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Unraveling the Weak Hydrogen Bonds of Ethynylpyridines and Ethynylbenzene with Trimethylphosphate – A Combined FT-Raman Spectroscopic and Quantum-chemical Study

The interplay of secondary hydrogen bonds of 2- and 3-ethynylpyridine or ethynylbenzene with trimethylphosphate in tetrachloroethene was elucidated using FT-Raman spectroscopy and MP2/6-311+G(d, p) calculations. The direct participation of C≡C moiety in the complex formation was demonstrated by the change in the shape of the C≡C stretching band and further characterized in terms of vibrational dephasing of C≡C stretching. With this aim, the complex band pattern in frequency domain was decomposed using analytical function, introduced by Egelstaff and Schofield and further disseminated by Kirillov, with analytical counterpart in the time domain. The amplitude of frequency fluctuations (M2), frequency modulation time ( taum ) and vibrational dephasing time ( tauv) were determined for both unassociated (C≡C) and associated (C≡C···) ethynyl moieties of 2- and 3-ethynylpyridine and ethynylbenzene. The differences in the dynamical parameters indicate broader distribution of the frequency fluctuations for C≡C··· moiety (M2 ~ 1-2 ps-2) than for C≡C moiety (M2 ≈ 0.5 ps-2), while the average time between perturbative events, as well as the time needed for the phase being completely lost, were shorter for C≡C··· ( taum ~ 0.2-0.7 ps, tauv ≈ 1 ps) than for C≡C moiety ( taum ~ 1.4-1.7 ps, tauv ≈ 2 ps). The shorter taum for C≡C··· moiety of 2-ethynylpyridine ( taum ≈ 0.23 ps), in comparison with analogous quantity of 3-ethynylpyridine and ethynylbenzene ( taum ≈ 0.6 and 0.68 ps), is attributed to more frequent hindering of the C≡C···HCH2 hydrogen bond by spatially close N-atom which competes for H-atom of CH3 group thus making the N···HCH2 hydrogen bond, as predicted by MP2 calculations. Additionally, a hydrogen bond between ortho H- atom of 3-ethynylpyridine and PO(CH3) group of trimethylphosphate is suggested from experimental FT-Raman spectra as well and also computationally verified.

ethynyl compounds ; hydrogen bonding ; vibrational dephasing ; C≡C moiety ; FT-Raman spectroscopy ; quantum chemical calculations

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