sym 2

Question 1

what to think when an operation starts with a big C,, Cn

Answer 1

u think that its a proper rotation!!!

Question 2

what should we think the n in Cn stands for

Answer 2

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.

Question 3

does the rotational operation, C, move clockwise or anticlockwsie

Answer 3

clockwise

Question 4

ammonia has a what rotational axis

Answer 4

it has a C3 axisss,, running through the N

the first C 1 3 rotation has a clockwise rotation of 120*

Question 5

superscript is added for what reason

Answer 5

to show how many times the rotation has been carried out

Question 6

E is what operation

Answer 6

E is the identity operation

Question 7

what is the E // identity operation

Answer 7

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

Question 8

E is the identity operation so we should think thisssss

Answer 8

makes the molcule end up in the same configuration it was to start with.

the axis that is used doesnt mater.

Question 9

any plane of symmetry is given the label

Answer 9

sigmaaa

Question 10

if more than one symmetry plane is added we use

Answer 10

dashes to tell them apart

Question 11

sigma’ is usually what element

Answer 11

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 .

Question 12

if sigma’ is the plane of symmetry thats in plane wirth the molecule,, what is normal sigma

Answer 12

the other plane!! (in h2o its the one that follows the C2 axis)

Question 13

how to figure out the end product after doing a sigma operation

Answer 13

u pretend its a mirror and draw the reflection

Question 14

in water,, do the 2 planes of symmetry meet,, aka do they make a cross

Answer 14

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

Question 15

when do we call a sigma plane a sigma v plane

Answer 15

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

Question 16

in water,, 2 x sigma gives

Answer 16

E

sigmav sigmav = E

Question 17

what do we think when we see ‘ the plane’

Answer 17

think of the plane of the molecule

aka the plane that the moelcule makes when its lying down

Question 18

BF3 has what rotarional element

Answer 18

C3

bc it rotates 3 times to get back to the beginning

Question 19

does BF3 have any other rotational elements

Answer 19

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

Question 20

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

Answer 20

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

Question 21

how many planes of symmetry does BF3 have

Answer 21

4

C3 going through the B

3C2
one C2 going through each BF bond

Question 22

how many planes of symmetry does BF3 have

Answer 22

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)

Question 23

what do we call the flat plane in BF3

Answer 23

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.

Question 24

in BF3,, what do we call the 3 tall planes

Answer 24

sigma v

sigma vertical

vertical plane of symmetry

bc they dont stay in the XY plane,, they have Z character like the principal axis.

Question 25

when we see sigma v what should we think

Answer 25

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

Question 26

sigma d is called a

Answer 26

dihedral plane of symmetry

Question 27

when are sigma d seen

Answer 27

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!!

Question 28

whats needed for ‘i’ to occur

Answer 28

a point of inversion

Question 29

whats the point of inversion

Answer 29

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

Question 30

whats the symbol for a proper rotation

Answer 30

C

Question 31

whats the symbol for an improper rotation

Answer 31

S

Question 32

what is an improper rotation

Answer 32

a combo of 2 operations,, a rotation around the Cn axis and then a reflection through a horizontal plane.

Question 33

when will an improper rotation be present

Answer 33

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)

Question 34

thinking of ethane in the staggered confirmation,, explain what a Sn operation will do

Answer 34

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.

Question 35

explain a S 1/6 operation in ethane staggered

Answer 35

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.

Question 36

sometimes a more complex symmetry operation has a simpler and equivalent version ,, which one should wr write

Answer 36

we write the simpler equivalence version.

Question 37

C2/2 can also be called what

Answer 37

E

bc the operation gets u to the beginning

Question 38

tips for Sn // improper rotations // rotation and mirror operation

Answer 38

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

Question 39

better explanation of C2 and sigmaD in ethane.

Answer 39

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

Question 40

what does it mean when molecules are in the same symmetry point group

Answer 40

it means that molecules can undergo an identical set of operations

both have C2, sigma v, sigma v’ etc.