Spectres de vibration des molecules BCl2CO et BX2NCS (X = Cl,Br, I), champs de forces et modes normaux

Experimental data on the vibrational spectra of BX₂¹⁴NCS (X = Cl, Br or I), BCl₂¹⁵NCS, BBr₂¹⁵NCS, and BCl₂NCO are reported. Analysis of the results shows that the boron atom is bonded to the nitrogen atom, and the BN, CN, CS and CO bonds are colinear. Force fields are calculated and found to reproduce the experimental frequencies and the ¹⁰B-¹¹B and ₁₄N-₁₅N isotopic effects. High values are obtained for the force constant of the B-N stretching vibration (about 6.5 mdyn Å^?1) and it is shown that, owing to electron transfer from nitrogen to boron, the B-N bond is intermediate between a single and a double bond. The force constant νBX is marginally greater than that for the corresponding BX₂SH compounds. The C-N bond is weaker than in the NCS ion, whereas the CS bond is stronger.Calculation of the normal modes and the potential energy distribution shows that for the BCl₂NCO molecule, the B, N, C, and O atoms are almost equally involved in all the modes of A, symmetry, especially those at 2270, 1525 and 1022 cm^?1. On the other hand, for BX₂NCS compounds, the potential energy is relatively localised on one coordinate. Consequently, the group vibration approximation is justified in this case.