vicinal

Vicinal is an adjective from the Latin vicinus, meaning neighboring or nearby. In science, it is used to describe atoms, groups, or features that are on adjacent positions within a molecule or a crystal lattice. The term emphasizes proximity rather than identity, distinguishing adjacent arrangements from remote or geminal (on the same atom) arrangements.

In organic chemistry, vicinal refers to substituents on neighboring carbon atoms. Common examples include vicinal dihalides (two halogen atoms on adjacent carbons) and vicinal diols (two hydroxyl groups on adjacent carbons). Vicinal amino alcohols and other vicinal functional group patterns are described likewise. Reactions often exploit this adjacency, such as dehalogenation of vicinal dihalides to form alkynes or dehydration of vicinal diols to give epoxides or carbonyl derivatives.

In NMR spectroscopy, vicinal coupling describes scalar coupling between protons on adjacent carbons (3JHH). The coupling constant depends on the dihedral angle between the vicinal protons and is commonly described by the Karplus relationship.

In crystallography and surface science, vicinal can describe a surface that is slightly tilted from a low-index plane, producing steps and terraces. Vicinal surfaces are studied to understand surface reactions, diffusion, and growth processes.

See also adjacent and neighboring. Etymology: from Latin vicinus, neighboring.