Microwave measurements and calculations of quadrupole coupling effects in CH₃I and CD₃I

The transitions J = 1 ← 0, K = 0; J = 2 ← 1, K = 0; and J = 2 ← 1, K = 1 of CH₃I and CD₃I were measured using a Stark-modulated microwave spectrometer. Iodine quadrupole coupling strengths were analyzed to determine variations with deuterium substitution on the methyl group and variations with centrifugal distortion. Quadrupole coupling strengths were described by the expression eQq₀ + aJ(J + 1) + bK² + cK⁴ J(J + 1). Explicit expressions are given for a, b, and c for a symmetric top in terms of molecular parameters. For CH₃I eQq₀ = -1934.11 ± 0.02 MHz and for CD₃I eQq₀ = -1928.95 ± 0.04 MHz. Rotational constants obtained are B(CH₃I) = 7501.274 ± 0.002 MHz and B(CD₃I) = 6040.298 ± 0.007 MHz. The observed fractional change in halogen quadrupole coupling of 0.0027 is related to previous results for methyl chloride and methyl bromide.