General and Inorganic Chemistry I. Lecture 1 István Szalai Eötvös University István Szalai (Eötvös University) Lecture 1 1 / 31 Outline István Szalai (Eötvös University) Lecture 1 2 / 31
Molecular Orbital (MO) Theory In molecular orbital theory, we postulate that the combination of atomic orbitals on different atoms forms molecular orbitals, so that the electrons in them belong to the molecule as a whole. Bonding orbitals: the energies of bonding orbitals is always lower (more stable) than the energies of combining orbitals. Antibonding orbitals: this is higher in energy than the original atomic orbitals leading to a repulsion between the two atoms. (number of bonding electrons) (number of antibonding electrons) 2. Bond order: The greater bond order of a diatomic molecule or ion, the more stable we predict it to be. The greater the bond order, the shorter is the bond length and the greater is the bond energy István Szalai (Eötvös University) Lecture 1 3 / 31 Molecular Orbital (MO) Theory István Szalai (Eötvös University) Lecture 1 4 / 31
Molecular Orbital (MO) Theory István Szalai (Eötvös University) Lecture 1 5 / 31 Molecular Orbital (MO) Theory István Szalai (Eötvös University) Lecture 1 6 / 31
Molecular Orbital (MO) Theory: H 2 Stability Stable Bond order 1 István Szalai (Eötvös University) Lecture 1 7 / 31 Molecular Orbital (MO) Theory: He 2 Stability Unstable Bond order 0 István Szalai (Eötvös University) Lecture 1 8 / 31
Molecular Orbital (MO) Theory: Li 2 Stability Stable (exist only in the vapor state) Bond order 1 István Szalai (Eötvös University) Lecture 1 9 / 31 Molecular Orbital (MO) Theory: Be 2 Stability Unstable Bond order 0 István Szalai (Eötvös University) Lecture 1 10 / 31
Molecular Orbital (MO) Theory: B 2 István Szalai (Eötvös University) Lecture 1 11 / 31 Molecular Orbital (MO) Theory: B 2 Stability Stable (exist only in the vapor state) Bond order 1 Magnetic properties Paramagnetic István Szalai (Eötvös University) Lecture 1 12 / 31
Molecular Orbital (MO) Theory: C 2 István Szalai (Eötvös University) Lecture 1 13 / 31 Molecular Orbital (MO) Theory: C 2 Stability Stable (exist only in the vapor state) Bond order 2 István Szalai (Eötvös University) Lecture 1 14 / 31
Molecular Orbital (MO) Theory: N 2 István Szalai (Eötvös University) Lecture 1 15 / 31 Molecular Orbital (MO) Theory: N 2 Stability Stable Bond order 3 István Szalai (Eötvös University) Lecture 1 16 / 31
Molecular Orbital (MO) Theory: O 2 István Szalai (Eötvös University) Lecture 1 17 / 31 Molecular Orbital (MO) Theory: O 2 Stability Stable Bond order 2 Magnetic properties Paramagnetic István Szalai (Eötvös University) Lecture 1 18 / 31
Molecular Orbital (MO) Theory: F 2 István Szalai (Eötvös University) Lecture 1 19 / 31 Molecular Orbital (MO) Theory: F 2 Stability Stable Bond order 1 István Szalai (Eötvös University) Lecture 1 20 / 31
Molecular Orbital (MO) Theory: Ne 2 István Szalai (Eötvös University) Lecture 1 21 / 31 Molecular Orbital (MO) Theory: Ne 2 Stability Unstable Bond order 0 István Szalai (Eötvös University) Lecture 1 22 / 31
Molecular Orbital (MO) Theory: CO Stability Stable Bond order 3 István Szalai (Eötvös University) Lecture 1 23 / 31 Multicentered Bonding Borane BH 3 Diborane Number of electron pairs: 6 Number of bonds:8 István Szalai (Eötvös University) Lecture 1 24 / 31
Intermolecular Forces Attractive forces between molecules Responsible for the bulk properties of matter (e.g. melting point) They are weaker then the the intramolecular forces (covalent bond). H 2 O 2H(g) + O(g) (absorbs 927 kj/mol) H 2 O(l) H 2 O(g) (absorbs 40.7 kj/mol at 100 C) István Szalai (Eötvös University) Lecture 1 25 / 31 Dipol-Dipol and Ion-Dipol Interactions Dipol-dipol interactions: Occur between polar covalent molecules because of the attraction of the δ+ atoms of one molecules to the δ atoms of another molecule. Average dipol-dipol interaction energies are approx. 4 kj/mol of bonds. E dipol dipol = 2kT µ A 2 µ B 2 3R 6 A B Dipol-dipol interactions become less important as temperature increases. δ+ Br δ F Br δ+ δ F Ion-dipol interactions: Occur between polar covalent molecules and ions. Cl δ+ Br δ F István Szalai (Eötvös University) Lecture 1 26 / 31
Dispersion Forces Dispersion forces result from the interaction of the positively charged nucleus of one atom for the electron cloud of an atom in nearby molecules. This induces temporary dipoles in neighboring atoms or molecules. As electron clouds become larger and more diffuse, they are attarcted less strongly by their own nuclei. Thus, they are more easily disorted, or polarized, by adjacent nuclei. ion-induced dipole dipole-induced dipole (hydrates of noble gases Xe H 2 O) induced dipole-induced dipole (responsible for the condensation of noble gases) E disp = 2α2 A α2 B EI AEI B 4R 6 A B (EI A + EI B ) István Szalai (Eötvös University) Lecture 1 27 / 31 Hydrogen Bonding Hydrogen bonds are a special case of very strong dipol-dipol interaction. Strong hydrogen bonding occurs among polar covalent molecules containing H and one of the three small, highly electrinegative elements F, O, or N. δ F δ+ H δ F δ+ H bonding energy (kj/mol) ionic 100 3800 covalent 100 900 H-bond 10 40 dispersion 0.1 10 István Szalai (Eötvös University) Lecture 1 28 / 31
Hydrogen Bonding Evidence for hydrogen bonding: anomaly in te boiling point series of similar compounds. Normal behaviour: boiling point increases with molar mass. H-bonded lightest member has the highest boiling point no individual molecules István Szalai (Eötvös University) Lecture 1 29 / 31 Hydrogen Bonding Intermolecular H-bond Intramolecular H-bond H 2 O: extended network, in ice highly ordered, lower density Very important in biological systems:proteins, DNA (two chains held together by H-bonding at well determined places; adenine-timine, guanine-cytosine) István Szalai (Eötvös University) Lecture 1 30 / 31
István Szalai (Eötvös University) Lecture 1 31 / 31