METHYL INTERNAL-ROTATION AND N-14 NUCLEAR-QUADRUPOLE COUPLING FROM THE ROTATIONAL SPECTRA OF ETHYL ISOCYANIDE, ISOPROPYL ISOCYANIDE AND GAUCHE-N-PROPYL AND TRANS-N-PROPYL ISOCYANIDE

The barrier heights (V3) hindering methyl internal rotation were determined with microwave Fourier transform spectroscopy from the ground vibrational state for the title molecules and found to be V3 = 3.336(52) kcal/mol for ethyl isocyanide, V3 > 3.1 kcal/mol for iso-propyl isocyanide, V3 = 2.894(23)kcal/mol for gauche-n-propyl isocyanide and V3 = 2.954 (22) kcal/mol for trans-n-propyl isocyanide. The quadrupole coupling constants of iso-propyl isocyanide are chi(aa) = 179.3(31) kHz, chi(bb) = -140(15) kHz and chi(cc) = -39(15) kHz; the constants of trans-n-propyl isocyanide were determined to be chi(aa) = 268.1 (71) kHz, chi(bb) = -108(23) kHz and chi(cc) = -160(23) kHz.