Geometries of the Halocarbene anions HCF− and CF2−: ab initio calculation and Franck–Condon analysis

Vibrationally high-resolved electronic spectra of MCl2 (M=C, Si, Ge, Sn, Pb) and photoelectron spectra of MCl2(.)

We systematically studied the vibrational-resolved electronic spectra of group IV dichlorides using the Franck-Condon approximation combined with the Duschinsky and Herzberg-Teller effects in harmonic and anharmonic frameworks (only the simulation of absorption spectra includes the anharmonicity). Calculated results showed that the band shapes of simulated spectra are in accordance with those of the corresponding experimental or theoretical ones. We found that the symmetric bend mode in progression of absorption is the most active one, whereas the main contributor in photoelectron spectra is the symmetric stretching mode. Moreover, the Duschinsky and anharmonic effects exert weak influence on the absorption spectra, except for PbCl2 molecule. The theoretical insights presented in this work are significant in understanding the photophysical properties of MCl2 (M=C, Si, Ge, Sn, Pb) and studying the Herzberg-Teller and the anharmonic effects on the absorption spectra of new dichlorides of this main group.

Fluorescence excitation and single vibronic level emission spectroscopy of the ÃA″1←X̃A′1 system of CHCl

We report new fluorescence excitation and single vibronic level emission spectra of the A (1)A(")<-->X (1)A(') system of CHCl. A total of 21 cold bands involving the pure bending levels 2(0) (n) with n=1-7 and combination bands 2(0) (n)3(0) (1)(n=4-7), 2(0) (n)3(0) (2)(n=4-6), 1(0) (1)2(0) (n)(n=5-7), 1(0) (1)2(0) (n)3(0) (1)(n=4-6), and 1(0) (1)2(0) (n)3(0) (2)(n=4) were observed in the 450-750 nm region; around half of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis typically yielded band origins and rotational constants for both isotopomers (CH(35)Cl,CH(37)Cl). The derived A (1)A(") vibrational intervals are combined with results of Chang and Sears to determine the excited state barrier to linearity [V(b)=1920(50) cm(-1)]. The A (1)A(") state C-H stretching frequency is determined here for the first time, in excellent agreement with ab initio predictions. Following our observation of new bands in this system, we obtained the single vibronic level (SVL) emission spectra which probe the vibrational structure of the X (1)A(') state up to approximately 9000 cm(-1) above the vibrationless level. The total number of X (1)A(') levels observed is around three times than that previously reported, and we observe five new a (3)A(") state levels, including all three fundamentals. The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with the previous experimental and recent high level ab initio studies, are reported. Our data confirm the previous assignment of the a (3)A(") origin, and our value for T(00)(a-X)=2172(2) cm(-1) is in excellent agreement with theory. By exploiting SVL spectra from excited state levels with K(a) (')=1, we determine the effective rotational constant (A-B) of the triplet origin, also in good agreement with theory. Our results shed new light on the vibrational structure of the X (1)A('), A (1)A("), and a (3)A(") states of CHCl, and, more generally, spin-orbit coupling in the monohalocarbenes.

Fluorescence excitation and emission spectroscopy of the ÃA″1←X̃A′1 system of CHBr

We report fluorescence excitation and emission spectra of CHBr in the 450-750 nm region. A total of 30 cold bands involving the pure bending levels 2(0)(n) with n=2-8 and combination bands 2(0)(n)3(0)(1)(n=1-8), 2(0) (n)3(0)(2)(n=1-6), 2(0)(n)3(0)(3)(n=1-2), 1(0)(1)2(0)(n)(n=5-7), 1(0)(1)2(0)(n)3(0)(1)(n=4-6), and 1(0)(1)2(0)(n)3(0)(2)(n=5) in the A (1)A(")<--X (1)A(') system were observed, in addition to a number of hot bands. The majority of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis yielded band origins and rotational constants for both bromine isotopomers (CH (79)Br,CH (81)Br). The derived A (1)A(") vibrational intervals are combined with results of [Yu et al. J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2(n), 2(n)3(1), and 2(n)3(2) progressions. The A (1)A(") state C-H stretching frequency is determined here for the first time, and the observed nu(3) dependence of the (79)Br-(81)Br isotope splitting in the A(1)A(") state is in good agreement with theoretical expectations. Our dispersed fluorescence spectra probe the vibrational structure of the X(1)A(') state up to approximately 9000 cm(-1) above the vibrationless level; the total number of levels observed is more than twice that previously reported. As first reported by [Chen et al. J. Mol. Spectrosc. 209, 254 (2001)], these spectra reveal numerous perturbations due to spin-orbit interaction with the low-lying a(3)A(") state. The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with previous experimental and theoretical studies, are reported. Our results lead to several revised assignments, including the X (1)A(') C-H stretching fundamental. Globally, the vibrational frequencies of X(1)A('), a(3)A("), and A(1)A(") are in excellent agreement with theoretical predictions.

Electronic spectroscopy of the Ã1A″← X̃1A′ system of CDF

To further investigate the Renner-Teller (RT) effect and barriers to linearity and dissociation in the simplest singlet carbene, we recorded fluorescence excitation spectra of bands involving the pure bending levels 2(n)(0) with n = 0-9 and the combination states 1(1)(0)2(n)(0) with n = 1-8 and 2(n)(0)3(1)(0) with n = 0-5 in the A(1)A''<-- X(1)A' system of CDF, in addition to some weak hot bands. The spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotationally analyzed to yield precise values for the band origins and rotational constants; fluorescence lifetimes were also measured to probe for lifetime lengthening effects due to the RT interaction. The derived A state parameters are compared with previous results for CHF and with predictions of ab initio electronic structure theory. The approach to linearity in the A state is evidenced in a sharp increase in the A rotational constant with bending excitation, and a minimum in the vibrational intervals near 2(9). A fit of the vibrational intervals for the pure bending levels yields an A state barrier to linearity in good agreement both with that previously derived for CHF and ab initio predictions. From the spectra and lifetime measurements, the onset of extensive RT perturbations is found to occur at a higher energy than in CHF, consistent with the smaller A constant.

The Singlet-Triplet Separation in CF2: State-of-the-Art Ab Initio Calculations and Franck-Condon Simulations Including Anharmonicity

Geometrical parameters, vibrational frequencies and relative electronic energies of the X2B1 state of CF2- and the X1A1 and ã3B1 states of CF2 have been calculated. Core-electron effects on the computed minimum-energy geometries and relative electronic energies have been investigated, and relativistic contributions to the computed relative electronic energies calculated. Potential energy functions of the X2B1 state of CF2- and the X1A1 and ã3B1 states of CF2 have been determined, and anharmonic vibrational wavefunctions of these states calculated variationally. Franck-Condon factors including anharmonicity and Duschinsky rotation have been computed and used to simulate the ã-X emission spectrum of CF2 determined by S. Koda [Chem. Phys. Lett. 1978, 55, 353] and the 364 nm laser photodetachment spectrum of CF2- obtained by R. L. Schwartz et al. [J. Phys. Chem. A 1999, 103, 8213]. Comparison between theory and experiment shows that the theoretical approach benchmarked in the present study is able to give highly reliable positions for the CF2(X1A1) + e <-- CF2-(X2B1) and CF2(ã3B1) + e <-- CF2-(X2B1) bands in the photoelectron spectrum of CF2- and a reliable singlet-triplet gap for CF2. It is therefore concluded that the same theoretical approach should give reliable simulated CCl2(X1A1) + e <-- CCl2-(X2B1) and CCl2(ã3B1) + e <-- CCl2-(X2B1) bands in the photodetachment spectrum of CCl2- and a reliable singlet-triplet gap for CCl2.

Dispersed fluorescence spectroscopy of jet-cooled HCF and DCF: Vibrational structure of the X̃A′1 state

We recorded dispersed fluorescence (DF) spectra following excitation of the pure bending levels 2(0) (n) and the combination states 1(0) (1)2(0) (n) and 2(0) (n)3(0) (1) in the A 1A"<--X 1A' system of HCF and DCF. Spectra were measured with a 0.3 m spectrograph equipped with a gated intensified charge coupled device (CCD) detector and obtained under jet-cooled conditions using a pulsed discharge source. The DF spectra reveal rich detail concerning the vibrational structure of the X state up to 10 000 cm(-1). For HCF, resonances among the nearly degenerate levels 1(1)2n, 2n+13(1), and 2n+2 produce a polyadlike structure in the spectrum, and the usual effective spectroscopic Hamiltonian (Dunham expansion) poorly reproduces the experimental term energies. In contrast, this Hamiltonian works well for the term energies of DCF. Density functional calculations of the ground state vibrational frequencies were performed; the results are in excellent agreement with the experimentally derived vibrational parameters. The search for perturbations involving the low-lying a 3A" state is described.