CH⋅⋅⋅O Hydrogen Bonds in Cyclohexenone Reveal the Spectroscopic Behavior of CH and CH Donors

Abstract

C-H...O hydrogen bonds in liquid 2-cyclohexen-1-one are studied to assess the vibrational spectroscopic behavior of the Csp2-H and Csp3-H donors. The presence of a pseudo-isosbestic point in the vC = O region supports the assignment of the two observed bands to two species in equilibrium, considered to be the free and 1:1 associated forms. The values of deltaH degrees =-18.5 +/- 0.6 kJmol(-1) and deltaS degrees = -76 +/- 2 J K(-1) mol(-1) for the dimerization through C-H...O hydrogen bonds were obtained from the dimerization constant at different temperatures. The concentration-dependent intensity of the vCH2 band profile is ascribed to the presence of a blue-shifted band from the hydrogen-bonded Csp3-H group. However, the most surprising result is the absence of concentration- or temperature-dependent intensities in the bands assigned to the stretching modes of the Csp2-H donors.

Crystal structure landscapes from combined vibrational spectroscopy and ab initio calculations: 4-(Dimethylamino)benzaldehyde

“…In particular, for molecules containing the carbonyl group acceptor, the mC=O stretching mode was found to be particularly sensitive to the presence of hydrogen bonds, even the weak C-HÁ Á ÁO type, as shown in several works [17,[21][22][23][24][25][26][27][28].…”

Computationally‐Assisted Approach to the Vibrational Spectra of Molecular Crystals: Study of Hydrogen‐Bonding and Pseudo‐Polymorphism

“…This approach-vibrational spectra assignment by comparison with suitable simulated spectra-has been successfully applied for a couple of systems [25][26][27][28] and is generally recognized as a reliable approach. [24] On the other hand, the B3LYP/6-31G* level is known to provide a good "quality/computational cost ratio" for both geometry optimizations and vibrational spectra simulations in organic systems.…”

“…Despite their weakness, CÀH···O hydrogen bonds are known to play a significant role in crystal packing and have major effects on the vibrational spectra of molecular systems. [25,27,28] Table 1 presents the predicted ab initio wavenumbers for the TP monomer and the wavenumber shifts promoted by considering the different molecular pairs of Figure 3 (only the modes that are significantly affected by the intermolecular contacts are shown).…”

Can a Blue Shift of the CH Stretching Modes of Inactivated C Groups be an Indicator of Weak CH⋅⋅⋅O Hydrogen Bonds?

“…The blueshift criteria was then extended to inactivated C groups, such as alkyl groups attached to none or only one electron-withdrawing species, as an indicator for this kind of hydrogen bond, CÀH···Y, in many experimental studies. [10,11] However, our recent investigations reveal that this is not necessarily evident. [12] Hydrogen bonds formed at the carbonyl group are found to play an important role in the methyl CÀH blue shift in N,N-dimethylformamide-water mixtures.…”