Infared Spectroscopy

Question 1

What are the modes of vibration?

Answer 1

*symmetric stretch
*asymmetric stretch
*symmetric bend
*asymmetric bend

Question 2

What does the position on an IR spectrum tell us?

Answer 2

Type of bond/functional group

Question 3

What is the section below 1200cm^-1 called on an IR spectrum?

Answer 3

Fingerprint region

Question 4

What are the two main types of instruments used in IR?

Answer 4

*Dispersive- has a monochromator that selects each wavelength in turn to monitor intensity after radiation passes through sample.
*Fourier Transform- uses a Michelson interferometer (moving mirror). Intensity measured as function of moveable mirror position.

Question 5

How is an IR sample prepared?

Answer 5

*2mg of sample is ground with 300mg KBr and pressed to a thin disc
OR
*small drop of liquid sample sandwiched as film between two KCl or NaCl windows
OR
*solid sample ground to a paste with Nujol (liquid paraffin) and sandwiched between two KCl or NaCl windows. DOWNSIDE-Nujol has own peaks
OR
*attenuated total reflectance- sample placed on zinc selenide and reflected light measured (gels creams)

Question 6

What is IR used for?

Answer 6

*appearance/disappearance of functional group sensitively and cheaply monitored
*allows effects of excipients to be examined
*quality control- unexpected functional groups present might suggest impurities
*fingerprinting of drugs- isomers have diff fingerprints