I_ab

Derivation of differential equations describing evolution of spin concentrations

1. Reaction rates and partial conversion rates

3. Net conversion rates

4. Expression in terms of spin (monomer) concentrations

5. Final result

Conclusions

Back to Contents

clean up workspace

Write properly balanced reactions equations:

Transition A:

(1)          (2)

Ra<=>Rb

Constants: k_1_A (forward), k_2_A (reverse).

Write reaction rates

Introduction.

We distinguish reaction rates (Rate, elementary reaction acts per unit time) and conversion rates (dc/dt, number of moles of the specific species consumed/produced per unit time). Conversion rates, dc/dt, for species are related to reaction rates, Rate, through molecularity coefficients.

To compute conversion rates, we need to distinguish partial conversion rates from net (overall) conversion rates. The net conversion rate is actual rate of change in measured concentration of the species. Partial conversion rate is a conversion rate of the species observed along the specific branch of the reaction mechanism. Summing partial conversion rates of the species one obtains the net conversion rate for this species.

Isomerization (forward transition on A: 1_A)

a reaction rate

a partial conversion rate of Ra: one reaction act uses one molecule of Ra

The final form

a partial conversion rate of Rb: one reaction makes one molecule of Rb

The final form

Isomerization (reverse transition on A: 2_A)

a reaction rate

a partial conversion rate of Ra: one reaction act makes one molecule of Ra

The final form

a partial conversion rate of Rb: one reaction act uses one molecule of Ra

The final form

Back to Contents

To define evolution of the species we need to compute concentrations  as a function of time. To this end, we will write differential equations for conversion rates of all species.

In a reversible process both forward and reverse reaction occur simultaneously. Thus, the net conversion rate of the species is a difference between partial conversion rates resulting from forward and reverse reactions, summed along all branches.

Net conversion rate of  Ra

Sum all pertaining partial conversion rates

Substitute using expressions for partial conversion rates

Net conversion rate of  Rb

Sum all pertaining partial conversion rates

Substitute using expressions for partial conversion rates

not needed here because we do not have oligomerization reactions: one spin in Ra is converted to one spin in Rb.

Summarize the derivation results

Assign order to species

Same order for net rates

Restate the equations in terms of numbered species

Prepare results for transfer to MATLAB

To avoid typing errors when transfering derived K matrix to MATLAB we type it in here and then directly test against the derivation result from above. After that the K matrix may be transfered to MATLAB by cut-and-paste of the MuPad output.

Enter the K-matrix looking at the above results (collect terms at correspondingly numbered species).

Simple rules that allow catching mistakes in K matrix derivation:

   (1) a sum of each column should be zero (so each constant must appear with both positive and negative sign), and 

   (2) each row has to have complete pairs of constants (i.e., if k12

    appears there must be k21 in the same row with an opposite sign and so on).

Create a column vector containing concentrations of species in numbered notation

Check correctness of the entered K matrix by multiplying with P and comparing to the above equations:

Multiply K and P:

Collect right-hand-side parts of equations

Compare the derivation result to manual input

=> If TRUE ---the typed K-matrix is correct.

Use this K-matrix  (copy-paste output to MATLAB)

Back to Contents

I derived differential equations governing spin populations. The K matrix has been prepared for transferring to MATLAB.

Back to Contents