Nonlinear hydrogen bonds of the type (CH2)(2)Z center dot center dot center dot HY: The rotational spectrum of a complex of methylenecyclopropane andhydrogen bromide

The rotational spectra of the isotopomers mecp ... (HBr)-Br-79, mecp ... (H Br)-Br-81 and mecp ... (DBr)-Br-79 (mecp=methylenecyclopropane) of a comple x formed between mecp and hydrogen bromide were observed by using a pulsed- nozzle, Fourier-transform microwave spectrometer. Accurate values of the ro tational constants A(0), B-0 and C-0, the centrifugal distortion constants Delta(J), Delta(JK) and delta(J), and the components chi(aa), (chi(bb)-chi( cc)) and chi(ab) of the Br nuclear quadrupole coupling tensor were determin ed in each case. These spectroscopic constants were interpreted to show tha t mecp ... HBr has C-s symmetry, with the HBr subunit lying in the principa l inertial plane ab, which coincides with the molecular symmetry plane. The H atom of HBr forms a hydrogen bond to the centre (*) of the mecp pi bond. The angle phi included by the unique C atom of the cyclopropyl ring, the c entre of the pi bond and the H atom of HBr is 88.8(10)degrees and the dista nce r(*... H)=2.353(18) Angstrom. The hydrogen bond *... H-Br deviates from linearity by theta=18.03(23)degrees in a direction that suggests a seconda ry interaction of Brdelta- with two of the protons of the cyclopropyl ring. The properties phi, theta, r(Z ... X) and r(H-ring... Y) of the complexes (CH2)(2)Z ... XY, where Z=O, S or C=CH2 and XY=HCl, HBr or ClF are compared and the reason why the hydrogen bonds Z ... H-Y are significantly nonlinea r, while the 'chlorine' bonds Z ... Cl-F remain close to linearity, is cons idered.