Beam maser spectroscopy on J=1→2, K=1, and K=0 transitions in CH ₃CN and CH₃ ¹³CN

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 CH ₃CN and CH₃ ¹³CN as follows: CH₃CN CH₃¹³CN B 9 198 899.3±1.0 9 194 347.1±0.7 KHz D_J 3.7±0.3 3.3±0.3 KHz D_JK 176.9±1.0 177.3±0.7 KHz.