Chapter 9 Flashcards
The study of the interaction of energy with matter. Energy can be absorbed, emitted, scattered, transmitted, or cause a chemical change (break bonds) when applied to matter. Can be used to probe molecular structure
Spectroscopy
A spectroscopic method for measuring the absorption of radio frequency radiation by certain nuclei when the nuclei are in a strong magnetic field; the two types of spectra provide structural information about the carbon framework of the molecule, and about the number and environment of hydrogen atoms attached to each carbon atom
Nuclear magnetic resonance (NMR) spectroscopy
An NMR method in which a pulse of energy in the radio frequency region of the electromagnetic spectrum is applied to nuclei whose nuclear magnetic moment is precessing about the axis of a magnetic field
Fourier transform NMR (FT-NMR)
An NMR signal comprised of three peaks in a 1:2:1 area ratio, caused by signal splitting from two neighboring NMR-active spin 1/2 nuclei
Triplet
An NMR signal comprised of four peaks in a 1:3:3:1 area ratio, caused by signal splitting from three neighboring NMR-active spin 1/2 nuclei
Quartet
The position in an NMR spectrum, relative to a reference compound, at which a nucleus absorbs. The reference compound is generally tetramwthylsilane (TMS) and is designated zero; x-axis in the NMR spectrum
Chemical shift
A numerical value representing the relative area under a signal in an NMR spectrum. In 1HNMR, the value is proportional to the number of hydrogens producing a given signal
Integration
In NMR, the splitting of the energy levels of a nucleus under observation by the energy levels of nearby NMR-active nuclei, causing characteristic splitting patterns for the signal of the nucleus being observed; hydrogen (n + 1) rule
Coupling
Splitting of an NMR signal into multiple peaks, in patterns such as doublets, triplets, quartets, etc., caused by interactions of the energy levels of the magnetic nucleus under observation with the energy levels of nearby magnetic nuclei
Signal splitting
Hydrogens on adjacent carbons, separated by three bonds from the hydrogens producing the signal
Vicinal hydrogens
Hydrogens bonded to the same carbon
Geminal hydrogens
An NMR signal with only a single, unsplit peak
Singlet
An NMR signal comprised of two peaks with equal intensity, caused by signal splitting from one neighboring NMR-active nucleus
Doublet
Effects observed in NMR spectra caused by the circulation of sigma and pi electrons within the molecule. Shielding causes signals to appear a lower frequencies (upfield), deshielding causes signals to appear at higher frequencies (downfield)
Shielding and deshielding
Chemical shift for ________________ is typically in the range of 0.8 to 1.8 ppm
Hydrogens of unsubstituted alkanes
The chemical shift of __________________ is typically in the range of 3.1-4.0 ppm
Hydrogens bonded to a carbon bearing oxygen or a halogen
The _____________ absorb at 7.27 ppm
Hydrogens of benzene
___________________ have chemical shifts in the range 6.0-8.5, depending on the substituents
Hydrogens bonded to substituted benzene rings
The chemical shift of ____________ is typically in the range of 4.0-6.0 ppm
Alkene hydrogens
The chemical shift of a(n) ___________ is typically in the range of 2.5-3.1 ppm
Alkyne hydrogen
Atoms in a molecule where replacement of one or another results in the same compound; are chemical shift equivalent in NMR spectroscopy
Homotopic (chemically equivalent atoms)
Atoms is a molecule where replacement of one or the other leads to a new compound; aren’t chemical equivalent in NMR spectroscopy
Heterotopic (chemically nonequivalent atoms)
If replacement of each of two hydrogens (or ligands) by the same group yields compounds that are enantiomers, the two hydrogen atoms (or ligands) are said to be enantiotopic
Enantiotopic hydrogens (ligands)
If replacement of each of two hydrogens (or ligands) by the same groups yields compounds that are diastereomers, the two hydrogen atoms (or ligands) are said to be diastereotopic
Diastereotopic hydrogens (or ligands)
An effect observed in NMR spectra; results in a signal appearing as a multiplet (i.e., doublet, triplet, quartet, etc.) and are caused by magnetic couplings of the nucleus being observed with nuclei of nearby atoms
Spin-spin splitting
The splitting of an NMR signal caused by hydrogen atoms in adjacent carbons
Vicinal coupling
The separation in frequency units (hertz) of the peaks of a multiplet caused by spin-spin coupling between atoms a and b
Coupling constant (J)
The frequency of a wave
Hertz (Hz)
The angle between two atoms (or groups) bonded to adjacent atoms, when viewed as a projection down the bond between adjacent atoms
Dihedral angle
An empirical correlation between the magnitude of an NMR coupling constant and the dihedral angle between two coupled protons. The dihedral angles derived in this manner can provide information about molecular geometries
Karplus correlation
In the context of NMR, transfer of protons bonded to heteroatoms from one molecule to another, broadening their signal and eliminating spin-spin coupling
Chemical exchange
Protons that can be transferred rapidly from one molecule to another. These protons are often attached to electronegative elements such as oxygen or nitrogen
Exchangeable protons
Technique that causes spin-spin splitting not to be observed in NMR spectra
Spin decoupling
Distortionless enhanced polarization transfer (DEPT) 13C spectra indicate how many hydrogen atoms are bonded to a given carbon atom
DEPT 13C NMR spectroscopy
Attached proton test (APT) 13C NMR spectra indicate by the up or down direction of signals whether each carbon has an odd or even number of hydrogens bonded to it. CH3 and CH carbon signals (odd number of hydrogens) point in one direction and CH2 and C signals (even) point in the other direction
APT 13C NMR spectroscopy
A method of eliminating carbon-proton coupling by irradiating the sample with a wide-frequency (“broadband”) energy input in the frequencies in which protons absorb energy
Broadband (BB) proton decoupling
A technique, useful in structure elucidation and quantitative analysis, that involves the generation of ions from a molecule, the sorting and detecting of the ions, and the display of the result in terms of the mass/charge ratio and relative amount of each ion
Mass spectrometry (MS)
The most intense peak in a mass spectrum
Base peak
Conversion of neutral molecules to ions (charged species)
Ionization
A method of ion formation in mass spectrometry whereby the sample to be analyzed (analyte) is placed in a high vacuum and, when in the gas phase, bombarded sighs beam of high-energy electrons. A valence electron is displaced by the impact of the electron beam, yielding a species called the molecular ion (if there has been no fragmentation), with a +1 charge and an unshared electron (a radial cation)
Electron impact (EI)
The cation produced in a mass spectrometer when one electron is dislodged from the parent molecule; symbolized M.+
Molecular ion (M.+)
A chemical species containing an unshared electron and a positive charge
Radial cation
Cleavage of a chemical species by the breaking bonds, as in the formation of fragments during mass spectrometric analysis
Fragmentation
When the bond linking the functional group to the adjacent, or alpha position, of that group breaks
Alpha-cleavage (a-cleavage)
The resonance-stabilized cation
Acylium ion
An analytical method that couples a gas chromatograph (GC) with a mass spectrometer (MS). The GC separates the components of a mixture to be analyzed by sweeping the components, in the gas phase, through a column containing an adsorbant called a stationary phase. The gaseous molecules will cling to the surface of the stationary phase (be adsorbed) with different strengths. Those molecules that cling (adsorb) weakly will pass through the column quickly: those that adsorb more strongly will pass through the column more slowly. The separated components of the mixture are then introduced into the mass spectrometer, where they are analyzed
GC/MS analysis
A method of ion formation in mass spectrometry whereby a solution of the sample to be analyzed (analyte) is sprayed into the vacuum chamber of the mass spectrometer from the tip of a high-voltage needle, imparting charge to the mixture. Evaporation of the solvent in the vacuum chamber yields charged species of the analyte, some of which may have charges greater than +1. A family or m/z peaks unique to the formula weight itself can be calculated by computer
Electrospray ionization (ESI)
A method in mass spectrometry for ionizing analyses that don’t ionize well by electrospray ionization. The analyte is mixed with low molecular weight organic molecules that can absorb energy from a laser and then transfer this energy to the analyte, producing ions which are then analyzed by the mass spectrometer
Matrix-assisted laser desorption ionization (MALDI)
