Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol

Abstract

Rotational microwave jet spectroscopy studies of the monoterpenol α-fenchol have so far failed to identify its second most stable torsional conformer, despite computational predictions that it is only very slightly higher in energy than the global minimum. Vibrational FTIR and Raman jet spectroscopy investigations reveal unusually complex OH and OD stretching spectra compared to other alcohols. Via modeling of the torsional states, observed spectral splittings are explained by delocalization of the hydroxy hydrogen atom through quantum tunneling between the two non-equivalent but accidentally near-degenerate conformers separated by a low and narrow barrier. The energy differences between the torsional states are determined to be only 16(1) and 7(1) cm−1hc for the protiated and deuterated alcohol, respectively, which further shrink to 9(1) and 3(1) cm−1hc upon OH or OD stretch excitation. Comparisons are made with the more strongly asymmetric monoterpenols borneol and isopinocampheol as well as with the symmetric, rapidly tunneling propargyl alcohol. In addition, the third—in contrast localized—torsional conformer and the most stable dimer are assigned for α-fenchol, as well as the two most stable dimers for propargyl alcohol.

Sniffing out camphor: the fine balance between hydrogen bonding and London dispersion in the chirality recognition with α-fenchol

“…10,25 Subsequent investigation employing FTIR and Raman jet spectroscopy and helium as carrier gas revealed the second and third conformer. 53 However, it was concluded that the hydroxy hydrogen atom is highly delocalized between positions A and B through a tunnelling interaction facilitated by an energetic near-degeneracy as well as a low and narrow interconversion barrier. The difference between the two lowest torsional states was determined as 0.19(1) kJ mol −1 , which represents an upper bound for the difference between the calculated hypothetical localized conformers A and B due to the also contributing tunnelling splitting.…”

“…This means this level of theory somewhat overestimates the E (B)– E (A) difference, as other tested DFT methods do. 53 In the complex with camphor, the tunnelling is expected to be quenched.…”

“…53 An intense signal downshifted at 3499 cm −1 was attributed to the most stable dimer consisting of two ( R )-fenchol molecules with conformer A as the donor and C as the acceptor for the hydrogen bond. 53 These three fenchol absorption bands remain visible when camphor is added. The admixture of ( S )-camphor results in an additional signal at 3517 cm −1 , while in contrast the addition of ( R )-camphor leads to a band at 3504 cm −1 , with signal-to-noise ratios of about 4 : 1 for both.…”

“…Two bands at 3675 and 3666 cm À1 were assigned to monomer transitions from the vibrational Table 1 Computational results at the B3LYP-D3(BJ)/def2-TZVP level of theory for the lower-energy RR and SR complexes with relative energies within 3.0 kJ mol ground state to different torsional levels of the OH stretch excited state. 53 An intense signal downshifted at 3499 cm À1 was attributed to the most stable dimer consisting of two (R)fenchol molecules with conformer A as the donor and C as the acceptor for the hydrogen bond. 53 These three fenchol absorption bands remain visible when camphor is added.…”

Vibrational dynamics of formic acid and its dimer: FTIR and Raman jet spectroscopy and theory

“…5.4, labelled "1D(ICP)". They were obtained with a recent computer implementation of the Chung-Phillips ansatz [257] that was published by Medel et al [258]. Note that there are deviations 178 to recently published values by Hull et al [107].…”