spectroscopy

Question 1

Describe the relationship of wavelength, frequency and energy for electromagnetic radiation (light).

Answer 1

Energy is directly proportional to frequency and inversely proportional to wavelength. These relationships can be described by Plank’s equations E = hv = hc/lambda

Question 2

Define wavenumber and describe its relationship to energy. What are the typical units used for wavenumber?

Answer 2

The wavenumber is determined by calculating the reciprocal of the wavelength. This means that the wavenumber (like frequency) is directly proportional to energy and inversely proportional to the wavelength. The typical units are cm^-1.

Question 3

In what region of the electromagnetic spectrum (in terms of wavelength) do we find the infrared, visible, and ultraviolet regions.

Answer 3

Infrared is considered to be above 700 nm, visible between 700-400 nm, and finally ultraviolet below 400 nm.

Question 4

Describe the relationship of IR absorption frequency (as measured in wavenumbers) to bond strength and reduced mass.

Answer 4

The absorption frequency in directly related to the (square root of) bond strength and inversely related to the (square root of) reduced mass. This means triple bonds in general will have a higher absortion frequency than single bonds between the same atoms. Further, we would expect a C-H bond (smaller reduced mass) to have a higher absorption frequency than a C-Cl bond (larger reduced mass).

Question 5

In what region of an IR spectrum will you typically see absorptions due to single bonds to hydrogen?

Answer 5

2500 to 4000 cm^-1

Question 6

In what region of an IR spectrum will you typically see absorptions due to triple bonds?

Answer 6

2000-2500 cm^-1

Question 7

In what region of an IR spectrum will you typically see absorptions due to double bonds?

Answer 7

1500-2000 cm^-1

Question 8

In what region of an IR spectrum will you typically see absorptions due to single bonds?

Answer 8

500-1500 cm^-1

Question 9

Define “fingerprint region”. What region of an IR spectrum typically corresponds to the fingerprint region?

Answer 9

The finger print region of an IR spectrum is typically found below 1500 cm^-1. Although this region corresponds to common single bonds, usually this region has complex absorption patterns. As such, it is often not possible to correlate an absorption peak with a particular bond vibration. The fingerprint region is nonetheless a useful part of the IR spectrum in that the complex set of absorption patterns is unique to a particular compound. Therefore, comparing the fingerprint region spectrum of an unknown compound with the spectrum from the literature, can lead to a positive identification.

Question 10

What is the effect of increasing “s character” on the absorption freqency (in wavenumber) for a C-H bond.

Answer 10

The greater the “s character”, the greater the absorption frequency (in wavenumbers). The means that a C-H bond on an sp hybridized carbon will absorb at a higher frequency (wavenumber) than a C-H bond on a sp³ hybridized carbon.

Question 11

Will the following molecule exhibit a signal to the left of 3000 cm^-1?

Answer 11

Yes - there are C-H bonds where the carbon is sp² hybridized.

Question 12

Will the following molecule exhibit a signal to the left of 3000 cm^-1?

Answer 12

No - there are no C-H bonds where the carbon in question is sp² or sp hybridized.

Question 13

Will the following molecule exhibit a signal to the left of 3000 cm^-1?

Answer 13

No - there are no C-H bonds, where the carbon is sp² or sp hybridized.

Question 14

Which carbonyl (a. or b.) will show a lower absorbance frequency? Justify your choice.

Answer 14

Carbonyl b. will show a lower absorbance frequency. This is due to the resonance experienced by the carbonyl in relation to the C=C. This resonance results in some single bond character and therefore an absorption of IR radiation at a lower frequency (wavenumber).

Question 15

Which alkyne (a. or b.) will show an absorbance frequency in the 2000-2500 cm^-1 region?

Answer 15

Only alkyne b. will show an absorbance in an IR spectra. An absorbance will only be detected if the bond vibration results in a change in dipole moment. Alkyne a. is perfectly symmetrical about the triple bond and therefore the carbon to carbon triple bond vibration will not be detected.

Question 16

What is the effect of hydrogen bonding on the signal for an O-H bond?

Answer 16

Hydrogen bonding causes the signal to be large and broad.

Question 17

How many signals are associated with a primary amine in the region of 3350-3500 cm^-1?

Answer 17

A primary amine is associated with two signals (bunny ears) which is caused by the fact that two types a stretching vibrations are possible (symmetrical and asymmetrical)

Question 18

Under what conditions will a signal for a bond vibration NOT be seen, even though the bond exists in a molecule?

Answer 18

An absorption for a bond vibration will only occur if the bond vibration itself results in a change of bond dipole. A change in bond dipole will only occur if there is some degree of assymmetry about the bond. By contrast, therefore, completely symmetrical bonds (all on groups on either side of the bond are identical), will show no absorption signal. We have seen this in certain examples of double and triple bonds.

Question 19

How is a proton NMR signal (for a hydroen environment) affected when the hydrogen environment is near an electron withdrawing group?

Answer 19

A proton NMR signal is shifted downfield (higher ppm) when the hydrogen in question is near an electron withdrawing group. We say such a proton is deshielded. That is, it will experience a higher effective magnetic field and therefore requre a higher radio frequency for energy absorption.

Question 20

How many proton environments (and therefore how many proton NMR signals) would be expected for the following molecule?

Answer 20

There are three environments which would integrate as 1:1:1.

Question 21

How many proton environments (and therefore how many proton NMR signals) would be expected for the following molecule?

Answer 21

There are two environments (which would integrate 4:2 or 2:1).

Question 22

How many proton environments (and therefore how many proton NMR signals) would be expected for the following molecule?

Answer 22

There is only one proton environment.

Question 23

What feature of proton NMR provides information about the number different proton environments?

Answer 23

Number of signals.

Question 24

What feature of proton NMR provides information about the number of protons in a chemical environment?

Answer 24

Integration of signal peak.

Question 25

What feature of proton NMR provides information about the number of nonequivalent protons adjacent to signal in question?

Answer 25

Splitting patterns.

Question 26

What feature of proton NMR provides information about the nature of a chemical environment (i.e. presence of electron withdrawing groups)?

Answer 26

Chemical shift.

Question 27

Describe the predicted integration patterns for the following molecule (a:b:c).

Answer 27

The predicted integration patterns are 6:2:3

Question 28

Describe the predicted splitting patterns for proton environment a.

Answer 28

There would be no splitting of the signal for proton environment a.

Question 29

Describe the predicted splitting patterns for proton environment b.

Answer 29

The signal for proton environment b. would be split into a quartet.

Question 30

Describe the predicted splitting patterns for proton environment c.

Answer 30

The signal for proton environment c would be split into a triplet.