posted on 2024-05-09, 04:29authored byBlagoj Achevski, Ljupcho Pejov
In an attempt to gain further insights
into the intermolecular
interactions implied by Rizzo’s group’s cautionary tale
related to molecular tagging in infrared multiple photon dissociation
(IRMPD) spectroscopy with molecular messengers [Masson, A. . J. Chem. Phys. 2015, 143, 104313], in the present study, we provide
an in-depth analysis of the noncovalent interaction between the molecular
hydrogen and protonated betaine molecule in the gas phase. We aim
to shed some new light on the fundamental issues concerning the wide
diapason of hydrogen-bonding-type intermolecular interactions, with
a wide variety of proton acceptors. We demonstrate that in the course
of tagging the protonated betaine with molecular hydrogen from the
OH group side, it is the σ bond of molecular hydrogen that plays
the role of hydrogen-bonding proton acceptor. The tagging thus induces
a small yet significant red shift of the protonated betaine O–H
stretching mode. We investigate the performance of a wide range of
density functional theory (DFT) functionals for the calculation of
anharmonic vibrational frequency shifts of the studied system, which
are essential for the correct interpretation of the experimental IRMPD
data. For an accurate prediction of the OH stretching frequency shifts,
specifically designed functionals such as Handy’s group HCTH/407
should be applied. The empirical dispersion correction enhances the
systematic overestimation of the anharmonic frequency shift, characteristic
of the most widely used DFT functionals. Combining the full-wave function
approach with the charge field perturbation and natural bond orbital
(NBO) deletion analyses, we demonstrate that the frequency shift in
the OH-tagged structure is governed by the σHH →
σ*OH intermolecular charge transfer. This interaction
stabilizes the OH-tagged dimer as well, in contrast to the dipole–quadrupole
electrostatic interaction energy term. Topological analysis of the
electron density reveals the presence of an intermolecular bond critical
point with a positive value of the density Laplacian very close to
the lower limit for hydrogen bonds. NCI analyses demonstrate that
the OH···H2 interaction is weaker than the
intramolecular CH···O one within the protonated betaine
molecule, with the through of reduced density gradient appearing at
less negative sign(λ2)·ρ values. Analyzing
the O–H stretching vibrational potential with the second-generation
absolutely localized molecular orbitals energy decomposition analysis
(ALMO-EDA 2) revealed that in the case of betaineH(+) tagged from
the OH group side, the permanent electrostatics (ΔEelec), polarization (ΔEpol), and charge-transfer (ΔEct) contributions
to the total intermolecular interaction energy contribute favorably
to the weak hydrogen bond formation and to the red shift of the fundamental
O–H stretching frequency, the ΔEct contribution being the most significant in the last context.
The Pauli repulsion term, on the other hand, favors an O–H
stretching frequency blue shift as a consequence of the vibrational
confinement effects.