Isotope effects on hydrogen bonding in acetylene ⋅⋅⋅ trimethylamine complexes isolated in liquid krypton: IR and MP2 study

In this study, the C–H ⋅⋅⋅ N hydrogen-bonded complexes formed by acetylene isotopologues (C2H2, C2D2, C2DH) and trimethylamine [N(CH3)3] were thoroughly investigated using cryosolvent infrared (IR) spectroscopy and anharmonic quantum-chemical calculations at the MP2/6-311++G(d,p) level. Experimental IR spectra recorded at 125 K in krypton, revealed a significant red shift Δν, and a sharp increase in intensity of the ν(CH) and ν(CD) stretching vibrations upon complex formation. For the first time, the study explains the observed difference between red shifts Δ(Δν) induced by the isotopic substitution (H → D) in acetylene in terms of the different changes in force constants ΔK. Furthermore, isotopic substitution was found to have a considerable effect on the value of the dipole moment derivative with respect to the normal coordinate ∂μ/∂Qk, which confirms that the changes in IR intensities are linked to changes in the complex’s electrical properties. The results obtained provide an important scientific contribution to understanding the mechanism of isotope effects in weakly hydrogen-bonded systems.