shown superior to treatments based on EHT, CNDO, INDO, or ab initio SCF procedures.

Change of free energy in heteromolecular nucleation: Electrostatic energy contribution

The presence of foreign particles in vapor is known to promote the formation of clusters and phase transition at lower supersaturation. Our primary object here is to assess the contribution of the electrostatic energy to the total free energy change owing to the presence of a charged or uncharged foreign particle (atom or molecule) during the formation of heteromolecular clusters. Unlike the Thomson’s ’’electrothermodynamic’’ theory, we introduce the contribution of the higher electric moment (dipole) and polarization of the foreign particle, in addition to its monopole charge contribution. The present theory predicts: (1) there is no strong charge sign preference in the promotion of nucleation, (2) the size (’’radius’’) differences of ions or neutral foreign particles largely determine differences in the free energy change and thus in the promotion of nucleation, and (3) the differences in the dipole moments and polarizabilities of the charged or uncharged foreign particles are also important for such di...

Calculation of the vibrational frequencies of polyethylene and polyethylene-d4 by the MNDO semi-empirical SCF method

Abstract The vibrational frequencies of polyethylene and polyethylene- d 4 have been calculated by the MNDO semi-empirical SCF MO method. The results are in good agreement with experiment.

MINDO/3 calculations of hyperpolarizabilities of fluoromethanes

Abstract Calculations of electric polarizabilities of molecules by the MINDO/3 semi-empirical SCF MO method have been extended to hyperpolarizabilities. Results for CH 3 F, CH 2 F 2 , and CHF 3 agree much better with experiment than those previously reported.

A MNDO study of the electronic band structure of polyethylene

Abstract The electronic band structure of polyethylene has been calculated using the tight-binding LCAO MO approximation together with a recently developed SCF MO procedure (MNDO). The calculated band structure agrees much better with a recent ab initio calculation than those from previous semi-empirical methods and the numerical results are in better agreement with experiment than the ab initio ones.