Microwave Spectrum of Cyclopropyl Bromide

The microwave spectra of four isotopic species of cyclopropyl bromide, namely C3H579Br, C3H581Br, C3H4D79Br, and C3H4D81Br have been analyzed in the frequency region of 19 000–31 000 Mc/sec. The rotational constants obtained are as follows (given in megacycles per second): ABCC3H579Br16 504.9 ± 652579.87 ± 0.032457.68 ± 0.03C3H581Br16 239.6 ± 1602560.49 ± 0.032440.12 ± 0.03C3H4D79Br15 139.2 ± 792527.52 ± 0.042455.93 ± 0.05C3H4D81Br15 046.8 ± 1842502.84 ± 0.052430.73 ± 0.04 Two structures compatible with these data have been obtained for this molecule. Structure I was calculated by assuming for the hydrogen parameters values obtained for cyclopropyl chloride, whereas Structure II was derived with the assumption that all d C–C = 1.080 Å and the ∠H–C–H's are bisected by the ring. Structure IAssumedCalculatedd C–H(trans): 1.086 Åd C1–C2: 1.521 ± 0.004 Åd C–H(cis): 1.074 Åd C1–C3: 1.512 ± 0.007 Åd C–H(sec): 1.078 Åd C3–Br: 1.905 ± 0.007 Å∠C2–C1–H(cis): 116°50′∠H(sec)–C3–Br: 116° ± 50′∠C2–C1–H(trans): 118°56′∠C2–C3–Br: 118°54′ ± 1.0°∠C3–C1–H(cis): 115°20′∠H(cis)–C1–H(trans): 115°59′ Structure IIAssumedCalculatedd C–H: 1.080 Åd C1–C2: 1.518 ± 0.004 Å∠C2–C1–H(cis): 117°37′d C1–C3: 1.507 ± 0.010 Å∠C2–C1–H(trans): 117°37′d C3–Br: 1.899 ± 0.005 Å∠C3–C1–H(cis): 117°6′∠H(sec)–C3–Br: 115°15′ ± 50′∠H(cis)–C1H(trans): 116°∠C2–C3–Br: 119°30′ ± 1.0° The analysis of the hyperfine structures of the spectra gives the following quadrupole coupling constants. For C3H579Br eQqm = 462.3 ± 3.5 Mc/sec, η = −0.23 ± 0.04; C3H581Br eQqm = 391.2 ± 1.0 Mc/sec, η = −0.20 ± 0.02; C3H4D79Br eQqm = 451.0 ± 14 Mc/sec, η = −0.24 ± 0.12; C3H4D81Br eQqm = 404.1 ± 4 Mc/sec, η = −0.08 ± 0.10. The ionicity of the bond was estimated to approximately 22%.