Rotational Isomerism in 1hyphen;Pentyne from the Microwave Spectrum

摘要

The gashyphen;phase microwave spectrum of 1hyphen;pentyne has been investigated in the region 8ndash;40 GHz. The spectrum can be attributed to two rotational isomers in which the methyl group is, respectively,transandgaucheto the acetylene substituent. The rotational constants of thetransform areA equals; 23thinsp;340 plusmn; 200,B equals; 2230.566 plusmn; 0.020, andC equals; 2116.385 plusmn; 0.020MHz, and those for thegaucheform areA equals; 9921.112 plusmn; 0.030,B equals; 3172.780 plusmn; 0.010, andC equals; 2634.025 plusmn; 0.010MHz. From these data a plausible structure has been derived which gives thegauchedihedral angle as115deg; plusmn; 3deg;from thetransposition. For each form several vibrationally excited states have been assigned to the torsional motion around the central carbonmdash;carbon bond, the methyl internal rotation, and theCsngbnd;Ctrpbnd;Cbending motion. The torsional vibration frequency is106 plusmn; 10cmminus;1in thetransform and114 plusmn; 10cmminus;1in thegauche. Stark effect measurements yieldmgr;transequals; 0.842 plusmn; 0.010Dandmgr;gaucheequals; 0.769 plusmn; 0.028D. Thegauchehyphen;form ground state is27 plusmn; 36cmminus;1below thetransground state. Four coefficients in the Fourier expansion of the potential energy for rotation about the centralCsngbnd;Cbond have been derived:V1equals;0.940,V2equals;minus;1.045,V3equals;3.518, andV4equals;0.071 kcalsol;mole. It is suggested that the slightly greater stability of thegaucheform may be due to an attractive dipolemdash;dipole interaction.