Torsional rotation terms

CONFORMATIONAL POPULATION INCREMENT (POP) TORSIONAL CONTRIBUTION (TOR) TRANSLATION/ROTATION TERM (T/R)... 

The functional form for dihedral angle (torsional) rotation is identical to that shown in equation (13) on page 175. The barrier heights are in kcal/mol and are in the file pointed to by the Fouri-erTorsion entry for the parameter set in the Registry or the chem. ini file, usually called tor.txt(dbf). If more than one term is... 

The parameter redundancy is also the reason that care should be exercised when trying to decompose energy differences into individual terms. Although it may be possible to rationalize the preference of one conformation over another by for example increased steric repulsion between certain atom pairs, this is intimately related to the chosen functional form for the non-bonded energy, and the balance between this and the angle bend/torsional terms. The rotational banier in ethane, for example, may be reproduced solely by an HCCH torsional energy term, solely by an H-H van der Waals repulsion or solely by H-H electrostatic repulsion. Different force fields will have (slightly) different balances of these terms, and while one force field may contribute a conformational difference primarily to steric interactions, another may have the... 

It is common practice to describe torsional rotations around single bonds and those around multiple bonds with the same type of potential function but with very different force constants. The function must be able to describe multiple minima. Generally, a Fourier expansion of the torsional angle with only cosine terms is used (Eq. 2.23),... 

According to the classical stereochemical definition, the term conformation describes the different spatial arrangements of atoms in molecules that result solely from torsions (rotations) around single and/or partial double bonds. However in the domain of supramole-cular chemistry, molecular motions arise from the competitive noncovalent interactions between the components. 

There is a strong correlation between the parameters of different potential energy functions so that they should not be developed or refined in isolation. For example, the barrier to rotation about a bond can be modified by changing the explicit torsion angle term or by changing the nonbonded interactions. Thus, the effect of any change on a force field parameter needs to be tested extensively, i.e.,... 

The third term torsion provides the energy associated with torsional rotations. It is generally written as... 

A variety of different methods have been used to measure V, V, and (LS59, OM07) only a few of the more important will be discussed here. For asymmetric rotors, both the pure rotational spectrum and its torsion-rotation counterpart are electric dipole allowed and are affected in lowest order by the leading terms in the torsional Hamiltonian. Both types of spectra have been used extensively to determine (LS59). For symmetric tops with a single torsional degree of freedom, either the permanent electric dipole moment vanishes, as in CH CH, or the normal rotational spectrum is independent of 17 in lowest order, as in CH SiH. In... 

Hirsch [65] studied the elongation of hair under a steadily increasing load, together with a rotational movement. He attempted to explain this combination of stretching and torsion in terms of molecular structure. See the section on the torsional properties of hair in this chapter. 

The NR force field included both steric (nonbonded) and torsional terms, and conformational energies were computed without allowance for any geometry optimization (molecular relaxation or deformation) following torsional rotations about the backbone bonds. The PR force field as applied in the study of [-Si(CH3)2 ] was nearly identical to the NR force field except that the former optimized the geometry with respect to torsional rotations about the pendant Si-CH3 bonds for each backbone conformation considered. 

As indicated, the heat of formation (HFO) is taken as the sum of three terms. The first is the steric energy (E). The second is the bond enthalpy (BE), which is calculated from normal bond increments. The third is a partition function increment (PFC), which is itself the sum of three terms a conformational population increment (POP), a torsional contribution term (TOR), and a translation / rotation term (T / R). Since the molecular mechanics calculation has considered only one conformation, the POP and TOR terms are defined to be zero for this calculation (but see below). The T/R term is a molecular translational and rotational term that is always taken to be 2.4kcal/mol at room temperature, since even an anfi-butane molecule that is not rotating about the C2-C3 bond will still be undergoing translation and rotation of the entire molecule as a unit. Therefore, HFO is the total of the bond increment contributions (BE = —35.08) plus the steric energy (E = 2.17) plus the partition function increment (PFC = 2.4). The sum, -30.51 kcal/mol, is slightly lower than the literature value of -30.15 kcal/mol. ... 

A popular approach to modeling the interactions and conformations of Itu-ge molecules, particularly in solution, is to assume an energy U that is a sum of terms (1) coulombic interactions between charged atoms, (2) spring forces that stretch and bend bonds, (3) periodic potentials for torsional rotations around bonds, and (4) a Lennard-Jones potential for nonbonded interactions [3, 4, 5). For example. 

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