ALBERT

Notes: Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n–π* transition of jet-cooled 4- and 5-methylpyrimidine (4-mp and 5-mp) have been recorded and analyzed. In 5-mp, methyl substitution preserves many of the spectroscopic signatures of the unsubstituted pyrimidine molecule. Dispersed fluorescence spectra are used to assign most of the major features in the first 1000 cm−1 of the excitation spectrum. High resolution scans at the origin reveal a 0.20 cm−1 splitting of the origin arising from the 0a'1–0a1 and 1e‘–1e‘ internal rotor transitions. From this small splitting we deduce that the nearly free internal rotation of the methyl group in the ground state is carried over to the S1 state as well. In 4-methylpyrimidine, the reduction in symmetry accompanying methyl substitution (G12 to G6 ) results in allowed transitions to all in-plane fundamentals. The methyl group is seen to participate in the electronic transition to a greater degree in 4-mp than in 5-mp.We observe clear activity in both the C–CH3 stretch and C–CH3 in-plane bend in the dispersed fluorescence from the origin of 4-mp. 4-mp also differs remarkably from 5-mp in the magnitude of the barrier to methyl internal rotation in S0 and S1. By fitting the positions and intensities of internal rotor structure in ground and excited states we deduce a ground state barrier to internal rotation of V‘3=95±5 cm−1 and a best-fit excited state barrier of V3=745 cm−1, V'6=−100 cm−1. Ab initio calculations on 4-mp which reproduce both the magnitude and shape of the experimental barrier to internal rotation in the ground state. The lowest energy methyl conformation places a hydrogen atom in the plane of the ring pointing away from the nitrogen lone pair. Finally, in both molecules we observe spectroscopic signatures of vibrational state mixing in the S1 state. Density-of-states calculations on both molecules using the experimentally determined internal rotor energy levels predict a similar density of same-symmetry states in the two molecules at a given energy. Experimental evidence is presented that the small V6 barrier in 5-mp leads to modest vibration/internal rotation coupling matrix elements of ∼1 cm−1. The high V3 barrier in 4-mp is observed to give strong vibration/internal rotation coupling in the case of the 6a10(0a1) level which shifts its position by 17 cm−1 from its companion 6a10(1e) level due to interaction with an X1(3a2) vibration/internal rotation combination band.

Notes: We compare two time-dependent methods (time-dependent Hartree and time-dependent density functional methods) with a time-independent density functional method for the calculation of the frequency dependent polarizability and resulting absorption spectrum of two interacting quantum confined particles (quantum dots). The system is examined within the dipole approximation and the methods are evaluated in terms of the optical absorption spectrum. The spectral noise generated by time-dependent methods is a sensitive measure of the degree of broken correlation between the quantum degrees of freedom and the time-dependent density functional method may help to quantify the efficacy of correlation-exchange potentials that are used in density functional models. With respect to the quantum confinement issue, we find that increasing the interaction energy between nearest neighbor quantum dot sites represented in our model tends to shift absorption intensity to higher energy transitions. © 1997 American Institute of Physics.

Notes: The products of diacetylene's ultraviolet photochemistry over the 245–220 nm region are directly determined for the first time. At these wavelengths, the photochemistry is thought to proceed from a metastable excited state of C4H2 rather than by direct photolysis. The experimental method employs a small reaction tube attached to a pulsed nozzle. C4H2 is excited within the reaction tube where collisions of C4H*2 with C4H2 form products which are subsequently ionized by vacuum ultraviolet radiation (118 nm) in the ion source of a time-of-flight mass spectrometer. The C4H*2 + C4H2 reaction produces C6H2 (+C2H2), C8H2 (+2H,H2), and C8H3 (+H), all in good yield. An extensive set of experiments is carried out to ensure that the products observed are initial products formed by single-photon excitation of gas phase C4H2. Under certain conditions, secondary products formed by subsequent reaction of the initially formed products with C4H2 are also observed. These are dominated by C10H3 and C12H3. Thermochemical arguments are made which point to C8H3+C4H2 as the source of these secondary products. Collisional deactivation of C4H*2 from its initially excited energy (∼120 kcal/mol above the ground state) to the lower levels of the metastable state (∼74 kcal/mol) is important in determining the relative amounts of C8H2 and C8H3 products. In cases where C4H*2 undergoes extensive deactivation prior to reaction, C8H3+H products dominate. When collisional deactivation is minimized, much of the C8H3 products are formed with enough energy to subsequently dissociate further to form C8H2+2H. Mechanisms are postulated for the observed reactions and some suggestions for further work to assess the importance of these reactions in planetary atmospheres are given.

Notes: Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n–π* transition of jet-cooled 2-methylpyrimidine have been recorded and analyzed. This work extends our earlier study of the spectroscopic and photophysical effects of methyl substitution in 4- and 5-methylpyrimidine. An unusual Fermi resonance involving the 6an0 progression forms the focus of the present study. The 6a10 vibronic transition is observed to be split into a triad of transitions. Dispersed fluorescence spectra are used to identify the dark background state responsible for the Fermi resonance coupling as the 16b1(3a''2) vibration/internal rotation combination level. This level is selectively coupled by symmetry constraints to 6a1(0a1), leaving the 6a1(1e‘) level unperturbed. The positions and intensities of the triad of peaks in the excitation spectrum allow a quantitative determination of the 6a1(0a'1)↔16b1(3a2) coupling matrix element of V=4.1 cm−1. This vibration/internal rotation Fermi resonance is thus typical of the new types of routes to vibrational state mixing which are opened by methyl substitution. Higher members of the 6an0 progression are also involved in Fermi resonance mixing. However, in addition, these levels experience weaker, less state-specific coupling to a bath of same-symmetry states at that energy. The excitation spectrum provides an estimate of the average coupling matrix element of this second tier coupling of ∼1 cm−1.