sym 2 Flashcards
what to think when an operation starts with a big C,, Cn
u think that its a proper rotation!!!
what should we think the n in Cn stands for
the order of the rotational operation
aka the amount of positions in a molecule that will lead to it looking the same after a rotation.
does the rotational operation, C, move clockwise or anticlockwsie
clockwise
ammonia has a what rotational axis
it has a C3 axisss,, running through the N
the first C 1 3 rotation has a clockwise rotation of 120*
superscript is added for what reason
to show how many times the rotation has been carried out
E is what operation
E is the identity operation
what is the E // identity operation
the rotation where the moelcule ends up in an identical configuration to the starting point
360* rotation
the same as doing nothing to the molecule
E is the identity operation so we should think thisssss
makes the molcule end up in the same configuration it was to start with.
the axis that is used doesnt mater.
any plane of symmetry is given the label
sigmaaa
if more than one symmetry plane is added we use
dashes to tell them apart
sigma’ is usually what element
sigma’ is usually the plane of symmetry that is in plane with the molecule.
think of ‘ as a plane so its in plane wirth the moelcule .
if sigma’ is the plane of symmetry thats in plane wirth the molecule,, what is normal sigma
the other plane!! (in h2o its the one that follows the C2 axis)
how to figure out the end product after doing a sigma operation
u pretend its a mirror and draw the reflection
in water,, do the 2 planes of symmetry meet,, aka do they make a cross
yesss they make it and this is also the C2 axis,, aka they form a cross and this is where the C2 axis happens to be..
bc the C2 axis is the only rotational axis,, meaning its the principle axis that sets the vertical
bc the planes meet here we say that they are sigma v planes
when do we call a sigma plane a sigma v plane
when they contain the rotational axissss
aka think of h20 and the C2 axis.
the sigma’ (in the plane) has the C2 in it
and the sigma (other plane) also has the C2 axis in it,, so its also a v plane
in water,, 2 x sigma gives
E
sigmav sigmav = E
what do we think when we see ‘ the plane’
think of the plane of the molecule
aka the plane that the moelcule makes when its lying down
BF3 has what rotarional element
C3
bc it rotates 3 times to get back to the beginning
does BF3 have any other rotational elements
yes!!
theres a C2 going through every B - F bond
so three C2 bonds
theyre in identical chemical environments tho so ne need for dashes 💗 we just write 3C2
if theres multiple rotational axis,, like in BF3,, but theyre in the same environment (bc when we rotate it looks the same) do we need to use dashes
nope!! u just write how many of them there are
in the case on BF3 theres 1 C3 axis going through B
ad 3C2 axis,, going through each B-F bond ,, bc the moelcule is planar
how many planes of symmetry does BF3 have
4
C3 going through the B
3C2
one C2 going through each BF bond
how many planes of symmetry does BF3 have
it has 4 planes of symmetry
one through each BF bond (3D one) so 3 in total
1 in the plane of the molecule (flat one)
what do we call the flat plane in BF3
sigma h
sigma horizontal
horizontale plane of symmetry
bc its not in the Z plane like the C3 principal axis. it stays in the XY plane and has no Z character so we call it h.
in BF3,, what do we call the 3 tall planes
sigma v
sigma vertical
vertical plane of symmetry
bc they dont stay in the XY plane,, they have Z character like the principal axis.
when we see sigma v what should we think
think where the principal axis is and what wazy its pointing
then realise that the planes of symmetry must have vertical character,, aka they should grow tall in the same direction as the principal axis
sigma d is called a
dihedral plane of symmetry
when are sigma d seen
think of ethane in eclipsed position
theres a principal axis of C3
theres also a C2 axis that is horizontal
dihedral planes are planes that are between the different axis!!
dihedral planes are between the C2 axis!! staggered ethane,, theres a 3 sigma d cutting through the atoms
whats needed for ‘i’ to occur
a point of inversion
whats the point of inversion
the inversion centre
where everything that is gonna be inverted meets. imagine a line joining each atom to the centre of the molecule.
the centre is the point of inversion.
think of all the atoms going into the inversion centre and then carrying on moving out the other way.
in PtCl4,,
Cl1 –> Cl4
Cl2 –> Cl3
whats the symbol for a proper rotation
C
whats the symbol for an improper rotation
S
what is an improper rotation
a combo of 2 operations,, a rotation around the Cn axis and then a reflection through a horizontal plane.
when will an improper rotation be present
an improper rotation will be present if theres a Cn axis (rotation) with a horizontal plane of symmetry (aka the plane is perpendicular to the C axis)
thinking of ethane in the staggered confirmation,, explain what a Sn operation will do
think of newman projection.
the Hs at the front will rotate (C 1/6) so they end up where the back H’s were.
the plane of symmetry then kicks in and sigma h is donee.
so the back H’s come to the front and the front H’s go to the back.
explain a S 1/6 operation in ethane staggered
front H’s rotate C1/6 to go to where the back H’s were. and the back H’s rotate to where the front H’s were.
the symmetry operation then occurs and the back H’s become the front H’s and the front H’s become the back H’s.
sometimes a more complex symmetry operation has a simpler and equivalent version ,, which one should wr write
we write the simpler equivalence version.
C2/2 can also be called what
E
bc the operation gets u to the beginning
tips for Sn // improper rotations // rotation and mirror operation
draw the staggered confirmation,, u then dont need to think, ‘turn, flip, turn flip’ but if it says S4/6 u just pick an atom and move 4 times,, bc the atoms//bonds will already be arranged with the flips
better explanation of C2 and sigmaD in ethane.
the C2 will be where we thought the sigma d will be. (like in staggered confirmation they would be between each atom on the newman projection)
theres 3C2 bc they kinda extend to the other side.
and then theres 3 sigmaD. sigma means symmetry plane so try remmeber that it the sigma plane will cut through every atom in the staggered position 💗
symmetry otterbein dnd ethane
what does it mean when molecules are in the same symmetry point group
it means that molecules can undergo an identical set of operations
both have C2, sigma v, sigma v’ etc.
