Foundations of Chemical Kinetics Lecture 12: Transition-state - PowerPoint PPT Presentation

Foundations of Chemical Kinetics Lecture 12: Transition-state theory: The thermodynamic formalism Marc R. Roussel Department of Chemistry and Biochemistry

Breaking it down ◮ We can break down an elementary reaction into two steps: Reaching the transition state, and going through it into the product valley. R ⇋ TS → P ◮ For a high barrier, there will be a Boltzmann distribution of reactant energies which is only slightly disturbed by the leak across the top of the barrier. ◮ This implies that we can treat the step R ⇋ TS as being in equilibrium. K ‡ − TS k ‡ − − ⇀ R → P ↽ − − v = k ‡ [TS] (roughly)

Strategy K ‡ − TS k ‡ ⇀ − − R ↽ → P − − v = k ‡ [TS] ◮ This is an elementary reaction (R → P), so its rate ought to be v = k [R]. ◮ We will use the equilibrium condition to eliminate [TS]. This will bring thermodynamic quantities related to the equilibrium constant into the theory. ◮ We will use a quantum-statistical argument to get a value for the specific rate of crossing of the barrier k ‡ .

Review of some elementary thermodynamics Free energy ◮ The Gibbs free energy ( G ) is defined by G = H − TS H : enthalpy (∆ H = heat at const. p ) S : entropy (measure of energy dispersal) ◮ The Gibbs free energy change is the maximum non- pV work available from a system.

Review of some elementary thermodynamics Free energy (continued) ◮ For a reaction b B + c C → d D + e E, the Gibbs free energy change is ∆ r G m = ∆ r G ◦ m + RT ln Q where ∆ r G ◦ m is the free energy change under standard conditions: ∆ r G ◦ m = d ∆ f G ◦ (D) + e ∆ f G ◦ (E) − [ b ∆ f G ◦ (B) + c ∆ f G ◦ (C)] ◮ The reaction quotient Q is defined as Q = a d D a e E a b B a c C where a i is the activity of species i .

Review of some elementary thermodynamics Activity ◮ The activity also depends on the definition of the standard conditions: In the gas phase, a i = γ i p i / p ◦ where p ◦ is the standard pressure (usually 1 bar). For a solute, a i = γ i c i / c ◦ where c ◦ is the standard concentration. There are several different conventions used for standard concentrations, the most common being 1 mol/kg and 1 mol/L. γ i is the activity coefficient (sometimes known as a fugacity coefficient in the gas phase) of species i , a measure of the deviation from ideal behavior. γ i = 1 for an ideal substance.

Review of some elementary thermodynamics Equilibrium ◮ At equilibrium, ∆ r G m = 0, i.e. ∆ r G ◦ m = − RT ln K where K is the numerical constant such that Q = K at equilibrium. ◮ This equation can be rewritten − ∆ r G ◦ � � m K = exp . RT Note that K is related to ∆ r G ◦ m , the standard free energy change.

Correction: Statistical equilibrium constant in a general reaction Reaction: a A + b B ⇋ c C + d D Correct equation: K = Q c C Q d � − ∆ E 0 � N − ∆ n exp D Q a A Q b k B T B ◮ ∆ n = c + d − ( a + b ) (difference of stoichiometric coefficients) ◮ N is the number of molecules (dimensionless). ◮ The translational partition function depends on V . N and V are chosen to be consistent with the standard state. Example: For p ◦ = 1 bar at 25 ◦ C, p / RT = 40 . 34 mol m − 3 , so we could pick V = 1 m 3 and N = 40 . 34 × 6 . 022 × 10 23 = 2 . 429 × 10 25 .

The K ‡ equilibrium Case 1: First-order elementary reaction ◮ We are treating the “equilibrium” K ‡ − − ⇀ R − TS ↽ − ◮ For this equilibrium, K ‡ = a TS ⇒ a TS = K ‡ a R = a R ◮ If we assume ideal behavior or similar activity coefficients for R and TS, we get [TS] = K ‡ [R] ∴ v = k ‡ K ‡ [R] ∴ k = k ‡ K ‡

The K ‡ equilibrium Case 2: Second-order elementary reaction ◮ The equilibrium is K ‡ − − ⇀ X + Y − TS ↽ − ◮ Now, K ‡ = a TS ⇒ a TS = K ‡ a X a Y = a X a Y ◮ Since a i = γ i c i / c ◦ , this becomes [TS] = K ‡ γ X γ Y [X][Y] c ◦ γ TS ◮ Assuming ideal behavior, we get [TS] = K ‡ c ◦ [X][Y] which gives k = k ‡ K ‡ c ◦

The K ‡ equilibrium First- vs second-order rate constants ◮ Since the difference between the first- and second-order cases is just a factor of c ◦ , we treat the second-order case from here on.

Mathematical interlude: Taylor series ◮ For any “nice” function, ( x − a ) 2 + . . . + f ( n ) ( a ) f ( x ) ≈ f ( a )+ f ′ ( a )( x − a )+ f ′′ ( a ) ( x − a ) n 2 n ! ◮ For small x , take a = 0: f ( x ) ≈ f (0) + f ′ (0) x + 1 2 f ′′ (0) x 2 + . . . ◮ In practice we often stop at the first non-trivial term (i.e. the first term after f (0) that isn’t identically zero).

Units of frequency ◮ In our treatment of vibration, we have so far used the angular frequency ω , whose units can be thought of as rad s − 1 . ◮ We can also express frequencies in Hz, i.e. cycles per second, typically denoted ν . ◮ Since ω = 2 πν and � = h / 2 π , � ω = h ν .

Statistical thermodynamic considerations ◮ From statistical thermodynamics, and neglecting non-ideal effects, we have = c ◦ [TS] Q TS � − ∆ E 0 � K ‡ = N exp [X][Y] , Q X Q Y RT where Q TS is the partition function of the transition state. � � ∴ [TS] = [X][Y] Q TS − ∆ E 0 N exp c ◦ Q X Q Y RT ◮ Writing X + Y ⇋ TS → P involves an implicit assumption, namely that all transition states decay to product. ◮ A complex that has reached the top of the activation barrier has no intrinsic bias toward reactants or products. Thus, half of those complexes will, all other things being equal, proceed to products.

Statistical thermodynamic considerations (continued) ◮ Define the concentration of transition states leading to product formation as [TS → P ] = 1 2[TS] = 1 [X][Y] � − ∆ E 0 � Q TS N exp 2 c ◦ Q X Q Y RT ◮ The reaction rate is therefore correctly cast as v = k ‡ [TS → P ] = 1 2 k ‡ [X][Y] Q TS � − ∆ E 0 � N exp c ◦ Q X Q Y RT which gives k ‡ � � k = 1 Q TS − ∆ E 0 N exp c ◦ 2 Q X Q Y RT

Statistical thermodynamic considerations (continued) ◮ We assume that the transition-state partition function factors, i.e. that motion along the reactive normal mode is independent of other molecular motions: Q TS = Q ‡ Q r where Q r is the part of the partition function associated with the reactive normal mode (i.e. the motion through the saddle) while Q ‡ is the rest of the partition function.

Statistical thermodynamic considerations (continued) ◮ Assume we can treat the reactive mode as a vibration, with partition function Q r = [1 − exp( − h ν r / k B T )] − 1 ◮ Since the reactive mode is “loose”, assume h ν r / k B T is small. ◮ Taylor expansion for small x : 1 − e − x ≈ x 1 − e − x � − 1 ≈ x − 1 � ∴ ∴ Q r ≈ k B T h ν r

Statistical thermodynamic considerations (continued) ◮ The rate constant becomes k ‡ Q ‡ � � k = 1 k B T − ∆ E 0 N exp 2 c ◦ h ν r Q X Q Y RT ◮ ν r represents the frequency for a full “vibrational” cycle of the reactive mode (back and forth). ◮ k ‡ is the frequency for crossing the saddle in one direction only. ∴ k ‡ = 2 ν r Q ‡ � − ∆ E 0 � ∴ k = k B T N exp c ◦ h Q X Q Y RT

Statistical formula for the rate constant Interpretation Q ‡ k = k B T � − ∆ E 0 � N exp c ◦ h Q X Q Y RT ◮ Q ‡ is the partition function for the transition state omitting the reactive mode. ◮ ∆ E 0 is the difference in zero point energies between the reactants and transition state. � � Q ‡ − ∆ E 0 Q X Q Y N exp is of the form of an equilibrium constant ◮ RT with one mode (the reactive mode) removed.

Statistical formula for the rate constant Application Q ‡ k = k B T � − ∆ E 0 � N exp c ◦ h Q X Q Y RT In principle, we can use this equation to compute rate constants. In practice, this isn’t so easy because: ◮ We need to know the geometry of the transition state. ◮ We need to know the height of the barrier and zero-point energies of the reactants and transition state. ◮ We need to know the vibrational spectrum of the transition state. All of this information is in principle available either from very good quantum chemical calculations or from transition-state spectroscopy.

Thermodynamic interpretation Q ‡ � − ∆ E 0 � k = k B T N exp c ◦ h Q X Q Y RT ◮ Define Q ‡ � � − ∆ E 0 K ‡ = N exp Q X Q Y RT ◮ Note that this isn’t quite a normal equilibrium constant because we have removed one mode from the transition state partition function. ◮ We can still write − ∆ ‡ G ◦ � � K ‡ = exp m RT

Thermodynamic interpretation (continued) − ∆ ‡ G ◦ � � k = k B T m c ◦ h exp RT � ∆ ‡ S ◦ − ∆ ‡ H ◦ � � � ∴ k = k B T m m c ◦ h exp exp R RT

Relationship to Arrhenius parameters From ln k = ln A − E a / RT , we have d ln k = E a / RT 2 dT or E a = RT 2 d ln k dT .

Relationship to Arrhenius parameters (continued) ◮ For the transition-state theory expression, + ∆ ‡ S ◦ − ∆ ‡ H ◦ � k B T � m m ln k = ln c ◦ h R RT ∂ ∆ ‡ S ◦ ∂ ∆ ‡ H ◦ + ∆ ‡ H ◦ ∂ ln k � = 1 T + 1 � 1 � � m � m � m − � � � RT 2 ∂ T ∂ T ∂ T R RT � � � p p p