Khamis Siam

Investigation of the molecular structure of catechol by combined microwave spectroscopy and AB initio calculations

Abstract The molecular structure of catechol (1,2-dihydroxybenzene) was studied by microwave spectroscopy combined with ab initio geometry refinements of the 4-21G level, and 4-21G augmented with d -functions (4-21G*). It is shown that the experimental rotational constants of catechol can be reproduced by a geometry in which the Cue5f8O bond lengths are 1.409 and 1.395 A. This is in contrast to the results of a previous investigation [M. Onda,J. Mol. Struct., 159 (1987) 243] in which an exceedingly large difference of 0.08 A between the two Cue5f8O bonds was reported together with an unusually short Cue5f8O bond length of 1.323 A. In that investigation, the magnitude of the two Cue5f8O bonds depended on assumptions made in the data analysis, but the difference between them was found to be definitive on the basis of statistical criteria. In order to obtain an updated correction for the 4-21G Cue5f8C bonds, a new r o structure was refined for benzene ( r (Cue5f8C)=1.3974(3) A. r (Cue5f8H)=1.9788(16) A) using recently reported data from Bauder [J. Mol. Struct., 190 (1988) 31].

Electronic and Photophysical Properties of Platinum(II) Biphenyl Complexes Containing 2,2′-Bipyridine and 1,10-Phenanthroline Ligands

Pt(bph)(bpy) and Pt(bph)(phen), where bph is the 2,2-biphenyl dianion, bpy is 2,2-bipyridine, and phen is 1,10-phenanthroline, crystallize in the space groups I4(1)/a and P2(1)/c, respectively, in two different configurations as X-shaped and bowed (B). The distance between Pt centers is 3.5 Å indicative of π-π stacking. The complexes are optically active, absorb light at 440 nm, and emit in the solid state at room temperature and in the solid glass phase at 77 K. The emission maxima for both in the glass occur near 581 nm but are red-shifted to ∼700 nm in the solid state. Both complexes exhibit solvatochromism in nitrile-based solvents with the Pt(bph)(phen) complex showing greater excited state dipole character compared to the Pt(bph)(bpy) derivative. Frontier orbitals for the HOMO determined by DFT calculations contain electronic contributions from the biphenyl ligand and the platinum center. The LUMO orbitals primarily reside on the diimine ligands. TDDFT calculations indicate the low-energy transitions occur from the metal/bph combination to the diimine ligand.

Ab initio studies of structural features not easily amenable to experiment: Part 71. Conformational analysis and structural study of valine and threonine

Abstract The molecular structures of valine and threonine were studied by ab initio gradient optimization at the 4-21G level. The conformational effects associated with the χ1 [C(α)-C(β)] torsion were determined in valine. Conformationally dependent changes of several hundredths of an angstrom were found for bond distances, and up to 5 ° for bond angles, and the χ1 potential was found to be asymmetric. Ten structures were optimized for threonine, displaying different types of hydrogen bonding, including polycyclic networks established by hydrogen bonds in some forms. Quantitative differences in bond distances and angles are given for diastereoisomers of threonine which differ in chirality at the β-carbon.

Ab initio studies of structural features not easily amenable to experiment: Part 69. Conformational analysis and structural study of cysteine

Abstract The geometries of fourteen conformations of serine were determined by ab initio gradient geometry optimization using the 4-21G basis set. It is found that it is possible to design conformations with significantly different features but similar energies. Simultaneous H-bonding between different groups in the molecule is an important aspect of serine conformational stability and leads to the formation of bi-, tri- and tetra-cyclic structures. Heavy atom bond distances, such as C(α)-N, C(α)-C(β), and C(α)-C′ display conformationally dependent structural variations of up to 0.02 A. Corresponding angles, C(β)-C(α)-N, and C′-C(α)-N range from 107 to 115° in the investigated series.

Ab initio studies of structural features not easily amenable to experiment: Part 67. The 4-21G optimized structure of a novel cage dimer, C22H24, and comparison with its crystal structure

Abstract The geometry of the cage dimer, C22H24(1), was determined by ab initio gradient optimization on the 4-21G level. A previously described multiplicative integral approximation (MIA) was used together with the direct SCF approach. MIA is effective in determining the geometries of large molecules, at the same time achieving the accuracy of conventional SCF methods. The calculations determine the unperturbed bond distance and angle patterns of 1 with a resolution which is currently not afforded by any experimental method. The relative C-C“ bond distances show the expected correlations with C-C-C”-C torsions and are in good agreement with X-ray crystallographic parameters, but crystal-packing effects are noticeable at the 0.01-A level; this is less than the packing effects on structures previously found for molecules with higher dipole moments. Thus, comparisons of this kind may provide estimates of the extent to which stronger intermolecular interactions in the crystal lead to larger deviations of the crystal structure from the isolated equilibrium structure. It is a special feature of 1 that the local geometry of the sp2 carbons which link the two cages are asymmetric and pyramidal.

HOMO-LUMO energy gap control in platinum(ii) biphenyl complexes containing 2,2'-bipyridine ligands.

A series of platinum(ii) biphenyl 2,2-bipyridine complexes containing electron-donating and electron-withdrawing moieties on the 4 and 4 positions of the bipyridine ligand exhibit emission from excited states in the 600 nm region of the spectrum upon excitation in the metal-to-ligand charge transfer transition located near 450 nm. These complexes are distorted from planarity based on both single crystal structure determinations and density functional theory (DFT) calculations of isolated molecules in acetonitrile. The DFT also reveals the geometry of the lowest-lying triplet state (LLTS) of each complex that is important for emission behavior. The LLTS are assigned based on the electron spin density distributions and correlated with the singlet excited states to understand the mechanism of electronic excitation and relaxation. Time-dependent DFT calculations are performed to compute the singlet excited state energies of these complexes so as to help interpret their UV-Vis absorption spectra. Computational and experimental results, including absorption and emission energy maxima, electrochemical reduction potentials, LLTS, singlet excited states, and LUMO and HOMO energies, exhibit linear correlations with the Hammett constants for para-substituents σp. These correlations are employed to screen complexes that have not yet been synthesized. The correlation analysis indicates that the electronic structure and the HOMO-LUMO energy gap in Pt(ii) complexes can be effectively controlled using electron-donating and electron-withdrawing moieties covalently bonded to the ligands. The information presented in this paper provides a better understanding of the fundamental electronic and thermodynamic behavior of these complexes and could be used to design systems with specific applications.

Structural study and conformational analysis of t-butyl ethyl ether by gas electron diffraction, ab initio calculations and vibrational spectroscopy

Abstract The molecular structure and conformational properties of t -butyl ethyl ether were studied by gas electron diffraction. Structural constraints of the data analysis were obtained from HF/4-21G ab initio geometry optimizations, and mean amplitudes of vibration and shrinkage corrections were calculated from vibrational spectra recorded for this analysis. At room temperature the compound exists in a conformational equilibrium of two forms, trans and skew, in a ratio of 70: 30 ± 24%. Subject to the constraints of the analysis, the following structural parameters were found for the trans conformer; r g (Oue5f8C) t -Bu = 1.436(3) A; r g (Oue5f8C) t -Bu − r g (Oue5f8C) Et = 0.014A (assumed); r g (Cue5f8C) t -Bu = 1.532(2)A (averaged value); r g (Cue5f8C) t -Bu − r g (Cue5f8C) Et = 0.008A (assumed); r g (Cue5f8H) = 1.121(3)A; ∠ α Cue5f8Oue5f8C = 119.9(12)°; ∠ α Cue5f8Cue5f8C t −Bu = 111.1(5)°; ∠ α Oue5f8Cue5f8C Et = 109.3(15)°; the tilt angle of the t -butyl group τ α = 4.5(8)°. Parenthesized values are error estimates (3σ) referring to the last significant digit. The effects of steric repulsion on the structural parameters are discussed.

Conformational geometry functions: additivity and cooperative effects

Abstract The conformational geometry changes associated with torsional motion about Cue5f8C single bonds are reported for some 47 basic organic molecules of the type (X,Y,Z)Cue5f8C(X′,Y′,Z′), and for the ,ψ space of the model dipeptides N-acetyl N′-methyl amides of glycine and alanine. All structures were determined by ab initio HF 4−21 G geometry optimizations. Employing natural cubic spline parameters, a program was written for use in empirical modeling parameter development, which calculates the bond lengths and angles of the reported compounds as functions of the associated torsional angles at any point in conformational space. The additivity of the conformational geometry functions is explored and illustrates how cooperative effects emerge in complex molecules. The results are instructive for procedures in which the properties of molecular fragments are used to derive force field parameters for the empirical modeling of complex molecules.

AB initio calculations of structural features not easily amenable to experiment: Part 68. Structural trends in some substituted nitriles and acetylenes

Abstract The geometries of a number of alkynes, nitriles, and dinitriles were determined by SCF gradient optimization using the 4-21G, 4-21G ∗ , 5-31G ∗ , and 3-321G ∗ basis sets. The systems investigated include: YH3-Cue5fb-C-X compounds, with Y = Si and C, and X = H, F, Cl, and Cue5fbN; the geminally disubstituted systems, 1,4-pentadiyne, H 2 C-(Cue5fcCH) 2 , and 1,1-dicyanocarbonyl, Oue5fbC(Cue5fcN) 2 ; X 2 Z-(Cue5fbN) 2 homologs with X = H, F, and Cl, and Z = Si and C; and various auxiliary structures (e.g. silylcyanide, methoxyacetylene, and S -methylacetylene) needed to establish structural trends with specific basis sets. The Cue5fbC bond lengths in Cue5fc C-X systems are found to decrease with increasing electronegativity of X. The Si-H bond lengths decrease in the series SiH 4 , SiH 3 -CN, SiH 2 -(CN) 2 in contrast to C-H bond lengths in the methyl homologs. The characteristic non-linearity of the C-Cue5fcC-H and Z-Cue5fcN (Z = Si and C) moieties in geminally disubstituted Y 2 Z(Cue5fcX) 2 systems is considered in detail and found to depend on the electronegativity of Y. The calculated structures are in good agreement with previously published experimental geometries, some of which were derived from a limited number of experimental data.

Ab initio studies of structural features not easily amenable to experiment: Part 73. Structure-conformation relations in some substituted ethane derivatives

Structure-conformation relations have been determined for a number of substituted ethane derivatives at the ab initio HF/4-21G level with standard gradient procedures. The systems studied include methyl ethyl ether (MEE), ethylene glycol, propane-1,2-diol, 2-fluoroethanol, N-methylethylenediamine, 2-aminoethanal, 2-hydroxyethanal, propanone-al, 1-amino-2-fluoroethane and 1-fluoro-1-chloro-2-ethanol. In each case the structures have been refined for different points of the main C-C torsional angle (C-O torsion for MEE). It is the purpose of this paper to augment the existing collection of 4-21G structure-conformation functions which, in the recent past, have found considerable utility in various applications. For reasons of completeness, conformationally dependent geometry trends calculated for fluoromethanol and formic acid (C-O torsions) are included in this paper.