Orbital Overlap Formation of a bond by overlapping orbitals on two different atoms A, B Occupied by a pair of electrons Y = YAxYB Overlap conditions: • Proper symmetry, sign of wave functions • Suitable energies, comparable, not too disparat Types of Orbital Overlap Sigma bond, a Electron density localized on the connection line between atoms, customarily assigned as z Pi bond, 71 Electron density localized above and below the connection line between atoms, one nodal plane 1 2 Types of Orbital Overlap Delta bond, 5 Electron density localized off the connection line between atoms, two nodal planes Types of Orbital Overlap Sigma Bond, a Electron density localized on the connection line between atoms 5 Pi vazba, n Electron density localized above and below the connection line between atoms One nodal plane Same for px and p, Effectivity of Orbital Overlap Shorter distance = Better orbital overlap At the same distance : a > n > 8 Bonding Parameters Anion Bond Order Bond Distance, Ä cio- 1.0 1.67 cio2- 1.50 1.58 CIO3- 1.67 1.49 cio4- 1.75 1.43 CI02+ 2.0 ▼ 1.39 ▼ Bond Order increases Bond Distance decreases Bonding Parameters Bond Distance A] Energy [kJ moh1] C-C 1.54 348 C=C 1.34 612 C=C 1.20 837 C-0 1.43 360 C=0 1.23 743 C=0 1.13 1074 N-N 1.47 163 N=N 1.24 409 NeN 1.10 944 11 Bond E, kJ mol-1 Bond Polarity H-H 431 Nonpolar F-F 155 Nonpolar H-F 565 Polar C-Br 240 276 Bond polarity increases C-Ci 339 C-F 485 m.D, °C Ge-Ge 188 937 Si-Si 226 1412 C-C 347 3827 Bonding Parameters Bond E, kJ moh1 Length, A C-l 240 2.16 C-Br 276 1.91 C-CI 339 1.79 C-F 485 1.40 Pauling ED(AB) = {ED(AA) x ED(BB)}* + A A = 96.48 (Xa -Xb)2 Schomaker-Stevenson rab ~ ra + rb 009 Xa-Xb i3 Bond angles 90° are rare for main group elements, common values are 109, 120, 180° Hybridization = energetical mixing and directional change of atomic orbitals on the same atom Number of hybrid orbitals = Number of mixed atomic orbitals sp-Hybridization sp-Hybridization sp-Hybridization Acetylene • 2 a bonds - C(sp)-H(s) overlap • 1 a bond - C(sp)-C(sp) overlap • 2 perpendicular %- bonds (x, y), C(p)-C(p) overlap sp2-Hybridization sp2-Hybridization 22 sp2-Hybridization Ethylene • 4 a bonds - C(sp2)-H(s) overlap • 1 a bond - C(sp2)-C(sp2) overlap • 1 7i bond - C(px)-C(px) overlap 24 Benzene • Each C uses 3 sp2 orbitals for 3 a-bonds 2 C-C bonds and 1 C-H bond • One 2px orbital on each C remains unused for a-bondjng Benzene 26 sp3dn - Hybridization Bonding in C032_ and N03~ 9 (6+3) bonding electron pairs / 6 bonds Bond Order = 1.5 34 Electronegativity and Bonding Angles Bonding Angle Hybridization NH3 107.3° sp3 PH3 93.8° AsH3 91.8° SbH3 91.3° s + 3p NF3 102.5° Incr. Ax decreases bonding angles OH2 OF2 104.5° 103.2° 35 Bent's Rule More electronegative substituents prefer hybrid orbitals with smaller s-contribution and conversely, electropositive substituents (better donors) prefer hybrid orbitals with higher s-contribution Better donors occupy equatorial plane in TBP and basal plane in SP Free electron pair is the best donor = substituent with zero electronegativity Hybridization and Electronegativity t t s-character p.character Electron in an s-orbital is bound more strongly than in a p-orbital Orbital with higher s-character has a higher electronegativity PES = Photoelectron Spectroscopy hv = IE + E, X-ray Photoelectron Spectroscopy (XPS) - Soft X-rays (200-2000 eV) ejects inner e UV Photoelectron Spectroscopy (UPS) - vacuum UV (10-45 eV) ejects valence e measured Octahedron Virus of common cold Virus of polio Icosahedron Virus of mouth and foot disease 42 Symmetry of Molecules Placement of atoms in moleculs = equivalent atoms Symmetry Elements and Operations Element Operation Identity Identity No change, (= 1) Inversion center Inversion Every point x,y,z translated to -x,-y,-z Proper axis Proper rotation Rotation about axis by 360/n degrees Symmetry plane Reflection Reflection through plane Improper axis Improper rotation 1. rotation by 360/n 2. reflection through plane perpendicular to rotation axis 44 Inversion Center New situation is indistinguishable from the original 45 Inversion Center J^^^^J Proper Axis C2 ^^fi Rotation about axis by 360/2 degrees. New situation is indistinguishable from the original 1 1 D*«c - bjmc1 |c3> <9d b9 *^AI 1(a) 1 Ic^T 1 dQT "•<=! 48 Proper Axis C3 Rotation about axis by 360/3 degrees New situation is indistinguishable from the original Proper Axis C4 Rotation about axis by 360/4 degree > ^ ^ ^4^ — ^ New situation is indistinguishable from the original Proper Axis Cn The main (principal) axis = the highest n Symmetry Plane a Any planar molecule has a symmetry plane = perpendicular to the main axis H is negative or positive end of dipole 63 Dipole Moment in Diatomic Molecules Enlace XA-XB u(D) Enlace XA-XB u(D) Enlace XA-XB u(D) C-H 0.30 0.4 N=C 0.9 Br-P 0.68 0.36 N-H 0.87 1.31 N=C 3.5 l-P 0.15 0 N-D 1.30 O-C 1.00 0.74 0=As 4.2 H-P 0.14 0.36 o=c 2.3 F-As 1.90 2.03 H-As 0 0.10 c-s 0.06 0.9 CI-As 0.63 1.64 H-Sb 0.38 0.08 c=s 2.6 Br-As 0.54 1.27 O-H 1.30 1.51 C-Se 0.02 0.8 I-As 0.01 0.78 O-D 1.50 C-Te 0.49 0.6 Cl-Sb 1.01 0.78 S-H 0.24 0.68 F-C 1.60 1.41 Br-Sb 0.92 1.9 F-H 1.90 1.94 Cl-C 0.33 1.46 l-Sb 0.39 0.8 Cl-H 0.63 1.08 Br-C 0.24 1.38 o=s 2.8 Cl-D 1.09 C-l 0.29 1.19 Cl-S 0.39 0.7 Br-H 0.54 0.78 O-N 0.43 0.3 O-CI 0.67 0.7 l-H 0.01 0.38 0=N 2.0 F-CI 1.27 0.88 C-C 0 0 F-N 1.03 0.17 F-Br 1.36 1.3 c=c 0 o=p 2.7 CI-Br 0.09 0.57 c^c 0 S=P 3.1 Br-I 0.53 1.2 N-C 0.57 0.22 Cl-P 0.77 0.81 67