An experimental and ab initio study of the electronic spectrum of the jet-cooled F2BO free radical.

We have studied the B̃ (2)A1-X̃ (2)B2 laser-induced fluorescence (LIF) spectrum of the jet-cooled F2BO radical for the first time. The transition consists of a strong 0(0)(0)band at 446.5 nm and eight weak sequence bands to shorter wavelengths. Single vibronic level emission spectra obtained by laser excitation of individual levels of the B̃ state exhibit two electronic transitions: a very weak, sparse B̃-X̃ band system in the 450-500 nm region and a stronger, more extensive set of B̃ (2)A1-à (2)B1 bands in the 580-650 nm region. We have also performed a series of high level ab initio calculations to predict the electronic energies, molecular structures, vibrational frequencies, and rotational and spin-rotation constants in the X̃ (2)B2, à (2)B1 and B̃ (2)A1 electronic states as an aid to the analysis of the experimental data. The theoretical results have been used as input for simulations of the rotationally resolved B̃ (2)A1-X̃ (2)B2 0(0)(0) LIF band and Franck-Condon profiles of the LIF and single vibronic level emission spectra. The agreement between the simulations obtained with purely ab initio parameters and the experimental spectra validates the geometries calculated for the ground and excited states and the conclusion that the radical has C2v symmetry in the X̃, Ã, and B̃ states. The spectra provide considerable new information about the vibrational energy levels of the X̃ and à states, but very little for the B̃ state, due to the very restrictive Franck-Condon factors in the LIF spectra.

An experimental and theoretical study of the electronic spectrum of HPS, a second row HNO analog.

The à (1)A" - X (1)A' electronic spectra of jet-cooled HPS and DPS have been observed for the first time, using a pulsed discharge jet source. Laser induced fluorescence spectra were obtained in the 850-650 nm region. Although the 0(0)(0) band was not observed, strong 3(0)(n) and 2(0)(1)3(0)(n) progressions and 3(1) hot bands could be assigned in the HPS LIF spectrum. Single vibronic level emission spectra were also recorded, resulting in the determination of all three HPS ground state vibrational frequencies. High level ab initio calculations were used to help confirm the vibronic assignments by calculation of transition energies, anharmonic vibrational frequencies, and anharmonic Franck-Condon factors. Ab initio potential energy surfaces gave an equilibrium structure for the X (1)A' state of r"(PH) = 1.4334 Å, r"(PS) = 1.9373 Å, θ" = 101.77° and for the à (1)A" state of r'(PH) = 1.4290 Å, r'(PS) = 2.0635 Å, and θ' = 91.74°. The rotational contours observed are consistent with these structures, confirming that the bond angle of HPS decreases on electronic excitation. Although the bond angles of HNO and HNS open in the excited state, in accord with the Walsh predictions for 12 valence electron HAB molecules, HPO, HAsO and now HPS all show the opposite behavior.

Toward an improved understanding of the AsH2 free radical: laser spectroscopy, ab initio calculations, and normal coordinate analysis.

Spectra of the Ã(2)A(1)-X̃(2)B(1) transition of the jet-cooled AsD(2) and AsHD isotopologues of the arsino radical have been studied by laser induced fluorescence and wavelength resolved emission techniques. A high-resolution spectrum of the AsD(2) 0(0)(0) band has been recorded, and an improved r(0) structure [r(0)(') = 1.487(4) Å, θ(0)(') = 123.0(2)°] for the à state has been determined from the rotational constants. To aid in the analysis of the vibrational levels, an ab initio potential energy surface of the X̃(2)B(1) state has been constructed and the rovibronic energy levels of states on that potential have been determined using a variational method. The vibrational levels observed in wavelength resolved emission spectra have been fitted to a local mode Hamiltonian with most anharmonic parameters fixed at ab initio values, and the resulting harmonic frequencies have been used to perform a normal coordinate analysis which yielded an improved set of quadratic force constants and an estimate of the equilibrium ground state structure.

Heavy atom nitroxyl radicals. V. An experimental and ab initio study of the previously unknown H2PS free radical.

The B̃(2)A(')-X̃(2)A(') transition of the prototypical thiophosphoryl radical, H(2)PS, was observed for the first time using laser-induced fluorescence and single vibronic level emission spectroscopy. H(2)PS and its deuterated isotopologues, D(2)PS and HDPS, were produced in a pulsed supersonic discharge jet from a precursor mixture of Cl(3)PS and H(2) or D(2) or an H(2)/D(2) mixture in high-pressure argon. High level ab initio calculations of the lowest three doublet electronic states helped in the definitive assignment of the B̃-X̃ transition, which involves electron promotion from the π to the π∗ orbital. Vibrational frequencies were determined for several modes of each isotopologue in the X̃ and B̃ states and found to be in accord with theoretical predictions. Although a line-by-line rotational analysis was not possible, the observed band contours are consistent with the geometries obtained from our ab initio calculations. Theory indicates that PS bond length increases upon electronic excitation, while the pyramidalization of the radical does not change significantly.

The electronic spectrum of the previously unknown HAsO transient molecule.

The Ã(1)A('')-X̃(1)A(') electronic spectrum of the jet-cooled transient molecule HAsO and its deuterated isotopologue has been observed for the first time by pulsed discharge jet laser spectroscopy. The techniques of laser-induced fluorescence and single vibronic level emission were employed to probe the electronic properties of the species. The bending and AsO stretching frequencies have been determined in both states. A rotational analysis of the 0(0)(0) bands of both HAsO and DAsO has been completed and the following effective (r(0)) structures were derived: r(")(HAs) = 1.576(3) Å, r(")(AsO) = 1.8342(5) Å, and θ(") = 101.5(4)°; and r(')(HAs) = 1.569(4) Å, r(')(AsO) = 1.7509(9) Å, and θ(') = 93.1(10)°. In the rotational analysis, lines induced by axis-tilting were observed, and calculated spectra with an axis tilting angle of 3.0(5)° reproduced the intensity of these lines. The change in geometry on electronic excitation is similar to that observed for the molecule HPO, with an increase in the X-O bond length and a decrease in the HXO angle, but contrary to the predictions of the Walsh diagram for generic HAB triatomic molecules. Our ab initio calculations show that the correlation between orbital energy and bond angle changes upon electronic excitation, resulting in the atypical angle change.

Electronic spectroscopy of the jet-cooled arsenic dicarbide (C2As) free radical.

The (2)Delta(r)-X (2)Pi(r) band system of the jet-cooled arsenic dicarbide (C(2)As) free radical has been recorded by laser-induced fluorescence (LIF) techniques in the 685-588 nm region. The radical was produced in a pulsed electric discharge jet using a precursor mixture of AsCl(3) vapor and methane in high pressure argon. A series of weak bands involving all three excited state vibrations was observed for both (12)C(2)As and (13)C(2)As. High-resolution spectra of the (2)Pi(12) component of the 0(0)(0) bands of both isotopomers were rotationally analyzed, leading to the conclusion that the upper state is (2)Delta with a small spin-orbit splitting (A = 2.78 cm(-1)). Ground and excited state molecular structures of r(0)(")(CC,ab initio) = 1.2933 A, r(0)(")(CAs) = 1.734(4) A and r(0)(')(CC,ab initio) = 1.2276 A, r(0)(')(CAs) = 1.830(3) A were derived from the B values and our density functional predictions of the C-C bond lengths. Single vibronic level emission spectra were recorded for many of the LIF bands and these were used to obtain the ground state vibrational frequencies and spin-orbit splittings. These data were satisfactorily fitted to a Renner-Teller model which gave (12)C(2)As parameters of epsilon = 0.695(8), omega(1) = 1704.8(20) cm(-1), omega(2) = 161.6(8) cm(-1), omega(3) = 663.6(12) cm(-1), and a spin-orbit constant A = 857.7(11) cm(-1).