Stretch-bend coupling

We have seen that resonance couplings destroy quantum numbers as constants of the spectroscopic Hamiltonian. Widi both the Darling-Deimison stretch coupling and the Femii stretch-bend coupling in H2O, the individual quantum numbers and were destroyed, leaving the total polyad number n + +... 

The CCSD(T)/aug-cc-pV(T d)Z calculated values of the G-matrix and force constant parameters used in Eqs. (5) and (9) are given in Table 8.1. From these parameters the effective stretch-bend coupling constants are calculated to be /r =... 

For many composite applications the laminate is not orthotropic, but anisotropic, though this can exist in many particular forms, one of which occurs when an orthotropic laminate is loaded in a direction which does not coincide with one of the principal axes, or when the lay-up is symmetric but not balanced about a principal reference axis. A [0°/+45°/90°]s laminate is balanced and quasi-isotropic. As a general rule, all laminates should be symmetrically laid up about their mid-plane to avoid stretching bending coupling. 

To establish a connection between the stretch-bend potential coupling and the decay rate constant from overtone excited HC bonds in benzene, we have studied a two-mode model system describing a HC fragment of benzene. The Hamiltonian, which is expressed in internal curvilinear coordinates, includes both kinetic and potential stretch-bend coupling(13) ... 

These results, together with the dynamical behavior described above, show that there is a cancellation of the kinetic and potential stretch-bend coupling terms in the Hamiltonian. 

The difficulty with this approach is that it is very time consuming and typically unsystematic. For polyatomics, the functional forms may suffer from missing or inadequate stretch-stretch or stretch-bend couplings. Even for triatomics, the barrier shape may depend strongly on the chosen functional form. One requires many electronic structure calculations and considerable care to create an accurate (or even useful) surface with all the important internal-coordinate couplings and without artifacts or spurious features. 

Buchanan EG, Sibert EL, Zwier TS (2013) Ground state conformational preferences and CH stretch-bend coupling in a model alkoxy chain 1,2-diphenoxyethane. J Phys Chem A 117 2800... 

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