Block 2

Question 1

Why does molecular spectroscopy work

Answer 1

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

Question 2

in Mass Spectra, what is the parent ion M+’ and why is it useful

Answer 2

M+’ (of the signal furthest along the x axis) corresponds to the molar mass of the molecule

Question 3

What are two things that MS can’t distinguish

Answer 3

compounds with the same molecular formula (isomers) or different molecular formula with the same integer mass.

Question 4

What happens to the molecules when they absorb IR radiation and the two types

Answer 4

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.

Question 5

What is IR good for

Answer 5

rapid identification of functional groups and fingerprint of substances

Question 6

What is IR not good for

Answer 6

distinguishing between molecules with the same functional groups

Question 7

What does UV-VIS radiation do to molecules

Answer 7

It raises electrons (generally those with pi electrons) from lower energy bonding (or non bonding) molecular orbitals to higher energy anti bonding molecular orbitals

Question 8

Conjugated systems will exhibit different UV-VIS absorbance than non conjugated systems. What is conjugation?

Answer 8

When there is only one single bond between double bonds - eg. double- single - double, pi- sigma- pi

Question 9

C=C in non conjugated systems will show an absorption at … due to what transition?

Answer 9

170 nm, due to a pi to pi excited state transition

Question 10

C=O in non conjugated systems will show a weak extra absorption at … due to what transition?

Answer 10

280 nm due to non bonding electron on O to pi excited state transition

Question 11

What bonds can be found out using UV-VIS

Answer 11

C=C and C=O

Question 12

Which change in energy is the greatest, n-pi* or pi-pi* ?

Answer 12

pi- pi*

Question 13

Energy of radiation is proportional to

Answer 13

frequency

Question 14

What is Beer-Lamberts Law

Answer 14

For a particular compound specified at wavelength, Absorbance (A) is proportional to concentration (molL-1) and path length (cm).

Question 15

Tell me all the units and things of A= ebc

Answer 15

A = absorbance (no units) , e= molar absorptivity (L mol–1 cm-1. b= pathlength i(cm). c = concentration (molL-1)

Question 16

In conjugated systems, the change in energy for electron excitement is smaller or bigger than non conjugated?

Answer 16

smaller

Question 17

If you see an absorption more than 200 nm in UV-VIS, the molecule contains

Answer 17

conjugated double bonds

Question 18

What effect will the more conjugation a molecule has on the lamda max

Answer 18

it will make it at a greater wavelength

Question 19

What types of spectroscopic techniques tell you about the carbon/ hydrogen skeleton

Answer 19

H and C NMR

Question 20

Which particular atoms does NMR interact with and what property do they have in magnetic fields

Answer 20

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

Question 21

What happens when the atoms absorb a particular NMR

Answer 21

The nuclear spin flips from lower to higher energy state

Question 22

What is the relationship between chemical shift and shielding of the nucleus of the atom

Answer 22

As the chemical shift increases (increasing frequency) the nucleus is becoming increasingly deshielded.

Question 23

What does deshielded nucleus mean?

Answer 23

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.

Question 24

In CNMR, what does each line mean?

Answer 24

A unique carbon environment.

Question 25

If you see a signal in CNMR at between 0-90ppm what does it mean

Answer 25

that carbon environment has a sp3 carbon (4 groups attached)

Question 26

If you see a signal from 100-200 ppm in CNMR what does it mean?

Answer 26

that carbon environment has a sp2 carbon (3 groups attached)

Question 27

What are the 4 things you have to look at when you’re looking at HNMR

Answer 27

the number of signals, position of signals. relative areas under absorption signals and splitting pattern for absorption signals

Question 28

What does the number of signals in the HNMR tell us

Answer 28

the number of unique hydrogen environments in a molecule.

Question 29

How many signals would you expect to see for a monosubstituted benzene ring

Answer 29

usually 1, although can be 3 sometimes. Check to see if theres 5 Hs in the signal area.

Question 30

Describe the relationship between increased electronegativity of atom bonded to the carbon that the H is bonded to— and the ppm value

Answer 30

As the electronegativity increases, there is more deshielding of the H so therefore the ppm increases.

Question 31

For Hydrogens bonded to a sp2 carbon where will the signals be and why

Answer 31

They will be at higher ppm values because substituents containing double bonds de-shield

Question 32

What does the relative areas under absorption signal mean?

Answer 32

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.

Question 33

Which of the 4 criteria of HNMR will be given to you in the test

Answer 33

relative area under the signal

Question 34

What does the number of lines an absorption signal is split into tell us?

Answer 34

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.

Question 35

What is the equation for the number of split lines from an absorption signal in HNMR

Answer 35

An absorption signal is split into N+1 lines with N= number of neighbouring/vicinal hydrogens.

Question 36

What does the pascals triangle tell us about

Answer 36

the different height ratios of the split lines for an absorption signal.

Question 37

When you have more than 4 DBE what do you need to check for

Answer 37

a phenyl group present (benzene ring - 1H). To establish the number of substituents on the ring check the Ar zone of HNMR)

Question 38

What is the DBE formula

Answer 38

1/2 (2C + N-H +2)