ABSORPTION SPECTRA OF HCCl AND DCCl

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Abstract
A long progression of absorption bands appearing in the region 5 500–8 200 Å has been observed in the flash photolysis of dibromochloromethane, HCClBr2. Observation of chlorine and deuterium isotope shifts, together with the results of the rotational analyses, have shown that the carrier of the bands is the chloromethylene radical, HCCl.The ground state of HCCl is a 1A′ state in which the molecule has an equilibrium angle of ~103°, a C—H bond length of 1.12 Å, and a C—Cl bond length of 1.69 Å. In the upper state, of character 1A″, unusual patterns of the vibrational and K-type rotational energy levels indicate that the molecule, although bent, has a low barrier to linearity and is easily "straightened" by vibration. The vibrational levels of the upper state may be fitted quite accurately, using a potential function for the bending vibration of the type suggested by Thorson and Nakagawa (1960); our calculations show that the equilibrium angle in the upper state is ~135° and that the top of the barrier lies 2 250 cm−1 above the lowest vibrational level.The large change of angle in the transition causes the appearance of subbands with ΔK = 0 and −2 in addition to those allowed by the strict type-C selection rules. The intensities of the forbidden subbands are well explained by the axis-switching mechanism suggested by Hougen and Watson (1965).

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