Characterization of ON-ON by resonance Raman and infrared spectroscopy

Abstract

The colored species (λ[subscript max] ~ 500 nm) formed form the reaction of nitric oxide with Lewis acids (BF₃, BCl₃, SiF₄, TiCl₄, and SnCl₄), SO₂, or HCL at 77°K has been characterized by resonance Raman and infrared spectroscopy. Detailed analysis of the vibrational data using ¹⁵N and ¹⁵N¹⁸O isotopic substitution strongly suggests that an asymmetric form of dimerized nitric oxide, ON-ON, has been formed, although an unorthodox T-shaped ON--[superscript O subscript N] structure could not be ruled out. The proximity of the electronic absorption band to the laser frequencies used in the experiment results in overtone and combination band progressions characteristic of resonance Raman spectroscopy. The fundamental frequencies observed for O¹⁶N¹⁶ON (for ¹⁶O¹⁵N¹⁶O¹⁵N and ¹⁸O¹⁵N¹⁸O¹⁵N in parentheses), resulting from reacting NO with an of the above acids or from the photolysis of ONCl, are ν₁ (N=O stretch) at 1778 (1750 and 1695), ν₂ (N=O stretch) at 1690 (1661 and 1610), ν₃ (N-O stretch) at 482 (475 and 466), ν₄ (ONO bend) at 184 (183 and 179), and ν₅ (NON bend) at 116 (115 and 114) cm⁻¹. The overtone and combination bands included ν₁ - ν₃, ν₁ + ν₃, ν₁ + 2ν₃, ν₁ + 3ν₃, ν₁ + (ν₃ + ν₄), 2ν₁, 2ν₁ + ν₄, 2ν₁ + ν₅, ν₂ + ν₄, ν₃ - ν₄, ν₃ + ν₄, ν₃ + 2ν₄, ν₃ + 3ν₄, 2ν₃, 2ν₃ - ν₄, 2ν₃ - 2ν₄, 2ν₃ + ν₄, 2ν₃ + 2ν₄, 2ν₄, 3ν₄, 4ν₄, 5ν₄, and ν₄ + ν₅ (or ν₃ - ν₄). Only ν₁, ν₂, and ν₁ + ν₂ were observed in the infrared spectra in addition to unshifted acid bands. Harmonic frequencies and anharmonicity constants for the ν₂, ν₃, and ν₄ fundamentals were determined and the magnitudes of the latter indicate little deviation of these vibrations form the simple harmonic approximation. Finally, the force field determined for this molecular species indicates that both NO groups retain strong multiple bond character and are joined by a weak NO single bond.