A comparative ab initio study of the structures,dipole moments,force fields,and anharmonic frequencies of formamide and thioformamide

The structures, dipole moments, force fields, and anharmonic frequencies for the planar conformation of formamide and thioformamide were calculated using the unscaled 4-31G basis set, augmented with a full set of d functions on the sulfur, and full geometry optimization. Extensive comparison of the geometries are made, especially the CO and CS bond lengths, with both the experimental values for the amides and values calculated in previous studies on the acids and other carbonyl compounds. Comparison of the dipole moments calculated using the optimized and experimental geometries with the experimental values suggest there is some inconsistancy in the experimental geometry for thioformamide. Quadratic, cubic, and quartic force constants are calculated for both amides, and hence the fundamental vibration frequencies. Critical comparisons are made with the assignments based on experimental observations. Differences in the bond lengths and stretching force constants for the two NH bonds are shown to be consistent with a hydrogen-bonding type of interaction between the proximal NH and CO and CS groups, like that in the acids.     

Evolution of enzymes and pathways for the biosynthesis of cofactors

The evolution of metabolic pathways is discussed with reference to the biosynthesis of a number of vitamins and cofactors. Retrograde and patchwork models are highlighted and their relevance to our knowledge of pathway processes and enzymes is examined. Pathway complexity is explained in terms of the acquisition of broad specificity enzymes.     

Elementary particle physics from general relativity

This paper presents a qualitative comparison of opposing views of elementary matter—the Copenhagen approach in quantum mechanics and the theory of general relativity. It discusses in detail some of their main conceptual differences, when each theory is fully exploited as a theory of matter, and it indicates why each of these theories, at its presently accepted state, is incomplete without the other. But it is then argued on logical grounds that they cannot be fused, thus indicating the need for a third revolution in contemporary physics. Toward this goal, the approach discussed is one of further generalizing the theory of general relativity in a way that incorporates the inertial manifestations of matter in covariant fashion, with quantum mechanics serving as a low-energy, linear approximation. Such a theoretical extension of general relativity will be discussed, with applications in elementary particle physics, such as the appearance of mass spectra in the microdomain, as an asymptotic feature of matter, mass doublets (electron-muon and proton-heavy proton), the explanation of pair annihilation and creation from a deterministic field theory, charge quantization, and features of pions.This paper is based on a seminar given at the International Centre for Theoretical Physics, Trieste, Italy, in the summer, 1980. I thank Prof. Abdus Salam and the faculty of the Centre for their hospitality in this period.     

On Einstein's later view of the twin paradox

It is shown that Einstein abandoned his earlier view that there are material consequences, such as asymmetric aging, implied by the space-time transformations of transformations of relativity theory.     

An AB initio study of the cis- and trans -conformers of 1,3-butadiene,acrolein and glyoxal: evidence for a stabilizing interaction in cis-acrolein

The cis- and trans-conformers of 1,3-butadiene, acrolein and glyoxal have been studied using the 4-31G and (7, 3) basis sets with full geometry optimization. The changes in geometry in going from the cis- to the trans-structures indicate that the terminal carbon and oxygen atoms in cis-acrolein are closer together than would otherwise be expected from the butadiene and glyoxal data, and that the hydrogens of the CH₂ group are also affected. These results, together with a comparison of the changes in force constants for key structural elements, suggest that some sort of attractive interaction is present in cis-acrolein. ΔE_T, ΔE_K, ΔV_ee, ΔV_nn and ΔV_en are reported for the cis-trans isomerizations, the special geometrical features of the cis-conformer being reflected in ΔV_nn for the acrolein reaction. The positive ΔE_T values for the disproportionation reactions of both conformers2 acrolein → 1,3-butadiene + glyoxal show that the acrolein framework provides the most stable combination of the H₂C, CH- and O structural elements, while the bigger value for the cis-conformers affords additional evidence that the interaction in cis-acrolein is stabilizing in nature. However, in going from the trans- to the cis-structure neither the change in f_C-H,C-H for the C-H that would be involved in an H- bond, nor the change in charge on the atoms in the C-H ? O unit. based on Mulliken population analyses, are in accord with what would be expected for a hydrogen-bonding type of interaction. Instead the stabilization may be a manifestation of the different pattern of charge distribution along the polyene chain of acrolein, i.e. with the ends oppositely charged, compared to butadiene and glyoxal, in which the ends carry the same charge.     

A theoretical study of the structure and spectrum of 1,3,5-hexatriene,Part II

The results of ab initio STO-3G and STO-4G calculations are reported on tTt- and tCt-1,3,5-hexatriene, including a STO-3G partial geometry optimization of all C/C bond lengths in both isomers. Results are also reported from a partial INDO geometry optimization on.the tTt-isomer, and complete optimizations on both isomers using empirical consistent force Meld programs. Significant discrepancies are found to exist between these calculated structures and those based on the electron diffraction analysis of Traetteberg. In addition, further results of a CNDO/S spectral analysis are presented and compared with experiment.     

An ab initio study of the geometry and energy of six planar conformers of β-hydroxyacrolein

Ab initio calculations with full geometry optimization have been carried out on the planar cCc, cTc, tTc, tCt, tTt, and cCt conformers of β-hydroxyacrolein using the 4-21G basis set, and on the cCc and cCt conformers using the 4-31G basis set. The hydrogen-bonded cCc conformer is the most stable and the cCt conformer the least stable, with the other conformers following the above sequence. β-Hydroxy substitution has scarcely any influence on the geometry of the trans-acrolein structure, whereas the geometry of the cis-acrolein structure shows significant changes which depend on whether the O? H group is cis or trans with respect to the CHO group about the C?C bond. The ΔE_T values for cis → trans isomerization about the C? C bond in cCt and cTc support the hypothesis that these changes in geometry are the result of a destabilizing interaction in cCt and a stabilizing interaction in cTc. The geometry of the hydrogen-bonded structure cCc sets it apart from all the other conformers: it has by far the longest C?C, the longest C?O, the longest O? H, the shortest C? C, and the shortest C? O. Its formation from cCt involves a lengthening of C?C, C?O, and O? H and a shortening of C? C and C? O, indicating a delocalization of charge within the ring. 4-21G calculations have also been made for a distorted cCt structure that has the same bond lengths and angles as the equilibrium cCc structure, and the distortion energy, cCt (equm. geom.) → cCt (distorted geom.), is found to be +13.1 kJ mole^?1. Taking the energy of this distorted cCt structure as the baseline, the hydrogen-bonding energy in cCc is found to be —80.3 kJ mole^?1.     

An ab initio study of the relationship between the O?H bond length and the harmonic and anharmonic stretching force constants for planar molecules containing C?OH,N?OH,and O?OH groups

The O? H bond length and the quadratic, cubic, and quartic stretching force constants, calculated ab initio using the unscaled 4-31G basis set with full geometry optimization, are reported for 30 planar conformers of ten molecules contaning either the C? OH, N? OH, or O? OH group. The data are analyzed in terms of the general form of Clark's equation, and the power functions and exponential functions proposed by Herschbach and Laurie. In the case of the quadratic constants, significant trends are found in the values of the parameters depending on whether the O? H group is bonded to carbon, nitrogen, or oxygen, and whether it is non-hydrogen-bonded or involved in intramolecular hydrogen bond formation in four-, five-, or six-membered rings. Using data for diatomic molecules, O? H, and C? H bonds, and the C?O and C? C bonds in planar monosubstituted carbonyl compounds, the parameter d_ij in the power function equation for quadratic constants, which can be regarded as the distance of closest approach of the two nuclei, is shown to increase progressively along the series (i) diatomic molecule; (ii) similar bond in a polyatomic environment with one of the two atoms covalently bonded to a neighboring atom; (iii) as in (ii) but with the second atom hydrogen bonded; and (iv) with both atoms covalently bonded to neighboring atoms.     

A reassessment of some restricted Hartree–Fock limit molecular energies and an investigation of the applicability of Ermler and Kern's procedure for their estimation

New estimates of Hartree–Fock limit energies (E_RHF) for selected AH and AH_n hydrides, diatomic and linear polyatomic molecules have been made utilizing E_SCF values recently reported in the literature for HF, N₂, CO, NH₃, and CH₄ which are very close to the respective limits. These new values have been used to investigate the applicability of Ermler and Kern's procedure for estimating E_RHF: i.e., a factor f is first evaluated from data for reference molecules, where f = E_RHF/E_SCF, which is then used with E_SCF values for other molecules to obtain their E_RHF values. f has been evaluated for three groups of reference molecules? HF, H₂O, NH₃, CH₄, N₂, and CO; CH₄, C₂H₂, C₂H₄, and C₂H₆; and C₂H₂, HCN, and N₂? utilizing E_SCF data in the literature for many Gaussian-type orbital (GTO) basis sets together with some new values calculated at the (9,5,1) to (13,8,2) levels. Trends in the variation of f within each group of reference molecules from one basis set to another, and the trends in f from one group of reference molecules to another, are discussed in detail. To minimize the influence of these effects in an E_RHF estimate it is recommended that the f value should be derived from reference molecules which possess a similar combination of structural features, i.e., bonded hydrogen, single, double, or triple bonds, and the number of lone-pair electrons. Further calculations show that an f value based on data for closed-shell molecules is not applicable to open-shell species.