Probing spin–orbit mixing and the singlet–triplet gap in dichloromethylene via K_a-sorted emission spectra

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.

Single vibronic level emission spectroscopy of the system of dibromocarbene

“…4 shows expanded views of the 4700-6600 cm À1 region (i.e., centered on the theoretical position of the triplet origin) for spectra obtained by pumping K 0 a = 0 and 2 for the C 79 Br 81 Br isotopomer in the 2 15 0 band. From the DK a = ±1 selection rule, levels in the K 0 a = 0 spectra are not split, while those in the K 0 a = 2 spectra are split by 8ðA 00 À B 00 Þ [49], the calculated rotational constants for the vibrationless levels of theX 1 A 1 andã 3 B 1 states [40] suggest splittings of $10 and 19 cm À1 , respectively, for pure (unmixed) singlet and triplet levels. However, due to the spectral congestion, we were unable to determine the ðA 00 À B 00 Þ constants in this region; the 4800 cm À1 polyad (p = 25) illustrates the problem.…”

“…To probe for triplet levels in the high energy region of the spectrum, we used K a -sorted emission spectroscopy [49], a method previously used by us to probe for the presence of triplet levels in CHCl and C 35 Cl 2 . This method takes advantage of the large ($20°) increase in equilibrium bond angle for the triplet state, which leads to a larger A rotational constant for triplet levels.…”

“…Due to extensive interference from HCBr and Br 2 , emission spectra were only obtained for two bands, 2 12 0 and 2 13 0 (labels follow our revised assignment of the electronic origin) [50]. These authors reported that the pure bending progression (0, n, 0) did not appear in their emission spectrum, which is surprising given that: (1) this progression appears in the emission spectrum of every other triatomic carbene [16,20,21,49], (2) it was reported in the initial matrix emission studies of CBr 2 [51,52], and (3) a long bending progression is observed in the excitation spectrum of CBr 2 [53][54][55]. In addition, they reported congested, unassigned structure beginning $3650 cm À1 above thẽ X 1 A 1 origin which was attributed to levels of theã 3 B 1 state, leading to an estimated DE ST of 10.4 kcal mol À1 .…”

“…Miller, Chang and co-workers obtained emission spectra of CCl 2 which probed the vibrational structure of theX 1 A 1 state [16], and reported unassigned structure beginning $5000 cm À1 ($14 kcal mol À1 ) above theX 1 A 1 origin which they attributed to levels of theã 3 B 1 state. In a recent study [49], we tested for the presence ofã 3 B 1 levels in the high energy region of the CCl 2 emission spectrum using K 0 a -sorted emission spectra; these discriminate between singlet and triplet levels via the ðA 00 À B 00 Þ rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle of the triplet state. In the region between $3500 and 9000 cm À1 above the vibrationless level of theX 1 A 1 state, we found only a very modest increase in ðA 00 À B 00 Þ, and $86% of the lines observed between 5000 and 9000 cm À1 could be assigned toX 1 A 1 levels within 3 standard deviations of our Dunham expansion fit, which included more than 140 levels in total.…”

“…A successful approach to determining DE ST for the monohalocarbenes CHCl and CHBr is single vibronic level emission spectroscopy [10][11][12]14,15,17,18,[20][21][22], and this technique has recently been extended to the dihalocarbenes [15,16,48,49]. Miller, Chang and co-workers obtained emission spectra of CCl 2 which probed the vibrational structure of theX 1 A 1 state [16], and reported unassigned structure beginning $5000 cm À1 ($14 kcal mol À1 ) above theX 1 A 1 origin which they attributed to levels of theã 3 B 1 state.…”