Block 2 Flashcards
Why does molecular spectroscopy work
certain molecules can enter higher energy states only when it absorbs radiation at a specific frequency/wavelength because the E= hf that it requires is specific. From seeing the specific absorbance this can reveal the structure of a molecule
in Mass Spectra, what is the parent ion M+’ and why is it useful
M+’ (of the signal furthest along the x axis) corresponds to the molar mass of the molecule
What are two things that MS can’t distinguish
compounds with the same molecular formula (isomers) or different molecular formula with the same integer mass.
What happens to the molecules when they absorb IR radiation and the two types
Molecules have an increasing amplitude of vibration. Bond vibrations can either symmetric or assymetric stretching. Or bond angles can change: inplane scissoring or rocking as well as out of plane twisting.
What is IR good for
rapid identification of functional groups and fingerprint of substances
What is IR not good for
distinguishing between molecules with the same functional groups
What does UV-VIS radiation do to molecules
It raises electrons (generally those with pi electrons) from lower energy bonding (or non bonding) molecular orbitals to higher energy anti bonding molecular orbitals
Conjugated systems will exhibit different UV-VIS absorbance than non conjugated systems. What is conjugation?
When there is only one single bond between double bonds - eg. double- single - double, pi- sigma- pi
C=C in non conjugated systems will show an absorption at … due to what transition?
170 nm, due to a pi to pi excited state transition
C=O in non conjugated systems will show a weak extra absorption at … due to what transition?
280 nm due to non bonding electron on O to pi excited state transition
What bonds can be found out using UV-VIS
C=C and C=O
Which change in energy is the greatest, n-pi* or pi-pi* ?
pi- pi*
Energy of radiation is proportional to
frequency
What is Beer-Lamberts Law
For a particular compound specified at wavelength, Absorbance (A) is proportional to concentration (molL-1) and path length (cm).
Tell me all the units and things of A= ebc
A = absorbance (no units) , e= molar absorptivity (L mol–1 cm-1. b= pathlength i(cm). c = concentration (molL-1)
In conjugated systems, the change in energy for electron excitement is smaller or bigger than non conjugated?
smaller
If you see an absorption more than 200 nm in UV-VIS, the molecule contains
conjugated double bonds
What effect will the more conjugation a molecule has on the lamda max
it will make it at a greater wavelength
What types of spectroscopic techniques tell you about the carbon/ hydrogen skeleton
H and C NMR
Which particular atoms does NMR interact with and what property do they have in magnetic fields
H1 and C13 isotope have the property of nuclear spin. Without a magnetic field they arrange themselves randomly. In magnetic field they can align themselves with (low energy) or against (high energy) the magnetic field
What happens when the atoms absorb a particular NMR
The nuclear spin flips from lower to higher energy state
What is the relationship between chemical shift and shielding of the nucleus of the atom
As the chemical shift increases (increasing frequency) the nucleus is becoming increasingly deshielded.
What does deshielded nucleus mean?
the carbon or hydrogen is bonded to an electronegative atom that is drawing electrons to itself and away from C or H so it is getting de-shielded.
In CNMR, what does each line mean?
A unique carbon environment.
If you see a signal in CNMR at between 0-90ppm what does it mean
that carbon environment has a sp3 carbon (4 groups attached)
If you see a signal from 100-200 ppm in CNMR what does it mean?
that carbon environment has a sp2 carbon (3 groups attached)
What are the 4 things you have to look at when you’re looking at HNMR
the number of signals, position of signals. relative areas under absorption signals and splitting pattern for absorption signals
What does the number of signals in the HNMR tell us
the number of unique hydrogen environments in a molecule.
How many signals would you expect to see for a monosubstituted benzene ring
usually 1, although can be 3 sometimes. Check to see if theres 5 Hs in the signal area.
Describe the relationship between increased electronegativity of atom bonded to the carbon that the H is bonded to— and the ppm value
As the electronegativity increases, there is more deshielding of the H so therefore the ppm increases.
For Hydrogens bonded to a sp2 carbon where will the signals be and why
They will be at higher ppm values because substituents containing double bonds de-shield
What does the relative areas under absorption signal mean?
These refer to the relative heights of the signals, which shows the ratio of how many hydrogens are contributing to that environment compared to the other signals.
Which of the 4 criteria of HNMR will be given to you in the test
relative area under the signal
What does the number of lines an absorption signal is split into tell us?
It tells us about the number of vicinal hydrogens there are- the number of hydrogens attached to the atoms directly bonded to the atom bearing the H in that environment.
What is the equation for the number of split lines from an absorption signal in HNMR
An absorption signal is split into N+1 lines with N= number of neighbouring/vicinal hydrogens.
What does the pascals triangle tell us about
the different height ratios of the split lines for an absorption signal.
When you have more than 4 DBE what do you need to check for
a phenyl group present (benzene ring - 1H). To establish the number of substituents on the ring check the Ar zone of HNMR)
What is the DBE formula
1/2 (2C + N-H +2)
