1+2 book Flashcards
what is IR spectra a result of
a result of radiation exciting vibrational modes in a molecule
what does the number of IR signals depend on
depends on the number of vibrational modes for a molecule
total degrees of freedom
3N
where N is the total amount of atoms in a molecule
amount of vibrational modes for a non linear molecule
3N - 6
3 trans and 3 rot
amount of vibrational modes for a linear molecule
3N - 5
3 trans and 2 rot
will all vibrational modes be seen as IR signals
nope!!!
what happens to the amount of IR vibrational modes as the degree of symmetry increases
more symmetry = less vibrational modes in spec
what is used to predict the number of absorption lines in spectra
group theory is used to predict the amount of absorption bands in the spectra
give 2 examples of a symmetry element
planes and lines
what is a group point symmetry
where all the lines of symmetry meet
if smt undergoes a symmetry operation and ends up looking the same,, what does it mean about the 2 atoms
they’re in the same chemical environment
label for a rotational element + operation
C
element is theeee
line or plane
operation issss
what is done to the molecule
what label do we need for a rotating element or an operation
C
subscript is theee
bottom number
tells us how many rotations are possible for the molecule to still look identical
subscript can also be called
the order of rotation axisssss
the superscript issss
the amount of the total rotations that was done to get a certain arrangement
what’s one way to think of superscripts and subscripts when looking at a rotation operation label
think of it like 2/3
so we rotated it 2 of the total 3 ways we could
what is the principal axis
this is the line of symmetry with the largest order // subscript value
between C5 and C2 which would be the principe axis
C5
it’s the largest order
making it the best principle axis
what do we do about the principal axis
we set this as the vertical axis
we say it’s aligned with the z axis
do symmetry elements move when operations occur
nope!!
the lines and planes stay in the same place,, only the molecule moves
C6 can be split intoooo
C1 6
C5 6
C2 3
C1 3
C2
if both elements are called C2 how can we tell them apart
use
C2’ if it’s going through an atom
C2’’ if it’s going through a bond
arbitrary principal axis
when u choose which axis will be the principal one
u just need to make sure u make it known using cartesian axis
C2 (y or x or z)
what is the plane of symmetry symbol and when is it seen
it’s a sigma sign
it’s seen when a reflection gives the same orientation
when the plane of symmetry is in the plane of the molecule,, how do we write it
sigma ‘
when the plane is not in the plane of the molecule, how do we write it
just the sigmaaaa
if there’s only one axis,, what will be the principal one
that one!!
that one would align with the z axis and be set as vertical
how do we tell if a plane is vertical or horizontal
sigma v ‘
sigma v
remember that z is set to the principal axis,, which is the axis withhhh
the highest order axis
plane of symmetry with the principal axis in it issss
vertical
sigma v
when do u add ‘
if the line goes through an atom
if the symmetry plane is in plane with the molecules plane
it would be going through z and y
which axis is perpendicular to z
x axisssss
how to say plane in the zy axis in the plane of the molecule
sigma for the plane of symmetry
v for it being vertical
‘ for it matching the plane
(zy)
plane of symmetry which has the principal axis within it issss
sigma v
bc principal is the new vertical
plane of symmetry that matches the plane of the molecule
sigma h
bc it’s not vertical bc it doesn’t have the principal axis which would make that plane vertical
plane of symmetry that is perpendicular to the principal axis
sigma h
bc it’s not vertical bc it doesn’t have the principal axis which would make that plane vertical
