High-resolution microwave spectrum of the weakly bound helium-pyridine complex.

High-resolution rotational spectra of the helium-pyridine dimer were obtained using a pulsed molecular beam Fourier transform microwave spectrometer. Thirty-nine R-branch (14)N nuclear quadrupole hyperfine components of a- and c-type dipole transitions were observed and assigned. The following spectroscopic parameters were obtained: rotational constants A=3875.2093(48) MHz, B=3753.2514(45) MHz, and C=2978.4366(81) MHz; quartic centrifugal distortion constants D(J)=0.124 08(55) MHz, D(JK)=0.1200(43) MHz, D(K)=-0.2451(25) MHz, d(1)=0.004 27(27) MHz, and d(2)=0.000 16(10) MHz; sextic centrifugal distortion constants H(J)=0.003 053(35) MHz, H(JK)=-0.006 598(47) MHz, and H(K)=0.004 11(59) MHz; (14)N nuclear quadrupole coupling constants chi(aa)((14)N)=-4.7886(76) MHz, chi(bb)((14)N)=1.4471(76) MHz, and chi(cc)((14)N)=3.3415(43) MHz. Our analyses of the rotational and (14)N quadrupole coupling constants show that the He atom binds perpendicularly to the aromatic plane of C(5)H(5)N with a displacement angle of approximately 7.0 degrees away from the c axis of the pyridine monomer, toward the nitrogen atom. Results from an ab initio structure optimization on the second order Moller-Plesset level are consistent with this geometry and gave an equilibrium well depth of 86.7 cm(-1).