High resolution probe of spin-orbit coupling and the singlet-triplet gap in chlorocarbene

Abstract

Among the most important of chemical intermediates are the carbenes, characterized by a divalent carbon that generates low-lying biradical (triplet) and spin-paired (singlet) configurations with unique chemical reactivities. The "holy grail" of carbene chemistry has been determining the singlet-triplet gap and intersystem crossing rates. We report here the first high resolution spectra of singlet-triplet transitions in a prototypical singlet carbene, CHCl, which probe in detail the triplet state structure and spin-orbit coupling with the ground singlet state. Our spectra reveal a pronounced vibrational state dependence of the triplet state spin-spin splitting parameter, which we show is a sensitive probe of spin-orbit coupling with nearby singlet states. The parameters derived from our spectra, including a precise determination of the singlet-triplet energy gap, are in excellent agreement with recent ab initio calculations.

Spectroscopy and dynamics of the predissociated, quasi-linear S2 state of chlorocarbene

“…Recently, our group reported spectroscopic studies of chlorocarbene (CHCl) that represented the first rotationally resolved spectrum of singlettriplet transitions in a carbene. 3,4 In addition to providing a spectroscopically precise value for the singlet-triplet gap that was within several cm −1 (∼0.01 kcal/mol) of recent high level ab initio predictions, our experiments showed a pronounced vibrational state dependence of the triplet spin-spin coupling constant, reflecting interactions with nearby singlet levels. 4 This provided a sensitive probe of the spin-orbit coupling, and our derived value for the electronic spin-orbit coupling matrix element was in excellent agreement with theoretical predictions.…”

“…Approximately 300-400 ns later, a pulse from a second Nd:YAG pumped dye laser (Spectra-Physics INDI/Syrah CobraStretch), counter-propagating with the first, either excited specific transitions in the S 1 → S 2 system or stimulated emission from S 1 → S 0 . The latter process has been extensively studied in previous work from our lab, 3,4 and the S 0 level structure is well known. The decrease in fluorescence caused by the second (probe) laser was measured using a dual boxcar system (SRS 250), where a portion of the fluorescence decay was integrated before and after the second laser pulse.…”

“…For calculations, the ground atomic states for hydrogen H( 2 S g ) and chlorine Cl( 2 P u ) were combined with the three lowest atomic states of carbon C( 3 P g ), C( 1 D g ), and C( 1 S g ) to form 27 singlet (14 1 A + 13 1 A ) and 36 triplet (18 3 A + 18 3 A ) molecular states in C S symmetry. Calculations relating to any particular molecular state of interest were performed using a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately).…”

“…3,4,[8][9][10]20 Briefly, CHCl (CDCl) was produced using a pulsed discharge nozzle through a ∼1%-2% mixture of CH 2 Cl 2 (CD 2 Cl 2 ) in high purity He. Specific rotational transitions in various bands of the S 0 → S 1 system 21,22 were excited by a tunable Nd:YAG pumped dye laser (Continuum P7010/Lambda Physik Scanmate 2E).…”

Towards a global model of spin-orbit coupling in the halocarbenes

“…Turning now to CH 35 Cl, a (model 1) fit to 53 levels derived from SVL emission 23,26 and SEP 33,72 measurements yielded a MAD of 2.1 cm −1 , which is a more than twofold improvement over a simple Dunham expansion fit to the observed singlet levels. A fit using model 2 does a similar job and returns a MAD of 2.0 cm −1 .…”