CH 16-Infrared Spectroscopy Flashcards

Flashcards based on lecture notes about Infrared Spectroscopy.

What type of analysis is IR spectroscopy used for?

Qualitative analysis for identifying compounds and, with FTIR, quantitative analysis is also possible.

What instrument is dominantly used since the 1990s?

FTIR (Fourier Transform Infrared Spectroscopy)

What are the three regions of Infrared radiation, their wavenumber ranges (cm-1), and their primary uses?

Near IR (12000 – 4000 cm-1, quantitative analysis), Mid IR (4000 – 400 cm-1, qualitative analysis), Far IR (400 – 10 cm-1, limited use)

What is the definition of Infrared spectroscopy?

The measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample to excite molecular vibrations.

What is required for a molecule to absorb IR radiation?

The vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule.

What units are used for Mid IR wavelengths, and why?

Wavenumbers (cm-1) are used due to the very long wavelengths in mid IR, instead of nm.

What region of a typical Mid IR spectrum is the fingerprint region?

Below 1500 cm-1

What molecular processes are excited by Mid IR light?

Vibration and Rotation

Describe molecular rotations in IR spectroscopy.

Rotational transitions are of little use in liquids or solids because lines broaden into a continuum, but useful in gases where line spectra are observed.

What is Hooke's Law in the context of vibrational transitions?

F=-ky, which models vibrations where K = force constant (stiffness of the spring) and y=axis.

What are the two main types of vibrational modes?

Stretching and Bending

What is the selection rule for vibrational transitions in IR spectroscopy?

Δν = ± 1, and there must be a change in the dipole moment during the vibration.

How are normal mode vibrations calculated for polyatomic molecules?

For linear molecules: 3N-5, Non-linear molecules: 3N-6 where N is the number of atoms.

What factors influence vibrational coupling?

Common atom, common bond, stretching bond forms one side of the angle that bends, individual energies that are nearly equal, vibrations are of the same symmetry species.

What does an infrared spectrophotometer do?

Passes infrared light through an organic molecule and produces a spectrum that plots the amount of light transmitted against the wavelength of infrared radiation.

What is the Michelson interferometer principle used in FTIR?

It involves a beamsplitter, stationary mirror, and movable mirror to create interference patterns based on optical path differences.

What is the purpose of the HeNe laser in modern FTIR?

It provides the “laser fringe reference system” to regulate the Sampling Interval.

Describe the Nernst Glower source.

Made from rare earth oxides, heats to 1200-2200K, requires pre-heating.

Describe the Glowbar source.

Silicon carbide rod heated to 1300-1500 K, provides better output below 5 μm, requires water cooling.

What are the three main types of IR transducers (detectors)?

Pyroelectric transducers, Thermal transducers, and Photoconducting transducers

What is the purpose of thermal transducers?

Black body, small, very low heat capacity- DT=10-3 K, housed in vacuum, signal is chopped

What materials are commonly used for IR sample holders, and why?

Salts (KBr) or ZnSe, because they are transparent to IR radiation.

How are liquid samples prepared for IR spectroscopy?

Neat, 1 drop on salt plates or dissolved in volatile solvents (0.1-10%).

How are solid samples prepared for IR spectroscopy?

KBr pellets or mulling (dispersions).

Name four FTIR Sampling Techniques

KBr Pellets, Salt Plates, Diffuse Reflectance, ATR

Describe ATR (Attenuated Total Reflectance).

The Electric Vector of the evenescent wave penetrates the surface of the crystal and interacts with the sample.

How do photo-conducting transducers work?

By absorbing radiation and promoting non-conducting valence electrons into a higher energy conducting state, decreasing the resistance of the semiconductor.

What are the IR spectra for ALKENES?

=C—H bond, “unsaturated” vinyl (sp2) 3020-3080 cm- 1 + 675-1000 RCH=CH2 + 910-920 & 990-1000 R2C=CH2 + 880-900 cis-RCH=CHR + 675-730 (v) trans-RCH=CHR + 965-975 C=C bond 1640-1680 cm- 1 (v)

What are the IR spectra for BENZENEs?

=C—H bond, “unsaturated” “aryl” (sp2) 3000-3100 cm- 1 + 690-840 mono-substituted + 690-710, 730-770 ortho-disubstituted + 735-770 meta-disubstituted + 690-710, 750-810(m) para-disubstituted + 810-840(m) C=C bond 1500, 1600 cm- 1

What are the IR spectra for ALCOHOLS & ETHERS?

C—O bond 1050-1275 (b) cm- 1 1o ROH 1050 2o ROH 1100 3o ROH 1150 ethers 1060-1150 O—H bond 3200-3640 (b)