Related Experiment Video
Updated: Jul 19, 2026

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization
Published on: August 6, 2018
Probing spin-orbit mixing and the singlet-triplet gap in dichloromethylene via Ka-sorted emission spectra
Calvin Mukarakate1, Yulia Mishchenko, Danielle Brusse
1Department of Chemistry, Marquette University, Milwaukee, WI 53201-1881, USA.
Abstract:
The magnitude of the singlet-triplet gap in dichloromethylene (CCl(2)) has been a point of controversy in the recent literature. In this study, we report single vibronic level emission spectra of the A(1)B(1)-->X[combining tilde](1)A(1) system of the carbene C(35)Cl(2), which probes the vibrational structure of the X[combining tilde](1)A(1) state up to approximately 10,000 cm(-1) above the vibrationless level. By the careful selection of bands where complete isotope and K(a)' selectivity in excitation was possible, we measured K(a)'-sorted emission spectra in order to test the previously established hypothesis [M.-L. Liu, C.-L. Lee, A. Bezant, G. Tarczay, R. J. Clark, T. A. Miller and B.-C. Chang, Phys. Chem. Chem. Phys., 2003, 5, 1352] that unassigned lines lying above approximately 5,000 cm(-1) belong to levels of the ã(3)B(1) state. The K(a)'-sorting method discriminates between singlet and triplet levels via the (A''-B[combining macron]'') rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle. In the region between 3,500 and 9,000 cm(-1) above the vibrationless level of the X[combining tilde](1)A(1) state, we find only a very modest increase in (A''-B[combining macron]''), and approximately 86% of the lines observed between 5,000 and 9,000 cm(-1) can be assigned to X[combining tilde](1)A(1) levels within 3 standard deviations of our Dunham expansion fit, which included more than 140 levels in total. A nearly complete set of Dunham parameters was determined for the C(35)Cl(2) isotopomer, and the X[combining tilde](1)A(1) state term energies up to 4,000 cm(-1) are in excellent agreement with recent variational calculations of Tarczay, et al. [G. Tarczay, T. A. Miller, G, Czakó and A. G. Császár, Phys. Chem. Chem. Phys., 2005, 7, 2881]. Finally, the implication of our results for the singlet-triplet gap in dichloromethylene is discussed.
Related Concept Videos
Spin–Spin Coupling Constant: Overview
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
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...
UV–Vis Spectroscopy: Molecular Electronic Transitions
Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals
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.
Molecular Spectroscopy: Absorption and Emission

