Experimental Evidence for Blue-Shifted Hydrogen Bonding in the Fluoroform–Hydrogen Chloride Complex: A Matrix-Isolation Infrared and ab Initio Study

Abstract

The 1:1 hydrogen-bonded complex of fluoroform and hydrogen chloride was studied using matrix-isolation infrared spectroscopy and ab initio computations. Using B3LYP and MP2 levels of theory with 6-311++G(d,p) and aug-cc-pVDZ basis sets, the structures of the complexes and their energies were computed. For the 1:1 CHF3-HCl complexes, ab initio computations showed two minima, one cyclic and the other acyclic. The cyclic complex was found to have C-H · · · Cl and C-F · · · H interactions, where CHF3 and HCl sub-molecules act as proton donor and proton acceptor, respectively. The second minimum corresponded to an acyclic complex stabilized only by the C-F · · · H interaction, in which CHF3 is the proton acceptor. Experimentally, we could trap the 1:1 CHF3-HCl cyclic complex in an argon matrix, where a blue-shift in the C-H stretching mode of the CHF3 sub-molecule was observed. To understand the nature of the interactions, Atoms in Molecules and Natural Bond Orbital analyses were carried out to unravel the reasons for blue-shifting of the C-H stretching frequency in these complexes.

Interpretation of Spectroscopic Markers of Hydrogen Bonds

“…Rather than the red shift of the OH stretching frequency in the water dimer,t he CH stretchs hifts to the blue. This reversal of the usual pattern in the case of CH···O HBs is not unexpected, based on numerous reports [21,[67][68][69][70][71][72] in the literature for this and similar systems. More importantly,j ust as in the case of the water dimer,t he trendsi nb oth Ds and Dn become more pronounced as the CH 4 andO H 2 molecules are squeezed together,a nd the potential reverses from attractive to repulsive.…”

Assessment of the Presence and Strength of H-Bonds by Means of Corrected NMR

“…Spectroscopic measurements offer a second opportunity to examine a system for possible HBs. Within the context of IR and Raman spectra, the stretching frequency of the A-H covalent bond commonly undergoes a substantial red shift, and the band is intensified and broadened (this pattern can be reversed in certain HBs, largely those involving the CH donor [26,27,28,29,30]). The NMR signal of the bridging proton typically shifts downfield by several ppm upon formation of a HB [20,21,31].…”

Evidence of blue-shifting N–H⋯N hydrogen bonding despite elongation of the N–H bond

“…The phenomenon of 'blue-shiing hydrogen bonding', where the stretching frequency of X-H (n X-H ) is shied to a higher value in a hydrogen bonded X-H/Y type cluster, has been extensively discussed in recent literature. In most of the studies reported to date, a very weakly acidic C-H group has been shown to display blue-shiing H-bonding either with classical acceptors, i.e., the electronegative atom Y having lone pair(s) of electrons [5][6][7][8][11][12][13][14][15][16][17][18][19][20]22,23,[25][26][27][29][30][31][32] or non-classical acceptors with p-electron systems, e.g., aromatic group. 6,14,25 The proposed mechanisms, with many different approaches, were aimed at shortening of X-H bond, and the blue shi of n X-H was considered a direct consequence of X-H bond shortening.…”