Spectroscopiċ investigations of hydrogen bonding interactions in the gas phase. X. Properties of the hydrogen-bonded heterodimer HCN∙ ∙ ∙HF determined from hyperfine coupling and centrifugal distortion effects in its ground-state rotational spectrum

The rotational spectra of six isotopic species of HCN∙ ∙ ∙HF in their vibrational ground states have been observed by pulsed-nozzle, Fourier-transform, microwave spectroscopy and have been analysed to yield the rotational constants B0, centrifugal distortion constants DJ and various hyperfine coupling constants Xi(D), X3(14N) and D4, 5 as follows (subscripts refer to the numbering scheme H(1)C(2)N(3)∙ ∙ ∙H(4)F(5)). isotopic species B0/ MHZDJ/kHZXi(D)/kHZX3(14N)/MHZ D4, 5(H, F)/kHZ HC15N∙ ∙ ∙HF 3573 5874(2) 6.97(2) ─ ─ –242(6) HC15N∙ ∙ ∙DF 3551.5110 6.83 269(4)(i = 4) ─ ─ HC14N∙ ∙ ∙HF 3591.1552 6.99 ─ –4.098(4) –218(6) HC14N∙ ∙ ∙DF 3569.6576 6.86 259(2)(i = 4) –4.096(1) ─ DC15N∙ ∙ ∙HF 3360.3549 5.85 186(5)(i = 1) ─ –244(4) DC15N∙ ∙ ∙DF 3338.0824 5.81 { 181(10) (i = 1) ─ ─ 283(7) (i = 4) The B0 values lead to conclusions about the dimer geometry, the DJ values allow the hydrogen bond stretching force constant kσ to be determined and the hyperfine coupling constants provide information about the internal dynamics of the subunits. An analysis of the D nuclear quadrupole coupling and H, F nuclear spin nuclear-spin coupling constants demonstrates that the H—F bond lengthens by 0.14 Å when the heterodimer HCN∙ ∙ ∙HF is formed. (1 Å = 10–10 m = 10–1 nm.)