Showing papers in "Journal of Molecular Structure in 1996"

Relationships between dissociation energies and electrostatic potentials of CNO2 bonds: applications to impact sensitivities

Abstract: : New data for nitroalkanes support earlier results lor nitroheterocycles indicating that C-NO2 dissociation energies are related to the computed electrostatic potential maxima on the molecular surfaces above these bond regions. This presumably explains the well-established involvement of the electrostatic potential in impact sensitivity correlations; a second factor in the latter, for unsaturated systems, is the internal charge separation in the molecule. jg p1

The missing conformers of glycine and alanine: relaxation in seeded supersonic jets

Abstract: Certain conformers of glycine and alanine that are predicted by ab initio calculations to be among those of the lowest energy have not been detected in jet spectroscopy studies of these amino acids, although others of similar or higher predicted energy are observed. Selective conformational relaxation of the missing conformers to lower energy species during the free jet expansion, broadly consistent with the criteria established by Ruoff et al. and by Felder and Gunthard, has been found to provide a satisfactory explanation. Relevant regions of the molecular potential energy hypersurface have been modelled via ab initio calculations [ MP 2 6–31 G(d,p) ] and these show that glycine 4 (see Fig. 3) and glycine 7 can relax to glycine 1 , leading to the conclusion, in accordance with observation, that only the latter and glycine 2 should be detected in the jet. In the case of alanine, the ab initio study of the molecular potential energy hypersurface has yielded revised data on predicted spectroscopic constants for some conformers. Because of the combined effects of relaxation and conformational coalescence through large-amplitude vibrational motion, the revised data do not change the interpretation of the previous spectroscopic study of alanine. The barrier separating conformers 2 and 3 (see Fig. 8) is predicted to be no more than 129 cm −1 , so that in addition to alanine 1 only a single additional coalesced conformer { 2 , 3 }, undergoing a large-amplitude motion in a double-minimum potential, is expected to appear in the jet spectrum rather than two separate species. The ab initio calculations on alanine indicate that conformers 4 and 5 are separated by a low energy barrier and a similarly low barrier separates 4 from the much lower energy conformer 1 . The resulting coalesced { 4 , 5 } is effectively a “ledge” on one wall of the potential well containing 1 , and is occupied via an excited vibrational state of 1 . In this light, the conformer { 4 , 5 } is not expected to be observed in the jet, as the vibrational relaxation of low-frequency vibrational modes is known to be very efficient in free jet expansions. Overall, no conflicts are found to remain between the experimental observations of jet spectra and the ab initio calculations for glycine and alanine, provided that the scope of such calculations is broadened from a narrower focus on equilibrium structures to consideration of the potential hypersurface. This approach should be important in the correct interpretation of the spectra of other multi-conformational species in free jet expansions.

Rheology of complex fluids measured by dynamic light scattering

Abstract: We introduce a method for using dynamic light scattering to measure the frequency-dependent linear viscoelastic moduli of complex fluids. The technique exploits the fluctuation dissipation theorem, which relates the relaxation of thermal excitations of a probe particle to the viscoelastic properties of the surrounding medium. The relaxation of the thermal excitations of probe particles are determined by measuring the time evolution of the mean square displacement using dynamic light scattering. A Langevin equation with a time-dependent damping term is used to relate this mean square displacement to the dynamic shear modulus of the medium. This method probes the linear viscoelastic moduli over a much larger frequency range than traditional mechanical means, and in particular, easily extends their measurement to much higher frequencies.

One-electron photoreduction of flavin mononucleotide: time-resolved resonance Raman and absorption study

Abstract: Resonance Raman and absorption spectra of the lowest excited triplet state T1, and semiquinone neutral and anion radicals of flavin mononucleotide (FMN) have been obtained by time-resolved Raman spectroscopy and laser flash photolysis. Both spectra have revealed that photoreduction of FMN is initiated by the process in which the T1 state of FMN abstracts an electron from substrates such as EDTA to generate the semiquinone anion radical FMN−· both in acidic and basic solutions over the pH range 5–9, and when the pH is smaller than 8.4, FMN−· is further converted into the semiquinone neutral radical FMNH· by combining with H+. The electron transformation is much faster in basic solutions than in acidic solutions. In more strongly acidic solutions pH

Alkali carbonates: Raman spectroscopy, ab initio calculations, and structure

Abstract: Equilibrium structures of Li 2 CO 3 and K 2 CO 3 were calculated using ab initio molecular orbital calculations carried out at the Hartree-Fock (HF) level. Of the four structures considered for Li 2 CO 3 and K 2 CO 3 , the most stable was a structure with all five atoms in a plane. The harmonic frequencies were also calculated and found to be in agreement with the present Raman measurements. Structure factors, calculated from the ab initio data for each of the four structures considered, are compared with existing X-ray results.

High temperature Raman study of phase transitions in antiferroelectric NaNbO3

Abstract: The Raman spectra of antiferroelectric NaNbO3 have been studied at temperatures up to 480°C. Two phase transitions were observed, one at 376°C in consistence with the previous observations, and the other being a new phase transition at 190°C. There is no evident hysteresis for this transition, which indicates that the phase transition is of second order.

Structural and dynamic properties of urea and thiourea inclusion compounds

Abstract: There is currently considerable interest in the chemistry and physics of solid inclusion compounds as a consequence of the wide range of important physico-chemical properties associated with them. In part, scientific interest in these materials is motivated by the desire to compare the structural, dynamic, and chemical properties of organic “guest” molecules embedded within different crystalline environments, and to investigate how the properties of the guest molecule may be influenced by the structural characteristics of its environment. This paper is focused upon urea and thiourea inclusion compounds containing a variety of different types of organic guest molecule. Aspects of the structural and dynamic properties of these inclusion compounds have been probed using solid-state NMR spectroscopy, X-ray diffraction, EXAFS spectroscopy, incoherent quasielastic neutron scattering, Raman spectroscopy, computer simulation and mathematical modelling. On the basis of the results from these investigations, the current level of understanding of the structural and dynamic properties of these materials is assessed.

Vibrational spectroscopic study of light-induced metastable states of ethylenediaminenitrosylruthenium(II) complexes

Abstract: Light-induced metastable states were found in trans - and cis -[RuCl(en) 2 NO]Cl 2 , trans - and cis -[RuBr(en) 2 NO]Br 2 , trans -[Ru(H 2 O)(en) 2 NO]Cl 3 , and a mixture of fac - and mer -[RuCl 3 (en)NO]. These states are formed when the polycrystalline samples are irradiated with blue or green laser lines from an Ar ion laser at 77 K. They decay upon irradiation with red light or a rise in temperature. The NO stretching frequency that is observed in the range from 1865 to 1904 cm −1 in the ground (normal) state of each compound shifts to the lower frequency side by approx. 100–140 cm −1 in the metastable states. These characteristics are similar to those of the metastable states of Na 2 [Fe(CN) 5 NO]·2H 2 O, K 2 [RuHCl 5 NO] and K 2 [Ru(NO 2 ) 4 (OH(NO)]. Decay temperatures of the ethylenediaminenitrosyl-ruthenium(II) complexes are notably higher (approx. 230–270 K) than those of the substances examined in earlier studies (approx. 180–190 K).

Recent aspects of the proton transfer reaction in H-bonded complexes

Abstract: Proton transfer processes cover a very wide range of situations and time scales and they are of great interest from the viewpoint of chemical reactions in solution. These processes can occur via thermally activated crossing or tunneling. This review considers various aspects of this many-faceted field. Spectroscopic, dielectric, colligative and energetic properties and structures of various species with H-bonds are examined. Proton transfer reactions in water and organic solvents, and the contribution of various H-bonded species and ions to these processes are discussed. Among other topics, this survey includes the effects of solvent, acid-base stoichiometry, concentration, temperature and impurity on proton transfer reactions in complexes of phenols and carboxylic acids with amines, pyridines and pyridine N -oxides. The contribution of the nonstoichiometric acid-base complexes and ionic species to the reversible proton transfer mechanism is discussed.

Symmetry, pseudosymmetry and hypersymmetry of organic crystals

Abstract: Topological aspects of the structure of molecular crystals and the distribution of organic crystals among structural classes (SCs) are considered. As the space group alone does not characterise a molecular arrangement sufficiently, the list of orbits occupied by molecules should be specified to give the SC. The method of symmetry of potential functions (MSPF) is proposed to explain molecular arrangements which occur in organic crystals and the distribution of these crystals among SCs. Some specific effects typical for molecular crystals are also considered, including the existence of pseudosymmetric subsystems of different types (island agglomerates, chains, layers, three-dimensional subsystems). Sometimes molecules retain their intrinsic symmetry, partially or fully, in groups of pseudosymmetry of such subsystems, while it is lost in the true space group. In polysystem crystals, where molecules occupy two or more orbits, one can usually (but not always) observe the phenomenon of hypersymmetry, i.e. the transformation of symmetrically independent molecules into each other by means of non-trivial (hypersymmetrical) operations. Genuine hypersymmetry cannot be reduced to pseudosymmetry (or local symmetry).

Neutron scattering characterization of homopolymers and graft-copolymer micelles in supercritical carbon dioxide

Abstract: Superficial fluids (SCF) are becoming an attractive alternative to the liquid solvents traditionally used as polymerization media [1]. As the synthesis proceeds, a wide range of colloidal aggregates form, but there has hitherto been no way to measure such structures directly. We have applied small-angle neutron scattering (SANS) to characterize such systems, and although SCF polymerizations are carried out at high pressures, the penetrating power of the neutron beam means that typical cell windows are virtually transparent. Systems studied include polymers soluble in CO 2 such as poly(1,1-dihydroperfluorooctyl acrylate) (PFOA), poly(hexafluoropropylene oxide) (PHFPO) and poly(dimethyl siloxane) (PDMS). PFOA has previously [2] been shown to exhibit a positive second virial coefficient ( A 2 ), though for PHFPO, A 2 appears to be close to zero ( 10 4 A 2 ⋍ 0 ± 0.2 cm 3 g −2 mol ). PDMS is soluble on the molecular level only in the limit of dilute solution and seems to form aggregates as the concentration increases (c > 0.01 g cm −3 ). Typical hydrocarbon polymers are insoluble in CO 2 , but it has been found that polymerizations may be accomplished via the use of a stabilizer [3]. For example, styrene has been polymerized in CO 2 by means of a polystyrene-b-PFOA block copolymer surfactant, which forms micelles in CO 2 and is also amenable to SANS characterization. Other amphiphilic surfactant molecules that form micelles include PFOA-g-poly(ethylene oxide) (PFOA-g-PEO) graft copolymers, which swell as the CO 2 medium is saturated with water. These systems have been characterized by SANS, by taking advantage of the different contrast options afforded by substituting D 2 O for H 2 O. This paper illustrates the utility of SANS to measure molecular dimensions, thermodynamic variables, molecular weights, micelle structures etc. in supercritical CO 2 .

Hydrogen-bonded structure and 13C NMR chemical shift tensor of amino acid residue carbonyl carbons of peptides and polypeptides in the crystalline state. Part I

Abstract: 13 C chemical shift tensor components ( δ 11 δ 22 and δ 33 ) of glycine (Gly), l -valine (Val), l -leucine (Leu), and l -asparagine (Asp) residue carbonyl carbons (C  O) of peptides and polypeptides covering a wide range of hydrogen-bond lengths ( R N…O ) in the crystalline state have been measured by slow magic-angle-spinning solid-state 13 C NMR. From these experiments, it is found that δ 22 , which lies approximately along the amide C  O bond, moves linearly downfield with a decrease in R N…O and the slope and intercept of the variation of δ 22 against R N…O depend on the amino acid residue. Using this relationship, the R N…O values for polypeptides were determined by observation of the δ 22 of the guest Gly residue incorporated into host polypeptides. δ 11 , and δ 33 are found to be insensitive to the change in R N…O and amino acid residues. Moreover, it is found that the sum of δ 11 and δ 33 is almost constant (337.5 ± 3.5 ppm) and is independent of the amino acid residue. The quantum-chemical calculation on the 13 C shielding constant for a peptide model compound was carried out by the finite perturbation theory within the INDO framework. This calculation acceptably explains the experimental results.

Structural behaviour of cobaloximes: planarity, an anomalous trans-influence and possible implications on Co-C bond cleavage in coenzyme-B12-dependent enzymes

Abstract: Deviations from planarity of the 17 non-hydrogen atoms forming the equatorial moiety in cobaloximes can be described by six significant distortion modes. No significant correlation has been found between these deformations and changes in the axial Co-C and Co-N distances. Analysis of the Co-R and Co-B distances reveals that, in a series of related RCo(DH)2B complexes, the two distances lengthen or shorten simultaneously but to different degrees. This anomalous structural trans-influence is quantified with the help of an additive scheme based on the nature of the axial ligands R and B, and is discussed in terms of a simple Huckel molecular-orbital model.

Stretching frequency vs. bond distance correlation of hydrogen bonds in solid hydrates: a generalized correlation function

Abstract: Supplementary uncoupled \ gn(OD) frequencies of isotopically dilute HDO molecules (from 75 K IR measurements) are reported for OH(D)⋯Cl and OH(D)⋯Br hydrogen bonds in solid hydrates that improve the statistical significance of the corresponding frequency vs. R(O⋯Y) distance correlations. The question of establishing a uniform frequency vs. distance correlation function that is approximately valid for all kinds of O⋯Y hydrogen bonds in solid hydrates is reconsidered. It is shown that the previous single correlation functions, which contain two physically meaningless Y-dependent regression parameters, can consistently be replaced by a uniform correlation function, which, besides general parameters, contains the electronegativity and an appropriately defined hydrogen bond contact radius, rHB (Y), of the Y-proton acceptors.

Infrared and Raman spectra, conformational stability, vibrational assignment, and ab initio calculations of chloromethyl isocyanate

Abstract: The infrared (3200 to 400 cm −1 ) and Raman (3200 to 20 cm −1 ) spectra of gaseous and solid chloromethyl isocyanate, CICH 2 NCO, have been recorded. Also, the Raman spectrum of the liquid and qualitative depolarization values have been obtained. Additionally, infrared spectra have been recorded at various temperatures with the sample dissolved in liquid xenon. An assignment of the fundamental vibrations based on the infrared band contours, depolarization values and group frequencies is given and discussed. This assignment is supported by a normal coordinate analysis utilizing a force field obtained from ab initio calculations with the RHF/6-31G ∗ basis set as well as with electron correlation, MP2/6-31G ∗ . A complete equilibrium geometry has been determined by ab initio gradient calculations employing the RHF/3-21G ∗ , RHF/6-31G ∗ and MP2/6-31G ∗ basis sets. The potential surface governing internal rotation about the C-N bond is calculated to be consistent with a single minimum corresponding to a structure having the chlorine atom cis or near-cis to the NCO moiety. These results are compared with the corresponding quantities for ethyl isocyanate, CH 3 CH 2 NCO, and related compounds.

Ab initio vibrational analysis of isotopic derivatives of aqueous hydrogen-bonded trans-N-methylacetamide

Abstract: Normal modes have been calculated for a CH 3 CONHCH 3 (H 2 O) 2 [NMA(H 2 O) 2 ] system and its CCD 3 , NCD 3 , 13 C 15 N, and ND derivatives. Fully optimized structures were obtained with the 6–31+ G ∗ basis set. The scale factors for the NMA force constants were obtained by optimizing the scale factors for the isolated molecule to experimental frequencies of aqueous NMA. HOH bend scale factors were found that are consistent with observed resonance Raman enhancements and infrared intensities of the components of the amide I doublet. Its frequency splitting can be reproduced quantitatively by a reaction field calculation. The calculated frequency difference between the two allowed NMA conformers confirms the assignment of the two observed amide II bands to the presence of these conformers in aqueous solution. An evaluation is made of the assignments of other bands.

Theoretical and experimental studies of the structure and vibrational spectra of NTO

Abstract: The structure and vibrational spectra of the high explosive 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) have been determined by ab initio molecular orbital calculations at the Hartree-Fock and second-order Moller-Plesset levels and by density functional theory (B3LYP). Experimental frequencies for the molecule have been determined from infrared spectra of pure NTO films and NTO molecules isolated in an argon matrix at 21 K. A force field for gas phase NTO has been obtained based on calculated results at the MP2/6–311G∗∗ level. In addition, a force field for solid state NTO has been constructed using the experimental vibrational frequencies for NTO films and scaled ab initio vibrational frequencies. Differences between the solid state and gas phase results indicate that the environment and preparation procedure exert a marked influence on the spectral characteristics of the NTO molecule.

The infrared spectra and structure of acetylsalicylic acid (aspirin) and its oxyanion: an ab initio force field treatment

Abstract: The structures of acetylsalicylic acid (aspirin) (I) and its oxyanion (II) have been studied by means of infrared spectra and ab initio 3–21 G force field calculations. The 3100-1100 cm −1 region bands of both the aspirin molecule and its oxyanion have been assigned. The theoretical infrared data for the free aspirin anion are in good agreement with the experimental data for aspirin alkali-metal salts in dimethyl sulfoxide- d 6 . The theoretical geometrical parameters for the isolated aspirin molecule are close to the literature X-ray diffraction data for its dimer in the solid state, except for those of the carboxy group, which participates directly in hydrogen bond formation. The changes in both the spectral and geometrical parameters, caused by the conversion of the aspirin molecule into the anion, are essential, but they are localized mainly within the carboxy group and the adjacent C-Ph bond. This is also true for the changes in the corresponding bond indices and electronic charges.

High-temperature vapor-phase Raman spectra and assignment of the low-frequency modes of trans-stilbene and 4-methoxy- trans-stilbene

Abstract: The vapor-phase Raman spectra of trans -stilbene at 330°C and 4-methoxy- trans -stilbene at 350°C have been recorded using a specially constructed high temperature cell. Together with the dispersed fluorescence spectra, these spectra have been used to assign the eight low-frequency vibrational modes in each molecule. For trans -stilbene, strong polarized bands at 273 and 152 cm −1 were ascribed to the totally symmetric ν 24 (C e -phenyl bend) and ν 25 (in-plane phenyl wag), respectively. A depolarized band at 207 cm −1 was assigned to the B g phenyl flap ( ν 47 ), and a polarized shoulder near 125 cm −1 is believed to result from both the B g phenyl torsion ( ν 48 ) and the overtone of the A u phenyl flap ( ν 36 ). For 4-methoxy- trans -stilbene the C e -phenyl bend and in-plane phenyl wag were shifted to 234 and 144 cm −1 , respectively. These assignments were critical for understanding the fluorescence spectra associated with the torsional motions.

In situ anomalous small-angle X-ray scattering and X-ray absorption near-edge structure investigation of catalyst structures and reactions

Abstract: Synchrotron radiation is applied to the combined anomalous small angle X-ray scattering (ASAXS) and X-ray absorption near-edge structure (XANES) in situ studies of platinum electrocatalysts, whose metal content is highly dispersed in the form of small crystallites on high surface area porous carbon supports. Combined measurements with X-ray energies near the Pt-L3 X-ray absorption edge have been performed in an electrochemical cell to study in situ the anodic oxidation process in cyclic voltammetry both for the anodic oxidized and reduced state of the platinum. The reduction process was monitored with XANES. From ASAXS, the sizes of the platinum catalyst particles have been found to increase during the oxidation process at about 1.1 V vs. Ag AgCl . From this increase one can infer a shell model with a platinum core and an oxide surface layer with a thickness of about 1 nm for the oxidized particles.

Electronic structures and spectra of two antioxidants: uric acid and ascorbic acid

Abstract: Electronic absorption and fluorescence spectra of aqueous solutions of two well known antioxidants, uric acid and ascorbic acid (vitamin C), have been studied at different pH. The observed spectra have been interpreted in terms of neutral and anionic forms of the molecules with the help of molecular orbital calculations. The N3 site of uric acid has been shown to be the most acidic. Fluorescence of uric acid seems to originate from an anion of the molecule in a wide pH range. Around pH 3, both the neutral and anionic forms of ascorbic acid appear to be present in aqueous solutions. In aqueous media, ascorbic acid appears to get converted easily to its dehydro form and this conversion does not seem to be reversible. An anion of dehydroascorbic acid seems to be formed on heating dehydroascorbic acid in aqueous solutions.

The significance of intra-molecular hydrogen bonding in the ß-O-4 linkage of lignin

Abstract: During our molecular mechanics investigation of the three-dimensional structure of lignin, the need for a more accurate model chemistry became apparent. Selected conformers of 2-phenoxy-1,3-propanediol, a simplified p-coumaryl s-O-4 model structure, were analyzed with both MM3(92) and ab initio (HF/6–31G∗) calculations. The poor agreement between these computational methods led to the proposal that hydrogen bonding to the aromatic ether oxygen changes the C AR O bond order which, in turn, is predicted to change the bond's rotational characteristics. Ab initio computations on methoxybenzene indicate a significant lowering of the rotation barrier due to an inter-molecular hydrogen-bonded water molecule. We suggest that, in the p-coumaryl s-O-4 linkage, intra-molecular hydrogen bonds from the α- and γ-hydroxyl groups to the ether oxygen effect the rotational characteristics, and thus the preferred orientation, of the aromatic ring.

The hydrogen bond as a design element in crystal engineering. Two- and three-dimensional building blocks of crystal architecture

Abstract: The specificity and the widespread occurrence of hydrogen bonds make them a special constructuion element in crystal engineering. A concise presentation of the chemical and crystallographic principles for controlled molecular association into structural building blocks, and their further organisation into three-dimensional crystal structures, is given and several examples are discussed.

Hydrogen/deuterium isotope effects on the 15N NMR chemical shifts and geometries of low-barrier hydrogen bonds in the solid state☆

Abstract: In this study, hydrogen/deuterium isotope effects on the geometry and the 15N NMR chemical shifts of strongly hydrogen bonded, solid hydrogen-bisisocyanide salts of the type [MCN⋯L⋯NCM]−X+ 1 (L  H, D) are described both experimentally and theoretically. The theoretical studies include PM 2 6–31+ G(d,p) ab initio calculations of geometries and subsequent calculations of the proton and the deuteron vibrational states in the hydrogen bond of the model compounds [CN⋯L⋯NC]−Li+ 1a (L  H) and 1b (L  D). The Li+, at various fixed CLi distances, represents the external electric field. Furthermore, the 15N NMR chemical shifts were calculated using the Individual Gauge for Localized Orbitals method. The calculated isotope effects depend strongly on the asymmetry of the hydrogen bond caused by the influence of the Li+ cation. These results are compared with those of a solid state 15N cross polarization, magic angle spinning NMR study of the metal-stabilized salts 1c and 1d, where M  Cr(CO)5, X+  AsPh+4 and L  H or D, as well as of those of 1e and 1f, M  Cr(CO)5, and X+  NnPr+4. In the case of 1c and 1d the hydrogen bond is symmetric and isotope effects on the 15N NMR chemical shifts are not observed, as predicted theoretically. By contrast, and in agreement with the calculations, large isotope effects are observed for 1e and 1f, where the hydrogen bond symmetry is lifted by a strong interaction with the counterion. So far, solid state hydrogen/deuterium isotope effects on the NMR chemical shifts of hydrogen bonded nuclei have, to our knowledge, not been observed and may be used as a promising novel tool in hydrogen bond research.

Conformational equilibria in quinolizidine alkaloids

Abstract: A method involving 1H and 13C NMR spectroscopy was used for quantitative determination of conformational equilibria in 12 bis-quinolizidine alkaloids and their two salts. In view of the three criteria, i.e. 13C chemical shifts for C12 and C14 as well as the coupling constant H7H17β, the results were consistent. The coupling constant H7H17β was valid also for three-ring quinolizidine-piperidine alkaloids. Factors influencing the equilibria were discussed.

FTIR studies of hydrogen bonding between α,β-unsaturated esters and alcohols

Abstract: The enthalpy (and entropy) of hydrogen bond formation has been measured between the ester carbonyl groups of the two α,β-unsaturated esters thienylacryloyl (TAOMe) and 5-methylthienylacryloyl (5MeTAOMe) methyl ester and the hydrogen bond donors ethanol, phenol and 3,5-dichlorophenol in CCl4. For the esters, the hydrogen bonding strengths were measured by quantitating the amount of bound and unbound donor, using the OH stretching region, as a function of temperature and applying the van't Hoff equation. The decrease in νCO of the ester carbonyl group upon hydrogen bond formation ΔνCO has also been measured and correlated with the enthalpy of hydrogen bond formation. A linear correlation is observed between the enthalpy of hydrogen bond formation −ΔH and ΔνCO, with −ΔH = 1.36ΔνCO − 16.1, where ΔH is measured in kJ mol−1 and Δν in cm−1. Comparison with data for other carbonyl acceptor compounds indicates that the carbonyl group of the above α,β-unsaturated esters is more readily polarized than the carbonyl group of saturated esters or ketones. The quantitative relationship between −ΔH and ΔνCO derived here has been used to determine the change in the enthalpy of hydrogen bond formation between substrate and enzyme groups in a series of acylserine proteases.

Colloidal structure of ionomer solutions in polar solvents

Abstract: The structure of ionomer solutions in polar solvents which are not able to dissolve the neutral polymer analogue was investigated using small-angle X-ray and neutron scattering. The colloidal nature of these solutions is demonstrated for two different families of ionomers. The perfluorinated ionomers form rod-like particles. The value of the radius varies from 15 to 25 A depending on the nature of the ionomer and on the solvent. The polyethylene co-methacrylic acid partially neutralized ionomers form ellipsoidal particles. The semi-major axis and the semi-minor axis are, respectively, 125 A and 350 A. In both cases, no solvent penetrates the particles.

Microwave spectrum, conformation, ab initio calculations, barrier to internal rotation and dipole moment of propionamide

Abstract: The microwave spectrum of propionamide has been investigated in the 21.4–39 GHz spectral range. One conformer has been assigned. This rotamer has a C s equilibrium conformation and the methyl group is syn to the carbonyl group. The dipole moments are μ a = 2.121(20), μ b = 11.66(10) and μ tot. = 11.85(10) × 10 −30 C m. The nuclear quadrupole coupling constants of the 14 N nucleus are χ aa = 2.2(8) and χ bb = 2.3(5)MHz. The barrier to internal rotation of the methyl group is 9.1(5) kJ mol −1 . Six vibrationally excited states of the torsion around the C2–C6 bond were assigned and this fundamental frequency was found to be 45(7)cm −1 . These vibrationally excited states were used to approximate the potential function for torsion near its bottom as V = 7.0 (〈 z 4 〉 + 2.0〈 z 2 〉) cm −1 . This function implies that there is no (or a very small) potential hump at the heavy-atom planar conformation. The first vibrationally excited state of the methyl-group torsion was also assigned. Ab initio computations have been made at the 6–31G ∗ , 6&3ndash;11G ∗∗ , 6–311+ G ∗ , MP 2 6–311 ++ G ∗∗ and MP 3 6–311 ++ G ∗∗ levels of theory. The ab initio calculations imply that there is no second stable form of propionamide. These computations also predict a non-planar heavy-atom conformation at all levels of theory. The Hartree-Fock calculations predict a very small barrier to planarity which is not in complete disagreement with the microwave data. However, the computations involving electron correlation are in obvious disagreement with the experimental findings because a rather high barrier to planarity of 1.6 kJ mol −1 is predicted.

Molecular structure and vibrational IR spectra of the 2-hydroxypyridine2(1H)-pyridinone system and its thio and seleno analogs: density functional theory versus conventional ab initio calculations

Abstract: The molecular parameters (optimized geometries, rotational constants, dipole moments) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of the 2- hydroxypyridine 2(1H)- pyridinone ( 2 OHP 2 OP ) system and its thio ( 2 SHP 2 SP ) and seleno analogs ( 2 SeHP 2 SeP ) are predicted by density functional theory (DFT) with the Becke3-LYP functionals (DFT(B3-LYP) method) using a valence split double-ζ basis set augmented by polarization functions on all atoms. The results are compared with available experimental data and with the data calculated at the conventional ab initio (Hartree-Fock, MP2) levels. Molecular parameters (rotational constants, dipole moments) of 2-oxopyridine tautomers predicted by the DFT(B3-LYP) method agree well with recent gas-phase microwave data and with the parameters pedicted by the MP2 calculations. The vibrational IR spectra of the tautomers calculated by the DFT(B3-LYP) method agree much better with the recorded experimental spectra than the spectra predicted at the Hartree-Fock level and are also better than those predicted at the MP2 level. The DFT(B3-LYP) results for 2-thiopyridine tautomers are of a similar accuracy. The relative internal energies at 0K of the tautomeric species seem to be better predicted by conventional ab initio calculations than by the DFT(B3-LYP) method.