Organic Chemistry - Spectral Methods

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infrared (IR) spectroscopy

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deals with light that has a longer wavelength and lower frequency than visible light; used to identify and study chemicals

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nuclear magnetic resonance (NMR) spectroscopy

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a research technique that exploits the magnetic properties of certain atomic nuclei; determines the physical and chemical properties of atoms or the molecules in which they are contained

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ultraviolet-visible

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absorption spectroscopy in the UV-vis spectral region

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mass spectrometry

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an analytical technique that produces spectra of the weights of the molecules comprising a sample of material

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X-ray diffractometry

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analysis of the structure of a material from the scattering pattern produced when a beam of radiation interacts with the instrument

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spectral database

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freely accessible source of extensive spectroscopic data for organic compounds

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vibrational-rotational level

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different molecular states may exist within the same electronic state

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functional group

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lexicon-specific collection of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules

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nuclear spin state

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excitations of electrons that produce transitions in a magnetic field; associated with NMR spectroscopy

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nearest neighbor

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number determined in 1H NMR spectroscopy regarding hydrogen atoms

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conjugated

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systems in which two or more multiple bonds are directly linked by single bonds

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wavenumber

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reciprocal centimeter; measurement of wavelength

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reciprocal centimeter

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wavenumber; measurement of wavelength

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stretching

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a change in the length of a bond

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twisting

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a change in the angle between the planes of two groups of atoms

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bending

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a change in the angle between two bonds

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Hooke's law

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a principle of physics that states that the force needed to extend or compress a spring by some distance is proportional to that distance

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harmonic oscillator

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a system that, when displaced from its equilibrium position, experiences a restoring force proportional to the displacement

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force constant

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reflects the strength of the bond between two atoms

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dipole moment

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a measure of the separation of positive and negative charges (polarity)

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transmittance

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the amount of light diffused through a sample

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absorbance

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a logarithmic ratio of the radiation falling upon a material, to the radiation transmitted through the material

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infrared spectra

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plots of transmittance

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incident radiation

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light hitting a specific surface

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amplitude

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a measure of a variable's change over a single period

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fixed-thickness cell

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constructed of two transparent salt plates separated by a plastic or metal gasket, which defines the width of the sample contained in the compartment

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a neat sample

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IR spectrum of a pure liquid

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solution

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a homogeneous mixture composed of only one phase

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casting

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a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify

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KBr pellet

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a small, compressed, hard chunk of matter used in infrared spectroscopy

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microporous polyethylene film

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separation between a cathode and an anode with a fuse function

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attenuated total reflectance

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a sampling technique used in conjunction with infrared spectroscopy which enables samples to be examined directly in the solid or liquid state without further preparation

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matched cells

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two fixed-thickness compartments having identical pathlengths

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sample cell

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compartment containing the solution

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reference cell

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compartment containing pure solvent

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Fourier transform (FT) IR

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a technique which is used to obtain a spectrum of absorption, emission, photoconductivity or Raman scattering of a solid, liquid, or gas; collects spectral data in a wide spectral ragne

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hygroscopic

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attracted to and able to hold water molecules from the surrounding environment

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reflectance spectroscopy

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measures the difference in the echoes of two beams of light that are shone in normal incident on a surface with different linear polarizations

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functional group region

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absorptions in the approximate range of 4000-1250 cm^-1

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superimposability

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the intensity, shape, and location of every absorption in two spectra must be identical

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fingerprint region

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the portion of a spectrum (1250-500 cm^-1) that is typically unique for a particular compound

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nuclear spin

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an intrinsic form of angular momentum carried by elementary particles, composite particles, and atomic nuclei

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applied magnetic field

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a magnetic field only due to free currents

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magnetogyric ratio

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the ratio of a particle's magnetic dipole moment to its angular momentum

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radio-frequency (rf) range

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3 kHz - 300 GHz

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resonance condition

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when the relationship between the magnetic field meets a certain set of requirements

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continuous-wave

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an electromagnetic wave of constant amplitude and frequency

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Fourier transform

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a mathematical transform with many applications in physics and engineering

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proton magnetic resonance

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the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei with the molecules of a substance, in order to determine the structure of its molecules

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chemical shift

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the resonant frequency of a nucleus relative to a standard

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spin-spin splitting

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indirect or scalar coupling (magnetic interaction) between nuclei with a non-zero spin

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peak integration

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used to determine relative number of protons producing the resonances

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upfield

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further to the right on the horizontal axis

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downfield

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further to the left on the horizontal axis

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delta scale

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used for 1-H NMR spectra, in units of ppm

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parts per million

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the units of the delta scale

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tetramethysilane

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TMS, an inert volatile liquid that is added directly to solutions of the sample

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internal standard

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a chemical substance that is added in a constant amount to samples, the blank and calibration standards in a chemical analysis

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connective equivalency

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all protons having identical molecular connectivities

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chemical equivalent

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having identical chemical shifts

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heterotopic

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chemically nonequivalent

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substitution test

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replacement of a hydrogen nucleus with a probe nucleus that can be distinguished form other nuclei in the same set

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probe nucleus

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used in substitution tests

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homotopic

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chemically equivalent

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enantiotopic

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the relationship between two groups in a molecule which, if one or the other were replaced, would generate a chiral compound

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center of chirality

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an atom bearing groups such that an interchanging of any two groups leads to a stereoisomer

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restricted rotation

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a phenomenon usually associated with the presence of a pi bond or a ring

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diastereomers

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stereoisomers that are not enantiomers

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diastereotopic

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the relationship between two groups in a molecule which, if replaced, would generate compounds that are diastereomers

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electronic shield

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provided by an externally applied magnetic field and circulation of electrons within a molecule

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internal electric field

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induces an internal magnetic field about various nuclei in a molecule

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internal magnetic field

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induced by an internal electric field

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effective magnetic field

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at a given nucleus is less than H0 in the presence of an internal magnetic field

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ring-current effect

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the protons of aromatic compounds resonate at lower fields than the vinylic protons of alkenes because the induced magnetic field responsible for deshielding is greater owing to the cyclic nature of the circulation of pi electrons

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nearest neighbors

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elements that have nuclear spin and, in most cases, are no more than three bonds, or two atoms, away from the atom of interest

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magnetic equivalency

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every nucleus in a particular set of chemical shift-equivalent nuclei is coupled equally to every other nucleus of the spin system

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splitting pattern

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an NMR resonance will be split into n+1 peaks where n = number of hydrogens on the adjacent atom or atoms

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coupling constant (J)

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a number that determines the strength of the force exerted in an interaction

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first-order analysis

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construction of a reverse coupling tree, by removing each of the couplings in turn, starting with the smallest

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second-order

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complicate analysis because delta V / J < 5

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long-range coupling

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proton-proton couplings over more than three bonds

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conjugated pi bonds

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connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability

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geminal coupling

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coupling between magnetically different protons on the same atom

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vicinal coupling

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coupled nuclei are on adjacent atoms

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dihedral angle

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angle between two planes

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relative ratio

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given by converting the number of hydrogen atoms present to whole numbers by multiplying each value by an integer

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absolute ratio

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corresponding to the absolute number of hydrogen nuclei present