Problem+set+4 KEY CHEM+4320 PDF

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CHEM 4320 Problem Set #4 1. A) Construct a molecular orbital diagram for Si2. Assume that 3d orbitals are not used and assume the same s/p mixing as is observed for C2. Label the MOs as σ/σ* or π/π*. B) Use your MO diagram to predict a bond order for Si2. C) Would oxidizing Si2 by a single electron increase or decrease the Si-Si bond distance? A) The MO diagram of Si2 is shown below. Si

Si σ*

π*

3p

3p

σ

π

σ* 3s

3s

σ

B) The bond order of Si2 is 2 … ½ (6-2) = 2. C) Oxidizing Si2 by a single electron would increase the Si-Si bond distance by reducing the bond order to 1.5 2. On the basis of molecular orbitals, predict the shortest bond, and provide a brief explanation for: A) Li2 or Li2+

B) F2 or F2 +

C) He2 +, [H-He]+or H2+

A) Li2 has shorter bond. The bond order of Li2 is 1 whereas the bond order of Li2+ is 0.5. B) F2+ would have shorter bond. The bond order of F2 is 1 whereas the bond order of F2+ is 1.5.

C) [H-He]+ would have the shorter bond. The bond order of [H-He]+ is 1 whereas the bond orders of He2+ and H2+ are 0.5. 3. Predict the strongest bond among the series … HCl, HBr or HI. HCl > HBr > HI 4. Use MO diagrams to predict the bond order and relative bond strengths of O2, O2and O22-. For each, predict whether they would be paramagnetic or diamagnetic. 2Bond order of O2, O2- and O2 is respectively 2, 1.5, and 1. 2Relative bond strengths order: O2 > O2- > O2 .

O2 and O2- are paramagnetic while O22- should be diamagnetic. The MO diagram of O2 is shown below. O

O

σ*

π*

2p

2p π σ

σ* 2s

2s σ

5. For the molecular HF, the F 2s orbital does not mix to any appreciable extent with the H 1s orbital. For HCl, do you think that the mixing between the H 1s and Cl 3s orbtials will be less than the HF mixing, about the same or more than? Briefly explain your answer. The mixing between the H 1s and Cl 3s orbitals will be more substantial than the HF mixing. Cl 3s orbital potential energy is closer to H 1s orbital energy compared to F 2s orbital, which leads to better orbital mixing. 6. Construct a molecular orbital diagram for HCl and predict the character of the HOMO(s). Table 5.2 in your textbook provides the energy of valence atomic orbitals. The HOMOs are more localized on Cl. The MO diagram is shown below. The 3s orbital will mix a little into 1s, but the mixing is not substantial due to differences in potential energy. H

Cl σ*

p orbitals (px & py) 3p

σ 3s

1s

7. The SALCs for the H 1s orbitals of methane are shown below. Using one of the T2 SALCs, draw the σ* molecular orbitals.

A1

T2

The σ* molecular orbitals are shown below.

8. Construct of MO diagram for NO (note that it will be similar to CO). Compare the bond orders of NO, NO+ and NO-. The MO diagram of NO is shown below. O

N σ*

π* 2p 2p σ

π σ* 2s 2s σ

Molecule NO+ NO NO-

Bond order 3 2.5 2

9. Construct of MO diagram for NF. Assume s/p mixing similar to homonuclear diatomics of the 2nd row before O2. The MO diagram of NF is shown below. F

N σ*

π* 2p 2p

σ π

σ*

2s

2s σ

10. What happens to the orbital overlap and, hence, energy of the MO, as the bond angle for the MOs shown below is increased toward 180 degrees?

Orbital overlap Energy of MO

Decreases Destabilized Less bonding

Increases Stabilized More bonding

11. The SALCs for the 1s orbitals of BH3 (D3h point group) are of A'1 and E' symmetry. Using the D3h point group, construct of MO diagram using the SALCs and the 2s and three 2p orbitals of boron. You do not need to place symmetry labels on the final MOs, but you should clearly indicate which atomic orbitals are used. One of the four boron atomic orbitals will remain non-bonding, which one? The MO diagram is shown below. The pz orbital of B remains non-bonding. B

H z

σ*

y

a1!∗ x

z

σ*

z

y

e!∗

y x

x

z 2p

a2 # y x 1s z

z 2s

x

x z σ

y

y

e!

σ

a1 !

y x...