Helicopter molecules

S. Zilberg and Y. Haas, The photochemistry of 1,4-cyclohexadiene in solution and in the gas phase Conical intersections and the origin of the helicopter-type motion of H2 photogenerated in Che isolated molecule, PCCP 4, 34 (2002). 

Alignment results reported for NO trapping/desorption from Pt(l 11). At high was positive implying a propensity for helicopter-type desorbing molecules. (Adapted from Re/ 29.)... 

Using (1 + 1) REMPI, the quadrupole alignment Aq2) (E, v, J, Ts = 925 ) has also been measured for D2 associative desorption from Cu(lll) [229] and some of the results are given in Figure 3.24. Aq2) favors helicoptering alignment, in agreement with a PES that favors dissociation of molecules preferentially parallel to the surface (and detailed balance). It increases substantially with J but decreases with Ef. The... 

Orientational hindrance does not occur only for molecules with angular momentum vectors parallel to the surface, i.e. cartwheel rotating molecules. The PES topography also depends strongly on the azimuthal orientation of the molecular bond. For example, in the H2/Cu(l 11) system the lowest dissociation barrier occurs at the bridge site when the H atoms are directed towards adjacent 3-fold hollow sites, but the barrier is substantially higher if the molecule is rotated 90° so that the atoms are directed towards atop sites. In this case, there is also substantial orientational hindrance of dissociation for helicopter states (those with angular momentum perpendicular to the surface). 

Zilberg S, Haas Y. The photochemistry of 1,4-cyclohexadiene in solution and in the gas phase conical intersections and the origin of the helicopter-type motion of H-2 photo-generated in the isolated molecule. Phys Chem Chem Phys. 2002 4 34-42. Tamura H, Nanbu S, Nakamura H, Ishida T. A theoretical study of cyclohexadiene/ hexatriene photochemical interconversion multireference configuration interaction potential energy surfaces and transition probabilities for the radiationless decays. Chem Phys Lett. 2005 401 487-491. 

Increased rotational energy helps to dissociate the molecule, as Figure 6.13 shows. Calculations indicate that rotational motion in a plane perpendicular to the surface (the cartwheel mode) is restricted, because it affects the interaction between the molecule and the surface. Rotation parallel to the surface (the helicopter mode ) is virtually free, however, and allows for stretching of the H-H bond as required for dissociation. 

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