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Six Lectures on the Nature of the Hydrogen Bond

Lecture 1Introduction to the Hydrogen Bond:Basic Concepts and Summary of Our First Studies

from 1989 to 2002

Edited byPaola Gilli (paola.gilli@unife.it)Researcher of Physical Chemistry,

andGastone Gilli (gastone.gilli@unife.it; www.ggilli.com)

Freelance, Former Professor of Physical Chemistry

Department of Chemical and Pharmaceutical Sciencesand Centre for Structural Diffractometry,

University of Ferrara, Italy

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Modern Hydrogen Bonding TheoryModern Hydrogen Bonding TheoryGastone Gilli

23rdEuropeanCrystallographic

Meeting

6-11 August 2006Leuven, Belgium

The topics of the present lecture have been previouslypresented to other meetings and, in particular, to:

CUSO Summer School2012

on Hydrogen Bonding

20-24 August 2012Villars sur Ollon,

Switzerland

Six Lectures on the Nature of the Hydrogen BondSix Lectures on the Nature of the Hydrogen Bond

Gastone Gilli

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Hydrogen Bond DefinitionsHydrogen Bond Definitions

A Three-Center-Four-Electron InteractionRDH . . . . :AR

where D is the HB Donor {an electronegative atom such as F, O, N, C, S, Cl, Br, I}and :A the HB Acceptor or Lone Pair Carrier {A second electronegative atom or

a multiple bond, that is-bond}

Alternatively:A Proton Sharing Interaction

RD: . . . H+ . . . :ARbetween two electron pairs

located on two adjacent electronegative atoms

Two Important HB PropertiesTwo Important HB Properties The HB acceptor is not an atom but an electron pair located on that atom Since both D and A must be more electronegative than H, all HBs have polarity

RDH++++ . . . .. . . .. . . .. . . . :AR

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Electrostatic and Covalent HBs: The PaulingElectrostatic and Covalent HBs: The Paulings Models Model

In The Nature of the Chemical Bond, L. Pauling describes two types of HBs:Weak and dissymmetric HBs of electrostatic nature. "It is recognized that the hydrogen atom,with only one stable orbital (the 1s orbital), can form only one covalent bond, that the hydrogenbond is largely ionic in character, and that it is formed only between the most electronegativeatoms." (HB Chapter, p. 1)

Strong and symmetric HBs of covalent nature: The exceptions. These exceptions aredescribed in terms of VB theory as: . . . the hydrogen bond in the [HF2]

ion lies midway thetwo fluorine atoms and may be considered to form a half-bond with each. (HB Chapter, p. 49)

[F..H..F] [O..H..O] [O..H..O]+ [H2O..H..OH2]+

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TheThe CoulsonCoulsons VBs VB TreatmentTreatment..

TheThe Standard HB ModelStandard HB Model

Paulings ideas acquired theoretical weightwith the VB treatment by Coulson and Danielsson(1954) where the O-H...O bond is depicted as a

mixture of three main VB forms, two covalent andone ionic.

This line of thought was also adopted byPimentel and McClellan in their famous book TheHydrogen Bond(1960).

They wrote: At the 1957 LjubljanaConference one of the important points of fairlygeneral accord was that the electrostatic modeldoes not account for all of the phenomena

associated with H bond formation.

O H :O< COV1, NCT

O: +H :O