Structural and Electronic Properties of the Charge-Transfer Complex H3P⋅⋅⋅Br2 Determined by Fourier Transform Microwave Spectroscopy

Abstract

The ground-state rotational spectra of six isotopomers of the symmetric-top complex H3P...Br2 have been measured by the technique of pulsed-nozzle, Fourier transform microwave spectroscopy. The spectroscopic constants B0, DJ, DJK, chiaa(Brx) and Mbb(Brx), x=i (inner) or o (outer) bromine atom, were obtained from analysis of the spectra. Interpretation of these constants with the aid of models revealed that the pre-reactive complex has an intermolecular bond of length r(P...Br) = 3.0440(4) A between the P atom of PH3 and one Br atom of Br2 and that this bond is a relatively strong one, as measured by the intermolecular stretching force constant ksigma-9.8 Nm(-1). The complex was discovered to have a significant contribution from charge transfer in the ground state by establishing the fraction of intermolecular charge transferred from P to Bri[sigmai = 0.077(23)] and the fraction of intramolecular charge transferred from Bri to Bro [sigmap(Br)=0.11(1)].

Halogen Bonding with Phosphine: Evidence for Mulliken Inner Complexes and the Importance of Relaxation Energy

“…Plotting δ i against the “appropriate” ionization energy I B of B produces a roughly exponential decrease of the form δ i = A exp­{− bI B }, where A and b are coefficients fitted to experimental data. , As phosphine has a low ionization energy, it follows that the amount of electron transfer should be large and hence result in strong interactions, particularly with the heterodihalogens. Experimental rotational spectroscopy studies of complexes of phosphine with Cl 2 , Br 2 , BrCl, and ICl confirmed these trends, with 0.144 of an electron transferred in the H 3 P···ICl case. − …”

“…Phosphine complexes with Cl 2 , Br 2 , BrCl, and ICl have been produced in the gas phase and characterized using rotational spectroscopy. − Of particular interest to the present investigation is that these previous works determined intermolecular bond distances and the strength of the intermolecular bond in terms of intermolecular force constants. These experimental data are compared to theoretical results in Table .…”

“…This is perhaps unsurprising as a direct proportionality between intermolecular dissociation energy [calculated at the CCSD­(T)-F12c/cc-pVDZ-F12 level] and intermolecular quadratic stretching force constants ( k σ ) for a number of halogen-bonding complexes has been previously established . Experimental estimates of the fraction of an electron transferred from the Lewis base B to X (δ i ) were extracted from changes in the halogen nuclear quadrupole coupling constants when the complex is formed, which is based on the Townes–Dailey model. , The experimental values for XY = Cl 2 , Br 2 , BrCl, and ICl are 0.01, 0.077(23), 0.100(5), and 0.144(7) e , respectively. ,,, A comparison with Table shows that as the experimental δ i increases so does ΔXY (the bond elongation on complex formation), which appears logical on the basis of electron density being donated from the P lone pair to the XY antibonding orbital.…”

The SN2 reaction and its relationship with the Walden inversion, the Finkelstein and Menshutkin reactions together with theoretical calculations for the Finkelstein reaction

“…The structure of dibromo-λ 5 -phosphane, Br-PH 3 -Br trigonal bipyramidal (Fig. 2), differs from that determined by Legon [65] and calculated by Kraka and Cremer [66], H 3 P••• Br-Br, a halogen-bonded complex, although both are minima (no imaginary frequencies). We have reported in a previous work that the P(V) derivatives are much more stable than the P(III) derivatives, although the last ones are stabilized by an halogen bond [67].…”

Untitled

“…10 Other dihalogen adduct systems reported include diiodine adducts of thiocarbonyls, 11 aryl phosphites 12 and phenyltelluride; 13 further comments on the structure of the dibromine adduct of N-methylbenzothiazole-2-selone were reported last year. 14 Finally, in a continuing investigation of the gas-phase adducts of halogens with donors, the ground-state rotational spectrum for H 3 P…Br 2 is reported, 15 and calculations on the intermolecular interaction between ammonia and F 2 , ClF, and Cl 2 have been described. 16 To date most of the studies of adducts of dihalogens have concentrated on their reactions with Group 15 and Group 16 containing Lewis bases.…”