Inorganic chemistry: Test 3
What happens to internuclear bond length btw B2 and F2?
internuclear bond length shorten up to N2 then lengthen afterwards as antibonding MOs begin to fill (page 138)
What is significance of LUMO (lowest unoccupied orbital)?
large, empty..often antibonding...orbitals can act as lewis acids or electron acceptors for example, LUMO antibonding 2pz on boron (in BF3) can accept electrons from HOMO of NH3
What is an antibonding MO?
destructive interaction btw two AOs ie. AO's interact with different signs..dark + light part blas
What does MO look like when 2px or 2py (on bond axis) interact constructively?
pi (π)
Molecular orbital diagram for N3- (azide):
-Like CO2, HOMO are pair of nonbonding π orbitals -unlike CO2 all atoms have same initial orbital energies; so best combos formed by identical AOs e.g. 3 x 2s or 3 x pz etc -2s interactions not shown, but form σ , nonbonding σ, and σ* pairs -nb σ is higher in energy than nb π bc of sp interaction...remember σ are pointed towards nuclei
What does MO diagram look like for heteronuclear diatomic molecules like HF?
-MO receives unequal contrib from different AOs -more electronegative atom (fluorine) lower on diagram!!! (lower in energy) -F also contributes more to bonding MO, so closer on diagram -2pz of F (-19eV) and 1s of H (-14eV) have similar energy, so interact to form σ - 2px and spy of F remain nonbonding...guess still fill pi (π) MOs
Steps to identifying MO of complex polyatomic molecules?
1. Determine point group. For C∞v and D∞h, use C2v and D2h respectfully. 2. Assign x,y,z axes to atoms. Z axis for highest rotation axis. Y axis of outer atoms point towards central atom. 3. Construct reducible representations for orbitals of outer atoms ie. 2s, 2pz, 2py, 2px 4. Reduce each to irreducible representation..use chart and formula ...1/order x Σ [ (order of operation) x (reducible Γ total) x (irreducible value from table)] 5. Identify atomic orbital of central atom with same symmetries i.e. Ag + Bu ...can eyeball it (see page 154) 6. Combine AO of central atom and those of group orbital to form MO. Total number of MO formed must equal number of AO from all atoms
How do u calculate bond order of molecule?
1/2 (# e's in bonding MOs - # e's in antibonding MOs) = bond order ...a bond order > 0 indicates molecule stable relative to separate atoms ...higher bond order = more stable
Molecular orbital diagram for H2O: Symmetry for AO of central atom (oxygen).
2s: A1..bonding 2pz: A1..bonding 2px: B1..bonding 2py: B2..not symmetric with H-H Both s and pz orbitals on oxygen match with the A1 SALC, so s-p mixing can be expected.
Molecular orbital diagram for NH3+: Symmetry for AO of central atom (nitrogen).
2s: A1..bonding 2pz: A1..bonding 2px: E..bonding 2py: E..bonding based on symmetry all capable of bonding, but do they?
Molecular orbital diagram for NH3+: Draw MO diagram. How many MO are formed?
4 •hydrogen GO's have symmetry A1+E •central atom: s (A1), pz (A1), px+py (E) (book says 3 bc A1 from 2pz is nonbonding, but other sources say bonding MO) ...book also later calls 2pz A1 "slightly bonding" and labels it as the HOMO
Why isn't Helium a diatomic molecule (He2)?
4 electrons to place in σ (bonding) and σ* (antibonding) MO's. Stabilization from bonding MO is cancelled out by antibonding MO. And has zero bond order..1/2 (2-2) = 0..suggests doesn't exist.
Do MO diagram for N2 (N=7e). Which fill first... σ (of p orbitals) or pi (of p orbitals)?
5 VE x 2 = 10 electrons 4 in 2s orbitals (2 σ and 2 σ*) 6 in 2p (2 σ and 4 pi) Notice pi (of 2px, 2py orbitals) appears below the σ (2pz)
Do MO diagram for O2. Paramagnetic or diamagetic?
6 VE x 2 = 12 e's 4 in 2s orbitals (2 σ and 2 σ*) 8 in 2p (2 σ, 4 pi, 2 σ*!!) ~paramagnetic b/c two highest energy electrons are unpaired (pic is correct except π should be above σ MO!)
With sp3 hybridization of CH4, combining all 4 atomic orbitals of central atom (one 2s and three 2p) into 4 equivalent hybrid orbitals, one directed to each H atom. Using C-H σ bonds as vectors, make reducible representation. Using Td table, which symmetry groups (A1, A2, T1, T2, E) represents S and P orbitals respectively?
A1 for s orbital All p orbitals degenerate and have T2 symmetry notice if add together = hybridized representation
What is a molecular orbital? How do molecular orbital form?
Atomic orbitals combine to form molecular orbitals which surround the molecule. Similar to AO's, MO's are wave functions (probability distribution etc). Each molecular orbital can only have 2 electrons, each with an opposite spin. Form when to atoms in close proximity and electrostatic (attractive) forces btw electrons and + nuclei hold them together e.g. covalent bond. (MO's only form btw orbitals of atoms bonded to each other?)
Draw molecular orbital diagram for H2O:
B1 formed with 2px-1s B2 is nonbonding antibonding A1 from 2s-1s & 2pz-1s merged for whatever reason
Using B-F bonds as vectors, make reducible representation. Using D3h table, which symmetry groups (A1', A2' etc) represents S & P orbitals?
Easy to figure out reducible representation of planar molecule. And atomic orbitals in hybrid have same symmetry as A1' and E'. A1' for s orbital E' for 2px and 2py of central atom
When drawing MO diagram, which filled first...which is lower, σ (of 2pz orbitals) or pi (of 2px, 2py orbitals)? Which filled first... σ* (of 2s orbitals) or lowest 2p orbital?
For nitrogen and elements before it, pi (of 2px, 2py orbitals) appears below the 2p σ. For elements after nitrogen on period table, σ (2pz) molecular orbital is LOWER on diagram. For all, σ* (of 2s orbitals) filled before all 2p MOs.
Which elements form diatomic molecules under standard conditions?
H2, N2, O2, F2 also Cl2, Br2, I2 (7 total and looks like 7 on periodic table)
Molecular orbital diagram for NH3+ (ammonia): Construct reducible representations for orbitals of outer H atoms. Reduce irreducible representation(s). ...make xz the σv plane w/z axis C3 rotational axis
Only 1 bc just 1s orbital. Γ GO = A1 + E
Molecular orbital diagram for H2O: Construct reducible representations for orbitals of H atoms. Reduce irreducible representation(s). ...make xz plane of molecule w/z axis C2 rotational axis
Only 1 bc just 1s orbital. Γ GO = A1 (both 1s same orientation) + B1 (diff signs)
Molecular orbital diagram for CN-?
VE = 10 bond order= 1/2 (8-2) = 3
{skip} T or F: In Molecular Orbital theory molecule is pictured as collection of nuclei with electron orbitals 'delocalized' over entire molecule.
True - in contrast to valence bond theory in which says there's localized overlap of valence shell AO's
T or F: more electronegative atom (fluorine) lower in energy and appear lower on diagram.
True, also contributes more to bonding MO, so closer on diagram
Molecular orbital diagram for CO2?
assafasf
What happens to bond order as fill anti bonding orbitals?
bond order goes down
bonding MO vs antibonding MO vs original atomic orbitals..which has lower energy?
bonding MO = lower energy, more stable...which is why appears lower on diagram thingy bonding MO < original AO < antibonding MO
Make sense of this.... Decompose the representation into its component irreducible representations to determine the symmetry species of the SALCs. The number of SALCs, including members of degenerate sets, must equal the number of AOs taken as the basis for the representation.
count em
{skip} What is a SALC?
like the symmetry of group orbitals e.g. Ag+B1u ....SALCs (Symmetry Adapted Linear Combinations) are the linear combinations of basis sets composed of the stretching vectors of molecule. The SALCs of molecule can help determine binding schemes and symmetries.
You found that both central atom AO and group orbital have E1 symmetry. As usual, E1 is degenerate symmetry for x and y. What should you be careful of when making MO diagram?
need to designate 4 electrons (2 pair) as bonding on diagram...even if form a σ interaction
According to book p.129, why is destructive σ* bond (2pz) considered addition and constructive σ (of 2pz) called subtraction?
no idea
If bond order is 3, is molecule always triple bond?
not sure
What kind of hybridization is SF6 molecule? Point group?
octahedral (Oh group) is d2sp3 or sp3d2
How do you draw a polyatomic MO diagram?
orbitals from central atom on left, group orbitals of surrounding atoms on far right, and resulting MO in middle
Which molecular orbital is formed by side to side overlap and is formed by two parallel [unhybridized] 2px orbitals: sigma (σ) or pi (π)?
pi (π) fyi- a double bond e.g. ethylene always consists of one sigma and one pi (π) bond perpendicular to eachother
What does MO look like when 2px or 2py (on bond axis) interact destructively?
pi* (π*)
Which 2p orbital interacts with 2s orbital?
pz + s orbital (no mixing btw s & px or py)
What if one of SALCs (GO symmetries) has no match on central atom?
remains nonbonding...put in center of diagram btw bonding and anti-bonding MOs remember if it is perpendicular to plane of molecule (would be π orbital), need to designate 4 electrons (2 pair) as nonbonding on diagram
If compound is diamagnetic, what does this mean?
repelled by magnetic field (e.g. nitrogen is diamagnetic)...d.r. incontrast O2 is paramagnetic..attracted or can be contained in magnetic field
What are 5 d orbitals? (skip)
see pic (to help visualize orbitals and match up with representations on character table)
Using S-F bonds as vectors, make reducible representation. Using Oh table, which symmetry groups represent orbitals involved?
see pic A1g for s orbital Eg for d orbtals (z2 , x2-y2) T1u for p (x, y, z)
What are 2 types of molecular orbitals?
sigma (σ) pi (π)
Which molecular orbital is formed by end to end overlap, has highest density along bond axis (btw nuclei), and is formed by combination of overlapping 's' or 'p' orbitals: sigma (σ) or pi (π)?
sigma (σ) - does not restrict molecular rotation
What kind of hybridization is BF3 which trigonal planar? What would point group be?
sp2 formed by combination of 2s, 2px, and 2py D3h
What kind of hybridization is NH3+?
sp3 (as if tetrahedral) also...lone pair counts as 4th orbital (vector)..so combine one 2s and three 2p into 4 equivalent hybrid orbitals
What does MO look like when 2pz (on bond axis) interact constructively?
σ (actually called 3σ?) ~atomic orbital combine to MO shown here ~this is fluorine molecule but will always look same
Molecular orbital diagram for CO? Which is lower on diagram σ (of 2pz orbitals) or pi? Why?
σ above π (like smaller atom) which apparently has some bullsh*t to do with large amount of interaction btw 2pz of oxygen and 2s & 2p on carbon VE = 10 bond order= 1/2 (8-2) = 3 (C-O triple bond)
What does MO look like when 2pz (on bond axis) interact destructively?
σ* 2pz AOs do not overlap properly
Molecular orbital diagram for FHF-?
•FHF- is linear •1s of H only interacts with 2pz-2pz group orbital of F-F (forms a^g MO) •PE differences btw 1s of H and 2s of F prevents interaction •other electron pairs of F atoms are nonbonding and fill 2px and 2py bonding atomic orbitals •no anti bonding MO are filled
