Blue-Shifted Hydrogen Bonding in the Gas Phase CH/D₃CN···HCCl₃ Complexes

Abstract

Blue-shifting H-bonded complexes between CHCl 3 and CH/D 3 CN have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. The change in FTIR peak intensity of the mixture of the two components as a function of temperature and composition provides the basis for identification of the H-bonded band in the infrared spectrum. On complex formation with CH 3 CN and CD 3 CN, the C−H stretching frequency of CHCl 3 shifts to the blue by +8.7 and +8.6 cm −1 , respectively. The molecular electrostatic potential calculation at the MP2/6-311++G** level has been used to arrive at the geometry of the complex. It has been reported in the literature that CHCl 3 and CH/D 3 CN form red-shifting H-bonded complex in Ar matrix. The red shifting has been verified by doing ab initio calculations in the presence of Ar atoms, which has been attributed to the matrix effect at low temperature. The interaction of Ar with CH 3 CN makes the CH 3 CN more basic and as a result it becomes better hydrogen bond acceptor and causes red shift. The potential energy scans and NBO analysis of the Cl 3 CH•••NCCH 3 complex have been compared with those of F 3 CH•••NCCH 3 and Cl 3 CH•••NH 3 complexes. The change in electron density of the CHCl 3 as a function of C−H•••N distance shows that the approach of CH 3 CN to CHCl 3 induces a shift in electron density from the H atom to the Cl atoms of CHCl 3 which leads to C−H bond contraction and blue shifting of C−H stretching frequency. However, in the complex Cl 3 CH•••NH 3 , where frequency shift to the red is reported, charge transfer and electrostatic interaction dominate.

Importance of water and intramolecular interaction governs substantial blue shift of Csp2–H stretching frequency in complexes between chalcogenoaldehydes and water

“…[25][26][27][28][29][30][31][32][33] More recently, in 2019, a very slight C-H blue-shi of 8.7 cm À1 was even observed by Fourier transform IR spectroscopy for the C-H/N H-bond in the binary Cl 3 CH/NCCH 3 complex. 34 Similar to covalent C sp 3 -H bonds (C sp 3 refers to the tetrahedral carbon), the stretching frequency blue-shis were recently observed in C sp 2-H bonds upon the formation of hydrogenbonded complexes to a larger extent. 35,36 Indeed, the data at the B3LYP/6-311++G(d,p) level of theory reported a large increase in the stretching frequency of C sp 2 -H covalent bond involving in C sp 2 -H/O H-bond of ca.…”

“…A considerable C sp 2 –H blue-shift, up to 81–96 cm −1 in the C sp 2 –H⋯O H-bonds formed by the interactions of formaldehydes and thioformaldehydes with formic acid has recently been reported. 36 In the complexes between HCHO and one or two H 2 O molecules, the blue-shifts of C sp 2 –H stretching frequency were found to be 45 and 66 cm −1 , respectively, 37 which are also significantly greater than those of C sp 3 –H bond in Cl 3 CH⋯NCCH 3 (Δ ν CH = 8.7 cm −1 ) 34 and F 3 CH⋯OH 2 (Δ ν CH = 20.3 cm −1 ). 38 …”

“…A considerable C sp 2-H blue-shi, up to 81-96 cm À1 in the C sp 2-H/O H-bonds formed by the interactions of formaldehydes and thioformaldehydes with formic acid has recently been reported. 36 In the complexes between HCHO and one or two H 2 O molecules, the blue-shis of C sp 2 -H stretching frequency were found to be 45 and 66 cm À1 , respectively, 37 which are also signicantly greater than those of C sp 3 -H bond in Cl 3 CH/NCCH 3 (Dn CH ¼ 8.7 cm À1 ) 34 and F 3 CH/OH 2 (Dn CH ¼ 20.3 cm À1 ). 38 Furthermore, the strength and characteristics of O-H/S, S-H/O/S H-bonds in the systems such as (H 2 O) 2 , (H 2 S) n (n ¼ 2-4), and H 2 O/H 2 S were realized both computationally and experimentally.…”

“… 25–33 More recently, in 2019, a very slight C–H blue-shift of 8.7 cm −1 was even observed by Fourier transform IR spectroscopy for the C–H⋯N H-bond in the binary Cl 3 CH⋯NCCH 3 complex. 34 …”

Probing Blue-Shifting Hydrogen Bonds with Adiabatic Energy Decomposition Analysis

“…While a typical hydrogen-bonding complex X–H···Y is associated with an increased X–H bond length and a red-shift in the XH stretch frequency relative to those in a free XH molecule, there exist exceptions where the X–H bond is contracted and the XH stretch frequency is blue-shifted. Those exceptional cases were referred to as “improper, blue-shifting” H-bonds by Hobza and Havlas. , The blue-shifted XH stretch frequencies can be characterized experimentally, − and due to its apparent peculiarity there has been a plethora of computational studies aiming to unravel the origin of blue-shifting H-bonds, but no consensus has been reached as the conclusions vary with the analysis tools employed and the interpretation of results. To list a few, the origin of the bond contraction and frequency blue-shifting has been attributed to dispersion interactions, ,, charge transfer (CT), ,, electrostatics, − Pauli repulsion, − electron density redistribution/polarization, ,− and so forth.…”