What is a HOMO (orbital) and LOMO, and a NODAL SURFACE?- please help thanks?
it's LUMO not LOMO Highest Occupied Molecular Orbital vs Lowest Unoccupied Molecular Orbital they go from HOMO to LUMO as they absorb energy. The difference between the two is called "band gap" energy and critical to semiconductor device design.
HOMO and LUMO?
HOMO - highest occupied molecular orbital LUMO - lowest unoccupied molecular orbital due to the energy change that is involved.
What is the relationship between HOMO & LUMO and bonding & antibonding?
To start off, molecular orbitals are the linear combination of atomic orbitals. For every atomic orbital you put in, you get a molecular orbital out. So if I put in 1 AO from atom A and 1 AO from atom B, I end up with 2 MO's shared by both atoms - one bonding, one antibonding. HOMO is the highest occupied molecular orbital; LUMO is the lowest occupied molecular. Putting electrons into the LUMO takes energy - some compounds can do this with heat, others can do it with light - it just depends on the molecular orbital arrangement. I'll get back to this in a second... Remember that bond order is defined as (Electrons in B.O. - Electrons in Anti-bonding Orbitals)/2 Bonding orbitals are orbitals whose overlap allow for the formation of a bond; anti-bonding orbitals are orbitals that, when an electron is placed into them, reduce the bond order. The reason I'm making this distinction is because having an electron in an antibonding orbital does not necessarily mean that your bonds will be broken - it is only when the number of electrons in anti-bonding orbitals equal the number of electrons in bonding orbitals.. So if you had 2 bonding electrons and 2 antibonding electrons, your bond order would be zero. (In other words, you wouldn't have a bond.) But if you had 4 bonding electrons and 2 anti-bonding electrons, then your bond order would be 1 - you would still have a bond (A single one, albeit, but still a bond.) You can have bond orders of 1.5, 2.5, anywhere in between, it just depends on the molecular orbitals available to that molecule. Hope this helps.
How do I find the HOMO and LUMO?
When you've drawn the molecular orbital diagrams, whether it be a full diagram such as you can do for N2, O2 and maybe about as far as H2O and CH4 or even hybridised orbital diagram which obviously are a lot easier to deal with, for the HOMO just simply look at which orbital is occupied and has highest energy a tip it is usually the pi orbital if it is a double bond. And for LUMO just look at the unoccupied orbitals and see which one is lowest in energy, tip is usually the pi* orbital in double bonds. But what if there is no double bond and there is an atom such as CHNH2IF, so there are 3 different bonds the C-I, the C-F and the & C-N (ignoring C-H). In this situation since bonds are sigma you have to think which unoccupied sigma* orbital will have lowest energy (LUMO)? Or which occupied sigma orbital will have highest energy (HOMO)? The pleasing thing to know is nearly always the LUMO is just the a-posing antibonding orbital to bonding. There are 2 factors to consider the 1st is electronegativity for going across a period, a simple rule is the more electronegative the orbital the lower the energy of the orbitals, this means the C-F sigma bonding orbital is definitely not the HOMO because it will have lower energy than the other 2. Now then the 2nd factor to consider is what period is it in, what orbitals are involved in bonding? Although Cl is very slightly more electronegative than N, the orbitals involved in bonding for nitrogen are 2p orbitals which are much lower in energy than the orbitals involved for Cl which are the 3p orbitals. So we can now conclude that the C-Cl bonding MO is the HOMO and the anti-bonding MO is the LUMO. * I have ignored the C-H bond because these bonds have a very short bond length and are way too strong to actually be broken except by very powerful bases. Also the orbital involved in bonding for H is the 1s orbital which i'm not 100% sure where hydrogens 1s fits - energy wise.
Not understanding HOMOS and LUMOS :( HELP PLEASE!!?
H*OMO energy levels: highest (least stable to lowest) lone pairs (n) > π > σ(p) > σ(s) (consider overlaps); the lone pair attached to the least electronegative element will have highest energy (less attraction for central atom). LUMOs lowest energy follow the reverse order n (e.g. B empty 2p)< π*< σ* Two examples acetone: H*OMO : O lone pair; LUMO: π* (C=O) there is a huge absorption in the UV just outside the vis attributed to the n → π* e- transition. 2aminoethanol H*OMO: N lone pair (N less electronegative than O: NH3 is stronger base than H2O) LUMO: difficult but probably O-H σ* (from advanced theory: O-H weaker σ bond) Fukui Principle: reaction occurs by the H*OMO (see above) on one species interacting with the LUMO (O-H δ+ σ MO); so for 2aminoethanol the H^+ binds to the R-NH2 → R-NH3^+. You should be able to work the others out. ;)