The Micro wave Spectrum of Bromoacetylene ; rs-Structure , Dipole Moment, Quadrupole Coupling Constants and Excited Vibration States

Abstract

Abstract The microwave spectrum of bromoacetylene has been investigated in the frequency range from 7 GHz to 35 GHz. From Stark effect measurements the dipole moment has been determined as μ = 0.23 + 0.01 D, and the restructure has been derived in four independent ways from twelve isotopic species: C - H = 1.0553 Å, C ≡ C = 1.2038 Å, C - Br = 1.7913 Å (internal consistency better than ± 0.0003 Å). Quadrupole coupling constants have been determined for eleven isotopic forms and are e q Q = 541.47 MHz and 648.00 MHz for HCCBr81 and HCCBr79, respectively. Rotation spectra have also been observed for excited states of the three lowest normal modes and for several isotopic forms. For the Br81 (Br79) species the rotation-vibration coefficients are α5 = -10.98 (-11.02) MHz, α4 = -1.57 (- 1.61) MHz and α3 = + 12.88 (+ 13.40) MHz. For the bending vibrations, ν5 and ν4 , l-type doubling constants are obtained as ql5 = 4.14 (4.17) MHz and ql4 = 2.6 MHz. Analysis of the Fermi resonance between the first excited state of ν3 and the l = 0 component of the second excited state of ν5 gives the mixing ratio of these two states as a/b = 1.45 (1.51) and the interaction energy as W3,5 = 1.313 (1.181) δ3,5 for the Br81 (Br79) species. With an approximate value of δ3,5 ≌ 25 cm-1, the cubic force constant is obtained as κ3,55 ≌ 44 cm-1. The results are discussed in relation to the molecular properties of other halogen acetylenes and halogen cyanides.