Bonding Weaker than Covalent

Intramolecular forces (bonds) Hold atoms together in molecules Have high energy associated with them it’s difficult to break molecules into their individual atoms Different types based upon what is going on with the electrons (electron clouds)

Types of bonds: Ionic Covalent Ionic attraction between fully charged molecules/ atoms NaCl, made from Na+ and Cl-; or Ca(OH)2, made from Ca2+ and 2OH- Covalent electrons are shared between atoms, water (H2O) and sugar (C6H12O6) Can be polar or nonpolar Based on electronegativity VSEPR geometry (shape)

Intermolecular forces (IMFs) Hold molecules together Much weaker than intramolecular forces Intramolecular bonds are usually 100x or even 1000x stronger

Intermolecular forces exist between molecules Intermolecular forces exist between molecules. Bonds exist within molecules.

Intermolecular forces (IMFs)

HYDROGEN BONDING A hydrogen bond is a bond between a functional group A-H and an atom or group of atoms B in the same or a different molecule. With exceptions, hydrogen bonds are assumed to form only when A is oxygen, nitrogen, or fluorine and when B is oxygen, nitrogen, or fluorine. The oxygen may be singly or doubly bonded and the nitrogen singly, doubly, or triply bonded. The bonds are usually represented by dotted or dashed lines

HYDROGEN BONDING

HYDROGEN BONDING

HYDROGEN BONDING

HYDROGEN BONDING

HYDROGEN BONDING When two compounds whose molecules form HB with each other are both dissolved in water, the HB between the two molecules is usually greatly weakened or completely removed. In amides, the oxygen atom is the preferred site of protonation or complexation with water. In the case of dicarboxylic acids, there is little or no evidence for strong HB in aqueous solution, although recent studies concluded that strong, intramolecular HB can exist in aqueous acetone solutions of hydrogen maleate and hydrogen cis-cyclohexane-1,2-dicarboxylate.

Geometry Hydrogen Bonds In most (though not all) cases, the hydrogen is on or near the straight line formed by A and B. This is true both in the solid state, and in solution. The vast majority of intramolecular HB occurs where 6-membered rings can be formed, in which linearity of the HB is geometrically favorable, while 5-membered rings, where linearity is usually not favored, are much rarer.

Bifurcated or Three-Center HB In certain cases, X-ray crystallography has shown that a single H–A can form simultaneous hydrogen bonds with two B atoms

Detection of HB HB has been detected by measurements of dipole moments, solubility behavior, freezing-point lowering, and heats of mixing, but one important way is by the effect of the HB on IR. The IR frequencies of groups, such as O–H or CO, are shifted when the group is HB. HB always moves the peak toward lower frequencies, for both the AH and the B groups, though the shift is greater for the former. In dilute solution, there is partial HB, that is, some OH groups are free and some are HB. In such cases, two peaks appear. IR can also distinguish between inter- and intramolecular HB, since intermolecular peaks are intensified by an increase in concentration while intramolecular peaks are unaffected.

Detection of HB Raman, electronic, and NMR have been used for the detection of HB. Since HB involves a rapid movement of protons from one atom to another, nmr records an average value. HB can be detected because it usually produces a chemical shift to a lower field. For example, carboxylic acid–carboxylate systems arising from either mono- or diacids generally exhibit a downfield resonance (16–22 ppm), which indicates ‘‘strong’’ HB. in anhydrous, aprotic solvents.

Importance of HB 1. Intermolecular HB raises boiling points and frequently melting points. 2. If HB is possible between solute and solvent, this greatly increases solubility and often results in large or even infinite solubility where none would otherwise be expected. 3. HB causes lack of ideality in gas and solution laws. 4. As previously mentioned, HB changes spectral absorption positions. 5. HB, especially the intramolecular variety, changes many chemical properties.

π–π INTERACTIONS

π–π INTERACTIONS

CATION–π INTERACTIONS

Van der Waals Force

Supramolecular Chemistry

ADDITION COMPOUNDS Electron donor–acceptor complexes When the reaction of two compounds results in a product that contains all the mass of the two compounds, the product is called an addition compound. Electron donor–acceptor complexes Complexes formed by crown ethers and similar compounds, Inclusion compounds Catenanes

Electron Donor–Acceptor Complexes In EDA, there is a donor (D) and an acceptor (A) molecule. The D may donate an unshared pair (an n donor) or a pair of electrons in a p orbital of a double bond or aromatic system (a π donor). These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Complexes in Which the A Is A Metal Ion and the D an Alkene or an Aromatic Ring Complexes in Which the A Is an Organic Molecule

Complexes in Which the A Is A Metal Ion and the D an Alkene or an Aromatic Ring

Complexes in Which the A Is A Metal Ion and the D an Alkene or an Aromatic Ring

Complexes in Which the A Is an Organic Molecule Picric acid forms addition compounds with many aromatic hydrocarbons, aromatic amines, aliphatic amines, alkenes, and other compounds. These addition compounds are usually solids with definite melting points. The bonding in these cases is more difficult to explain than in the previous case, and indeed no really satisfactory explanation is available. The difficulty is that although the donor has a pair of electrons to contribute, the acceptor does not have a vacant orbital. Simple attraction of the dipole-induced dipole type accounts for some of the bonding, but is too weak to explain the bonding in all cases; for example, nitromethane, with about the same dipole moment as nitrobenzene, forms much weaker complexes.

Crown Ether Crown ethers are large-ring compounds containing several oxygen atoms, usually in a regular pattern. 12-crown-4 15-crown-5 Dicyclohexano-18-crown-6

Crown Ether Thiacrown ethers Azacrown ethers

Crown Ether az Molecular Device V. Balzani, A. Credi, M. Venturi Molecular Devices and Machines - A Journey into the Nano World. 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Podands The host compounds in which two or more arms come out of a central structure

Lariat Ethers Lariat ethers are compounds containing a crown ether ring with one or more side chains

Cryptates Bicyclic molecules like 20 can surround the enclosed ion in three dimensions, binding it even more tightly than the monocyclic crown ethers. Bicyclics and cycles of higher order are called cryptands and the complexes formed are called cryptates

Calixarenes

Spherands

Starands

Molecular Capsule

Inclusion Compounds In this case the host forms a crystal lattice that has spaces large enough for the guest to fit into. There is no bonding between the host and the guest except van der Waals forces. There are two main types, depending on the shape of the space. The spaces in inclusion compounds are in the shape of long tunnels or channels, while the other type, often called clathrate, or cage compounds have spaces that are completely enclosed. In both types, the guest molecule must fit into the space and potential guests that are too large or too small will not go into the lattice, so that the addition compound will not form

Inclusion Compounds

Inclusion Compounds

Cyclodextrins

Cyclodextrins

Molecular Devices V. Balzani, A. Credi, M. Venturi Molecular Devices and Machines - A Journey into the Nano World. 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Molecular Devices V. Balzani, A. Credi, M. Venturi Molecular Devices and Machines - A Journey into the Nano World. 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Macroscopic and Molecular Brakes. V. Balzani, A. Credi, M. Venturi Molecular Devices and Machines - A Journey into the Nano World. 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Catenanes and Rotaxanes These compounds contain two or more independent portions that are not bonded to each other by any valence forces but nevertheless must remain linked. [n]- Catenanes (where n corresponds to the number of linked rings) are made up of two or more rings held together as links ina chain, while in rotaxanes a linear portion is threaded through a ring and cannot get away because of bulky end groups.

Rotaxanes

A. Altieri et al. J. Am. Chem. Soc. 2003, 125, 8644

Cucurbit[n]uril-Based Gyroscane

Cucurbit[n]uril-Based Gyroscane