Multidimensional Large Amplitude Dynamics in the Pyridine–Water Complex

Abstract

Aqueous pyridine plays an important role in a variety of catalytic processes aimed at harnessing solar energy. In this work, the pyridine-water interaction is studied by microwave spectroscopy and density functional theory calculations. Water forms a hydrogen bond to the nitrogen with the oxygen tilted slightly toward either of the ortho-hydrogens of the pyridine, and a tunneling motion involving in-plane rocking of the water interconverts the resulting equivalent structures. A pair of tunneling states with severely perturbed rotational spectra is identified and their energy separation, ΔE, is inferred from the perturbations and confirmed by direct measurement. Curiously, values of ΔE are 10404.45 and 13566.94 MHz for the HO and DO complexes, respectively, revealing an inverted isotope effect upon deuteration. Small splittings in some transitions suggest an additional internal motion making this complex an interesting challenge for theoretical treatments of large amplitude motion. The results underscore the significant effect of the ortho-hydrogens on the intermolecular interaction of pyridine.

Noncovalent Interactions and Internal Dynamics in Pyridine–Ammonia: A Combined Quantum‐Chemical and Microwave Spectroscopy Study

“…Our best‐estimated, ZPE‐ and BSSE‐corrected dissociation energy is 11.5 kJ mol −1 , and can be considered a reliable reference value, undoubtedly fulfilling chemical accuracy requirements. This value is perfectly in line with those derived previously for σ‐type pyridine–CH n F 4− n freons complexes (Figure ) …”

“…This value is perfectly in line with those derived previously for s-type pyridine-CH n F 4Àn freons complexes (Figure 1). [41] Experimental Section Methods A1%m ixture of anhydrous NH 3 (> 99.99 %, commercial sample) in Helium at as tagnation pressure of 0.2 MPa was streamed over as ample of pyridine (> 99.8 %, commercial sample) kept at room temperature, and expanded through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry-PØrot-type cavity.T he rotational spectrum in the range 6.5-18.5 GHz was recorded by means of aC OBRA-type [34] pulsed-jet Fourier-transform microwave (FTMW) spectrometer, [35] which is described elsewhere. [36] Rest frequencies were obtained as the arithmetic mean of the frequencies of the two Doppler components.…”

How Aromatic Fluorination Exchanges the Interaction Role of Pyridine with Carbonyl Compounds: The Formaldehyde Adduct

“…Studies on isolated model adducts of hexafluorobenzene⋅⋅⋅water (C 6 F 6 ⋅⋅⋅H 2 O), [15,16] pentafluoropyridine⋅⋅⋅water (C 5 F 5 N⋅⋅⋅H 2 O) [17] or the related complexes with ammonia (C 6 F 6 ⋅⋅⋅NH 3 and C 5 F 5 N⋅⋅⋅NH 3 ) [18] have unambiguously proved the interaction between the oxygen or nitrogen lone pairs and the electron‐deficient π‐clouds of the fully fluorinated aromatic rings. It should be noted that in these cases, the complexes with the corresponding non‐fluorinated rings are stabilized by hydrogen bond interactions [19–23] …”

“…It should be noted that in these cases, the complexes with the corresponding non‐fluorinated rings are stabilized by hydrogen bond interactions. [ 19 , 20 , 21 , 22 , 23 ]…”

“…[ 24 , 29 , 30 ] The structural data obtained from this technique have shown the fundamental differences between interactions like the C−H⋅⋅⋅π bond in benzene⋅⋅⋅H 2 O[ 19 , 20 ] or corannulene⋅⋅⋅H 2 O [31] and the lone pair⋅⋅⋅π‐hole contact responsible for the formation of C 6 F 6 ⋅⋅⋅H 2 O. [16] In the same way, one could compare the differences between the O−H⋅⋅⋅N hydrogen bond, forming pyridine⋅⋅⋅H 2 O, [21] the lone pair⋅⋅⋅π‐hole contact observed in C 5 F 5 N⋅⋅⋅H 2 O [17] or the n →π* interaction dominating the pyridine⋅⋅⋅H 2 CO adduct. [24] Furthermore, the rotational spectrum shows the rich intermolecular dynamics associated with the internal rotation and other intermolecular vibrations of the adduct, ranging from free rotation in C 6 F 6 ⋅⋅⋅H 2 O [16] or corannulene⋅⋅⋅H 2 O [31] to hindered rotations as occur in pyridine⋅⋅⋅H 2 O [21] or triacetonetriperoxyde⋅⋅⋅H 2 O.…”