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MS, IR, and NMR

  • An IR Spectrum is a plot of the amount of transmitted light vs. its wavelength

  • Each peak corresponds to a particular kind of bond and each bond type occurs at a characteristic frequency

  • The spectrum is divided into two sections, the functional group region (1500 and above) and the fingerprint region (below 1500)
  • Determine the location of the peaks in the spectrum
  • Compare the location of the peaks in the spectrum with the approximate locations of the absorptions of bond types from the table and match the numbers to the bond types
  • Look for characteristic rounded peaks if there are any
  • C NMR spectra are easier to analyze because the signals are not split

  • Each type of carbon appears as a single peak so the number of signals is the same as the number of different types of carbon atoms in the molecule

  • The structural information is provided by:
  • Number of signals observed
  • Chemical shifts of the signals

IR Spectrum

Mass Spectrometry

Functional Group Region vs. Fingerprint Region

Problem Solving Tips for IR Spectra

Understanding Mass Spectra

The OH group always appears as a rounded peak between 3600-3200. A carbon bonded to a Hydrogen that is sp3 hybridized appears between 3000-2850. Carbons that are hybridized with a lower number of orbitals, appear at higher ends of the spectrum. The sp2 hybridized carbon appears at higher frequencies.

  • The fingerprint region consists of the single bonds with Carbon
  • MS is a technique used for measuring the molecular weight and determining the molecular formula of an organic compound

  • The ionization of the molecule forms an unstable radical cation, symbolized by M+

  • Because M+ is unstable, it decomposes to form fragments of radicals and cations that have a lower molecular weight than M+

  • The mass spectrometer analyzes the masses of cations, then forms a Mass Spectrum
  • A Mass Spectrum is a plot of the amount of each cation versus its mass to charge ratio m/z where m is the mass and z is the charge

  • The tallest peak in the Mass Spectrum is called the Base Peak

  • The M Peak is the peak that represents the weight of the molecular ion

  • If the molecular ion is unstable, it will split into other cations and radical cations containing fewer hydrogens than the original molecule, which will cause several smaller peaks in the Mass Spectrum of the molecule

Peaks in Mass Spectrum

The Nitrogen Rule

  • M+1 Peak:
  • Occurs in compounds which contain atoms that have two isotopes with one of the isotopes being one gram heavier, such as Carbon-12 and Carbon -13
  • The size of the peak depends on the ratio of occurrence between the isotopes

  • M+2 Peak:
  • Occurs in alkyl halides such as those that contain Chlorine or Bromine
  • The naturally occurring isotopes of Chlorine and Bromine differ by two grams of mass (Cl-35, Cl-37 3:1 ratio) (Br-79, Br-81 1:1 ratio)

The nitrogen rule states that:

  • Hydrocarbons, as well as compounds that contain only C, H, and O atoms will always have an even mass

  • A compound with an odd molecular ion contains an odd number of nitrogen atoms, but a compound with an even number of nitrogen atoms gives an even molecular ion

The 3:1 ratio is shown for the compound that contains Chlorine. The 3:1 ratio implies that there are two lines and that one of the lines is a third of the height of the other line

Regions of an IR Spectrum

H NMR Spectra

Nuclear Magnetic Resonance Spectroscopy (NMR)

The green hydrogen signal is a singlet because there are no nearby hydrogens. The blue hydrogen signal is a triplet because there are two equivalent hydrogens which follows the n+1 rule. The red hydrogen signal is a quartet because there are three equivalent hydrogens which follow the n+1 rule.

Bonds absorb in 4 predictable regions of an IR spectrum:

  • Single bonds at 400-1500
  • Double bonds at 1500-2000
  • Triple bonds at 2000-2500
  • Lighter atoms at 2500-4000
  • The percent s character effects the wavelength of absorption; the higher the percent s character corresponds to a stronger bond

  • Symmetrical molecules do not absorb IR light. They are IR inactive because they do not have a change in their dipole moment during vibration
  • NMR is an analytical technique used to characterize organic molecules by identifying carbon-hydrogen bonds within molecules

  • Two common types of NMR are used:
  • H NMR - used to determine the type and number of H atoms in a molecule
  • C NMR - used to determine the type of C atoms in a molecule
  • An H NMR spectra is a plot of the intensity of a peak vs. its chemical shift
  • Absorptions appear as sharp peaks and most protons absorb between 0-10 ppm
  • Upfield and downfield are used to describe the location of peaks; upfield is to the right, in the higher magnetic field and downfield is to the left, in the lower magnetic field

This C NMR shows a compound that has four different chemical environments.

C NMR Spectrum

Structural Information for the H NMR Spectra

There are four descriptive characteristics of spectra:

  • Number of signals - which indicate the number of different types of hydrogens in a molecule
  • Position of signals - which indicate what types of hydrogen the molecule contains
  • Intensity of signals - which indicates how many of each kind of hydrogen is in the molecule
  • Spin-spin splitting signals - which indicates the neighboring environment for the various hydrogens

Spin-Spin Splitting Signals

Number of Signals

Intensity of Signals

Position of Signals

  • Single signals are singlets, singals split in two peaks are doublets, and signals split into three peaks are triplets

  • Occurs only between nonequivalent protons on the same carbon or adjacent carbons

  • The coupling constant is the frequency difference between the two peaks of a doublet

  • Splitting is not usually observed between protons separated by more than three sigma bonds
  • The area under a signal is proportional to the number of absorbing protons

  • The height of each step is proportional to the area under the peak which is proportional to the number of absorbing protons

  • The ratio of integrals gives the ratio of absorbing protons in the spectrum
  • When comparing the H atoms on a ring or double bond, two protons are equivalent if they are cis or trans to the same groups

  • Enantiopic protons result when the substitution of two H atoms by Z forms enantiomers because the two H atoms are equivalent

  • Diastereotopic protons rest when the substitution of two H atoms by Z forms diastereomers because the two H atoms are not equivalent
  • Near the nucleus, the magnetic field generated by the electron opposes the applied field and decreases the external magnetic field that the proton is exposed to which results in the electron needing a lower frequency to achieve resonance and shifting of the absorption upfield

  • A less shielded nucleus (deshielded) needs a higher frequency to achieve resonance and shifts the absorption downfield

  • Protons in a given environment absorb in a predictable region
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