J. Pfab

Photodissociation of jet-cooled methyl and t-butyl nitrite near 380 nm

Abstract A one-colour photolysis-ionisation study of the photodissociation of methyl and t -butyl nitrite near 380 nm in a supersonic jet has been performed. The NO photoproduct has been characterised by state-resolved probing by (2 + 1) REMPI via the C→X δ bands of NO. The photodissociation of methyl nitrite yields predominantly vibrationally cold NO, while t -butyl nitrite photodissociation shows a propensity for formation of NO in the first excited vibrational level. The NO fragments are rotationally highly excited in both cases, and the rotational population distributions can be fitted by Gaussian curves in agreement with recent dynamical calculations. The different vibrational energy disposal is attributed to the different excitation of a rapidly predissociating complex. Previous results that were at variance with knowledge of the dynamics of the predissociation of alkyl nitrites may have been affected by experimental problems.

The 355 nm laser photolysis of jet-cooled methyl nitrite (CH3ONO). Internal energy distributions of the NO fragment

Abstract The photodissociation of methyl nitrite (CH 3 ONO) at 355 nm in a supersonic free-jet expansion has been studied by state-resolved two-proton laser-induced fluorescence of the NO fragment. The vibrational population distribution of the nascent NO is much colder than reported for the photodissociation of 300 K vapour and can be understood on the basis of the direct electronic dissociation of a very short-lived excited complex. The conformational isomerism of CH 3 ONO and the assignment of the structure of its electronic absorption spectrum prove to be important for the interpretation of the observed dissociation dynamics in the jet and at 300 K.

Structured absorption spectrum and vibrational state-selectivity in the photodissociation of methyl nitrite in the near-UV

The gas-phase electronic absorption spectrum of methyl nitrite (CH 3 ONO) in the near-UV has been re-examined. Both temperature dependence and comparison of the vibrational structure with nitrous acid (HONO) and tertiary-butyl nitrite ( t -BuONO) show that syn-anti isomerism leads to two shifted, overlapped band systems. The origin of the prominent S 1 N=O stretch progression of the syn-rotamer has been re-assigned to the medium strong feature at 364.3 nm. The vibrational energy disposal into NO is reported for photolysis of jet-cooled CH 3 ONO at 387,380 and 355 nm. Photolysis at all three wavelengths favours vibrational disposal into NO (v n =0). All currently available experimental results indicate a preference for an adiabatic or direct mechanism as the predominant dissociation route.

Photodissociation of methyl and t-butyl thionitrite near 450 nm

Nascent NO from the primary dissociation step in the 450 to 470 nm photodissociation of the title compounds has been detected and characterised by two-photon laser-induced fluorescence. The NO photofragment is vibrationally cold but rotationally hot with a strongly inverted rotational population distribution, and the two 2Π spin-orbit components are not in equilibrium. Dissociation proceeds from a repulsive S2(n, π∗) state and gives rise to a non-statistical rotational population distribution.

Laser photofragment spectroscopy of jet-cooled CClF2NO

Abstract Photofragment yield spectra of jet-cooled CClF2NO in the visible region have been recorded using pulsed laser dissociation in conjunction with two-photon LIF probing of the nascent NO fragment. Photodissociation of the cold parent at 646.6 nm yields NO with a statistical rotational state distribution but a colder than statistical spin-orbit distribution. A dissociation energy of 13500 ± 350 cm−1 has been evaluated for the C-N bond of the parent nitroso compound.

Laser-induced fluorescence spectroscopy of jet-cooled 2-methylanthracene S1 (π,π*). Evidence for methyl conformation change upon electronic excitation

Abstract The laser-induced fluorescence excitation spectrum of jet-cooled 2-methylanthracene (2-MA) near the S 1 —S o origin reveals a prominent torsional progression due to the internal rotation of the methyl group or its torsional vibration against the aromatic ring frame. From the torsional level spacings a low, threefold hindered internal rotation barrier of 70 cm −1 has been determined for S 1 (π,π * ). A Frank—Condon intensity analysis of the progression in the excited state torsion suggests that the most stable rotamer of 2-MA changes from an eclipsed—staggered (e—s) conformation in the electronic ground state to a (s—e) conformation in the first singlet excited state. This 60° change in the dihedral conformation can be rationalized by the change of the π-bond order of the adjacent ring bonds upon electronic excitation, such that one of the CH methyl bonds prefer to eclipse the adjacent CC bond with the stronger π-bond character.

Fourier-transform electronic spectroscopy in the near-infrared: Torsional barrier of the Ā1A″ state of nitrosobenzene

Abstract The Fourier-transform absorption spectrum of nitrosobenzene vapour in the near-infrared has been recorded, and the vibrational bands near the 869 nm electronic origin of the Ā 1 A″ ← X 1 A′ (n,π * ) transition have been re-analysed. The prominent torsional structure observed is shown to represent sequence transitions in the torsional mode. Electronic excitation leads to an increase of the V 2 barrier hindering internal rotation from roughly 1300 cm −1 in the ground to 2500 cm −1 in the (n,π * ) excited state.

The visible spectrum of jet-cooled CF3NO

Abstract The electronic origin of the A A″ ← X A′ transition of trifluoronitrosomethane (CF3NO) has been reassigned to the very weak feature near 717.9 nm in the fluorescence excitation spectrum of the jet-cooled molecule. The prominent torsional progression has been reinpreted and the height of the threefold torsional barier in the A( n,π* ) state has been revised to 601.5 ± 10 cm−1.

The state-selected predissociation dynamics of CClF2NO Ã(n, π)

The photodissociation of gaseous CClF2NO at 300 K and <20 K in a supersonic jet has been studied in the wavelength range 568 to 705 nm of the S1(n,π*)←S0 electronic transition. Energy disposal into, rates of formation and product-state distributions of the nascent NO(X2Πi) photofragment have been measured in detail, and have been compared with statistical (Prior) calculations. The rotational population distributions of the NO fragment are statistical up to the highest excess energies investigated, while the spin-orbit branching ratios are very non-statistical and non-thermal, favouring the lower component. The lack of evidence for state-specific effects in the energy disposal and appearance times of the NO indicate that excess energy randomises prior to dissociation. Intersystem crossing to T1 dominates the predissociation of all but the lowest S1 levels. Levels close to the O,O origin predissociate either by internal conversion to the ground state or by vibrational predissociation on an S1 potential energy surface that is weakly bound in the dissociation coordinate. A bond dissociation energy of 13 700 ± 350 cm−1 (164 kJ mol−1) has been evaluated for the C—N bond.