Ultra-violet absorption spectrum of isocyanic acid

Abstract

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...

Isocyanic acid in the atmosphere and its possible link to smoke-related health effects

“…It is possible that there are mechanisms, such as adduct formation, that are important at low temperatures, but we are not aware of any studies of that phenomenon. HNCO is expected to have a very low absorption at near-UV to visible wavelengths that constitute the solar actinic region based on measurements at wavelengths shorter than 280 nm (35,36). The major dissociation channel (HNCO ⇒ H þ NCO) has a threshold of 261 nm which corresponds to a bond energy D o ðH-NCOÞ ¼ 109.6 AE 0.4 kcal∕mole (37), and a channel that forms triplet NH has a 332 nm threshold (38), however the absorption cross-section in that region is quite low.…”

Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO

“…Because the two pathways have markedly different anisotropy parameters, deconvolution of the spectrum could be achieved, which is also shown in Figure 3. We can find that although direct dissociation on S 1 is already open at this excitation energy (E avail = 10955 cm −1 ), the S 0 pathway still makes a major contribution. Only 36 ± 5% products come from S 1 pathway, whereas for the photolysis wavelength 210.02 nm, less than 10% products come from direct dissociation pathway on S 1 .…”

Imaging the Pair-Correlated HNCO Photodissociation: The NH(a1Δ) + CO(X1Σ+) Channel

“…10,15,32 Schinke et al 33 have calculated a large portion of the S 1 surface in five dimensions. The S 1 electronic state correlates with channels (2) and (3). For energies below the threshold of channel (3), dissociation should be in the ground electronic state S 0 , following IC from S 1 to S 0 .…”