Related Experiment Video
Updated: May 19, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
A tale of two carenes: intrinsic optical activity and large-amplitude nuclear displacement
Priyanka Lahiri1, Kenneth B Wiberg, Patrick H Vaccaro
1Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
Abstract:
The specific rotation for two isomeric members of the terpene family, (S)-(+)-2-carene and (S)-(+)-3-carene, has been investigated under complementary solvated and isolated conditions, where the latter vapor-phase work has been performed at excitation wavelengths of 355 and 633 nm by means of ultrasensitive cavity ring-down polarimetry (CRDP). Linear-response computations of dispersive optical activity built upon analogous density-functional (B3LYP/aug-cc-pVTZ) and coupled-cluster (CCSD/aug-cc-pVDZ) levels of theory have been enlisted to unravel the structural and electronic origins of observed behavior. The six-membered portion of the bicyclic skeleton in the nominally rigid 3-carene system is predicted to be near-planar in nature, with calculated and measured rotatory powers for the isolated (gas-phase) species shown to be in excellent agreement. In contrast, the inherent flexibility of 2-carene gives rise to two quasidegenerate conformations that are interconnected by a large-amplitude ring-puckering motion and exhibit antagonistic chiroptical properties. Various approaches to simulate the intrinsic response evoked from a thermally equilibrated ensemble of gaseous (S)-(+)-2-carene molecules have been considered, including implicit averaging over independent conformers and explicit (albeit restricted) averaging over nuclear degrees of freedom. A polarizable continuum model for implicit solvation was found to describe solvent-dependent trends reasonably well in the case of (S)-(+)-2-carene, but failed to reproduce the specific-rotation patterns emerging from polarimetric studies of (S)-(+)-3-carene.
Related Concept Videos
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
¹H NMR: Interpreting Distorted and Overlapping Signals
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
¹H NMR: Long-Range Coupling
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
¹H NMR: Complex Splitting
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Inductive Effects on Chemical Shift: Overview

