Nuclear spin blockade of laser ignition of intramolecular rotation in the model boron rotor B13+11

Abstract

The boron rotor 11 B + 13 consists of a tri-atomic inner "wheel" that may rotate in its pseudo-rotating ten-atomic outer "bearing"-this concerted motion is called "contorsion." 11 B + 13 in its ground state has zero contorsional angular momentum. Starting from this initial state, it is a challenge to ignite contorsion by a laser pulse. We discover, however, that this is impossible, i.e., one cannot design any laser pulse that induces a transition from the ground to excited states with non-zero contorsional angular momentum. The reason is that the ground state is characterized by a specific combination of irreducible representations (IRREPs) of its contorsional and nuclear spin wavefunctions. Laser pulses conserve these IRREPs because hypothetical changes of the IRREPs would require nuclear spin flips that cannot be realized during the interaction with the laser pulse. We show that all excited target states of 11 B + 13 with non-zero contorsional angular momentum have different IRREPs that are inaccessible by laser pulses. Conservation of nuclear spins thus prohibits laser-induced transitions from the non-rotating ground to rotating target states. We discover various additional constraints imposed by conservation of nuclear spins, e.g., laser pulses can change clockwise to counterclockwise contorsions or vice versa, but they cannot stop them. The results are derived in the frame of a simple model.

Nuclear versus electronic ring currents in oriented torsional molecules induced by magnetic fields. I. Nuclear currents of toluene

“…which are related to the z components of the canonical angular momenta of the fragments A and B as [1].…”

“…which was derived in Chapter IIa of the Supplemental Material of Ref. [1]. The field-free torsional/rotational Hamiltonian…”

“…The field induces rotation of the CH 3 and C 6 H 5 groups around the torsion axis since the torsion barrier is very low. The rotations of the two groups is associated with magnetically induced torsional angular momenta [1]. The nuclei rotating with the two fragments generate a magnetically induced nuclear ring current, which is here investigated for the torsional/rotational ground state of toluene.…”

“…This work is motivated by a recent discovery that external magnetic fields can induce torsional angular momenta in molecules consisting of two fragments (A and B) that can rotate around an intramolecular torsion axis [1]. Depending on the height of the torsion barrier, A can rotate relatively to B.…”

Vibrational band-structures caused by internal rotations of the boron Wankel rotor B11−

“…5 It should be noted that nuclear quantum effects are in general important for large amplitude motions 18 such as the internal rotations of boron clusters. [19][20][21][22] In 2017 a universal mechanism for the internal rotations of planar boron clusters B 11 À , B 13 + , B 15 + , and B 19 À was proposed, 19 with explicit treatment of nuclear quantum effects. Accordingly the inner wheel of each boron cluster rotates with respect to its pseudo-rotating outer bearing.…”