Ch 2

Card Set Information

Ch 2
2013-01-12 13:48:54
IR Spectroscopy

Chapter 2
Show Answers:

  1. The study of the interaction of energy with matter, which can be used to elucidate the structure of a molecule.
  2. Various forms of electromagnetic waves that propagate at the speed of light; includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
    Electromagnetic radiation
  3. A massless packet of electromagnetic energy, with energy equal to Planck's constant times the frequency: E = h ∙ υ.
  4. The number of complete wave cycles that pass a fixed point in a second, or the number of reversals of the electromagnetic field per second; usually expressed in Hertz (Hz), which is cycles per second (cps or s-1 ).
    Frequency (v)
  5. A measure of the frequency of a wave in cycles per second (cps or s-1 )
    Hertz (Hz)
  6. The distance between consecutive peaks (or troughs) of a wave; often expressed in μm; the product of the wavelength and the frequency is the speed of light: c = λ ∙ υ.
    Wavelenght (λ)
  7. The frequency of a wave, expressed as the number of wavelengths that fit into one centimeter; often expressed in cm-l ; the product of the wavenumber (in cm-l ) and the wavelength (in μm) is 10,000
  8. In a spectrometer, the amount of light transmitted through a sample divided by the amount of light transmitted through air; percent transmittance converts the fraction into a percent: %T = ( I / I0 ) ∙ 100, where I is the intensity of radiation transmitted through the sample at a particular wavelength, and I0 is the intensity of radiation incident on the sample at the same wavelength. The lower is %T, the greater the amount of incident radiation being absorbed by the sample.
  9. A type of optical spectroscopy that measures the absorption of infrared radiation ( 4000 - 400 cm-1, or 2.5 - 25 μm ), and it provides structural information about functional groups present in the compound being analyzed
    IR Spectroscopy
  10. An instrument that passes infrared light through both the sample and a scanning interferometer to give an interference pattern (interferogram), which is digitized, and the Fourier transformed spectrum is calculated.
    Fourier transform infrared spectrometer (FT–IR)
  11. The light measuring portion of an FT–IR spectrometer. The light is split into two beams, and one beam is reflected from a stationary mirror, while the other beam is reflected from a moving mirror, then the beams are recombined to form an interference pattern, called an interferogram, then fourier transformation of the interferogram gives the spectrum.
  12. A measure of the degree of charge separation in a molecule, which is the product of the positive charge, Q, in electrostatic units (esu) and the distance, d, that separates them, in centimeters: μ = Q ∙ d , with units in debyes, D, where 1 D = 1 x 10-18 esu ∙ cm = 3.336 x 10-30 C ∙ m (coulomb meter). The larger the dipole moment, the more polar the molecule.
    Dipole moment
  13. The number of chemical bonds between a pair of atoms, which is an index of bond strength. In molecular orbital theory, bond order is defined as the difference between the number of bonding electrons and the number of antibonding electrons, divided by two. For example, the bond order for a carbon-carbon bond is 1 for ethane, 2 for ethene, 3 for ethyne, and 1.5 for benzene.
    Bond order
  14. Motions of two atoms in a molecule that take place along the bond axis, due to absorption of IR light; the IR absorption frequency decreases with increasing atomic mass, and the frequency increases with increasing bond energy
    Stretching vibrations
  15. Motions of three atoms in a molecule that result in bond angle deformations due to absorption of IR light.
    Bending vibrations
  16. A vibration of a bond that changes the dipole moment of a molecule, and thus can absorb IR light.
    IR-active vibration
  17. A vibration of a bond that does not change the dipole moment of a molecule, and thus cannot absorb IR light.
    IR-inactive vibration
  18. A weak absorption in the IR spectrum that occurs at approximately twice the fundamental vibration frequency of a very strong absorption, due to harmonic oscillations.
    Overtone in IR
  19. A weak absorption in the IR spectrum that occurs at approximately the sum of the vibration frequencies of two strong absorptions.
    Combination band in IR
  20. The right one-third of an infrared spectrum, in the region 1400 - 400 cm-1 of an infrared spectrum, where most of the bending vibrations and many complex vibrations occur; no two different compounds (except enantiomers) have exactly the same absorptions in this region
    Fingerprint region of IR
  21. The left two-thirds of an infrared spectrum, in the region 4000 - 1400 cm-1, where most of the functional groups undergo stretching vibrations and produce reliable absorptions in this region
    Functional group region of IR
  22. If the intensity, shape and location of every absorption in two IR spectra are the same, then there is a high probability that the two compounds are the same.
    Superimposability of IR spectra
  23. A sample cell used in IR spectroscopy that is constructed of two transparent salt plates separated by a gasket, which defines the thickness of the sample contained in the cell.
    Fixed-thickness cell