Ab initio calculations on monohalogenophosphanes PH2X (X=F,Cl,Br,I), and experimental detection and characterization of PH2F and PH2Cl by high resolution infrared spectroscopy

Abstract

The harmonic and anharmonic force fields of the title compounds have been calculated at the ab initio self-consistent-field level using effective core potentials and polarized double-zeta basis sets. Additional calculations for PH2F employ larger basis sets and include electron correlation. Many rovibrational constants are predicted theoretically. The infrared spectra generated from the ab initio data have guided the experimental identification of PH2F and PH2Cl in the gas phase. High resolution Fourier transform infrared spectra of these unstable molecules have been recorded for the first time. Rotational analyses for several bands are reported which provide accurate ground state constants and a precise characterization of a number of vibrationally excited states. The accuracy of the ab initio predictions for PH2F and PH2Cl is evaluated by comparisons with these experimental data.

High-Resolution Infrared Spectrum of PH2Br: Rotational Analysis of the ν3 and ν4 Bands

“…Therefore the fit was started with ζ c 3,6 fixed to the ab initio value, and the ν 6 wavenumber was varied in an interval that produced a K a , K a + 1 level crossing of ν 3 and ν 6 between K a (ν 3 ) = 1 and 2. This was intuitively assumed in view of PH 2 Cl and PH 2 I where the crossing occurs between K a = 2 and 3 and 0 and 1, respectively (6,9). This hypothesis was indeed confirmed by the experimental data, Fig.…”

“…The spectral ranges in which absorptions of fundamental bands of PH 2 Br will occur are rather well predictable either by interpolation from PH 2 Cl (6,14) and PH 2 I (9), or on the basis of ab initio predictions (6,7,11,17). Accordingly, the fundamentals ν 1 and ν 5 (ν PH) are expected to lie near 2310 cm −1 , while ν 2 (δ HPH) should be very close to 1100 cm −1 .…”

“…Following the first mention of PH 2 I which, according to NMR studies, was reported to be a product of the scrambling reaction between PH 3 and PI 3 in CS 2 (1) or CFCl 3 (2), and the detection of PH 2 F among the products of the reaction of PH 3 with F 2 in an Ar matrix at 16 K (3), we have initiated a broad experimental study to characterize the whole PH 2 X family by rotational and high-resolution infrared spectroscopy. These experimental studies were closely accompanied by ab initio theoretical calculations providing structures and spectroscopic parameters on the basis of the harmonic and anharmonic force fields (4)(5)(6)(7). The concerted efforts allowed to measure the millimeter-wave (MMW) spectra of PH 2 F (5), PH 2 Cl (5), PH 2 Br (8), and PH 2 I (9), and some of their isotopic species and to record and analyze high-resolution FTIR spectra of PH 2 F (6, 10), PH 2 Cl (6, 10), and PH 2 I (9).…”

“…Several reasons are responsible for this deficiency. First, in contrast to PH 2 F and PH 2 Cl, the bromo species cannot be prepared by reaction of PH 3 F 2 with Lewis-acidic bromides (5,6,10). Second, the feasible gas phase reaction of P 2 H 4 with HBr yielding PH 3 and PH 2 Br proceeds only slowly.…”

“…Third, in view of small isotopic shifts, except for ν 4 , a highresolution spectrum of PH 2 Br is difficult to analyze owing to the presence of the isotopes 79 Br and 81 Br in almost equal abundance. Finally, the rotational structure of the ν 3 band is perturbed by a Coriolis resonance between the ν 3 state and the unobservable, dark state ν 6 . In view of all these deficiencies it took some time after the first gas phase detection of PH 2 Br by MMW spectroscopy (8) to obtain, and successfully analyze, the first highresolution IR spectrum of PH 2 Br in the ν 3 and ν 4 ranges near 810 and 400 cm −1 .…”