Probing the global potential energy minimum of (CH2O)2: THz absorption spectrum of (CH2O)2 in solid neon and para-hydrogen

Abstract

The true global potential energy minimum configuration of the formaldehyde dimer (CHO), including the presence of a single or a double weak intermolecular CH⋯O hydrogen bond motif, has been a long-standing subject among both experimentalists and theoreticians as two different energy minima conformations of C and C symmetry have almost identical energies. The present work demonstrates how the class of large-amplitude hydrogen bond vibrational motion probed in the THz region provides excellent direct spectroscopic observables for these weak intermolecular CH⋯O hydrogen bond motifs. The combination of concentration dependency measurements, observed isotopic spectral shifts associated with H/D substitutions and dedicated annealing procedures, enables the unambiguous assignment of three large-amplitude infrared active hydrogen bond vibrational modes for the non-planar C configuration of (CHO) embedded in cryogenic neon and enriched para-hydrogen matrices. A (semi)-empirical value for the change of vibrational zero-point energy of 5.5 ± 0.3 kJ mol is proposed for the dimerization process. These THz spectroscopic observations are complemented by CCSD(T)-F12/aug-cc-pV5Z (electronic energies) and MP2/aug-cc-pVQZ (force fields) electronic structure calculations yielding a (semi)-empirical value of 13.7 ± 0.3 kJ mol for the dissociation energy D of this global potential energy minimum.

Evidence of C–H···O Interactions in the Thiophene:Water Complex

“…A few examples of small-molecule C−H•••O interactions include dimethoxyethane dimer, 14 intermolecular interaction of 1-methoxy-2-(dimethylamino)ethane, 15 and formaldehyde dimer. 16 Additionally, a competition between C−H•••O and O−H•••π was observed in the C 2 H 2 :H 2 O and C 2 H 4 :H 2 O complexes. 17 In the furan:water complex, the oxygen heteroatom is very electronegative; thus, it was a strong director for the final geometry of the complex.…”

“…Despite its familiarity in larger systems, C–H···O interactions are not well documented in small-molecule studies in which only one or two interactions are observed. A few examples of small-molecule C–H···O interactions include dimethoxyethane dimer, intermolecular interaction of 1-methoxy-2-(dimethylamino)­ethane, and formaldehyde dimer . Additionally, a competition between C–H···O and O–H···π was observed in the C 2 H 2 :H 2 O and C 2 H 4 :H 2 O complexes .…”

The effect of alkylation on the micro-solvation of ethers revealed by highly localized water librational motion

“…The cryostat contains a rotatable vacuum shroud with positions for a CsI window and a gate valve for the inlet system. The inert gas matrix was obtained by simultaneous deposition of neon gas (99.999%, Air Liquide) with the sample vapour through separate inlet tubes onto a gold plated oxygen-free high thermal conductivity copper mirror at 3 K. The experimental sensitivity was increased by means of a specially designed dual-pass optical arrangement guiding the focused probe beam onto and back from the cold copper mirror of the cryostat and has previously been employed for investigations of weakly bound molecular complexes [18][19][20] . The neon was supplied via a MKS mass flow controller at 8 sccm flow rate, passing a LN 2 -cooled spiral, depositing approximately 20 mmol of host gas per hour.…”

A computational study on multiple formaldehyde complexes and their possible chemical reactions as well as the catalytic effect in the gas phase

“…The electronic structures and spectra of formaldehyde dimers were studied by several researchers experimentally and theoretically [1,2,17,[26][27][28] Dornshuld et al investigated the minima structures of formaldehyde and thioformaldehyde dimers such as Karpfen studied the minima structures of some dimers such as trans-glyoxal, trans-acrolein, and formaldehyde. He optimized all stationary points at the MP2/aug-cc-pVTZ level and the precise energies computed at the CCSD(T)/ aug-cc-pVTZ level.…”