Hydrogen Bonding and Chemical Reactivity

van der Waals Volumes and Radii

1964 • 18,329 citations

Enzyme structure and mechanism

1978 • 2,897 citations

The solvatochromic comparison method. I. The .beta.-scale of solvent hydrogen-bond acceptor (HBA) basicities

1976 • 1,808 citations

Proton Transfer, Acid‐Base Catalysis, and Enzymatic Hydrolysis. Part I: ELEMENTARY PROCESSES

1964 • 1,743 citations

Dissociation Constants of Organic Bases in Aqueous Solution

1965 • 1,503 citations

Hydrogen bonding in solids correlation of spectroscopic and crystallographic data

2007 • 1,384 citations

Very strong hydrogen bonding

1980 • 1,086 citations

Evidence for resonance-assisted hydrogen bonding from crystal-structure correlations on the enol form of the .beta.-diketone fragment

1989 • 1,082 citations

Soft modes in ferroelectrics and antiferroelectrics

1974 • 1,059 citations

Hydrogen bonding and biological specificity analysed by protein engineering

1985 • 1,050 citations

Why do molecules interact? The origin of electron donor-acceptor complexes, hydrogen bonding and proton affinity

1977 • 949 citations

Stretching Frequencies as a Function of Distances in Hydrogen Bonds

1955 • 898 citations

Theory of the hydrogen bond

1972 • 693 citations

Structural basis of the action of thermolysin and related zinc peptidases

1988 • 675 citations

Partially deuterated water dimers: Microwave spectra and structure

1980 • 642 citations

The origin of hydrogen bonding. An energy decomposition study

1977 • 630 citations

Carboxypeptidase A

1989 • 589 citations

Effective Molarities for Intramolecular Reactions

1980 • 560 citations

Spectroscopic Studies of the Hydrogen Bond. II. The Shift of the O–H Vibrational Frequency in the Formation of the Hydrogen Bond

1937 • 547 citations

Hydrogen Bonding in Solids

1969 • 492 citations

Solute-solvent interactions

1970 • 486 citations

Structure and mechanism of chymotrypsin

1976 • 449 citations

Structure of thermolysin refined at 1.6 Å resolution

1982 • 443 citations

“Proton Sponges” and the Geometry of Hydrogen Bonds: Aromatic Nitrogen Bases with Exceptional Basicities

1988 • 364 citations

Strain effects on amine basicities

1989 • 350 citations

Stretching frequency versus bond distance correlation of OD(H)⋯Y (Y  N, O, S, Se, Cl, Br, I) hydrogen bonds in solid hydrates

1986 • 345 citations

Keto-Enol Tautomerism in β-Dicarbonyls Studied by Nuclear Magnetic Resonance Spectroscopy.¹ I. Proton Chemical Shifts and Equilibrium Constants of Pure Compounds

1964 • 339 citations

Hydrogen Bonding

1971 • 338 citations

Double-Minimum Potentials in Hydrogen-Bonded Solids

1962 • 313 citations

Intermolecular interactions of the carbon-fluorine bond: the crystallographic environment of fluorinated carboxylic acids and related structures

1983 • 302 citations

Solvent effects on chemical reactivity. Evaluation of anion- and cation-solvation components

1983 • 299 citations

Thermodynamics and kinetics of the gas-phase reactions H3O+(H2O)n-1 + water = H3O+(H2O)n

1982 • 292 citations

Acid salts of carboxylic acids, crystals with some ‘very short” hydrogen bonds

2008 • 275 citations

Correlations between proton chemical shift tensors, deuterium quadrupole couplings, and bond distances for hydrogen bonds in solids

1980 • 267 citations

On the source of intramolecular and enzymatic reactivity

1985 • 266 citations

Infrared spectra of strongly hydrogen-bonded systems

1965 • 261 citations

An interactive computer graphics study of thermolysin-catalyzed peptide cleavage and inhibition by N-carboxymethyl dipeptides

1984 • 249 citations

The ionic hydrogen bond and ion solvation. 1. NH+.cntdot..cntdot..cntdot.O, NH+.cntdot..cntdot..cntdot.N, and OH+.cntdot..cntdot..cntdot.O bonds. Correlations with proton affinity. Deviations due to structural effects

1984 • 248 citations

The composition, structure and hydrogen bonding of the β-diketones

2007 • 240 citations

A thermodynamic analysis of solvation effects on the basicities of alkylamines. An electrostatic analysis of substituent effects

1976 • 233 citations

The O–H vs O.O distance correlation, the geometric isotope effect in OHO bonds, and its application to symmetric bonds

1978 • 227 citations

Active site titration and aminoacyl adenylate binding stoichiometry of aminoacyl-tRNA synthetases

1975 • 226 citations

Gas-phase basicities of amines. Hydrogen bonding in proton-bound amine dimers and proton-induced cyclization of .alpha., .omega.-diamines

1973 • 217 citations

A simple quantitative model of hydrogen bonding

1986 • 205 citations

What makes proton transfer fast

1975 • 202 citations

The energetics of neighbouring group participation

1973 • 185 citations

Proton Transfer Reactions;

1976 • 185 citations

The Chemical Physics of Solvation

1986 • 184 citations

Spectroscopic investigations of hydrogen bonding interactions in the gas phase. I. The determination of the geometry, dissociation energy, potential constants and electric dipole moment of the hydrogen-bonded heterodimer HCN • • • HF from its microwave rotational spectrum

1980 • 182 citations

Proton, deuterium, and tritium nuclear magnetic resonance of intramolecular hydrogen bonds. Isotope effects and the shape of the potential energy function

1978 • 176 citations

Hydrogen bonding and specificity. Fluorodeoxy sugars as probes of hydrogen bonding in the glycogen phosphorylase-glucose complex

1986 • 175 citations

Strong hydrogen bonding in gas-phase anions. An ion cyclotron resonance determination of fluoride binding energetics to Broensted acids from gas-phase fluoride exchange equilibrium measurements

1983 • 174 citations

Gas phase ion equilibriums studies of the hydrogen ion by methanol, dimethyl ether, and water. Effect of hydrogen bonding

1973 • 172 citations

The Chemistry of Enzyme Action

1984 • 172 citations

Relationships between apparent binding energies measured in site-directed mutagenesis experiments and energetics of binding and catalysis

1988 • 172 citations

Bond distances and thermal motion in ferrous fluosilicate hexahydrate: a neutron diffraction study

1962 • 169 citations

Hydration of OH- and O2- in the gas phase. Comparative solvation of OH- by water and the hydrogen halides. Effects of acidity

1970 • 166 citations

Reconstruction by site-directed mutagenesis of the transition state for the activation of tyrosine by the tyrosyl-tRNA synthetase: a mobile loop envelopes the transition state in an induced-fit mechanism

1988 • 158 citations

Structures and isotopic fractionation factors of complexes, A1HA2-

1980 • 158 citations

The Brønsted relation – recent developments

1973 • 158 citations

Isotope Effects on Nuclear Shielding

1984 • 156 citations

Photoionization of dimeric polyatomic molecules: Proton affinities of H2O and HF

1977 • 154 citations

Simple model of hydrogen bonding

1975 • 149 citations

Tyrosyl-tRNA synthetase forms a mononucleotide-binding fold

1982 • 149 citations

A large increase in enzyme–substrate affinity by protein engineering

1984 • 147 citations

The correlation between hydrogen-bond lengths and proton chemical shifts in crystals

1986 • 146 citations

The molecular structure of acetylacetone as studied by gas-phase electron diffraction

1987 • 140 citations

Concertedness in acyl group transfer in solution: a single transition state in acetyl group transfer between phenolate ion nucleophiles

1987 • 136 citations

Quantitative evaluation of intramolecular strong hydrogen bonding in the gas phase

1973 • 135 citations

Demonstration of two reaction pathways for the aminoacylation of tRNA. Application of the pulsed quenched flow technique

1975 • 134 citations

Structures and isotopic fractionation factors of complexes AHA-1

1977 • 133 citations

Gas-phase bihalide and pseudobihalide ions. An ion cyclotron resonance determination of hydrogen bond energies in XHY- species (X, Y = F, Cl, Br, CN)

1984 • 131 citations

Normal-Coordinate Analyses of Hydrogen-Bonded Compounds. V. The Enol Forms of Acetylacetone and Hexafluoroacetylacetone

1966 • 127 citations

Hydrogen-bonding energies to negative ions from gas-phase measurements of ionic equilibriums

1971 • 126 citations

Concerted acetyl-group transfer between substituted phenolate ion nucleophiles: variation of transition-state structure as a function of substituent

1989 • 121 citations

Molecular structure of acetylacetone. A crystallographic determination

1983 • 120 citations

Crystal structure of the covalent complex formed by a peptidyl .alpha.,.alpha.-difluoro-.beta.-keto amide with porcine pancreatic elastase at 1.78 .ANG. resolution

1989 • 118 citations

Site-directed mutagenesis shows that tyrosine 248 of carboxypeptidase A does not play a crucial role in catalysis

1985 • 117 citations

Demonstration of two active sites on a monomeric aminoacyl-tRNA synthetase. Possible roles of negative cooperativity and half-of-the-sites reactivity in oligomeric enzymes

1975 • 117 citations

Formation, thermochemistry, and relative stabilities of proton-bound dimers of oxygen n-donor bases from ion cyclotron resonance solvent-exchange equilibrium measurements

1982 • 117 citations

Crystal structure of a deletion mutant of a tyrosyl-tRNA synthetase complexed with tyrosine

1987 • 117 citations

Use of binding energy in catalysis analyzed by mutagenesis of the tyrosyl-tRNA synthetase

1986 • 116 citations

O-H and O-D stretching vibrations in isotopically dilute HDO molecules in some solid hydrates

1978 • 116 citations

A simple relationship between the infra-red stretching frequencies and the hydrogen bond distances in crystals

1969 • 114 citations

Structure and thermal properties associated with some hydrogen bonds in crystals I. The isotope effect

1939 • 110 citations

The crystal structure of tyrosyl-transfer RNA synthetase at 2.7 Å resolution

1976 • 109 citations

Theoretical study of SN2 reactions. Ab initio computations on HF and CI level

1976 • 107 citations

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

1980 • 105 citations

Infrared diode laser study of the hydrogen bifluoride anion: FHF− and FDF−

1986 • 101 citations

Single-crystal, time-of-flight, neutron-diffraction structure of hydrogen cis-diacetyltetracarbonylrhenate, [cis-(OC)4Re(CH3CO)2]H: a metallaacetylacetone molecule

1984 • 99 citations

Proton and deuterium NMR of hydrogen bonds: Relationship between isotope effects and the hydrogen bond potential

1976 • 96 citations

Spectroscopic study of hydrogen bonding in the enol form of β-diketones—I. Vibrational assignment and strength of the bond

1979 • 96 citations

Mechanisms of Proton Transfer between Oxygen and Nitrogen Acids and Bases in Aqueous Solution

1986 • 95 citations

Spectroscopic study of hydrogen bonding in the enol form of β-diketones—II. Symmetry of the hydrogen bond

1979 • 95 citations

Refinement of Peterson and Levy's Neutron Diffraction Data on KHF2

1964 • 94 citations

Ab initio calculations on the very strong hydrogen bond of the biformate anion and comparative esterification studies

1978 • 94 citations

Hydrogen bond studies. 71. Ab initio calculation of the vibrational structure and equilibrium geometry in HF−2 and DF−2

1972 • 93 citations

Ligand binding and enzymic catalysis coupled through subunits in tyrosyl-tRNA synthetase

1975 • 88 citations

Molecular orbital study of the complexes (AHn)2H+ formed from ammonia, water, hydrogen fluoride, phosphine, hydrogen sulfide, and hydrogen chloride

1985 • 87 citations

β-diketone interactions

1987 • 83 citations

The bihydroxide (H3O2-) anion. A very short, symmetric hydrogen bond

1979 • 83 citations

A Single Crystal Neutron Diffraction Determination of the Hydrogen Position in Potassium Bifluoride

1952 • 82 citations

Proton chemical shift tensors in hydrogen-bonded dimers of RCOOH and ROH

1983 • 82 citations

Correlation between two isotope effects in hydrogen-bonded crystals: transition temperature and separation of two equilibrium sites

1981 • 81 citations

Nature of the Hydrogen Bond in Sodium Acid Fluoride

1963 • 81 citations

Some Calculations on the Hydrogen Bond

1967 • 79 citations

Role of symmetry in the tunneling of the proton in double-minimum potentials

1982 • 79 citations

The Fluoronium Ions H₂F^⊕ and H₃F^⊕₂: Characterization by Crystal Structure Analysis

1988 • 78 citations

Differences in the Hydrogen and Deuterium Bonds

1981 • 76 citations

Investigation of Keto–Enol Tautomerism by Nuclear Magnetic Resonance Spectroscopy

1971 • 76 citations

The role of crystal-structure determination in the study of structural phase transitions

1980 • 74 citations

Symmetry and tunneling in the intramolecular proton exchange in naphthazarin, methylnaphthazarin, and dimethylnaphthazarins

1982 • 74 citations

Linear solvation energy relationships. 29. Solution properties of some tetraalkylammonium halide ion pairs and dissociated ions

1985 • 74 citations

Heterobihalide ions. Nuclear magnetic resonance spectroscopy of strong hydrogen bonds

1974 • 73 citations

Proton transfer from intramolecularly hydrogen-bonded acids

1984 • 72 citations

The “OH stretching” band of acid salts of carboxylic acids and the symmetry of the hydrogen bond

1973 • 71 citations

Proton Magnetic Resonance Spectra and Electronic Effects in Substituted 1,3-Diketones

1966 • 71 citations

Strong Hydrogen Bonds. II. The Hydrogen Difluoride Ion

1964 • 70 citations

Neutron diffraction study of the hydrogen bond in trisodium hydrogenbissulphate and a survey of very short O–H...O bonds

1982 • 68 citations

Pressure-induced transition of the hydrogen bond in the ferroelectric compounds KH2P04 and KD2P04

1989 • 68 citations

Polynuclear complexes with hydrogen-bonded bridges. Dinuclear complex of N,N'-bis(2-hydroxyethyl)-2,4-pentanediimine with copper(II)

1976 • 68 citations

Isotope Effect on the Hydrogen-Bond Length

1969 • 68 citations

Structure of 1,8-bis(dimethylamino)naphthalene hydrobromide dihydrate

1983 • 67 citations

Energy surfaces and proton polarizability of hydrogen-bonded chains: an ab initio treatment with respect to the charge conduction in biological systems

1988 • 67 citations

Hydrogen bonding. 8. Preparation, properties, and low-temperature infrared structural analysis of ammonium and alkylammonium trihydrogen tetrafluorides and tetramethylammonium dihydrogen trifluoride

1977 • 67 citations

Mechanism and Stereoelectronic Effects in the Lysozyme Reactio

1987 • 64 citations

The crystal structure of the enol tautomer of 1,3-diphenyl-1,3-propanedione (dibenzoylmethane) by neutron diffraction

1976 • 64 citations

Matrix isolation investigation of the hydrogen bihalide anions

1982 • 63 citations

Gas-phase bifluoride ion. An ion cyclotron resonance determination of the hydrogen bond energy in fluoride ion (FHF-) from gas-phase fluoride transfer equilibrium measurements

1982 • 61 citations

Mechanism of proton transfer in solution. Factors determining whether the activated complex has an equilibrated environment

1972 • 61 citations

Quino[7,8‐h]quinoline, a New Type of “Proton Sponge”

1987 • 61 citations

A Short, Slightly Asymmetrical, Intramolecular Hydrogen Bond: A Neutron Diffraction Study of Bis(2-amino-2-methyl-3-butanone Oximato)Nickel(II) Chloride Monohydrate {Ni(C5H11N2O)2H}+Cl−·H2O

1971 • 61 citations

Theoritical study on an anomalous isotopic effect on the position and integrated intensity of the ir stretching vibration band for medium-strong and strong hydrogen bonds

1981 • 60 citations

Evidence for a double-minimum potential for intramolecular hydrogen bonds of aqueous hydrogen maleate and hydrogen phthalate anions

1989 • 60 citations

Natural variation of tyrosyl-tRNA synthetase and comparison with engineered mutants

1986 • 60 citations

Binding of tyrosine, adenosine triphosphate and analogues to crystalline tyrosyl transfer RNA synthetase

1978 • 59 citations

Protolytic Processes in H2O-D2O Mixtures

1969 • 58 citations

Correlation effects on hydrogen-bond potentials. SCF Cl calculations for the systems HF−2 and H3O−2

1975 • 57 citations

Malondialdehyde and acetylacetone. An AM1 study of their molecular structures and keto–enol tautomerism

1989 • 57 citations

4,5‐Bis(dimethylamino)phenanthrene and 4,5‐Bis(dimethylamino)‐9,10‐dihydrophenanthrene: Syntheses and “Proton Sponge” Properties

1986 • 57 citations

Proton transfer in and proton polarizability of hydrogen bonds: IR and theoretical studies regarding mechanisms in biological systems

1988 • 57 citations

The electron-density distribution in ammonium bifluoride

1960 • 56 citations

Fluorinated Carbohydrates

1988 • 56 citations

The Hydrogen Bond System in Potassium Trihydrogen Bis-selenite, KH3(SeO3)2, and in Potassium Trideuterio Bis-selenite KD3(SeO3)2, as Determined by Neutron Diffraction.

1971 • 56 citations

Nuclear magnetic resonance studies of enol-enol and keto-enol equilibria in substituted benzoylacetones

1969 • 56 citations

Proton affinities of hydrogen halides determined by the molecular beam photoionization method

1979 • 55 citations

THE CRYSTAL STRUCTURE OF POTASSIUM HYDROGEN FLUORIDE

1923 • 55 citations

A new method for obtaining isotopic fractionation data at multiple sites in rapidly exchanging systems

1985 • 55 citations

Fluorinated and deoxygenated substrates as probes of transition state structure in glycogen phosphorylase

1989 • 55 citations

Ionic species in hydrofluoric acid-antimony pentafluoride solutions. An infrared spectroscopy investigation

1980 • 55 citations

LCAO–MO–SCF Studies of HF2− and the Related Unstable Systems HF2 and HeF2

1970 • 54 citations

The Crystal Structures of Alkali Metal Bifluorides

1956 • 53 citations

IR continua of hydrogen bonds and hydrogen-bonded systems, calculated proton polarizabilities and line spectra

1989 • 53 citations

The Crystal Structure of Potassium Fluoroiodate, KIO₂F₂

1940 • 53 citations

Structure and thermal properties associated with some hydrogen bonds in crystals. III. Further examples of the isotope effect

1939 • 52 citations

Evidence against a crucial role for the phenolic hydroxyl of Tyr-248 in peptide and ester hydrolyses catalyzed by carboxypeptidase A: comparative studies of the pH dependencies of the native and Phe-248-mutant forms

1986 • 51 citations

Poly(hydrogen Fluorides) with the Tetramethylammonium Cation: Preparation, Stability Ranges, Crystal Structures, [H_nF_n+1]⁻ Anion Homology, Hydrogen Bonding F‐H…︁F [2]

1987 • 51 citations

The lattice energies of potassium, rubidium and caesium bifluorides and the strength of the bonds in the bifluoride, HF ? 2 , ion

1958 • 51 citations

Isotope and potential energy surface effects in vibrational bonding

1984 • 50 citations

Hydrogen Isotope Fractionation in Acidified Solutions of Protium and Deuterium Oxide

1963 • 50 citations

Reactions of coordinated molecules. V. Preparation of cis-(OC)4Re[C(CH3)O...H...OC(CH3]: the enol tautomer of a metalloacetylacetone molecule

1976 • 50 citations

The infrared spectrum of the hydrogen bifluoride anion: unprecedented variation with level of theory

1986 • 50 citations

4,5‐Bis(demethylamino)fluorene, a New “Proton Sponge”

1983 • 50 citations

The photodissociation and high resolution laser photodissociation of halogen-substituted toluene cations

1976 • 50 citations

Infra-red spectra and the solid state. III. Potassium bifluoride

1951 • 49 citations

The structure of 1,6-diazabicyclo[4.4.4]tetradecane and of its inside protonated ion

1983 • 48 citations

Asymmetry of tyrosyl-tRNA synthetase in solution

1988 • 48 citations

Asymmetric single-minimum hydrogen bond in the bifluoride ion. Neutron diffraction study of p-toluidinium bifluoride

1973 • 48 citations

A rapid, stereospecific synthesis of 2-deoxy-2-fluoro-D-glucose using the fluoride ion

1982 • 47 citations

Crystal structures of the isomorphous tris(hexafluoroacetylacetonato)copper(II) and tris(hexafluoroacetylacetonato)magnesium salts of monoprotonated 1,8-bis(dimethylamino)naphthalene

1972 • 47 citations

Hydrogen bond studies. 85. A very short, asymmetrical, intramolecular hydrogen bond: A neutron diffraction study of pyridine-2,3-dicarboxylic acid (C7H5NO4)

1974 • 46 citations

Fluorides and fluoro acids. 12. Complex-anion homology and isomerism in the crystal structures of two potassium poly(hydrogen fluorides), KF.cntdot.2.cntdot.5HF and KF.cntdot.3HF

1986 • 46 citations

Über den Einfluß von Wasserstoffbrücken-Struktur und elektrostatischer Wechselwirkung auf die Geschwindigkeit protolytischer Reaktionen

1963 • 45 citations

Tyrosyl-tRNA synthetase acts as an asymmetric dimer in charging tRNA. A rationale for half-of-the sites activity

1988 • 45 citations

The preferential direction of enolization of some asymmetric 1,3-dicarbonyl compounds in solution: a study by multinuclear NMR spectroscopy

1990 • 44 citations

Amino acid activation in crystalline tyrosyl-tRNA synthetase from Bacillus stearothermophilus

1981 • 44 citations

Intermolecular interactions in crystals of carboxylic acids

1979 • 44 citations

Stable simple enols. 5. Solvent dependence of the conformation of the OH group in 2,2-dimesitylethenol and several 1,2,2-triarylethenols

1984 • 43 citations

The Infrared Spectrum of KHF2

1951 • 43 citations

A Redetermination of the Fluorine-Fluorine Distance in Potassium Bifluoride

1939 • 43 citations

Solvent isotope effects on the ionization of hydrofluoric acid

1973 • 42 citations

The Crystal and Molecular Structure of Dimedone.

1974 • 42 citations

Steric and electronic effects on the neighboring general acid catalyzed hydrolysis of methyl phenyl acetals of formaldehyde

1970 • 41 citations

A neutron diffraction study of the diaquohydronium ion in yttrium oxalate trihydrate

1975 • 41 citations

Hydrogen bonding. 13. Infrared study of the HF2- anion in p-toluidinium and tetraalkylammonium hydrogen difluorides

1982 • 40 citations

β-diketone interactions

1990 • 39 citations

Internal thermodynamics of position 51 mutants and natural variants of tyrosyl-tRNA synthetase

1986 • 39 citations

Das Allopolarisierungsprinzip und seine Anwendungen, V. ¹³ CNMR‐Spektren von Enolaten und verwandten Carbanionen

1981 • 38 citations

The Polarized Infrared Spectrum of Potassium Bifluoride at − 185°C

1951 • 38 citations

High Resolution Nuclear Magnetic Resonance Spectra of Bifluoride Ion and its Homologues

1971 • 37 citations

Application of the Hammett Equation to Fused Ring Systems

1954 • 37 citations

Synthesis, structure and basicity of 1,9-bis(dimethylamino)-dibenzothiophene and 1,9-bis(dimethylamino)-dibenzoselenophene1,2)

1988 • 36 citations

17O n.m.r. studies of enol and phenol compounds with intramolecular hydrogen bonds

1985 • 36 citations

The Crystal Structure of Dimedone

1975 • 35 citations

The crystal and molecular structure of the enol form of 1-phenyl-1,3-butanedione (benzoylacetone) by neutron diffraction

1976 • 35 citations

Vibrational spectra of acetylacetone and its anion

1970 • 35 citations

β-Diketone interactions Part 12. The structure and properties of 3-(3′,4′,5′- trimethylphenyl)-pentane-2,4-dione, C14H18O2; the hydrogen-bond energy of the enol tautomers of β-diketones

1990 • 35 citations

Studies on Sulfate Esters. II. Carboxyl Group Catalysis in the Hydrolysis of Salicyl Sulfate

1966 • 35 citations

Infrared Absorption Associated with Strong Hydrogen Bonds

1958 • 34 citations

High resolution nuclear magnetic resonance spectra of hydrogen fluoride in solution and in bihalide ions. Nuclear spin coupling in strong hydrogen bonds

1974 • 34 citations

Isotope effects—I

1977 • 34 citations

Hydrogen bond studies. CXLII. Neutron diffraction study of the two very short hydrogen bonds in lithium hydrogen phthalate–methanol

1981 • 34 citations

The crystal structure of bis(m-bromobenzoyl)methane

1962 • 33 citations

Primary Photoproduct of 2,6-Dimethyl-4-Aminopyrimidine

1963 • 33 citations

Conditions for the Formation of Hydrogen Bonds

1953 • 33 citations

X–N deformation density studies of the hydrogen maleate ion and the imidazolium ion

1980 • 32 citations

The Anomalous Deuterium Isotope Effect on the Chemical Shift of the Bridge Hydrogen in the Enol Tautomer of 2,4-Pentanedione

1970 • 31 citations

Strong hydrogen bonding in the polyfluorides: ab initio calculations and a new method of preparing potassium hydrogendifluoride

1981 • 31 citations

The crystal structure of deuterated oxalic acid dihydrate (COOD)₂.2D₂O, by neutron diffraction analysis

1967 • 31 citations

Infrared matrix isolation studies of the M+HF2- and M+XHF- ion pairs, for X = chlorine, bromine, and iodine

1979 • 31 citations

Proton and deuteron magnetic resonances of the strongly hydrogen-bonded bisformate complexes (HCO2)2H- and (HCO2)2D- in aqueous solution. The primary isotope effect on chemical shift, and isotopic fractionation of labile hydrogen in hydrogen-deuterium mixtures

1984 • 30 citations

Structure and thermal properties associated with some hydrogen bonds in crystals. IV. Isotope effects in some acid phosphates

1942 • 30 citations

Physical Organic Model Systems and the Problem of Enzymatic Catalysis

1975 • 29 citations

β-diketone interactions

1988 • 29 citations

Structure of a mutant of tyrosyl-tRNA synthetase with enhanced catalytic properties

1987 • 29 citations

Intramolecular general acid catalysis in the hydrolysis of acetals with aliphatic alcohol leaving groups

1979 • 29 citations

An ab initio study of the keto-enol tautomerism

1979 • 29 citations

Spectroscopic Studies on Enols. II. Proton Magnetic Resonance and Infrared Spectra of Diacetoacetic and Cinnamoylacetoacetic Esters.

1960 • 28 citations

Intramolecular unsymmetrical oxygen-hydrogen-oxygen (OHO) bonds. Thermochemistry

1983 • 28 citations

Intermediates in nucleophilic aromatic substitution. XIV. Spiro Meisenheimer complexes derived from N,N-dimethylethylenediamine. Kinetics in aqueous dimethyl sulfoxide

1975 • 28 citations

Intramolecular carboxyl group participation in acetal hydrolysis

1971 • 28 citations

NMR study of the influence of β‐substituents on the intramolecular hydrogen bond in β‐dicarbonyl compounds

1973 • 27 citations

Crystal structure of calcium 2-fluorobenzoate dihydrate: indirect calcium...fluorine binding through a water-bridged outer-sphere intermolecular hydrogen bond

1984 • 27 citations

Proton affinities of the hydrogen, methyl, and trifluoromethyl halides from pulsed electron beam high pressure mass spectrometric equilibria measurements

1985 • 27 citations

The N1s ESCA Spectrum of Protonated 1,8‐Bis‐(Dimethylamino) Naphthalene (“proton sponge”)

1972 • 27 citations

Intrabridgehead hydrogen-bonded ions: spectroscopic characteristics and the question of single vs. double minimum potentials

1983 • 27 citations

The Crystalline Hydrates of Hexafluorosilicic Acid: A Combined Phase‐Analytical and Structural Study

1988 • 27 citations

Hydrogen/deuterium fractionation factors of the aqueous ligand of cobalt in Co(H2O)62+ and Co(II)-substituted carbonic anhydrase

1989 • 27 citations

Kinetics of proton-transfer reactions in aqueous solution. IV. Broensted slope for internally hydrogen-bonded weak acids

1971 • 26 citations

A precise n.m.r. method for measuring isotope effects on acid–base equilibria: effects of solvent, solvent isotopic composition, and ionic strength on hydrogen and oxygen isotope effects in formic acid

1983 • 26 citations

The Crystal Structure of 2,2'-Bipyridinium(1+) mu-Hydrogen-bis[(2,2'-bipyridine)oxodiperoxovanadate](1-)-x-hydrogen peroxide-(6-x)-water, (Hbipy)[H{VO(O2)2bipy}2].xH2O2.(6-x)H2O, x ~= 0.5, at -100 degrees C.

1984 • 26 citations

The Crystal Structure of Phenylmalondialdehyde at -162 degrees C.

1977 • 26 citations

Hydrogen bond studies. XXIII. The crystal structure of potassium hydrogen diformate

1968 • 26 citations

Protonated dimethyl sulphoxide, [Me2SO ? H ? OSMe2]+; a novel hydrogen-bridged structure: X-ray crystal structure of trans-[H(Me2SO)2][RhCl4(Me2SO)2]

1980 • 26 citations

135. The constitution of the acetylacetonate derivatives of sulphur, selenium, and tellurium

1964 • 25 citations

Infrared spectra of cyanoacetylacetone and the free enolate ions of acetylacetone and cyanoacetylacetone

1965 • 25 citations

The anomalous gas phase acidity of ethyl fluoride. An abinitio investigation of the importance of hydrogen bonding between F⁻ and sp² and sp C—H bonds

1985 • 25 citations

Infrared spectra and structure of the argon-matrix isolated hydrogen difluoride(1-) anion

1978 • 25 citations

The Crystal Structure of Hypofluorous Acid: Chain Formation by OH ⃛O Hydrogen Bonds

1988 • 25 citations

Deuterium oxide solvent isotope effects on nitrogen-hydrogen oxygen, oxygen-hydrogen...nitrogen, and nitrogen-hydrogen...nitrogen intramolecular hydrogen bonds

1967 • 25 citations

X-Ray crystallographic and NMR evidence for a uniquely strong OH ? N hydrogen bond in the solid state and solution

1989 • 25 citations

Spectroscopic Studies of the Symmetric O–H–O Hydrogen Bonding in CsH(CF3CO2)2 and KH(CF3CO2)2

1972 • 24 citations

β-diketone interactions

1987 • 23 citations

Hydrogen bonding in the gas phase. Infrared spectroscopic investigation of hydrogen fluoride alcohol–heterodimers

1979 • 23 citations

Reactivity of alkenes at a diruthenium centre: carbon–carbon bond formation via HF elimination

1989 • 23 citations

Three-dimensional Structure of Usnic Acid.

1974 • 22 citations

Deuterium fractionation factor for the gas-phase hydronium ion

1986 • 22 citations

Neutron diffraction study of deuterated imidazolium hydrogen maleate: an evaluation of isotope effect on the hydrogen-bond length

1980 • 22 citations

Gas phase negative ion chemistry of alkylchloroformates. A convenient method for bimolecular generation of chloride ion adducts and determination of chloride affinities from ion cyclotron resonance thermal equilibrium measurements

1984 • 22 citations

Über die fraktionierte Destillation des Acetessigesters. (Über Keto‐Enol‐Tautomerie, XII.)

1920 • 22 citations

Fluoride und Fluorosäuren, VI. Verbindungsbildung und Kristallstrukturen im System Ammoniak—Fluorwasserstoff [1, 2] / Fluorides and Fluoro Acids, VI. Compound Formation and Crystal Structures in the System Ammonia-Hydrogen Fluoride

1984 • 22 citations

Neutronenbeugungsuntersuchung der extrem kurzen Wasserstoffbrücke im Kupfer-Dihydrogen-Diphthalat-Dihydrat

1980 • 22 citations

Use of various nuclei as probes in a new NMR method for obtaining proton/deuteron fractionation data

1986 • 22 citations

Carboxyl group participation in acetal hydrolysis. Hydrolysis of disalicyl acetals

1973 • 22 citations

Stable simple enols. Part 21. Solvent and .alpha.- and .beta.-substituent effects on the association with hydrogen bond accepting solvents and the hydroxyl conformation of stable simple enols

1989 • 21 citations

High resolution NMR investigation of enolization of ethyl α-alkylacetoacetates and 3-alkylacetylacetones with branched substituents

1966 • 21 citations

Strongly basic medium-ring diamines which mimic gas phase behaviour in solution: 1,6-dimethyl-1,6-diazacyclodecane

1988 • 21 citations

The crystal and molecular structure of tris(thiourea)copper(I) hydrogen-o-phthalate. A monomeric trigonal planar copper(I) complex

1977 • 21 citations

Intramolecular hydrogen bonding: synthesis and crystal structure of nickel(II) and copper(II) complexes of a tetradentate amine oxime

1986 • 20 citations

The crystal structure of 3,3'-trithiobis-(2,4-pentadione)

1971 • 20 citations

Structure-dependent intermolecular hydrogen-bond effects on carbon-13 NMR chemical shifts in enol forms of 1,3-diketones

1987 • 20 citations

Intramolecular Hydrogen Bonding in cis-Cyclopropanepolycarboxylic Acids¹

1965 • 20 citations

The crystal structure of LiHF2

1962 • 20 citations

The mechanism of proton transfer from intramolecularily hydrogen-bonded acids. Differences between nitrogen-to-oxygen and nitrogen-to-nitrogen proton transfer

1981 • 20 citations

Neutron diffraction study of diammonium trihydrogen periodate, (NH₄)₂H₃IO₆, and its deuterium analogue, (ND₄)₂D₃IO₆

1980 • 19 citations

Neutron diffraction study of quinolinic acid recrystallized from D₂O: evaluation of temperature and isotope effects in the structure

1979 • 19 citations

Substituted uranocenes with alkoxy and amino groups

1977 • 19 citations

A note on the correlation between infrared frequences and hydrogen-bond lengths in hydroxyl chains

1983 • 19 citations

The crystal structure of potassium pentafluoroperoxotantalate(V)–potassium hydrogendifluoride, K2[Ta(O2)F5].KHF2

1976 • 18 citations

Measurement of solvent isotope effects with the glass electrode. Part I. The ionic product of D₂O and D₂O–H₂O mixtures

1967 • 17 citations

Properties of a strong intramolecular OHO hydrogen bond in 2-(N,N-diethylamino-N-oxymethyl)-4,6-dichlorophenol

1986 • 17 citations

Darstellung der Enolformen aus Acetessigester und Acetyl‐aceton. (Über Keto‐Enol‐Isomerie, XIV)

1921 • 17 citations

Vibrational spectrum of solid sodium hydrogen bis(formate). An unsymmetrical very short OH...O bond in a complex containing spectroscopically distinct formate ion and formic acid entities

1983 • 17 citations

Equilibrium isotope effects on the hydration of gas-phase ions. The effect of hydrogen bond formation on deuterium isotopic fractionation factors for H3O+, H5O2+, F(H2O)-, and Cl(H2O)-

1988 • 17 citations

The Infra-Red Absorption and Reflection Spectra of KHF2 and of KDF2

1941 • 17 citations

Catalyse acide intramoléculaire dans l'hydrolyse des acétals I. Position du proton dans l'état de transition

1976 • 16 citations

Association of .alpha.-alkyl-.beta.,.beta.-dimesitylethenols with hydrogen-bond-accepting solvents

1988 • 16 citations

Formation of intrabridgehead hydrogen-bonded ions from medium-ring bicyclic diamines

1983 • 16 citations

NMR Studies of Chemical Exchange Processes in Intramolecularly Hydrogen Bonded o-Hydroxy-carbonyl Compounds.

1974 • 16 citations

General base catalysis, isotope effects, activation parameters, and the mechanism of removal of the hydrogen-bonded proton from protonated 1,8-bis(diethylamino)-2,7-dimethoxynaphthalene

1984 • 16 citations

Nucleophilic addition to olefins. 12. Solvent-induced change in the rate-limiting step of the hydrolysis of benzylidenemalononitrile in acidic dimethyl sulfoxide-water solution

1985 • 16 citations

The Nature of the Hydrogen Bond in KHF2

1947 • 15 citations

[Bis(dimethylsulphoxide) protium][trans-bisdimethylsulphoxidetetrachloroiridate(III)]

1988 • 15 citations

Hydrogen bonding. II. Complex fluoride salts of arylammonium ions

1974 • 15 citations

Strong hydrogen bonding in gas-phase ions: a high-pressure mass-spectrometric study of formation and energetics of methyl fluoride proton-bound dimer

1986 • 15 citations

Crystal and Molecular Structure of Hexaaquocobalt(II)–Hexafluorosilicate(IV), [Co(H₂O)₆][SiF₆]

1973 • 15 citations

The Raman spectrum of solid hypofluorous acid HOF

1982 • 15 citations

Structuresof 2,2′-bis(dimethylamino)biphenyl and its monohydrobromide)

1988 • 14 citations

Tetraphenylphosphonium hydrogendifluoride: a new source of fluoride ion

1985 • 14 citations

Fluoride und Fluorosäuren, IX [1]. Verbindungsbildung im System NOF–HF sowie Kristallstrukturen von NOF·3 HF und NOF·4 HF / Fluorides and Fluoro Acids, IX [1]. Compound Formation in the System NOF–HF and Crystal Structures of NOF·3HF and NOF·4HF

1984 • 14 citations

Slow proton transfer involving amino-groups

1973 • 14 citations

Hydrogen-bond energies of FHF^–and HOHF^–. Ab initio studies of strong hydrogen bonds involving fluoride, using improved fluoride energies

1983 • 14 citations

The Double Minimum Problem Applied to Potassium Hydrogen Fluoride

1942 • 14 citations

Intramolecular nucleophilic catalysis in the hydrolysis of substituted aspirin acids

1968 • 14 citations

The infrared spectrum of solid sodium hydrogen bifluoride. Doubling of the HF stretching band of dilute NaHF2 in NaDF2

1980 • 13 citations

Short hydrogen bonds: diadducts of substituted pyridine N-oxides: synthesis, spectroscopic studies, and X-ray structure

1985 • 13 citations

Nuclear magnetic resonance studies of the difluoride ion. Part 1.—Single crystal study of potassium difluoride

1975 • 13 citations

A Redetermination of the Parameters in Ammonium Bifluoride

1940 • 13 citations

Intra- and intermolecular association and i.r. spectra of dicarbonyl compound enol forms

1975 • 13 citations

The i.r. spectrum of solid hypofluorous acid, HOF

1981 • 13 citations

Deuterium fractionation factor for unhydrated hydronium ion. Deuterium isotope effects on proton-transfer equilibria in acetonitrile

1984 • 13 citations

INTRAMOLECULAR HYDROGEN BONDING OF AZO DYES IN AQUEOUS SOLUTION

1968 • 13 citations

β-Diketone interactions. Part 1. Pentane-2,4-dione–diethylamine adduct. X-Ray structure reveals novel hydrogen-bonded dimer

1986 • 12 citations

Nuclear magnetic resonance studies of the difluoride ion. Part 2.—Difluorides of the alkali metals and alkaline earths

1977 • 12 citations

1H and 19F NMR spectra of the HF2− anion

1969 • 12 citations

A neutron diffraction study of the effect of deuteration in bis(3-amino-3-methyl-2-butanone oximato)nickel(II) chloride monohydrate

1980 • 12 citations

A simple but accurate method for measuring deuterium chemical shifts relative to the corresponding proton shifts

1982 • 12 citations

The Crystal Structure of Ammonium Hydrogen Fluoride, NH₄HF₂

1933 • 12 citations

Deuterium isotope effects in solutions of 2,4‐pentanedione

1977 • 12 citations

Self-association in HOF and HNF2. Which atoms are the better proton acceptors, fluorine, oxygen or nitrogen?

1987 • 12 citations

Tripotassium cis-dichloro(hydrogendisulfito)platinate(II) K₃[Pt{(SO₃)₂H}Cl₂]. A case of an extremely short hydrogen bond O...H...O

1980 • 12 citations

The kinetics of the deparotonation of tropaeolin O

1975 • 12 citations

An ion cyclotron resonance study of competitive solvation of gas phase anions

1979 • 12 citations

Caesium fluoride solutions in carboxylic acids

1976 • 12 citations

The fractionation of hydrogen and deuterium isotopes in solutions of sodium methoxide

1969 • 12 citations

Carbon-13 and low temperature proton nuclear magnetic resonance study of the interaction of acetylacetone with diethylamine and triethylamine

1977 • 11 citations

Infrared spectra of 3-methylthio-2,4-pentanedione and its deuterated compound

1971 • 11 citations

The Problem of Negative Anharmonicity and the Nature of the Hydrogen Bond in Potassium Hydrogen Fluoride

1958 • 11 citations

Isotope effects in proton-transfer reactions. An ion cyclotron resonance determination of the equilibrium deuterium isotope effect in the bichloride ion

1987 • 11 citations

Deuterium Oxide Solvent Isotope Effects on Fast Reactions of Substituted Malonic Acids¹

1966 • 11 citations

Carboxylate anion stabilization of a developing carbonium ion in acetal hydrolysis. Hydrolysis of phthalaldehydic acid acetals

1977 • 11 citations

¹H and ¹³C NMR study of the enol–enolic tautomerism of some cyclic β‐ketoaldehydes

1975 • 11 citations

Solutions of potassium fluoride in glacial acetic acid

1971 • 11 citations

Deuteron magnetic resonance study of kd(CF3COO)2

1972 • 11 citations

Isotopic fractionation factor and hydrogenic potential in 2-hydroxy-1,1,1,5,5,5-hexafluoro-2-penten-4-one

1981 • 10 citations

The Crystal Structure of Trisodium Pentafluoroperoxoniobate(V) Hydrogendifluoride, Na3[HF2][NbF5(O2)].

1981 • 10 citations

Base-catalyzed proton transfer from an intramolecularly hydrogen-bonded naphthylammonium ion in 70% dimethyl sulfoxide-water (v/v)

1978 • 10 citations

Crystal geometry of the bifluoride anion

1975 • 10 citations

Tetrafluoroboric Acid‐Methanol (1/2), Cyclic Molecules by Hydrogen Bonds between Ions

1981 • 10 citations

Study of Short Hydrogen Bonds. I. Structures of 4-Methylpiperidinium Hydrogen Bis(p-methylbenzoate) and Piperidinium Hydrogen Bis(p-methylbenzoate)

1986 • 9 citations

Internal hydrogen bonding and positions of protonation in the monoprotonated forms of some 1,3- and 1,4-diamines

1975 • 9 citations

What kind of potential does a H-bond proton feel? An electrostatic study

1979 • 9 citations

Crystal structure ofp-toluidinium bifluoride

1971 • 9 citations

Mechanism of proton removal from intramolecularly hydrogen-bonded (phenylazo)resorcinol monoanions

1983 • 9 citations

Berechnung der Gitterenergie der Alkalihydrogenfluoride KHF₂, RbHF₂ und CsHF₂

1971 • 9 citations

Study of short hydrogen bonds. III. Structures of pyrrolidinium hydrogen bis(p-methylbenzoate) and hexamethyleneiminium hydrogen bis(p-methylbenzoate)

1989 • 9 citations

The Crystal Structure of 1,10-Phenanthrolinium Pentafluoroperoxoniobate(V), (C12H10N2)[NbF5(O2)].

1982 • 8 citations

Strong and symmetric halogen bonding in coordination compounds of carboxylic acid derivatives; the crystal structure of (3,6-dithiaoctanedioato-S,S′-(3,6-dithiaoctanedioic acid-S,S′)-copper(I) redetermined at 173 K

1984 • 8 citations

Molecular conformations, hydrogen-bond strengths and electronic structure of usnic acid: an AM1 and CNDO/S study

1990 • 8 citations

Ion model independent studies of hydrogen bonding and the thermochemistry of bifluoride salts

1977 • 8 citations

Equilibrium and Kinetic Studies of the Deprotonation of the Monoanion of Several Dicarboxylic Acids in Water and in Deuterium Oxide¹

1965 • 8 citations

Erdalkalihydrogenfluoride SrF(HF2) und BaF(HF2)

1983 • 8 citations

Strongly hydrogen-bonded molecular solids with 'highly anomalous' infrared OH absorption

1974 • 8 citations

Very short OHO hydrogen bonds in a binuclear transition-metal complex. Neutron-diffraction study at 120 K of [Co₂{H₂N(CH₂)₂OH}₃{H₂N(CH₂)₂O}₃][ClO₄]₃·0.5H₂O

1986 • 8 citations

1,6-Diazabicyclo[4.4.4]tetradecane and its inside-protonated ion: changes in space groups

1984 • 7 citations

The infrared spectrum of isotopically diluted potassium hydrogen bifluoride. Doubling of the HF stretching and of the v3 + v1 band

1977 • 7 citations

Equilibrium constants of HF, F-, HF-2 and H2F2 species in formic acid.

1977 • 7 citations

Protonation of a β-diketone; an intramolecular hydrogen bond with an extremely deshielded proton

1988 • 7 citations

Study of alkali halide/FHF− systems at 10 – 290 K, 0 – 8 kBAR

1984 • 7 citations

Solvent Deuterium Isotope Effects on Intramolecularly Hydrogen-Bonded Dicarboxylic Acid Monoanions¹

1966 • 7 citations

Electronic structure and hydrogen bonding of valence-ionized states of β-hydroxyacrolein

1983 • 7 citations

Deuteriation of an asymmetric short hydrogen bond. X-Ray crystal structure of KF·(CH₂CO₂D)₂

1981 • 6 citations

Carboxyl group participation in sulfate and sulfamate group transfer reactions

1982 • 6 citations

A comparison between predicted and measured structural parameters in symmetric hydrogen bonds

1981 • 6 citations

Röntgenuntersuchungen des Ammoniumbifluorids (NH₄HF₂)

1932 • 6 citations

The mechanism of the reaction between hydroxyl ions and some internally hydrogen-bonded acids

1976 • 6 citations

Change in rate-limiting step in proton removal from an intramolecularly hydrogen-bonded acid and the rate of opening and closing of a hydrogen bond

1985 • 6 citations

The hydrogen bond as a key factor in efficient intramolecular proton transfer catalysis

1987 • 6 citations

Hydrogen bonding in 2,3,4-trihydroxyacetophenone

1986 • 5 citations

Generation of carbenium ions by phase transfer catalysis. A convenient preparation of benzylic fluorides.

1977 • 5 citations

Stabile Enole von α‐[Aryl(Alkyl)oxy]dibenzoylmethanen

1981 • 5 citations

Isotope effects on hydroxide ion in aqueous solution

1972 • 5 citations

A calorimetric and PMR study of the interaction between hydrogen chloride and dimethylacetamide. Tetramethylurea and dimethylsulphoxide in 1,2-dichloroethane

1972 • 5 citations

VERY STRONG HYDROGEN BONDING OF PHOSPHATES: X-RAY CRYSTAL STRUCTURES OF 1,2,3-BENZOTRIAZOLIUM DIHYDROGEN PHOSPHATE [C₆H₄N₃H₂ ⁺][H₂PO⁻ ₄] AND 1,3-BENZIMIDAZOLIUM DIHYDROGEN PHOSPHATE [C₇H₅N₂H₂ ⁺][H₂PO⁻ ₄]

1988 • 5 citations

Tetra-alkylammonium fluorides and chlorides in glacial acetic acid solution: nuclear magnetic resonance studies, deuterium fractionation factors (φ), and anion hydrogen bonding

1986 • 4 citations

A negligibly small primary kinetic isotope effect for slow proton transfer from a protonated amine

1980 • 4 citations

Neutron diffraction study of the orthorhombic form of [3,3'-dimethyl-3,3'-(2-nitropropanediylidenediamino)bis(2-butanone oximato)-N,N',N'',N''']nickel(II)

1981 • 4 citations

PROTON-TRANSFER REACTION OF 2,4-DIHYDROXYAZOBENZENE IN AQUEOUS SOLUTION

1977 • 4 citations

Fractionation factor for hydrogen isotopes at the aqueous ligand of cobalt in cobalt(II)-substituted bovine carbonic anhydrase

1981 • 4 citations

Strong hydrogen bonding in aliphatic diol solutions of alkali-metal fluorides

1981 • 3 citations

Study of short hydrogen bonds. II. Structure of piperidinium hydrogen bis(p-bromobenzoate)

1989 • 3 citations

Inside-protonated 1,7-diazabicyclo[5.4.2]tetradecane picrate

1988 • 3 citations

Inside-protonated 1,7-diazabicyclo[5.5.2]tetradecane picrate

1988 • 3 citations

Mechanism of proton transfer from an intramolecularly hydrogen-bonded acid in aqueous solution

1976 • 3 citations

Inside-protonated 1,7-diazabicyclo[5.5.4]hexadecane p-nitrobenzoate p-nitrobenzoic acid solvate

1988 • 2 citations

Inside-protonated 1,7-diazabicyclo[5.4.3]tetradecane trifluoromethanesulfonate

1988 • 2 citations

Intramolecular proton transfer catalysis of ether hydrolysis

1987 • 2 citations

Inside-protonated 1,8-diazabicyclo[6.5.3]hexadecane picrate

1988 • 2 citations

Fatti per servire alla storia de’ fluoruri

1862 • 1 citations