Inter system crossing

Inter system crossing in (piezochemical) mechanosynthesis

Design target: Increasing inter system crossing rates. Goals:

• avoiding errors
• increasing reaction speeds
• reducing energy dissipation

Avoiding errors (omitted reactions, misreactions) due to too low singlet-triplet energy gap

• Singlet transition geometries often resemble triplet equilibrium geometries (Salem and Rowland 1972 – referenced by Nanosystems)
• Singlet state (paired) to low lying triplet state is undesired

To avoid that

• either (conservatively) get the energy gap up to above 145zJ (= k*300K*ln(10^15)]
• or (weaker prerequisite) get the accumulated ramp-on ISC transition rate to a probability above ln(P_err) prior
  to theΔV_s,t point where the geometry is no longer suitable for bond formation

Inadvertently slowing down by pressing to hard

Citations from Nanosystems:

• "As the radicals approach the gap between the triplet and singlet state energies grows, but this decreases the rate of intersystem crossing".
  
• "The condition that ΔV_s,t ≥145maJ imposes a significant constraint because k_isc varies inversely with the electronic energy difference ΔV_isc which (in the absence of mechanical relaxation) would equal the difference in equilibrium energies ΔV_s,t, and will frequently be of similar magnitude"

Avoiding dissipation during bond cleavage by speeding up ISC

If inter system crossing is slow compared to (tensile) bond cleavage then cleavage gives a singlet diradical
When pulling the cleaved parts further apart then the singlet triplet energy gap vanishes
and thermal excitations fill the triplet state (a loss in Helmholtz free energy of minus ln(2)kT – a loss of one bit information – undesired dissipation)

(TODO: That would cause local cooling that goes unused and thus gets dissipated by ambient heat flowing in? Could that be partially recuperated by a heat pump? Abd or exploited for deliberate cooling?)

If inter system crossing is fast compared to (tensile) bond cleavage then

• thermal excitations fill the (repulsive) triplet state already during bond cleavage
• => there is a reduction of the mean-force bond potential energy (?)
• there is no significant dissipation ☺

ISC rates in pi-bond twisting

Abstraction of a moiety to yield an aklene (accelerating Diels Adler and related reactions)

• resembles radical coupling
• requires spin pairing
• raises questions about inter-system crossing rates

Employing the "external heavy atom effect" to accelerate ISC

Nearby site integration of heavy elements.
E.g. Bismuth (Z=83) – since it likes to form 3 weak covalent bonds (?) (suggested in Nanosystems)

Given known examples for the "external heavy atom effect" (Nanosystems page 216) it should be possible to have:

• k_isc>10^9 with ΔV_s,t >145zJ and thus
• t_trans<10^-7s with P_err<10^-15

Inter system crossing – relevant parts

(wiki-TODO: Add the most relevant bits about the theory)

Nanosystems references

• 8.3.4. Preview: molecular manufacturing and reliability constraints – e. Meeting constraints on omitted reactions in a single trial. (P210 center)
• 8.4.4. Carbon radicals – b. Radical coupling and inter system crossing (P215 bottom, P216)
• 8.5.3. Tensile bond cleavage – c. Spin, dissipation, and reversibility. (P224 bottom)
• 8.5.6. Pi bond torsion (P231 bottom)

Related

• Piezochemical mechanosynthesis
• Mechanosynthesis
• Fun with spins – influencing spins by ligand-fields (crystal-fields) rather than spin-orbit coupling

External links

(Wikipedia: Intersystem crossing)