Chemical Bonding

Chemical Bonding Hydrogen atom based atomic orbitals a.k.a. hydrogen atom wavefunctions: 1s, 2s, 2p, 3s, 3p, 3d, ……. 1s = 1/()1/2(1/a0)3/2 exp[-r/a0], a0=Bohr Radius = 0.529 Angstroms Electron z (x,y,z)  r y Nucleus  r2=x2+y2+z2 x

Orbitals, Wavefunctions and Probabilities The orbital or wavefunction is just a mathematical function that can have a magnitude and sign (e.g. + 0.1 or -0.2) at a given point r in space. Probability of finding a 1s electron at a particular point in space is often not as interesting as finding the electron in a thin shell between r and r+dr.

Orbitals, Wavefunctions and Probabilities Probability of finding a 1s electron in thin shell between r and r+dr: Prob(r,r+dr) ~ 1s1s [r2]dr Volume of shell of thickness dr: r  dV = (4/3) [(r)3+3r2dr+ 3r(dr)2+(dr)3 - r3] r+dr dV≈(4)[r2dr] [r>>>dr  3r2dr>>> 3r(dr)2+(dr)3 ]

Bonding in Diatomic Molecules such as H2 Bonding Axis z1 z2   r1 r2 y1 y2   x2 x1 H Nucleus A H Nucleus B 1s(A) = 1/()1/2(1/a0)3/2 exp[-r1/a0], 1s orbital for atom A Note the two orbitals are centered at different points in space.

1s = C1[1s(A) + 1s(B)], Sigma 1s Bonding Molecular orbital. C1 is a constant. Note that probabilities for finding electron at some position in space scale like [1s]2 and [1s*]2: [1s*]2 = {C2[1s(A) - 1s(B)]}2 = (C2)2{[1s(A)]2 + [1s(B)]2 - 2[1s(A)][1s(B)]} “Non-interacting” part is result for large separation between nucleus A and B [1s]2 = {C1[1s(A) + 1s(B)]}2 = (C1)2{[1s(A)]2 + [1s(B)]2 + 2[1s(A)][1s(B)]}

Notational Detail Oxtoby uses two different notations for orbitals in the 4th and 5th editions of the class text: 1s in the 4th edition becomes g1s in the 5th edition 1s* in the 4th edition becomes u1s* in the 5th edition The addition of g and u provides some extra identification of the orbitals and is the one encountered in the professional literature. g and u are from the German “gerade” and “ungerade”

+ H 2 2 2 [ s [ s * ] ] 1s 1s A B * A B [y - y ] s = C 2 1s 1s 1s A B [y + y ] s = C 1 1s 1s 1s 2 2 2 [(y ) + (y ) ] A B (n.i.) ~ y 1s 1s Wave Functions Electron Densities 1s(A) + B ANTIBONDING - Pushes e- away from region between nuclei A and B A -1s(B) - 2[1s(A)][1s(B)] + 2[1s(A)][1s(B)] BONDING + + Pushes e- between nuclei A and B A B A B 1s(A) 1s(B) + + NON-INTERACTING A B A B

+ Potential Energy of H 2 ∆Ed= 255 kJ mol-1 H R H V(R) Separated H, H+ *1s H + H+ R 0 1s Single electron holds H2+ together 1.07 Å

CORRELATION DIAGRAM Energy Ordering: s1s < 1s < s1s* H2 s1s* E 1s 1s (H Atom A) (H Atom B) Atomic Orbital Atomic Orbital s1s H2 Molecular Orbitals

CORRELATION DIAGRAM Z for He =2 He2 s1s* E 1s 1s (He Atom A) (He Atom B) Atomic Orbital Atomic Orbital s1s He2 Molecular Orbitals

Bonding for Second Row Diatomics Involves the n=2 Atomic Shell Lithium atomic configuration is 1s22s1 (Only the 2s electron is a valence electron.) [(1s)2(1s*)2] (2s)2 = [KK](2s)2 Li2 dimer has the configuration: CORRELATION DIAGRAM For Li2 (2s)2, Bond order = (1/2)(2-0) = 1 Li2 s2s* E 2s 2s (Li Atom A) (Li Atom B) Atomic Orbital Atomic Orbital s2s Li2 Molecular Orbitals

Bonding for Second Row Diatomics: the Role of 2p Orbitals Once the 2s, 2s* molecular orbitals formed from the 2s atomic orbitals on each atom are filled (4 electrons, Be2), we must consider the role of the 2p electrons (B2 is first diatomic using 2p electrons). There are 3 different sets of p orbitals (2px, 2py, and 2pz), all mutually perpendicular. If we choose the molecular diatomic axis to be the z axis (this is arbitrary), we have a picture like this:

Bonding Axis Bonding Axis x1 x1 x2 x2 z1 z1 z2 z2     y2 y2 y1 y1 Nucleus A Nucleus A Nucleus B Nucleus B 2pz obital on atom 1 and atom 2 2pz orbitals point at each other. - + - + 2px orbitals are parallel to each other. 2px orbital on atom 1 and atom 2 + + - -

Chemical Bonding