Electronic spectroscopy of the ÃA″1↔X̃A′1 system of CDBr

Abstract

We report fluorescence excitation and single vibronic level emission spectra of jet-cooled CDBr in the 450-750 nm region. A total of 32 cold bands involving the pure bending levels 2(0)n with n=3-10 and combination bands 2(0)n3(0)1 (n=2-10), 2(0)n3(0)2 (n=2-9), 1(0)(1)2(0)n (n=7-10), and 1(0)(1)2(0)n3(0)(1) (n=6,8-9) in the A1A" <-- X1A' system of this carbene were observed; most of these are reported and/or rotationally analyzed here for the first time. Rotational analysis yielded band origins and effective (B) rotational constants for both bromine isotopomers (CD79Br and CD81Br). The derived A1A" vibrational intervals are combined with results of Yu et al. [J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n3(1), and 2n3(2) progressions. The A1A" state C-D stretching frequency (2350 cm(-1)) is determined for the first time, in excellent agreement with theory, as are the 79Br-81Br isotope splittings in the excited state. Our emission spectra probe the vibrational structure of the X1A' and a3A" states up to approximately 9000 cm(-1) above the vibrationless level of the X1A' state; the total number of levels observed is around twice that previously reported. Unlike CHBr, where even the lowest bending levels are perturbed by spin-orbit interaction with the triplet origin, the term energy of every level save one below 3000 cm(-1) in CDBr is reproduced by a Dunham expansion to within a standard deviation of 1 cm(-1), and a spin-orbit coupling matrix element of approximately 330 cm(-1) is derived from a deperturbation analysis of the triplet origin. The multireference configuration interaction (MRCI) calculations of Yu et al. [J. Chem. Phys. 115, 5433 (2001)] well reproduce triplet perturbations in the pure bending manifold, and globally, the vibrational frequencies of X1A', a3A", and A1A" are in excellent agreement with theoretical predictions.

Single vibronic level emission spectroscopy of the system of dibromocarbene

“…The apparatus, pulsed discharge nozzle, and data acquisition procedures have been described in detail in recent studies [20][21][22]. In previous discharge studies [15,48], the carbene CBr 2 was produced using a pulsed electrical discharge through a mixture of CBr 4 and CHBr 3 seeded in high purity He.…”

“…First, as the observation of spin-orbit perturbations does not necessarily coincide with the position of the triplet origin, the discrepancy with theory is uncomfortably large (i.e., >6 kcal mol À1 or 2200 cm À1 ). To explain this, the authors invoke a large spin-orbit matrix element; however, in the related monohalocarbene CHBr the derived matrix elements are on the order of $330 cm À1 [13,22]. Second, the observedX 1 A 1 state vibrational term energies up to $5200 cm À1 were fit to a Dunham expansion with a standard deviation of $2.3 cm À1 [48], i.e., within the experimental uncertainty, which suggests a lack of spin-orbit perturbations in this region.…”

“…Thus, the magnitude of the singlet-triplet gap (DE ST ) is an important quantity needed to predict the reactivity of carbenes in environments where both states can be populated. The mono-and dihalocarbenes have served as important test cases for comparing experimental and theoretically derived DE ST 's, and recent progress has been made in determining DE ST for a number of carbenes [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. For example, in CHCl the experimental value for T 00 ðã ÀX Þ of 2172(2) cm À1 , derived via single vibronic level emission spectroscopy [17,21], is within 2 cm À1 of a theoretical estimate at the CCSD(T)/aug-cc-pVQZ level which included relativistic, core correlation and diagonal Born-Oppenheimer corrections [19].…”

“…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.…”

Electronic spectroscopy of the system of CDCl

“…Building upon the seminal work of Merer and Travis in the 1960's [9,10], and later work from the groups of Hirota [11][12][13][14][15][16][17][18], Sears [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], Kable [34][35][36], and Chang [37][38][39][40][41][42][43], our group has recently undertaken a systematic study of the monohalocarbenes CHF, CHCl, CHBr and their deuterated isotopomers, using a variety of methods including fluorescence excitation spectroscopy, lifetime measurements, polarization and Zeeman quantum beat spectroscopy, and single vibronic level emission spectroscopy [8,[44][45][46][47][48][49][50][51][52][53][54]. Most recently, we reported comprehensive surveys ...…”