Initio MO Method Research Articles

Theoretical analysis of magnetic parameters between donors in BEDT-TTF, BETS, TMTTF and TMTSF crystals

Intermolecular magnetic interaction of BEDT-TTF, BETS, TMTTF and TMTSF donors in some crystals are studied theoretical calculations by using ab initio MO and DFT methods. The two and one dimensional magnetic networks are essential for BEDT-TTF, BETS and TMTTF, TMTSF donors, respectively. The J ab values succesfully evaluated and discussed relationship between J, J' and T SC .

Theoretical Studies on Network Systems with Interspin Interactions by Using the Genetic Algorithm

In 1960’s, Holland has proposed genetic algorithms (GAs) as learning methods which can adapt wide variety of environments. 1) Though GAs are used for many chemical problems, there are few studies for Ising model spin networks, 2), 3) where fixed J values were used. Calculated Jab values were not adopted. Recently, we calculated interspin magnetic interactions by ab initio and semiempirical MO methods. 4) In this study, magnetizations of spin clusters are calculated by Metropolis methods introduced GAs procedures, using calculated Jab values.

Calculation of the hyperfine constants of phosphorus-containing radicals

The hyperfine coupling constants of some small radicals containing phosphorus (PH, PH2, PF2, PCl2, PH3 +, PH4, H3PF, HPF3) and for which experimental values are available are investigated using ab initio MO and DFT methods. Geometries were obtained at the UMP2, UQCISD and B3LYP levels, in combination with the 6-311G(d,p) basis set. The isotropic hyperfine coupling constants were calculated using the above methods with a variety of basis sets, in particular for the B3LYP method. For the smallest radicals (PH, PH2, PH3 + and PH4), UQCISD(T) calculations also were performed using a finite field method. 4 Anistropic coupling constants were calculated using the PWP density functionals. The influence of geometry is discussed, pointing out that UMP2 yields geometries closer to the UQCISD than the B3LYP for this type of molecules. It is shown again that DFT, when used in conjunction with purposely tailored basis sets, is a very economic alternative to highly correlated MO methods for computing the hyperfine properties of phosphorus-containing radicals. Such a basis set, which is being introduced in this work, seems to provide a better cancellation of errors and is small enough to be used in large molecular computations while still giving reliable hyperfine coupling constants. The improved basis set is then applied to the study of three somewhat larger radicals, namely PF4, PCl4 and P2 H6 +.

Theoretical Studies of Direct Exchange Couplings Between Transition Metal Ions I. Naked Binuclear Chromium(II) and Molybdenum (II) Systems

Abstract The effective exchange integrals (Jab) and bond orders of naked Cr(II)-Cr(?) and Mo(?)-Mo(?) dimers were calculated by ab initio UHF MO and density functional (DFT) methods. Reduction of formal quadruple bond orders for these species by strong electron correlation effects are examined by these computational procedures. The calculated Jab values are compared with the experimental results for binuclear Cr(II) complexes with different interatomic distances.

Energy Band Structure for Metallic Polyacetylene

ABSTRACTThe optimized molecular structures of doped polyacetylene model compounds are studied by the ab initio SCF MO method. The calculated structures are assigned to polaron, charged soliton and poison unit. The one dimensional energy band structures of the charged soliton-antisoliton and the polson-antipolson lattices are investigated. The latter one gives a quasi-metallic energy band. The results of the energy band calculation are consistent with the electrical properties and ultraviolet photoemission spectra of the doped polyacetylene.

Structure and dynamics of dicyandiamide: A theoretical study

AbstractAb initio MO methods have been used to study the structures and energetics of dicyandiamide, [(NH2)2CNCN], its isomers, protonated species, radical anions, transition structures for internal conformational change and transition structures for isomerization. Structures were optimized at the HF/STO‐3G, HF/3–21G and HF/6–31G* levels; selected barrier heights for smaller analogues were also computed at the MP4SDTQ/6–31G* level. The most stable isomer of dicyandiamide has the cyano group on the imine nitrogen [1, (NH2)2CNCN]; the other isomer [2, HNC(NH2)NHCN] lies 12.8 kcal mol−1 higher. Inversion at the imino nitrogen proceeds by a linear, in plane process with a barrier of 32.5 kcal mol−1. The amino rotation barriers are 19 kcal mol−1 (single NH2) and 40 kcal mol−1 (both NH2 in a conrotaory or a disrotatory fashion; if the NH2 groups are allowed to pyramidalize the disrotatory barrier drops to 20 kcal mol)−1. Protonation occurs preferentially on the imine nitrogen (PA = 219.7 kcal mol−1 for 1); the proton affinities PA of the amino nitrogens are 25–30 kcal mol−1 lower. Isomerization between 2 and 1 would go via a 1,3‐sigmatropic hydrogen shift, but the barrier is high (48.3 kcal mol−1); protonation reduces the hydrogen shift barrier by ca 15 kcal mol−1. However, the most likely mechanism for isomerization involves protonation of the imine nitrogen in 2 followed by deprotonation of the cyano‐substituted nitrogen to form 1, circumventing the energetically costly 1,3‐sigmatropic hydrogen shift. When an electron is transferred to dicyandiamide, a sizeable fraction of the resonance stabilization of the guanidine moiety is lost.

Ab initio study of vibrational frequencies, force field and normal coordinate analysis of the cis- and trans-isomets of nitrosomethanol

In this paper, the equilibrium geometries of two isomers of the newly found compound — nitrosomethanol—have been optimized by ab initio SCF MO method with 3–21G basic set by gradient technique. And the second derivatives of potential energy (i. e. the force constant matrix elements) have been calculated analytically. Hence the entire force fields of the two isomers of nitrosomethanol have been obtained theoretically. The theoretical vibrational frequencies and the corresponding normal modes were obtained and compared with the experimental values, and the structures of two isomeric forms of nitrosomethanol are established.

The ab initio quantum chemical calculation of MgH+ and Mg2H+

The potential energy curve of MgH~+ and the potential energy surface of Mg_2H~+ are calculated by quantum chemical ab initio SCF MO method with STO-3G basis set. The electronic wave functions and populations are obtained. The equilibrium internuclear distance of MgH~+ is 1.60A. There are two possible configurations of Mg_2H~+: C_(∞v) and D_(∞h). The former is[Mg—Mg—H]~+, with the bond lengths r_(Mg)-Mg=2.41A, r_(Mg-H)=1.63A, and the latter is [Mg—H—Mg]~+, with the bond length r_(Mg-H)=1.73 A. The total energy of the former is 50 kcal/mol lower than that of the latter. The cause of the stability of these species are discussed. The result of calculation about bond length of MgH~+ consists reasonably well with experimental value. The conclusion about the stability of [Mg—Mg—H]~+ ion supports the hypothesis proposed by Porter, based on the result of thermo-dynamical calculation.

Molecular orbital study of proton transfer energetics in the active site of papain by using methanethiol-imidazole-formic acid complex as a model.

The energetics of proton transfers in the methanethiol-imidazole-formic acid complex were analyzed from a quantum chemical point of view in order to elucidate the nature of the enzymatic reaction of papain and the electronic structure in the active site. Methanethiol, imidazole and formic acid were used as models of the side chains of Cys 25, His 159 and Asp 158. The calculations were performed within the closed-shell-LCAO-ab initio MO method by using a 4-31G basis set. The hydrogen-bonding structure including the anion form of the methanethiol, the cation form of the imidazole and the anion form of the formic acid was the most stable of the various hydrogen-bonding structures. This result is in good agreement with experiments on the electronic structure in the active site of papain, and it supports the mechanism of the enzymatic reaction proposed by Angelides and Fink.

Intrinsic acidities of substituted phenols and benzoic acids determined by gas-phase proton-transfer equilibriums

Intrinsic acidities of substituted phenols and benzoic acids determined by gas-phase proton-transfer equilibriums

Theoretical study of the barriers to internal rotation in nitrous acid

Nitrous acid, HONO, has been studied for three geometries by the ab initio LCAO SCF MO method with a basis of accurate gaussian atomic orbitals. The trans geometry is correctly predicted to be most stable, lying about 2 kcal/mole lower than the cis form, and 9 kcal/mole lower than the 90° form (experimental estimates being 0.4 and 11.6 kcal/mole, respectively). Population analysis, dipole moment components, and properties related to nuclear-nuclear and nuclear-electron potentials all show a partial breaking of the hydroxyl oxygen-nitrogen bond at 90° compared to cis and trans, as well as the effects of electronic rearrangement for nuclear screening in the high nuclear repulsion cis form. The cis to 90° barrier is dominated by the attractive components of the total energy, while the trans to 90° one is dominated by repulsive components, in agreement with our analysis and an earlier prediction by Allen.