High-resolution measurements of the kinetic energies of hydrogen atom fragments formed during ultraviolet photolysis of imidazole, pyrrole, and phenol in the gas phase confirm that N(O)-H bond fission is an important nonradiative decay process from their respective 1pisigma* excited states. The measurements also reveal that the respective cofragments (imidazolyl, pyrrolyl, and phenoxyl) are formed in very limited subsets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photoinduced evolution from parent molecule to ultimate fragments.

T r a n s-HONO is optically prepared in specific −N=O stretching vibrational levels (2n, n=1,2,3) of the à state at 369, 355, and 342 nm. The ejected OH fragment is completely characterized by Doppler and polarization laser excitation spectroscopy. In this manner the OH translational energy, angular distribution, rotational alignment, and internal state distribution (vibration, rotation, spin-orbit and Λ-doubling components) are probed through the OH A–X system. The OH fragment is found to be translationally hot (∼0.5 eV) with a nearly sin2 θ angular distribution about the electric vector of the photolysis laser. The corresponding line shapes are Doppler split. However, the line shapes and widths do not noticeably depend on either fragment rotation or parent vibration. The internal motion of the OH fragment is vibrationally and rotationally cold; the spin-orbit components and the Λ doublets are not in equilibrium. The OH fragment is aligned and its π lobe lies preferentially in the plane of rotation. With increasing rotational excitation, these effects become more pronounced. This information allows us to construct a detailed photodissociation mechanism. The fragmentation is prompt and the trajectories of the recoiling fragments lie close to the initial HONO plane. The impulse associated with the central O–N bond fission contributes predominantly to OH translation while the rotation appears to arise from the zero-point motion of the parent in-plane bending and torsional vibrations. The OH energy content is found to be quite insensitive to the parent ν2 vibration, suggesting that the à state surface is rather ‘‘flat’’ along the −N=O stretch compared to the steep fragmentation coordinate.

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH 4 (CD 4 ) at 121.6 nm. Contrary to the previous consensus view, we find simple C-H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH 3 fragments are formed with very high levels of internal excitation: Some (~25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H + H2 + CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH 4 (A IT 2 ) molecules undergo rapid internal conversion (promoted by the Jahn-Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH 4 photodissociation at 121.6 nm yields CH 3 (and CH) fragments, rather than methylene radicals, will necessitate some revision ofcurrent models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.2054

The effects of polymyxin B and polymyxin B nonapeptide (PMBN) on cell envelope integrity in Escherichia coli were compared. Both compounds caused loss of proteins from E. coli K-12 3300(pBR322), although PMBN released less protein than did polymyxin B. The origin of the released protein was determined both by polyacrylamide gel electrophoresis and by using specific enzyme markers (beta-lactamase in periplasm, beta-galactosidase in cytoplasm). The proteins released by both compounds were derived principally from the periplasm, accompanied in the case of polymyxin B by a low level of cytoplasmic proteins. Although polymyxin B and PMBN both caused release of periplasmic proteins, the individual proteins released by the compounds differed. The periplasmic fraction contained six principal polypeptides with molecular weights between 62,000 (polypeptide 1) and 29,000 (polypeptide 6). Polypeptide 6 was identified as the pBR322-encoded betalactamase, but the other proteins were not specifically identified. Polymyxin B caused considerable release of polypeptides 1, 2, and 5 with some release of polypeptides 4 and 6. PMBN released polypeptide 1 (trace), 3, 4, and 6 (trace). Scanning electron microscopy showed that polymyxin B and PMBN both caused surface damage in E. coli. However, polymyxin B produced greater morphological changes than PMBN.

The absorption spectrum of HNCO between 2000 and 2800 A has been photographed with high resolution and absorption paths of up to 8 m atm. None of the bands are completely sharp, but many show coarse rotational structure. Long progressions of bands are assigned to excitation of an NCO bending vibration. The rotational structure shows that the NCO chain is strongly bent in the excited state, and that either LNCO = 119" and rNc+rco = 264A if the excited state is trans, or LNCO = 129" and r~c+uco = 2-60A if the excited state is cis. Comparison of the observed band contours with computed asymmetric top band syntheses proves that the bands have type C polarization. The electronic and geometrical structure of HNCO are discussed with the aid of a diagram correlating the molecular orbitals of HABC and AB2 molecules.Isocyanic acid (HNCO) is isoelectronic with 16 valency electron AB2 molecules such as CO,. The lowest energy electronic transition of these molecules is predicted to involve a considerable change in geometry from the linear ground state to a bent excited state. For COz the absorption spectrum, lying in the region 1400 -1700& is diffuse but the emission spectrum indicates that the excited state has an apex angle of 12212". We have found evidence in the absorption spectrum of another isoelectronic molecule, ketene (CH,CO), for a similar large decrease in apex angle in the first excited state.The only previous observation of the electronic absorption spectrum of HNCO was made at low resolution with a short absorbing path through the v a p~u r ,~ and showed a series of diffuse bands in the region 2250-2570 A merging into a continuum at the short wavelength end. Infra-red and microwave studies 5-7 have shown that in its ground state the molecule has a linear NCO group and an HNC angle of 128". EXPERIMENTALThe product was purified by two distillations from -78°C to -197°C discarding first and last fractions. The vapour pressure of the resulting HNCO agreed well with the published values,* and the i.r. spectrum closely corresponded with the published ~pectrum.~Of the impurities to be expected in the crude product only cyanogen absorbs at wavelengths longer than 2200 kl and this absorption is so weak as to require 5 m atm for its ob~ervation.~The product was kept at -78°C at which temperature it is stable for long periods though the vapour slowly polymerizes at room temperature. A sample of the deuterated acid was prepared in the same way after several exchanges of the cyanuric acid with D 2 0 . The i.-r. spectrum indicated that the product was roughly 70 % DNCO.Low resolution spectra were recorded on a Unicam SP700 spectrophotometer using a 100 cm cell and up to 10 cm Hg pressure. High resolution spectra of HNCO were observed in the third order of the Sheffield University 21 ft concave grating spectrograph,2 using a high pressure xenon arc lamp to provide the background continwm. The sample was contained in a 3 m absorption cell equipped with a multiple traversal mirror system enabling path lengths of up to 48 m to be...