Isotope Effects

Question 1

Which term of the eyring equation accounts for the vibrational frequency of bonds in the transition state?

Answer 1

KbT/h

Question 2

What are the types of energy levels in an atom?

Answer 2

Vibrational, rotational and translational

Question 3

_ energy levels are so close together that they can be treated as a continuum

Answer 3

Translational

Question 4

Quantization of energy

Answer 4

Atoms have discrete levels from their electronic energy levels.

Question 5

Electronic levels

Answer 5

• Electronic transitions occur between bonding and anti-bonding orbitals or between nonbonding and bonding orbitals
• These are principally π & n and π* orbitals
• n ⟶ π* and π ⟶ π* transitions usually appear in the visible and near UV region

Question 6

Vibrational levels

Answer 6

• Vibrational levels are quantised and curves show likely length of bond
• IR-spectroscopy - different groups vibrate at different frequencies
• Various models (harmonic, anharmonic, Morse)

Question 7

Equation for reduced mass

Answer 7

µ - = (m_1 m_2)/(m_1+m_2 ) in kg

Question 8

Heavier atoms vibrate _

Answer 8

slower

Question 9

rotational levels

Answer 9

Transitions usually in the microwave region
Raman (inelastic scattering) is usually used to examine these transitions.
Rigid interior rotor

Question 10

Temperature and energy levels

Answer 10

Follows Boltzman distribution
If degeneracy isn’t considered, then occupation of levels decreases exponentially with increasing temperature
At body temperature all molecules can be considered to be in the vibrationally ground state

Question 11

What is the zero point energy?

Answer 11

Lowest vibrational energy level
When v=0

Question 12

What is the molar heat capacity?

Answer 12

The molar heat capacity (at constant pressure) is the energy required to raise the temperature of 1 mole of a substance by 1 K

Question 13

Enthalpic change with temperature

Answer 13

Enthalpy change at new temperature is related to the enthalpy change at the old temperature and the change in temperature combined with the difference in heat capacity between reactants and products.

Question 14

Effect of deuterating a sample

Answer 14

bond strength doesn’t change but µ effectively doubles so Ev is lower

Question 15

In a sample containing deuterium, a deuterated sample, the energy levels have much _ intervals.

Answer 15

Smaller

Question 16

Δ‡H and isotopic substitution

Answer 16

CHanges, usually more negative

Question 17

Δ‡S and isotopic substitution

Answer 17

Unchanged

Question 18

The deuterated sample starts with a _ ΔG

Answer 18

Lower

Question 19

What is the kinetic isotope effect?

Answer 19

The difference in rates between the two isotopes.

Question 20

If the rate is significantly higher than predicted by the kinetic isotope effect, this is _

Answer 20

Quantum tunnelling

Question 21

Uses of isotope effects

Answer 21

• Sometimes effects can be seen even if it isn’t the substituted atoms in bond making/ breaking
• Solvents are also affected (of different isotopes) particularly if involved in mechanism
• Enzyme reactions can be more difficult to follow as so many protons (and exchange to solvent)

Question 22

Limitation of studying isotope effects

Answer 22

Some bonds are labile and so the heavier atom may swap and move around.