IR Spectroscopy

Why do we use wavenumbers (cm-1) instead of wavelength on the x-axis of the IR spectrum? a) wavenumber is directly proportional to energy b) the numbers are very small and interpretable c) wavenumber is inversely related to energy d) wavenumbers correspond to the length of the bond

a

How could IR spectroscopy be used to distinguish between the following pair of compounds: C6H6 (benzene) vs. C6H12 (cyclohexane)? Select all that apply. a) Compound 1 would have sp2 C-H greater than 3000 cm-1 b) Compound 2 would have peaks indicative of an aromatic system c) Compound 2 would have sp3 C-H greater than 3000 cm-1 d) Compound 2 would have sp3 C-H less than 3000 cm-1 e) Compound 1 would have sp2 C-H less than 3000 cm-1 f) Compound 1 would have a smaller fingerprint region because of less hydrogens

a and e

___________stretching occurs at a higher energy than ___________stretching

asymmetric; symmetric

IR spectroscopy provides valuable information about: a) melting point b) functional groups c) conjugation d) molecular weight

b

The region of the IR spectrum which contains the most complex vibrations (600-1400 cm-1) is called the __________ region of the spectrum. a) thumbprint region b) fingerprint region c) aromatic ring region d) ill-defined region

b

What are the simplest modes of vibration?

bending and stretching

The IR spectrum of a sample contains absorptions at 3050, 2950, and 1620 cm-1. To what class of organic compound does this sample most likely belong? a) ester b) alcohol c) alkene d) alkane e) alkyne

c

When running an experiment, you are rushed for time and forget to dry your product. All methods of analysis indicate that no alcohol containing compounds were made. However, when you check using IR, you notice a broad absorption around 3200-3500 cm-1. If there is no alcohol, why is there still absorption in this region? a) the salt plate also absorbs in the 3200-3500 region b) a high amount of sp3 hybridized carbons causes the peak to shift to this region c) there is water in the sample d) the other spectra analyses are inaccurate

c

Compounds with what kinds of bonds will absorb infrared radiation?

covalent

Which compound would be expected to show intense IR absorption at 2352 cm-1? a) Prop-1-ene b) CH3COOH c) CH3CH2SCH3 d) C6H5CN

d

Only bonds that have a changing ____________ during vibration are capable of absorbing IR.

dipole moments

How are FTIR and monochromatic IR different?

monochromatic produces and detects absorbance one wavelength at a time, while FTIR does them all at once and then transforms them with math

Will symmetric bonds absorb IR strongly?

no

_____________occurs at higher energies than _____________

stretching; bending