Chapter 9 Notecards

NMR spectroscopy applied to carbon. Carbon-13 is NMR active, whereas carbon-12 is not and therefore cannot be studied by NMR. Only 1.1% of all naturally occurring carbon is carbon-13.

A two-dimensional NMR method used to display the coupling between hydrogen atoms.

The most intense peak in a mass spectrum

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. This energy input causes the protons to remain in the high energy state, eliminating coupling with carbon nuclei.

In the context of NMR, transfer of protons bonded to heteroatoms from one molecule to another, broadening their signal and eliminating spin-spin coupling.

The position in an NMR spectrum, relative to a reference compound, at which a nucleus absorbs. The reference compound most often used is tetramethylsilane (TMS), and its absorption point is arbitrarily designated zero. The chemical shift of a given nucleus is proportional to the strength of the magnetic field of the spectrometer. The chemical shift in delta units, d, is determined by dividing the observed shift from TMS in hertz multiplied by 106 by the operating frequency of the spectrometer in hertz

A two-dimensional NMR method that displays coupling relationships between protons in a molecule.

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. The signal from an NMR-active nucleus will be split into (2nI + 1) peaks, where n = the number of equivalent neighboring magnetic nuclei and I = the spin quantum number. For hydrogen (I = 1/2) this rule devolves to (n + 1), where n = the number of equivalent neighboring hydrogen nuclei.

The separation in frequency units (hertz) of the peaks of a multiplet caused by spin–spin coupling between atoms a and b.

Distortionless enhanced polarization transfer (DEPT) 13C NMR spectra indicate how many hydrogen atoms are bonded to a given carbon atom

Effects observed in NMR spectra caused by the circulation of sigma and pi electrons within the molecule. Shielding causes signals to appear at lower frequencies (upfield), deshielding causes signals to appear at higher frequencies (downfield)

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

The angle between two atoms (or groups) bonded to adjacent atoms, when viewed as a projection down the bond between the adjacent atoms.

An NMR signal comprised of two peaks with equal intensity, caused by signal splitting from one neighboring NMR-active nucleus

Any area or signal in an NMR spectrum that is to the left relative to another. (See Upfield for comparison.) A signal that is downfield of another occurs at higher frequency (and higher d and ppm values) than the other signal.

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 with a 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 radical cation).

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 of m/z peaks unique to the formula weight of the analyte results, from which the formula weight itself can be calculated by computer

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.

Protons that can be transferred rapidly from one molecule to another. These protons are often attached to electronegative elements such as oxygen or nitrogen.

An NMR method in which a pulse of energy in the radiofrequency region of the electromagnetic spectrum is applied to nuclei whose nuclear magnetic moment is precessing about the axis of a magnetic field. This pulse of energy causes the nuclear magnetic moment to “tip” toward the xy plane. The component of the nuclear magnetic moment in the x–y plane generates (“induces”) a radiofrequency signal, which is detected by the instrument. As nuclei relax to their ground states this signal decays over time; this time-dependent signal is called a “Free Induction Decay” (FID) curve. A mathematical operation (a Fourier transform) converts time-dependent data into frequency dependent data–the NMR signal

Cleavage of a chemical species by the breaking of covalent bonds, as in the formation of fragments during mass spectrometric analysis

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 compounds, 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. These 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.

The frequency of a wave. Now used instead of the equivalent cycles per second (cps)

A two-dimensional NMR method used to display the coupling between hydrogens and the carbons to which they are attached.

Atoms in a molecule where replacement of one or the other leads to a new compound. Heterotopic atoms are not chemical shift equivalent in NMR spectroscopy.

Atoms in a molecule where replacement of one or another results in the same compound. Homotopic atoms are chemical shift equivalent in NMR spectroscopy

A numerical value representing the relative area under a signal in an NMR spectrum. In 1H NMR, the integration value is proportional to the number of hydrogens producing a given signal.

Conversion of neutral molecules to ions (charged species).

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

A technique based on NMR spectroscopy that is used in medicine.

A method in mass spectrometry for ionizing analytes that do not 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 +1 ions which are then analyzed by the mass spectrometer.

A technique, useful in structure elucidation, 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.

The cation produced in a mass spectrometer when one electron is dislodged from the parent molecule, symbolized MD+.

A spectroscopic method for measuring the absorption of radio frequency radiation by certain nuclei when the nuclei are in a strong magnetic field. The most important NMR spectra for organic chemists are 1H NMR spectra and 13C NMR spectra. These 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.

An electronic technique used in 13C NMR spectroscopy that allows decoupling of spin–spin interactions between 13C nuclei and 1H nuclei. In spectra obtained in this mode of operation all carbon resonances appear as singlets.

A chemical species containing an unshared electron and a positive charge.

Effects observed in NMR spectra caused by the circulation of sigma and pi electrons within the molecule. Shielding causes signals to appear at lower frequencies (upfield), deshielding causes signals to appear at higher frequencies (downfield)

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.

An NMR signal with only a single, unsplit peak

The study of the interaction of energy with matter. Energy can be absorbed, transmitted, emitted or cause a chemical change (break bonds) when applied to matter. Among other uses, spectroscopy can be used to probe molecular structure.

An effect that causes spin–spin splitting not to be observed in NMR spectra.

An effect observed in NMR spectra. Spin–spin splittings result 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.

A method of illustrating the NMR signal splittings in a molecule by drawing “branches” from the original signal. The distance between the branches is proportional to the magnitude of the coupling constant. This type of analysis is especially useful when multiple splittings (splitting of already split signals) occur due to coupling with non-equivalent protons

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

NMR techniques such as COSY and HETCOR that correlate one property (e.g., coupling), or type of nucleus, with another. (See COSY and HETCOR.)

Any area or signal in an NMR spectrum that is to the right relative to another. (See Downfield for comparison.) A signal that is upfield of another occurs at lower frequency (and lower d and ppm values) than the other signal.

The splitting of an NMR signal caused by hydrogen atoms on adjacent carbons.