Donor-acceptor complexes formed by perfluoro-organo bromides and iodides with nitrogenous and other bases. IV. Analysis of the infrared spectra of CF₃I.N(CH₃)₃ and CF₃Br.N(CH₃)₃ and related complexes

Abstract

The infrared spectra of complexes of CF₃I and CF₃Br with trimethylamine, reported in Part III, are considered in detail. The spectra are compared with those of related complexes. Considerable regularities are found. It is suggested that published assignments to δ_a1(Nme₃) and δ_E(Nme₃) in certain trimethylamine complexes should be reversed. The intensity of δ_A1(Nme₃) is considerably greater in its complexes with CF₃I and CF₃Br than in the free base: an explanation in terms of variation of the extent of electron transfer with vibrational coordinate is suggested. Force constants of the complex are calculated for several simplified models of the complex and possible errors are considered in some detail. The N-I bond stretching force constant in the complex CF₃I.N(CH₃)₃ for the room temperature gaseous, solution in carbon tetrachloride, and liquid phases and 80 K solid phase are respectively 0.19 ± 0.05, 0.29 ± 0.03, 0.31 ± 0.03 and 0.53 ± 0.09 mdyn/Ǻ. Corresponding values for the C-I stretching force constant in the complex and (in parenthesis) values for free CF₃I are: 2.14 ± 0.04(2.44 ± 0.02); 2.02 ± 0.03(2.42 ± 0.02); 2.00 ± 0.03 (2.42 assumed); 1.85 ± 0.04 (2.32 ± 0.02). For CF₃Br.N(CH₃), (solid) N-Br and C-Br force constants are 0.29 ± 0.05, 2.59 ± 0.05 (3.15 ± 0.02). Less definite information was obtained about the other force constants of the complex. The trimethylamine C-N stretching force constant decreases on formation of the complex; the decrease is greatest for solid CF₃I.N(CH,₃),. Potential energy dis- tributions are calculated. Possible causes of the changes of the A₁ and E CF₃ stretching frequencies on formation of the complexes are considered; several factors appear to be involved.