Showing papers on "Hydrogen bond published in 1980"

Intramolecularly hydrogen-bonded peptide conformations.

Abstract: (1980). Intramolecularly Hydrogen-Bonded Peptide Conformation. Critical Reviews in Biochemistry: Vol. 9, No. 1, pp. 1-44.

Quantum and statistical mechanical studies of liquids. 7. Structure and properties of liquid methanol

Abstract: Statistical mechanics simulations of liquid methanol at 25/sup 0/C have been carried out using an intermolecular potential function derived from ab initio molecular orbital calculations with added dispersion corrections. Detailed structural and thermodynamic information has been obtained and compares favorably with the available experimental results including x-ray and infrared data. The liquid primarily consists of long, winding hydrogen bonded chains. Roughly linear hydrogen bonding dominates. Most monomers (56%) are in two hydrogen bonds; 24 and 17% are in one and three hydrogen bonds, respectively, and there is 2.5% monomer present in the liquid. Stereoplots give little indication of dimers, though various higher oligomers are evident. The singly hydrogen bonded monomers occur at the ends of chains, while the monomers with three hydrogen bonds are at Y junctions. For the first time in a liquid simulation, the binding energy distribution is found to be bimodal rather than unimodal. The higher energy peak is proved to be due to the singly hydrogen bonded monomers. Overall, the results confirm the capabilities of computer-simulation techniques to successfully model complex fluids.

Highly structured water network in crystals of a deoxydinucleoside---drug complex.

Abstract: X-ray diffraction analysis of crystals of the intercalative complex between the deoxyribonucleoside phosphate d(CpG) and the mutagen proflavine shows a highly structured arrangement of water molecules linked together by newtworks of hydrogen bonds to form four edge-linked pentagons per asymmetric unit. These pentagons have a general role in maximizing hydrogen bonding at 3.4-A intervals. The conformation of the deoxyribose sugar ring at the 3' end of one strand can depend on its local aqueous environment.

Unusual metal-carbon-hydrogen angles, carbon-hydrogen bond activation, and α-hydrogen abstraction in transition-metal carbene complexes

Abstract: Alkylidene complexes of electron-deficient transition metals display an interesting structural deformation in which the carbine appears to pivot in place while the C α -H bond weakens. A molecular orbital analysis of these carbine complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbine lone pair. Bulky substituents on the metal and carbine protect the metal center from intermolecular reactions and control the extent of carbine pivoting. While a secondary interaction weakens the C-H α bond and attracts the a-hydrogen to the metal, full transfer of hydride to the metal is a forbidden reaction, at least for a five-coordinate, 14-electron complex. The metal-hydrogen bonding interaction guides the hydride to a neighbouring alkyl group, facilitating an α-elimination mode characteristic of the reactions of these compounds. The complexed carbene centers are unusually electron-rich, nucleophilic, by comparison with 18-electron d 6 stabilized carbene complexes. This is a consequence of an extremely effective Ta-C overlap.

The water–water pair potential near the hydrogen bonded equilibrium configuration

Abstract: An analysis is presented of the contributions to the potential energy of interaction between water molecules near the hydrogen bonded equilibrium configuration. From this analysis we synthesize an effective potential that includes the influences of long‐ranged and nonadditive interactions. The influence of the hydrogen bond bending on the anharmonic properties of ice I and on the spectral properties of amorphous solid water, the denser ice polymorphs, and liquid water are used to test the consistency of the effective potential. We also use that potential to estimate the quasistatic distribution of hydrogen bond angles in liquid water, which distribution is an important characteristic of the random network model of water.

Infrared studies on the conformation of synthetic alamethicin fragments and model peptides containing alpha-aminoisobutyric acid.

Abstract: Infrared studies of synthetic alamethicin fragments and model peptides containing a-aminoisobutyric acid (Aib) have been carried out in solution. Tripeptides and larger fragments exhibit a strong tendency to form /3 turns, stabilized by 4 - 1 10-atom hydrogen bonds. Dipeptides show less well-defined structures, though C5 and C7 conformations are detectable. Conformational restrictions imposed by Aib residues result in these peptides populating a limited range of states. Integrated intensities of the hydrogen-bonded N-H stretching band can be used to quantitate the number of intramolecular hydrogen bonds. Predictions made from infrared data are in excellent agreement with nuclear magnetic resonance and X-ray diffraction studies. Assignments of the urethane and tertiary amide carbonyl groups in the free state have been made in model peptides. Shifts to lower frequency on hydrogen bonding are observed for the carbonyl groups. The 1-6 segment of alamethicin is shown to adopt a 310 helical structure stabilized by four intramolecular hydrogen bonds. The fragments Boc-Leu-Aib-Pro-Val-Aib-OMe (1 2-1 6) and Boc-Gly-Leu-Aib-Pro-Val-Aib-OMe (1 1-1 6) possess structures involving 4 - 1 and 5 - 1 hydrogen bonds. Supporting evidence for these structures is obtained from proton nuclear magnetic resonance studies.

Strong, positive‐ion hydrogen bonds: The binary complexes formed from NH3, OH2, FH, PH3, SH2, and ClH

Abstract: A systematic, a b i n i t i oelectronic structure analysis of the strong, positive‐ion hydrogen bond is reported. Energies and wave functions have been obtained at the 4‐31G level for the twenty‐one complexes, (B⋅⋅⋅H–A)+, where A,B=NH3, OH2, FH, PH3, SH2, ClH. The A–H bond length (r 1), the B⋅⋅⋅A separation (R(, and the angle (ϑ) measured relative to the symmetry axis of B have been optimized. Calculated dimerization energies E D are found to be in reasonable agreement with experiment. Charge density difference plots of these complexes exhibit a remarkable similarity to the pattern of alternating charge gain and loss known for the neutral H‐bonded dimers. The proton donor is characterized by a charge gain region between A and the proton and charge loss on the proton; the electron donor by a charge loss between B and the proton. Four of the complexes (HF⋅⋅⋅H⋅⋅⋅FH)+, (H2O⋅⋅⋅H⋅⋅⋅OH2)+, (HCl⋅⋅⋅H⋅⋅⋅ClH)+, (HCl⋅⋅⋅H⋅⋅⋅FH)+, have unusually short internuclear separations and show large charge gain around the protons. This is the first theoretical evidence of a transition from predominantly electrostatic to predominantly covalent binding in hydrogen bonding and it corroborates a recent experimental X–N study. An estimate of the amount of charge lost from the proton, Δq H, has been obtained from the difference plots and is found to bear a linear relation with the dimerization energy E D for a series of complexes with a single proton donor. The inverse relation between E D and the difference in monomerproton affinities, ΔPA, reported in the literature for substituted pyridinium ions, is shown to hold as well for all A and B. Our calculated results also give a quantitative demonstration of the recently proposed inverse relationship between proton position and ΔPA. Several useful new organizing principles have been found: (a) R varies linearly with r 2, the B⋅⋅⋅H distance. Covalent bonding in the four complexes noted above is indicated by deviation from this line. Crystal structures taken from the literature also obey this relationship and the satisfactory agreement between experiment and calculations show that the 4‐31G basis is adequate for predicting strong H‐bond geometries. (b) R N /R?1.2, where R N is the A⋅⋅⋅B separation in neutral hydrogen bonds. (c) E D displays a smooth inverse relation to r 2 for a sequence of complexes with a single electron donor.

Infrared spectrum of the water formaldehyde complex in solid argon and solid nitrogen

Abstract: Infrared spectra of the water formaldehyde complex in argon and nitrogen matrices have been obtained. The complex shifts of the water fundamentals clearly show that water forms a hydrogen bond with formaldehyde. HDO prefers to form a D bond, but a metastable H‐bonded complex is observed in nitrogen matrices below 20 K.

Effect of hydrogen bonding on electronic spectra and reactivity of flavins.

Abstract: Riboflavin tetrabutyrate undergoes characteristic spectral changes, in both the first and second absorption band regions, upon hydrogen bonding with trichloroacetic acid of trifluoroacetic acid. On the basis of the calculated electron densities, hydrogen bonding at the heteroatoms of the isoalloxazine nucleus is considered to occur with increasing concentrations of the proton donor, first at N(1), then at O(12), O(14), and N(3)H, and finally at N(5). The idea that the major effect of the hydrogen bonding at the N(1), N(3)H, and oxygen atoms of the flavin nucleus is to facilitate the electrophilicity of the N(5) position, which was predicted by molecular orbital calculations, was supported by the observation that the hydrogen-bonded flavin in its triplet state abstracts hydrogen from the donor N-benzyl-n,n'-dimethylethylenediamine at a faster rate than do the non-hydrogen-bonded species in CCI4. The implications of the present study in the spectroscopic and catalytic properties of flavoproteins are briefly discussed.

The rotational spectra of weakly bound dimers of carbon monoxide and the hydrogen halides HX (X=F, Cl, and Br)

Abstract: A microwave spectrometer was used to measure the molecular constants of the dimers and to establish the linearity of the atom arrangement. (AIP)

Ab Initio Relativistic Effective Core Potential Studies of Metal–Metal and Metal–Hydrogen Bonding in Pd2, Pt2, PdH and PtH

Abstract: The electronic structure of the palladium and platinum homonuclear diatomics (M2) and diatomic hydrides (MH) have been calculated using relativistic effective core potentials in an ab initio multi-configuration self-consistent field framework. Calculated spectroscopic properties (Re and ωe) of the diatomic hydrides are in close agreement with experiment except for the calculated harmonic force constant in PdH. This latter discrepancy is attributed to inaccuracies in the relative energies of the metal atom in the dissociation limit. The calculated M–M and M–H bond strengths follow the expected trend of the Pt atom forming stronger such bonds than Pd. The metal–hydrogen bonding interaction involves primarily the metal nd orbitals for both Pd and Pt, in contrast to NiH where the main metal bonding orbital has been found to be the 4s. On the other hand, as in Ni2, the Pd–Pd bond is essentially a (n + 1)s electron pair bond, while the Pt–Pt bond has a substantial 5d orbital contribution. This quantitative difference between M–M and M–H bonding interactions found for the group VIII transition metals is supported by experimental photoemission energy distribution and difference spectra for the clean metal and hydrogen chemisorbed metal surfaces.