Alkynes as CH/π Acceptors: Microwave Spectra and Structures of the CH₂F₂···Propyne and CH₂ClF···Propyne Dimers

Abstract

Rotational spectra of weakly bound complexes of chlorofluoromethane (CH2ClF) and difluoromethane (CH2F2) with propyne (HCCCH3) have been measured using chirped-pulse and resonant-cavity Fourier-transform microwave spectroscopy, adding to a relatively small body of high resolution spectroscopic data on propyne complexes. Both dimers contain CH/π contacts, as well as secondary contacts between one or both halogen atoms and the methyl group of propyne. A detailed structural determination for CH2F2···propyne has been made by study of the normal, one deuterated and four (13)C substituted isotopologues, with the second lowest energy configuration predicted from ab initio calculations agreeing well with the observed structure. Experimental rotational constants for the most abundant isotopologue of CH2F2···propyne are A00 = 5815.5858(15) MHz, B00 = 1341.1191(5) MHz, C00 = 1099.2040(4) MHz (uncorrected for internal rotation effects), and the dipole moment components, determined by Stark effect measurements, are μa = 1.568(2) D, μb = 0.587(2) D, and μtot = 1.674(3) D. For CH2ClF···propyne, only (35)Cl and (37)Cl isotopologues have been assigned, providing rotational constants and chlorine atom coordinates consistent with the lowest energy structure from a series of ab initio predictions. Rotational constants for the (35)Cl isotopologue are A = 3423.639(7) MHz, B = 1253.7562(20) MHz, and C = 1200.4828(15) MHz and the diagonal and two off diagonal components of the quadrupole coupling tensor have also been determined.

Rotational spectra and conformer geometries of 2-fluorophenol and 3-fluorophenol

“…Fourier transform microwave (FTMW) spectroscopy, in particular, is an ideal tool for such investigations as the compound of interest is interrogated in a collision-free supersonic jet. A number of FTMW studies of weakly bound dimers involving small fluorinated molecules 5,6,7 have been reported in recent years but similar high resolution investigations of intramolecular interactions with fluorine are scarce. In such cases, the classification of an interaction as a hydrogen bond may be ambiguous as the optimal 180 o angle for X-H••• F may not be possible.…”

Fourier Transform Microwave Spectroscopic and ab Initio Study of the Rotamers of 2-Fluorobenzaldehyde and 3-Fluorobenzaldehyde

“…As expected, the largest geometric changes were reported near the site of fluorination and these changes typically involved an increase in the ring angle at that site by 3–4° and a shortening of the adjacent C–C bonds by 0.005–0.010 Å. Although a number of studies of weakly bound dimers involving small fluorinated molecules − have provided important insights into the nature of noncovalent interactions involving fluorine, the observation of these effects may be masked in intramolecular systems as the optimal 180° angle for X–H···F is often unfeasible . Despite these geometric constraints, however, recent studies of ortho fluorinated phenols and thiophenols have successfully identified subtle structural changes that are indicative of weak intramolecular forces involving F. These noncovalent interactions resulted in the stabilization of one planar conformer over the other.…”