The Electronic Absorption Spectra of NH2 and ND2

The absorption spectra of ¹⁴NH₂, ¹⁵NH₂ and ¹⁴ND₂ have been photographed in the region 3900 to 8300 angstrom with a 21 ft. concave grating spectrograph. The radicals are produced by the flash photolysis of ¹⁴NH₃, ¹⁵NH₃ and ¹⁴ND₃ respectively. A detailed study of the ¹⁴NH₂-¹⁵NH₂ isotope shifts suggests that the molecule has a linear configuration in the excited state and that the spectrum consists of a long progression of the bending vibration in this state. These conclusions have been confirmed by detailed rotational and vibrational analyses of the ¹⁴NH₂ and ¹⁴ND₂ spectra. The spectra consist of type C bands for which the transition moment is perpendicular to the plane of the molecule. For NH₂, sixteen bands of the progression (0, v₂', 0) <- (0, 0, 0) have been identified with v₂' = 3, 4,..., 18. In addition four bands of a subsidiary progression (1, v₂', 0) <- (0, 0, 0) have been found; these bands derive most of their intensity from a Fermi-type resonance between (0, v₂', 0) and (1, v₂'-4, 0) levels in the excited state. The interaction constant W_ni is 72 ± 3 cm^-1. For ND₂, fourteen bands of the principal progression (v₂' = 5 to 18) and one band of the subsidiary progression have been identified. The upper state vibration frequencies ω ₁^0' and ω ₂^0' are 3325 cm^-1 and 622 cm^-1 for NH₂ and 2520 cm^-1 and 422 cm^-1 for ND₂ respectively. The bending frequencies are unusually low; moreover, the anharmonicities of the bending vibration are unusually large and negative (x₂₂^0' = 11\cdot 4 cm^-1 for NH₂ and 8\cdot 1 cm^-1 for ND₂). The origin of the system lies in the region of 10 000 cm^-1. Ground-state rotational term values have been derived from observed combination differences; values for the rotational constants A₀₀₀^{' '}, B₀₀₀^{' '} and C₀₀₀^{' '} and for the centrifugal distortion constants D_A^{' '}, D_B^{' '} and D_C^{' '} have been determined. The bond lengths and bond angles for NH₂ and ND₂ agree and are 1\cdot 024 ± 0\cdot 005 angstrom and 103 degrees 20' ± 30' respectively. Small spin splittings have been observed. In the excited state an unusual type of vibronic structure has been found. Successive levels of the bending vibration consist alternately of Σ , Δ , Gamma ,... and Pi , Φ ,... vibronic sub-levels with large vibronic splittings. The origins of the vibronic sub-bands may be represented by the formula ν ₀^K = ν ₀-GK², where G is ~ 27 cm^-1 for NH₂ and ~ 19 cm^-1 for ND₂. The rotational levels show both spin and K-type doubling. No simple formula has been found to fit the energies of the Pi , Δ , Φ and Gamma rotational levels; the Σ levels fit the formula F(N) = BN(N+1)-DN²(N+1)², though with a negative value for D. By extrapolating the B values for the Σ levels to v₂' = 0 we obtain B₀₀₀' = 8\cdot 7₈ cm^-1 for NH₂ and 4\cdot 4₁ cm^-1 for ND₂. These values are consistent with a linear configuration with a bond length of 0\cdot 97₅ angstrom. The significance of this short bond length is discussed. An explanation of the complex vibronic structure is given. The two combining states are both derived from an electronic Pi state which is split by electronic-vibrational coupling for the reasons advanced by Renner. A detailed correlation diagram is given. A quantitative treatment of this effect by Pople & Longuet-Higgins gives good agreement with the experimental data.