Chapter 2 - Principles of ionization and ion dissociation - part 2 Flashcards
Typically, ion dissociations are endothermic, and thus
each fragmentation step consumes some of the ions’ internal energy.
In fragmentation, simple bond cleavages cause
radical losses.
Rearrangement fragmentations typically lead to losses of
intact neutral molecules.
In homolytic bond cleavages, the activation energy of the reverse reaction (E0r) is
close to zero because an ion and a radical are created from the cleavage (very reactive with each other).
In heterolytic bond cleavages, the activation energy of the reverse reaction (E0r) is
greater than zero because a very stable neutral molecule is formed from heterolytic cleavage.
Kinetic energy release (KER)
A portion of the total excess energy is converted into translational motion of the fragments in the direction of the bond that is cleaved.
A larger value of KER is expected when
there is a larger sum of total excess energy (excess energy + reverse activation energy).
The observed KER consists of two components,
one from excess energy (Eex), and one from reverse activation energy (E0r).
Degrees of freedom effect (DOF)
The ratio of Etrans*/Eex decreases as the size of the fragmenting ion increases.
Isotope effects
Any effect exerted by the introduction of isotopes.
Intermolecular isotope effects example
Comparing D radical loss from CD4 to H radical loss from CH4.
Intramolecular isotope effect example
Comparing H radical to D radical loss from CH2D2.
The term “primary kinetic isotope effect” applies if
the effect is exerted on a bond where the isotope itself is involved during the reaction.
The kinetic isotope effect for heavier elements
decreases.
Isotope effects in mass spectrometry are usually
kinetic isotope effects resulting from different rate constants of bond breaking and bond formation without (kH) or with (kD) a heavier isotope involved, respectively.
Kinetic isotope effects are called normal if ____, and inverse if ______.
kH/kD > 1
kH/kD < 1
Inverse kinetic isotope effects are
rarely observed.
Secondary kinetic isotope effects are observed if
an isotopic label is located adjacent to or remote from the bond that is being broken or formed during the reaction.
The critical slope method was developed to
overcome the unpredictability of the actual onset of ionization.
An electron monochromator is a device for
selecting nearly monoenergetic electrons from an electron beam.
The absorption of UV light by a neutral can result in
electronically excited states that undergo relaxation either by emission of light or by emission of an electron.
In photoionization (PI), the ______ serves the same purpose as the energetic electron in EI.
photon
The normal procedure to achieve PI is
multiphoton ionization (MUPI)
Zero kinetic energy photoelectron spectroscopy is based on
distinguishing excited ions from ground state ions.
Kinetic shift denotes the
overestimation of AEs due to the contribution of excess energy in the transition state necessary to yield rate constants larger than 10^6 s^-1.
The rate of a first order reaction (change in concentration of precursor ion over time) is solely determined by
the concentration of the precursor ion, and its internal energy.
Dimension of a first order reaction
s^-1 (or Hz)
In a bimolecular reaction, the rate of a second order reaction is dependent on
both the concentration of the respective molecular ions and that of the neutrals.
