spectrum

Examples of spectrum in the following topics:

• The Electromagnetic Spectrum

  • The visible spectrum constitutes but a small part of the total radiation spectrum.
  • This electromagnetic spectrum ranges from very short wavelengths (including gamma and x-rays) to very long wavelengths (including microwaves and broadcast radio waves).
  • The following chart displays many of the important regions of this spectrum, and demonstrates the inverse relationship between wavelength and frequency (shown in the top equation below the chart).
  • The energy associated with a given segment of the spectrum is proportional to its frequency.

• Emission Spectrum of the Hydrogen Atom

  • The emission spectrum of atomic hydrogen is divided into a number of spectral series.
  • The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by an atom's electrons when they are returned to a lower energy state.
  • Each element's emission spectrum is unique, and therefore spectroscopy can be used to identify elements present in matter of unknown composition.
  • The emission spectrum of atomic hydrogen is divided into a number of spectral series, with wavelengths given by the Rydberg formula: $\frac { 1 }{ \lambda_{vac} } =RZ^2( \frac { 1 }{ {n_1 }^{ 2 } } -\frac { 1 }{ { n_2 }^{ 2 } })$,
  • Explain how the lines in the emission spectrum of hydrogen are related to electron energy levels.

• Electromagnetic Spectrum

  • The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation.
  • The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation.
  • Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum.
  • Thus, we refer to a spectrum, but divide it up based on the different interactions with matter.
  • Below are the regions of the spectrum and their main interactions with matter:

• Introduction

  • As noted in a previous chapter, the light our eyes see is but a small part of a broad spectrum of electromagnetic radiation.
  • On the immediate high energy side of the visible spectrum lies the ultraviolet, and on the low energy side is the infrared.
  • An example of such a spectrum is that of the flavoring agent vanillin, shown below.
  • The complexity of this spectrum is typical of most infrared spectra, and illustrates their use in identifying substances.
  • The gap in the spectrum between 700 & 800 cm-1 is due to solvent (CCl4) absorption.

• UV-Visible Absorption Spectra

  • Commercial optical spectrometers enable such experiments to be conducted with ease, and usually survey both the near ultraviolet and visible portions of the spectrum.
  • The energies noted in the Electromagnetic Spectrum concept are sufficient to promote or excite a molecular electron to a higher energy orbital.
  • The resulting spectrum is presented as a graph of absorbance (A) versus wavelength, as in the isoprene spectrum shown below.
  • Since isoprene is colorless, it does not absorb in the visible part of the spectrum and this region is not displayed on the graph.
  • The diagram that follows the spectrum below shows the change in units.

• Characteristics of Mass Spectra

  • Even with simple compounds like these, it should be noted that it is rarely possible to explain the origin of all the fragment ions in a spectrum.
  • Since a molecule of carbon dioxide is composed of only three atoms, its mass spectrum is very simple.
  • The molecular ion of propane also has m/z=44, but it is not the most abundant ion in the spectrum.
  • The third strongest ion in the spectrum has m/z=39 (C3H3).
  • Spectrum diagrams are followed by the fragmentations leading to the chief fragment ions.

• Carbon NMR Spectroscopy

  • Even when numerous C-H groups are present, an unambiguous interpretation of a proton nmr spectrum may not be possible.
  • The spectrum of camphor, shown on the left below, is typical.
  • Furthermore, a comparison with the 1H nmr spectrum on the right illustrates some of the advantageous characteristics of carbon nmr.
  • The only clearly identifiable signals in the proton spectrum are those from the methyl groups.
  • Because of this, the number of discrete signals and their chemical shifts are the most important pieces of evidence delivered by a carbon spectrum.

• Vibrational Spectroscopy

  • The four-atom molecule of formaldehyde, the gas phase spectrum of which is shown below, provides an example of these terms.
  • We expect six fundamental vibrations (12 minus 6), and these have been assigned to the spectrum absorptions.
  • The general regions of the infrared spectrum in which various kinds of vibrational bands are observed are outlined in the following chart.

• Fragmentation Patterns

  • Without a molecular ion peak as a reference, the difficulty of interpreting a mass spectrum increases markedly.
  • The mass spectrum of dodecane shown below illustrates the behavior of an unbranched alkane.
  • All of the significant fragment ions in this spectrum are even-electron ions.
  • The following mass spectrum of cocaine demonstrates how a forensic laboratory might determine the nature of an unknown street drug.

• Spin-Spin Splitting

  • The nmr spectrum of 1,1-dichloroethane (below right) is more complicated than we might have expected from the previous examples.
  • The ethyl acetate spectrum on the left displays the typical quartet and triplet of a substituted ethyl group.
  • A smaller distortion of this kind is visible for the A and C couplings in the ethyl acetate spectrum.
  • Try to associate each spectrum with a plausible structural formula.
  • When you have made an assignment you may check your answer by by viewing the spectrum itself.