Showing papers on "Molecule published in 1971"
01 Jan 1971
TL;DR: In this article, the experimental, theoretical, and empirical correlations between functional organic groups and the infrared spectrum were discussed and many examples of spectra of inorganic compounds in the solid phase were presented.
Abstract: This chapter discusses the experimental, theoretical, and empirical correlations between functional organic groups and the infrared spectrum. The application of infrared spectroscopy to the identification of inorganic compounds is less successful. In obtaining infrared spectra of inorganic solids, an experimental complication arises from possible chemical reaction between the inorganic compound and the infrared window material or support medium. The chapter presents many examples of spectra of inorganic compounds in the solid phase. The majority of these compounds are crystalline solids in which the crystallographic unit cell contains several polyatomic ions or molecules. Optical modes called lattice modes of vibration result from the motion of one polyatomic group relative to another within the unit cell. Lattice modes occur in the region 400–10 cm −1 and are characteristic of specific crystal geometry. They are used as fingerprints for an inorganic compound in much the same way as the internal modes of vibration of organic compounds are used in the region 4000–400 cm −1 .
1,578 citations
451 citations
TL;DR: Triatomic hydrogen positive ion surface crossing effects in chemical reactions based on potential energy surfaces calculation using diatomics-in-molecules approach were investigated in this paper, where the potential energy surface calculation was carried out using a graph-based approach.
Abstract: Triatomic hydrogen positive ion surface crossing effects in chemical reactions based on potential energy surfaces calculation using diatomics-in- molecules approach
384 citations
TL;DR: In this paper, it was shown that only slight changes in rotation occur on changing the solvent for conformationally rigid molecules, and the magnitude of the anomeric effect was influenced as expected by changes in the polarity of the solvent but hydrogen bonding with the acetal oxygen atoms had a more pronounced effect.
Abstract: The empirical rules for estimating molar rotations of carbohydrate structures proposed by Whiffen and elaborated by Brewster provided interpretations of solvent effects on optical rotation with changes in conformational equilibria which in certain cases were confirmed by n.m.r. Conversely, it was found that only slight changes in rotation occur on changing the solvent for conformationally rigid molecules. Thus, studies involving optical rotations measured at the D-line of sodium and n.m.r. spectra provided the experimental support for the following solvation phenomena which are related to the role of unshared pairs of electrons in conformational equilibria. (a) The reverse anomeric effect was substantiated by the effect of introducing a positive charge on the imidazole ring of certain N-glycosides of imidazole. (b) The magnitude of the anomeric effect was influenced as expected by changes in the polarity of the solvent but hydrogen bonding of the solvent with the acetal oxygen atoms had a more pronounced effect. (c) The orientation of oxygen atoms in gauche relationship appears particularly favourable with water as solvent. (d) An intramolecular hydrogen bond between two hydroxyl groups is strengthened by hydrogen bonding of the free hydrogen to a basic solvent. le) The non-bonded interaction between two opposing axial oxygen atoms is dependent on the nature of the substituents. The repulsion is substantially greater when the oxygen atoms are either bonded to methyl groups or hydrogen-bonded to the solvent than when attached to acetyl, benzoyl or methane-sulphonyl groups. Tim first statement that unshared pairs of electrons may play an important role in establishing conformational preferences was made by Edward' with reference to the apparent stability of that anomeric form for a glycosyl halide which has the halogen in axial orientation. The statement made was, as seen in Figure 1, that by going axial the polar C, to X bond avoids an interaction with the axially oriented orbital of the ring oxygen. Later this was discussed by Kabayama and Patterson2 who pointed out that the unfavourable interactions could, indeed, involve repulsions between the orbitals occupied by lone pairs of electrons in the aglycon X with those of the ring oxygen. Such interactions equivalent to syn-axial interactions, would be released on passing from the equatorial to axial anomer.
285 citations
TL;DR: The symmetry properties of molecular orbitals and of reaction coordinates can be used to decide on the feasibility of selected chemical reaction mechanisms as mentioned in this paper, where reaction paths are shown to have a large energy barrier and are said to be "forbidden by orbital symmetry".
Abstract: The symmetry properties of molecular orbitals and of reaction coordinates can be used to decide on the feasibility of selected chemical reaction mechanisms. Some reaction paths are shown to have a large energy barrier and are said to be “forbidden by orbital symmetry.” The reactions of molecules with no symmetry can also be analyzed by being compared to related symmetric molecules, where the molecular orbitals are topological identical.
247 citations
TL;DR: A computer treatment is developed for determining the conformations of flexible molecules, such as nucleotides, in aqueous solution.
Abstract: A computer treatment is developed for determining the conformations of flexible molecules, such as nucleotides, in aqueous solution.
244 citations
TL;DR: The electron density map has been calculated at 6 A resolution for a human γG1 immunoglobulin molecule and is interpreted in terms of four possible models, of which a T-shaped model is favored.
225 citations
TL;DR: The synthetic macrocycles represent an intriguing new area of coordination chemistry, the systematic study of which should lead to many interesting and useful chemical applications in the field of metal complexation in solution.
Abstract: The existence of synthetic macrocyclic molecules with hydrophilic cavities containing multiple binding atoms and with hydrophobic exteriors gives rise to extraordinary possibilities with respect to the design and synthesis of molecules with specific cation and anion binding properties. The preparation of many new macrocyclic compounds has recently been reported, but few practical applications for them have been suggested. From the information available, it is becoming clear that it should be possible to synthesize macrocycles that will have specified, or selected, ion binding properties. Cavity size can be varied to accommodate only those cations or anions within a specified narrow band of sizes. Numbers and types of coordinating atoms can be chosen to give essentially electrostatic or covalent bonding or a combination of the two in a metalmacrocycle complex. The metal ligand bond appears to be predominantly ionic in the case of the cyclic polyethers but the covalent character increases on substitution of sulfur or nitrogen for oxygen donor atoms. The essential hydrophobic exteriors of the macrocycles can be modified by the addition of side chains and groups to facilitate the solution of anions and cations in organic solvents. The structures of many macrocycles can be made to approximate naturally occurring molecules, that is, cyclic polyethers similar to macrocyclic antibiotics of the valinomycin and nonactin types and cyclic polyamines similar to porphyrins. Macrocycles are also useful as model compounds for the study of metal interactions with biological systems. The synthetic macrocycles thus represent an intriguing new area of coordination chemistry, the systematic study of which should lead to many interesting and useful chemical applications in the field of metal complexation in solution.
218 citations
TL;DR: In this paper, the N.M. spectrum of o-chlorotoluene, dissolved in the nematic phase of 4-methoxybenzylidene-4-amino-α-methylcinnamic acid n-propylester, was analyzed and the shape of the proton skeleton determined.
Abstract: The N.M.R. spectrum of o-chlorotoluene, dissolved in the nematic phase of 4-methoxybenzylidene-4-amino-α-methylcinnamic acid n-propylester is analysed and the shape of the proton skeleton determined. The position of lowest energy of the potential barrier to methyl rotation is found to be that in which the chlorine atom is staggered to the methyl group, and the height of the barrier is of the order of 5·0 kJ mol-1 (1200 cal/mole).
207 citations
TL;DR: In this paper, the presence or absence of a double-well potential depends on the repulsive or attractive nature of the A−H1 and H1−B potentials, depending on the relative position of the molecular fragments A and B, and on the attractive or repulsive interaction of the fragments B and A. In particular, it is suggested to use the designation "hydrogen bridge" as a more general designation for what is commonly referred to as hydrogen bond.
Abstract: In this work we analyze molecular complexes with two hydrogen bonds (A=H2=B) and with three hydrogen bonds (A≡H3≡B). In an introductory discussion of molecular complexes with a single hydrogen bond (A–H1–B), it is argued that the presence or absence of a double‐well potential depends primarily on (a) the repulsive or attractive nature of the A–H1 and H1–B potentials, (b) on the relative position of the molecular fragments A and B, (c) on the attractive or repulsive interaction of the fragments A and B. Since the A–H1–B complex can be stable if one of the potentials for A–H1 or H1–B is repulsive, provided that the attraction of A and B compensates the above repulsions, it is suggested to use the designation “hydrogen bridge” as a more general designation for what is commonly referred to as “hydrogen bond.” The guanine–cytosine (G–C) base pair is taken as an example of three hydrogen bridges (h–b's). A number of computations were performed to assess the shape of the potential curve for the h–b in the G–C pa...
196 citations
TL;DR: From the morphology of the polymers it could be concluded that the four subunits of the avidin molecules were arranged with 222 symmetry and that they were grouped in two pairs at opposite ends of the short axis of the molecule whose dimensions were 55Ax55Ax41A.
Abstract: A series of bisbiotinyl diamines was synthesized with between 9 and 25 bonds between the carboxyl groups of the two biotin residues. It was found that only one of the two biotin residues could combine with avidin when there were fewer than 12 bonds between the biotin residues. Compounds with longer chains behaved in a bifunctional manner and gave rise to linear polymers of avidin, which were characterized by electron microscopy and by gel filtration. The polymers formed with the shorter-chain reagents (12, 13 or 14 bonds) were relatively unstable and could be depolymerized by weakly bound analogues of biotin. The polymers of longer-chain reagents were not depolymerized under these conditions and were only slowly affected by added biotin. When the chain length of the reagent reached 23 bonds the polymers became much shorter, suggesting that the reagent was now able to link two subunits within the same avidin molecule. From the morphology of the polymers it could be concluded that the four subunits of the avidin molecules were arranged with 222 symmetry and that they were grouped in two pairs at opposite ends of the short axis of the molecule whose dimensions were 55A×55A×41A.
TL;DR: The positions of the five hydrogen atoms in the mineral brushite, CaHPO4,2H2O, which crystallizes with Z = 4 in the non-centrosymmetric monoclinic space group la(Cs4), have been established by neutron-diffraction analysis based on 632 independent reflexions from synthetic crystals as mentioned in this paper.
Abstract: The positions of the five crystallographically independent hydrogen atoms in the mineral brushite, CaHPO4,2H2O, which crystallizes with Z= 4 in the non-centrosymmetric monoclinic space group la(Cs4), have been established by neutron-diffraction analysis based on 632 independent reflexions from synthetic crystals. Anisotropic three-dimensional least-squares refinement of all the atoms in the structure to R 0·033 gives final estimated standard deviations of 0·002 for calcium ions, ca. 0·006 for phosphorus and oxygen atoms, and 0·010–0·015 A for hydrogen atoms. The final Fourier difference synthesis shows no significant spurious peaks and there is no evidence that any hydrogen atom occupies several sites; the anionic hydrogen is 1·00(1)A from O(1), which has the longest [1·608(6)A] P–O bond. The two water molecules co-ordinate somewhat differently; both are essentially coplanar with the oxygen atoms to which they are hydrogen bonded, but water (2) has an unusually long bond of 3·09 A to water (1). The other four distinct hydrogen bonds in the structure, which closely resembles that of the arsenic analogue, pharmacolite, have O ⋯ O lengths ranging from 2·68 to 2·83 A.
TL;DR: In this paper, the electron diffraction patterns of gaseous acetic acid at 160 °C for the monomer and at 24 °C to obtain the molecular geometry were obtained (first value for monomer, second for dimer, standard deviations in parentheses).
TL;DR: The capacity of humic acids to bind metal ions is, at most, equal to the number of titratable H+-ions divided by the valency of the concerning metal ion as mentioned in this paper.
TL;DR: In this paper, the effect of paramagnetic Mn 2+ and Eu 3+ ions on the NMR spectra of sonicated lecithin dispersions in water and benzene has been investigated.
TL;DR: In this paper, the influence of a molecule's dipole moment on its ability to capture an electron into a stable bound state is examined, and it is proved that, in the fixed-nuclei approximation, a value greater than 1·625 Debye for the Dipole moment suffices to guarantee the existence of a discrete spectrum of negative ion states.
Abstract: The influence of a molecule's dipole moment on its ability to capture an electron into a stable bound state is examined. It is proved that, in the fixednuclei approximation, a value greater than 1·625 Debye for the dipole moment suffices to guarantee the existence of a discrete spectrum of negative ion states. Implications for Hartree-Fock calculations of negative ions are discussed. The interaction of electronic, vibrational and rotational motions in negative dipolar ions is studied, and conclusions are drawn for real molecules.
TL;DR: In this article, the stretching vibrations of water in montmorillonite, hectorite, saponite and vermiculite are split into two components, similar to those seen in perchlorate solutions.
Abstract: The stretching vibrations of water (H2O, HDO and D2O) in montmorillonite, hectorite, saponite and vermiculite are split into two components, similar to those seen in perchlorate solutions. Examination of lower hydrates and pyridine complexes of the layer silicates shows that the higher frequency component corresponds to hydrogen bonds to oxygens of Si—O—Si linkages, the lower frequency component to water-water bonds and hydrogen bonds to oxygens of Al—O—Si linkages. The observations are explained in terms of chains of hydrogen-bonded water molecules which form dielectric links between interlayer cations and oxygens on the silicate anion surface. This concept, together with information obtained on the distribution of charge on the surface oxygens, provides a qualitative explanation of the hydration properties of layer silicates, and is extended to account for the stability of organic complexes of montmorillonite and hectorite. Some analogies between interlayer complexes and ionic solutions are proposed.
TL;DR: In this paper, the angular dependence of pure resonance scattering is rather flat and not in accordance with the simplest theoretical model, and the rotational structure of the resonance in a somewhat extended model has been performed.
Abstract: Abstract Elastic scattering, vibrational excitation to v=1, 2, 3, 4 of the electronic ground state, and electronic excitation to the states a1Δ g and b1Σg+ of O2 have been measured in a crossed beam apparatus for collision energies from nearly 0 eV to 4 eV. Differential and integral cross sections have been determined and calibrated on an absolute scale. From 15 vibrational levels of O2-, which could be observed as resonances in the cross sections, the spectroscopic constants for the vibrational structure of O2- have been derived: ωe = 135 meV and ωeχe = 1 meV. The cross sections for vibrational excitation have the order of 10-18 cm2. eV for the larger resonance peaks. Detailed cross sections have been listed in Table 1. The half width of the resonance can be estimated to Γ ≈ 0.5 meV, which corresponds to a lifetime tof 10-12 sec for the O2- states. The angular dependence of pure resonance scattering is rather flat and not in accordance with the simplest theoretical model. An analysis of the angular dependence and of the rotational structure of the resonance in a somewhat extended model have been performed. - No electronically excited O2-states could be detected in the energy range up to 3 eV.
TL;DR: In general, physical and chemical results, as well as the base homologies in different tRNA molecules, support Holley's “clover leaf” model and it should be possible to determine molecular conformations in solution by studying the hydrogen bonds directly.
Abstract: SINCE the determination of their primary sequence the secondary and tertiary structures of tRNA molecules have been studied by various techniques (reviewed in ref. 1). In general, physical and chemical results, as well as the base homologies in different tRNA molecules, support Holley's “clover leaf” model2. There is also evidence of tertiary structure, and different models have been suggested3. Because both secondary and tertiary structures depend on hydrogen bonding, it should be possible to determine molecular conformations in solution by studying the hydrogen bonds directly.
TL;DR: In this paper, the first experimental determinations of orbital energy spectra of electrons in chemisorption bonds were presented, and a transition density function which is essentially the local density of states in the surface region of the solid was determined.
Abstract: This work presents the first experimental determinations of orbital energy spectra of electrons in chemisorption bonds. Our method, ion neutralization spectroscopy, determines a transition density function which is essentially the local density of states in the surface region of the solid. For the particular surface formed by chemisorption of the chalcogens in ordered, surface‐crystalline arrays on Ni(100), the local density of states includes a strong component from the orbital energy spectrum of the electrons in the bonds of the surface molecules formed from the adsorbate atom and Ni atoms presented by the substrate lattice. The orbital levels of electrons in the surface bonds are broadened resonances or virtual states in whose energy range local wavefunction magnitude and hence transition probability for ion neutralization are enhanced. These orbital energy spectra, coupled with measurement of work‐function change on adsorption, suggest, in many instances, plausible identifications of local bonding str...
TL;DR: In this article, the potential functions for rotation about the CC single bonds in glyoxal, 1,3-butadiene, benzaldehyde, biphenyl, and 2,2′-difluorobiphenyl have been calculated by the CNDO/2 method.
TL;DR: In this paper, the atomic beams of potassium and rubidium with oxygen molecules at high dilution in argon together on a salt window at 15°K produces the KO2 and RbO2 molecules, as identified by infrared spectra.
Abstract: Deposition of atomic beams of potassium and rubidium with oxygen molecules at high dilution in argon together on a salt window at 15°K produces the KO2 and RbO2 molecules, as identified by infrared spectra. Secondary reaction yields the KO2K and RbO2Rb molecules. Oxygen isotopic mixtures and simultaneous deposition of two different alkali‐metal atoms verify the molecular composition. Detailed concentration changes and oxygen isotropic mixtures identify the new molecular species O2KO2 and O2RbO2, the potassium and rubdium disuperoxide molecules which likely have the D2d structure. Force constants for the molecules synthesized here are compared with those reported earlier for LiO2 and NaO2.
TL;DR: Atomic coordinates and backbone torsion angles are tabulated for the active form of subtilisin BPN′ (or Novo) and for well defined solvent molecules inside the structure and in the neighborhood of the catalytic site.
TL;DR: In this article, the potential surfaces for CC stretch and CH2 twisting of ethylene obtained by the combined SCF CI method lead to the prediction that nonvertical transitions are responsible for the absorption maximum in the VN bands and that the broad diffuse nature of the spectrum in this region could well be caused by the proximity of two states of the same symmetry of the twisted molecule.
TL;DR: In this paper, the existence of phase transitions in several kinds of two-component lattice gases has been proved, some of which are isomorphic to spin systems and/or to fluids composed of asymmetrical molecules which can have different orientations.
Abstract: We prove the existence of phase transitions in several kinds of two‐component lattice gases: Some of these are isomorphic to spin systems and/or to fluids composed of asymmetrical molecules which can have different orientations. Among the models studied is one with infinite repulsion between particles of different species (hard cores), extending over arbitrarily many neighboring lattice sites. Some of these systems have been investigated previously in the mean field approximation and numerically.
TL;DR: In this article, the authors show that with water, acetone and 3-pentanone, expansion of the montmorillonites increased in a step-wise manner with increasing numbers of interlayer cations qualitatively in accord with the field strength of the cations and the dipole moments of the molecules.
Abstract: Li-, Na-, K- and Ca-saturated Wyoming montmorillonites have been prepared and used to obtain Li.Na-, Li,K-, and Li,Ca-montmorillonites with a range of Li contents. These were heated at 220°C for 24 hr, causing the Li+ ions to migrate mainly into the layer structure and leaving varying amounts of Li+, Na+, K+, and Ca2+ ions in the interlayer positions as determined by exchange with NH4+ ions. The results are only partially consistent with a migration of the Li+ ions into vacant octahedral sites up to the limit of the octahedral layer charge. Solvation of the resulting clays with water and various organic liquids showed the following results: With water, acetone and 3-pentanone, expansion of the montmorillonites increased in a step-wise manner with increasing numbers of interlayer cations qualitatively in accord with the field strength of the cations and the dipole moments of the molecules. With ethanol, ethylene glycol and morpholine, swelling with each liquid was practically independent of the number of interlayer cations, within the limits of the prepared materials. It is suggested that for the second group of liquids some mechanism additional to cation-dipole interactions, such as hydrogen bonding to silicate oxygen surfaces, may play an important part.
TL;DR: It is concluded that the O -methyloxime derivative effectively “labels” the carbonyl portion of the surgar, and probably contributes to the stability of ions containing that part of the molecule.
TL;DR: The three-dimensional structure of the disodium salt of adenosine triphosphate (Na 2 ATP) has been determined from an X-ray diffraction study to a resolution of 0.9 A as discussed by the authors.
Abstract: The three-dimensional structure of the hydrated disodium salt of adenosine triphosphate (Na 2 ATP) has been determined from an X-ray diffraction study to a resolution of 0.9 A. The crystals are orthorhombic, space group P 2 1 2 1 2 1 , with a = 30.45(4), b = 20.88(3), c = 7.07(1) A. There are two molecules of ATP, four sodium ions, and six water molecules in the asymmetric unit. The structure was solved by direct methods and refined to R = 12.3 % using 1118 intensities measured on an automatic diffractometer. The triphosphate chain is in the folded conformation in each of the two crystallographically independent molecules. However, in one molecule (A) it is folded so as to form part of a lefthanded helix, while in the other part of a right-handed helix. There are corresponding differences in the conformations of the ribose rings. The ring is in the envelope conformation with C39 endo in molecule A and C29 endo in molecule B. Two of the sodium ions coordinate the two molecules through the phosphate oxygens and N7 to form an almost centrosymmetric ‘dimer’ which is the fundamental structural unit. The adenine bases show considerable overlap and are stacked in the c axis direction. Details of the hydrogen bonding and the role of water molecules in the structure are discussed.