Substituted Methanes. XXIII. Infrared Spectral Data, Rotational Constants, Normal Coordinate Treatments, and Thermodynamic Properties for CD3Br and CH3Br

Abstract

Infrared spectra were obtained between 400 and 4000 K with NaCl and KBr prisms. The a₁ fundamentals for CD₃Br are 2157, 993, and 578 K; for CH₃Br they are 2972, 1305, and 611 K. For CD₃Br, a least squares analysis of the observed wave numbers of the sub‐bands of σ₆(e) gave for B₁,A₁,ζ, and σ₀ the values 0.255 K, 2.529 K, 0.150, and 712.3 K; for σ₅(e) the values were 0.257 K, 2.530 K, —0.339, and 1056.2 K. For σ₄(e)=2293 K, the sub‐bands were not resolved, but the sum rule gave ζ₄=0.240. The microwave value of B₀ was used. The signs of B₁—B₀ for σ₆ and σ₅ are the same as those previously obtained for CH₃I by use of the high‐dispersion records of Lagemann and Nielsen. For CH₃Br, a least squares analysis of the grating data of Bennett and Meyer gave for σ₆(e) : 0.315 K, 5.144 K, 0.217, and 955.4 K; for σ₅(e) : 0.304 K, 5.062 K, —0.242, and 1448.2 K; and for σ₄(e) : 0.321 K, 5.153 K, 0.064, and 3055.6 K. The sign of B₁—B₀ for σ₆ is the same as for CH₃I, but the signs for σ₅ and σ₄ are reversed. This inconsistency is presumably due to the limited accuracy with which the two small constants B₁—B₀ and (A₁—B₁) — (A₀—B₀) can be determined by the least squares method. The most probable values of the band origins are believed to be, 954.5, 1443.8, and 3056.7 for CH₃Br and 711.6 and 1056.4 K for CD₃Br (σ₄ was not resolved, so no band origin could be obtained). Values of the principal potential constants are: f_b=2.8631 md/A, f_h=5.1199 md/A, f_hh=0.53535 md A/rad², and f_bh=0.62871 md A/rad². Values are given for 13 interaction constants. For CD₃Br, the values of (H⁰—E₀⁰)/T, — (F⁰—E₀⁰)/T, S⁰, and C_p⁰ at 100, 273.16, 298.16, 900, and 1550°K are, respectively : 7.95, 42.66, 50.61, 8.00; 8.75, 50.86, 59.61, 11.07; 8.98, 51.63, 60.61, 11.68; 14.23, 64.09, 78.32, 20.38; and 17.57, 72.74, 90.31, 23.44 cal deg^—1 mole^—1. For CH₃Br, the corresponding values are : 7.95, 41.41, 49.36, 7.97, 8.37, 49.51, 57.88, 9.72; 8.50, 50.25, 58.75, 10.14; 12.63, 61.51, 74.14, 18.19; and 15.87, 69.25, 85.12, and 22.00. Values for 27 other temperatures in this range are given.