The Electronic Absorption Spectra of NH$_{2}$ and ND$_{2}$

The absorption spectra of $^{14}$NH$\_{2}$, $^{15}$NH$\_{2}$ and $^{14}$ND$\_{2}$ 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 $^{14}$NH$\_{3}$, $^{15}$NH$\_{3}$ and $^{14}$ND$\_{3}$ respectively. A detailed study of the $^{14}$NH$\_{2}$-$^{15}$NH$\_{2}$ 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 $^{14}$NH$\_{2}$ and $^{14}$ND$\_{2}$ spectra. The spectra consist of type C bands for which the transition moment is perpendicular to the plane of the molecule. For NH$\_{2}$, sixteen bands of the progression (0, v$\_{2}^{\prime}$, 0) $\leftarrow $ (0, 0, 0) have been identified with v$\_{2}^{\prime}$ = 3, 4,..., 18. In addition four bands of a subsidiary progression (1, v$\_{2}^{\prime}$, 0) $\leftarrow $ (0, 0, 0) have been found; these bands derive most of their intensity from a Fermi-type resonance between (0, v$\_{2}^{\prime}$, 0) and (1, v$\_{2}^{\prime}$-4, 0) levels in the excited state. The interaction constant W$\_{\text{ni}}$ is 72 $\pm $ 3 cm$^{-1}$. For ND$\_{2}$, fourteen bands of the principal progression (v$\_{2}^{\prime}$ = 5 to 18) and one band of the subsidiary progression have been identified. The upper state vibration frequencies $\omega \_{1}^{0\prime}$ and $\omega \_{2}^{0\prime}$ are 3325 cm$^{-1}$ and 622 cm$^{-1}$ for NH$\_{2}$ and 2520 cm$^{-1}$ and 422 cm$^{-1}$ for ND$\_{2}$ respectively. The bending frequencies are unusually low; moreover, the anharmonicities of the bending vibration are unusually large and negative (x$\_{22}^{0\prime}$ = 11$\cdot $4 cm$^{-1}$ for NH$\_{2}$ and 8$\cdot $1 cm$^{-1}$ for ND$\_{2}$). 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 A$\_{000}^{\prime \prime}$, B$\_{000}^{\prime \prime}$ and C$\_{000}^{\prime \prime}$ and for the centrifugal distortion constants D$\_{A}^{\prime \prime}$, D$\_{B}^{\prime \prime}$ and D$\_{C}^{\prime \prime}$ have been determined. The bond lengths and bond angles for NH$\_{2}$ and ND$\_{2}$ agree and are 1$\cdot $024 $\pm $ 0$\cdot $005 angstrom and 103 degrees 20$^{\prime}$ $\pm $ 30$^{\prime}$ 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 $\Sigma $, $\Delta $, $\Gamma $,... and $\Pi $, $\Phi $,... vibronic sub-levels with large vibronic splittings. The origins of the vibronic sub-bands may be represented by the formula $\nu \_{0}^{K}$ = $\nu \_{0}$-GK$^{2}$, where G is $\sim $ 27 cm$^{-1}$ for NH$\_{2}$ and $\sim $ 19 cm$^{-1}$ for ND$\_{2}$. The rotational levels show both spin and K-type doubling. No simple formula has been found to fit the energies of the $\Pi $, $\Delta $, $\Phi $ and $\Gamma $ rotational levels; the $\Sigma $ levels fit the formula F(N) = BN(N+1)-DN$^{2}$(N+1)$^{2}$, though with a negative value for D. By extrapolating the B values for the $\Sigma $ levels to v$\_{2}^{\prime}$ = 0 we obtain B$\_{000}^{\prime}$ = 8$\cdot $7$\_{8}$ cm$^{-1}$ for NH$\_{2}$ and 4$\cdot $4$\_{1}$ cm$^{-1}$ for ND$\_{2}$. These values are consistent with a linear configuration with a bond length of 0$\cdot $97$_{5}$ 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.