P. Russegger

UV-, VIS- and IR-light-induced isomerization of HSNO in a low-temperature matrix

Abstract An infrared spectroscopic investigation has been performed on the trans and cis isomers of thionitrous acid (HSNO) and their D- and 15 NO-isotopic modifications in argon matrices at 12 K. The substances were prepared photolytically from thionylimide (HNSO) isotopes in the matrix. With UV (250 nm), VIS (585 nm), and IR irradiation the cis → trans or the trans → cis isomerization of HSNO was induced, allowing an unequivocal distinction between the closely resembling IR spectra of the trans and cis isomers. Complete sets of fundamental frequencies of both rotamers were obtained and assigned by normal coordinate analysis using the transferable valence force field (TVFF) approach. Parallel to this analysis ab initio calculations on the SCF- and CI-levels were performed to predict energy, geometry, and barrier of internal rotation for the two HSNO rotamers.

Hydrogen-bonded complex between HNO and formaldehyde. Photolysis of methyl nitrite in an argon matrix

Abstract Nitroxyl and formaldehyde have been prepared by photolyzing methyl nitrite and six of its isotopomers in an argon matrix at low temperature. Under thes

Quantum beats in the fluorescence decay of propynal

Abstract The oscillatory fluorescence decay for a number of rovibronic levels in the S1(nπ*) state of propynal (HCCCHO) was investigated in a supersonic jet apparatus. The quantum beats of propynal, which arise from coherent excitation of mixed S1 and T1 states, were analysed (Fourier transformed) and the intramolecular coupling (υSO = 0.3–7 MHz) determined. Calculations were performed to examine the position, the interstate coupling, and the density of vibronic T1 states in close vicinity of a given S1 state. According to these calculations vibronic spin-orbit coupling is favored over direct coupling in propynal.

Deuterium isotope effect in the T1 → S0 radiationless transition of propynal in the gas phase

Abstract The phosphorescence lifetimes of propynal- h 1 and propynal- d 1 have been measured at room temperature in the 40 mTorr-1 Torr pressure range The reciprocal of the zero-pressure lifetime ( k 0 ) is (3.10 ± 0.05) × 10 3 and (1.70 ± 0.04) × 10 3 s −1 for propynal- h 1 and propynal- d 1 For both compounds the rate constant for self-quenching between triplet and ground-state molecules is k SQ = (1 2±007) × 10 3 Torr −1 s −1 The deuterium isotope effect is attributed to the T 1 → S 0 radiationlcss decay, for which k H ISC / k D ISC = 2 4

A calculation of the temperature dependent radiationless decay of the propynal triplet state T1

Abstract The strong increase in the propynal triplet decay rate upon IR laser heating, having been attributed to temperature sensitive radiationless processes, is investigated by theoretical calculations. For the temperature range ≈100–800 K the relative rate constants k ISC (T 1 ⇝S 0 ) are calculated and, introducing the concept of the “mean energy” accepted or promoted by a mode as a function of the excess energy, the efficiency of the individual modes towards the radiationless processes is analysed.

The deactivation of the S1(nπ*) state of propynal and its deuterated compounds in the dilute gas phase

Abstract The fluorescence life times of propynal, propynal- d 1 , and propynal- d 2 have been measured at room temperature in the pressure range 0.2–10 mTorr. In conjunction with these results and those of the pressure-dependent fluorescence to phosphorescence intensity ratio, the collision-free deactivation of the Si(nπ * ) state of propynal is discussed.

Photoisomerization about carbon—nitrogen double bonds. I. Kinetic and potential energy for ground and excited states of methylenimine

Abstract The cis-trans photoisomerization of molecules containing the CN double bond is investigated on the basis of potential surfaces calculated within MO-SCF scheme. It is shown that for the isomerization of methylenimine two coordinates have to be considered to describe properly the rearrangement process in the various excited states. A method is presented for the construction of the vibrational adiabatic Born—Oppenheimer states, which divides the coordinates according to their importance for dynamic processes. The hamiltonian derived, should be appropriate for the construction of the rovibronic states.

The triplet state decay (T1(nπ*) → S0) of benzaldehydes in the dilute gas phase

Abstract The triplet T 1 (nπ * ) decay of benzaldehyde (B) and its isotopomers and were investigated in the dilute vapour phase (≤0.5 Torr) at room temperature. Following excitation the quantum yields of the phosphorescence and photodecomposition, and the rate constants of the phosphorescence and the radiationless T 1 → S o process were determined. Based on these results and in conjunction with theoretical calculations of T 1 → S o rates and previous data obtained on propynal, the decay mechanism of benzaldehyde was analyzed. It is shown that the important accepting modes of the non-radiative T 1 → S a decay are the wagging and the CO stretching modes. In spite of the close vicinity of the T 2 (ππ * ) and the T 1 (nπ * ) states, the non-adiabatic coupling (communication between ring and carbonyl vibrations is not sufficient to influence the relaxed T 1 (nπ * ) decay significantly.

Vibronic state and vibronic energy dependence of the intersystem crossing process T1⇝So in a polyatomic molecule. Propynal

Abstract Theoretical calculations on popynal (HCCCHO) were performed to examine the vibronic state and the vibronic energy dependence of the T 1 &-rarrw; S o process in an isolated molecule.

Vibrational wavefunctionals for the thermal and photochemical syn—anti isomerization of imines

Abstract Employing the partitioning scheme for the nuclear coordinates, we recently have discussed the vibration—isomerizational hamiltonian appropriate for the syn—anti rearrangement of imines. Now we present the calculations of the vibrational wavefunctions and energies which were needed for the investigation of the horizontal and vertical isomerization processes. The energy splitting of the adiabatic zero-order states shows clearly the importance of both inversion and rotation, for the rearrangement process in the ground and first triplet state of methylenimine. Using these zero-order states we then have investigated the question as to what extent nonadiabatic coupling influences the rate of the horizontal and vertical reactions. Further we have discussed two mechanisms for the nonadiabatic effects concerning the tunneling frequency, which depend on whether the nonrelevant coordinated are “symmetric” or “antisymmetric” with respect ot the symmetry operator describing the isomerizational states. The use of the vibrational adiabatic states for the calculation of the photoisomerization rates has also been suggested.