Beam maser spectroscopy on J = 1 → 2, K = 1, and K = 0 transitions in CH3CN and CH3 13CN

Abstract

J = 1 → 2 transitions near 36.8 GHz in CH₃CN and CH₃ ¹³CN were measured using a beam maser spectrometer. K = 1 transitions were observed with a 10 kHz linewidth (FWHM) and K = 0 transitions were observable with a two‐cavity Ramsey system with about 3 kHz resolution. The measured difference between a axis and c axis nitrogen spin–rotation tensor components is C_a–C_c = 4.6±1.0 kHz for CH₃CN and 4.5±1.0 kHz for CH₃ ¹³CN. The c axis nitrogen spin–rotation tensor component is C_c = 2.0±0.4 kHz. Nitrogen chemical shift tensor components of σ_a = 247 ppm and σ_c = −165 ppm are obtained from the data and calculated diamagnetic contributions. The measured paramagnetic contributions to the nitrogen chemical shift tensor are σ_a^p = −98±15 ppm and σ_c^p = −600±120 ppm. Nitrogen quadrupole coupling strength is eqQ = −4224.3±4.0 kHz and rotational transition frequencies are: 36 795 476.9±3.0 kHz (CH₃CN, K = 0), 36 794 769.3±1.5 kHz (CH₃CN, K = 1), 36 777 282.6±3.4 kHz (CH₃ ¹³CN, K = 0), and 36 776 573.3±1.6 kHz (CH₃ ¹³CN, K = 1). This data, combined with previous J = 0 → 1 data yields accurate rotation and distortion constants for CH₃CN and CH₃ ¹³CN as follows: