IR Tutor Exercises

                                                                                                                              Name__________________________

This worksheet is to guide you through the IR Tutor tutorial so you can get maximum benefit from it. Work through this problem set as you work through the IR Tutor.

When infrared light is passed through a sample, the absorption varies with the _________________.

An electromagnetic wave consists of a varying _____________ and _____________field. Light that vibrates in only one plane is said to be ___________________. Using the slider on the electromagnetic spectrum and your mouse; a wavelength of 1.1mm corresponds to __________ cm^-1 and energy of ____________ J. A wavenumber of 6000 cm^-1 corresponds of a wavelength of _______ mm and an energy of ____________J.  An energy of 3.00x10^-19 J corresponds to a wavelength of ___________ nm and a wavenumber of ______________.  Calculate the energy of infrared light that corresponds to a wavenumber of  1700 cm^-1 corresponds of a wavelength of ____________nm and an energy of ___________J. (show calculation below)

What is another name for the detector for a dispersive IR spectrometer?_______________

For a dispersive spectrophotometer, what is used to disperse the light instead of a prism? ___________ 

What determines the magnitude of the signal of a dispersive IR spectrometer?___________________

What are three advantages of an FTIR over a classical dispersive IR? __________________________________________________________________

___________________________________________________________________

Using the equations given calculate the frequency and energy for the C-C bond in the lowest energy state. What distance, x, do the carbon’s stretch in the lowest energy state? Remember to use SI units. The spring constant for a C-C bond is approximately 440 Newtons/meter. Show calculation:

What is the overtone wavenumber for the C-C bond? Show calculation.

What wavelength and wavenumber of light must be absorbed to make the vibrational state of the C-C bond increase from the n=0 to n=1 vibrational quantum states?

What happens to the wavenumber for this transition as the bond order increases? __________________

For a bond stretching the potential energy well is different. Explain. What happens to the energy levels in this anharmonic oscillator? ___________________________________________________________

What must change as in order for infrared light to be absorbed? ________________________________

What are the three vibrational modes for water? _____________________________________________

Why does the right side of the potential energy well become level? ______________________________

For CO₂ there are four normal modes of vibration, two bending and two stretching. Which stretching mode does not show an IR absorption band? Why?___________________________________________

Assign vibrational types and modes for each major peak for the two IR spectra at the right. I.e. Methyl asymmetric stretch. Explain the difference between hexane and cyclohexane CH stretching peaks referring to the wavenumbers for each.

Explain the difference between the peaks around 2900 cm^-1 and those around 1400 cm^-1.

What kind of vibrational modes happen at about 720 cm^-1?___________________________________

Why are there two umbrella bending peaks for 2,3-dimethyl butane?____________________________

For 1-hexene the sp² C-H stretching modes appear in two places. Where? ________________________

Why is it difficult to see the symmetric mode? ______________________________________________

What is the spring constant for the C=C bond in 1-hexene? Show calculation.

What can be said about the ≡C-H bond in 1-heptyne in relation to other C-H bonds based on the IR spectra beyond the obvious larger wavenumber?

What is the spring constant for the C≡C of 1-heptyne?

Terminal alkynes have what two peaks at ________ cm^‑1 and _______ cm^-1 that non-terminal alkynes do not. These peaks are due to  _________________________________________________.

What are the characteristic absorption wavenumbers peculiar to the benzene ring in toluene?   Give each a type and mode where appropriate. I.e. 3050 cm^-1 = sp² C-H stretch. No mode such as symmetric or out of plane for this one.

What peak is peculiar to 1-hexanol?_____________Why is the peak so broad?____________________

Why is it hard to see the O-H bending mode?______________________________________________

Is it possible to tell a 1^o, 2^o, and 3^o alcohol from an IR? If so, how, and if not why not?

From the IR spectra of hexylamine, what appears to be the stronger bond, C-H or N-H? What is your reasoning? Check a bond strength table; is what you said true?

Why will the NH₂ scissoring and wagging modes not be observed in 2^o or 3^o amines?

Overlay other spectra hexane, 3-heptanone, and heptanoic acid onto the heptanaldehyde spectra to answer this question. What peak is peculiar to heptanaldehyde?__________________ Is the wavenumber larger or smaller that the similar peak for the ketone heptanone?_________________

Why is the carbonyl C-H stretch so low in frequency?

The 1408 cm^-1 peak for 3-heptanone is due to the scissor bending of a CH₂ group adjacent to a carbonyl. Why is this wavenumber less than for a CH₂ not adjacent to a carbonyl?_________________________

Heptanoic acid has a very prominent OH stretch. Why is the absorption of lower wavenumber than an alcohol OH stretch? ___________________________ Why is the OH peak so broad? _____________