Infrared spectroscopy of hydrogen-bonded CHCl3–SO2 in the gas phase

Abstract

A molecular association between chloroform and sulfur dioxide in the gas phase at room temperature was studied by Fourier transform infrared spectroscopy. Since the intensity of the CH-stretching fundamental vibration of monomer chloroform is very weak but much stronger upon complexation, a simple subtraction procedure isolated the CH-stretching vibration spectrum of the complex. The presence of a 1:1 complex was confirmed by two dilution series, where the monomer concentrations were varied. The molecular association manifested itself as a shift of the peak absorbance of the CH-stretching vibration of CHCl3-SO2 by +7 cm(-1) and of the CD-stretching vibration of CDCl3-SO2 by +5 cm(-1) to higher wave numbers compared to monomer chloroform, accompanied by a considerable broadening of the band contour. In agreement with previous ab initio calculations, this indicates a "blueshifting" or more appropriately, a "C-H contracting" hydrogen bond between chloroform and sulfur dioxide. An estimate of the complex concentration was made based on ab initio calculations for the integrated band strength and the measured spectrum. With this estimate, the equilibrium constant Kp (295 K)=0.014 (po=10(5) Pa) for the dimerization was calculated, providing one of the very few cases where the formation of a hydrogen-bonded gas phase complex at room temperature could be quantitatively studied by infrared spectroscopy.

CH⋅⋅⋅N Hydrogen‐Bonding Interaction in 7‐Azaindole:CHX3 (X=F, Cl) Complexes

“…They reported CÀH···O interactions [8] in complexes of acetone, oxirane, and dimethylether with CHCl 3 and CHF 3 . [12] In the case of CÀH···p complexes such as complexes of CHX 3 (X = F, Cl) with fluorobenzene, [37] p-cresol, [38] and 3-methylindole [11] the CÀH stretching frequency was found to undergo a blueshift. The CHF 3 complexes showed larger blueshifts of 26.7 and 24.1 cm À1 for acetone and oxirane, respectively.…”

“…Figure 2 b shows the FDIR spectrum corresponding to the 7-AI:CHCl 3 complex. [12,14,24] This indicates that the CÀH stretching transition of the CHCl 3 moiety in this complex is red shifted by 82 cm À1 . An intense and broad peak was observed centered at 2951 cm À1 .…”

“…However, in the case of CÀH···O and CÀH···p interactions, a blueshift has been reported in the CÀH stretching mode in a few cases [7][8][9][10][11][12] instead of the usual redshift. These shifts are much smaller than those observed in stronger H-bonds such as OÀH···O and OÀH···N, in which the vibrational frequency shifts of the hydrogen-bond donor are often greater than 100 cm À1 .…”

“…Another characteristic of strongly hydrogenbonded systems is that these shifts are towards the lowenergy side (redshift). However, in the case of CÀH···O and CÀH···p interactions, a blueshift has been reported in the CÀH stretching mode in a few cases [7][8][9][10][11][12] instead of the usual redshift. In short, in CÀH···O and CÀH···p systems the CÀH vibrational frequency can be expected to undergo a small shift in either direction, that is, red or blue.…”

The effect of large amplitude motions on the vibrational intensities in hydrogen bonded complexes

“…[14][15][16][17][18][19][20][21][22] With this method, equilibrium constants of complex formation are determined from the definition of the equilibrium constant…”