The cold boroncarbide free radical (BC X 4Σ−) has been produced in a pulsed discharge free jet expansion using a precursor mixture of trimethylborane in high pressure argon. High resolution laser induced fluorescencespectra have been obtained for the B 4Σ−–X 4Σ− and E 4Π–X 4Σ− band systems of both 11BC and 10BC. An optical-optical double resonance (OODR) scheme was implemented to study the finer details of both band systems. This involved pumping a single rotational level of the B state with one laser and then recording the various allowed transitions from the intermediate B state to the final E state with a second laser by monitoring the subsequent E–X ultraviolet fluorescence. In this fashion, we were able to prove unambiguously that, contrary to previous studies, the spin-spin constant λ is negative in the ground state and positive in the B 4Σ−excited state. It has been shown that λ″ < 0 is in fact expected based on a semiempirical second order perturbation theory calculation of the magnitude of the spin-spin constant. The OODR spectra have also been used to validate our assignments of the complex and badly overlapped E 4Π–X 4Σ− 0-0 and 1-0 bands of 11BC. The E–X 0-0 band of 10BC was found to be severely perturbed. The ground state main electron configuration is …3σ24σ25σ11π22π0 and the derived bond lengths show that there is a 0.03 Å contraction in the B state, due to the promotion of an electron from the 4σ antibonding orbital to the 5σ bonding orbital. In contrast, the bond length elongates by 0.15 Å in the E state, a result of promoting an electron from the 5σ bonding orbital to the 2π antibonding orbitals.
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This material is based upon work supported by the National Science Foundation under Grant No. CHE-1106338.
Sunahori, Fumie X.; Nagarajan, Ramya; and Clouthier, Dennis J., "Optical-Optical Double Resonance, Laser Induced Fluorescence, and Revision of the Signs of the Spin-Spin Constants of the Boron Carbide (BC) Free Radical" (2015). Chemistry Faculty Publications. 72.