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An anti glycosidic conformation is a base–sugar orientation in the anti region of the glycosidic torsion angle $\chi$. Under the IUPAC-IUB atom convention, this region is centered at $180^{\circ }$ and extends90degrees on either side. Equivalently, for an angle reported in $[0^{\circ },360^{\circ })$, the broad anti region lies between90 and270degrees.^[ref]

For a purine nucleoside, $\chi$ uses O4′–C1′–N9–C4; for a pyrimidine, it uses O4′–C1′–N1–C2. The ordered atoms specify the dihedral angle. An angle reported using different outer atoms cannot be assigned to this region by its numerical value alone.^[ref]

For an original example, $\chi =-160^{\circ }$ and $\chi =200^{\circ }$ describe the same orientation modulo one full turn, and both are anti. Moving to $\chi =60^{\circ }$ changes the orientation into the syn glycosidic conformation region while retaining the base–sugar bond connectivity. Conformational change and a change in covalent constitution are different descriptions.

The word anti here does not describe how two bases pair with one another. RNA base-pair geometry can separately classify the relative orientations of two glycosidic bonds. Nor does a single anti torsion determine the entire nucleotide or RNA structure: sugar geometry and the other backbone angles remain to be specified.

An antiperiplanar conformation places two specified bonds in approximately the same plane with a dihedral angle of $180^{\circ }$. The designation concerns those particular bonds, not the planarity of the entire molecule.

In many E2 elimination reactions, the carbon-hydrogen bond being broken and the carbon-leaving-group bond align antiperiplanar. This arrangement supports the orbital overlap required to form the new pi bond. In a cyclohexane chair, the condition often requires a trans-diaxial pair, linking conformational geometry directly to which elimination pathway is accessible.

The dihedral angle measures the angle between two intersecting planes. Molecular torsion angles use an oriented four-atom convention to specify a related signed angle.

In a nucleoside or nucleotide, the glycosidic torsion angle $\chi$ describes rotation of the nucleobase relative to the sugar about their N-glycosidic bond. It is a signed dihedral angle defined by four ordered atoms:The middle two atoms specify the glycosidic bond; the outer atoms define the two reference planes. Thus the atom sequence, not the symbol $\chi$ alone, fixes the angle convention.^[ref]

The recommended broad regions are syn glycosidic conformation, centered at $0^{\circ }$ with a90degree range on either side, and anti glycosidic conformation, centered at $180^{\circ }$ with the corresponding range. For example, $\chi =60^{\circ }$ lies in the syn region and $\chi =-160^{\circ }$ in the anti region. These describe base–sugar orientation; they do not by themselves specify sugar-ring shape or the complete RNA conformation.^[ref]

An original angular comparison illustrates periodicity. Measurements of $179^{\circ }$ and $-179^{\circ }$ differ by only2degrees around the circle, not358degrees. A signed shortest difference can be calculated asusing radians inside the trigonometric functions. Interpreting the two near-boundary values as distant conformations would be an artifact of the angle's reporting interval.

The convention is important when comparing structural models or older literature. Alternative choices using C8 for a purine or C6 for a pyrimidine can shift the numerical angle by approximately180degrees. Numerical torsions should therefore be compared only after checking their defining atoms.^[ref]

The relevant quantity is the H–C–C–H dihedral angle, through the Karplus relation, rather than the labels cis and trans alone. Couplings tend to be larger near eclipsed or antiperiplanar arrangements and smaller nearer a right angle. The observed value averages the accessible conformations.

In A, the saturated five-membered ring is puckered. A preferred envelope-like conformation can place the trans hydrogens closer to an antiperiplanar arrangement, giving a slightly larger coupling than the cis pair. In B, the double bond restricts ring geometry toward a flatter arrangement: the cis hydrogens are more nearly eclipsed, whereas the trans pair has an oblique dihedral angle. The cis coupling is then larger. Approximate Newman projections illustrate these trends; exact angles cannot be recovered from four coupling constants without a calibrated relation and conformational populations.

Torsional strain is an increase in the energy of a molecule associated with an unfavourable torsion angle, relative to a stated reference conformation. Rotation about a bond can change the relative orientations of substituents without changing which atoms are bonded. The IUPAC definition of strain explicitly includes unfavourable dihedral angles alongside bond-length and bond-angle distortions.^[ref]

Consider ethane viewed along its central carbon–carbon bond. An eclipsed conformation aligns projected carbon–hydrogen bonds, whereas a staggered conformation places them between one another. An idealized threefold potential energy model iswith $\varphi =0$ chosen eclipsed. It gives $V(0)=V_3$ and $V(\pi /3)=0$: a finite rotational barrier separates equivalent minima. The model is a teaching parametrization; $V_3$ must be obtained independently rather than read from the drawing.

The 1937 paper by J. D. Kemp and K. S. Pitzer explicitly connected the entropy of ethane and the third law of thermodynamics with hindered methyl-group rotation.^[ref] This historical connection shows why internal rotation is a measurable molecular degree of freedom, not simply unrestricted motion around a single bond. In a ring, geometric closure can restrict accessible torsion angles; ring strain can therefore include torsional strain as well as other contributions. Assigning a separate torsional energy is model-dependent, rather than a complete account of every electronic interaction.

Vicinal coupling is spin-spin coupling between nuclei separated by three bonds, often hydrogens on neighboring carbons, and is denoted ${}^{3}J$. Its dependence on the intervening dihedral angle is described approximately by a Karplus relation.

This makes a measured coupling useful for estimating molecular conformations. The relationship also depends on substituents and bonding, and different angles can give similar coupling magnitudes. A single coupling therefore rarely determines a conformation uniquely; several measurements and stereochemical constraints are normally combined.